Metal cation effects on the structural, optical and thermal properties of double tungstates AM(WO4)2 (A = Li, Na, K; M = Y, La, Ce, Pr, Nd, Sm, Bi)

Abstract

Alkali metal rare earth double tungstates AM(WO₄)₂ (A = alkali metal; M = trivalent metal cation) are an extensively researched class of materials with applications not only as phosphors but also in photonics and biomedicine. However, the structural, optical and thermal properties of these materials are yet to be investigated. The crystal structures of the double tungstates are elucidated and the role of the Bi³⁺ lone pair activity of ABi(WO₄)₂ (A = Li, Na, K) is discussed. Furthermore, the optical properties of AM(WO₄)₂ are studied using UV-vis and luminescence spectroscopy. The bismuth compounds KBi(WO₄)₂ and NaBi(WO₄)₂ revealed s–p fluorescence from the trivalent bismuth cations, while the other compounds dependent on the trivalent cation M show either broad ligand to metal charge transfer transitions or typical f–f transitions from the rare earth cations. For most of the compounds, the energy transfer from the tungstate moieties towards the rare earth elements was successful, verifying an effective antenna. The luminescence behaviour of the polymorphs LiLa_1−xPr_x(WO₄)₂ shows different intensities of the respective f–f transitions of Pr³⁺ with respect to the doping concentration, especially when excited at the charge transfer transition of the tungstate units. Furthermore, the thermal properties of the double tungstates were investigated by elucidating the phase transition temperatures for those polymorphs exhibiting two temperature phases. For LiM(WO₄)₂ (M = La, Ce, Pr, Nd), the phase transition temperature revealed a trend for different sized rare earth elements from the low- to the high-temperature polymorphs. Moreover, MAPLE calculations were carried out for those double tungstates with single crystal data reported for the first time.