Self-diffusion in the intermetallic compound CoGa

Abstract

In the ordered B2 compound CoGa, self-diffusion coefficients of both ⁶⁰Co and ⁶⁷Ga have been measured for five compositions, namely 60·0, 56·0, 50·0, 48·0 and 45·2 at.% Co, covering a large diffusivity range from 10⁻⁸ to 10⁻¹⁴ cm²/s. In addition, a small number of self-diffusion coefficients has been determined for four intermediate compositions, namely 58·0, 54·0, 52·0 and 47·0 at.% Co. The Arrhenius plots for the Co and Ga diffusion in every composition are curved and can be fitted to the sum of two exponentials: D = D _0a exp (-Q _a/kT) + D _0b exp (-Q _b/kT). The lower activation energies and frequency factors for Co diffusion have average values of 2·4 eV and 0·7 cm²/s respectively. This Co process has been attributed to diffusion via single vacancies and nearest-neighbour jumps. For every composition the other Co activation energy equals one of the Ga activation energies, within error limits, at values of about 3·1 eV. The associated Co frequency factors are of the order of 1000 cm²/s and the Ca frequency factors are smaller by a factor of about four. These processes have been identified as representing coupled diffusion of Co and Ga via divacancies. The second exponentials of the Ga diffusion are characterized by very large activation energies of about 4·4 eV and very large frequency factors of the order of 10⁷ cm²/s. The relation between the logarithm of the frequency factor and the activation energy turns out to be almost linear for all components, diffusion mechanisms and compositions involved. The combined diffusion of Co and Ga has been worked out in more detail in view of the special type of defect structure in CoGa. This leads to the concept of the triple defect as a special and predominant type of divacancy. Finally the triple defect is discussed as an alternative to the six-jump cycle for the interpretation of diffusion measurements in intermetallic compounds with the B2 structure.