Advances in the accuracy and efficiency of first-principles electronic structure calculations have allowed detailed studies of the energetics of materials under high pressures. At the same time, improvements in the resolution of powder x-ray diffraction experiments and more sophisticated methods of data analysis have revealed the existence of many new and unexpected high-pressure phases. The most complete set of theoretical and experimental data obtained to date is for the group-IVA elements and the group-IIIA--VA and IIB--VIA compounds. Here the authors review the currently known structures and high-pressure behavior of these materials and the theoretical work that has been done on them. The capabilities of modern first-principles methods are illustrated by a full comparison with the experimental data.

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High-pressure phases of group-IV, III–V, and II–VI compounds

The kinetics of a first-order, solid-solid phase transition were investigated in the prototypical nanocrystal system CdSe as a function of crystallite size. In contrast to extended solids, nanocrystals convert from one structure to another by single nucleation events, and the transformations obey simple unimolecular kinetics. Barrier heights were observed to increase with increasing nanocrystal size, although they also depend on the nature of the nanocrystal surface. These results are analogous to magnetic phase transitions in nanocrystals and suggest general rules that may be of use in the discovery of new metastable phases.

https://science.sciencemag.org/content/sci/276/5311/398.full.pdf

Size dependence of structural metastability in semiconductor nanocrystals

The currently known structures and properties of group?IV elements and III-V compounds at high pressure are reviewed. Structural properties of various phases, as determined by experimental techniques, predominantly x-ray crystallography using diamond anvil cells, are covered first. The relative equilibrium stability of these phases, as determined by theoretical methods, is also discussed. Metastable phases and the processing techniques by which they can be made are examined, introducing the importance of phase transition kinetics in determining what is actually seen. Elastic and vibrational properties are then considered, looking at how elastic constants and phonon frequencies are affected by increasing pressure and how this can help us to understand the phase diagram and transition kinetics. Finally, using these ideas, it is shown how one can formulate equilibrium pressure-temperature equations of state for these materials. Throughout, the review draws on both experimental and theoretical work, and emphasizes features which seem to be generic to these tetrahedral semiconductors and their high-pressure phases.

http://iopscience.iop.org/article/10.1088/0034-4885/64/4/202/pdf

High-pressure phases of group IV and III-V semiconductors

Nanocrystalline ZnS was coarsened under hydrothermal conditions to investigate the effect of particle size on phase transformation kinetics. Although bulk wurtzite is metastable relative to sphalerite below 1020 °C at low pressure, sphalerite transforms to wurtzite at 225 °C in the hydrothermal experiments. This indicates that nanocrystalline wurtzite is stable at low temperature. High-resolution transmission electron microscope data indicate there are no pure wurtzite particles in the coarsened samples and that wurtzite only grows on the surface of coarsened sphalerite particles. Crystal growth of wurtzite stops when the diameter of the sphalerite−wurtzite interface reaches ∼22 nm. We infer that crystal growth of wurtzite is kinetically controlled by the radius of the sphalerite−wurtzite interface. A new phase transformation kinetic model based on collective movement of atoms across the interface is developed to explain the experimental results.

Size-dependent phase transformation kinetics in nanocrystalline ZnS.

The progressive stacking of chalcogenide single layers gives rise to two-dimensional semiconducting materials with tunable properties that can be exploited for new field-effect transistors and photonic devices. Yet the properties of some members of the chalcogenide family remain unexplored. Indium selenide (InSe) is attractive for applications due to its direct bandgap in the near infrared, controllable p- and n-type doping and high chemical stability. Here, we reveal the lattice dynamics, optical and electronic properties of atomically thin InSe flakes prepared by micromechanical cleavage. Raman active modes stiffen or soften in the flakes depending on which electronic bonds are excited. A progressive blue-shift of the photoluminescence peaks is observed for decreasing flake thickness (as large as 0.2 eV for three single layers). First-principles calculations predict an even larger increase in the bandgap, 0.40 eV, for three single layers, and as much as 1.1 eV for a single layer. These results are promising from the point of view of the versatility of this material for optoelectronic applications at the nanometer scale and compatible with Si and III-V technologies.

