The crystal chemistry of MTO₄ compounds with the zicron, scheelite, and monazite structure types

The crystal structures of zircon, scheelite, and monazite are very closely related. All three have chains of alternating polyhedra and planes of closest packed or pseudo-closest packed cations. Using these similarities the unit cells of these structures can be placed in analogous orientations. This in turn leads to a better understanding of the geometrical aspects of the reconstructive phase transformations that occur among the structure types as functions of temperature and pressure. In essence the phase transformations require the cation planes of one structure to the cation planes of another. Phase transformations also occur via compositional pathways.

Crystal structure parameters were modeled for compounds with the zircon, scheelite and monazite structure types using multiple regression techniques. Data consisted of structure refinements of 26 zircon-, 13 scheelite-, and 13 monazite-type compounds. These compounds include but are not limited to the lanthanide vanadates and phosphates, the alkali earth molybdates and tungstates, and KTcO₄. The structural parameters studied included bond lengths, bond angles, polyhedral volumes, unit cell edge lengths, tetrahedral quadratic elongations and atomic coordinates of individual atoms; they were modeled as a function of the Shannon radii of the cations and the product of the M and T cation charges. Correlation coefficients for these regressions exceeded 0.9 for nearly all parameters studied except for the y coordinate of M, the z coordinate of O1, and T-O1-M2 angle of the monazite compounds.