Landau theory and giant room-temperature barocaloric effect in M F3 metal trifluorides
Abstract
The structural phase transitions of M F3 (M = Al, Cr, V, Fe, Ti, Sc) metal trifluorides are studied within a simple Landau theory consisting of tilts of rigid M F6 octahedra associated with soft antiferrodistortive 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 find that the tilt fluctuations of the M F6 octahedra increase with metal cation size. We apply our model to predict giant barocaloric effects in Sc-substituted TiF3 of up to about 15 JK -1kg-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, d Tc/d P =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.
- Publication:
-
Physical Review Materials
- Pub Date:
- October 2017
- DOI:
- 10.1103/PhysRevMaterials.1.053601
- arXiv:
- arXiv:1708.01158
- Bibcode:
- 2017PhRvM...1e3601C
- Keywords:
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- Condensed Matter - Materials Science
- E-Print:
- 11 pages, 8 figures