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Crystal structures and properties of Ba4Fe2Ti10O27 and Ba3Fe10TiO20
PII: SOO22-3697(97)00019-X
Pergamon
1. Phys. Ckm Solids Vol 58. No. 9. pp. 140-1415. 1997 Published by Elwvier Science Ltd Prinkd in Great Britain 0022-3697197 $17.00 + 0.00
CRYSTAL STRUCTURES AND PROPERTIES OF Ba4Fe2Ti,,,0z7 AND Ba3FeIoTiO2o T. A. VANDERAHa’*,
W. WONG-NG”,
Q. HUANGb,
R. S. ROTH”,
R. G. GEYER’
and R. B. GOLDFARBC “Materials Science and Engineering Laboratory, National Institute of Standards and Technology, Gaithersburg, MD 20899, U.S.A. bDepartment of Materials and Nuclear Engineering, University of Maryland, College Park, MD 10742, U.S.A. ‘Electronics and Electrical Engineering Laboratory, National Institute of Standards and Technology, Boulder, CO 80303, U.S.A. (Received 26 August 1996; accepted 16 December 1996) Abstract-Crystal structures, dielectric properties, and magnetic behaviour of the newly prepared compounds BalFezTi ,002, and BalFeloTi10 are described. Structural studies were carried out by single-crystal X-ray diffraction and neutron powder diffraction. Ba.+FezTi 1002~crystallizes with an eight-layer close-packed stycture having a monoclinic unit cell [space group C2/m, No. 12; a = 19X28( I), b = 11.436(I),c = 9.9020(6) A, fi = 109.30(l)“; ocalc= 4.93 g/cm”] and is isostructural with Ba.+A12Ti ,002,. BalFe IOTi020 is structurally analogous to BatAl loTi and exhibits an open-framework type structure with a monoclinic unit cell [/2/m, No. 12; a = 15.358(l), b = 11.818(l), c = 5.1771(3) A, fl= 91.231(4)“; pcalc = 4.73 g/cm-‘]. Neutron diffraction studies of Ba3Fe ,oTiOzo at room temperature revealed a magnetic lattice with reduced symmetry; an additional magnetic structure was observed at I3 K and below magnetic susceptibility measurements indicated that Bape*Ti ,,,02, is essentially paramagnetic; in contrast, Ba3FeloTiOzo displays complex magnetic behaviour with transitions at 45 and 5 K. For Ba4Fe2Ti02, the values obtained for the permittivityand dielectricloss (tans) are 39 and I X IO-“; for Ba3Fe ,~TiO~~, I5 and 5 X 10m4, respectively. The dielectric properties of both compounds exhibited minimal dependence on frequency in the measured regions (above 6 GHz). Published by Elsevier Science Ltd. Keywords: barium iron titanium oxides, Ba.,Fe?Ti ,002,, BajFe loTiO 20,dielectric properties, magnetic properties, crystal structure, magnetic structure
1. INTRODUCTION
2. EXPERIMENTAL
Magnetic dielectric ceramics are of interest in communications systems as signal circulators and isolators. Desirable properties include high dielectric constant, low dielectric loss, and high saturation magnetization. A study of the Ba0:Fe203Ti02 system was undertaken in order to determine the phase relations, crystal chemistry, and property trends that occur between the technically important barium polytitanates and magnetic ironcontaining oxides. In considering what compounds might form in this system, the existence of the title compounds was suggested by the formation of stoichiometric analogs in the BaO:A1203:TiO* system [l]. Subsequent experimental studies confirmed the formation of 16 ternary phases [2] including Ba4FezTi 1oO27(4:l:lO) and Ba3FeloTi020 (3:5:1) described herein. The crystal and magnetic structures of those compounds were investigated by single-crystal X-ray and neutron powder diffraction. Polycrystalline samples were used to evaluate dielectric properties (permittivites and loss tangents) and magnetic behaviour.
*Author to whom correspondence should be addressed. PCS s-l:%?-0
METHODS
Ba4Fe2Ti10027 (4:l:lO) and BajFeloTiOz,, (3:5:1) were prepared in polycrystalline from by solid state reaction in air of stoichiometric quantities of reagent grade BaC03, Fe203, and phosphate-free Ti02. Before each heating the sample was ground 15-20 min with an agate motar and pestle, pelletized, and placed on a bed of sacrifical powder of the same composition in an alumina combusion boat. The samples were first heated at 1ooO”Cfor -24 h and then twice at 1250°C for 166 and 186 h, respectively. Sample purity was confirmed by analysis of X-ray powder diffraction data using unit cells determined from precession photographs. Single crystals were grown in platinum capsules (approximately 2.5 mm outer diameter, 25 mm long) partly open to the air. Crystals of the 4:l: 10 compound were obtained by heating an offstoichiometric specimen (0.2581:0.0322:0.7097 BaCOs: Fe203:Ti02, fully reacted prior to use as described above) at 1340°C for 6 h, then cooling to 1270°C at 2”/h, followed by furnace cooling to 850°C and then air quenching. 4: 1: 10 crystals were mechanically separated from the nearcomplete melt. Crystals of the 3:5:1 compound were obtained by grain growth: the stoichiometric singlephase polycrystalline sample was heated, with minimal melting, at 1285°C for 63 h followed by air quenching.
1403
T. A. VANDERAH et al.
1404
Table I. X-ray crystallographic, structure solution, and refinement details for Ba$erTi 10027and BajFetaTiOra Ba.+Fe2Ti,a0rr (4:l:lO)
Ba3Fe ,,,TiO2s(3:5: I)
Monoclinic C2/m (No. 12) a = 19.828(l) b= 11.436(l) c = 9.9020(6) p = 109.3O(lj” 4 4.93 g/cm’ 0.71073 = 295 K
Monoclinic 12/m (No. 12) a = 15.358(l) b = 11.818(l) c = 5.1771(3) 6 = 91.231(4)” 2 4.73 g/cm’ 0.71073 = 295 K
25 13.5 < 0 < 24.7 graphite 27 -29 < h < 0 -17
25 10.2 < 8 < 20.7 graphite 37 -26 < h < 26 -1
4357 3827 2905
2936 2465 1765
22.4-35.6% on I 12.54 11.54x lo-4l
26.9-37.4% on I 14.42 12.74 X 10-r\
0.0432 0.1 I57 l/[o’(F:) + (0.0576P)’ + 58.38P] P = (F; + 2F;)/3 1.036 148 8.40 eA_r -4.53 eA_’
0.0326 0.0743 l/[u’(F:) + (0.0285P)2 + 1.83PJ P = (F: + 2F;)/3 1.039 65 1.74 eA_r -1.70eA-’
Crystal system Space group Cell parameters
2
Density (talc) MoKol radiation Temperature Cell parameter LSQ: n reflections angle settings(‘) Monochromator Maximum 0(“) Data range Reflections: total unique Refinements (F, > 4oF,) Corrections: Absorption (ellipsoid) firn (mm-‘) Extinction Refinement on FZ R (F, > 4uFJ WR(F’) W
s # parameters 6P mar SP nlar
Polyctystalline samples were characterized by X-ray powder diffraction using an automated vertical diffractometer equipped
with a theta compensating
graphite post-monochromator.
slit and a
Data were collected using
obtained from least squares refinements using the setting angles of 25 reflections in the range of 10” < 13< 16” for Ba.+Fe2Ti1a027 and 13 < 0 < 24” for BasFetsTi02e. Three
standard
reflections
used
for monitoring
the
Cu Kar radiation in steps of 0.02” 28 at 2 s/step and were
stability of the crystals showed negligible variations in
corrected for systematic error using Si and W as external
intensities.
calibrants.
version of the SHELXTL program suite [4]. The data reduction package included corrections for Lorentz and
crystals
Prior
to
structure
were characterized
(with Zr-filtered
determination,
single
by the precession
method
MO KCYradiation)
All calculations
were carried out using a PC
to assess quality,
polarization effects and for absorption using the empirical
approximate cell parameters, and space group. Unit cells were refined from powder data by the least squares
ellipsoidal approach; the bond lengths and temperature factors obtained for both compounds were reasonable,
method (program CELLSVD [3]).
indicating that the ellipsoidal absorption correction is an
Single-crystal data collection was performed with a VAX300 computer-controlled CAD-4 diffractometer? in
acceptable approximation to the analytical absorption correction approach. Initial refinements of the structures
the w - 28 scan mode using graphite monochromated MO Kor (X = 0.71068 A, Zr filter) radiation. Experimental
were carried out using the published single-crystal data for presumably isostructural Ba3A12Ti ro02, [S] and
conditions and information on the data reduction and refinement results are summarized in Table 1. Cell constants and orientation
matrices for data collection
were
Pb$eAl
,sO2e [6]. Atomic scattering factors were taken
from the International Extinction corrections
Tables for Crystallography [7]. were applied for both data sets
during the last few cycles of refinements. tCertain commercial equipment is identified in order to adequately specify the experimental procedure; recommendation or endorsement by the National Institute of Standards and Technology is not therein implied.
During the
refinements of both structures it was assumed that Ti4+ and Fe3+ would mix on all 6-coordinated (octahedral) metal sites whereas 4-coordinated (tetrahedral) metal sites would be preferentially (100%) occupied by Fe3+.
Structures and properties of barium iron titanium oxides
I
I
I
I
I
I a0
100
I
1405
I
I
4ooo-
I
I
I
I
20
40
60
I
I
120
I
I
140
160
I
2Ww)
Fig. 1. Plot of observed (crosses) and calculated (continuous line) neutron powder diffraction intensity profiles for Ba.,Fe*Ti I0O27at room temperature. The vertical lines indicate the angular positions of Bragg reflections. The lower part of the figure shows the difference plot, Itobsj - Itcalk
The atomic coordinates and thermal parameters of the mixed Fe/Ti octahedral sites were constrained to be the same during the refinements. Since the X-ray scattering factors for Fe and Ti are similar whereas for neutrons they differ in sign, the occupancies of the mixed Fe/Ti sites were taken from the neutron powder diffraction results. Within experimental error, both compounds were found to be stoichiometric. Neutron powder diffraction intensity data were collected using the 32-detector high resolution diffractometer at the National Institute of Standards and Technology. A copper (311) monochromator was employed at a wavelength of 1.5396 A. The horizontal collimations used were 15’, 20’, and 7’ (full width at half maximum) before and after the monochromator, and after the sample, respectivly. Data were collected every 0.05” over a 28 angular range from 3 to 165”. The profile refinements were carried out using the General Strucrture Analysis System (GSAS) program of Larson and Von Dreele [8]. Neutron scattering amplitudes for Ba, Fe, Ti, and 0 were 0.525.0.954, - 0.344, and 0.582 (IO“’ cm), respectively. The refinements were carried out using as initial parameters those obtained from the single-crystal X-ray experiments. The observed neutron powder diffraction pattern of BadFetTii0027 at room temperature could be readily indexed using the monoclinic (CUm) unit cell obtained in the X-ray studies (Table 1); no extra lines were observed. In the refinements the atomic coordinates were fixed at the values obtained from the X-ray experiments. The occupancy parameters of the M(Fe/Ti) sites were varied assuming full occupancy by a mixture of Fe and Ti atoms. Due to the high correlation,
the temperature factors of Ba, M, and 0 were constrained to be equal. The final refinement gave good agreement factors of R, = 3.35, R,, = 4.10, and x2 = 1.084. The temperature factors U, for Ba, M, and 0 sites are 0.014( 1), 0.04(2), and 0.0087(2) (A*), respectively. The agreement between observed and calculated intensities is shown in Fig. 1. Neutron powder diffraction data for the BasFe iaTi02a compound were obtained at room temperature and also at 15, 6.5, and 1.8 K. The lattice indicated by the singlecrystal X-ray experiments (nlm) symmetry) obeys the condition h + k + 1 = 2n. As shown in Fig. 2 (top), reflections with h + k + 1 = 2n + 1 are present in the neutron diffraction pattern. These observations allow us to conclude that the powder sample used in the neutron diffraction experiment has the same nuclear structure as the single crystal used in the X-ray diffraction study and that the magnetic structure is antibody-centred. The refinements were therefore carried out considering both the nuclear and magnetic ordering and assuming that the M (F&i) sites were fully occupied by disordered Fe and Ti atoms. The 4-coordinated M 1 and M4 sites were found to be preferentially occupied by Fe (less than 3% Ti), as had been assumed in the single-crystal X-ray experiments. The R-factors for the final refinement were unchanged with the occupancies of Ml and M4 fixed at 1.0 Fe. The room temperature magnetic structure of BajFeisTi02a exhibits P/2’ symmetry. The good fit between observed and calculated intensities (Fig. 2, bottom) indicates that this model, with indicated moments for the Fe ions of 3.28(3) pa at the Fe1 and Fe4 sites and 2.8( 1) pa at the M2 and M3 sites, adequately
1406
T. A. VANDERAH et al.
2WW
I 80
I
100
1
2Ww)
Fig. 2. Observed (crosses) and calculated (continuous line) neutron powder diffraction intensity profiles for BasFe loTiOmat room temperature. Top: A portion of the pattern with only the nuclear structure model used in the fit; the solid line is the calculated curve and a number of extra lines as observed. The longer vertical lines below the intensity profile indicate the positions of the Bragg reflections assuming the symmetry of space group 12/m, in which the reflections obey the condition h + k + I = 2n + 1; the shorter vertical lines show the positions of reflections with h + k + I= 2n + I. Bottom: Observed intensity profile including both the nuclear and magnetic structures. The lower part of the figure shows the difference plot, ICobs) - Itcal).
