Physical properties of YB66 and consideration of possible use for high-resolution X-ray optics

A combined microscopic, i.e., powder and single crystal x-ray diffraction and inelastic x-ray scattering, and macroscopic, i.e., heat capacity, characterization was carried out on YB 66 between 10 and 400 K. The system crystallizes in a rather large unit cell with a lattice parameter of about 23.4 Å at room temperature. The volume thermal expansion coefficient is found to be 3.9 × 10 − 5 K − 1 at 295 K. A high Debye temperature above 1000 K was determined using several methods, 1043 ( 25 ) K, from heat capacity measurements, 1430 ( 59 ) K, from elastic constants measurements, and 1690 ( 198 ) K, from thermal expansion measurements. The thermal transport is modeled as a collection of Einstein oscillators with an Einstein temperature of 194 ( 2 ) K. The C 11 and C 44 elastic constants are above 400 and 150 GPa, respectively. The average speed of sound in YB 66 is estimated to be 9631 ( 380 ) m / s, similar to the average speed of sound in corundum, 10 000 m / s. The large volume of the unit cell and the stiffness of the crystal structure make YB 66 a promising candidate for high-resolution x-ray monochromatization applications. Candidate reflections for high-resolution resonant inelastic x-ray scattering are indicated and potential pitfalls relevant to the quality of crystals and multi-beam cases are mentioned.