Magnetic, electronic, and structural properties of MFe 2 O 4 (M = Mg,Zn,Fe) ferric spinels have been studied by 57 Fe Mössbauer spectroscopy, electrical conductivity, and powder and single-crystal x-ray diffraction (XRD) to a pressure of 120 GPa and in the 2.4-300 K temperature range. These studies reveal for all materials, at the pressure range 25-40 GPa, an irreversible first-order structural transition to the postspinel CaTi 2 O 4 − type structure (Bbmm) in which the HS Fe 3+ occupies two different crystallographic sites characterized by six-and eightfold coordination polyhedra, respectively. Above 40 GPa, an onset of a sluggish second-order high-to-low spin (HS-LS) transition is observed on the octahedral Fe 3+ sites while Fe 3+ occupying bicapped trigonal prism sites remain in the HS state. Despite an appreciable resistance decrease, corroborating with the transition to the LS state, MgFe 2 O 4 and ZnFe 2 O 4 remain semiconductors at this pressure range. However, in the case of Fe 3 O 4 , the second-order HS-LS transition on the Fe 3+ octahedral sites corroborates with a clear trend to a gap closure and formation of a semimetal state above 50 GPa. Above 65 GPa, another structural phase transition is observed in Fe 3 O 4 to a new Pmma structure. This transition coincides with the onset of nonmagnetic Fe 2+ , signifying further propagation of the gradual collapse of magnetism corroborating with a sluggish metallization process. With this, half of Fe 3+ sites remain in the HS state. Thus, this paper demonstrates that, in a material with a complex crystal structure containing transition metal cation(s) in different environments, a HS-LS transition and delocalization/metallization of the 3d electrons does not necessarily occur simultaneously and may propagate through different crystallographic sites at different degrees of compression.

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In this work, we present a high-pressure diffraction results of the Mo-based MAX phase, Mo2GaC. A diamond anvil cell was used to compress the material up to 30 GPa, and X-ray diffraction was used to determine the structure and unit cell parameters as a function of pressures. Somewhat surprisingly, we find that at 295±25 GPa, the bulk modulus of Mo2GaC is the highest reported of all the MAX phases measured to date. The c/a ratio increases with increasing pressure. At above 15 GPa, a splitting in the (1 0 0) reflection occurs. This result, coupled with new DFT calculations, show that a second order phase transition to possibly a mixture of a hexagonal and monoclinic structures may explain this splitting. Such experimentally and theoretically supported phase transition was not predicted in previously published calculations.

The precision of melting curve measurements using laser-heated diamond anvil cell (LHDAC) is largely limited by the correct and reliable determination of the onset of melting. We present a novel image analysis of speckle interference patterns in the LHDAC as a way to define quantitative measures which enable an objective determination of the melting transition. Combined with our low-temperature customized IR pyrometer, designed for measurements down to 500K, our setup allows studying the melting curve of materials with low melting temperatures, with relatively high precision. As an application, the melting curve of Te was measured up to 35 GPa. The results are found to be in good agreement with previous data obtained at pressures up to 10 GPa.

We describe an experimental setup designed for transport measurement of thin disordered superconducting films as a function of pressure up to several GPa. We use a specially designed single screw diamond anvil cell that allows the gradual increase of high pressure at cryogenic temperatures. By depositing amorphous films of disordered superconducting indium oxide directly on the diamond, we avoid the effect of pressure-induced structural changes in the substrate. Using this technique, we are able to drive thin films through a pressure tuned superconductor–insulator transition.