Tunable Hypersonic Bandgap Formation in Anisotropic Crystals of Dumbbell Nanoparticles

Phononic materials exhibit mechanical properties that alter the propagation of acoustic waves and are widely useful for metamaterials. To fabricate acoustic materials with phononic bandgaps, colloidal nanoparticles and their assemblies allow access to various crystallinities in the submicrometer scale. We fabricated anisotropic crystals with dumbbell-shaped nanoparticles via field-directed self-assembly. Brillouin light spectroscopy detected the formation of direction-dependent hypersonic phononic bandgaps that scale with the lattice parameters. In addition, the local resonances of the constituent nanoparticles enable metamaterial behavior by opening hybridization gaps in disordered structures. Unexpectedly, this bandgap frequency is robust to changes in the dumbbell aspect ratio. Overall, this study provides a structure-property relationship for designing anisotropic phononic materials with targeted phononic bandgaps.

Thermal Expansion of Rutile from 100 to 700 °K

The thermal expansion of a single crystal of rutile (TiO₂) was determined in directions parallel to its crystallographic axes. A unique macroscopic technique was used wherein a cube-shaped specimen was cut from the anisotropic crystal, each of its six faces polished flat and nearly parallel to the face opposite it, and the expansion measured along each of its three body axes with an Abbe-Pulfrich interferometer. The relationship between the expansion along the body axes of the specimen and that in the crystallographic directions was determined from a Laue x-ray pattern. The unusual behavior of the thermal expansion that was observed can be explained by assuming an acoustical and optical contribution where the corresponding Grüneisen constants are 2.80 and 0.75 respectively.