Third order elastic constants of cubic K2Zn(CN)4 and K2Hg(CN)4

Abstract

Abstract The complete third order elasticity tensor has been determined for K 2 Zn(CN) 4 and K 2 Hg(CN) 4 by measuring stress induced shifts of resonance frequencies in thick plates. K 2 Hg(CN) 4 , and in a smaller extent also K 2 Zn(CN) 4 , possess highly anomalous third order elastic constants, not observed until now in any other crystalline material. The most surprising properties of K 2 Hg(CN) 4 are: a negative pressure derivative of the inverse bulk compressibility, an extreme deviation from Cauchy's relation, and a high anisotropy of the pressure derivative of Poisson's ratio under uniaxial stress. The components c 112 and c 123 exhibit large positive values whereas c 111 is much smaller in magnitude than in other stable crystals like alkali halides. The anomalous behaviour still increases at lower temperatures. There exists a relation between the anomalous transversal interactions as expressed in Poisson's ratio and the structural differences in the high pressure (or low temperature) ferroelastic phase and the corresponding cubic prototype. A certain similarity to the third order elastic behaviour of fused quartz is present.