CRYSTAL-GROWTH OF FERROELECTRIC AND FERROELASTIC K-3[NB3O6-VERTICAL-BAR(BO3)(2)] AND CRYSTAL-STRUCTURE OF THE ROOM-TEMPERATURE MODIFICATION

The anhydrous potassium niobate berate K-3[Nb3O6\(BO3)(2)] has been gr own from potassium berate flux to crystals of good optical grade and d imensions of up to 6 x 6 x 20 mm(3). Incorrectly described earlier as trigonal P31m, K-3[Nb3O6\(BO3)(2)] normally forms multidomain crystals at room temperature, composed of three different orientation states. The reported crystal structure of the room-temperature modification of K-3[Nb3O6\(BO3)(2)], determined using a single-domain crystal, shows that the real symmetry is orthorhombic with space group P2(1)ma (No. 2 6) (a = 17.506(2) Angstrom, b = 15.162(3) Angstrom, c = 3.968(4) Angst rom, Z = 4, D-x = 3.845 g/cm(3), mu(MoKalpha) = 4.122 mm(-1), R(F) = 0 .030 for 2752 unique reflections (I > 4 sigma(I)).