ORIENTED CRYSTAL STUDIES: ORIENTING THE CRYSTALS

Raman spectroscopic and X-ray diffraction studies have been performed since the early 1900's. These techniques have defined the way the scientific community looks at crystal structures. At the University of Arizona we are able to perform oriented Raman spectroscopic studies by orienting crystals on a machine that has been dubbed “The Orienter”. The process for orienting crystals is fairly straight forward. A crystal as large as 50 mm is mounted on a goniometer head and placed in our Bruker X8 Single-Crystal Diffractometer with a CCD and Kappa geometry. Several scans are taken in a wide range of orientations, and the locations of diffraction spots are recorded. The spots are indexed and their locations determined. An orientation matrix is then computed that transforms crystallographic indicies to laboratory Cartesian space. The crystal is placed on The Orienter and the matrix is loaded into a computer program that computes the motor steps needed to drive the crystal so that a given crystallographic vector is aligned parallel to a pin, and another crystallographic vector is aligned perpendicular to a fiducial mark on the pin. When the motors have been driven to their respective places, the pin is then aligned in a unique way with the crystal. The pin is slowly pushed towards the crystal until it barely makes contact. Once in contact, optical adhesive is applied and set with a UV light. The oriented crystals are then analyzed with the Raman spectrometer to record the vibrational spectrum. After careful analysis of the oriented spectra, it is possible to compute many thermodynamic properties, or even seismic properties of a mineral.