Thermostructural and Elastic Properties of PbTe and Pb0.884Cd0.116Te: A Combined Low-Temperature and High-Pressure X-ray Diffraction Study of Cd-Substitution Effects

Crystals 11(9), 1063 (2021). doi:10.3390/cryst11091063
Rocksalt-type (Pb,Cd)Te belongs to IV–VI semiconductors exhibiting thermoelectric properties.With the aim of understanding of the influence of Cd substitution in PbTe on thermostructuraland elastic properties, we studied PbTe and Pb$_{0.884}$Cd$_{0.116}$Te (i) at low temperatures (15 to 300 K)and (ii) at high pressures within the stability range of NaCl-type PbTe (up to 4.5 GPa). For crystalstructure studies, powder and single crystal X-ray diffraction methods were used. Modelingof the data included the second-order Grüneisen approximation of the unit-cell-volume variation,V(T), the Debye expression describing the mean square atomic displacements (MSDs), (T), andBirch–Murnaghan equation of state (BMEOS). The fitting of the temperature-dependent diffractiondata provided model variations of lattice parameter, the thermal expansion coefficient, and MSDswith temperature. A comparison of the MSD runs simulated for the PbTe and mixed (Pb,Cd)Tecrystal leads to the confirmation of recent findings that the cation displacements are little affectedby Cd substitution at the Pb site; whereas the Te displacements are markedly higher for the mixedcrystal. Moreover, information about static disorder caused by Cd substitution is obtained. Thecalculations provided two independent ways to determine the values of the overall Debye temperature,qD. The resulting values differ only marginally, by no more than 1 K for PbTe and 7 K forPb$_{0.884}$Cd$_{0.116}$Te crystals. The $\Theta_D$ values for the cationic and anionic sublattices were determined. The Grüneisen parameter is found to be nearly independent of temperature. The variations of unit-cellsize with rising pressure (the NaCl structure of Pb$_{0.884}$Cd$_{0.116}$Te sample was conserved), modeledwith the BMEOS, provided the dependencies of the bulk modulus, K, on pressure for both crystals. The $K_0$ value is 45.6(2.5) GPa for PbTe, whereas that for Pb$_{0.884}$Cd$_{0.116}$Te is significantly reduced,33.5(2.8) GPa, showing that the lattice with fractional Cd substitution is less stiff than that of purePbTe. The obtained experimental values of $\Theta_D$ and $K_0$ for Pb$_{0.884}$Cd$_{0.116}$Te are in line with the trendsdescribed in recently reported theoretical study for (Pb,Cd)Te mixed crystals.
Published by MDPI, Basel