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The crystal structure of Eu2BaZnO5 and the optical and magnetic properties of some related oxides
Journal of the Less-Common
Metals, 169 ( 199
1)2 17-225
The crystal structure of Eu,BaZnO, properties of some related oxides
217
and the optical and magnetic
M. Taibi and J. Aride Laboratoire de Physico-Chimie Rabat (Morocco)
des Matiriaux, Ecole Normale SupPrieure de Takaddoum B.P. 51 I&
J. Darriet Laboratoire de Chimie du Solide du CNRS, 351 cows de la libt%ation, 33405, Talence Cedex (France)
A. Boukhari Laboratoire de Chimie du Solide Appliqute, (Morocco)
Facultl des Sciences, Avenue Ibn Batouta, Rabat
(Received July 19,199O)
Abstract The structure of the Eu,BaZnO, oxide has been solved for a single crystal. The orthorhombic cell parameters are a= 12.537(11) A, b=5.791(8) 8, and c=7.186(11) A. The space group is Prima. In the refinement 1197 independent reflections (I > 34 I)) have been used. The R factor value is 0.033 (R, = 0.036). The europium ions occupy two different sites with seven oxygen bonds in the lattice. The Ba2+ ions are eleven-fold surrounded. The zinc ions are located in a distorted square-based pyramidal environment. An optical study of Er,BaZnO, and the neodymium-doped LnzBaZnO, (Ln E Er, Tm) has confirmed that the rare-earth atom occupies two sites in these compounds. Magnetic susceptibility measurements show the effect of the crystal field at low temperature.
1. Introduction In the investigation of Ln-Ba-M-O systems (Ln = rare earth and M = Co, Ni, Cu, Zn, Pt and Pd), several oxides were isolated with the general formula Ln,BaMO, [ 1- 171. The structure of these compounds usually depends on the Ln and M. The metal could occupy different sites in the Ln2BaM0, phases: tetrahedral, square-plane, octahedral or square-based pyramidal polyhedra. The lanthanides are surrounded by seven or eight oxygen atoms. In a recent paper we reported the crystal structure of Nd,BaZnO, [18]; the zinc ions in the lattice occupy a tetrahedral environment. The Eu,BaZnO, is isostructural to Y,BaCuO, which appears as an impurity phase in the synthesis of the high-temperature superconductors YBa,Cu,O, _ x [ 191. The present results are related to the Eu,BaZnO, crystal structure refinement from a single-crystal X-ray diffraction analysis. 0022-5088/91/$3.50
0 Elsevier Sequoia/Printed
in The Netherlands
218
The structural relationship with Ln,BaMO, oxides is discussed. The optical and magnetic properties of some related compounds are also investigated.
2. Experimental details A powder sample of Eu,BaZnO, was synthesized by heating a mixture of Eu203, ZnO and BaC03 in air, first at 900 “C for 12 h, then at 1100 “C for 24 h and finally by quenc~ng to room temperature. The purity of the compounds was controlled using an X-ray diffraction technique. Single crystals were prepared by heating the powder at 1650 “C in a platinum crucible in a furnace equipped with an automated program. The temperature was decreased at a rate of 10 “C h-r until 1350 “C was reached then at 20 “C to 1100 “C. After cooling, golden prismatic crystals were obtained. The crystal selected for X-ray analysis was mounted on an Enraf Nonius CAD4. The lattice parameters were refined from 25 reflections obtained using a double scan technique. The conditions of the diffraction experiment are summarized in Table 1. The observed reflection conditions hk0; h = 2n and Ok/; k + 1= 2n indicate the space groups Pnm or Pn2,a. The intensities, corrected for any Lorentz polarization effects as well as for absorption using the empirical corrections in the structure determination program (SDP), were averaged and led to a value I?,, of 0.036. Scattering factors were taken from ref. 20 and the influence of any anomalous dispersion [2 l] was included. The optical absorption spectra of Ln,BaZnO, compounds in powder form were recorded at 10 K through a Huet 2B spectrograph. The spectral lines were measured with respect to the iron arc and a wavelength corrected for vacuum. The magnetic susceptibility measurements were carried out using a Faraday balance. All values were corrected from the diamagnetic contribution of the constituent ions. TABLE