describes the magnetic ordering in this structure at room temperature. The final refinement gave good agreement factors of R, = 4.32, R,, = 5.08, and x2 = 1.115. For analysis of the patterns collected at 15.6.5, and 1.8K, the room temperature model of the nuclear structure was also used. The results indicated no observable structural transition down to 1.8 K. The extra magnetic peaks, however, could not be fitted using the model obtained for the room temperature magnetic lattice. Magnetic measurements of randomly oriented polycrystalline samples of Ba.@erTi to027 and Ba3Fe raTi02a
were acquired in stepped fields using a commercial magnetometer equipped with a superconducting quantum interference device (SQUID). Mass magnetization was measured as a function of temperature (300-I .75 K) and applied field (cycled between t 80 kA/m and up to 5.6 MA/m). To characterize irreversible effects, low field mass susceptibility (calculated as mass magnetization divided by applied field) was measured as a function of increasing temperature after zero field cooling (ZFC) and field cooling (FC). For the ZFC data the sample was ac demagnetized at room temperature before cooling to I .75 K.
Structures and properties of barium iron titanium oxides
Table 2.
Atom Bal Ba2 Ba3 Ml M2 M3 M4 M5 M6 M7 M8 M9
StrucNrat
parameters
Site
8j 4i 2a 2d 4i 8j 4h 8j
4F 4i
011 012 013 014 015 016
8j 8j 4i
x
1.0 1.o 1.o 0.07( 1)/0.93( 1) 0.00(3)/l .00(3) 0.29(2)/0.7 l(2) 0.30(2)/0.70(2) 0.2 1(2)/0.79(2) 0.097(8)/0.903(8) 0.13(2)/0.87(2) 0.164(8)/0.836(8) 0.32(1)/0.68( 1) 0.30(1)/0.7q1) 1.o 1.0
4i
Ml0 01 02 03 04 05 06 07 08 09 010
for Ba4FezTi tsO2r (4: 1: IO) from single-crystal X-ray diffraction; occupancies are from the neutron diffraction results occ FefTi
81 4i
8j 8j 8j 4i 4i
8j
0.27286(4) O.OOOOO 0.500008 0.25837( 11) O.OOOOO O.OOOOO 0.50000 0.50000 0.63392( 11) 0.22808( 14) 0.86219(11) 0.25000 O.OOOOO 0.7531(4) 0.2524(4) 0.6304(4) O.OOOOO 0.50000 0.1287(5) 0.2533(5) 0.3749(5) 0.1194(4) O.OOOOO 0.6332(5) 0.1232(4) 0.50000 0.3668(5) 0.00008 0.1150(5)
ES(l2) 0.24718(7) O.OOOOO 0.36842(7) 0.25008 0.11407(13) 0.3019(3) 0.1800(3) 0.0592(3) 0.3169(4) 0.2989(4) 0.4376(3) 0.4341(3) 0.4402(3) 0.0560(3) 0.4327(4) 0.1922(3) 0.3278(3) 0.1879(4) 0.3131(3) O&69(4) 0.1893(3)
1.0 1.0 1.0
8j 4i
Y
0.065 14(2) 0.2026q3) 0.42634(3) 0.37684(6) 0.38732( 10) O.OflOOO
1.0 1.o 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0
8j 8j 8j 8j 4i
Permittivity and dielectric loss tangent measurements were performed at C- and X-band frequencies (6- 12 GHz) at ambient temperature using sintered disks (= 11 mm diameter) of polycrystalline BahFezTi to027and Ba3Fe 1~ TiOzo. A dielectric resonator technique unilizing higherorder TE,, modes was used; this new technique offers the capability to perform dielectric measurements on a single sample at several contiguous frequency subbands. The higher-order mods are tuned by changing the distance between the conductor ground planes of the dielectric resonator. Identification of higher-order TE,,
Table 3. Anisotropic
1407
thermal parameters
U, for the non-exygen
M represents a mixed FeITi site;
Z
0.21622(J) 0.12992(7) 0.28759(6) 0.07670( 13) 0.0776(2) O.OOOOO 0.50000 0.48 18(2) 0.21626(13) 0.50000 0.34144(13) OSOCNI 0.3960(2) 0.3628(5) 0.1150(5) 0.4688(5) 0.3816(g) 0.3485(g) 0.0527(6) 0.301 l(5) 0.05 15(6) 0.443 l(6) 0.3025(7) 0.3694(5) 0.1395(5) 0.1145(g) 0.1238(6) 0.2002(S) 0.3978(6)
U.”
(A*)
0.00986( 12) 0.00867( 14) 0.00784(13) 0.0062(2) 0.0054(3) 0.0132(5) 0.0068(4) 0.0179(4) 0.0062(2) 0.0062(3) 0.0061(2) 0.0218(4) 0.0180(4) 0.0038(8) 0.0059(9) 0.0056(9) 0.0069(13) 0.0094( 14) 0.0084(9) 0.0064(9) 0.0102(1) 0.0071(9) 0.0052( 12) 0.0073(9) 0.0051(S) 0.0062( 12) 0.0072(9) 0.0105(14) 0.0102(10)
resonant modes allows properities to be determined over a broad frequency spectrum. The test sample constitutes a cylindrical dielectric resonator situated between two parallel, silver-coated plates that can be adjusted with an accuracy of ? 0.01 mm. Adjustable coupling loops are used that permit precise control of the coupling coefficients between the sample and the external loads. The sample is placed at the centre of the lower conducting ground plane. The permittivity and dielectric loss tangent are then determined by mode-matching techniques using the separation of the parallel plates, the measured
atoms in Ba4FesTi t@27 (from single-crystal
X-ray diffraction) u12
Bal Ba2 Ba3 Ml M2 M3 M4 M5 M6 M7 MS M9 Ml0
0.0078(2) 0.0076(2) 0.0088(3) 0.0069(5) 0.0067(7) 0.016ql3) 0.0076( 1) 0.0196(10) 0.0074(5) 0.0070(7) 0.0063(5) 0.0161(8) 0.0241(10)
The form of the anisotropic 2hlo’c*U,, + 2klb’c’U23).
0.0141(2) 0.0078(2) 0.0061(2) 0.005 l(5) 0.0036(7) 0.0096( 1 I) 0.0043(9) 0.0177(9) 0.0053(S) 0.0049(7) 0.0059(5) 0.0361(1 I) 0.0094(8) displacement
0.0089(2) O.OlOO(2) 0.0077(3) 0.0079(5) 0.0072(7) 0.0181(13) 0.0078( 10) 0.018(l) 0.0071(5) 0.0078(7) 0.0065(5) 0.0154(g) 0.0300( 1 I) parameters
-0.00008(2) 0.00000 O.OOOOO 0.0002(4) O.OOC00 O.OOOOO O.OOOOO 0.00800 -0.0003(4) O.OOOOO -0.0002(4) 0.0065(8) O.OOOOO
is: exp[ - 2r2[hza’2(U,,
0.0044ql4) 0.0021(2) 0.0015(2) 0.0044(4) 0.0039(6) 0.0111(10) 0.0013(8) 0.0084(8) 0.0041(4) 0.0039(6) 0.0027(4) O.OOSO(7) 0.0218(9)
-0.0015(2) O.OOOOO O.OOOOO 0.0002(4) O.OOOOO O.OOOOO O.OOOOO 000000 0.0003(4) 0.00080 -0.0007(4) 0.0034(8) O.OOOOO
+ k2b*2U22+ 12c’*U3s +Zhk~‘b*U,~
+
T. A. VANDERAH
1408
218
618
31%
?I8
et al.
Fig. 4. A portion of the Ba4FezTi ,o02, (4: 1: 10) structure, viewed along < 100 > , that includes two cp Ba-0 layers about x = 0. Lighter shaded spheres are oxygen, the darker are Ba. Octahedra denote interstices occupied by Fe’+/Ti4+. The coordination number of Bal is eleven owing to the cp vacancy that occurs between each pair of Bal ions: two Ba substitute for three exygens in a row, as is also observed in the structures of the barium polytitanates [ 11. The displacement of Bal towards the vacancy is noted.
3. RESULTS AND DISCUSSION
3.1. Description
of the structures
3.1.1. Ba4FezTi10027. The Fig. 3. Crystal structure of 8L Ba4FeZTi10027 (4:l:lO); slices along the a-axis illustrating the pattern formed within each cp layer. Ba ions (shown as spheres) pack with oxygen ions to form the cp structural matrix. The octahedra are occupied by Fe” and Ti4+ with some preferential ordering. In this structure Fe’+ displays 6-coordination only.
parameters
structural
for the 4:l: 10 compound
are colected
Ti1002, is isostructural (presumably)
frequencies
and
Q-factors
of
the
Conductive
in
distributed
with Ba4A12Ti1,-,02, [ 1, 51 and
Ba4ZnTi,,0z,
[I 11. Fe3+ and Ti4+ are
among 10 crystallographically
sites with some preferential
distinct octa-
ordering;
compound Fe3+ displays 6-coordination modes, and the sample dimensions.
thermal
Table 2 and Table 3. These results confirm that Ba4Fe2.
hedral resonant
and
in this
only. The struc-
TE,,
tural motif is similar to that of the barium polytitanates[ I]
losses
and is comprised
of distorted
close-packed
O/Ba-0
are taken i?to account by measuring their surface resis-
layers in an eight-layer
tance using a sapphire dielectric rod resonator machined
patterns formed within each layer along the close-packing
for TEoll resonance at 10 GHz. For other frequencies,
the
a-direction
to the square
Selected
surface resistance is scaled proportionately root of the frequency. dielectric
For low loss, precisely machined
samples this system can be used with uncer-
tainties of + 0.2% in permittivity
and
? 1 X lo-’
in
dielectric loss tangent. For the sintered disks used in this study, corrections to crystallographic porated by estimating
sequence.
