We measured the temperature dependence of the elastic constant C 11 of a 12 C diamond monocrystal using picosecond ultrasonics between 10 and 613 K. We found that C 11 is almost temperature independent below room temperature; the temperature coefficient around 300 K is À6.6 MPa/K. Our results show a significantly higher Einstein temperature than reported values by $30%, indicating that diamond has a larger zero-point energy, which remains dominant around ambient temperature. We also calculated

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... he temperature dependence of the elastic constants using ab-initio methods, resulting in good agreement with measurements. Our study shows that below-ambient-temperature measurements are not sufficient to extract the Debye temperature and the Gr€ uneisen parameter of high-Debye-temperature materials.