Octahedral Tilting and Cation Ordering in Perovskite-Like Ca4Nb2O9 = 3 Center Dot Ca(Ca1/3Nb2/3)O3 Polymorphs

Published

February 1, 2000

Author(s)

Igor Levin, Leonid A. Bendersky, James P. Cline, Robert S. Roth, Terrell A. Vanderah

Abstract

Four Ca₄Nb₂O₉ polymorphs with perovskite-related A(B'_1/3B_2/3)O₃ structures and different arrangements of B-site cations were identified and characterized by X-ray powder diffraction and transmission electron microscopy. The B-cation ordering in three of these phases is combined with a⁺b^-b^- octahedral tilting. Two high-temperature forms of Ca₄Nb₂O₉ are separated by a displacive octahedral tilting phase transition which occurs between 1500 C and 1600 C. Both high-temperature phases exhibit 1:1 ordering (ordering vector 1/2[111]_c*) of Ca/Nb cations on the B-sites. Upon cooling, the 1:1 ordered structure transforms to a metastable P1 triclinic structure with lattice parameters {square root of 6}a_c x {square root of 2}a_c x 2{square root of 2}a_c, α {approximately equal to} γ {approximately equal to} 90 , Β {approximately equal to} 125 (a_c refers to cubic perovskite) and ordering vector 1/4[111]_c*. This transformation occurs by reordering of cations on [111] planes. The metastable P1 structure transforms upon annealing to a stable 2:1 ordered P2₁/c structure with lattice parameters {square root of 6}a_c x {square root of 2}a_c x 3{square root of 2}a_c, Β = 125 (ordering vector 1/3[111]_c*). This first-order transition occurs by precipitation and growth of the 2:1 ordered phase, which is stable below 1400 C.

Citation

Journal of Solid State Chemistry

Volume

150

Issue

No. 1

Pub Type

Journals

Keywords

cation ordering, dielelectrics, octahedral tilting, perovskite, transmission electron microscopy

Citation

Levin, I. , Bendersky, L. , Cline, J. , Roth, R. and Vanderah, T. (2000), Octahedral Tilting and Cation Ordering in Perovskite-Like Ca₄Nb₂O₉ = 3 Center Dot Ca(Ca_1/3Nb_2/3)O₃ Polymorphs, Journal of Solid State Chemistry (Accessed May 8, 2024)