The
are illustrated [ 121 in Fig. 3. bond distances,
site distortion
bond valence sums are collected hedral distortions
observed
described by Schmachtel A13+ analog
ratios, and
in Table 4. The poly-
here are the same as those
and Miiller-Buschbaum
Ba4A12Ti10027 [5]; hence
for the
they are not
density were incor-
caused by electronic factors related to the d-shell contig-
the pore volume of the sample
uration of Fe”. Rather, the highly distorted arrangement is a compromise between the conflicting requirements of
(from size and mass) and applying the Bruggeman effective medium formulation [9] for two-phase composite. Inaccuracy
(8L) chhcchhc
in the measurement
of pore volume
suggests a realistic error limit for the permittivity values of ? 10%. The permittivity and dielectric loss
chemical bonding and spatial filling [ 131. For example, Ba” ions require 12 nearest-neighbour oxygen ions to achieve ideal coordination, yet for electrostatic reasons cannot occupy sites within the cp (close-packed)
layers
(tan 6) values measured for a sintered polycrystalline disk of LaA103 prepared in the same manner as the test
that form interstices occupied by Fe3+/Ti4+ cations. One
samples were 22 and 1.5 X 10V4, respectively, in good agreement with the corresponding values of 23.7 and 1.1 X lo4 obtained at 18 GHz and 298 K for single-crystal
vacancies in the cp layers of the barium polytitanates for three oxygens in a row, the Ba ions. This occurs
LaA103 [IO].
possible
result of this constraint
is the occurrence
of
as observed in the structures [I]; i.e. two Ba ions substitute leaving a cp vacancy between for Bal in the structure of
2.773(5) 2.875(5) 2.997(6) 2.903(5) 3.125(l) 2.783(5) 2.753(5) 2.999(5) 3.115(6) 2.698(5) 3.102(6) 2.920 A 0.863 2.17 V.U. 2.918(5) X 2.759(7) 3.151(6) X 2.828(5) X 2.866(5) X 2.995(8) 3.049(6) X 2.948 A 0.876 2.14 vu. 2.82q5) X 2.786(g) 2.831(6) X 2.869(5) X
2 2
2
2
2 2 2
2
Ba3-014 Ba3-015 Ba3-09 average: distortion v: Ml-02 Ml-06 Ml-07 Ml-012 Ml-08 Ml-014 average: distortion V/[V]: Ti-06 Ti-012 Ti-010 Ti-013 average: distortion V/[V]: M3-08 M3-015 average: distortion V/[V]:
ratio:
ratio:
ratio:
ratio:
1.974 A 0.872 3.98/3.93 1.839(5) 2.057(5) 2.108(7) 1.992(7) 1.982 ii 0.872 3.99/4.o0 2.029(6) 1.987(8) 2.015 A 0.979 3.36l3.71 V.U.
V.U. X4 X 2
V.U. X 2 X 2
1.934(5)
2.13q5) 2.03 l(5) 1.903(6)
1.974(5)
2.755(5) X 2 3.178(5) 2.312(5) X 2 2.863 A 0.867 2.64 V.U. 1.863(5) M4-03 M4-010 average: distortion V/[V]: M5-04 MS-05 M5-011 M5-016 average: distortion V/[V]: M6-01 M6-02 M6-011 M6-014 M6-05 M6-013 average: distortion V/[V]: M7-03 M7-07 M7-09 average: distortion V/[V]: ratio:
ratio:
ratio:
ratio:
v.u.
x 2 x 2
V.U.
x 4 x 2
2.055(S) 1.99W5) 1.989 A 0.854 3.84/3.90 2.081(5) 1.983(S) 1.873(S) 1.979 A 0.900 3.88/3.87
V.U.
vu. x 2 x 2 x 2
1.828(5)
1.984(5) 1.969(7) 1.979 A 0.992 3.6913.70 1.957(8) 2.104(8) 1.913(5) 2.052(6) I .988 A 0.909 3.62t3.79 2.025(J) 1.895(5) 2.14q5) M8-01 M8-03 MS-04 M8-010 M8-012 MS-07 average: distortion ratio: VI[V]: M9-01 I M9-01 M9-016 average: distortion ratio: V/[V]: MlO-09 MlO-05 MlO-015 Mlo-016 average: distortion ratio: V/[V]:
1.954(5) I .988(4) 2.141(5) 19W5) 1.98q5) I .974 A 0.877 3.9013.84 1.946(5) 1.957(5) 2.013(6) 1.972 A 0.967 3.7q3.68 1.939(5) 2.548(8) 1.854(8) 1.984(6) 2.041 A 0.728 3.58I3.70
V.U.
X 2
V.U. X 2
V.U. x 2 x 2 X 2
1.877(S)
site distortion ratios (defined as the ratio of the shottest to thelongest bond distance), bond valence sums* (V), and expected bond valence sums* ([VI) (vu.) about ech of the cation sites in BaJFezTi ,aOr,
*Bond valence sums (V) were calculated from the observed bond distances using the formalism and parameters given in [20]: for each mixed Fe/l? site, a weighted average of the sums calculated using the Fe3+;Ti4+ parameters was taken according to the observed occupancies (from neutron diffraction). For comparison, the expected bond valence sums ([VI) for the mixed M sites were calculated from the formal valences of Fe3+, Ti4+ and the observed occupancies.
Bal-03 Bal-06 Bal-08 Bal-09 Bal-015 Bal-02 Bal-06 Bal-07 Bal -08 Bal-011 Bal-016 average: distortion ratio: v: Ba2-02 Ba2-04 Ba2-08 Ba2-012 Ba2-014 Ba2-015 Ba2-016 average: distortion ratio: V: Ba3-07 Ba3-05 Ba3-08 Ba3-09
Table 4. Selected bond distances (r\) (from X-ray diffraction),
20
4i
8i
4h
26
8i
4i
4i
V
8li
8i
8j
Bal
Ba2
Fe1
M2
M3
Fe4
01
02
03
04
05
06
*Iry = 0.
Site
Atom
Table 5. Structural parameters
1.0
1.0
1.0
1.0
1.0
1.0
1.0
0.458(8)/0.542(8)
0.855(8)/O. 145(8)
1.0
I.0
1.0
occ FeEi
0.3629(2) 0.4356(3) 0.4260(3) -0.0979(3) -0.0983(3) 0.2604(2) 0.261 l(2) 0.0836(2) 0.0836(2) 0.1410(2) 0.1406(2) -0.0722(2) -0.0719(2)
0 0 0.28256(2) 0.2824(4) 0.14477(3) 0.1449(l) 0.5 0.5 0.5 :::6310(3)
x 0 0 0 0 0.13784(5) 0.1375(2) 0.13246(7) 0.1342(3) 0 0 -0.21318(5) -0.2137(2) 0 0 0 0 -0.1377(3) -0.1375(4) 0.2565(3) 0.2560 0.1442(3) 0.1445(3) -0.3818(2) -0.3818(3)
Y
results; occupancies
0.0172(l) 0.016(2) 0.01508(9) 0.013(l) 0.0086( 1) 0.0084(3) 0.0079( 1) 0.007( 1) 0.0083(2) 0.007(l) 0.0087( 1) 0.0084(3) 0.0094(6) 0.009( 1) 0.0153(g) 0.012(l) 0.0167(6) 0.0179(8) 0.0129(5) 0.01 l7(8) 0.0141(5) O.OlOl(7) 0.0098(4) 0.0105(8)
ue,(A*)
are X-ray single-crystal
0 0 0.0261 l(8) 0.0265(9) 0.4856( 1) 0.4866(4) 0 0 -0.5 -0.5 -0.4723( 1) -0.4728(4) -0.1797(7) -0.1795(9) 0.4238(8) 0.4239(9) -0.4222(6) -0.421 l(7) 0.6488(5) 0.6504(7) 0.1276(5) 0.1282(6) -0.1833(5) -0.1847(7)
Z
for BalFe ,,,Ti02,, (35: 1); M represents a mixed Fe/I? site; first line of positional parameters parameters are neutron diffraction results
-3.06(4)
-2.8( 1)
-2.8( 1)
-3.06(3)
P&B)*
0
1.17(7)
B&B)
and tbe second line of positional
3 .?
141 I
Structures and properties of barium iron titanium oxides
Table 6. Anisotropic thermal parameters U,, for the non-oxygen atoms in BajFeI OTiOra (from single-crystal X-ray diffraction)
Bal Ba2 MI M2 Fe3 Fe4
0.0099(2) 0.0099(I)
0.0239(3) 0.0163(2) 0.0086(2) 0.0089(3) 0.0090(5) 0.0096(2)
O.o090(2) 0.0075(3) 0.0092(4) 0.0088(2)
The * form of the *anisotropic
displacement
0.0180(2) 0.0193(2) 0.0082(2) 0.0072(3) 0.0068(4) 0.0076(2)
O.OOOOO O.OOOOO -0.0005(2) O.ooooO O.OOOOO 0.0009(2)
parameters is: exp[-2*2{hZa”2(U,,
0.0037(2) 0.0048( 1) 0.0017(2) 0.@304(2) 0.001 l(3) 0.0003(2)
O.OOOOO O.OOOOO 0.0008(2) O.OOOOO O.OOOOO -0.0003(2)
+ k26*2U22 + t2c*2U3A + 2hb*b*t~,~
+
2hla’c U,j + 2klb*c Ur3).
Ba4FezTi
ta027 and is illustrated
in Fig. 4: between each
pair of Bal sites a cp vacancy occurs, thus lowering its coordination
number
to 11 instead of 12, as seen in
As seen in Table 2, Ti4+ and Fe3+ exhibit preferential
some
ordering among the ten 6-coordinated
metal
sites; site M2 contains only Ti4+. Comparison with Table
Table 4. The absence of the 12th oxygen is compensated
4 reveals no obvious correlation of the occupancies
by shortening of the Bal - 0 bond distances resulting in sum of 2.17 V.U. for Bal. The other bond
the site distortion radios. Instead, the distribution patterns ofTi4+andFe3+, which differ more in formal charge than
valence sums given in Table 4 are in good agreement with those calculated from the formal valences of the
in ionic size (-6% [ 16]), may result from differences in the second coordination spheres of the various M sites:
a valence
cations except for Ba3 and M3 with valence sums of 2.64
higher valent Ti preferentially
and 3.36 v.u., respectively.
These deviations
from the
has the fewest
valence
that the structure
contains
longest
sum rule suggest
residual bond strain [13] despite from ideal packing and polyhedral
the large distortions symmetry.
Ba3 is
and
M3
also
occupies site M2, which
next-nearest-neighbour
distances.
The chemically
exhibit
with
unusual
cations strained
second
at the
sites Ba3
coordination
spheres; both have the largest number of next-nearest-
overbonded and in compressive stress (bonds too short) while M3 is underbonded and in tensile stress (bonds too
neighbour cations as compared to the other Ba and M sites. For example, M3 has 14 cations in its second
long). Brown has described how such residual bond strain
coordination on average.
can result
in important
ferroelectricity
physical
properties
[ 131 and, if large enough,
such as
3.1.2. BajFeloTiOze
structural
instability [ 13- 151.
sphere whereas the other M sites have 8-9
parameters
The
structural
for the 3:5:1 compound
and
thermal
are collected
in
Table 7. Selected bond distances (A) (from X-ray diffraction), site distortion ratios (defined as the ratio of the shortest to the longest bond distance), bond valence sums* (V), and expected bond valence sums* ([VI) (v.u.) about each of the cation sites in BalFe, cTiO,s; bond distances on the second line in brackets are from the neutron diffraction results
average: distortion ratio: v:
3.312(4) X 2 [3.314(5) x 21 2.686(4) X 2 [2.690(5) X 21 2.823(3) X 4 [2.820(3) X 41 3.199 0.7 I 1.76V.U.
Ba2-01
2.602(4)
Ba2-05
2.820(3) X 2 [2.826(6) X 21 2.849(3) X 2 [2.843(6) X 21 3.31 l(3) x 2 [3.31 l(6) X 21 3.00(7) x 2 [3.008~6) X 21 2.953 A 0.78 I .76 V.U.
Bal-02 Ba-02 Ba-05
Fel-02 Fel-03
1.846(2) [1.839(3)] I .829(3) W
Fel-05 average: distortion ratio: V/[Vl:
]]I .857$4)1 1.857 A 0.96 3.08/3.00 v.u.
I]
Ba2-03 Ba2-06 average: distortion ratio: V:
average: distortion ratio: V/[V]: Fe4-03
Fe4-04 Fe4-05
P.a1(7)1
Ba2-03
M3-06
.838(4)1
1.897(3) 1.gW4)1 1.855(3)
Fel-04
M3-01
M2-01 M2-04 M2-06 average distortion ratio V/[V]:
2.06(2) x 2 [2.074(4) X 21 1.980(3) x 2 [1.974(4) x 21 2.007(2) X 2 [1999~4)X 2 2.016 A 0.96 3.08/3.14 V.U.
Fe4-06
1.949(4) [1.946(5) 2.010(3) [2.009{3) 1.990A 0.97 3.5113.54
x 2 X 21 X 4 X 41
V.U.
