Synthesis and investigation of neptunium zirconium phosphate, a member of the NZP family: crystal structure, thermal behaviour and Mössbauer spectroscopy studies

A new double neptunium zirconium phosphate of the type M x Zr 2 (PO 4 ) 3 (M = Np), crystallizing in the structure type NaZr 2 (PO 4 ) 3 (NZP, NASICON), was synthesized by solid state reactions at high temperatures and characterized by X-ray diffraction, infrared spectroscopy and Mössbauer spectroscopy. The Rietveld refinement of the XRD pattern together with the analysis of the IR spectra of the sample confirmed the space group P 3&cmb.macr; c , the same as that for the lanthanide analogues Ln 0.33 Zr 2 (PO 4 ) 3 . However, Mössbauer studies revealed the presence of neptunium in the two oxidation states +3 and +4, indicating a two-phase NZP system with different crystallographic environments of the neptunium atoms. The thermal behaviour of the sample was followed up to 1400 °C by thermogravimetric analysis. We synthesized a new NZP-type neptunium zirconium phosphate Np 0.29 Zr 2 (PO 4 ) 3 and investigated it by Rietveld analysis, infrared spectroscopy and Mössbauer spectroscopy.