Negative thermal expansion coefficient of Al pnictides -A systematic realistic pressure-dependent lattice dynamical study

In weakly and strongly bonded semiconductors, the knowledge of accurate linear thermal expansion coefficients alpha(T) and Gruneisen constants gamma(T) play crucial roles for attaining the basic understanding of lattice anharmonicity and appraising their use as thermal management materials in designing different electronic devices. By adopting a realistic rigid-ion-model (RIM), we have reported the results of a comprehensive study on the phonon dispersions roj(q ->) and thermodynamical properties of weakly bonded III-V AlX (X = P, As, and Sb) pnictides at ambient (P = 0 GPa) and high pressure (P = 8 GPa). The results are compared/contrasted with the strongly bonded III-Ns (AlN, GaN, and InN) and other materials. Specifically, we have linked the simulated negative thermal expansion (NTE) coefficient in AlX to the weakness of bond-bending forces which caused large softening in partial differential roTA partial differential P for gamma(T) and alpha(T) to attain more negative values at low T. In III-Ns, however, there is only a weak sign of the TA phonon softening which results in no negative values of gamma(T) and alpha(T). Unlike earlier first-principles studies in AlX indicating small NTE, our comprehensive RIM simulations have exhibited large negative values of alpha(T) and gamma(T) for AlP, AlAs and AlSb. In both weakly and strongly bonded solids the room temperature results of specific heat Cv(T), Debye temperature theta D(T), thermal expansion coefficient alpha (T), and Gruneisen constant gamma(T) provided very good agreement with the experimental and recent first-principles response function calculations. We strongly feel that this understanding of basic traits in different tetrahedral materials will be beneficial for specific device applications.