A first-principles investigation of the linear thermal expansion coefficients of BeF2: giant thermal expansion.

We present the results of a theoretical investigation of the linear thermal expansion coefficients (TECs) of BeF2, within a direct Grüneisen formalism where symmetry-preserving deformations are employed. The required physical quantities such as the optimized crystal structures, elastic constants, mode Grüneisen parameters, and phonon density of states are calculated from first-principles. BeF2 shows an extensive polymorphism at low pressures, and the lowest energy phases [α-cristobalite with space group (SG) P41212 and its similar phase with SG P43212] are considered in addition to the experimentally observed α-quartz phase. For benchmarking purposes, similar calculations are performed for the rutile phase of ZnF2, where the volumetric TEC (α v), derived from the calculated linear TECs along the a (α a ) and c (α c ) directions, is in very good agreement with experimental data and previous theoretical results. For the considered phases of BeF2, we do not find any negative thermal expansion (NTE). However we observe diverse thermal properties for the distinct phases. The linear TECs are very large, especially α c of the α-cristobalite phase and its similar phase, leading to giant α v (∼175 × 10-6 K-1 at 300 K). The giant α v arises from large Grüneisen parameters of low-frequency phonon modes, and the C 13 elastic constant that is negatively signed and large in magnitude for the α-cristobalite phase. The elastic constants, high-frequency dielectric constants, Born effective charge tensors, and thermal properties of the above phases of BeF2 are reported for the first time and hence serve as predictions.

Vibrational and thermal properties of ScN and YN: quasi-harmonic approximation calculations and anharmonic effects.

The structural, vibrational and thermal properties of rocksalt ScN and YN are investigated by using a first-principles plane-wave approach. The results are discussed in comparison with the similarly calculated results for rocksalt MgO and zincblende AlN. The thermal expansivity (α(V)) computed within the quasi-harmonic approximation shows that there are significant anharmonic effects in ScN and YN, which are comparable to those in MgO. Since no experimental results are available for α(V) of either ScN or YN, the anharmonic effects are accounted for by a variant of the very recently introduced effective semiempirical ansatz (Phys. Rev. B 2009 79 104304) for calculating anharmonic free energy, which does not require any input from experiment. The validity of this very simple approach is demonstrated first by applying it to MgO. For the considered phase of AlN, the quasi-harmonic approximation is valid up to very high temperatures, and the thus obtained α(V) is in good agreement with experiment. The values of α(V) for semiconductor transition metal nitrides that crystallize in the rocksalt phase are higher than those for the zincblende phase of group-IIIB nitrides, and a major part of these differences is due to the crystal structure.