Elastic behaviour of the chalcopyrite CdGeAs₂

Abstract

The components (at 290K C 11 =9.45, C 12 =5.96, C 13 =5.97, C 33 =8.34, C 44 =4.21, C 66 =4.08, in units of 10 10 N m -2 ) of the elastic stiffness tensor of the chalcopyrite structure crystal CdGeAs 2 have been obtained between 4.2K and 290K from measurements of ultrasonic wave velocities by the pulse echo overlap technique. These elastic moduli, used to calculate the bulk and Young's moduli, Poisson's ratio and the Debye temperature ( theta D el =257+or-2K), define the elastic behaviour of an A II B IV C 2 V chalcopyrite. Measurements of the hydrostatic pressure dependence of the ultrasonic wave velocities up to about 10 8 Pa have been used to obtain the pressure derivatives of the elastic moduli at room temperature. Negative values of delta C 44 / delta P and delta ( 1 / 2 (C 11 -C 12 ))/ delta P show that the long-wavelength transverse q(110), e(110); q(100), e(001) and q(001), e(100) acoustic phonon modes soften under high pressure. Expressions for the acoustic mode Gruneisen parameters in the long-wavelength limit in the anisotropic continuum model have been developed for 4/mmm Laue group tetragonal crystals; the calculated Gruneisen parameters for CdGeAs 2 establish quantitatively an anomalous decrease of the frequencies of certain shear modes. The high pressure transition at 60 GPa may be associated with weakening of the interatomic binding forces against shear distortion.