The elastic behaviour of tetrahedral materials with vacant sites

Abstract

As part of a study of defect tetrahedral structure compounds, the elastic constants of single crystal specimens of Hg 3Ga 2□Te 6 and HgIn 2□Te 4 have been measured between 77 K and room temperature using the pulse superposition technique. These compounds are included in the series HgTe, Hg 5In 2□Te 8, Hg 3In 2□Te 6 and HgIn 2□Te 4 and HgTe, Hg 5Ga 2□Te 8, Hg 3Ga 2□Te 6 where there is a progressive increase in the concentration of vacant sites. While the other compounds studied are cubic, HgIn 2□Te 4 has a tetragonal structure with a c a ratio of 2.0. The components of the elastic stiffness tensor of this material at 77 K are (in units of 10 11 dyne cm −2) C 11 = 4.31 C 12 = 2.54 C 44 = 2.14 C 33 = 4.47 C 13 = 2.18 C 66 = 2.41. In a cubic material C 11 = C 33, C 44 = C 66 and C 12 = C 13: the elastic behaviour of this tetragonal compound closely resembles that of a cubic material, as might be anticipated from its structure. This similarity is further illustrated by reference to the symmetry of phase velocity and Young's modulus surfaces. Examination of the elastic constants and reduced elastic constants of these compounds shows a regular trend, the elastic stiffness decreases as the number of vacant sites increases. There is an approximately linear relationship between the reduced bulk modulus and the number of sited vacancies.