Metal Cation Effects on the Structural, Optical and Thermal Properties of the Double Tungstates AM(WO4)2 (A = Li, Na, K; M = Y, La, Ce, Pr, Nd, Sm, Bi)

Abstract

The 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 biomedics. However, there are still yet not investigated structural, optical and thermal properties of these materials. The crystal structures of the double tungstates are elucidated and the role of the Bi 3+ lone pair activity of ABi(WO4)2 (A = Li, Na, K) is discussed. Further, the optical properties of AM(WO4)2 are studied by UV-vis and fluorescence spectroscopy. The bismuth compounds KBi(WO4)2 and NaBi(WO4)2 revealed s-p luminescence from the trivalent bismuth cations, while the other compounds dependend 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 were succesfull, verifying an effective antenna. The luminescence behaviour of the polymorphs LiLa1-xPrx(WO4)2 shows different intensities of the respective f-f transitions of Pr 3+ with respect of the doping concentration, especially when excited at the charge transfer transition of the tungstate units. Further, the thermal properties of the doublt tungstates were investigated by elucidating the phase transition demperatures 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 of the transition temperature 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.