Elucidation of the Antiferroelectricity Mechanism in CsBi(MoO$_4$)$_2$

Although the antiferroelectric compound CsBi(MoO$_4$)$_2$ has been known for a long time, the underlying mechanism remained poorly understood. No temperature-dependent crystallographic investigation was performed to solve this. In this work, a neutron diffraction study at 150 K was used to solve the crystal structure of the antiferroelectric phase existing between 135 and 330 K. X-ray absorption spectroscopy at room temperature and temperature-dependent diffraction studies between 150 K and the melting point were used to elucidate the mechanism driving the phase transition. The antiferroelectricity mechanism of CsBi(MoO$_4$)$_2$ is revealed to be driven by the temperature-dependent Bi displacement caused by the Bi 6s$^2$ lone pair. The thermal expansion of CsBi(MoO$_4$)$_2$ between 150 K and its melting point is determined, as well. The current work solves the long-standing question of the origin of the antiferroelectricity in CsBi(MoO$_4$)$_2$.