Isotope Effects in the Lattice Constant and Thermal Expansion ofNe20andNe22Single Crystals

Single crystals of ${}_{}{}^{20}{}_{}{}^{}\mathrm{Ne}$ and ${}_{}{}^{22}{}_{}{}^{}\mathrm{Ne}$ were grown by directional solidification from the liquid, and the x-ray lattice constants were measured by means of a rotating-camera method. The volume coefficients of thermal expansion were determined over the interval 2.5 to 24.3°K with a precision ranging from 5 ppm °${\mathrm{K}}^{-1\mathrm{}}$ at low temperatures to 18 ppm °${\mathrm{K}}^{-1\mathrm{}}$ at the highest temperature. The lattice constants extrapolated to 0°K are 4.46440 ${\mathrm{\AA }}^{*}$ for ${}_{}{}^{20}{}_{}{}^{}\mathrm{Ne}$ and 4.45589 ${\mathrm{\AA }}^{*}$ for ${}_{}{}^{22}{}_{}{}^{}\mathrm{Ne}$, within an estimated maximum error 0.00008 ${\mathrm{\AA }}^{*}$. Several current lattice-dynamical models adequately describe the isotopic difference in the lattice constant at 0°K, while at finite temperatures the description is unsatisfactory.