MODULATED CRYSTAL-STRUCTURE OF K4(NBO)2[SI8O21]

K4(NbO)2[Si8O21] was synthesized in the hydrothermal system K2O-Nb2O5- SiO2-H2O (T=450-500-degrees-C; P=1000-1200 bar; Cu capsule; KOH concen tration, 5%-10%; 0.5<Nb2O5: SiO2<1). The triclinic structure was estab lished from X-ray single-crystal diffraction: space group, P1BAR; a=33 .92(2) angstrom, b=10.950(6) angstrom, c=7.737(4) angstrom; alpha=76.4 2(4)-degrees, beta=72.10(5)-degrees, gamma=89.81(5)-degrees; V=2650.8 angstrom3; D(x)=2.927 g cm-3; Z=5; R(aniso)=0.042 for 8855 reflections . The new phase is isotypic with Cs4(NbO)2[Si8O21]; however, it is cha racterized by one-dimensional commensurable structure modulation along the a axis: 0.2a of K,Nb silicate is approximately the a axis of Cs,N b silicate. The modulation is related to the changes in tilt angle bet ween two parts of the ''loop-branched dreier double chains'' Of SiO4 t etrahedra, and to the shifts of K atoms.