An Original Polymorph Sequence in the High-Temperature Evolution of the Perovskite Pb 2 TmSbO 6 Sebastia ´n A. Larre ´ gola, † Jose ´ A. Alonso,* ,‡ Denis Sheptyakov, § Miguel Alguero ´, ‡ Angel Mun ˜ oz, | Vladimir Pomjakushin, § and Jose ´ C. Pedregosa † A ´ rea de Quı ´mica General e Inorga ´nica, Departamento de Quı ´mica, Facultad de Quı ´mica, Bioquı ´mica y Farmacia, UniVersidad Nacional de San Luis, Chacabuco y Pedernera, 5700 San Luis, Argentina, Instituto de Ciencia de Materiales de Madrid, C.S.I.C., Cantoblanco, 28049 Madrid, Spain, Laboratory for Neutron Scattering, PSI Villigen, CH-5232 Villigen PSI, Switzerland, and Dpto. de Fı ´sica Aplicada, EPS, UniVersidad Carlos III, AVda. UniVersidad 30, E-28911, Legane ´s-Madrid, Spain Received May 30, 2010; E-mail: ja.alonso@icmm.csic.es Abstract: The synthesis, crystal structure, and dielectric properties of the novel double perovskite Pb 2 TmSbO 6 are described. The room-temperature crystal structure was determined by ab initio procedures from neutron powder diffraction (NPD) and synchrotron X-ray powder diffraction (SXRPD) data in the monoclinic C2/c (No. 15) space group. This double perovskite contains a completely ordered array of alternating TmO 6 and SbO 6 octahedra sharing corners, tilted in antiphase along the three pseudocubic axes, with an a - b - b - tilting scheme, which is very unusual in the crystallochemistry of perovskites. The lead atoms occupy a highly asymmetric void with 8-fold coordination due to the stereoactivity of the Pb 2+ lone electron pair. This compound presents three successive phase transitions in a narrow temperature range (at T1 ) 385 K, T2 ) 444 K, and T3 ) 460 K in the heating run) as shown by differential scanning calorimetry (DSC) data. The crystal structure and temperature-dependent NPD follow the space-group sequence C2/c f P2 1 /n f R3 j f Fm3 j m. This is a novel polymorph succession in the high-temperature evolution of perovskite-type oxides. The Tm/Sb long-range ordering is preserved across the consecutive phase transitions. Dielectric permittivity measurements indicate the presence of a paraelectric/antiferroelectric transition (associated with the last structural transition), as suggested by the negative Curie temperature obtained from the Curie-Weiss fit of the reciprocal permittivity. 1. Introduction The perovskite-type oxides exhibit the general formula ABO 3 , where A represents a large electropositive cation and B stands for a small transition metal ion. The prototypical perovskite structure (aristotype) is cubic, and it can be described as a framework of corner-sharing BO 6 octahedra with the A cations located at the 12-fold coordinated voids within the octahedra. The so-called double perovskite A 2 B′B′′O 6 oxides contain two suitable B′ and B′′ cations at the octahedral positions. Double perovskites may present different kinds of cationic ordering at the octahedral sites as it has been reviewed by Anderson et al. 1 The most common ordering is a rock-salt arrangement of the B′O 6 and B′′O 6 octahedra in a perfectly alternated disposition along the three directions of the crystal. The layered ordering is another possibility, which is mostly observed in copper- containing perovskites, where alternating layers of B′O 6 and B′′O 6 octahedra compose the crystal structure. The tolerance factor for A 2 B′B′′O 6 double perovskites is defined as t ) (r A + r O )/[2[(r B′ + r B′′ )/2 + r O ]]. The ideal double perovskite also shows a cubic symmetry (t ) 1), space group Fm3 j m, with a doubled unit-cell edge with respect to that of the ABO 3 aristotype. If there is a mismatch between the A-O and the average (B′,B′′)-O bond lengths, that is, t < 1 or t > 1, the structure of the double perovskites experiences a distortion from the cubic symmetry, giving rise to a superstructure. Just like ABO 3 perovskites, the most frequent distortions in A 2 B′B′′O 6 oxides are due to the tilting of the B′O 6 /B′′O 6 octahedra. On the basis of Glazer’s description of the tilt systems, Woodward 2 has considered the cation ordering and octahedral tilting occurring simultaneously and derived 13 possible space groups for double perovskites. Howard et al. 3 identified, using the group-theoretical analysis, 12 space groups under the same conditions of octahedral tilting and cation ordering. Despite such a theoretical variability, only a few space groups have been actually observed in the real world, that is, a 0 a 0 a 0 (Fm3 j m), a 0 a 0 c - (I4/m), a - a - a - (R3 j ), a 0 b - b - (I2/m), and a - a - b + (P2 1 /n). In addition, for many double perovskites the deviation from the ideal cubic structure (due to compositional, temperature, or pressure changes) in general follows either the sequence Fm3 j m f I4/m f I2/m f P2 1 /n or the sequence Fm3 j m f R3 j f I2/m f P2 1 /n. † Universidad Nacional de San Luis. ‡ C.S.I.C. § ETH Zurich and PSI Villigen. | Universidad Carlos III. (1) Anderson, M. T.; Greenwood, K. B.; Taylor, G. A.; Poeppelmeier, K. R. Prog. Solid State Chem. 1993, 22, 197. (2) Woodward, P. M. Acta Crystallogr., Sect. B 1997, 53, 32. (3) Howard, C. J.; Kennedy, B. J.; Woodward, P. M. Acta Crystallogr., Sect. B 2003, 59, 463A. Published on Web 09/24/2010 10.1021/ja104417f  2010 American Chemical Society 14470 9 J. AM. CHEM. SOC. 2010, 132, 14470–14480