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Structural characterization and electronic properties of A-substituted LaNi0.8M0.2O3 (A = Ca, Sr; M = Te, W) perovskites
a, __ .__ @
SOLID STATE
El SEWJEK
Solid State Ionics 93
( 1997)
IONICS
329-334
Structural characterization and electronic prbperties of A-substituted LaNiO.,M,.,O, (A = Ca, Sr; M = Te, W) perovskites I. Alvarez”,
M.L. Vejga, C. Pica
Abstract The synthesis. structural characterization perovskite-like
and electronic propertics of the A,
in both the A and R sublatticcs of the perovskitc structure: Ni’ A ones. Orthorhombic resistivity
symmetry
(S.G.
measurements show semiconductor
Perovskitcs: Nickel
behavior in all cases (E,, < 0.2 eV). Ferromagnetic
PI1
author.
01997
SOl67-2738(96)00531-O
interactions with Curie
oxides
LaNiO, oxide is well known to have a rhombohedrically distorted perovskite structure and to behave as a metal down to 4.2 K, showing electrical and magnetic behavior which indicate a high electronic correlation [l-3]. In the recent past years, this material and some derived systems have attracted considerable attention and have been the sub.ject of extensive studies. Having in mind the discovery of superconductivity in Cu-containing perovskites at relatively high temperatures, this interest was in principle directed to the synthesis of Ni-containing similar phases or, at least, to a better understanding of this type of phenomena. Although superconductivity has not been found in these systems, it was soon stated however that they can be considered as a
0167-2738/97/Sl7.00
cations by divalent
dilTraction data. for all the title phase>. Electrical
K are detected from susceptibility mcasuremcnts.
1. Introduction
*Corresponding
by partial substitution
’ is substituted by M”- cations and La’-
Pbnm) is found, from X-ray
temperatures ranging between 180 and 250 Kcyvords:
,La
system are reported. These compounds can be regarded as being derived from LaNiO,
Hsevicr
Science H.V. All rights reserved
new family of cbmpounds with characteristic +atures opening a field of research in materials science and justifying by themselves the concentration of’ intensive efforts. The most important properties displayed by these oxides are the metal-insulator (M-I) transitions observed in some cases, the stabiliLation of magneti’c interactions and attiactive features such as magnetoresistence effects 141. In this context, systems like LaNi,.,,MXO, (M = Cr. Mn, Fe. Co, Cu, W, Tc, MO), RNiO, and (R = rare earth; M = Sr, -J-h), R, ,M,NiO, La ,, I INi,, \,,,_, IS-101 have been studied. In some of them, composition, temperature and structure driven M-l transitions have been reported. Jn the casts in which Ni cations are present in mixed-valent state, electrical arid magnetic proper+ could be qualitatively related to the relative amoudts of Ni’ ’ and Ni3’ taking into accouit the more or less probability of’ferro or antifcrromagnetic interactions
330
I. Alvarez. et al. I Solid State tonics 93 (1997) 329-334
and the evolution of the electronic localization in the systems. In this sense, both structural aspects and electronic correlation effects seem to be responsible for the observed behavior. We have previously reported structural and electronic results for the LaNi I _xW,O, system [ 11,121 in which a M-I transition governed by the composition at x > 0.10 has been observed. For the less substituted phases, a M-I transition governed by temperature was evidenced at - 50 K. In this work, we concentrate on the hole doping of these systems, for a given composition of M, x = 0.20, which could permit, a priori, the amplification of the metallic limit of these phases conserving the cations distribution and concentration in the B-sublattice of the perovskite structure. In this sense, our aim has been focused on the study of structural, electrical and magnetic features of the title compounds, comparing the obtained results with those corresponding to the non-doped phases and establishing the most important parameters which drive the observed physical phenomena.
2. Experimental The A,La, _,Ni,,,M,,,O, (A = Ca, Sr; 0 5 x 5 0.2) materials were prepared by the ceramic method starting from stoichiometric amounts of Ca(NO,), * La(NO,), * 6H,O, Ni(NO,), 4H,O, Sr(NO,),, 6H,O, WO, and H,TeO,, at 1423 K, for several days. During the thermal treatment, the samples were reground in each step and the process was monitored by X-ray diffraction until single phases were obtained. The X-ray diffraction patterns were recorded using a Philips X’Pert MPD diffractometer with a D-5000 (3051 /OO) goniometer. A step scan of 0.04 and a counting time of 15 s for each step were employed. The goniometer was connected to a PC controlled by the commercial program PC-APD (Analytical Powder Diffraction Software, 4.0). The obtained data were analyzed by the Rietveld method by means of the Fullprof program [13]. For the electronic resistivity measurements, pelletized samples were sintered at temperatures somewhat lower than the respective synthesis temperatures. Pycnometric measurements showed that, for all the samples, the densities were greater than 90% the
crystallographic values. Electronic resistivity was measured using the Van der Pauw method [143, which avoids contact effects. Contacts were made by covering with platinum paint the circular faces of the sample pellets. The magnetic susceptibility measurements were done by using a DSM8 pendulum susceptometer based on the Faraday method. The maximum applied magnetic field was 15 kG with H(dHldz) = 29 kG* cm-‘. The equipment was calibrated with Hg(Co(SCN),) and Gd,(SO,), * 8H,O, with x independent of the field in the temperature range of measurements. The magnetic susceptibility data have been corrected taking into account the ionic diamagnetic contribution from Ref. [15].
