Unusual elasticity of monoclinic β-Ga_2O_3

摘要

This paper determines all the 13 elastic constants C-ij of monoclinic beta-Ga2O3, which has never been achieved since the discovery of this crystal about 100 years ago. We used resonant ultrasound spectroscopy with laser-Doppler interferometry; where the resonant-mode identification was unambiguously made by comparing measured and calculated displacement distributions on the vibrating specimen surface. Using more than 110 resonance frequencies, we inversely determined the C-ij. C-11 = 242.8 +/- 2.9, C-22 = 343.8 +/- 3.8, C-33 = 347.4 +/- 2.5, C-44 = 47.8 +/- 0.2, C-55 = 88.6 +/- 0.5, C-66 = 104.0 +/- 0.5, C-12 = 128 +/- 0.1, C-13 = 160 +/- 1.5, C-23 = 70.9 +/- 2.1, C-15 = -1.62 +/- 0.05, C-25 = 0.36 +/- 0.01, C-35 = 0.97 +/- 0.03, and C-46 = 5.59 +/- 0.69 GPa. We also performed a density-functional-theory calculation arid found that the local density approximation yields both the lattice parameters and C-ij closer to the measurements than the generalized gradient approximation. Strong elastic-stiffness anisotropy is found in the diagonal elastic constants: C-11 is smaller than C-22 and C-33 by, similar to 30%, and the difference between C-44 and C-66 exceeds 50%. Our measurements also reveal anomalous Poisson's ratios: nu(21) and nu(31) exceed 0.5, and nu(23) and nu(32) are almost zero. We explain these unusual elastic properties with the truss-like deformation of the tetrahedra-octahedra network and confirm this view with the ab- initio calculation. Published by AIP Publishing.