In closed thermodynamics system the influence of any external thermodynamics force causes change of all thermodynamics parameters of the system [1], which within the linear approaching can be described by the generalized system of equations:
dx_i =M_ij dX_j (1)
where dX_j is the increment of thermodynamics forces (temperature, magnetic field, stress dT,dH,dσ), dx_i is the change of thermodynamics parameters of system (entropy, magnetized, deformation dS,dJ,dε), and M_ij is the matrix of thermodynamics coefficients, including the elastic modules of solid, coefficients of thermal expansion, magnetic receptivity, pyro- and piezomagnetic coefficients (S_λμ, α_i, γ_ik, ρ_i, b_iμ).
In one’s turn external influences cause the return reaction of the system. As a result own «internal thermodynamics forces» were activated in a body: internal stresses, temperature, own magnetic field. The internal stress state was selected from thermodynamic parameters as the most sensible parameter to structure of solid. Internal stresses is the integral feature of solid given by chemical bond and related to the location of electronic clouds of neighboring atoms in a lattice. These stresses are changed under external mechanical forces, heating and influence of the magnetic fields according to equation:
dσ_λ = s_λμ (α_μdT + b_kμdH_k + s_μvdσ_v) (2)

The value of internal stresses are enough great and achieve a few GPa (according to results [1,2]). Structural phase transition will be realized, when some critical value of internal stresses, being parameter of the material, is achieved. There are different ways to attain the critical value of stresses: by change of any parameters H, T and P according to (2) and results [1]. The analysis of experimental data [1] allowed to make a conclusion about the deciding role of internal stresses in the nature of phase transitions.

1. P.I.Polyakov, S.S.Kucherenko. MMM, 278, (2004), p. 138-155.
2. Table of Physical quantities. Ed.N.K.Kikoin, Moscow, (1976), -1008 p.

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1. Role of elastic stresses in the magnetic properties of solids
2. Influence of internal stresses on the properties of solids
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