Synthesis, structural, electrical and optical properties of LiPr(PO3)4

Abstract

A lithium praseodymium polyphosphate LiPr(PO3)4 sample was synthesized by flux method, and its crystal structure was refined by the Rietveld method.The studied sample crystallizes in an isotopic structure with other polyphosphates, with the general formula LiLn(PO3)4 [Ln ​= ​La, Ce, Nd, Sm and Eu].Refinement results have allowed us to establish the location of the atoms in the monoclinic system of space group C2/c as follows: Li/Pr (4e) and P/O (8f); being the unit cell parameters a ​= ​16.462(4) Å; b ​= ​7.076(2) Å; c ​= ​9.774(3) Å and β ​= ​126.186(15)°. The structure of LiPr(PO3)4 sample can be best described in terms of a three-dimensional framework formed by an infinite spiral (PO3)n chains linked by PrO8 polyhedra and in which Li+ cations are in tunnels that run along the [101] direction. The structural model was supported by the charge distribution (CHARDI) and Bond Valence Sum (BVS) models. The ionic conductivity of this phosphate has also been determined by complex impedance spectroscopy measurements in pressed pellets and the obtained data suggest the unique contribution of Li+ mobile cations with activation energies of 0.49 and 1.07eV.The movement of the Li+ cations analyzed by the Bond Valence Site Energy model finds Li+ cations that move in 1D routes along [101] with an empirical activation energy of approximately 0.58 ​eV, value very close to that determined by impedance spectroscopy measurements. Finally, different optical parameters (absorption and emission spectra, refractive index, the band interval and color coordinates) were measured to investigate the optical properties of this polyphosphate. A persistent luminescence in the yellow-green region due to 3P0 → 3H5, 3H6, 3F2 and 3F4 transitions of Pr3+ ions, being the decay time from 3P0 level of 61.76 ns.