X-ray diffraction study of La3Ga5.5Ta0.5O14 and La3Ga5.5Nb0.5O14 langasite-type single crystals

The structure of langatate (as-grown and vacuum-annealed: LGT-I and LGT-II, respectively) and langanite (seed- and tail-end portions: LGN-III and LGN-IV, respectively) single crystals grown by the Czo-chralski technique from charges of nominal composition La3Ga5.5M0.5O14 = La-3(Ga0.5M0.5)(1)Ga-3(2)Ga-2(3)O-14 (M = Ta5+, Nb5+) is studied by x-ray diffraction. The LGT-I and LGT-II crystals are shown to differ in the Ga and Ta distributions over crystallographic sites: La-3(Ga0.52Ta0.48(2)5+)Ga-3(Ga0.94Ta0.06(1)3+)(2)O-14 in LGT-I (Ta5+ in the octahedral site Ga(1) and Ta3+ in the trigonal-pyramidal site Ga(3)) and La-3(Ga0.55Ta0.45(2)5+)Ga3Ga2O13.93(2)square(0.07) in LGT-II (Ta5+ in the octahedral site Ga(1) and oxygen vacancies in O(1)). The increased Ta content is responsible for the lower structural perfection of LGT-I. LGN-III and LGN-IV have essentially identical compositions, La-3(Ga0.47Nb0.53(1)5+)Ga5O14 and La-3(Ga0.48Nb0.52(1)5+)Ga5O14, respectively, but differ in polarity.