Landau theory and giant room-temperature barocaloric effect in MF$_3$ metal trifluorides

TL;DR

This study uses a Landau theory model to analyze phase transitions in MF$_3$ metal trifluorides, predicting giant room-temperature barocaloric effects driven by structural distortions and cation size, with potential applications in cooling technologies.

Contribution

The paper introduces a Landau theory model for MF$_3$ trifluorides that predicts large barocaloric effects near room temperature, highlighting the influence of cation size on tilt fluctuations.

Findings

Predicted barocaloric effect of up to 15 JK$^{-1}$kg$^{-1}$ in Sc-substituted TiF$_3$.

Large temperature range of over 140 K for the barocaloric effect.

Rate of change of transition temperature with pressure exceeds typical materials.

Abstract

The structural phase transitions of MF$_3$ (M=Al, Cr, V, Fe, Ti, Sc) metal trifluorides are studied within a simple Landau theory consisting of tilts of rigid MF$_6$ octahedra associated with soft antiferrodistoritive optic modes that are coupled to long-wavelength strain generating acoustic phonons. We calculate the temperature and pressure dependence of several quantities such as the spontaneous distortions, volume expansion and shear strains as well as $T-P$ phase diagrams. By contrasting our model to experiments we quantify the deviations from mean-field behavior and found that the tilt fluctuations of the MF$_6$ octahedra increase with metal cation size. We apply our model to predict giant barocaloric effects in Sc substituted TiF$_3$ of up to about $15\,$JK$^{-1}$kg$^{-1}$ for modest hydrostatic compressions of $0.2\,$GPa. The effect extends over a wide temperature range of over $140\,$K (including room temperature) due to a large predicted rate $dT_c/dP = 723\,$K GPa$^{-1}$, which exceeds those of typical barocaloric materials. Our results suggest that open lattice frameworks such as the trifluorides are an attractive platform to search for giant barocaloric effects.