THERMODYNAMIC PROPERTIES OF C-13-DIAMOND

Changes in thermodynamic properties of diamond at isotopic substitutio n C-12 --> C-13 have been rigorously calculated at room and higher tem peratures by using the first order of the thermodynamic perturbation t heory. At 25 degrees C, the relative changes in the lattice constant o f diamond, in coefficient of thermal expansion, in isothermal bulk mod ulus, and in heat capacity are equal to -(1.54 +/- 0.15) x 10(-4), 7.5 8 x 10(-2), 7.58 x 10(-4) and 9.56 x 10(-2), respectively. It has been shown that the isotopic effect in the lattice constant of diamond sli ghtly decreases with increasing pressure.