1.835(3) [ 1.828(4)] 1.882(3) [ 1.878(4)] 1.868(3) [ 1.860(4)] 1.866(3)
[I .878(4)] average: distortion ratio: V/[V]:
1.863 A 0.98 3.02l3.00
V.U.
*Bond valence sums (V) were calculated from the observed bond distances using the formalism and parameters given in [20]. For site, a weighted average of the sums calculated using the Fe’+;Ti + parameters was taken according to the observed occupancies (fmm neutmn diffraction). For comparison, the expected bond valence sums ([VI) for the mixed M sites were calculated from the formal valences of Fe’+, Ti4+ and the observed occupancies.
each mixed FeJTi
T. A. VANDERAHet al.
1412
(a) Fig. 5. Crystal structure of Ba3Fe10Ti020(351) along the (a) c-direction and (b) a-direction. In this framework structure the tetrahedral sites arc preferentially occupied by Fe’+; octahedral sites contain a mixture of Fe”’ and Ti4+. Ba ions are shown as spheres.
Table 5 and Table 6. Selected bond distances and bond valence sums about each cation site are given in Table 7. Our results confirm that this phase is isostrucural with PblAI roGeOzo[61, Pb3AI uSi 161,BaXFeloSn02a 1171,
Ba3AlIoTi02u [l], and SrjAIIOTi020 [18]. The crystal structure features four sites for the smaller cations, two with tetrahedral coordination that are occupied by Fe3+ and two octahedral sites with mixed Fe3+/Ti4’ occupation. As shown in Fig. 5, the structural motif features infinite strips (alternately one- and two-octahedra wide) of edge sharing octahedra interconnected by vertex sharing [Fe041 tetrahedra to form a framework-type structure with channels accommodating the Ba ions. Bal is 8-coordinated and occupies rectangular-shaped channels, shown in Fig. 5(a) near the centre of the unit cell; Ba2 is 9-coordinated and occupies pentagonal-like channels. Both Ba sites have low bond valence sums of 1.76 vu. in this open-framework type structure. The valence sums about the tetrahedral Fe and mixed octahedral Fe/Ti sites agree well with those expected from the formal valences; both polyhedra exhibit only slight distortions from ideal symmetry. The magnetic structure observed for Ba3Fe roTiO2aat room temperature
exhibits P/2’ symmetry
and is illus-
trated in Fig. 6. The magnetic arrangement
features two
sets of collinear spins (Fe1 + Fe4) and (M2 + M3), with antiferromagnetic ordering within each set. The two sets
Magnetic lattice Pt
Fig. 6. The magnetic structure model for Ba?Fe ,aTiOzOfrom room temperature neutron powder diffraction studies. The magnetic lattice exhibits P/2’ symmetry and, unlike that indicated by X-ray diffraction, is antibody-centted. The polyhedral linkage patterns among the magnetic sites (most Ba ions have been omitted for clarity) are illustrated in the bottom part of the figure. Fe1 and Fe4 comprise one set of collinear antiparallel spins residing in the vertex-linked tetrahedrq magnetic interactions occur via superexchange, with Fe-O-Fe angles ranging 120- 140”.Fe located in the edge sharing M2 and M3 octahedral sites comprise the second set of collinear antiparallel spins: direct interactions across shared edges are possible in addition to -90” superexchange via oxygen.
Structures and properties of barium iron titanium oxides correspond
to the two types of polyhedra
occupied
by
Fe3+, as illustrated in the bottom of Fig. 6. Segregation of
Ba0:Fe203:Ti02 4:l:lO
the spins into two sets may be caused by the different types
of exchange
pathways
permitted
1413
s
by the two
polyhedral linkage patterns [19]: The octahedral sites (M2 -I- M3) share edges, hence a pathway exists for direct Fe-Fe exchange superexchange
(via fzl orbitals)
in addition
to -90”
via shared oxygens. In contrast, the tetra-
hedral sites are vertex-linked
octahedra1 sites), thus magnetic superexchange
0 O.BlUum,zFC . 0.8Wm.FC
only (to each other and to interactions
only, with significantly
occur via
larger Fe-O-Fe
angles on the order of 120 to 140”. The neutron diffraction patterns collected
below room temperature,
and 1.8 K, exhibited
100
at 15, 6.5,
magnetic peaks that could not be
fitted using the model obtained for the room temperature magnetic
Ba0:Fe203:TI02
lattice. Some magnetic peaks split and could
not be indexed temperature
150 200 Temperature (K)
with a monoclinic
was successively
unit cell. As the
lowered,
increased
and changes
intensities
of the extra magnetic
4:i:lO
were observed
the splitting in the relative
peaks. These results
suggest that at least one additional
magnetic
structure
exists below room temperature. 3.2. Magnetic behaviour The magnetic susceptibility ture and magnetization
as a function of tempera-
as a function
of applied field
observed for BadFeZTi ,,,02, are shown in Fig. 7. In this compound the magnetic Fe3+ ions are diluted by a large proportion of nonmagnetic (top), the susceptibility
Ti4+ ions. As seen in Fig. 7 curve
indicates
paramagnetic
behaviour deviating somewhat from Curie-law temperature dependence.
The inset shows the somewhat
non-
linear reciprocal susceptibility vs temperature behaviour. Nevertheless, a linear least squares fit of susceptibility to the Curie-Weiss
function
over the entire temperature
range yields a Weiss consant of -2.5 of 5 ccs per formula compound
weight
is a semiconductor
K and a moment
Fig. 7. Magnetic behaviour of Ba.,FezTi1002, (4:l:lO). Top: Mass susceptibility as a function of temperature after zero field cooling (ZFC, open symbols) and field cooling (FC, closed symbols and curves). The measuring field was 0.8 kA/m (10 Oe). Inset: Reciprocal susceptibility vs temperature. The magnetic behaviour deviates from a Curie-law temperature dependence. Bottom: High field magnetization at several temperatures. Below 20 K, there is some curvature as a function of field. (Units: To convert susceptibility values in units of m3/kg to emu/g, multiply by 10’14%. To convert field values in unit& of A/m to Oe, multiply by 4610’. Magnetization values in units of Am*/kg and emu/g are numerically equal [21].)
Ba4Fe~Ti~~O~27.As this [2] this moment is attrib-
uted to Fe3+. The near-coincidence
of the field cooling
transition, such as a N&e1 point, up to 400 K. Low field hysteresis loops, obtained after zero field cooling, exhib-
and zero field cooling curves also reflects the absence of
ited little temperature dependence (Fig. 8, bottom). Given
substantial interactions among the diluted magnetic ions. The mass magnetization as a function of field exhibits
the results of the neutron powder diffraction experiments,
curvature below 20 K in fields up to 5.6 MA/m (Fig. 7,
room temperature
bottom), characteristic
of paramagnets
at low tempera-
which
indicated
two antiferromagnetic
sublattices
as well as the presence
magnetic peaks at liquid helium temperatures,
at
of extra the transi-
ture and high field. The magnetic data for Ba3Fe,oTiOzo are shown in
tions at 5 and 45 K are subtle and perhaps attributable to spin canting.
Fig. 8. Interactions among the magnetic ions are substantial and this compound exhibits irreversibility at all
3.3. Dielectric properties
temperatures. Field cooled and zero field cooled susceptibility curves indicate transitions at 5 and 45 K. The
Ti 10027 and was found to be effectively
transition temperatures
6.28 to 6.68 GHz. For Ba,Fe ,,,Ti02,, a permittivity
are independent
of the measuring
A permittivity
value of 39 was obtained for Ba4Fez. constant from of 15
field
as seen in Fig. 8 (top). Alternating field susceptibility measurements (not shown) in 80 A/m (1 Oe) at IO
was measured with minimal dependence on frequency from 10.4 to 12.0 GHz. BadFezTi 10027is more than twice
and ICKKlHz revealed a frequency-independent maximum at 45 K but no transition at 5 K and no other
as polarizable as BalFe IOTiO?o.Both compounds exhibited only moderate dielectric dispersion with losses
T. A. VANDERAH et al.
1414
Fe/Ti sites in tension. Ba3FeloTiOzo was found to exhibit the same open-framework type structure as Ba3AlloTiOZOand, for convenience, its structure is described in space group Wm. Neutron diffraction studies of Ba3FeIOTi020 revealed a magnetic lattice at room temperature with reduced symmetry; data collected at 15, 6.5, and 1.8 K indicated that at least one other magnetic structure occurs below room temperature. Magnetic susceptibility measurements as a function of temperature indicated that Ba4Fe2Ti 10O27is essentially paramagnetic, a result consistent with its low proportion of magnetic ions. In contrast, Ba3FeloTiOzo displays complex magnetic behaviour with transitions at 45 and 5 K, in agreement with the neutron diffraction studies. The permittivity of Ba4Fe2Ti10027 was 39 with a dielectric loss tangent of 1 X 10e3; the corresponding values obtained for Ba3Fe1,-,TiOZ0were 15 and 5 X lOa. For both compounds the dielectric properties exhibited minimal dependence on frequency in the measured regions above 6 GHz. Ba4FelTi10027 is more than twice as polarizable as Ba3Fe ,oTi02,, and exhibits greater dielectric loss. These results are consistent with the presence of residual bond strain, as indicated by violations of the valence sum rule, in the highly distorted structure of the former compound. Fig. 8. Magnetic behaviour of Ba3Fe 10Ti020(3:5: 1). Top: Mass susceptibility as a function of temperature after zero field cooling (ZFC, open symbols) and field cooling (FC, closed symbols and curves). Measuring fields were 0.8 kA/m (10 Ge) and 8 kA/m (100 Ge). Transitions are observed at 5 and 45 K. Bottom: Low field hysteresis loops at 300,100, and 5 K. Data points are shown only for the 100K data for clarity. Data for 50 K (not shown) are similar to those for 5 K. (Units: see caption to Fig. 6.)
effectively independent of frequency over the measures z;;;ranges(tand= 1 X 10-3for4:1:10and5 X lOA :: .
4. CONCLUSIONS
Crystal structures, dielectric properties, and magnetic behaviour of the newly prepared compounds Ba.+Fe2. Ti ,002, and Ba3Fe ,oTiOzo have been described. Structural studies were carried out by single-crystal X-ray diffraction and neutron powder diffraction. Both structures contain octahedral sites for the B-type cations that are mutually substituted by Fe3+ and Ti4+, with some preferential ordering. In the former compound Fe3+ displays 6-coordination only while in the latter this versatile cation occupies both tetrahedral and octahedral sites, BapezTi 1oO27was confirmed to be isotypic with Ba4A12. Ti ,002, and crystallizes with a highly distorted 8L closepacked (chhcchhc) structure in space group Wm. Deviations from the valence sum rule indicate that the structure contains residual bond strain despite the large distortions, with one of the Ba sites in compression and one of the
Acknowledgemenrs-The authors would like to thank J. M. Loezos for assistance in sample preparation and J. M. Honig, I. D. Brown, and A. Santoro for helpful discussions.
REFERENCES 1. Roth, R. S., Rawn, C. J., Lindsay, C. G. and Wong-Ng, W., Journal of Solid State Chemistry, 1993,104,99-l 18. 2. Vanderah, T. A., Loezos, J. M. and Roth, R. S., Journal of Solid Bare Chemisrry, 1996,121, 38-50. 3. Program CELLSVD by C. K. Lowe-Ma, Naval Air Warfare Centre Weapons Division Technical Publication 8128, September 1993. SHELXTL-v.5, Siemens Industrial Automation, Inc., 1994. Schmachtel, J. and Miiller-Buschbaum, H., Z. Anorg. A&. Chem., 1981.472.89. Vinek, H., Viillenkle, H. and Nowotny, H., MOM& Chem., 1970,101,275. Maflen, E. N.. Fox. A. G. and O’Keefe, M. A., Inremational Tables for X-Ray Crysrallography, Vol. C. Kluwer Academic Publications, 1992. 8. Larson, A. C. and Von Dreele, R. B., General Srrucrure Analysis Sysrem, Report LA-UR-86-748. Los Alamos National Laboratory, Los Alamos, NM 87545,199O. 9. Geyer. R. G., Mantese, J. and Baker-Jarvis, J., NIST Tech. Nore 1371, 1994. 10. Krupka, J., Geyer, R.G., Kuhn, M. and Hinken, J.H., IEEE Tmns. Microwave Theory Technology, 1886.42, 1994. 11. Lindsay, C. G., Rawn. C. J. and Roth, R. S., Powder Dijkcrion,
1994, 9, 56.
