Crystal structure and optical study of praseodymium polyphosphate Pr(PO3)3

Single crystals of praseodymium polyphosphate Pr(PO3)3 were grown for the first time using a flux method and characterized by single-crystal X-ray diffraction. Pr(PO3)3 is isostructural with Nd(PO3)3 and La(PO3)3. It crystallizes in the orthorhombic system with space group C2221 (D25), with lattice parameters: a=11.234(1) Å, b=8.5289(8) Å, c=7.3199(8) Å. The crystal structure was resolved with final R(F2)=0.0359 and Rw(F2)=0.0924 for 402 independent reflections (F02≥2σ(F02)). In the Pr(PO3)3 structure, oxygen atoms form a PO4 tetrahedron around each P atom and a PrO8 polyhedron around each Pr atom. The PO4 tetrahedra share corners to produce helical chains and the PrO8 polyhedra share edges to form zigzag chains. The shortest PrPr distance is 4.253 Å. The energies of the vibrational modes of the crystal are obtained from the infrared and Raman spectra. Measurements of emission spectra under pulsed laser excitation in the Pr3+ (4f2) 3PJ (J=0, 1, 2) levels and decay times of the observed emissions are reported both at room and liquid-helium temperatures. X-ray excitation leads to intense ultraviolet 4f15d1→4f2 band emissions in the 200–300 nm spectral range with decay times in the order of 6 ns both at room and low temperatures.