Anharmonic Effects in the {ital A}15 Compounds Induced by Sublattice Distortions

We demonstrate that elastic anomalies and lattice instabilities in the A15 compounds are describable in terms of first-principles local density approximation electronic structure calculations. We show that at T=0 V{sub 3}Si , V{sub 3}Ge , and Nb{sub 3}Sn are intrinsically unstable against shears with elastic moduli C{sub 11}{minus}C{sub 12} and C{sub 44} , and that the zone center phonons, {Gamma}{sub 2} and {Gamma}{sub 12} , are either unstable or extremely soft. We demonstrate that sublattice relaxation (internal strain) effects are key to understanding the behavior of the A15 materials. {copyright} {ital 1997} {ital The American Physical Society}