Crystal structure analysis of Scheelite and Zircon type thorium germanates: A neutron diffraction study

In this communication we report the detailed crystal structure of two polymorphs of ThGeO4 (scheelite and zircon type) by Rietveld refinement of the neutron diffraction data of polycrystalline samples. Both of crystal structures consist of ThO8 (bisdisphenoid) and GeO4 tetrahedra. Scheelite type structure is a meta-stable polymorph and densely packed lattice, with a little larger average bond lengths of Th-O and Ge-O bonds to accommodate better packing. This is responsible for zircon to scheelite transformation under pressure. However, the average anion-anion contact and cation-cation contacts are shorter in scheelite structure compared to the zircon structure, which would causes the meta-stability in the scheelite structure. (C) 2006 Elsevier B.V. All rights reserved.