Structur-Composition Relations and Fractional Site Occupancy of New M5Ge4 Compounds in the System Ge-Ta-Zr

Single crystals of synthetic Na3VB6O13 were obtained by heating a mixture of Na2CO3.H2O, V2O5, and H3BO3; its formula has been determined by the resolution of the structure from Xray diffraction data. The compound is orthorhombic, space group P212121; the unit cell parameters are a=7.723(7), b=10.155(4), c=12.505(4) ?, Z=4. The crystal structure was solved from 1535 reflections until R=0.029; it contains hexaborate units formed by three triangular BO3 (3delta) and three tetrahedral BO4 (3T). These hexaborate groups are joined together to form sheets which are linked by VO4 tetrahedra leading to a three-dimensional network. The shorthand notation of the vanadoborate ion is 6: infinity3 (3delta+3T)+VO4. The sodium atoms are inside the channels that exist in the compound, whose structural formula may be written Na3[B6O9(VO4)]. This compound melts incongruently; powder may be obtained by annealing a mixture of Na2B4O7 and V2O5 at 630°C. It is the first vanadoborate for which the formula and the structure have been unambiguously established.