Vibrational properties of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>CuInP</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="normal">S</mml:mi><mml:mn>6</mml:mn></mml:msub></mml:mrow></mml:math> across the ferroelectric transition
Sabine N. Neal, Sobhit Singh, Xiaochen Fang, Choongjae Won, Fei-Ting Huang, Sang‐Wook Cheong, Karin M. Rabe, David Vanderbilt, J. L. Musfeldt
Abstract
In order to explore the properties of a two-sublattice ferroelectric, we measured the infrared and Raman scattering response of ${\mathrm{CuInP}}_{2}{\mathrm{S}}_{6}$ across the ferroelectric and glassy transitions and compared our findings to a symmetry analysis, calculations of phase stability, and lattice dynamics. In addition to uncovering displacive character and a large hysteresis region surrounding the ferroelectric transition temperature ${T}_{\mathrm{C}}$, we identify the vibrational modes that stabilize the polar phase and confirm the presence of two ferroelectric variants with opposite polarizations. Below ${T}_{\mathrm{C}}$, a poorly understood relaxational or glassy transition at ${T}_{\mathrm{g}}$ is characterized by local structure changes in the form of subtle peak shifting and activation of low frequency out-of-plane Cu- and In-containing modes. The latter are due to changes in the Cu/In coordination environments and associated order-disorder processes. Moreover, ${T}_{\mathrm{g}}$ takes place in two steps with another large hysteresis region and significant underlying scattering. Combined with imaging of the room temperature phase separation, this effort lays the groundwork for studying ${\mathrm{CuInP}}_{2}{\mathrm{S}}_{6}$ under external stimuli and in the ultrathin limit.