- The procedure for the estimation of the defect-interaction parameters for solids is suggested. The vacancy-type defect (VTD) effective interaction is described by the mixing potential. The sign of this potential determines the solubility in the VTD-atom solid solution. The VTD thermodynamic parameters are evaluated by fitting to the temperature dependence of the self-diffusion coefficient. For the example of Nb, we illustrate the ability of the suggested approach to describe the stability of the VTD-atom ``alloy.''