K2Mo4O13 phases prepared by hydrothermal synthesis

Hydrothermal synthesis in the K-Mo oxide system was investigated as a function of the pH of the reaction medium. Four compounds were formed, including two K2Mo4O13 phases. One is a new low-temperature polymorph, which crystallizes in the orthorhombic, space group Pbca, with Z = 8 and unit cell dimensions a = 7.544(1) Angstrom, b = 15.394(2) Angstrom, c = 18.568(3) Angstrom. The other is the known triclinic K2Mo4O13, whose structure was re-determined from single crystal data; its cell parameters were determined as a = 7.976(2) Angstrom, b = 8.345(2) Angstrom, c = 10.017(2) Angstrom, alpha = 107.104(3)degrees, beta = 102.885(3)degrees, gamma = 109.760(3)degrees, which are the standard settings of the crystal lattice. The orthorhombic phase converts endothermically into triclinic phase at ca. 730 K with a heat of transition of 8.31 kJ/mol. (C) 2003 Elsevier Inc. All rights reserved.