The dynamic properties of zircon studied by single-crystal X-ray diffraction and Raman spectroscopy

An investigation of the dynamic properties of synthetic non-metamict zircon , ZrSiO4, was undertaken using X-ray single-crystal diffraction and polariz ed single-crystal Raman spectroscopy. The X-ray results at room temperature show that the Zr cation located on the triangular dodecahedral site is tig htly bonded. The atomic displacement parameters from different crystals and refinements on zircon are compared and shown to be different. This is prob ably a result of experimental problems associated with extinction and/or sl ight degrees of metamictization in natural samples. A calculation of the li brational motions of the rigid SiO4 tetrahedron give a mean-square libratio n of 3.3(3) degree(2) along the a axes and 6.9(6) degree(2) along the c axi s. The Raman spectrum shows some unusual features, such as the low wave num ber SiO4 bending Motion, upsilon (2), at 266 cm(-1), that can be explained by the structural properties of zircon. The structure is characterized by o pen 'channels' running parallel to [001] and they influence the energies of the SiO4 bending modes depending upon their symmetries. The Raman modes di ffer strongly in intensity and this can be explained by vibrational interac tions with the electronic state of Zr4+. In contrast to garnet, which share s a few structural similarities with zircon, the Zr cation residing in the large dodecahedral site shows little dynamic disorder. In addition, all the external modes in zircon are harmonic in comparison to a few modes in pyro pe garnet, for example, that soften with decreasing temperature.