Abstract
In the present work, we report a combined experimental and ab-initio study of the crystal structure, lattice dynamics and thermodynamic properties of the thermoelectric orthorhombic BaCu2Se2 compound. The thermal and pressure evolution of the crystal structure is obtained using synchrotron 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 Grüneisen parameter of about 2.03 is estimated. This rather large value together with the presence of low energy optical modes and rather low Debye temperature in orthorhombic BaCu2Se2 can explain its low thermal conductivity. Additionally, we have also studied the lattice dynamics, the thermodynamic properties and the stability of the metastable tetragonal BaCu2Se2 against the stable orthorhombic phase and found that it might be stabilized at high temperature or high pressure.