1
Crystal data and conditions of data collection for Eu,BaZnO, Symmetry: Orthorhombic. Space group Pnma CeIlparameters:a= 12.537(11),~=5.791(8), c=7.186(11), V=521.7 (A3),Z=4 Radiation MO Ku (graphite monochromatized) Detector aperture a + b tan f3with a = b = 2.5 Scanning mode w/2 B Range registered 2B,, 2@,,,,: 0.1 - 35 (deg); (~~~)~~~(20 9 11) Absorption coefficient p (cm-’ 339.5) Absorption correction empirical absorption corrections (SDP program) Transmission factors min. 77.17%, max. 99.8%, av. 88.43% Reflection measured: total 9140 with I > Sa(I) 7940 independent 1197 Number of refined parameters: 49 (with anisotropic thermal parameters)
219
3. Structural determination The structural analysis was performed using the SHELX program [22]. The structure has been solved for the centrosymmetric space group Prima. The initial atomic positions were those that have been found for the isostructural phase Y,BaZnO, [3]. The final positional and thermal parameters are reported in Table 2. The interatomic distances with the corresponding standard deviations are reported in Table 3. A table specifying the calculated and observed structural factors can be obtained on request from the authors. In the Eu,BaZnO, structure the europium atoms occupy two different sites. The Eu( 1) and Eu(2) ions are located in monocapped trigonal prisms with seven oxygen atoms. The same EuO, coordination polyhedra were observed in the Eu,BaNiO, oxide [lo]. The Eu-0 distances ranged from 2.355 to 2.430 A (Eu(l)-0) and 2.348 to 2.426 A (Eu(2)-0). The Eu-0 mean values are 2.384 and 2.393 A for Eu( 1) and Eu(2) respectively. They are comparable with the values found for Eu,BaNiO, (2.397 A) [lo]. The zinc ions are located in a distorted square-based pyramid. The Zn-0 bond lengths range from 1.993 to 2.130 A. This environment is similar to that observed for Cu2 + ions in the Y,BaCuO, phase [ 11. Barium ions are located in polyhedra of eleven oxygen atoms. This is the high coordination usually found in the Ln,BaMO, family (M 3 Co, Ni, Cu, Zn, Pt and Pd) [l-l 71. The Ba-0 distances ranged between 2.7 14 and 3.296 A. The Ba-0( 3) is the shortest distance (2.7 1 A). The 0( 3) atoms are very closely associated with the Ba2’ ions. The mean Ba-0 bond length is 3.02 A, which is greater than that observed for Ln,BaMO, compounds except for zinc and copper. It seems that the Ba* + ions are situated in a large cavity of oxygen atoms. Recently, we reported that Er,BaZnO, and Tm,BaZnO, are isostructural to Eu,BaZnO, [23]. Figure 1 shows the variation in orthorhombic cell parameters with respect to the rare-earth ionic radius for the I+n,BaZnO, compounds (Ln = Tm, Er, Ho, Dy, Gd, Eu, Sm) [3, 231. We observed a monotonic decrease in the crystallographic parameters as the ionic radius of the lanthanide decreases across the series. 4. Optical properties In order to complete the structural investigation and to confirm that the rareearth atoms can occupy two sites in Ln,BaZnO, (Ln = Eu, Er and Tm), we conducted an optical study of Er,BaZnO,. The absorption spectra were recorded at 10 K between 4000 and 8200 A. The transitions from the Er3+ ground state level 4I15,2have sharp lines similar to those observed for the free ion. For each isolated level 4F9,2 and 419,2we noted ten lines (Fig. 2) as is allowed by crystal field theory (J+ l/2 = 10) when the Er3 + occupies two sites in the structure. However for some other levels like 4Fs,2 we isolated many lines compared with the number allowed for the two sites (Table 4).