3. Results and discussion 3.1. Compositional
and structural characterization
Table 1 shows the Ni3+ concentration and the respective stoichiometry for the Te-containing compounds, obtained by iodometric titration. It can be observed, in general, that the oxygen content is somewhat lower than expected. This fact could be attributed, in part, to the synthesis method employed: the ceramic method implies the use of relatively high temperatures and, at these conditions, the reduction of Ni3+ to Ni2+ is more favourable. We have however used this method as it assures a good crystallinity degree in the samples and this condition is quite important in order to evaluate structural and electronic aspects. X-ray diffraction data for all the title compounds were analyzed by means of the Rietveld method and
Table 1 Compositional pounds Compound 0 0.1 Sr 0.2 Sr 0.2 Ca
data from
(x A)
iodometric
analysis
for the Te com-
za
Stoichiometry
0.24 0.25 0.28 0.28
La%.,Te,.,%,,
a z is the Ni3’ percentage A,La,_,Ni:+Ni~+,,_,M~:,O,..
Sr,,La,,,Nib.8Te,.*O,,,
Sr,.,La,,,Ni,,Te,.,O,,, Ca,.,La,.,Ni,.,Te,.,O*.~~
per one in the samples,
that
is:
I. Alvarez
331
et al. I Solid State Ionics 93 (1997) 329-334
they appear to be isostructural to the similar Wcontaining materials [ 111. The correspondent X-ray diffraction data were indexed and refined considering an orthorhombic symmetry, S.G. Pbmn (No. 62). Figs. 1 and 2 show, as an example, the observed and calculated X-ray diffraction profiles, the difference between them and the allowed reflections, for the Te phases with x = 0.20 Ca and x = 0.20 St-, respectively. In Tables 2 and 3 the crystal data and the Rfactors obtained in these refinements are gathered. As can be observed, profiles agreement and the
Table 2 Crystal data and R-factors
for the Te phase with x = 0.2Ca
Atom
Position
n
Y
2
CalLa Ni/Te O(1) O(2)
4c 4b 8d 4c
0.021(4) 0.5 0.281(3) -0.038(3)
0.0326(3) 0 0.294(3) 0.484(2)
0.25 0 0.051(2) 0.25
a (A) b (A) c (A) S.G. Pbnm
5.581(l) 5.604(l) 7.894( 1) (N” 62)
R, =7.8 R,, = 10.7 R,=6.1 x2 = 6.8
Table 3 Crystal data and R-factors
Fig. 1. Observed (dots) and calculated (solid line) X-ray diffraction profiles for the Te phase with .x=0.2 Ca.
I I
~lww-
2
5
o-
-‘Id
I I I
I I I I I I I II III
LA. I
.
A
1111 II 1111111 lllllll lllllll Ill1II 1111111 1llm
Atom
Position
x
Y
Z
SrlLa NilTe O(1)
4c 4b 8d
G(2)
4c
0.0203(5) 0.5 0.265(4) -0.288(3)
0.0235(4) 0 0.278(4) 0.493(2)
0.25 0 0.049(2) 0.25
a (A) b (A) c (A) S.G. Pbnm
5.592( 1) 5581(l) 7.906( 1) (N” 62)
R, = 8.2 R,,=11.2 R, =7.2 x*=6.5
R-factors obtained seem to confirm the validity of the proposed structural model. The most representative bond lengths for these compounds are given in Tables 4 and 5. The mean values and the sums of the Shannon ionic radii [ 161 are also included and, as can be observed, they agree reasonably. The X-ray diffraction data for the other title mixed oxides were also refined by the Rietveld method, and the results are compiled in Table 6. As a general trend, the respective cell-volume increases when x, i.e. the A concentration, is increased. This fact is in good agreement with the differences in ionic radii
Table 4 Bond lengths (in A) for the Te phase with n=0.2Ca Ca/La-0(
Ca/La-O(2)
Fig. 2. Observed (dots) and calculated (solid line) X-ray diffraction profiles for the Te phase with x=0.2 Sr.
for the Te phase with x=0.2.%
Mean Shannon
1)
2.59(1)X2 2.53(1)X2 2.89(1)X2 2.55( 1) 2.70(2) 2.66 2.55
(Ni/Te)-0( (Ni/Te)-O(2) Mean Shannon
1)
1.987(2)X2 2.08(2)X 2 1.99(2) X 2 2.02 2.03
332
I. Alvarez
et al.
