We present a computational study of the crystal structure and electric polarization of strained wurtzite III-V nitrides and II-VI oxides, performed in the context of density functional theory and the Berry phase method. The main goal is to investigate the degree to which the lattice parameters, piezoelectric polarization, and piezoelectric constant can be affected by compressive uniaxial strain along the hexagonal c-axis. We show that imposing such strain enhances the piezoelectric response, with both polarization and piezoelectric coefficient increasing from their equilibrium values. The internal parameter of the wurtzite structure also increases with uniaxial strain and eventually becomes equal to 0.5, resulting in a phase transition into the layered hexagonal structure. Furthermore, we discuss the physical origin behind the enhanced piezoelectricity, showing that the enhancement is caused by a strong increase in the response of the internal parameter to strain.

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In this work, we perform first-principles calculations based on density functional theory and the semi-classical Boltzmann method to study the structural, mechanical, electronic, and thermoelectric properties of rare earths filled skutterudites RECo4Sb12 (RE= Nd,Sm,Eu,Yb). It is found that these compounds are n-type semiconductors with high effective mass and narrow bandgap. The main focus here is to investigate the effect of filler rare earth elements on the thermoelectric response of binary skutterudite CoSb3. In doing so, we compute for each compound the Seebeck coefficient, electrical conductivity, electronic thermal conductivity, and figure of merit in the temperature range from 400 K to 1000 K. The relaxation time and lattice thermal conductivity are calculated as well. Our results reveal that low thermal conductivity and high Seebeck coefficient can be achieved at the same time in RE-filled skutterudites RECo4Sb12, thereby improving their thermoelectric performance which makes them attractive thermoelectric materials at high temperatures.

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We report the results of density functional theory calculations of several properties of wurtzite-structured InGaN and CdZnO alloys. It is shown that the investigated properties, including the internal parameter, bandgap, mechanical moduli, and electric polarization are nonlinear functions of alloy composition, as they deviate from the linear behavior predicted by Vegard’s law. Based on these results, InGaN and CdZnO are materials whose properties can be tuned via In and Cd concentrations. The spontaneous and piezoelectric polarizations considerably affect the properties of alloys-based devices due to the huge electric fields that build up at the heteroineterfaces. In this work we propose a method of controlling such fields by employing the composition dependence of the total polarization. We support this proposal by showing that, in the case of InGaN, an optimal alloy composition can be found that effectively reduces the polarization-induced electric fields, thereby improving the efficiency of optoelectronic applications.