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 Gruneisen formalism where symmetry-preserving deformations are employed. The required physical quantities such as the optimized crystal structures, elastic constants, mode Gruneisen parameters, and phonon density of states are calculated from first-principles. BeF2 shows an extensive polymorphism at low pressures, and the lowest energy phases [alpha-cristobalite with space group (SG) P4(1)2(1)2 and its similar phase with SG P4(3)2(1)2] are considered in addition to the experimentally observed alpha-quartz phase. For benchmarking purposes, similar calculations are performed for the rutile phase of ZnF2, where the volumetric TEC (alpha(v)), derived from the calculated linear TECs along the a (alpha(a)) and c (alpha(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 alpha(c) of the alpha-cristobalite phase and its similar phase, leading to giant alpha(v) (similar to 175 x 10(-6) K-1 at 300 K). The giant alpha(v) arises from large Gruneisen parameters of low-frequency phonon modes, and the C-13 elastic constant that is negatively signed and large in magnitude for the alpha-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.