Cation Ordering Types and Dielectric Properties in the Complex Perovskite Ca(Ca_(1/3)Nb_(2/3)O_3

摘要

In the present work the effects of cation ordering on dielectric properties were isolated by investigating three polymorphs of Ca(Ca,,3 Nb213)03 that feature different arrangements of the Ca2~ and Nb5~ cations on the B-sites of the p~rovskite 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 struc-tures with 1:1 and newly described k — ~ [11 1]~’ ordering. Riet-veld 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 dielectic properties. In all three polymorphs the cation ordering was combined with the same bbc~ octahedral tilt system, and the structural refinements yielded similar magnitudes of the tilting angles. The most signifi-cant 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 = ~ [11 l]~’ to the 2:1 ordered structure. The increased fraction of strongly compressed Nb—O bonds in the 2:1 structure asso-ciated with the large distortion was correlated with the decrease in dielectic 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.