12. ATOMS for Windows (version 3.0). computer program to display atomic structures, E. Dowty, Shape Software, Kingsport, Tennessee, 1994. 13. Brown, I. D., Acra Crysrallography, 1992, B48, 553. 14. Brown, I. D., Z. Krisrallogr., 1992, 199, 255.
Structures and properties of barium iron titanium oxides 15. Garcia-MuAoz, J. L. and Rodriguez-Carvajal, J., Journal Solid State Chemist?,
of
1995, 115,324.
16. Shannon, R. D., Actu Clystullography. 1976, A32, 75 I. Cadee, M. C. and Ijdo, D. J. W., Journal of Solid Slate
Goodenough, J. B., Magnetism and the Chemical Chapter 3. John Wiley, New York, 1963. 20. Brese, N. E. and O’Keeffe, M.,Acta Crystallography, 19.
Chemistry, I98
I, 36.3 14.
Zandbergen, H. W. and Ijdo, D. J. W., Materials Bulletin. 1983, 18, 371.
21. Research
Bond, 1991.
B47, 192.
17.
18.
1415
Goldfarb, R. B.,
Concise Encyclopedia of Superconducting and Magnetic Materials. Pergamon Press, Oxford,
1992.
Pergamon
1. Phys. Ckm Solids Vol 58. No. 9. pp. 140-1415. 1997 Published by Elwvier Science Ltd Prinkd in Great Britain 0022-3697197 $17.00 + 0.00
CRYSTAL STRUCTURES AND PROPERTIES OF Ba4Fe2Ti,,,0z7 AND Ba3FeIoTiO2o T. A. VANDERAHa’*,
W. WONG-NG”,
Q. HUANGb,
R. S. ROTH”,
R. G. GEYER’
and R. B. GOLDFARBC “Materials Science and Engineering Laboratory, National Institute of Standards and Technology, Gaithersburg, MD 20899, U.S.A. bDepartment of Materials and Nuclear Engineering, University of Maryland, College Park, MD 10742, U.S.A. ‘Electronics and Electrical Engineering Laboratory, National Institute of Standards and Technology, Boulder, CO 80303, U.S.A. (Received 26 August 1996; accepted 16 December 1996) Abstract-Crystal structures, dielectric properties, and magnetic behaviour of the newly prepared compounds BalFezTi ,002, and BalFeloTi10 are described. Structural studies were carried out by single-crystal X-ray diffraction and neutron powder diffraction. Ba.+FezTi 1002~crystallizes with an eight-layer close-packed stycture having a monoclinic unit cell [space group C2/m, No. 12; a = 19X28( I), b = 11.436(I),c = 9.9020(6) A, fi = 109.30(l)“; ocalc= 4.93 g/cm”] and is isostructural with Ba.+A12Ti ,002,. BalFe IOTi020 is structurally analogous to BatAl loTi and exhibits an open-framework type structure with a monoclinic unit cell [/2/m, No. 12; a = 15.358(l), b = 11.818(l), c = 5.1771(3) A, fl= 91.231(4)“; pcalc = 4.73 g/cm-‘]. Neutron diffraction studies of Ba3Fe ,oTiOzo at room temperature revealed a magnetic lattice with reduced symmetry; an additional magnetic structure was observed at I3 K and below magnetic susceptibility measurements indicated that Bape*Ti ,,,02, is essentially paramagnetic; in contrast, Ba3FeloTiOzo displays complex magnetic behaviour with transitions at 45 and 5 K. For Ba4Fe2Ti02, the values obtained for the permittivityand dielectricloss (tans) are 39 and I X IO-“; for Ba3Fe ,~TiO~~, I5 and 5 X 10m4, respectively. The dielectric properties of both compounds exhibited minimal dependence on frequency in the measured regions (above 6 GHz). Published by Elsevier Science Ltd. Keywords: barium iron titanium oxides, Ba.,Fe?Ti ,002,, BajFe loTiO 20,dielectric properties, magnetic properties, crystal structure, magnetic structure
1. INTRODUCTION
2. EXPERIMENTAL
Magnetic dielectric ceramics are of interest in communications systems as signal circulators and isolators. Desirable properties include high dielectric constant, low dielectric loss, and high saturation magnetization. A study of the Ba0:Fe203Ti02 system was undertaken in order to determine the phase relations, crystal chemistry, and property trends that occur between the technically important barium polytitanates and magnetic ironcontaining oxides. In considering what compounds might form in this system, the existence of the title compounds was suggested by the formation of stoichiometric analogs in the BaO:A1203:TiO* system [l]. Subsequent experimental studies confirmed the formation of 16 ternary phases [2] including Ba4FezTi 1oO27(4:l:lO) and Ba3FeloTi020 (3:5:1) described herein. The crystal and magnetic structures of those compounds were investigated by single-crystal X-ray and neutron powder diffraction. Polycrystalline samples were used to evaluate dielectric properties (permittivites and loss tangents) and magnetic behaviour.
*Author to whom correspondence should be addressed. PCS s-l:%?-0
METHODS
Ba4Fe2Ti10027 (4:l:lO) and BajFeloTiOz,, (3:5:1) were prepared in polycrystalline from by solid state reaction in air of stoichiometric quantities of reagent grade BaC03, Fe203, and phosphate-free Ti02. Before each heating the sample was ground 15-20 min with an agate motar and pestle, pelletized, and placed on a bed of sacrifical powder of the same composition in an alumina combusion boat. The samples were first heated at 1ooO”Cfor -24 h and then twice at 1250°C for 166 and 186 h, respectively. Sample purity was confirmed by analysis of X-ray powder diffraction data using unit cells determined from precession photographs. Single crystals were grown in platinum capsules (approximately 2.5 mm outer diameter, 25 mm long) partly open to the air. Crystals of the 4:l: 10 compound were obtained by heating an offstoichiometric specimen (0.2581:0.0322:0.7097 BaCOs: Fe203:Ti02, fully reacted prior to use as described above) at 1340°C for 6 h, then cooling to 1270°C at 2”/h, followed by furnace cooling to 850°C and then air quenching. 4: 1: 10 crystals were mechanically separated from the nearcomplete melt. Crystals of the 3:5:1 compound were obtained by grain growth: the stoichiometric singlephase polycrystalline sample was heated, with minimal melting, at 1285°C for 63 h followed by air quenching.
1403
T. A. VANDERAH et al.
1404
Table I. X-ray crystallographic, structure solution, and refinement details for Ba$erTi 10027and BajFetaTiOra Ba.+Fe2Ti,a0rr (4:l:lO)
Ba3Fe ,,,TiO2s(3:5: I)
Monoclinic C2/m (No. 12) a = 19.828(l) b= 11.436(l) c = 9.9020(6) p = 109.3O(lj” 4 4.93 g/cm’ 0.71073 = 295 K
Monoclinic 12/m (No. 12) a = 15.358(l) b = 11.818(l) c = 5.1771(3) 6 = 91.231(4)” 2 4.73 g/cm’ 0.71073 = 295 K
25 13.5 < 0 < 24.7 graphite 27 -29 < h < 0 -17
25 10.2 < 8 < 20.7 graphite 37 -26 < h < 26 -1
4357 3827 2905
2936 2465 1765
22.4-35.6% on I 12.54 11.54x lo-4l
26.9-37.4% on I 14.42 12.74 X 10-r\
0.0432 0.1 I57 l/[o’(F:) + (0.0576P)’ + 58.38P] P = (F; + 2F;)/3 1.036 148 8.40 eA_r -4.53 eA_’
0.0326 0.0743 l/[u’(F:) + (0.0285P)2 + 1.83PJ P = (F: + 2F;)/3 1.039 65 1.74 eA_r -1.70eA-’
Crystal system Space group Cell parameters
2
Density (talc) MoKol radiation Temperature Cell parameter LSQ: n reflections angle settings(‘) Monochromator Maximum 0(“) Data range Reflections: total unique Refinements (F, > 4oF,) Corrections: Absorption (ellipsoid) firn (mm-‘) Extinction Refinement on FZ R (F, > 4uFJ WR(F’) W
s # parameters 6P mar SP nlar
Polyctystalline samples were characterized by X-ray powder diffraction using an automated vertical diffractometer equipped
with a theta compensating
graphite post-monochromator.
slit and a
Data were collected using
obtained from least squares refinements using the setting angles of 25 reflections in the range of 10” < 13< 16” for Ba.+Fe2Ti1a027 and 13 < 0 < 24” for BasFetsTi02e. Three
standard
reflections
used
for monitoring
the
Cu Kar radiation in steps of 0.02” 28 at 2 s/step and were
stability of the crystals showed negligible variations in
corrected for systematic error using Si and W as external
intensities.
calibrants.
version of the SHELXTL program suite [4]. The data reduction package included corrections for Lorentz and
crystals
Prior
to
structure
were characterized
(with Zr-filtered
determination,
single
by the precession
method
MO KCYradiation)
All calculations
were carried out using a PC
to assess quality,
polarization effects and for absorption using the empirical
approximate cell parameters, and space group. Unit cells were refined from powder data by the least squares
ellipsoidal approach; the bond lengths and temperature factors obtained for both compounds were reasonable,
method (program CELLSVD [3]).
indicating that the ellipsoidal absorption correction is an
Single-crystal data collection was performed with a VAX300 computer-controlled CAD-4 diffractometer? in
acceptable approximation to the analytical absorption correction approach. Initial refinements of the structures
the w - 28 scan mode using graphite monochromated MO Kor (X = 0.71068 A, Zr filter) radiation. Experimental
were carried out using the published single-crystal data for presumably isostructural Ba3A12Ti ro02, [S] and
conditions and information on the data reduction and refinement results are summarized in Table 1. Cell constants and orientation
matrices for data collection
were
Pb$eAl
,sO2e [6]. Atomic scattering factors were taken
from the International Extinction corrections
Tables for Crystallography [7]. were applied for both data sets
during the last few cycles of refinements. tCertain commercial equipment is identified in order to adequately specify the experimental procedure; recommendation or endorsement by the National Institute of Standards and Technology is not therein implied.
During the
refinements of both structures it was assumed that Ti4+ and Fe3+ would mix on all 6-coordinated (octahedral) metal sites whereas 4-coordinated (tetrahedral) metal sites would be preferentially (100%) occupied by Fe3+.
