Anomalous isotopic effect on the lattice parameter of silicon.

The difference Aa = a ( 3 0 ) - a ( 2 8 ) of the lattice parameter of 3 0 Si and 2 8 Si 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 K. An anomalous behavior is observed: The relative difference ‖Δa/a‖ attains its maximum value of 56.8(5) ppm at T = 75(10) K. The anomalous behavior is attributed to the influence of phonon modes with negative Gruneisen parameters. At T = 700 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.