Abstract:

Tetragonal scheelite-like double tungstates belong to the family of ARE(WO4)2 compounds constituted by alkali (A = Li, Na, K) and rare earth (RE = Y, Sc, La – Lu) cations. The structures are disordered where the alkali and rare earth cations are statistically distributed at the same lattice site. Such cationic distribution strongly influences the optical properties of the materials generating a locally variable crystalline field which leads to the broadening of the absorption and luminescence lines of the dopant ions. This property is highly desired for solid state lasers. The double tungstates ARE(WO4)2 were often described as intermediate phases in the A2WO4 – RE2(WO4)3 binary systems. However, contradictory results relative to phase diagram for Li-La, about the melting behaviour of LiLa(WO4)2 were observed on the growth of single crystalline fibers for laser applications. In this work, we propose an experimental phase diagram for the La2W2O9–Li2W2O7 pseudo-binary section. The construction was based on differential thermal analysis and the phases were determined by X-ray powder diffraction with subsequent Rietveld analysis. The pseudobinary phase diagram La2W2O9–Li2W2O7 contains the 1:1 intermediate phase LiLa(WO4)2(ss) = x Li2W2O7 − (1 − x) La2W2O9 with a homogeneity region 0.48 ≤ x ≤ 0.546. LiLa(WO4) 2 undergoes peritectic melting at 998±5oC for x = 0.48. A eutectic point exists between LiLa(WO4)2 and Li2W2O7 at 722±5oC and 90mol% Li2W2O7