An original polymorph sequence in the high-temperature evolution of the perovskite Pb2TmSbO6

SEBASTIÁN A. LARRÉGOLA, JOSÉ A. ALONSO, DENIS SHEPTYAKOV, MIGUEL ALGUERÓ, ANGEL MUÑOZ, VLADIMIR POMJAKUSHIN AND JOSÉ C. PEDREGOSA

JOURNAL OF THE AMERICAN CHEMICAL SOCIETY

AMER CHEMICAL SOC

Año: 2010 p. 14470 - 14480

0002-7863

The synthesis, crystal structure, and dielectric properties of the novel double perovskite Pb2TmSbO6 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 TmO6 and SbO6 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 Pb2+ 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 P21/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.2TmSbO6 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 TmO6 and SbO6 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 Pb2+ 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 P21/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.