Cation Ordering types and Dielectric Properties in the Complex Perovskite Ca(Ca1/3Nb2/3)Od3^

Published

January 1, 2001

Author(s)

Igor Levin, Julia Y. Chan, Richard G. Geyer, James E. Maslar, Terrell A. Vanderah

Abstract

In the present work the effects of cation ordering on dielectric properties were isolated by investigating three polymorphs of Ca(Ca_1/3Nb_2/3)O₃ that feature different arrangements of the Ca²⁺ and Nb⁵⁺ cations on the B-sites of the perovskite structure. Dielectric measurements at frequencies above 1 GHz revealed a systematic dependence of the properties on the type of cation ordering. In particular, the structure with 2:1 ordering exhibited a lower dielectric constant and a significantly more negative temperature coefficient of resonance frequency than the structures with 1:1 and newly described k=1/4(111)_c ordering. Rietveld refinements of structural models for the three Ca₄Nb₂O₉ polymorphs were conducted using X-ray and neutron powder diffraction data to elucidate structural details that could be correlated with the changes in dielectric properties. In all three polymorphs the cation ordering was combined with the same b-b-b+ octahedral tilt system, and the structural refinements yielded similar magnitudes of the tilting angles. The most significant crystal-chemical difference between the three polymorphs was in the coordination environment of Nb⁵⁺. Analysis of the refined bond distances indicated increasing average distortion of the Nb nearest-neighbor environment in going from the 1:1 to the k=1/4(111)L_c to the 2:1 ordered structure. The increased fraction of strongly compressed Nb-O bonds in the 2:1 structure associated with the large distortion was correlated with the decrease in dielectric constant and more negative value of temperature coefficient of the resonant frequency obtained for this polymorph. Raman spectra obtained for the three polymorphs exhibited differences that were consistent with the observed structural chemistry.

Citation

Journal of Solid State Chemistry

Volume

156 No. 1

Pub Type

Journals

Keywords

cation ordering, dielectric properties, octahedral tilting, perovskite

Citation

Levin, I. , Chan, J. , Geyer, R. , Maslar, J. and Vanderah, T. (2001), Cation Ordering types and Dielectric Properties in the Complex Perovskite Ca(Ca_1/3Nb_2/3)Od3^, Journal of Solid State Chemistry (Accessed October 6, 2024)