Nonmonotonic Composition Dependence of the Dielectric Response of Ba1-xCaxZrO3

We use first-principles density functional theory calculations to investigate the dielectric response of BaZrO3 based perovskites. In this study we have substituted Ca for Ba over a range of compositions to understand what causes the recently reported (Levin et al., J. Solid State Chem. 2003, 175, 170) nonmonotonic dielectric response of Ba1-xCaxZrO3 (BCZ). We show that in all compositions studied there appear two new sets of IR active phonon modes that increase the dielectric constant epsilon. One set occurs below the normal IR active A-O modes, due to the "rolling cation" motion of Ca about its O-12 cage. The second set of new phonon modes occurs between the normal A-O and B-O mode frequencies, caused by the displacement of Ca along one Cartesian direction. As more Ca is added to BCZ, Ca-Ca interactions increase, leading to Ca displacements in all three Cartesian directions, stronger Ca-O bonds and increased O-6 tilts. Such structural deformations decrease epsilon. These effects are responsible for the anomalous compositional dependence of the BCZ dielectric response.