0.2922( 1) 0.0744( 1) 0.9010( 1) 0.6502( 1) 0.4348(4) 0.2260(5) 0.1004(6)
X
L/4
0.1193(l) 0.3972( 1) 0.9244( 1) 0.6921(2) 0.1706(9) 0.3557(9) 0.0712(14) 0.0060( 3) 0.0060( 3) 0.0094( 3) 0.0073(7) 0.0133(34) 0.0098( 30) 0.0167(49)
u22
T=exp[-2n2(h2a*2U,,
0.0021(2) 0.0018(2) 0.0065( 3) 0.0048( 5) 0.0060(21) 0.0106(23) 0.0034(30)
are related by theexpression
l/4 114 l/4 l/4 - 0.0070( 13) 0.5054( 12)
Y
Thevibrationalcoefficients
0, 0, 0,
Bu, Eu, Ba Zn
Atoms
+ k2b*2U22+
0.0017( 3) 0.0016( 3) 0.0077(4) 0.0028(6) 0.0052( 30) 0.0071(27) 0.0089(46)
Atomic parameters and the anisotropic temperature factors U,, and B,, (&) for Eu,BaZnO,
TABLE 2
12c*‘U~,
0 0 0 0 0.0007( 20) - 0.0037(22) 0
u,2
+2hla*c*U,,+2klb*c*U,31.
0.0001(2) -0.0001(2) -0.0014(2) 0.0022(4) 0.0005( 19) - 0.0042(23) 0.0029(30)
u,,
+2hka*b*U,z
-0.0010(26) -0.0071(24) 0
0 0 0 0
4?
0.26 0.25 0.62 0.39 0.64 0.72 0.76
221
TABLE 3 Mocin interatomic distances in Eu,BaZnO, M-O
Distance (A)
M-O
Distance (A)
Ba-0( 1) X 2 Ba-0(1)X2 Ba-0( 2) x 2 Ba-0( 2) x 2 Ba-0( 3) x 2 Ba-O(3) X 1 Eu( 1)-0( 1) x 2 Eu(l)-0(2)x2 Eu( 1)-O(2) x 2 Eu(l)-0(3)x 1
3.296(6) 3.089(6) 2.931(6) 3.082(6) 2.896( 8) 2.714(8) 2.355(6) 2.376(6) 2.401(6) 2.430( 8)
Eu(2)-O( 1) x 2 Eu(2)-O( 1) x 2 Eu(2)-0(2)x2 Eu(2)-O( 3) x 1
2.348(6) 2.419(6) 2.426(6) 2.365( 10)
Zn-0(1)x2 Zn-O(2) X 2 Zn-0( 3) X 1
2.023(6) 2.130(6) 1.993(7)
C
7.2
,
L Tm
Er
Ho Dy
Gd
Fig. 1. Variation in orthorhombic
Eu
Sm
r(Ld9
cell parameters
vs. rare-earth ionic radius of Ln,BaZnO,
phases.
The extra lines observed may be due to the mobility of the Er3+ ions or to the occupation of more than two sites by Er3 + . This problem can be solved by using Nd3 + ions as a structural probe, which is why we have synthesized the doped oxides ( 10% Nd3+ :Er,BaZnO,, 10% Nd+:Tm,BaZnO,) and recorded the absorption spectra at the liquid helium temperature (4.2 K). At this temperature it is possible to obtain the transition from the ground state Stark level of 419,2to 2P1,2 (this level is not decomposed by the
X(A)
Fig. 2. Adsorption spectra for Er,BaZnO,
at 10 K.
