Unusual elasticity of monoclinic β−Ga2O3

• Published in: Journal of applied physics 2018-08, Vol.124 (8)
• Main Authors: Adachi, K., Ogi, H., Takeuchi, N., Nakamura, N., Watanabe, H., Ito, T., Ozaki, Y.
• Format: Article
• Language: English
• Subjects: Applied physics; Approximation; Crystals; Deformation mechanisms; Density functional theory; Diffraction; Elastic anisotropy; Elastic deformation; Elastic properties; Elasticity; Gallium oxides; Lattice parameters; Mathematical analysis; Monoclinic lattice; Stiffness
• ISSN: 0021-8979; 1089-7550
• DOI: 10.1063/1.5047017

This paper determines all the 13 elastic constants Cij of monoclinic β-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 Cij: C11 = 242.8 ± 2.9, C22 = 343.8 ± 3.8, C33 = 347.4 ± 2.5, C44 = 47.8 ± 0.2, C55 = 88.6 ± 0.5, C66 = 104.0 ± 0.5, C12 = 128 ± 0.1, C13 = 160 ± 1.5, C23 = 70.9 ± 2.1, C15 = –1.62 ± 0.05, C25 = 0.36 ± 0.01, C35 = 0.97 ± 0.03, and C46 = 5.59 ± 0.69 GPa. We also performed a density-functional-theory calculation and found that the local density approximation yields both the lattice parameters and Cij closer to the measurements than the generalized gradient approximation. Strong elastic-stiffness anisotropy is found in the diagonal elastic constants: C11 is smaller than C22 and C33 by ∼30%, and the difference between C44 and C66 exceeds 50%. Our measurements also reveal anomalous Poisson's ratios: ν21 and ν31 exceed 0.5, and ν23 and ν32 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.