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 Na₂[B₄O₅(OH)₄] . 3H₂O (Tincalconite) (point group 32) and K₂[B₄O₅(OH)₄] . 2H₂O(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 ≈285 K for Na₂[B₄O₅(OH)₄] . 3H₂O.