Structures and properties of barium iron titanium oxides
I
I
I
I
I
I a0
100
I
1405
I
I
4ooo-
I
I
I
I
20
40
60
I
I
120
I
I
140
160
I
2Ww)
Fig. 1. Plot of observed (crosses) and calculated (continuous line) neutron powder diffraction intensity profiles for Ba.,Fe*Ti I0O27at room temperature. The vertical lines indicate the angular positions of Bragg reflections. The lower part of the figure shows the difference plot, Itobsj - Itcalk
The atomic coordinates and thermal parameters of the mixed Fe/Ti octahedral sites were constrained to be the same during the refinements. Since the X-ray scattering factors for Fe and Ti are similar whereas for neutrons they differ in sign, the occupancies of the mixed Fe/Ti sites were taken from the neutron powder diffraction results. Within experimental error, both compounds were found to be stoichiometric. Neutron powder diffraction intensity data were collected using the 32-detector high resolution diffractometer at the National Institute of Standards and Technology. A copper (311) monochromator was employed at a wavelength of 1.5396 A. The horizontal collimations used were 15’, 20’, and 7’ (full width at half maximum) before and after the monochromator, and after the sample, respectivly. Data were collected every 0.05” over a 28 angular range from 3 to 165”. The profile refinements were carried out using the General Strucrture Analysis System (GSAS) program of Larson and Von Dreele [8]. Neutron scattering amplitudes for Ba, Fe, Ti, and 0 were 0.525.0.954, - 0.344, and 0.582 (IO“’ cm), respectively. The refinements were carried out using as initial parameters those obtained from the single-crystal X-ray experiments. The observed neutron powder diffraction pattern of BadFetTii0027 at room temperature could be readily indexed using the monoclinic (CUm) unit cell obtained in the X-ray studies (Table 1); no extra lines were observed. In the refinements the atomic coordinates were fixed at the values obtained from the X-ray experiments. The occupancy parameters of the M(Fe/Ti) sites were varied assuming full occupancy by a mixture of Fe and Ti atoms. Due to the high correlation,
the temperature factors of Ba, M, and 0 were constrained to be equal. The final refinement gave good agreement factors of R, = 3.35, R,, = 4.10, and x2 = 1.084. The temperature factors U, for Ba, M, and 0 sites are 0.014( 1), 0.04(2), and 0.0087(2) (A*), respectively. The agreement between observed and calculated intensities is shown in Fig. 1. Neutron powder diffraction data for the BasFe iaTi02a compound were obtained at room temperature and also at 15, 6.5, and 1.8 K. The lattice indicated by the singlecrystal X-ray experiments (nlm) symmetry) obeys the condition h + k + 1 = 2n. As shown in Fig. 2 (top), reflections with h + k + 1 = 2n + 1 are present in the neutron diffraction pattern. These observations allow us to conclude that the powder sample used in the neutron diffraction experiment has the same nuclear structure as the single crystal used in the X-ray diffraction study and that the magnetic structure is antibody-centred. The refinements were therefore carried out considering both the nuclear and magnetic ordering and assuming that the M (F&i) sites were fully occupied by disordered Fe and Ti atoms. The 4-coordinated M 1 and M4 sites were found to be preferentially occupied by Fe (less than 3% Ti), as had been assumed in the single-crystal X-ray experiments. The R-factors for the final refinement were unchanged with the occupancies of Ml and M4 fixed at 1.0 Fe. The room temperature magnetic structure of BajFeisTi02a exhibits P/2’ symmetry. The good fit between observed and calculated intensities (Fig. 2, bottom) indicates that this model, with indicated moments for the Fe ions of 3.28(3) pa at the Fe1 and Fe4 sites and 2.8( 1) pa at the M2 and M3 sites, adequately
1406
T. A. VANDERAH et al.
2WW
I 80
I
100
1
2Ww)
Fig. 2. Observed (crosses) and calculated (continuous line) neutron powder diffraction intensity profiles for BasFe loTiOmat room temperature. Top: A portion of the pattern with only the nuclear structure model used in the fit; the solid line is the calculated curve and a number of extra lines as observed. The longer vertical lines below the intensity profile indicate the positions of the Bragg reflections assuming the symmetry of space group 12/m, in which the reflections obey the condition h + k + I = 2n + 1; the shorter vertical lines show the positions of reflections with h + k + I= 2n + I. Bottom: Observed intensity profile including both the nuclear and magnetic structures. The lower part of the figure shows the difference plot, ICobs) - Itcal).
describes the magnetic ordering in this structure at room temperature. The final refinement gave good agreement factors of R, = 4.32, R,, = 5.08, and x2 = 1.115. For analysis of the patterns collected at 15.6.5, and 1.8K, the room temperature model of the nuclear structure was also used. The results indicated no observable structural transition down to 1.8 K. The extra magnetic peaks, however, could not be fitted using the model obtained for the room temperature magnetic lattice. Magnetic measurements of randomly oriented polycrystalline samples of Ba.@erTi to027 and Ba3Fe raTi02a
were acquired in stepped fields using a commercial magnetometer equipped with a superconducting quantum interference device (SQUID). Mass magnetization was measured as a function of temperature (300-I .75 K) and applied field (cycled between t 80 kA/m and up to 5.6 MA/m). To characterize irreversible effects, low field mass susceptibility (calculated as mass magnetization divided by applied field) was measured as a function of increasing temperature after zero field cooling (ZFC) and field cooling (FC). For the ZFC data the sample was ac demagnetized at room temperature before cooling to I .75 K.
Structures and properties of barium iron titanium oxides
Table 2.
Atom Bal Ba2 Ba3 Ml M2 M3 M4 M5 M6 M7 M8 M9
StrucNrat
parameters
Site
8j 4i 2a 2d 4i 8j 4h 8j
4F 4i
011 012 013 014 015 016
8j 8j 4i
x
1.0 1.o 1.o 0.07( 1)/0.93( 1) 0.00(3)/l .00(3) 0.29(2)/0.7 l(2) 0.30(2)/0.70(2) 0.2 1(2)/0.79(2) 0.097(8)/0.903(8) 0.13(2)/0.87(2) 0.164(8)/0.836(8) 0.32(1)/0.68( 1) 0.30(1)/0.7q1) 1.o 1.0
4i
Ml0 01 02 03 04 05 06 07 08 09 010
for Ba4FezTi tsO2r (4: 1: IO) from single-crystal X-ray diffraction; occupancies are from the neutron diffraction results occ FefTi
81 4i
8j 8j 8j 4i 4i
8j
0.27286(4) O.OOOOO 0.500008 0.25837( 11) O.OOOOO O.OOOOO 0.50000 0.50000 0.63392( 11) 0.22808( 14) 0.86219(11) 0.25000 O.OOOOO 0.7531(4) 0.2524(4) 0.6304(4) O.OOOOO 0.50000 0.1287(5) 0.2533(5) 0.3749(5) 0.1194(4) O.OOOOO 0.6332(5) 0.1232(4) 0.50000 0.3668(5) 0.00008 0.1150(5)
ES(l2) 0.24718(7) O.OOOOO 0.36842(7) 0.25008 0.11407(13) 0.3019(3) 0.1800(3) 0.0592(3) 0.3169(4) 0.2989(4) 0.4376(3) 0.4341(3) 0.4402(3) 0.0560(3) 0.4327(4) 0.1922(3) 0.3278(3) 0.1879(4) 0.3131(3) O&69(4) 0.1893(3)
1.0 1.0 1.0
8j 4i
Y
0.065 14(2) 0.2026q3) 0.42634(3) 0.37684(6) 0.38732( 10) O.OflOOO
1.0 1.o 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0
8j 8j 8j 8j 4i
Permittivity and dielectric loss tangent measurements were performed at C- and X-band frequencies (6- 12 GHz) at ambient temperature using sintered disks (= 11 mm diameter) of polycrystalline BahFezTi to027and Ba3Fe 1~ TiOzo. A dielectric resonator technique unilizing higherorder TE,, modes was used; this new technique offers the capability to perform dielectric measurements on a single sample at several contiguous frequency subbands. The higher-order mods are tuned by changing the distance between the conductor ground planes of the dielectric resonator. Identification of higher-order TE,,
Table 3. Anisotropic
1407
thermal parameters
U, for the non-exygen
M represents a mixed FeITi site;
Z
0.21622(J) 0.12992(7) 0.28759(6) 0.07670( 13) 0.0776(2) O.OOOOO 0.50000 0.48 18(2) 0.21626(13) 0.50000 0.34144(13) OSOCNI 0.3960(2) 0.3628(5) 0.1150(5) 0.4688(5) 0.3816(g) 0.3485(g) 0.0527(6) 0.301 l(5) 0.05 15(6) 0.443 l(6) 0.3025(7) 0.3694(5) 0.1395(5) 0.1145(g) 0.1238(6) 0.2002(S) 0.3978(6)
U.”
(A*)
0.00986( 12) 0.00867( 14) 0.00784(13) 0.0062(2) 0.0054(3) 0.0132(5) 0.0068(4) 0.0179(4) 0.0062(2) 0.0062(3) 0.0061(2) 0.0218(4) 0.0180(4) 0.0038(8) 0.0059(9) 0.0056(9) 0.0069(13) 0.0094( 14) 0.0084(9) 0.0064(9) 0.0102(1) 0.0071(9) 0.0052( 12) 0.0073(9) 0.0051(S) 0.0062( 12) 0.0072(9) 0.0105(14) 0.0102(10)
resonant modes allows properities to be determined over a broad frequency spectrum. The test sample constitutes a cylindrical dielectric resonator situated between two parallel, silver-coated plates that can be adjusted with an accuracy of ? 0.01 mm. Adjustable coupling loops are used that permit precise control of the coupling coefficients between the sample and the external loads. The sample is placed at the centre of the lower conducting ground plane. The permittivity and dielectric loss tangent are then determined by mode-matching techniques using the separation of the parallel plates, the measured
atoms in Ba4FesTi t@27 (from single-crystal
X-ray diffraction) u12
Bal Ba2 Ba3 Ml M2 M3 M4 M5 M6 M7 MS M9 Ml0
0.0078(2) 0.0076(2) 0.0088(3) 0.0069(5) 0.0067(7) 0.016ql3) 0.0076( 1) 0.0196(10) 0.0074(5) 0.0070(7) 0.0063(5) 0.0161(8) 0.0241(10)
The form of the anisotropic 2hlo’c*U,, + 2klb’c’U23).
0.0141(2) 0.0078(2) 0.0061(2) 0.005 l(5) 0.0036(7) 0.0096( 1 I) 0.0043(9) 0.0177(9) 0.0053(S) 0.0049(7) 0.0059(5) 0.0361(1 I) 0.0094(8) displacement
0.0089(2) O.OlOO(2) 0.0077(3) 0.0079(5) 0.0072(7) 0.0181(13) 0.0078( 10) 0.018(l) 0.0071(5) 0.0078(7) 0.0065(5) 0.0154(g) 0.0300( 1 I) parameters
-0.00008(2) 0.00000 O.OOOOO 0.0002(4) O.OOC00 O.OOOOO O.OOOOO 0.00800 -0.0003(4) O.OOOOO -0.0002(4) 0.0065(8) O.OOOOO
is: exp[ - 2r2[hza’2(U,,
0.0044ql4) 0.0021(2) 0.0015(2) 0.0044(4) 0.0039(6) 0.0111(10) 0.0013(8) 0.0084(8) 0.0041(4) 0.0039(6) 0.0027(4) O.OOSO(7) 0.0218(9)
-0.0015(2) O.OOOOO O.OOOOO 0.0002(4) O.OOOOO O.OOOOO O.OOOOO 000000 0.0003(4) 0.00080 -0.0007(4) 0.0034(8) O.OOOOO
+ k2b*2U22+ 12c’*U3s +Zhk~‘b*U,~
+
T. A. VANDERAH
1408
218
618
31%
?I8
et al.
Fig. 4. A portion of the Ba4FezTi ,o02, (4: 1: 10) structure, viewed along < 100 > , that includes two cp Ba-0 layers about x = 0. Lighter shaded spheres are oxygen, the darker are Ba. Octahedra denote interstices occupied by Fe’+/Ti4+. The coordination number of Bal is eleven owing to the cp vacancy that occurs between each pair of Bal ions: two Ba substitute for three exygens in a row, as is also observed in the structures of the barium polytitanates [ 11. The displacement of Bal towards the vacancy is noted.
3. RESULTS AND DISCUSSION
3.1. Description
of the structures
3.1.1. Ba4FezTi10027. The Fig. 3. Crystal structure of 8L Ba4FeZTi10027 (4:l:lO); slices along the a-axis illustrating the pattern formed within each cp layer. Ba ions (shown as spheres) pack with oxygen ions to form the cp structural matrix. The octahedra are occupied by Fe” and Ti4+ with some preferential ordering. In this structure Fe’+ displays 6-coordination only.
parameters
structural
for the 4:l: 10 compound
are colected
Ti1002, is isostructural (presumably)
frequencies
and
Q-factors
of
the
Conductive
in
distributed
with Ba4A12Ti1,-,02, [ 1, 51 and
Ba4ZnTi,,0z,
[I 11. Fe3+ and Ti4+ are
among 10 crystallographically
sites with some preferential
distinct octa-
ordering;
compound Fe3+ displays 6-coordination modes, and the sample dimensions.
thermal
Table 2 and Table 3. These results confirm that Ba4Fe2.
hedral resonant
and
in this
only. The struc-
TE,,
tural motif is similar to that of the barium polytitanates[ I]
losses
and is comprised
of distorted
close-packed
O/Ba-0
are taken i?to account by measuring their surface resis-
layers in an eight-layer
tance using a sapphire dielectric rod resonator machined
patterns formed within each layer along the close-packing
for TEoll resonance at 10 GHz. For other frequencies,
the
a-direction
to the square
Selected
surface resistance is scaled proportionately root of the frequency. dielectric
For low loss, precisely machined
samples this system can be used with uncer-
tainties of + 0.2% in permittivity
and
? 1 X lo-’
in
dielectric loss tangent. For the sintered disks used in this study, corrections to crystallographic porated by estimating
sequence.
