Influence Of Elastic Anisotropy On Measured Sound Velocities And Elastic Moduli Of Polycrystalline Cubic Solids

Cubic solids such as NaCl, crystalline argon, or H2O-ice VII exhibit significant elastic anisotropy strongly increasing upon compression. As earlier recognized for solid argon and H2O-ice (both exhibiting Zener ratio A > 1), longitudinal sound velocities of their polycrystals, V-Lav, measured using Brillouin light scattering (BLS) or pulse-echo ultrasonics are much closer to V-L < 111 > than to V-L < 100 >, the V-L-extremes in any cubic single crystal. Here, we experimentally confirm, using the technique of time-domain Brillouin scattering, the same tendency for NaCl exhibiting the opposite anisotropy type, A < 1. To understand this tendency, we modelled orientational distribution and the frequency of occurrence of V-L values in texture-free polycrystalline samples of NaCl and solid argon. We found a remarkable and predictable asymmetry of V-L distributions with maxima at V-L < 110 > that is always much closer to V-L < 111 >. This asymmetry persists in BLS peaks but can be obscured in experiments. In the case of solid argon at 49 GPa, the asymmetry can lead to a moderate deviation of experimental V-Lav from V-LH (obtained from elastic-stiffness constants C-ij applying the Hill approximation) by similar to 7%. The latter can cause, however, a significant overestimation of the aggregate shear modulus by delta G/G similar to 50% or of the bulk modulus by delta B/B similar to 20% if just one BLS peak of longitudinal modes is detectable. A similar analysis, performed for transverse sound velocities, V-T and V-Tav, has shown that by the use of a BLS spectrum showing peaks of both longitudinal and transverse modes, overestimation of B is similarly high but that of G is much less dramatic. Published under an exclusive license by AIP Publishing.