Anomalous Isotopic Effect on the Lattice Parameter of Silicon

Abstract

The difference [Formula presented] of the lattice parameter of [Formula presented] and [Formula presented] 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 [Formula presented]. An anomalous behavior is observed: The relative difference [Formula presented] attains its maximum value of 56.8(5) ppm at [Formula presented]. The anomalous behavior is attributed to the influence of phonon modes with negative Grüneisen parameters. At [Formula presented] 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. © 2002 The American Physical Society.