Neutron Diffraction and X-Ray Diffraction Investigations of Langasite Crystals

Crystals of langasite La3Ga4(GaSi)O-14 grown by the Czochralski method are studied using neutron diffraction for the first time. It is established that the compositions of the upper and lower parts of an orange crystal grown from the La3Ga5.14Si0.86O14 seed in an (Ar + O-2) atmosphere (the (0001) growth direction) can be written as (La-2.85(2)square(0.15))(Ga-0.95(2)square(0.05))Ga-3(Ga1.15Si0.85(5))(O-13.72 square(0.28(7))) and (La-2.89(1)square(0.11)) center dot (Ga-0.98(1)square(0.02))Ga-3(Ga1.06Si0.94(4))(O-13.81 square(0.19(9))), respectively. The La content in the upper and lower parts of this crystal is lower and higher than the Ga content, respectively, and the Ga content exceeds the Si content in the (GaSi) position. By contrast, in a colorless crystal of the composition (La-2.97(4)square(0.03))Ga(1)(Ga-2.94(9)square(0.06))(Ga-0.7(1)square(1.3))(O-13.9 square(0.1(1))), which is grown from the La3Ga5SiO14 seed in an argon atmosphere (the < 01 (1) over bar1 > growth direction), the Ga content in the (GaSi) position is lower than the Si content. A relation between the Ga : Si ratio and the (Ga,Si)-3O interatomic distances is found.