CRYSTAL-STRUCTURE OF THE HIGH-TEMPERATURE K-3[NB3O6(BO3)(2)] PHASE REFINED BY THE RIETVELD METHOD

The anhydrous potassium niobate berate K-3[Nb3O6(BO3)(2)] undergoes si x phase transitions at different temperatures. The crystal structure o f the high-temperature K-3[Nb3O6(BO3)(2)] modification occurring above 505 degrees C is reported. The structure has been refined from X-ray powder diffraction data collected at 550(2)degrees C. The crystals are hexagonal, sp. gr. <P(6)over bar 2m> (a = 8.779(1) Angstrom, c = 3.98 3(1) Angstrom, Z = 1, D-X = 3.81 g/cm(3), transmission X-ray diffracti on, Rietveld refinement, R-wp = 4.70%, R-p = 3.65%, goodness-of-fit S = 1.01 for 35 variables, R-B = 2.40%).