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



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


Four Ca4Nb2O9 polymorphs with perovskite-related A(B'1/3B2/3)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}ac x {square root of 2}ac x 2{square root of 2}ac, α {approximately equal to} γ {approximately equal to} 90 , Β {approximately equal to} 125 (ac 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}ac x {square root of 2}ac x 3{square root of 2}ac, 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.
Journal of Solid State Chemistry
No. 1


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


Levin, I. , Bendersky, L. , Cline, J. , Roth, R. and Vanderah, T. (2000), Octahedral Tilting and Cation Ordering in Perovskite-Like Ca<sub>4</sub>Nb<sub>2</sub>O<sub>9</sub> = 3 Center Dot Ca(Ca<sub>1/3</sub>Nb<sub>2/3</sub>)O<sub>3</sub> Polymorphs, Journal of Solid State Chemistry (Accessed December 8, 2023)
Created February 1, 2000, Updated February 19, 2017