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arXiv · 1708.01158

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

A. Corrales-Salazar·R. T. Brierley·P. B. Littlewood·G. G. Guzmán-Verri
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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.

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A. Corrales-Salazar, R. T. Brierley, P. B. Littlewood, G. G. Guzmán-Verri. 2017-10-17. Landau theory and giant room-temperature barocaloric effect in MF$_3$ metal trifluorides. https://doi.org/10.1103/physrevmaterials.1.053601

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