Giant Mechanocaloric Effects in Fluorite-Structured Superionic Materials

Mechanocaloric materials experience a change in temperature when a mechanical stress is applied on them adiabatically. Thus, far, only ferroelectrics and superelastic metallic alloys have been considered as potential mechanocaloric compounds to be exploited in solid-state cooling applications. Here we show that giant mechanocaloric effects occur in hitherto overlooked fast ion conductors (FIC), a class of multicomponent materials in which above a critical temperature, <i>T</i>_s, a constituent ionic species undergoes a sudden increase in mobility. Using first-principles and molecular dynamics simulations, we found that the superionic transition in fluorite-structured FIC, which is characterized by a large entropy increase of the order of 10² JK^–1 kg^–1, can be externally tuned with hydrostatic, biaxial, or uniaxial stresses. In particular, <i>T</i>_s can be reduced several hundreds of degrees through the application of moderate tensile stresses due to the concomitant drop in the formation energy of Frenkel pair defects. We predict that the adiabatic temperature change in CaF₂ and PbF₂, two archetypal fluorite-structured FIC, close to their critical points are of the order of 10² and 10¹ K, respectively. This work advocates that FIC constitute a new family of mechanocaloric materials showing great promise for prospective solid-state refrigeration applications

Electronic, Vibrational, and Structural Properties of the Natural Mineral Ferberite (FeWO₄): A High-Pressure Study

This paper reports an experimental high-pressure study of natural mineral ferberite (FeWO4) up to 20 GPa using diamond-anvil cells. First-principles calculations have been used to support and complement the results of the experimental techniques. X-ray diffraction patterns show that FeWO4 crystallizes in the wolframite structure at ambient pressure and is stable over a wide pressure range, as is the case for other wolframite AWO4 (A = Mg, Mn, Co, Ni, Zn, or Cd) compounds. No structural phase transitions were observed for FeWO4, in the pressure range investigated. The bulk modulus (B0 = 136(3) GPa) obtained from the equation of state is very close to the recently reported value for CoWO4 (131(3) GPa). According to our optical absorption measurements, FeWO4 has an indirect band gap that decreases from 2.00(5) eV at ambient pressure to 1.56(5) eV at 16 GPa. First-principles simulations yield three infrared-active phonons, which soften with pressure, in contrast to the Raman-active phonons. These results agree with Raman spectroscopy experiments on FeWO4 and are similar to those previously reported for MgWO4. Our results on FeWO4 are also compared to previous results on other wolframite-type compounds

New Polymorph of InVO₄: A High-Pressure Structure with Six-Coordinated Vanadium

A new wolframite-type polymorph of InVO₄ is identified under compression near 7 GPa by in situ high-pressure (HP) X-ray diffraction (XRD) and Raman spectroscopic investigations on the stable orthorhombic InVO₄. The structural transition is accompanied by a large volume collapse (Δ<i>V</i>/<i>V</i> = −14%) and a drastic increase in bulk modulus (from 69 to 168 GPa). Both techniques also show the existence of a third phase coexisting with the low- and high-pressure phases in a limited pressure range close to the transition pressure. XRD studies revealed a highly anisotropic compression in orthorhombic InVO₄. In addition, the compressibility becomes nonlinear in the HP polymorph. The volume collapse in the lattice is related to an increase of the polyhedral coordination around the vanadium atoms. The transformation is not fully reversible. The drastic change in the polyhedral arrangement observed at the transition is indicative of a reconstructive phase transformation. The HP phase here found is the only modification of InVO₄ reported to date with 6-fold coordinated vanadium atoms. Finally, Raman frequencies and pressure coefficients in the low- and high-pressure phases of InVO₄ are reported

Experimental and Theoretical Studies on α‑In₂Se₃ at High Pressure

α­(R)-In₂Se₃ has been experimentally and theoretically studied under compression at room temperature by means of X-ray diffraction and Raman scattering measurements as well as by <i>ab initio</i> total-energy and lattice-dynamics calculations. Our study has confirmed the α (<i>R</i>3<i>m</i>) → β′ (<i>C</i>2/<i>m</i>) → β (<i>R</i>3̅<i>m</i>) sequence of pressure-induced phase transitions and has allowed us to understand the mechanism of the monoclinic <i>C</i>2/<i>m</i> to rhombohedral <i>R</i>3̅<i>m</i> phase transition. The monoclinic <i>C</i>2/<i>m</i> phase enhances its symmetry gradually until a complete transformation to the rhombohedral <i>R</i>3̅<i>m</i> structure is attained above 10–12 GPa. The second-order character of this transition is the reason for the discordance in previous measurements. The comparison of Raman measurements and lattice-dynamics calculations has allowed us to tentatively assign most of the Raman-active modes of the three phases. The comparison of experimental results and simulations has helped to distinguish between the different phases of In₂Se₃ and resolve current controversies

Compression of Silver Sulfide: X-ray Diffraction Measurements and Total-Energy Calculations

Angle-dispersive X-ray diffraction measurements have been performed in acanthite, Ag₂S, up to 18 GPa in order to investigate its high-pressure structural behavior. They have been complemented by ab initio electronic structure calculations. From our experimental data, we have determined that two different high-pressure phase transitions take place at 5 and 10.5 GPa. The first pressure-induced transition is from the initial anti-PbCl₂-like monoclinic structure (space group <i>P</i>2₁/<i>n</i>) to an orthorhombic Ag₂Se-type structure (space group <i>P</i>2₁2₁2₁). The compressibility of the lattice parameters and the equation of state of both phases have been determined. A second phase transition to a <i>P</i>2₁/<i>n</i> phase has been found, which is a slight modification of the low-pressure structure (Co₂Si-related structure). The initial monoclinic phase was fully recovered after decompression. Density functional and, in particular, GGA+U calculations present an overall good agreement with the experimental results in terms of the high-pressure sequence, cell parameters, and their evolution with pressure