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

For a synthetic non-metamict zircon, XRD results show that the Zr cation on the triangular dodecahedral site is tightly bonded. A calculation of the librational motions of the rigid SiO (sub 4) tetrahedron gives a mean-square libration of 3.3 degrees (super 2) along the a axes and 6.9 degrees (super 2) along the c axis. The Raman spectra show some unusual features, such as the low wave number SiO (sub 4) bending motion, upsilon (sub 2) , at 266 cm (super -1) , that can be explained by the structural properties of zircons. The structure is characterized by open 'channels' [001] which influence the energies of the SiO (sub 4) bending modes depending on their symmetries. The Raman modes differ greatly in intensity, due to vibrational interactions with the electronic state of Zr (super 4+) . The Zr cation in the large dodecahedral site shows little dynamic disorder; all the external modes in zircon are harmonic, in comparison to a few modes in pyrope that soften with rising T.