Polymorphism of K(2)Co(2)Mo(3)O(12): variations in the packing schemes and changes in molybdenum coordination under high pressure.

Within systematic studies on the K-Co-Mo-O system so-called high-temperature and high-pressure modifications of K2Co2Mo3O12 were found. The Birch-Murnaghan fits for the ambient-conditions modification alpha (Z = 4) and the high pressure phase II (Z = 8) lead to V0 = 1193.09 (4) A3, K =30.8 (8) GPa, K'0= 5.4 (4) and V0 = 2170 (10) A3, K =51 (2) GPa with K'0 fixed at 4.0, respectively. The high-pressure phase transition is denoted as pseudoreconstructive [Wiesmann et al. (1997). J. Solid State Chem. 132, 88-97], because some [MoO4] tetrahedra turn into edge-sharing pairs of [MoO5] pyramids or face-sharing pairs of [MoO6] octahedra. The new phases are presented and compared with the alpha phase.