A. Faik et al. / Journal of Molecular Structure 920 (2009) 196–201
201
space between the BO6 and B0O6 octahedra, irrespective of having a
solid solution in the B-site, at least for the studied solid solution.
4. Conclusions
The Cd2+ cation progressive substitution by Ca2+ in the
Sr2CdWO6 double perovskite led to a continuous solid solution in
the whole (0 6 x 6 1) fraction range. The crystal structures, at
room-temperature, of all the members of the solid solution were
determined from the Rietveld refinements of laboratory X-ray
powder diffraction data: they crystallize in the P21/n space group.
The high-temperature study revealed the following two phase-
transition sequence, for all the members of the solid solution:
P21=n ! I4=m ! Fm3m. The cation substitution does not induce
a phase transition, and the phase-transition sequence observed in
the end members of the solid solution is also observed in the solid
solution. The phase-transition temperatures of both successive
phase-transitions showed
a linear behavior in the whole
(0 6 x 6 1) fraction range. It was observed that the transition tem-
peratures are higher in compounds with low tolerance factors, as
in the case of the members of the Sr2MWO6 family.
Fig. 6. Phase-transition temperatures of the successive phase transitions of the
Sr2Cd1ꢀxCaxWO6 solid solution, as a function of their respective tolerance factors at
room-temperature. Continuous line is a guide for the eyes.
Acknowledgements
CaxWO6 (0 6 x 6 1) solid solution is P21=n ! I4=m ! Fm3m. As we
pointed out in [8], the same phase-transition sequence is observed
in the Sr2BWO6 double perovskite oxide family, suggesting that the
temperature evolution of their structures is governed by the same
principles.
We thank Dr. Gateshki for the very helpful discussions during
the preparation of the manuscript. This work was done in part un-
der project numbers: UPV 0063.310-13564/2001-2006 and
FIS2005-07090. The authors thank the technician of SGIker, Dr. J.
P. Chapman, financed by the Programa Nacional de Potenciación
de Recursos Humanos del Plan Nacional de Investigación Científica,
Desarrollo e Innovación, Ministerio de Ciencia y Tecnología y Fon-
do Social Europeo (FSE), for the X-ray diffraction.
It is usually assumed [16] that the driving force of the structural
changes in this kind of materials is the mismatch between the size
of the A cation (Sr2+ in this case) and the interstitial space between
the BO6 and B0O6 octahedra. This mismatch is measured by means
pffiffiffi
dAAO
BAOþdB0
of the so-called tolerance factor [9]: t ¼ 2 ðd
Þ. In the present
AO
case, dBAO = (1 ꢀ x)dCdAO + x dCaAO. We have obtained the mean
dSrAO, dCdAO, dCaAO and dWAO bond lengths by calculating the
room-temperature distances that give the nominal oxidation states
of the cations in the bond-valence method [8,17]. In Fig. 6 we show
the phase-transition temperatures of the successive phase transi-
tions as a function of the tolerance factor t at room-temperature.
A clear linear behavior of the cubic-to-tetragonal phase-transi-
tion temperature is observed, in accordance with the result ob-
tained for the whole Sr2BWO6 (B = Ca, Cd, Mn, Zn, Mg, Co, Cu, Ni)
family [6–8]. The same is true for the tetragonal-to-monoclinic
transition, if the value for that phase transition in Sr2 CdWO6 is
revisited [8]. The actual temperature (1070 K) for the tetragonal-
to-monoclinic transition in Sr2CdWO6, belonging to the prepara-
tion presented in this work, is a slightly lower than the previously
reported one (1105 K) [8] in a sample from another preparation,
measured in another equipment, and that showed, at high-temper-
atures, a progressive decomposition which could be responsible for
a slightly different stoichiometry, affecting the symmetry, and, in
turn, influencing in the temperature value for the phase transition.
These results confirm that the most important factor governing
the appearance of octahedral tilts in the double perovskite struc-
ture and the temperature range in which they exist is a geometrical
one: the mismatch between the size of the A cation and interstitial
References
[1] K.-I. Kobayashi, T. Kimura, H. Sawada, K. Terakura, Y. Tokura, Nature 395
(1998) 677–680.
[2] M. DeMarco, H.A. Blackstead, J.D. Dow, M.K. Wu, D.Y. Chen, E.Z. Chien, H. Haka,
S. Toorongian, J. Fridmann, Phys. Rev. B 62 (2000) 14301–14303.
[3] Y. Todate, J. Phys. Chem. Solids B 60 (1999) 1173.
[4] P.M. Woodward, Acta Cryst. B53 (1997) 32.
[5] T. Hahn (Ed.), International Tables for Crystallography, vol. A, Kluwer,
Dordrecht, 2002.
[6] M. Gateshki, J.M. Igartua, E. Hernández-Bocanegra, J. Phys. Condens. Matter 15
(2003) 6199–6217.
[7] M. Gateshki, J.M. Igartua, J. Phys. Condens. Matter 16 (2004) 6639–6649.
[8] M. Gateshki, J.M. Igartua, A. Faik, J. Solid State Chem. 180 (2007) 2248–2255.
[9] V.M. Goldschmidt, Str. Nor. Vidensk-Akad. Oslo 1 (1926) 1.
[10] Q. Zhou, B.J. Kennedy, C.J. Howard, M.M. Elcombe, A.J. Studer, Chem. Mater. 17
(2005) 5357–5365.
[11] Q. Zhou, B.J. Kennedy, M.M. Elcombe, Physica B Condens. Matter 385–386
(2006) 190–192.
[12] M.C.L. Cheah, B.J. Kennedy, R.L. Withers, M. Yonemura, T. Kamiyama, J. Solid
State Chem. 179 (2006) 2487–2494.
[13] Q. Zhou, B.J. Kennedy, K.S. Wallwork, M.M. Elcombe, Y. Lee, T. Vogt, J. Solid
State Chem. 178 (2005) 2282–2291.
[14] P.J. Saines, B.J. Kennedy, J. Solid State Chem. 181 (2008) 298–305.
[15] J. Rodrı
´guez-Carvajal, Physica B 192 (1993) 55–69.
[16] N.E. Brese, M. O’Keeffe, Acta Cryst. B47 (1991) 192–197.
[17] Accumulated Table of Bond Valence Parameters Version 2.2 Prepared by I.D.
Brown, McMaster University, Hamilton, Ontario, Canada. Available from:
<www.ccp14.ac.uk/ccp/web-mirrors/i_d_brown>.