I Solid State Ionics
Table 5 Bond lengths (in A) for the Te phase with x=0.2% Sr/La-0(
I)
253(2)X 2 2.41(2)X2 2.97(2)X2 2.63( 1) 2.75(2) 2.65 2.58
Sr/La-O(2) Mean Shannon
Table 6 Cell parameters
(Ni/Te)-0(
1)
and cell-volume
The electronic conductivity variation with temperature for the AXLa, _xNi,8M,,,0, mixed oxides has been measured in the temperature range between 300-950 K. An exponential variation of conductivity with temperature has been observed and the experimental data have been fitted to an equation of the type: (1)
for A,La, _%Ni, ,M,, ?O,
u (A)
h (A)
c (A)
V(A’)
Te, Te, Te, Tc,
5.594( I ) 5.589( 1) 5.592( I ) 5.581(4) 5.533(l) 5.478(3)
5.564( I ) 5.572( I ) 5.581(l) 5.604(4) 5.568( 1) S.hOS(3)
7.859( I ) 7.881(2) 7.906( 1) 7.894( 1) 7.880( 1) 7.966(4)
244.6 2415.4 246.8 246.9 242.8 244.7
W, 0 W, 0.2Sr
measurements
u(T) = (A/T) . exp( - E,lkT)
M, xA 0 0.1Sr 0.2Sr 0.2Ca
329-334
3.2. Conductivity 1.972(2)x2 2.06(2) x 2 1.983(1)X2 2.01 2.03
(Ni/Te)-O(2) Mean Shannon
9.7 (1997)
between the concerned cations in each case. Table 7 shows the mean ionic radii of the cations present in the A and B sublattices of pcrovskite structure. In general, the cell-volumes of the Te-phases are greater than the W-phases ones. This fact could be related to the electronic features of Te cations, having no available d-orbitals, as pointed out previously for similar compounds [ 171. From the above structural results, we can conclude that A,La, _XNi,,8M,,,20, compounds have perovskite-type structures in which all cations are placed at random in the correspondent sublattice of the perovskite structure.
which is usually applied to a small polaron hopping mechanism [ 181. In Eq. (l), E, is the activation energy related to the hopping process, A is the preexponential factor and k is the Boltzmann constant. Fig. 3 shows the linear variation of ln(cr.7) with 1‘. ’ for the title compounds, which agrees well with the above assumption. It is therefore shown that all the studied phases arc thermally-activated semiconductors, in the temperature range explored. In order to analyze the influence of x and the A cation substituted in the electrical properties, we present separately the correspondent graphs for the Te compounds with different x values (Fig. 4a) and with different A cations (Fig. 4b). The activation energies, deduced from the above results for each sample, are gathered in Table 8. It can be deduced from Fig. 4a that conductivity values decrease and activation energies increase as the substitution of La cations by Sr ones is increased. On the other hand, Fig. 4b
Table 7 Mean ionic radii and cell-volumes Compound; Te. Te, Te, Te, W, W,
0 0.1% 0.2Sr 0.2Ca 0 0.2Sr
M, xA
v (A’)
mlr.“.h (AY
rnc:...Fl (A)”
244.6 245.4 246.8 246.9 240.9 244.7
1.16 1.17 1.18 1.15 1.16 1.18
0.624 0.630 0.630 0.633 0.646 0.620
’ Mean ionic radii of A-sublattice b Mean ionic radii of B-sublattice
cations (Sr/Ca cations (TclW
and La). and Ni).
-21 0.912
I
I
I
I
1.482
I.982
2.&82
2.982
t-1. to] , I(-‘1
Fig. 3. Variation of In(rr.T)
vs. T
’ for AzLa, .Ni,,&.O,.
I UQ
I. Alvarez
et al. 1 Solid State Ionics 93 (1997) 329-334
0 i= in 6
5 4
0.982
1.482
1.982
2.482
2.982
3.482
T-'.lti(K-'1 10 I
----I
-21 0982
I 1.482
I 1.982
I 2.482
I 2.982
I 3.482
l-'.ld (K-')
Fig. 4. Variation of ln(o-7’) (b) different A cations.
