Sapphire Ultraoptics for Sub-meV 121Sb and 125Te Phonon Spectroscopy

Nuclear resonance scattering of synchrotron radiation enables probing hyperfine interactions and element specific vibrational modes of nuclei that exhibit a Mössbauer transition. A prerequisite for this method is a monochromator with narrow bandwidth. Silicon, as the most commonly used crystal in monochromators, is not suitable for experiments above 30 keV photon energy. Using a sapphire single crystal in backscattering geometry is an alternative. An x-ray beam with narrow
bandwidth can be obtained from a back-reflection with a Bragg angle of a few arcsec to a few
arcmin smaller than =2. The development of a sapphire backscattering monochromator with high energy resolution, better than 1 meV, would permit detailed lattice dynamics characterization of novel functional materials.
Sapphire is a very rigid material with desirable optical properties, high chemical resistance, and
high heat conduction. Crystals of large size with high quality are of interest for many industrial applications, and suited for optics and optoelectronics operating under ambient condition or extreme conditions.
The purpose of the work in this dissertation is twofold. First, study the quality of sapphire single
crystals by modern high-resolution characterization techniques in order to acquire microstructural information and thereby a better understanding of the origin of lattice defects. The second goal is the study of the lattice dynamics in materials based on tellurium and antimony with Mössbauer energies of 35.49 keV and 37.13 keV using nuclear resonance scattering with energy resolution given by one of the highest quality sapphires.
White beam topography of more than thirty crystals, grown at the Shubnikov Institute of Crystallography in Moscow, revealed qualitative information about the distribution of lattice defects of which linear defects, i.e. dislocations, are the majority type. The lowest dislocation density of 10^2 -10^5 cm^-2 was found in C-plane crystals grown by the Kyropoulos and Bridgman techniques.
We carried out rocking curve imaging in backscattering geometry to estimate the lattice parameter variation and energy resolution from back-reflections. Minimum variations of the lattice parameters
on the order of 10^-8 were observed from spots with an edge length of 0.2-0.5 mm. There are a few spots with such a quality in one crystal which makes it suitable as backscattering monochromator in nuclear resonance scattering or as analyzer in resonant inelastic x-ray scattering.
The use of very high energy resolution nuclear inelastic scattering with 0.7 meV at the energy
of the nuclear transition in 121Sb and 125Te enables valuable insight into the phonon scattering of thermoelectric materials, that convert heat to prvide electricity and vice versa, composed of Sb and Te. A careful study is done on heat carrying acoustic modes in the partial density of states of (PbTe)mAgSbTe2, so called LAST-m alloys. An impressive mismatch in the phonon group velocities in the 2-5 meV range and di erence in the force constants for the Sb and Te density of phonon states is observed, a phonon mismatch predicted to be responsible for low lattice thermal conductivity in LAST-m.
An in-depth understanding of the element-specific dynamic properties of cubic and orthorhombic
antimony trioxides was achieved using nuclear resonance scattering with an energy resolution of 1 meV at the nuclear transition energy of 121Sb. A softening of the Sb bonds upon transformation from cubic molecular structured in alpha-Sb2O3 to chain structured orthorhombic in beta-Sb2O3 is observed. Furthermore, results on the lattice dynamics on alpha-TeO2, with quasi molecular structure, demonstrate strong interatomic Te bonds, comparable with the strong bonds in molecular structured alpha-Sb2O3. The nuclear resonance data is complemented with inelastic neutron scattering data that reveals the oxygen vibrational modes. In addition, the experimental results validate the calculations of the vibrational modes in these types of materials and serve as benchmark for the calculation.