Crystal growth and phase diagram of lanthanum trihydride, LaH 3− x

Abstract

Rare earth (RE) hydrides are very interesting both as components of alloys for hydrogen storage, as well as due to their exciting physical properties (composition dependent metal-semiconductor phase transitions in the homogeneity range La 3+H 2−La 3+H 3). Lack of single crystals has strongly hindered the progress in this field so that many structural and physical problems remain still unsolved. The existing T- x phase diagram of La-H 2 shows a critical point of the miscibility gap La + LaH 2 which, if true, would make very difficult the growth of single crystals of LaH x from the melt. In this paper we report about the P- x and T- x phase diagrams of the La-H 2 system. It is shown that contrary to the literature the phase diagram does not show a critical point of the miscibility gap at T=970°C and x H=0.45. The miscibility gap continues up to the highest measured temperature T=111°C and the slope of the phase boundaries makes very probable the existence of a peritectic equilibrium at T≈1200°C and P H≈4 atm. These values could be confirmed by the Van 't Hoff isochore from which a value of Δ G° 298 = 3±2 kcal/mol H 2 (870⩽ T⩽111°C) was determined. A reactor for these conditions can be constructed using sealed tungsten crucibles. Even with the quartz reactor ( T⩽1150°C, P H 2 ⩽6 bar) it was possible to grow small crystals of LaH 2 (3 × 3 × 3 mm) by slow cooling under H 2 pressure.