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A better understanding of the effects of temperature and pressure on the wide gap SiC semiconductor is necessary for both (i) an improvement of the performance of this compound in a variety of technological applications and (ii) a clarification of controversial issues related to the stability of its cubic polymorphs at high pressure and high temperature. Bearing in mind this double demand, we perform first-principles calculations of the phonon band structures, vibrational density of states, and thermal and mode Grüneisen parameters of the zinc blende (B3) and rock-salt (B1) cubic polymorphs of 3C-SiC covering pressures and temperatures up to 120 GPa and 3000 K, respectively. Under a martensitic description of the B3-B1 transformation, we found that the large hysteresis pressure range observed at room temperature (35-100 GPa) disappears at around 1100 K. The calculated Clapeyron slope of this transformation is slightly negative, with average values of -2.9 MPa K

D.E. acknowledges the financial support given by the Spanish Ministry of Science, Innovation, and Universities (MCIU) under grant nos. PID2019-106383GB-C41 and RED2018-102612-T (MALTA Consolider-Team network) and by Generalitat Valenciana under Grant Prometeo/2018/123 (EFIMAT). R.F. and A. Lobato are grateful to financial support from Spanish MCIU under grant PGC2018-094814-B-C22. We would like to thank TGCC under the allocation 2020-A0080910433 made by GENCI, the PMMS (Pôle Messin de Modélisation et de Simulation), the Tirant supercomputer (Universitat de Valencia), and the MALTA-Consolider facilities for providing us the computational resources. S.G. and M.B. also acknowledge financial support through the COMETE project (Conception in silico de Matériaux pour l’Environnement et l’Energie) co-funded by the European Union under the program “FEDERFSE Lorraine et Massif des Vosges 2014–2020”.

AM2X4 crystalline materials display important technological electronic, optical and magnetic properties that are sensitive to general stress effects. In this paper, the behavior under hydrostatic pressure of the ambient condition rhombohedral phase of MgIn2Se4 is investigated in detail for the first time. We carried out first-principles calculations within the density functional theory framework aimed at determining the pressure-induced polymorphic sequence of this selenide. To accurately evaluate transition pressures at room temperature, thermal corrections have been included after the computation of phonon dispersion curves in potential candidate phases, namely the initial rhombohedral Rm one, inverse and direct spinels, LiTiO2-type and defective I structures. Only the transition from the Rm to the inverse spinel phase was found to fulfill the thermodynamic and mechanical stability criteria. The direct spinel could appear as metastable if kinetic effects hinder the above transition. Additionally, electronic band structures and chemical bonding properties were analyzed from the outcome of our quantum-mechanical solutions reporting band gap values and ionicity and noncovalent interaction indexes. It is shown that the investigated compound keeps behaving as a semiconductor, loses its van der Waals interactions, and becomes more covalent as hydrostatic pressure is applied.

The structure, geometry, and vibrational frequencies of several isomers of small group III−V (MX)n clusters (n = 1, 2, 3; M = Al, Ga, In; X = P, As) have been investigated using density functional theory. The results reveal the same behavior as in the nitride clusters for monomers and dimers. The Al trimers exhibit a D3h structure like the nitride, but the gallium and indium trimers exhibit a three-dimensional structure of Cs symmetry. The existence of strong X−X bonds dominates both the structure and the vibrations of the Ga and In trimers.

Calculations based on density functional theory (DFT) were performed on various structural isomers of methyl silsesquioxanes, [MeSiO3/2]n where n = 4, 6, 8, 10, 12, 14, and 16, to study their structural and electronic properties. The calculated results find the stability of methyl silsesquioxanes, except [MeSiO3/2]4, against fragmentation and hydrolysis, and of one isomer of [MeSiO3/2]14 against hydrolysis. The deformation density plots show that chemical bonding in methyl silsesquioxanes is mainly determined by the building block unit, (MeSiO3/2) as also seen in hydridosilsesquioxanes (HSQ). However, unlike HSQ, the large cages of methyl silsesquioxanes do not develop a localized electronic state in the HOMO−LUMO gap.