Phase equilibria and prospects of crystal growth in the system LiF-GdF3-LuF3

Scheelite type LiGdF4, LiLuF4, and mixtures of both end members were prepared by a hydrofluorination route from the rare earth oxides and commercial LiF. The samples were purified by melting in HF/Ar mixtures, and were investigated by differential scanning calorimetry (DSC), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDX) techniques. Both end members show unlimited miscibility in the solid phase. Mixed crystals containing at least 65 mol-% LiLuF4 melt under direct formation of the liquid phase. The gap between solidus and liquidus is narrow. LiGdF4 and mixed crystals with less then 65mol-% LiLuF4 decompose peritectically under formation of (Gd,Lu)F-3. Crystal growth is expected to be possible either from Lu-rich melts with the appropriate scheelite composition or from Gd-rich melts containing an excess of LiF.