A New Alkali-Earth Metal Oxalatohydrophosphate Crystal Ba2(C2O4)(HPO4): Synthesis, Structure, Optical and Thermal Properties

Single crystals of barium oxalatohydrophosphate Ba₂(C₂O₄)(HPO₄) were successfully synthesized by a simple solvothermal method for the first time. The crystal structure was determined from single-crystal X-ray diffraction (XRD). Ba₂(C₂O₄)(HPO₄) crystallizes in the monoclinic $C2/m$ space group with unit cell parameters of a = 15.360(5) Å, b = 5.4418(18) Å, c = 8.969(4) Å, β = 100.48(3)^o, and Z = 4. In the crystal structure of Ba₂(C₂O₄)(HPO₄), C₂O₄ groups are arranged in a completely consistent orientation. Therefore, Ba₂(C₂O₄)(HPO₄) shows a large birefringence of 0.097 at 550 nm. UV-Vis-NIR diffuse reflectance and theory calculation show that Ba₂(C₂O₄)(HPO₄) is a direct bandgap semiconductor with a bandgap of 4.33 eV. Fourier transform infra-red (FTIR) and Raman spectra confirm the crystal structure of Ba₂(C₂O₄)(HPO₄).