Effect of isotope concentration on the lattice parameter of germanium perfect crystals

The difference in lattice parameters between germanium crystals of natural isotopic composition and an isotopically enriched crystal of ${}_{}{}^{74}{}_{}{}^{}\mathrm{Ge}$ was calculated and measured by using a double-beam triple-axis x-ray spectrometer. The measured relative fractional difference was (-6.3±0.3)×${10}^{\mathrm{-}6}$ and (-14.9±0.3)×${10}^{\mathrm{-}6}$ at room and liquid-nitrogen temperatures, respectively. The corresponding calculated values are -6×${10}^{\mathrm{-}6}$ and -12×${10}^{\mathrm{-}6}$. The agreement is very good, given uncertainties in the values of physical constants used for the theoretical calculations.