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a b s t r a c tIn this study, first principles calculation results of the second order elastic constants and lattice dynamics of two lanthanum monopnictides, LaN and LaBi, which crystallize in rock-salt structure (B1 phase), are presented. Calculations were based on plane wave basis sets and pseudopotential methods in the framework of Density Functional Theory (DFT) with generalized gradient approximation. Elastic constants are calculated by tetragonal and orthorhombic distortions on cubic structure. Phonon dispersion spectra was constructed in the linear response approach of the Density Functional Perturbation Theory (DFPT). The complete phonon softening with negative frequencies and large elastic anisotropy were observed for LaN single crystal as a sign of the structural instability. The phonon dispersion curve for LaBi is typical for lanthanum monopnictides and does not show any anomalous physical property. The calculated structural quantities for both LaN and LaBi systems agree well with the available experimental and theoretical data.

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The structural, electronic and phonon properties of the LaP and LaAs compounds in NaCl (B1) and CsCl (B2) structures are studied by means of density functional theory within the generalized gradient approximation. The calculated lattice constants, static bulk modulus and first-order pressure derivative of the bulk modulus are reported in both B1 and B2 structures and compared with previous experimental and theoretical calculations. Band structure and density of states calculations are carried out for LaP and LaAs . Then, a linear-response approach to the density functional theory is used to derive the phonon frequencies and density of states.

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