Soft phonon-driven structural phase transition in Ba2CaWO6

Double perovskites (A2BB′X6) represent a class of materials known for their compositional diversity and promising functionalities. A pivotal aspect underlying their functionality is the coupling between the structure and their physical properties. Here, we report a comprehensive investigation of the structural phase transition in Ba2CaWO6 (BCWO) using temperature-dependent Raman spectroscopy, x-ray diffraction (XRD), specific heat measurements, and density functional theory (DFT)-based calculations. Our Raman measurements reveal a soft mode (at ∼ 28 cm−1)-driven continuous phase transition from the low-temperature tetragonal (I4/m) to high-temperature cubic (Fm-3m) phase, accompanied by splitting and unusual shifts of modes in the tetragonal phase. Furthermore, our Raman data suggest that strong anharmonicity plays a role in phase transition, which is corroborated by specific heat data and further confirmed by XRD measurements exhibiting a change in octahedral tilt and unit cell parameters. DFT calculations reveal the presence of imaginary modes in the high-temperature cubic phase underscoring a possible structural phase transition. Our results shed light on the mechanism behind the phase transition and resolve previous ambiguities regarding the room temperature structural symmetry of BCWO.