To find out the main features of symmetry–energy and structural–entropy aspects of interaction of impurity and solvent, a configuration model for the diffusionless formation of ‘hybrid’ interstitial–substitutional solid solution Me–X, where the interacting non-metal (X) atoms may occupy both octahedral interstices and sites of the b.c.c.(t.) metal (Me) lattice with vacancies, is developed. The discrete (atomic–crystalline) lattice structure, the anisotropy of elasticity, strain-induced (‘size’, etc.) as well as ‘blocking’ effects in the interatomic interactions are taken into account. An example of the simplest alloy isostructural to the nonstoichiometric Fe–N-martensite with a maximal α″-Fe₁₆N₂-type long-range order, but with X atoms in the octahedral interstices and sites of the b.c.t.-Me, is considered. An adequate set of the temperature- and concentration-dependent interatomic-interaction-energy parameters in such a solution is used to answer the following two questions. (i) Which are the features of varying the relative concentration of X atoms in the octahedral interstices of the b.c.t.-Me and, thus, its tetragonality degree, when the temperature increases? (ii) How can the equilibrium concentration of residual vacancies at the sites (in a wide range of changing the total content of introduced X atoms) correlate with the concentration of thermally activated vacancies in the impurity-free b.c.c.-Me at a fixed temperature?