Table 8 Activation
energies
vs. T-’ for (a) different x values and
for A,La, _,Ni, ,M,,,O,
M, XA
E, (eV)
Te, Te, Te, Te,
0.15(l) 0.16(l) 0.21(2) 0.24( 1) 0.07( 1) 0.16(Z)
0 0.1.9 0.2.Q 0.2Ca
w, q W, 0.2Sr
shows that, for a given composition, Ca-containing sample is related with smaller electrical conductivity values. Finally, it was clear from Fig. 3 that Te phases are, in general, worse semiconductors than the W phases. Considering the compositional and structural aspects mentioned above, the observed
333
behavior can be interpreted as follows: as the cellvolume increases and the oxygen content of the phases is smaller, the crystal lattice is more expanded and also more disordered. Having in mind the band model proposed for explaining the electronic evolution from LaNiO, to the LaNi,,,M,.,O, (X= 0) perovskites [ 121, which considered the existence of narrow bands responsible of the semiconductor behavior observed, both expansion and disorder probably give rise to narrower bands or, alternatively, to a greater electronic localization of the system. In this sense, the expected amplification of the metallic limit in the non-doped samples, as a consequence of substitution, is frustrated as a result of the disorder and oxygen deficiency introduced in this substitution. In Fig. 5a and 5b, the variation of magnetic susceptibility with temperature for the Te and Wcontaining perovskites, respectively, is represented. A, complex behavior due to both the electronic localization and the relative amounts of Ni2+/Ni3+ stabilized in each case was expected: Ni2+-0-Ni2+ interactions are, in principle, antiferromagnetic in character, whereas’ Ni3+ -0-Ni2+ interactions could be ferromagnetic. In this sense, a certain competition between both types of magnetic interactions could be expected. The observed behavior shows, in principle, the stabilization of ferromagnetic interactions with Curie temperatures which range between 200 and 250 K, as can be estimated from Fig. 5. A similar behavior was found in the non-substituted perovskites [19] and was interpreted, from neutron diffraction experiments, as antiferromagnetic interactions with and important spin canting. In this context, it is therefore probable that similar complex magnetic interactions take place in the title mixed oxides. On the other hand, Fig. 5 shows that magnetic susceptibility values are smaller as x is increased and this fact could indicate that, in the x#O phases, the ferromagnetic component is smaller. These results are probably related to structural aspects governing a less marked canting in the ‘spin arrangement giving rise to a quantitatively less important ferromagnetic component (significative variation of bond distances, more or less structural distortion in the directions in which magnetic ordering takes place,...). Further neutron diffraction experiments will make these points clearer.
334
1. Alvarez et al. I Solid State Iminicv 93 (1997) 329-334
References
0
50
loo
150 1 fK1
250
zoo
:
I
b)
0.05
cl 00.2ow
F
*
5 0.04
z
0.20
srw
2 x
0.02 -\
0
0
50
100
150
200
250
300
1 (K) Fig. 5. Variation of the magnetic compounds and (b) W phases.
susceptibility
for the (a) Te
In conclusion, new A,La, _,Ni,,,,M,,70, perovskite-type mixed oxides have been synthesized, in which all the cations are placed at random in each sublattice. They are all semiconductors with activation energies lower than 0.2 eV and show, in principle, ferromagnetic interactions below -250 K.
Acknowledgments We thank the DGICYT (Proj. PB 94-0214) and CICYT (Proj. MAT 94-0374, Spain) for financial support.
