Crystal Structure, Lattice Dynamics, and Thermodynamic Properties of a Thermoelectric Orthorhombic BaCu₂Se₂ Compound

Abstract

In the present work, we report a combined experimental and ab initio study of the crystal structure, lattice dynamics, and thermodynamic properties of a thermoelectric orthorhombic BaCu 2 Se 2 compound. The thermal and pressure evolution of the crystal structure is obtained using synchrotron X-ray diffraction (XRD) experiments. The lattice dynamics is studied through Raman scattering experiments and ab initio supercell calculations and reveals that the lowest Raman-active and infrared-active optical modes are at about 50 cm −1 (7 meV) and the presence of an optically silent low-energy optical mode at about 5 meV. From the determination of the heat capacity, the thermal expansion, and the bulk modulus, a Gruneisen parameter of about 2.03 is estimated. This rather large value, together with the presence of low-energy optical modes and the rather low Debye temperature in orthorhombic BaCu 2 Se 2 , can explain its low thermal conductivity. Additionally, we have also studied the lattice dynamics, the thermodynamic properties, and the stability of the metastable tetragonal BaCu 2 Se 2 against the stable orthorhombic phase and found that it might be stabilized at high temperature or high pressure.

Growth and Photovoltaic Device Application of Cu2BaGe1–xSnxSe4 Films Prepared by Selenization of Sequentially Deposited Precursors

“…The samples selenized at lower temperatures (300−400 °C) show additional Raman peaks at different positions, which likely correspond to secondary phases. For instance, for the sample selenized at 400 °C, the Raman peaks at 140, 161, and 184 cm −1 might correspond to Raman-active modes in the BaCu 2 Se 2 phase, 27 whose presence was also suggested from EDS measurement (Figures S12 and S13). Also, the peak observed at ∼250−260 cm −1 likely corresponds to the Cu x Se phase, 28,29 which also appears in XRD measurements (Figure 2).…”