Crystal growth, thermal and linear optical properties of the non-centrosymmetric hydrated tetraborates Na2[B4O5(OH)4] • 3H2O (tincalconite) and K2[B4O5(OH)4] • 2H2O
Large single crystals of the non-centrosymmetric hydrated tetraborates Na-2[B4O5(OH)(4)].3H(2)O ("tincalconite") (point group 32) and K-2[B4O5(OH)(4)].2H(2)O (point group 222) were grown from aqueous solutions and the linear optical properties (refractive indices between 365 nm and 1530 nm and unpolarized absorption spectra) as a basis for nonlinear optical investigations were determined The uniaxial positive sodium salt is not phase matchable; in the orthorhombic potassium compound type I phase matching is possible in the near infrared region. Thermal investigations indicate a phase transition at similar to 285 K for Na-2[B4O5(OH)(4)].3H(2)O.