Influence of rare earth elements (REEs) on the structure and optical properties of lithium zirconium phosphate (LZP)

Abstract

The concept of realisation of different substitution models in REE–LZP crystal structures where REEs occupy either Li⁺ or Zr⁴⁺ crystallographic positions corresponding to triclinic Li_1−3xREE_xZr₂(PO₄)₃ or NASICON-like ion conductive rhombohedral Li_1+xZr_2−xREE_x(PO₄)₃ phases was verified. Different synthesis approaches (a polymerizable-complex Pechini method and a solid state reaction) were tested to obtain polycrystalline samples with REE = La and x = 0.1. The reaction mechanism in the mixtures was explained using thermal analysis and XRD. On the basis of the obtained results, the most suitable synthesis method with regard to the phase composition (PM) was applied to obtain a whole series of REE–LZPs in order to verify their crystal structure and characterise their optical properties for pigment application. It is shown that realisation of the different substitution models in LZP polycrystalline samples can be distinguished by high resolution X-ray powder analysis that confirmed the co-existence of both types of solid solutions. However, REE–LZP phases have limited thermal stability and under the studied conditions can be obtained only in a mixture with non-substituted β-LZP. Moreover, Li excess, typically used to achieve the formation of pure-phase LZP products, in the studied case induced REE replacement in the LZP structure producing the (REE)PO₄ phase. It is shown that some REEs (Nd, Tb, Ho and Pr) induce in LZP ceramics attractive colouration varying from purple to yellow and green shades, which is pre-determined by the crystal structure of the prevailing REE–LZP phase and in some cases is sensitive to ambient light due to the luminescence properties of REEs.