Synthesis, Rietveld refinements, Infrared and Raman spectroscopy studies of the sodium diphosphate NaCryFe1−yP2O7 (0 ≤ y ≤ 1)

• New series of NaCryFe1−yP2O7 (0 ≤ y ≤ 1) materials were prepared by the co-precipitation method.
• Structural characterization was performed by X-ray diffraction and vibrational spectroscopy.
• Refinement of the structure is done via Rietveld Method.
• Infrared and Raman assignments were discussed based on the structural analysis.

In the present study we report on the synthesis and crystal structure studies of NaCryFe1−yP2O7 sodium diphosphate solid solution (0 ≤ y ≤ 1). The X-ray diffraction shows that these compounds are isostructural with NaFeP2O7 and NaCrP2O7 (space group P21/c (C2h5) Z = 4). The Rietveld refinements based on the XRD patterns show the existence of a continuous solid solution over the whole composition range (0 ≤ y ≤ 1). A continuous evolution of the monoclinic unit cell parameters was obtained. The transition metal ions (Cr3+ and/or Fe3+) connect the diphosphate anions forming a three-dimensional network with cages filled by Na+ cations. IR and Raman spectra have been interpreted using factor group analysis. A small shift of the band frequencies is observed when Fe is substituted by Cr. The POP bridge angles are determined from Lazarev's relation and agree well with those deduced from the crystal structure refinement.