Composition, structural parameters, and color of langatate

Langatate crystals with the nominal composition La-3(Ga0.5Ta0.5)Ga5O14 grown by Czochralski pulling along aOE (c) 0001 > have been studied by X-ray and neutron diffraction before and after postgrowth annealing in air and vacuum. Their compositions are determined, the relationship between the Ga/Ta ratio and unit-cell parameters is established, and their color (colorless, yellow, and orange crystals) is shown to depend on their oxygen content and the growth and annealing conditions. Vacuum annealing at sufficiently high temperatures leads to precipitation of a green phase, La(Ta,Ga)O-3+(3).