[I] K.P. Rajeev. G.V. Shivashankar and A.K. Raychaudhuri, Solid State Commun. 79 (1991) 591. 121 K. Sreedhar, J.M. Honig, M. Darwin, M. McElfresh, P.M. Shand, J. Xu, B.C. Crooker and J. Spalek, Phys. Rev. B 46 ( 1992) 6382. 131 N.Y. Vasanthacharya, P Ganguly, J.B. Goodenough and C.N.R. Rao, J. Phys. C: Solid State Phys. 17 (1984)2745. [4] F. Damay, A. Maignan, N. Nguyen and B. Raveau, J. Sol. St. Chem. 124 (1996) 385; R. Mahesh, R. Mahendiran, A.K. Raychaudhuri and C.N.R. Rao, Mat. Res. Bull. 3 I (1996) 897 (and references therein). ISI J.B. Torrance, P. Lacorre, A.I. Nazzal, E.J. Ansaldo and Ch. Niedcrmayer, Phys. Rev. B 45 (1992) 8209. and C.N.R. 161 R. Mahesh, R. Mahendiran, A.K. Raychaudhury Rao, J. Solid State Chem. 114 (1995) 297. I71 Z. Zhang, M. Greenblatt and J.B. Goodenough, J. Solid State Chem. 108 (1994) 402. t’31C.N.R. Rao, P. Om and P. Ganguly, J. Solid State Chem. 15 (1975) 186. 191 G.V. Bazuev,V.N. Kasilnikov, N.A. Kirsanov and N.V Lukin. Physica C 230 (1994) 163. and M. Hidalgo, J. Solid llO1 J.A. Alonso, M.J. Martinez-Lope State Chem. 116 (1995) 146. (111 I. Alvarez, M.L. Veiga and C. Pica, J. Mat. Chem. 5 (1995) 1049. t121 1. Alvarez, J.L. Martinez, M.L. Veiga and C. Pica, J. Solid State Chem. 125 (19%) 47. Fullprof: A Program for Rietveld [I31 J. Rodriguez-Carvajal. refinement and pattern matching analysis. Abstracts of Satellite Meeting on Powder Diffraction of XV Cong. of International Union of Crystallography (International Union of Crystallography, Toulouse, 1990) p. 127 (revised version, 1994). [I41 L.J. van der Pauw, Philips Research Rep. 1 (1958) 13. 1151 L.N. Mulay and E.A. Boudreaux (eds.), in: Theory and Applications of Molecular Paramagnetism (Wiley, New York, 1976) p. 494. R.D. Shannon, Acta Cryst., Sect. A 32 (1976) 751. 1. Alvarez, M.L. Veiga and C. Pica, Solid State Ionics (1996). in press. PA. Cox, Transition Metal Oxides: An Introduction to their Structure and Properties (Clarendon Press, Oxford, 1992). I. Alvarez, Ph.D. Thesis, Univenidad Complutense, Madrid, 1995.
SOLID STATE
El SEWJEK
Solid State Ionics 93
( 1997)
IONICS
329-334
Structural characterization and electronic prbperties of A-substituted LaNiO.,M,.,O, (A = Ca, Sr; M = Te, W) perovskites I. Alvarez”,
M.L. Vejga, C. Pica
Abstract The synthesis. structural characterization perovskite-like
and electronic propertics of the A,
in both the A and R sublatticcs of the perovskitc structure: Ni’ A ones. Orthorhombic resistivity
symmetry
(S.G.
measurements show semiconductor
Perovskitcs: Nickel
behavior in all cases (E,, < 0.2 eV). Ferromagnetic
PI1
author.
01997
SOl67-2738(96)00531-O
interactions with Curie
oxides
LaNiO, oxide is well known to have a rhombohedrically distorted perovskite structure and to behave as a metal down to 4.2 K, showing electrical and magnetic behavior which indicate a high electronic correlation [l-3]. In the recent past years, this material and some derived systems have attracted considerable attention and have been the sub.ject of extensive studies. Having in mind the discovery of superconductivity in Cu-containing perovskites at relatively high temperatures, this interest was in principle directed to the synthesis of Ni-containing similar phases or, at least, to a better understanding of this type of phenomena. Although superconductivity has not been found in these systems, it was soon stated however that they can be considered as a
0167-2738/97/Sl7.00
cations by divalent
dilTraction data. for all the title phase>. Electrical
K are detected from susceptibility mcasuremcnts.
1. Introduction
*Corresponding
by partial substitution
’ is substituted by M”- cations and La’-
Pbnm) is found, from X-ray
temperatures ranging between 180 and 250 Kcyvords:
,La
system are reported. These compounds can be regarded as being derived from LaNiO,
Hsevicr
Science H.V. All rights reserved
new family of cbmpounds with characteristic +atures opening a field of research in materials science and justifying by themselves the concentration of’ intensive efforts. The most important properties displayed by these oxides are the metal-insulator (M-I) transitions observed in some cases, the stabiliLation of magneti’c interactions and attiactive features such as magnetoresistence effects 141. In this context, systems like LaNi,.,,MXO, (M = Cr. Mn, Fe. Co, Cu, W, Tc, MO), RNiO, and (R = rare earth; M = Sr, -J-h), R, ,M,NiO, La ,, I INi,, \,,,_, IS-101 have been studied. In some of them, composition, temperature and structure driven M-l transitions have been reported. Jn the casts in which Ni cations are present in mixed-valent state, electrical arid magnetic proper+ could be qualitatively related to the relative amoudts of Ni’ ’ and Ni3’ taking into accouit the more or less probability of’ferro or antifcrromagnetic interactions
330
I. Alvarez. et al. I Solid State tonics 93 (1997) 329-334
and the evolution of the electronic localization in the systems. In this sense, both structural aspects and electronic correlation effects seem to be responsible for the observed behavior. We have previously reported structural and electronic results for the LaNi I _xW,O, system [ 11,121 in which a M-I transition governed by the composition at x > 0.10 has been observed. For the less substituted phases, a M-I transition governed by temperature was evidenced at - 50 K. In this work, we concentrate on the hole doping of these systems, for a given composition of M, x = 0.20, which could permit, a priori, the amplification of the metallic limit of these phases conserving the cations distribution and concentration in the B-sublattice of the perovskite structure. In this sense, our aim has been focused on the study of structural, electrical and magnetic features of the title compounds, comparing the obtained results with those corresponding to the non-doped phases and establishing the most important parameters which drive the observed physical phenomena.