The
are illustrated [ 121 in Fig. 3. bond distances,
site distortion
bond valence sums are collected hedral distortions
observed
described by Schmachtel A13+ analog
ratios, and
in Table 4. The poly-
here are the same as those
and Miiller-Buschbaum
Ba4A12Ti10027 [5]; hence
for the
they are not
density were incor-
caused by electronic factors related to the d-shell contig-
the pore volume of the sample
uration of Fe”. Rather, the highly distorted arrangement is a compromise between the conflicting requirements of
(from size and mass) and applying the Bruggeman effective medium formulation [9] for two-phase composite. Inaccuracy
(8L) chhcchhc
in the measurement
of pore volume
suggests a realistic error limit for the permittivity values of ? 10%. The permittivity and dielectric loss
chemical bonding and spatial filling [ 131. For example, Ba” ions require 12 nearest-neighbour oxygen ions to achieve ideal coordination, yet for electrostatic reasons cannot occupy sites within the cp (close-packed)
layers
(tan 6) values measured for a sintered polycrystalline disk of LaA103 prepared in the same manner as the test
that form interstices occupied by Fe3+/Ti4+ cations. One
samples were 22 and 1.5 X 10V4, respectively, in good agreement with the corresponding values of 23.7 and 1.1 X lo4 obtained at 18 GHz and 298 K for single-crystal
vacancies in the cp layers of the barium polytitanates for three oxygens in a row, the Ba ions. This occurs
LaA103 [IO].
possible
result of this constraint
is the occurrence
of
as observed in the structures [I]; i.e. two Ba ions substitute leaving a cp vacancy between for Bal in the structure of
2.773(5) 2.875(5) 2.997(6) 2.903(5) 3.125(l) 2.783(5) 2.753(5) 2.999(5) 3.115(6) 2.698(5) 3.102(6) 2.920 A 0.863 2.17 V.U. 2.918(5) X 2.759(7) 3.151(6) X 2.828(5) X 2.866(5) X 2.995(8) 3.049(6) X 2.948 A 0.876 2.14 vu. 2.82q5) X 2.786(g) 2.831(6) X 2.869(5) X
2 2
2
2
2 2 2
2
Ba3-014 Ba3-015 Ba3-09 average: distortion v: Ml-02 Ml-06 Ml-07 Ml-012 Ml-08 Ml-014 average: distortion V/[V]: Ti-06 Ti-012 Ti-010 Ti-013 average: distortion V/[V]: M3-08 M3-015 average: distortion V/[V]:
ratio:
ratio:
ratio:
ratio:
1.974 A 0.872 3.98/3.93 1.839(5) 2.057(5) 2.108(7) 1.992(7) 1.982 ii 0.872 3.99/4.o0 2.029(6) 1.987(8) 2.015 A 0.979 3.36l3.71 V.U.
V.U. X4 X 2
V.U. X 2 X 2
1.934(5)
2.13q5) 2.03 l(5) 1.903(6)
1.974(5)
2.755(5) X 2 3.178(5) 2.312(5) X 2 2.863 A 0.867 2.64 V.U. 1.863(5) M4-03 M4-010 average: distortion V/[V]: M5-04 MS-05 M5-011 M5-016 average: distortion V/[V]: M6-01 M6-02 M6-011 M6-014 M6-05 M6-013 average: distortion V/[V]: M7-03 M7-07 M7-09 average: distortion V/[V]: ratio:
ratio:
ratio:
ratio:
v.u.
x 2 x 2
V.U.
x 4 x 2
2.055(S) 1.99W5) 1.989 A 0.854 3.84/3.90 2.081(5) 1.983(S) 1.873(S) 1.979 A 0.900 3.88/3.87
V.U.
vu. x 2 x 2 x 2
1.828(5)
1.984(5) 1.969(7) 1.979 A 0.992 3.6913.70 1.957(8) 2.104(8) 1.913(5) 2.052(6) I .988 A 0.909 3.62t3.79 2.025(J) 1.895(5) 2.14q5) M8-01 M8-03 MS-04 M8-010 M8-012 MS-07 average: distortion ratio: VI[V]: M9-01 I M9-01 M9-016 average: distortion ratio: V/[V]: MlO-09 MlO-05 MlO-015 Mlo-016 average: distortion ratio: V/[V]:
1.954(5) I .988(4) 2.141(5) 19W5) 1.98q5) I .974 A 0.877 3.9013.84 1.946(5) 1.957(5) 2.013(6) 1.972 A 0.967 3.7q3.68 1.939(5) 2.548(8) 1.854(8) 1.984(6) 2.041 A 0.728 3.58I3.70
V.U.
X 2
V.U. X 2
V.U. x 2 x 2 X 2
1.877(S)
site distortion ratios (defined as the ratio of the shottest to thelongest bond distance), bond valence sums* (V), and expected bond valence sums* ([VI) (vu.) about ech of the cation sites in BaJFezTi ,aOr,
*Bond valence sums (V) were calculated from the observed bond distances using the formalism and parameters given in [20]: for each mixed Fe/l? site, a weighted average of the sums calculated using the Fe3+;Ti4+ parameters was taken according to the observed occupancies (from neutron diffraction). For comparison, the expected bond valence sums ([VI) for the mixed M sites were calculated from the formal valences of Fe3+, Ti4+ and the observed occupancies.
Bal-03 Bal-06 Bal-08 Bal-09 Bal-015 Bal-02 Bal-06 Bal-07 Bal -08 Bal-011 Bal-016 average: distortion ratio: v: Ba2-02 Ba2-04 Ba2-08 Ba2-012 Ba2-014 Ba2-015 Ba2-016 average: distortion ratio: V: Ba3-07 Ba3-05 Ba3-08 Ba3-09
Table 4. Selected bond distances (r\) (from X-ray diffraction),
20
4i
8i
4h
26
8i
4i
4i
V
8li
8i
8j
Bal
Ba2
Fe1
M2
M3
Fe4
01
02
03
04
05
06
*Iry = 0.
Site
Atom
Table 5. Structural parameters
1.0
1.0
1.0
1.0
1.0
1.0
1.0
0.458(8)/0.542(8)
0.855(8)/O. 145(8)
1.0
I.0
1.0
occ FeEi
0.3629(2) 0.4356(3) 0.4260(3) -0.0979(3) -0.0983(3) 0.2604(2) 0.261 l(2) 0.0836(2) 0.0836(2) 0.1410(2) 0.1406(2) -0.0722(2) -0.0719(2)
0 0 0.28256(2) 0.2824(4) 0.14477(3) 0.1449(l) 0.5 0.5 0.5 :::6310(3)
x 0 0 0 0 0.13784(5) 0.1375(2) 0.13246(7) 0.1342(3) 0 0 -0.21318(5) -0.2137(2) 0 0 0 0 -0.1377(3) -0.1375(4) 0.2565(3) 0.2560 0.1442(3) 0.1445(3) -0.3818(2) -0.3818(3)
Y
results; occupancies
0.0172(l) 0.016(2) 0.01508(9) 0.013(l) 0.0086( 1) 0.0084(3) 0.0079( 1) 0.007( 1) 0.0083(2) 0.007(l) 0.0087( 1) 0.0084(3) 0.0094(6) 0.009( 1) 0.0153(g) 0.012(l) 0.0167(6) 0.0179(8) 0.0129(5) 0.01 l7(8) 0.0141(5) O.OlOl(7) 0.0098(4) 0.0105(8)
ue,(A*)
are X-ray single-crystal
0 0 0.0261 l(8) 0.0265(9) 0.4856( 1) 0.4866(4) 0 0 -0.5 -0.5 -0.4723( 1) -0.4728(4) -0.1797(7) -0.1795(9) 0.4238(8) 0.4239(9) -0.4222(6) -0.421 l(7) 0.6488(5) 0.6504(7) 0.1276(5) 0.1282(6) -0.1833(5) -0.1847(7)
Z
for BalFe ,,,Ti02,, (35: 1); M represents a mixed Fe/I? site; first line of positional parameters parameters are neutron diffraction results
-3.06(4)
-2.8( 1)
-2.8( 1)
-3.06(3)
P&B)*
0
1.17(7)
B&B)
and tbe second line of positional
3 .?
141 I
Structures and properties of barium iron titanium oxides
Table 6. Anisotropic thermal parameters U,, for the non-oxygen atoms in BajFeI OTiOra (from single-crystal X-ray diffraction)
Bal Ba2 MI M2 Fe3 Fe4
0.0099(2) 0.0099(I)
0.0239(3) 0.0163(2) 0.0086(2) 0.0089(3) 0.0090(5) 0.0096(2)
O.o090(2) 0.0075(3) 0.0092(4) 0.0088(2)
The * form of the *anisotropic
displacement
0.0180(2) 0.0193(2) 0.0082(2) 0.0072(3) 0.0068(4) 0.0076(2)
O.OOOOO O.OOOOO -0.0005(2) O.ooooO O.OOOOO 0.0009(2)
parameters is: exp[-2*2{hZa”2(U,,
0.0037(2) 0.0048( 1) 0.0017(2) 0.@304(2) 0.001 l(3) 0.0003(2)
O.OOOOO O.OOOOO 0.0008(2) O.OOOOO O.OOOOO -0.0003(2)
+ k26*2U22 + t2c*2U3A + 2hb*b*t~,~
+
2hla’c U,j + 2klb*c Ur3).
Ba4FezTi
ta027 and is illustrated
in Fig. 4: between each
pair of Bal sites a cp vacancy occurs, thus lowering its coordination
number
to 11 instead of 12, as seen in
As seen in Table 2, Ti4+ and Fe3+ exhibit preferential
some
ordering among the ten 6-coordinated
metal
sites; site M2 contains only Ti4+. Comparison with Table
Table 4. The absence of the 12th oxygen is compensated
4 reveals no obvious correlation of the occupancies
by shortening of the Bal - 0 bond distances resulting in sum of 2.17 V.U. for Bal. The other bond
the site distortion radios. Instead, the distribution patterns ofTi4+andFe3+, which differ more in formal charge than
valence sums given in Table 4 are in good agreement with those calculated from the formal valences of the
in ionic size (-6% [ 16]), may result from differences in the second coordination spheres of the various M sites:
a valence
cations except for Ba3 and M3 with valence sums of 2.64
higher valent Ti preferentially
and 3.36 v.u., respectively.
These deviations
from the
has the fewest
valence
that the structure
contains
longest
sum rule suggest
residual bond strain [13] despite from ideal packing and polyhedral
the large distortions symmetry.
Ba3 is
and
M3
also
occupies site M2, which
next-nearest-neighbour
distances.
The chemically
exhibit
with
unusual
cations strained
second
at the
sites Ba3
coordination
spheres; both have the largest number of next-nearest-
overbonded and in compressive stress (bonds too short) while M3 is underbonded and in tensile stress (bonds too
neighbour cations as compared to the other Ba and M sites. For example, M3 has 14 cations in its second
long). Brown has described how such residual bond strain
coordination on average.
can result
in important
ferroelectricity
physical
properties
[ 131 and, if large enough,
such as
3.1.2. BajFeloTiOze
structural
instability [ 13- 151.
sphere whereas the other M sites have 8-9
parameters
The
structural
for the 3:5:1 compound
and
thermal
are collected
in
Table 7. Selected bond distances (A) (from X-ray diffraction), site distortion ratios (defined as the ratio of the shortest to the longest bond distance), bond valence sums* (V), and expected bond valence sums* ([VI) (v.u.) about each of the cation sites in BalFe, cTiO,s; bond distances on the second line in brackets are from the neutron diffraction results
average: distortion ratio: v:
3.312(4) X 2 [3.314(5) x 21 2.686(4) X 2 [2.690(5) X 21 2.823(3) X 4 [2.820(3) X 41 3.199 0.7 I 1.76V.U.
