High-Pressure Vibrational and Structural Properties of Ni₃V₂O₈ and Co₃V₂O₈ up to 20 GPa

The vibrational and structural behaviors of Ni3V2O8 and Co3V2O8 orthovanadates have been studied up to around 20 GPa by means of X-ray diffraction, Raman spectra, and theoretical simulations. Both materials crystallize in an orthorhombic Kagome staircase structure (space group: Cmca) at ambient conditions, and no phase transition was found in the whole pressure range. In order to identify the symmetry of the detected Raman-active modes under high pressure, single crystal samples of those materials were used in a polarized Raman and infrared setup. Moreover, high-pressure powder X-ray diffraction measurements were performed for Co3V2O8, and the results confirmed the structure stability also obtained by other diagnostic techniques. From this XRD analysis, the anisotropic compressibilities of all axes were calculated and the unit-cell volume vs pressure was fitted by a Birch-Murnaghan equation of state, obtaining a bulk modulus of 122 GPa.