2. Experimental The A,La, _,Ni,,,M,,,O, (A = Ca, Sr; 0 5 x 5 0.2) materials were prepared by the ceramic method starting from stoichiometric amounts of Ca(NO,), * La(NO,), * 6H,O, Ni(NO,), 4H,O, Sr(NO,),, 6H,O, WO, and H,TeO,, at 1423 K, for several days. During the thermal treatment, the samples were reground in each step and the process was monitored by X-ray diffraction until single phases were obtained. The X-ray diffraction patterns were recorded using a Philips X’Pert MPD diffractometer with a D-5000 (3051 /OO) goniometer. A step scan of 0.04 and a counting time of 15 s for each step were employed. The goniometer was connected to a PC controlled by the commercial program PC-APD (Analytical Powder Diffraction Software, 4.0). The obtained data were analyzed by the Rietveld method by means of the Fullprof program [13]. For the electronic resistivity measurements, pelletized samples were sintered at temperatures somewhat lower than the respective synthesis temperatures. Pycnometric measurements showed that, for all the samples, the densities were greater than 90% the
crystallographic values. Electronic resistivity was measured using the Van der Pauw method [143, which avoids contact effects. Contacts were made by covering with platinum paint the circular faces of the sample pellets. The magnetic susceptibility measurements were done by using a DSM8 pendulum susceptometer based on the Faraday method. The maximum applied magnetic field was 15 kG with H(dHldz) = 29 kG* cm-‘. The equipment was calibrated with Hg(Co(SCN),) and Gd,(SO,), * 8H,O, with x independent of the field in the temperature range of measurements. The magnetic susceptibility data have been corrected taking into account the ionic diamagnetic contribution from Ref. [15].
3. Results and discussion 3.1. Compositional
and structural characterization
Table 1 shows the Ni3+ concentration and the respective stoichiometry for the Te-containing compounds, obtained by iodometric titration. It can be observed, in general, that the oxygen content is somewhat lower than expected. This fact could be attributed, in part, to the synthesis method employed: the ceramic method implies the use of relatively high temperatures and, at these conditions, the reduction of Ni3+ to Ni2+ is more favourable. We have however used this method as it assures a good crystallinity degree in the samples and this condition is quite important in order to evaluate structural and electronic aspects. X-ray diffraction data for all the title compounds were analyzed by means of the Rietveld method and
Table 1 Compositional pounds Compound 0 0.1 Sr 0.2 Sr 0.2 Ca
data from
(x A)
iodometric
analysis
for the Te com-
za
Stoichiometry
0.24 0.25 0.28 0.28
La%.,Te,.,%,,
a z is the Ni3’ percentage A,La,_,Ni:+Ni~+,,_,M~:,O,..
Sr,,La,,,Nib.8Te,.*O,,,
Sr,.,La,,,Ni,,Te,.,O,,, Ca,.,La,.,Ni,.,Te,.,O*.~~
per one in the samples,
that
is:
I. Alvarez
331
et al. I Solid State Ionics 93 (1997) 329-334
they appear to be isostructural to the similar Wcontaining materials [ 111. The correspondent X-ray diffraction data were indexed and refined considering an orthorhombic symmetry, S.G. Pbmn (No. 62). Figs. 1 and 2 show, as an example, the observed and calculated X-ray diffraction profiles, the difference between them and the allowed reflections, for the Te phases with x = 0.20 Ca and x = 0.20 St-, respectively. In Tables 2 and 3 the crystal data and the Rfactors obtained in these refinements are gathered. As can be observed, profiles agreement and the
Table 2 Crystal data and R-factors
for the Te phase with x = 0.2Ca
Atom
Position
n
Y
2
CalLa Ni/Te O(1) O(2)
4c 4b 8d 4c
0.021(4) 0.5 0.281(3) -0.038(3)
0.0326(3) 0 0.294(3) 0.484(2)
0.25 0 0.051(2) 0.25
a (A) b (A) c (A) S.G. Pbnm
5.581(l) 5.604(l) 7.894( 1) (N” 62)
R, =7.8 R,, = 10.7 R,=6.1 x2 = 6.8
Table 3 Crystal data and R-factors
Fig. 1. Observed (dots) and calculated (solid line) X-ray diffraction profiles for the Te phase with .x=0.2 Ca.
