Symmetry induced ferroelastic isostructural phase transition and sound propagation anomaly in potassium rich feldspars

The proper ferroelastic phase transitions extension - compression type in cubic crystals are considered. The Landau potential being a polynomial function of the order parameter components includes the term of third power. This symmetry induced statement results in the line of isostructural phase transitions (so-called symmetry induced IPT) situated on the phase diagram near the lines of symmetry changing phase transition. The physical meaning of phenomenological parameters of the Landau theory and a description of the physical quantities anomalies along definite thermodynamic paths, crossing these lines are presented in this paper. One of the consequences of the developed theory is the conclusion that the rest of elastic constants anomaly, accompaning IPT in the parent phase of feldspars, must display itself in nonmonotonic dependence of the longitudinal sound velocity on external conditions. Our experimental data on potassium rich feldspars confirm this consequence of our calculations.