The variation of thermoelastic properties under conditions of variation of isotopic composition of diamond

The parameters of interatomic interaction in diamonds of ¹²C and ¹³C isotopes are determined. The parameters are determined by two methods, namely, from the elastic modulus (potential no. 1) and from the sublimation energy at T = 0 K (potential no. 2). Based on these parameters, the Debye model is used to calculate the isobaric temperature dependences of the thermoelastic properties for isotopically different diamonds, namely, the coefficient of thermal expansion (CTE), the density, the elastic modulus B _T , and the specific heat capacity. It is demonstrated that the best agreement with the experimentally obtained dependences of the foregoing parameters is obtained when potential no. 1 is used in calculations. The values of CTE, density, and heat capacity increase in the case of transition from ¹²C diamond to ¹³C diamond. It is demonstrated that the inequality B _T (¹²C) < B _T (¹³C) is valid at T = 0 K, which is associated with the reduction of interatomic spacing in the case of transition from ¹²C diamond to ¹³C diamond. However, as the temperature increases, the volumetric isotopic effect in diamond decreases. It is found that the B _T (T) dependences for ¹²C and ¹³C diamonds intersect at a temperature below the Debye temperature. Therefore, at high temperatures, the elastic modulus for light-isotope diamond comes to exceed that for heavy-isotope diamond, B _T (¹²C) > B _T (¹³C). Experimental results are given which confirm this effect. The variation of the properties of isotopically different diamonds with increasing pressure is discussed.