The difference $\Delta a=a_{\left(30\right)}-a_{\left(28\right)}$ of the lattice parameter of ${}^{\mathrm{30}}\mathrm{S}\mathrm{i}$ and ${}^{\mathrm{28}}\mathrm{S}\mathrm{i}$ crystals is measured over a temperature range from 4.7 to 700 K. In disagreement with existing knowledge, the strongest isotopic effect is not detected at the lowest achieved temperature $T=4.7\mathrm{K}$. An anomalous behavior is observed: The relative difference $|\Delta a/a|$ attains its maximum value of 56.8(5) ppm at $T=75\left(10\right)\mathrm{K}$. The anomalous behavior is attributed to the influence of phonon modes with negative Grüneisen parameters. At $T=700\mathrm{K}$ the effect still amounts to 30% of the maximal value. The experimental data are consistent with an approach based on the density-functional perturbation theory.

• Received 19 June 2002

DOI:https://doi.org/10.1103/PhysRevLett.89.285901