SYNTHESIS AND CRYSTAL-STRUCTURE OF POTASSIUM AND SODIUM VANADIUM PHOSPHATES

The isomorphic crystal structures of K2VPO6 and Na2VPO6 were determined from single-crystal X-ray diffraction data. The two compounds crystallize in the monoclinic space group P2(1)/n with four formula units per unit cell. For K2VPO6, a = 6.863 (2) angstrom, b = 13.479 (5) angstrom, c = 7.505 (1) angstrom, beta = 111.02 (1)-degrees, V = 648.0 (3) angstrom3, R = 0.046, and R(w) = 0.071. For Na2VPO6, a = 6.1805 (7) angstrom, b = 12.436 (1) angstrom, c = 7.386 (1) angstrom, beta = 107.00 (1)-degrees, V = 542.9 angstrom3, R = 0.030, and R(w) = 0.046. The compounds may be described as infinite helical chains of vanadium, phosphorous, and oxygen running perpendicular to the 101 plane. Between these chains, there are two crystallographically distinct sites for the alkali cations, but both sites may be described as highly irregular 6-fold coordination. The potassium compound melts at 480-degrees-C, and the sodium compound melts at 466-degrees-C.