title: An original polymorph sequence in the high-temperature evolution of the perovskite Pb2TmSbO6.
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 → P2(1)/n → R3 → Fm3m. 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.
authors.affiliation: Área de Química General e Inorgánica, Departamento de Química, Facultad de Química, Bioquímica y Farmacia, Universidad Nacional de San Luis, Chacabuco y Pedernera, 5700 San Luis, Argentina.
authors.forename: Sebastián A
authors.initials: SA
authors.lastname: Larrégola
authors.forename: José A
authors.initials: JA
authors.lastname: Alonso
authors.forename: Denis
authors.initials: D
authors.lastname: Sheptyakov
authors.forename: Miguel
authors.initials: M
authors.lastname: Algueró
authors.forename: Angel
authors.initials: A
authors.lastname: Muñoz
authors.forename: Vladimir
authors.initials: V
authors.lastname: Pomjakushin
authors.forename: José C
authors.initials: JC
authors.lastname: Pedregosa