TABLE 4 Number of observed lines for different 2r+‘L, levels in Er,BaZnO, Level
at 10 K
Number of observed lines 13 5 10 10 “large” 7
crystal field: J+ l/2 = 1 for each site). Two lines are observed for each compound indicating that Nd3+ occupies two sites in Er,BaZnO, and Tm,BaZnO, (Fig. 3). This result confirms the X-ray diffraction study which showed two sites for the rare-earth ions in the isostructural oxide Eu,BaZnOS. The extra lines in the absorption spectrum of Er,BaZnO, are probably due to the mobility of Er3+. This mobility does not occur in the case of Nd3+ because its size is greater (t&3+ = 1.12 A, r&H = 0.99 A). The 2PI,2 energies deduced from the spectra (22856 and 22787 cm-l for Nd3+:Er,BaZn0,; 22 844 and 22 782 cm-’ for Nd3+:Tm,BaZn0,) fall between that for A-Nd,O, and that for Nd@,S, a domain which corresponds to the
223
(4
lb)
Fig. 3. Adsorption (b) ErBaZnO,.
-14
’
spectra
50
at 4.2 K (transition
100
150
%,,z+2P,,z)
for Nd7+ in (a) TmZBaZnO,,
and
TM)
Fig. 4. Variation in the reverse molar magnetic susceptibility x; ’ vs. T for Er,BaZnO,. tive moment vs. T.
Insert: effec-
224
x:,
?
lo-
d
/ /
5-
50
/I
-9
50
100
Fig. 5. Variation in x; ’ for Tm&hZnO,
occupation 124-261.
by neodymium
150
T(K)
vs. T. Insert: effective moment vs. T.
atoms of a site with a coordination
number of seven
5. Magnetic susceptibility The magnetic behaviour of the Eu,BaZnO,-related oxides Er,BaZnO, and Tm,BaZnO, has been measured using powder samples in the temperature range 4-150 K. The resuhs are given in Figs. 4 and 5. The plot of the reverse of the susceptibility x ; ’ vs. T, for the two compounds, follows the Curie-Weiss law at high temperatures ( T> 50 K) with negative 0, ( - 14 and - 9 K). The negative 0, value cannot be attributed to the magnetic interactions because of the large distance between the rare-earth ions ( L&,,~+-~~s+ = 3.38 A). At 120 K the experimental magnetic moment deduced from the formula, ,~~=2.828X~7; is 7.06 ,u, for Tm3+ and 9.46 ,ur, for the erbium ion. The theoretical values of the magnetic moment for the pure ground state level are 7.57 ,uu,for Tm” + and 9.59 ,uuBfor Er3 +. The difference between the theoretical and experimental values shows the influence of the crystal field which mixes the levels. This effect is clearly shown at low temperatures where the x -I curves deviate from the Curie-Weiss law, and peR falls rapidly in this temperature range (Figs. 4 and 5).
225
References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
21 22 23 24 25 26
C. Michel and B. Raveau, J. Solid State Chem., 43 (1982) 73. C. Michel and B. Raveau, J. Solid State Chem., 42 (1982) 176. C. Michei and B. Raveau, .I. Solid State Chem., 49 (1983) 49. C. Michel, L. Er-rakho and B. Raveau, Rev. Chim. Miner., 21(1984) 85. S. F. Watkins, F. R. Fronczek, K. S. Wheelock, R. G. Goodrich, W. 0. Hamilton and W. W. Johnson, Actu Ctystogr., C44 (1988) 3. S. Schiffler and H. Muher-Buschbaum, 2. Anorg. ANg. Chem., 523 (1985) 63. S. Schiffler and H. Miiller-Buschbaum, Z. Anorg. A& Chem., 532 (1986) 10. S. Schiffler and H. Miiller-Buschbaum, .I. Less-Common Met., 128 (1987) 117. S. Schiffler and H. Miiller-Buschbaum, Monatsh. Chem., II8 ( 1987) 7 14. H. Miiller-Buschbaum and C. Larg, J. Less-Common Met., I42 (1988) L 1. H. Mev and H. Miiller-Buschbaum, J. Less-Common Met., 152 (1989) 139. S. Schiffler and H. Miiller-Buschbaum, Z. Anorg Aflg. Chem., 540 (1986) 243. Y. Lahgaut, G. Ferey, M. Hervieu and B. Raveau, Eur. J. Solid