Labile states are the basis of functional materials

The available data refute the widespread postulate of thermodynamics, according to which labile states are physically unrealizable, unobservable and, thus, devoid of practical interest, since the transition to a stable state does not require overcoming a potential barrier, and a random fluctuation leads to an accelerated shift of the system from the initial state. The cases when a system remains in a labile state for an indefinite period of time are well known. The corresponding states are not only observable, but can be used to create functional materials. The article analyses low-temperature phase equilibria and spinodal behavior in a number of binary systems containing solid solutions with a fluorite structure, such as CaF2-SrF2, CaF2-BaF2, BaF2-RF3 (R = La, Nd), SrF2-LaF3, ZrO2-Y2O3. The investigation of low temperature phase formation in the BaF2-LaF3 system allowed to reveal the decomposition of the solid solution Ba1-xLaxF2+x with a binodal curve. In the SrF2-LaF3 system the equilibrium solubility curve of lanthanum fluoride in strontium fluoride is expressed at the inflection point on the solvus curve with a practically horizontal tangent, which corresponds to the bifurcation point – the practical coincidence of the critical point of the nonequilibrium binodal/spinodal with the solvus curve. The Ba1-xCaxF2 continuous solid solution obtained by the mechanochemical method and possessing high fluorine-ion conductivity, remains in a labile state for an indefinitely long period of time. Upon heating, it disintegrates with an exothermic effect at 420-450 °C. In all other fluoride systems, single crystals grown from the melt retain the functional characteristics of photonics materials for years and have no signs of degradation. Obviously, the technological stability of crystalline samples of the listed solid solutions is determined by the extremely low values of the cation diffusion coefficients. The systems are “falling”, but too slowly to detect it. The fine architecture of materials in a labile state is of considerable interest