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

Abstract Polycrystalline samples of Cu 3 NbSe 4 and Cu 3 TaSe 4 were obtained by the melt and anneal technique. X-ray diffraction patterns show that both materials are single phase and crystallize in the P 4 ¯ 3 m (215) space group. Thermal transitions were obtained by Differential Thermal Analysis (DTA), founding that Cu 3 TaSe 4 and Cu 3 NbSe 4 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 Cu 3 TaSe 4 and 2.14 eV (indirect gap) and 2.45 eV (direct gap) for Cu 3 NbSe 4 , respectively. For Cu 3 TaSe 4 , 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 Cu 3 TaSe 4 and Cu 3 NbSe 4 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.