I I
~lww-
2
5
o-
-‘Id
I I I
I I I I I I I II III
LA. I
.
A
1111 II 1111111 lllllll lllllll Ill1II 1111111 1llm
Atom
Position
x
Y
Z
SrlLa NilTe O(1)
4c 4b 8d
G(2)
4c
0.0203(5) 0.5 0.265(4) -0.288(3)
0.0235(4) 0 0.278(4) 0.493(2)
0.25 0 0.049(2) 0.25
a (A) b (A) c (A) S.G. Pbnm
5.592( 1) 5581(l) 7.906( 1) (N” 62)
R, = 8.2 R,,=11.2 R, =7.2 x*=6.5
R-factors obtained seem to confirm the validity of the proposed structural model. The most representative bond lengths for these compounds are given in Tables 4 and 5. The mean values and the sums of the Shannon ionic radii [ 161 are also included and, as can be observed, they agree reasonably. The X-ray diffraction data for the other title mixed oxides were also refined by the Rietveld method, and the results are compiled in Table 6. As a general trend, the respective cell-volume increases when x, i.e. the A concentration, is increased. This fact is in good agreement with the differences in ionic radii
Table 4 Bond lengths (in A) for the Te phase with n=0.2Ca Ca/La-0(
Ca/La-O(2)
Fig. 2. Observed (dots) and calculated (solid line) X-ray diffraction profiles for the Te phase with x=0.2 Sr.
for the Te phase with x=0.2.%
Mean Shannon
1)
2.59(1)X2 2.53(1)X2 2.89(1)X2 2.55( 1) 2.70(2) 2.66 2.55
(Ni/Te)-0( (Ni/Te)-O(2) Mean Shannon
1)
1.987(2)X2 2.08(2)X 2 1.99(2) X 2 2.02 2.03
332
I. Alvarez
et al.
I Solid State Ionics
Table 5 Bond lengths (in A) for the Te phase with x=0.2% Sr/La-0(
I)
253(2)X 2 2.41(2)X2 2.97(2)X2 2.63( 1) 2.75(2) 2.65 2.58
Sr/La-O(2) Mean Shannon
Table 6 Cell parameters
(Ni/Te)-0(
1)
and cell-volume
The electronic conductivity variation with temperature for the AXLa, _xNi,8M,,,0, mixed oxides has been measured in the temperature range between 300-950 K. An exponential variation of conductivity with temperature has been observed and the experimental data have been fitted to an equation of the type: (1)
for A,La, _%Ni, ,M,, ?O,
u (A)
h (A)
c (A)
V(A’)
Te, Te, Te, Tc,
5.594( I ) 5.589( 1) 5.592( I ) 5.581(4) 5.533(l) 5.478(3)
5.564( I ) 5.572( I ) 5.581(l) 5.604(4) 5.568( 1) S.hOS(3)
7.859( I ) 7.881(2) 7.906( 1) 7.894( 1) 7.880( 1) 7.966(4)
244.6 2415.4 246.8 246.9 242.8 244.7
W, 0 W, 0.2Sr
measurements
u(T) = (A/T) . exp( - E,lkT)
M, xA 0 0.1Sr 0.2Sr 0.2Ca
329-334
3.2. Conductivity 1.972(2)x2 2.06(2) x 2 1.983(1)X2 2.01 2.03
(Ni/Te)-O(2) Mean Shannon
9.7 (1997)
between the concerned cations in each case. Table 7 shows the mean ionic radii of the cations present in the A and B sublattices of pcrovskite structure. In general, the cell-volumes of the Te-phases are greater than the W-phases ones. This fact could be related to the electronic features of Te cations, having no available d-orbitals, as pointed out previously for similar compounds [ 171. From the above structural results, we can conclude that A,La, _XNi,,8M,,,20, compounds have perovskite-type structures in which all cations are placed at random in the correspondent sublattice of the perovskite structure.
which is usually applied to a small polaron hopping mechanism [ 181. In Eq. (l), E, is the activation energy related to the hopping process, A is the preexponential factor and k is the Boltzmann constant. Fig. 3 shows the linear variation of ln(cr.7) with 1‘. ’ for the title compounds, which agrees well with the above assumption. It is therefore shown that all the studied phases arc thermally-activated semiconductors, in the temperature range explored. In order to analyze the influence of x and the A cation substituted in the electrical properties, we present separately the correspondent graphs for the Te compounds with different x values (Fig. 4a) and with different A cations (Fig. 4b). The activation energies, deduced from the above results for each sample, are gathered in Table 8. It can be deduced from Fig. 4a that conductivity values decrease and activation energies increase as the substitution of La cations by Sr ones is increased. On the other hand, Fig. 4b
Table 7 Mean ionic radii and cell-volumes Compound; Te. Te, Te, Te, W, W,
0 0.1% 0.2Sr 0.2Ca 0 0.2Sr
M, xA
v (A’)
mlr.“.h (AY
rnc:...Fl (A)”
244.6 245.4 246.8 246.9 240.9 244.7
1.16 1.17 1.18 1.15 1.16 1.18
0.624 0.630 0.630 0.633 0.646 0.620
’ Mean ionic radii of A-sublattice b Mean ionic radii of B-sublattice
cations (Sr/Ca cations (TclW
and La). and Ni).