Electronic structure, optical properties, and lattice dynamics in atomically thin indium selenide flakes

Under hydrostatic pressure, cubic GaAs-I undergoes phase transitions to at least two orthorhombic structures. The initial phase transition to GaAs-II has been investigated by optical-transmittance measurements, Raman scattering, and x-ray absorption. The structure of pressurized samples, which are retrieved at ambient, has been studied by x-ray diffraction and high-resolution diffraction microscopy. Various criteria that define the domain of stability of GaAs-I are examined, such as the occurrence of crystalline defects, the local variation in atomic coordination number, or the actual change in crystal structure. These are shown not to occur at the same pressure at 300 K, the latter being observable only several GPa above the actual thermodynamic instability pressure of GaAs-I. Comparison of the evolution of these parameters on increasing and decreasing pressure locates the thermodynamic transition region GaAs-I\ensuremath{\rightarrow}GaAs-II at 12\ifmmode\pm\else\textpm\fi{}1.5 GPa and at 300 K that is lower than generally reported. The use of thermodynamic relations around the triple point, and of regularities in the properties of isoelectronic and isostructural III-V compounds, yields a phase diagram for GaAs which is consistent with this value.

High-pressure phase transition and phase diagram of gallium arsenide.

Compressibility, optical-absorption, Raman-scattering, and refractive-index measurements for GaSe are reported at 300 K and pressures up to 8 GPa. A model which separates intra- and interlayer contributions along the c axis is used, with adequate deformation potentials, to quantitatively reproduce the compressibility along the c axis, the refractive-index variation, and the shift of both direct and indirect gaps under pressure. The shape of the absorption edges are calculated by the Elliott-Toyozawa formalism at all pressures, between 1.6 and 2.4 eV, with only one constant, and two pressure-dependent parameters: the exciton Rydberg and the matrix element for the direct transition. The decrease of the former, together with the measured shift of the TO and LO modes are interpreted by a large increase of the longitudinal effective charge under pressure which can be assigned to either intralayer-to-interlayer charge transfer, or to an increase of the total ionicity, or both.

Optical properties of gallium selenide under high pressure

The properties of solid krypton have been measured as a function of pressure in a diamond-anvil cell using energy-dispersive x-ray diffraction and Brillouin scattering. A room-temperature equation of state and a complete set of elastic constants are deduced. The analysis of these results indicates a possible phase transition above 30 GPa.

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Solid krypton: Equation of state and elastic properties

A four‐probe technique for electrical and photoelectrical measurements in a gasketed diamond anvil cell is presented. The special anvil profile and the method for the gasket preparation used in these experiments are described. Insulation and continuity of the electrical leads are shown to be satisfactory at least up to 18 GPa. As an example we report photoconductivity measurements in GaAs beyond crossover, under pressure, between direct and indirect band‐gap configurations.

Electrical transport measurements in a gasketed diamond anvil cell up to 18 GPa

The optical transmittancy of cesium iodide single crystals has been studied up to 58 GPa, using xenon as a pressure-transmitting medium. The shape and position of the absorption edge under pressure has been measured under controlled stress-homogeneity conditions. It is found to differ significantly from previous results obtained on highly strained powder samples. The data are analyzed in terms of band-to-band transitions and show that band closing in cesium iodide is not to be expected below 90 GPa.

https://hal.archives-ouvertes.fr/hal-03059382/document

J. M. Besson

Papers

High-pressure phase transition and phase diagram of gallium arsenide.

Optical properties of gallium selenide under high pressure

Solid krypton: Equation of state and elastic properties

Electrical transport measurements in a gasketed diamond anvil cell up to 18 GPa

Optical-absorption edge of CsI up to 58 GPa