Elastic constants of acenaphthene determined from studies of thermal diffuse scattering of X-rays

The absolute values of all the nine elastic constants of acenaphthene single crystals, C₁₂H₁₀, belonging to the orthorhombic system, have been determined by the photographic method from the measurement of the intensities of thermal diffuse scattering observed in a series of Laue photographs taken with monochromatized Cu Kα radiation having no higher harmonics of λ_Kα. The direct beam was recorded photographically after being absorbed through nickel foils of known thickness, its intensity being reduced by a factor of 10⁶. Diffuse scattering domains near the reciprocal-lattice nodes 400, 080, 004, 105, 014 and 072 were investigated extensively. The diffusely scattered intensities were corrected for the general scattering, polarization and skew factors. Corrections for absorption, divergence and second-order diffuse scattering were found negligible. The values of the elastic constants in units of GN m^-2, with e.s.d.'s in parentheses, are C₁₁ = 11.05 (50); C₂₂ = 10.26 (30); C₃₃ = 9.40 (23); C₄₄ = 4.25 (08); C₅₅ = 3.69 (07); C₆₆ = 4.87 (10); C₁₂ = -1.93 (20); C₁₃ = 3.34 (12); C₂₃ = 2.34 (13). The accuracy of experimentally determined values of the elastic constants is discussed.