State Inorg. Chem., 25 ( 1988) 11 I. St. Schiffler, Hk. Miiller-Buschbaum, Mona&z. Chem., If 7( 1986) 465. Hk. Miiller-Buschbaum and I. Riiter, 2. Anorg. Allg. Chem., 572 (1989) 18 1. H. Mevs and Hk. Miiller-Buschbaum, Z. Anorg. Allg. Chem., 573 (1989) 128. H. Mevs and Hk. Miiller-Buschbaum, Z. Anorg. Al/g. Chem., 574 (I 989) 172. M. Taibi, J. Aride, J. Darriet, A. Moqine and A. Boukhari, J. Solid. State Chem., H6 (1990) 233. M. K. Wu, J. R. Ashbum, C. J. Torng, P. H. Hor, R. L. Meng, L. Gao, Z. J. Huang, Y. Q. Wang and C. W. Chu, I’hys. Rev. Lett., 58 (1987) 908. D. T. Cromer and J. D. Mann, Acta CrystulIogr. A, 24 ( 1968) 321. D. T. Cromer and D. Liberman, J. Chem. Phys., 53 ( 1970) 198 1. G. Sheldrick, SHELX 76, Program for Crystal Structure Determination, Cambridge, 1976. M. Taibi, J. Aride, E. Anti-Fidancev, M. Lemaitre-Blaise, P. Percher and P. Caro, J. Solid. State Chem., 74 (1988) 329. P. Caro, J. Derouet and L. Beaury, J. Chem. Whys., 70 ( 1979) 2542. P. Caro and J. Derouet, Bull. Sot. Chem. Fr., I (1972) 46. E. Anti-Fidancev, M. Lemaitre-Blaise and P. Caro, New J. Chem., If (1987) 467.
Metals, 169 ( 199
1)2 17-225
The crystal structure of Eu,BaZnO, properties of some related oxides
217
and the optical and magnetic
M. Taibi and J. Aride Laboratoire de Physico-Chimie Rabat (Morocco)
des Matiriaux, Ecole Normale SupPrieure de Takaddoum B.P. 51 I&
J. Darriet Laboratoire de Chimie du Solide du CNRS, 351 cows de la libt%ation, 33405, Talence Cedex (France)
A. Boukhari Laboratoire de Chimie du Solide Appliqute, (Morocco)
Facultl des Sciences, Avenue Ibn Batouta, Rabat
(Received July 19,199O)
Abstract The structure of the Eu,BaZnO, oxide has been solved for a single crystal. The orthorhombic cell parameters are a= 12.537(11) A, b=5.791(8) 8, and c=7.186(11) A. The space group is Prima. In the refinement 1197 independent reflections (I > 34 I)) have been used. The R factor value is 0.033 (R, = 0.036). The europium ions occupy two different sites with seven oxygen bonds in the lattice. The Ba2+ ions are eleven-fold surrounded. The zinc ions are located in a distorted square-based pyramidal environment. An optical study of Er,BaZnO, and the neodymium-doped LnzBaZnO, (Ln E Er, Tm) has confirmed that the rare-earth atom occupies two sites in these compounds. Magnetic susceptibility measurements show the effect of the crystal field at low temperature.
1. Introduction In the investigation of Ln-Ba-M-O systems (Ln = rare earth and M = Co, Ni, Cu, Zn, Pt and Pd), several oxides were isolated with the general formula Ln,BaMO, [ 1- 171. The structure of these compounds usually depends on the Ln and M. The metal could occupy different sites in the Ln2BaM0, phases: tetrahedral, square-plane, octahedral or square-based pyramidal polyhedra. The lanthanides are surrounded by seven or eight oxygen atoms. In a recent paper we reported the crystal structure of Nd,BaZnO, [18]; the zinc ions in the lattice occupy a tetrahedral environment. The Eu,BaZnO, is isostructural to Y,BaCuO, which appears as an impurity phase in the synthesis of the high-temperature superconductors YBa,Cu,O, _ x [ 191. The present results are related to the Eu,BaZnO, crystal structure refinement from a single-crystal X-ray diffraction analysis. 0022-5088/91/$3.50
0 Elsevier Sequoia/Printed
in The Netherlands
218
The structural relationship with Ln,BaMO, oxides is discussed. The optical and magnetic properties of some related compounds are also investigated.
