Effects of diagonal strains and H-bond geometry in antiferroelectric squaric acid crystals

TL;DR

This paper investigates how diagonal strains and hydrogen bond geometry affect the phase transition and physical properties of antiferroelectric squaric acid crystals, providing a refined proton ordering model.

Contribution

The study introduces modifications to the proton ordering model by incorporating diagonal lattice strains and hydrogen bond geometry effects, improving the understanding of squaric acid's properties.

Findings

Thermal expansion and specific heat are well described by the modified model.

Spontaneous strain $ extvarepsilon_1- extvarepsilon_3$ is accurately modeled.

Further model adjustments are needed to fully capture hydrostatic pressure effects.

Abstract

The proton ordering model of the phase transition and physical properties of antiferroelectric crystals of squaric acid is modified by taking into account the influence of diagonal lattice strains and of the local geometry of hydrogen bonds, namely of the distance $\delta$ between the H-sites on a bond. Thermal expansion, the spontaneous strain $\varepsilon_1-\varepsilon_3$, and specific heat of squaric acid are well described by the proposed model. However, a consistent description of hydrostatic pressure influence on the transition temperature is possible only with further modifications of the model.