Ultrasonic determination of electromechanical coupling and ionization energies in ${\mathrm{Cd}}_{1\mathrm{-}\mathrm{x}}$${\mathrm{Mn}}_{\mathrm{x}}$Te with 0≤x≤0.52 and ${\mathrm{Cd}}_{0.52}$${\mathrm{Zn}}_{0.48}$Te

We have measured the attenuation and velocity of piezoelectrically active ultrasonic shear waves in ${\mathrm{Cd}}_{0.48}$ ${\mathrm{Mn}}_{0.52}$Te and ${\mathrm{Cd}}_{0.55}$ ${\mathrm{Mn}}_{0.45}$Te, and the velocity of such waves in CdTe and ${\mathrm{Cd}}_{0.52}$ ${\mathrm{Zn}}_{0.48}$Te as a function of temperature. We find an attenuation maximum and/or velocity change associated with thermally activated electrical conductivity, which indicates that electromechanical coupling and the piezoelectric constant are much larger in our ${\mathrm{Cd}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Mn}}_{\mathrm{x}}$Te samples than in CdTe or ${\mathrm{Cd}}_{0.52}$ ${\mathrm{Zn}}_{0.48}$Te. This is due mainly to a smaller strain-induced shift of bonding charge in ${\mathrm{Cd}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Mn}}_{\mathrm{x}}$Te, which we suggest is caused by hybridization of Mn 3d states into the tetrahedral bonding orbitals. From the electrical conductivity determined by fitting calculated curves to our ultrasonic data, we deduce the ionization energies for the centers which provide the mobile charge carriers (holes) responsible for the conductivity. The energies are similar to those found for various centers identified by others in as-grown or doped ${\mathrm{Cd}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Mn}}_{\mathrm{x}}$Te and as-grown ZnTe.