-21 0.912
I
I
I
I
1.482
I.982
2.&82
2.982
t-1. to] , I(-‘1
Fig. 3. Variation of In(rr.T)
vs. T
’ for AzLa, .Ni,,&.O,.
I UQ
I. Alvarez
et al. 1 Solid State Ionics 93 (1997) 329-334
0 i= in 6
5 4
0.982
1.482
1.982
2.482
2.982
3.482
T-'.lti(K-'1 10 I
----I
-21 0982
I 1.482
I 1.982
I 2.482
I 2.982
I 3.482
l-'.ld (K-')
Fig. 4. Variation of ln(o-7’) (b) different A cations.
Table 8 Activation
energies
vs. T-’ for (a) different x values and
for A,La, _,Ni, ,M,,,O,
M, XA
E, (eV)
Te, Te, Te, Te,
0.15(l) 0.16(l) 0.21(2) 0.24( 1) 0.07( 1) 0.16(Z)
0 0.1.9 0.2.Q 0.2Ca
w, q W, 0.2Sr
shows that, for a given composition, Ca-containing sample is related with smaller electrical conductivity values. Finally, it was clear from Fig. 3 that Te phases are, in general, worse semiconductors than the W phases. Considering the compositional and structural aspects mentioned above, the observed
333
behavior can be interpreted as follows: as the cellvolume increases and the oxygen content of the phases is smaller, the crystal lattice is more expanded and also more disordered. Having in mind the band model proposed for explaining the electronic evolution from LaNiO, to the LaNi,,,M,.,O, (X= 0) perovskites [ 121, which considered the existence of narrow bands responsible of the semiconductor behavior observed, both expansion and disorder probably give rise to narrower bands or, alternatively, to a greater electronic localization of the system. In this sense, the expected amplification of the metallic limit in the non-doped samples, as a consequence of substitution, is frustrated as a result of the disorder and oxygen deficiency introduced in this substitution. In Fig. 5a and 5b, the variation of magnetic susceptibility with temperature for the Te and Wcontaining perovskites, respectively, is represented. A, complex behavior due to both the electronic localization and the relative amounts of Ni2+/Ni3+ stabilized in each case was expected: Ni2+-0-Ni2+ interactions are, in principle, antiferromagnetic in character, whereas’ Ni3+ -0-Ni2+ interactions could be ferromagnetic. In this sense, a certain competition between both types of magnetic interactions could be expected. The observed behavior shows, in principle, the stabilization of ferromagnetic interactions with Curie temperatures which range between 200 and 250 K, as can be estimated from Fig. 5. A similar behavior was found in the non-substituted perovskites [19] and was interpreted, from neutron diffraction experiments, as antiferromagnetic interactions with and important spin canting. In this context, it is therefore probable that similar complex magnetic interactions take place in the title mixed oxides. On the other hand, Fig. 5 shows that magnetic susceptibility values are smaller as x is increased and this fact could indicate that, in the x#O phases, the ferromagnetic component is smaller. These results are probably related to structural aspects governing a less marked canting in the ‘spin arrangement giving rise to a quantitatively less important ferromagnetic component (significative variation of bond distances, more or less structural distortion in the directions in which magnetic ordering takes place,...). Further neutron diffraction experiments will make these points clearer.
334
1. Alvarez et al. I Solid State Iminicv 93 (1997) 329-334
References
0
50
loo
150 1 fK1
250
zoo
:
I
b)
0.05
cl 00.2ow
F
*
5 0.04
z
0.20
srw
2 x
0.02 -\
0
0
50
100
150
200
250
300
1 (K) Fig. 5. Variation of the magnetic compounds and (b) W phases.
susceptibility
for the (a) Te
In conclusion, new A,La, _,Ni,,,,M,,70, perovskite-type mixed oxides have been synthesized, in which all the cations are placed at random in each sublattice. They are all semiconductors with activation energies lower than 0.2 eV and show, in principle, ferromagnetic interactions below -250 K.
Acknowledgments We thank the DGICYT (Proj. PB 94-0214) and CICYT (Proj. MAT 94-0374, Spain) for financial support.
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