Raman spectra and X-ray diffraction of merrillite at various temperatures

• Raman spectra and XRD of merrillite Ca 9 NaMg(PO 4 ) 7 were collected up to 1173 K. • No phase transformation was observed up to 1173 K in this study. • The temperature-dependence of the Raman-active vibrations was calculated. • The thermal expansion coefficient of merrillite was determined. • The isobaric mode Grüneisen parameters of the Raman-active modes were obtained. The Raman spectra and in-situ X-ray diffraction patterns of synthetic merrillite, Ca 9 NaMg(PO 4 ) 7 , have been measured up to 1173 K at ambient pressure. No phase transformation was observed in this study. The effect of temperature on the Raman spectra shows that all observed active bands continuously shift to lower wavenumber with increasing temperature. The temperature dependences of v 3 anti-symmetric P O stretching modes are larger than those of v 1 symmetric P O stretching, v 4 and v 2 O P O bending modes and the external modes. Lattice parameters and volume were analyzed, and the thermal expansion coefficients were obtained as 4.86(5) × 10 −5 K -1 , 1.46(1) × 10 −5 K -1 and 1.94(5) × 10 −5 K -1 for V , a and c , respectively. Thermal expansion of merrillite shows an axial anisotropy with a smaller expansion coefficient along the a -axis. The average isobaric mode Grüneisen parameter for the internal PO 4 modes of merrillite has been calculated as 0.57.