2. Experimental details A powder sample of Eu,BaZnO, was synthesized by heating a mixture of Eu203, ZnO and BaC03 in air, first at 900 “C for 12 h, then at 1100 “C for 24 h and finally by quenc~ng to room temperature. The purity of the compounds was controlled using an X-ray diffraction technique. Single crystals were prepared by heating the powder at 1650 “C in a platinum crucible in a furnace equipped with an automated program. The temperature was decreased at a rate of 10 “C h-r until 1350 “C was reached then at 20 “C to 1100 “C. After cooling, golden prismatic crystals were obtained. The crystal selected for X-ray analysis was mounted on an Enraf Nonius CAD4. The lattice parameters were refined from 25 reflections obtained using a double scan technique. The conditions of the diffraction experiment are summarized in Table 1. The observed reflection conditions hk0; h = 2n and Ok/; k + 1= 2n indicate the space groups Pnm or Pn2,a. The intensities, corrected for any Lorentz polarization effects as well as for absorption using the empirical corrections in the structure determination program (SDP), were averaged and led to a value I?,, of 0.036. Scattering factors were taken from ref. 20 and the influence of any anomalous dispersion [2 l] was included. The optical absorption spectra of Ln,BaZnO, compounds in powder form were recorded at 10 K through a Huet 2B spectrograph. The spectral lines were measured with respect to the iron arc and a wavelength corrected for vacuum. The magnetic susceptibility measurements were carried out using a Faraday balance. All values were corrected from the diamagnetic contribution of the constituent ions. TABLE
1
Crystal data and conditions of data collection for Eu,BaZnO, Symmetry: Orthorhombic. Space group Pnma CeIlparameters:a= 12.537(11),~=5.791(8), c=7.186(11), V=521.7 (A3),Z=4 Radiation MO Ku (graphite monochromatized) Detector aperture a + b tan f3with a = b = 2.5 Scanning mode w/2 B Range registered 2B,, 2@,,,,: 0.1 - 35 (deg); (~~~)~~~(20 9 11) Absorption coefficient p (cm-’ 339.5) Absorption correction empirical absorption corrections (SDP program) Transmission factors min. 77.17%, max. 99.8%, av. 88.43% Reflection measured: total 9140 with I > Sa(I) 7940 independent 1197 Number of refined parameters: 49 (with anisotropic thermal parameters)
219
3. Structural determination The structural analysis was performed using the SHELX program [22]. The structure has been solved for the centrosymmetric space group Prima. The initial atomic positions were those that have been found for the isostructural phase Y,BaZnO, [3]. The final positional and thermal parameters are reported in Table 2. The interatomic distances with the corresponding standard deviations are reported in Table 3. A table specifying the calculated and observed structural factors can be obtained on request from the authors. In the Eu,BaZnO, structure the europium atoms occupy two different sites. The Eu( 1) and Eu(2) ions are located in monocapped trigonal prisms with seven oxygen atoms. The same EuO, coordination polyhedra were observed in the Eu,BaNiO, oxide [lo]. The Eu-0 distances ranged from 2.355 to 2.430 A (Eu(l)-0) and 2.348 to 2.426 A (Eu(2)-0). The Eu-0 mean values are 2.384 and 2.393 A for Eu( 1) and Eu(2) respectively. They are comparable with the values found for Eu,BaNiO, (2.397 