Structural changes in Sr9In(PO4)(7) during antiferroelectric phase transition
Faculty of Sciences. Physics
, p. 176-185
University of Antwerp
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 Ca-3(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 P1O(4) 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 similar to 10(4) K) which is in between the value usually observed for pure displacive (C similar to 10(5) K) and that for orderdisorder phase transitions (C similar to 10(3) 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 Ca-3(VO4)(2)-type materials.