Cu₃TaSe₄ and Cu₃NbSe₄: X-ray diffraction, differential thermal analysis, optical absorption and Raman scattering

Polycrystalline samples of Cu₃NbSe₄ and Cu₃TaSe₄ were obtained by the melt and anneal technique. X-ray diffraction patterns show that both materials are single phase and crystallize in the P43m (215) space group. Thermal transitions were obtained by Differential Thermal Analysis (DTA), founding that Cu₃TaSe₄ and Cu₃NbSe₄ melts incongruently at 1328 and 1206 K, respectively; several transitions were observed at lower temperatures suggesting successive solid to solid transformations. The optical gap energy of the samples at ambient conditions were measured using the optical absorption technique, the obtained the values were 2.43 eV (indirect gap) for Cu₃TaSe₄ and 2.14 eV (indirect gap) and 2.45 eV (direct gap) for Cu₃NbSe₄, respectively. For Cu₃TaSe₄, the optical band gap was also measured under high pressure in the diamond anvil cell in the pressure range of 0-5.5 GPa, the resulting pressure coefficient is negative with a value of -0.032 eV/GPa. Focalized micro-Raman scattering under high pressure in a diamond anvil cell was performed on Cu₃TaSe₄ and Cu₃NbSe₄ polycrystalline samples. The variation of the frequencies as a function of pressure of the respective modes was obtained up to the phase transitions observed in the pressure range 13.7-16.2 GPa for both compounds.