A) [lo]. The zinc ions are located in a distorted square-based pyramid. The Zn-0 bond lengths range from 1.993 to 2.130 A. This environment is similar to that observed for Cu2 + ions in the Y,BaCuO, phase [ 11. Barium ions are located in polyhedra of eleven oxygen atoms. This is the high coordination usually found in the Ln,BaMO, family (M 3 Co, Ni, Cu, Zn, Pt and Pd) [l-l 71. The Ba-0 distances ranged between 2.7 14 and 3.296 A. The Ba-0( 3) is the shortest distance (2.7 1 A). The 0( 3) atoms are very closely associated with the Ba2’ ions. The mean Ba-0 bond length is 3.02 A, which is greater than that observed for Ln,BaMO, compounds except for zinc and copper. It seems that the Ba* + ions are situated in a large cavity of oxygen atoms. Recently, we reported that Er,BaZnO, and Tm,BaZnO, are isostructural to Eu,BaZnO, [23]. Figure 1 shows the variation in orthorhombic cell parameters with respect to the rare-earth ionic radius for the I+n,BaZnO, compounds (Ln = Tm, Er, Ho, Dy, Gd, Eu, Sm) [3, 231. We observed a monotonic decrease in the crystallographic parameters as the ionic radius of the lanthanide decreases across the series. 4. Optical properties In order to complete the structural investigation and to confirm that the rareearth atoms can occupy two sites in Ln,BaZnO, (Ln = Eu, Er and Tm), we conducted an optical study of Er,BaZnO,. The absorption spectra were recorded at 10 K between 4000 and 8200 A. The transitions from the Er3+ ground state level 4I15,2have sharp lines similar to those observed for the free ion. For each isolated level 4F9,2 and 419,2we noted ten lines (Fig. 2) as is allowed by crystal field theory (J+ l/2 = 10) when the Er3 + occupies two sites in the structure. However for some other levels like 4Fs,2 we isolated many lines compared with the number allowed for the two sites (Table 4).
0.2922( 1) 0.0744( 1) 0.9010( 1) 0.6502( 1) 0.4348(4) 0.2260(5) 0.1004(6)
X
L/4
0.1193(l) 0.3972( 1) 0.9244( 1) 0.6921(2) 0.1706(9) 0.3557(9) 0.0712(14) 0.0060( 3) 0.0060( 3) 0.0094( 3) 0.0073(7) 0.0133(34) 0.0098( 30) 0.0167(49)
u22
T=exp[-2n2(h2a*2U,,
0.0021(2) 0.0018(2) 0.0065( 3) 0.0048( 5) 0.0060(21) 0.0106(23) 0.0034(30)
are related by theexpression
l/4 114 l/4 l/4 - 0.0070( 13) 0.5054( 12)
Y
Thevibrationalcoefficients
0, 0, 0,
Bu, Eu, Ba Zn
Atoms
+ k2b*2U22+
0.0017( 3) 0.0016( 3) 0.0077(4) 0.0028(6) 0.0052( 30) 0.0071(27) 0.0089(46)
Atomic parameters and the anisotropic temperature factors U,, and B,, (&) for Eu,BaZnO,
TABLE 2
12c*‘U~,
0 0 0 0 0.0007( 20) - 0.0037(22) 0
u,2
+2hla*c*U,,+2klb*c*U,31.
0.0001(2) -0.0001(2) -0.0014(2) 0.0022(4) 0.0005( 19) - 0.0042(23) 0.0029(30)
u,,
+2hka*b*U,z
-0.0010(26) -0.0071(24) 0
0 0 0 0
4?
0.26 0.25 0.62 0.39 0.64 0.72 0.76
221
TABLE 3 Mocin interatomic distances in Eu,BaZnO, M-O
Distance (A)
M-O
Distance (A)
Ba-0( 1) X 2 Ba-0(1)X2 Ba-0( 2) x 2 Ba-0( 2) x 2 Ba-0( 3) x 2 Ba-O(3) X 1 Eu( 1)-0( 1) x 2 Eu(l)-0(2)x2 Eu( 1)-O(2) x 2 Eu(l)-0(3)x 1
3.296(6) 3.089(6) 2.931(6) 3.082(6) 2.896( 8) 2.714(8) 2.355(6) 2.376(6) 2.401(6) 2.430( 8)
Eu(2)-O( 1) x 2 Eu(2)-O( 1) x 2 Eu(2)-0(2)x2 Eu(2)-O( 3) x 1
2.348(6) 2.419(6) 2.426(6) 2.365( 10)
Zn-0(1)x2 Zn-O(2) X 2 Zn-0( 3) X 1
2.023(6) 2.130(6) 1.993(7)
C
7.2
,
L Tm
Er
Ho Dy
Gd
Fig. 1. Variation in orthorhombic
Eu
Sm
r(Ld9
cell parameters
vs. rare-earth ionic radius of Ln,BaZnO,
phases.
