Enhancement of thermal stability in bismuth phosphate by Ln3+ doping for tailored luminescence properties

Abstract

Bismuth phosphate represents one promising class of luminescent host materials, while its application is challenged by structural instability. In this work, a series of nanocrystals Bi_1−xLn_xPO₄ (x = 0.02–0.09; Ln = La, Ce, Nd, Eu, Er) were prepared with an aim to find new routes in improving the structural stability of BiPO₄ nanocrystals. Systematic sample characterization by XRD, SEM, TEM, and EDX indicates that doping a small amount of Ln³⁺ ions ≤9% could significantly stabilize the low temperature monoclinic phase (LTMP) by retarding the structure transition to the high temperature monoclinic phase (HTMP). For instance, after annealing at 900 °C, the relative contents of LTMP for co-doping of 5% La³⁺, 5% Nd³⁺, 5% Ce³⁺ with 2% Eu³⁺ were as high as 89%, 87%, and 96%, respectively, which are totally different from the un-doped BiPO₄ at this temperature where LTMP is absent completely. This phenomenon has never been reported before for improving the stability of BiPO₄, which has been explained in terms of lattice disorder due to the preference of the linking ways of rare earth–oxygen polyhedra and PO₄ tetrahedra. With this abnormal structural stabilization, the luminescent properties of the Ln³⁺ doped LTMP, including emission intensity, lifetime, and quantum efficiency, were further tailored through simply tuning the calcination temperatures.