Ba2-01
2.602(4)
Ba2-05
2.820(3) X 2 [2.826(6) X 21 2.849(3) X 2 [2.843(6) X 21 3.31 l(3) x 2 [3.31 l(6) X 21 3.00(7) x 2 [3.008~6) X 21 2.953 A 0.78 I .76 V.U.
Bal-02 Ba-02 Ba-05
Fel-02 Fel-03
1.846(2) [1.839(3)] I .829(3) W
Fel-05 average: distortion ratio: V/[Vl:
]]I .857$4)1 1.857 A 0.96 3.08/3.00 v.u.
I]
Ba2-03 Ba2-06 average: distortion ratio: V:
average: distortion ratio: V/[V]: Fe4-03
Fe4-04 Fe4-05
P.a1(7)1
Ba2-03
M3-06
.838(4)1
1.897(3) 1.gW4)1 1.855(3)
Fel-04
M3-01
M2-01 M2-04 M2-06 average distortion ratio V/[V]:
2.06(2) x 2 [2.074(4) X 21 1.980(3) x 2 [1.974(4) x 21 2.007(2) X 2 [1999~4)X 2 2.016 A 0.96 3.08/3.14 V.U.
Fe4-06
1.949(4) [1.946(5) 2.010(3) [2.009{3) 1.990A 0.97 3.5113.54
x 2 X 21 X 4 X 41
V.U.
1.835(3) [ 1.828(4)] 1.882(3) [ 1.878(4)] 1.868(3) [ 1.860(4)] 1.866(3)
[I .878(4)] average: distortion ratio: V/[V]:
1.863 A 0.98 3.02l3.00
V.U.
*Bond valence sums (V) were calculated from the observed bond distances using the formalism and parameters given in [20]. For site, a weighted average of the sums calculated using the Fe’+;Ti + parameters was taken according to the observed occupancies (fmm neutmn diffraction). For comparison, the expected bond valence sums ([VI) for the mixed M sites were calculated from the formal valences of Fe’+, Ti4+ and the observed occupancies.
each mixed FeJTi
T. A. VANDERAHet al.
1412
(a) Fig. 5. Crystal structure of Ba3Fe10Ti020(351) along the (a) c-direction and (b) a-direction. In this framework structure the tetrahedral sites arc preferentially occupied by Fe’+; octahedral sites contain a mixture of Fe”’ and Ti4+. Ba ions are shown as spheres.
Table 5 and Table 6. Selected bond distances and bond valence sums about each cation site are given in Table 7. Our results confirm that this phase is isostrucural with PblAI roGeOzo[61, Pb3AI uSi 161,BaXFeloSn02a 1171,
Ba3AlIoTi02u [l], and SrjAIIOTi020 [18]. The crystal structure features four sites for the smaller cations, two with tetrahedral coordination that are occupied by Fe3+ and two octahedral sites with mixed Fe3+/Ti4’ occupation. As shown in Fig. 5, the structural motif features infinite strips (alternately one- and two-octahedra wide) of edge sharing octahedra interconnected by vertex sharing [Fe041 tetrahedra to form a framework-type structure with channels accommodating the Ba ions. Bal is 8-coordinated and occupies rectangular-shaped channels, shown in Fig. 5(a) near the centre of the unit cell; Ba2 is 9-coordinated and occupies pentagonal-like channels. Both Ba sites have low bond valence sums of 1.76 vu. in this open-framework type structure. The valence sums about the tetrahedral Fe and mixed octahedral Fe/Ti sites agree well with those expected from the formal valences; both polyhedra exhibit only slight distortions from ideal symmetry. The magnetic structure observed for Ba3Fe roTiO2aat room temperature
exhibits P/2’ symmetry
and is illus-
trated in Fig. 6. The magnetic arrangement
features two
sets of collinear spins (Fe1 + Fe4) and (M2 + M3), with antiferromagnetic ordering within each set. The two sets
Magnetic lattice Pt
Fig. 6. The magnetic structure model for Ba?Fe ,aTiOzOfrom room temperature neutron powder diffraction studies. The magnetic lattice exhibits P/2’ symmetry and, unlike that indicated by X-ray diffraction, is antibody-centted. The polyhedral linkage patterns among the magnetic sites (most Ba ions have been omitted for clarity) are illustrated in the bottom part of the figure. Fe1 and Fe4 comprise one set of collinear antiparallel spins residing in the vertex-linked tetrahedrq magnetic interactions occur via superexchange, with Fe-O-Fe angles ranging 120- 140”.Fe located in the edge sharing M2 and M3 octahedral sites comprise the second set of collinear antiparallel spins: direct interactions across shared edges are possible in addition to -90” superexchange via oxygen.
Structures and properties of barium iron titanium oxides correspond
to the two types of polyhedra
occupied
by
Fe3+, as illustrated in the bottom of Fig. 6. Segregation of
Ba0:Fe203:Ti02 4:l:lO
the spins into two sets may be caused by the different types
of exchange
pathways
permitted
1413
s
by the two
polyhedral linkage patterns [19]: The octahedral sites (M2 -I- M3) share edges, hence a pathway exists for direct Fe-Fe exchange superexchange
(via fzl orbitals)
in addition
to -90”
via shared oxygens. In contrast, the tetra-
hedral sites are vertex-linked
octahedra1 sites), thus magnetic superexchange
0 O.BlUum,zFC . 0.8Wm.FC
only (to each other and to interactions
only, with significantly
occur via
larger Fe-O-Fe
angles on the order of 120 to 140”. The neutron diffraction patterns collected
below room temperature,
and 1.8 K, exhibited
100
at 15, 6.5,
magnetic peaks that could not be
fitted using the model obtained for the room temperature magnetic
Ba0:Fe203:TI02
lattice. Some magnetic peaks split and could
not be indexed temperature
150 200 Temperature (K)
with a monoclinic
was successively
unit cell. As the
lowered,
increased
and changes
intensities
of the extra magnetic
4:i:lO
were observed
the splitting in the relative
peaks. These results
suggest that at least one additional
magnetic
structure
exists below room temperature. 3.2. Magnetic behaviour The magnetic susceptibility ture and magnetization
as a function of tempera-
as a function
of applied field
observed for BadFeZTi ,,,02, are shown in Fig. 7. In this compound the magnetic Fe3+ ions are diluted by a large proportion of nonmagnetic (top), the susceptibility
Ti4+ ions. As seen in Fig. 7 curve
indicates
paramagnetic
behaviour deviating somewhat from Curie-law temperature dependence.
The inset shows the somewhat
non-
linear reciprocal susceptibility vs temperature behaviour. Nevertheless, a linear least squares fit of susceptibility to the Curie-Weiss
function
over the entire temperature
range yields a Weiss consant of -2.5 of 5 ccs per formula compound
weight
is a semiconductor
K and a moment
Fig. 7. Magnetic behaviour of Ba.,FezTi1002, (4:l:lO). Top: Mass susceptibility as a function of temperature after zero field cooling (ZFC, open symbols) and field cooling (FC, closed symbols and curves). The measuring field was 0.8 kA/m (10 Oe). Inset: Reciprocal susceptibility vs temperature. The magnetic behaviour deviates from a Curie-law temperature dependence. Bottom: High field magnetization at several temperatures. Below 20 K, there is some curvature as a function of field. (Units: To convert susceptibility values in units of m3/kg to emu/g, multiply by 10’14%. To convert field values in unit& of A/m to Oe, multiply by 4610’. Magnetization values in units of Am*/kg and emu/g are numerically equal [21].)
Ba4Fe~Ti~~O~27.As this [2] this moment is attrib-
uted to Fe3+. The near-coincidence
of the field cooling
transition, such as a N&e1 point, up to 400 K. Low field hysteresis loops, obtained after zero field cooling, exhib-
and zero field cooling curves also reflects the absence of
ited little temperature dependence (Fig. 8, bottom). Given
substantial interactions among the diluted magnetic ions. The mass magnetization as a function of field exhibits
the results of the neutron powder diffraction experiments,
curvature below 20 K in fields up to 5.6 MA/m (Fig. 7,
room temperature
bottom), characteristic
of paramagnets
at low tempera-
which
indicated
two antiferromagnetic
sublattices
as well as the presence
magnetic peaks at liquid helium temperatures,
at
of extra the transi-
ture and high field. The magnetic data for Ba3Fe,oTiOzo are shown in
tions at 5 and 45 K are subtle and perhaps attributable to spin canting.
Fig. 8. Interactions among the magnetic ions are substantial and this compound exhibits irreversibility at all
3.3. Dielectric properties
temperatures. Field cooled and zero field cooled susceptibility curves indicate transitions at 5 and 45 K. The
Ti 10027 and was found to be effectively
transition temperatures
6.28 to 6.68 GHz. For Ba,Fe ,,,Ti02,, a permittivity
are independent
of the measuring
A permittivity
value of 39 was obtained for Ba4Fez. constant from of 15
field
as seen in Fig. 8 (top). Alternating field susceptibility measurements (not shown) in 80 A/m (1 Oe) at IO
was measured with minimal dependence on frequency from 10.4 to 12.0 GHz. BadFezTi 10027is more than twice
and ICKKlHz revealed a frequency-independent maximum at 45 K but no transition at 5 K and no other
as polarizable as BalFe IOTiO?o.Both compounds exhibited only moderate dielectric dispersion with losses
T. A. VANDERAH et al.
1414
Fe/Ti sites in tension. Ba3FeloTiOzo was found to exhibit the same open-framework type structure as Ba3AlloTiOZOand, for convenience, its structure is described in space group Wm. Neutron diffraction studies of Ba3FeIOTi020 revealed a magnetic lattice at room temperature with reduced symmetry; data collected at 15, 6.5, and 1.8 K indicated that at least one other magnetic structure occurs below room temperature. Magnetic susceptibility measurements as a function of temperature indicated that Ba4Fe2Ti 10O27is essentially paramagnetic, a result consistent with its low proportion of magnetic ions. In contrast, Ba3FeloTiOzo displays complex magnetic behaviour with transitions at 45 and 5 K, in agreement with the neutron diffraction studies. The permittivity of Ba4Fe2Ti10027 was 39 with a dielectric loss tangent of 1 X 10e3; the corresponding values obtained for Ba3Fe1,-,TiOZ0were 15 and 5 X lOa. For both compounds the dielectric properties exhibited minimal dependence on frequency in the measured regions above 6 GHz. Ba4FelTi10027 is more than twice as polarizable as Ba3Fe ,oTi02,, and exhibits greater dielectric loss. These results are consistent with the presence of residual bond strain, as indicated by violations of the valence sum rule, in the highly distorted structure of the former compound. Fig. 8. Magnetic behaviour of Ba3Fe 10Ti020(3:5: 1). Top: Mass susceptibility as a function of temperature after zero field cooling (ZFC, open symbols) and field cooling (FC, closed symbols and curves). Measuring fields were 0.8 kA/m (10 Ge) and 8 kA/m (100 Ge). Transitions are observed at 5 and 45 K. Bottom: Low field hysteresis loops at 300,100, and 5 K. Data points are shown only for the 100K data for clarity. Data for 50 K (not shown) are similar to those for 5 K. (Units: see caption to Fig. 6.)
effectively independent of frequency over the measures z;;;ranges(tand= 1 X 10-3for4:1:10and5 X lOA :: .
4. CONCLUSIONS
Crystal structures, dielectric properties, and magnetic behaviour of the newly prepared compounds Ba.+Fe2. Ti ,002, and Ba3Fe ,oTiOzo have been described. Structural studies were carried out by single-crystal X-ray diffraction and neutron powder diffraction. Both structures contain octahedral sites for the B-type cations that are mutually substituted by Fe3+ and Ti4+, with some preferential ordering. In the former compound Fe3+ displays 6-coordination only while in the latter this versatile cation occupies both tetrahedral and octahedral sites, BapezTi 1oO27was confirmed to be isotypic with Ba4A12. Ti ,002, and crystallizes with a highly distorted 8L closepacked (chhcchhc) structure in space group Wm. Deviations from the valence sum rule indicate that the structure contains residual bond strain despite the large distortions, with one of the Ba sites in compression and one of the
Acknowledgemenrs-The authors would like to thank J. M. Loezos for assistance in sample preparation and J. M. Honig, I. D. Brown, and A. Santoro for helpful discussions.
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