Heat capacity of germanium crystals with various isotopic composition

The heat capacity of three pure ( n , p ⩽2×10 16 cm -3 ) germanium crystals with different isotopic compositions was measured in the temperature range from 2.8 K to 100 K. These samples, one made of enriched 70 Ge (95.6%), Ge of natural isotopic composition and n , p < 10 14 cm -3 , and one of the largest possible isotopic mass variance 70/76 Ge (43%/48%) with n , p <10 14 cm -3 , show a change of the molar heat capacity (and corresponding Debye temperature, θ D ) as expected from the average mass variation, corresponding to θ D ∝ M -0.5 ( M = molar mass) at low temperatures. The mass effect is best visible around 21.5 K, at the minimum of the corresponding Debye temperatures θ D , and amounts to Δθ D = 5.3 K for the difference between the Debye temperatures of 70 Ge and 70/76 Ge. The specific heat capacity of the natural Ge crystal agrees within 2% with the best data available in the literature taken on much larger masses of Ge.