Crystal chemistry of immobilization of trivalent Cr and Al on ceramic matrix of sodium zirconyl phosphates

Sodium zirconium phosphate (hereafter NZP) is a typical host material capable of immobilizing the light water reactor (LWR) fuel reprocessing waste in to a single-phase material with good stability, high integrity and long durability. The parent immobilization material sodium zirconium phosphate NaZr2P3O12 crystallizes in rhombohedral (hexagonal) symmetry with the space group R-3c.The NZP related phases of synthetic chromium and aluminum substituted sodium zirconium phosphates have partial substitution on zirconium site by Cr(III) and Al(III). The object of this investigation is to understand the properties of Cr/Al substituted NZP and compare them with those of the substrate matrix. The materials have been synthesized by calcinations of the finely powdered mixture of oxides in a muffle furnace at 1050 °C. The step analysis powder diffraction data of Na1.1Zr1.9Cr0.1(PO4)3 and Na1.1Zr1.9Al0.1(PO4)3 have been subjected to General Structural Analysis System software to arrive at a satisfactory structure fit with Rp = 3.75% and Rwp = 4.92% for Cr substituted NZP and Rp = 9.36% and Rwp = 12.88% for Al substituted NZP, respectively. The following unit cell parameters have been calculated: a = b = 8.78736, c = 22.7461 Å for Cr substituted NZP and a = b = 8.79701, c = 22.7700 Å for Al substituted NZP, α = β = 90.0° and γ = 120.0°. The structure model gives satisfactory values of interatomic distances and angles. Each phosphate group is on a twofold rotation axis and is linked to four ZrO6 octahedra whereas each zirconium octahedron lies on a threefold rotation axis and is connected to six PO4 tetrahedra.