Isotope effect on anharmonic thermal atomic vibration and κ refinement of ¹²C and ³C diamond

Isotope dependence of diamond structure of the mixed crystals of ¹³C_x¹²C(1−x), which were grown under high pressure, has been examined by diffraction study using synchrotron radiation. The lattice constant is expressed by:

where X = [¹³C]/([¹²C] + [¹³C]). Anharmonic thermal vibration of natural (X = 0.020) and ¹³C diamond (X = 0.985) was clarified by applying the Gram–Charlier series expansion to the temperature factors. The anharmonicity of natural diamond was larger than that of ¹³C diamond. The isotope effect on the localized electron density has also been elucidated by application of the κ refinement. ¹³C diamond has a larger κ parameter, 0.993 (1), than natural diamond, 0.931 (7), which proves that the valence electron of natural diamond is more widely distributed than that of ¹³C diamond.

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