The extra lines observed may be due to the mobility of the Er3+ ions or to the occupation of more than two sites by Er3 + . This problem can be solved by using Nd3 + ions as a structural probe, which is why we have synthesized the doped oxides ( 10% Nd3+ :Er,BaZnO,, 10% Nd+:Tm,BaZnO,) and recorded the absorption spectra at the liquid helium temperature (4.2 K). At this temperature it is possible to obtain the transition from the ground state Stark level of 419,2to 2P1,2 (this level is not decomposed by the
X(A)
Fig. 2. Adsorption spectra for Er,BaZnO,
at 10 K.
TABLE 4 Number of observed lines for different 2r+‘L, levels in Er,BaZnO, Level
at 10 K
Number of observed lines 13 5 10 10 “large” 7
crystal field: J+ l/2 = 1 for each site). Two lines are observed for each compound indicating that Nd3+ occupies two sites in Er,BaZnO, and Tm,BaZnO, (Fig. 3). This result confirms the X-ray diffraction study which showed two sites for the rare-earth ions in the isostructural oxide Eu,BaZnOS. The extra lines in the absorption spectrum of Er,BaZnO, are probably due to the mobility of Er3+. This mobility does not occur in the case of Nd3+ because its size is greater (t&3+ = 1.12 A, r&H = 0.99 A). The 2PI,2 energies deduced from the spectra (22856 and 22787 cm-l for Nd3+:Er,BaZn0,; 22 844 and 22 782 cm-’ for Nd3+:Tm,BaZn0,) fall between that for A-Nd,O, and that for Nd@,S, a domain which corresponds to the
223
(4
lb)
Fig. 3. Adsorption (b) ErBaZnO,.
-14
’
spectra
50
at 4.2 K (transition
100
150
%,,z+2P,,z)
for Nd7+ in (a) TmZBaZnO,,
and
TM)
Fig. 4. Variation in the reverse molar magnetic susceptibility x; ’ vs. T for Er,BaZnO,. tive moment vs. T.
Insert: effec-
224
x:,
?
lo-
d
/ /
5-
50
/I
-9
50
100
Fig. 5. Variation in x; ’ for Tm&hZnO,
occupation 124-261.
by neodymium
150
T(K)
vs. T. Insert: effective moment vs. T.
atoms of a site with a coordination
number of seven
5. Magnetic susceptibility The magnetic behaviour of the Eu,BaZnO,-related oxides Er,BaZnO, and Tm,BaZnO, has been measured using powder samples in the temperature range 4-150 K. The resuhs are given in Figs. 4 and 5. The plot of the reverse of the susceptibility x ; ’ vs. T, for the two compounds, follows the Curie-Weiss law at high temperatures ( T> 50 K) with negative 0, ( - 14 and - 9 K). The negative 0, value cannot be attributed to the magnetic interactions because of the large distance between the rare-earth ions ( L&,,~+-~~s+ = 3.38 A). At 120 K the experimental magnetic moment deduced from the formula, ,~~=2.828X~7; is 7.06 ,u, for Tm3+ and 9.46 ,ur, for the erbium ion. The theoretical values of the magnetic moment for the pure ground state level are 7.57 ,uu,for Tm” + and 9.59 ,uuBfor Er3 +. The difference between the theoretical and experimental values shows the influence of the crystal field which mixes the levels. This effect is clearly shown at low temperatures where the x -I curves deviate from the Curie-Weiss law, and peR falls rapidly in this temperature range (Figs. 4 and 5).
225
References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
21 22 23 24 25 26
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