Precise measurement of the lattice parameters of α-Al₂O₃ in the temperature range 4.5–250 K using the Mössbauer wavelength standard

The lattice parameters of α-Al₂O₃ have been measured in a temperature range from 4.5 to 250 K with a relative accuracy of better than 6 × 10⁻⁶. The experimental method uses Bragg backscattering and the recently proposed Mössbauer wavelength standard, i.e. the wavelength λ_M = 86.025474 (16) pm of the nuclear resonance radiation of ⁵⁷Fe (Shvyd'ko et al., 2000), which has previously been applied successfully to measure the lattice parameters of α-Al₂O₃ at temperatures between 286 and 374 K (Shvyd'ko et al., 2002). The experimental data in the range from 4.5 to 374 K are consistent with the Debye model of thermal expansion. At 4.5 K, the thermal expansion coefficient is as low as 1.2 (9) × 10⁻¹⁰ K⁻¹ in the a direction.