Giant optical rotation in piezoelectric crystals with calcium gallium germanate structure

Specific optical rotations rho in c(Z)-direction were measured in single crystals of lanthanum gallium silicon oxide (langasite, La3GaGa3(GaSi)O-14, LGS), as well as its niobium (langanite, La-3(Ga1/2Nb1/2)Ga3Ga2O14, LGN) and tantalum (langataite, La-3(Ga1/2Ta1/2)Ga3Ga2O14, LGT) analogues, and strontium niobium gallium silicon oxide (Sr3NbGa3Si2O14, SNGS), all belonging to the calcium gallium germanate structure (Ca3Ga(GaGe2)Ge2O14, CGG). The crystals were grown from stoichiometric melts by conventional Czoch-ralski technique and the specific optical rotations obtained by a novel technique via measuring the optical transmission at various wavelengths between crossed polarisers. All compounds investigated show unusually large values of rho, in particular SNGS.