Preparation, characterization, impedance and thermal expansion studies of Mn0.5MSb(PO4)3 (M=Al, Fe and Cr)

Nasicon type compositions Mn0.5MSb(PO4)3 (M = Al, Fe and Cr) are prepared by solid state method. All the compounds are characterized by powder X-ray diffraction, infrared, Raman and electron spin resonance techniques. The crystal structures of these materials are refined by Rietveld and least square minimization programs using X-ray powder diffraction data. Among these compounds, Mn0.5AlSb(PO4)3 and Mn0.5FeSb(PO4)3 crystallize in rhombohedral lattice with R 3 ¯ c and R 3 ¯ space group respectively, while Mn0.5CrSb(PO4)3 adopts monoclinic lattice with P21/n space group. The 31P-MAS nuclear magnetic resonance spectrum of Mn0.5AlSb(PO4)3 indicates only one type of phosphorous in the rhombohedral lattice. All the compounds exhibit characteristic PO4 vibrational bands in infrared and Raman spectra. The room temperature powder electron spin resonance spectra of these samples gave broad signal at g ≈ 2.0 for Mn2+. Frequency dependent impedance measurements are carried out. The thermal expansion of these samples is studied in the temperature range 30–500 °C and they exhibit near zero thermal expansion coefficients.