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

The proper ferroelastic phase transitions extension - compression type in c ubic crystals are considered. The Landau potential being a polynomial funct ion of the order parameter components includes the term of third power. Thi s symmetry induced statement results in the line of isostructural phase tra nsitions (so-called symmetry induced IPT) situated on the phase diagram nea r the lines of symmetry changing phase transition. The physical meaning of phenomenological parameters of the Landau theory and a description of the p hysical quantities anomalies along definite thermodynamic paths, crossing t hese lines are presented in this paper. One of the consequences of the deve loped theory is the conclusion that the rest of elastic constants anomaly, accompaning IPT in the parent phase of feldspars, must display itself in no nmonotonic dependence of the longitudinal sound velocity on external condit ions. Our experimental data on potassium rich feldspars confirm this conseq uence of our calculations.