High-pressure phase transitions in CaTe and SrTe

Advancing thermoelectric potential: strontium telluride under compression strain analyzed with HSE hybrid functional and Wannier interpolation

In the present era, the energy sector is undergoing an intense transformation, which encourages numerous research efforts aimed at reducing and reusing energy waste. One of the main areas of focus is thermoelectric energy, where telluride compounds have attracted researchers due to their remarkable ability to convert thermal energy into electrical energy. We focused this study on finding out how well strontium telluride (SrTe) can be used to generate thermoelectric power by testing it under up to 10% compression strain. We have used advanced computational approaches to increase the accuracy of our results, specifically the HSE hybrid functional with the Wannier interpolation method. This method is primarily employed to analyze electronic properties; however, our research extends its utility to investigate thermoelectric characteristics. Our findings provide accurate predictions for both electronic and thermoelectric properties. The above method has successfully achieved a significant improvement of 58% in the electronic band gap value, resulting in a value of 2.83 eV, which closely matches the experimental results. Furthermore, the Figure of Merit 0.95 is obtained, which is close to the ideal range. Both the band gap value and the thermoelectric figure of merit decrease when the compression strain is increased. These findings emphasize the importance of using SrTe under specific conditions. The findings of this work provide motivation for future researchers to investigate the environmental changes in the thermoelectric potential of SrTe.

Reverse charge transfer and decomposition in Ca-Te compounds under high pressure

Pressure alters the nature of chemical bonds and triggers novel reactions. Here, we employed first-principles calculations combined with the CALYPSO structural search technique to reveal the charge transfer reversal between Ca and Te under high pressure in the calcium-tellurium compound (CaxTe1-x, x = 1/4, 1/3, 1/2, 2/3). We predict several new phases with conventional and unconventional compounds and found an unfamiliar phenomenon: the Ca-Te compounds will reverse charge transfer between Ca and Te atoms and decompose into elemental solids under pressure. The Bader charge analyses indicate that the Ca2+ ion gains electrons and becomes an anion under high pressure. This leads to a weakened electrostatic interaction between Ca and Te and ultimately results in decomposition. The calculated band occupation number suggests that the occupation of Ca 3d orbitals under high pressure corresponds to this atypical phenomenon. Our results demonstrated the reverse charge transfer between Ca and Te and, in addition, clarified the mechanism of CaxTe1-x decomposition into solid Ca and Te elements under high pressure, providing important insights into the evolution of the properties of alkaline-earth chalcogenide compounds under high pressure.

Screening topological materials with a CsCl-type structure in crystallographic databases

CsCl-type materials have many outstanding characteristics, i.e. simple in structure, ease of synthesis and good stability at room temperature, thus are an excellent choice for designing functional materials. Using high-throughput first-principles calculations, a large number of topological semimetals/metals (TMs) were designed from CsCl-type materials found in crystallographic databases and their crystal and electronic structures have been studied. The CsCl-type TMs in this work show rich topological character, ranging from triple nodal points, type-I nodal lines and critical-type nodal lines, to hybrid nodal lines. The TMs identified show clean topological band structures near the Fermi level, which are suitable for experimental investigations and future applications. This work provides a rich data set of TMs with a CsCl-type structure.

A novel theoretical design of electronic structure and half-metallic ferromagnetism in the 3d (V)-doped rock-salts SrS, SrSe, and SrTe for spintronics

The crystal structure of a Sr_1−xV_xZ supercell of 16 atoms with Z = S, Se and Te and concentration x = 0.125 of V.

Pressure induced structural phase transitions—A review

The contemporary status of experimental as well as theoretical advances within the special view of structural phase transitions is reviewed. A brief outline of phase transitions and its classification is presented first. High-pressure experimental techniques developed for studying the structural phase transitions and elastic properties are reviewed. The complete set of theoretical and experimental data obtained is for the group II–IV alkaline earth chalcogenides. Here the authors review the currently used calculations and high-pressure behavior of these materials and the theoretical work that has been done on them.

Simulating solid state phase transitions with the roots of transformation matrices

We have studied the pressure induced B1-B2 phase transition within the Buerger mechanism. A transition path was generated using roots of the transition matrix between the B1 and the B2 structure. The enthalpies of activation [Formula: see text] were obtained for this path for the typical examples of NaCl and of CaO from first principle calculations. The results were compared to [Formula: see text]-values for optimized transition paths either reported in the literature or obtained from our own test calculations. They were very similar to the values obtained from the matrix root method. The latter method is, however, computationally very efficient because no optimization procedure of the transition path is necessary.

The role of lattice distortions in determining the thermal properties of electron doped CaMnO(3)

We have investigated the thermal properties of electron doped perovskite manganite CaMnO(3), the end member ([Formula: see text]) of the Ruddlesden-Popper (RP) calcium manganates series with cation doping at the A-site. In this paper the functional relation between the lattice distortions and the thermal properties is determined and compared to available reports. The temperature dependence of the lattice specific heat (C(v(lattice))) of Ca(1-x)Ln(x)MnO(3) (x = 0.05, 0.10, 0.15, 0.20) with Ln(= La, Ce, Pr, Nd, Th, Bi) doping at the A-site has been studied as a function of temperature (10 K≤T≤500 K) by means of a rigid ion model (RIM) after modifying its framework to incorporate the van der Waals interactions. Strong electron-phonon interactions are present in these compounds, which are responsible for the variation of the lattice specific heat with cation doping of varying size and valency. We have found that the calculated thermal properties reproduce well the corresponding experimental data, implying that modified RIM represents properly the nature of these perovskite manganite systems. We demonstrate that the electron concentration, size mismatch and Jahn-Teller (JT) effects are the dominant factors, whereas charge mismatch and buckling of Mn-O-Mn angle influence the thermal properties to a lesser degree in the ferromagnetic state. In the insulating paramagnetic state, JT distortions vary linearly and influence the thermal properties. These specific heat results can be further improved by including the ferromagnetic spin wave and charge order contributions to the specific heat.

Energy band gaps and lattice parameters evaluated with the Heyd-Scuseria-Ernzerhof screened hybrid functional

This work assesses the Heyd-Scuseria-Ernzerhof (HSE) screened Coulomb hybrid density functional for the prediction of lattice constants and band gaps using a set of 40 simple and binary semiconductors. An extensive analysis of both basis set and relativistic effects is given. Results are compared with established pure density functionals. For lattice constants, HSE outperforms local spin-density approximation (LSDA) with a mean absolute error (MAE) of 0.037 A for HSE vs 0.047 A for LSDA. For this specific test set, all pure functionals tested produce MAEs for band gaps of 1.0-1.3 eV, consistent with the very well-known fact that pure functionals severely underestimate this property. On the other hand, HSE yields a MAE smaller than 0.3 eV. Importantly, HSE correctly predicts semiconducting behavior in systems where pure functionals erroneously predict a metal, such as, for instance, Ge. The short-range nature of the exchange integrals involved in HSE calculations makes their computation notably faster than regular hybrid functionals. The current results, paired with earlier work, suggest that HSE is a fast and accurate alternative to established density functionals, especially for solid state calculations.