Structure and optical properties of the RbNbB2O6 family of mixed borates

The mixed niobate-borate compound RbNbB2O6 (RNB) and its isomorphs have been produced in single crystal form using the spontaneous nucleation and top seeded solution growth (TSSG) techniques. X-ray diffraction studies show that RNB, KNB and certain K+, Na+ and Cs+ substituted variations of the pure compounds have monoclinic symmetry with the P11n space group. These crystals are transparent in the wavelength range 275 nm to 3.0 mu m and have exceptionally high resistance to optical damage by high intensity laser radiation. The three indices of refraction of RNB and KNB have been measured as a function of wavelength by a polarised interferometric technique and observed to be intermediate between those of such compounds as beta-BaB2O4 and LiNbO3. However, the birefringences of both RNB and KNB are smaller than those of other pure borate and niobate crystals. These results are discussed in terms of the structures of these different materials, in particular the role of odd-parity distortions of the NbO6 octahedra and B2O5 molecular ions. (C) 1997 Elsevier Science B.V.