Structural changes in Sr9In(PO4)7 during antiferroelectric phase transition

Abstract

Structural changes in Sr₉In(PO₄)₇ during the antiferroelectric (AFE) phase transition are studied by X-ray powder diffraction, electron microscopy, second-harmonic-generation, and dielectric measurements. Sr₉In(PO₄)₇ complements a group of Ca₃(VO₄)₂-type ferroelectric (FE) phosphates and vanadates and is the first example of an AFE material in this family. Antiparallel shifts of Sr atoms from their average positions and ordering of the P1O₄ tetrahedra form two contributions in the structural mechanism of the AFE phase transition: a displacive contribution and an order-disorder constituent, respectively. The displacive and order-disorder type of structural changes may account for the obtained value of the Curie–Weiss constant (C ~ 10⁴ K) which is in between the value usually observed for pure displacive (C ~ 10⁵ K) and that for orderdisorder phase transitions (C ~ 10³ K). The structural mechanism of the AFE phase transition in Sr₉In(PO₄)₇ is very similar to that of the FE phase transition in Ca₉R(PO₄)₇ and Ca₉R(VO₄)₇. Both displacive and orderdisorder contributions are responsible for the physical properties of the Ca₃(VO₄)₂-type materials.