Thermodynamic behaviour of the high-temperature P1- I1 phase transition along the CaAl2Si2O8 - SrAl2Si2O8 join

In situ high-temperature synchrotron radiation powder diffraction patterns were taken from room temperature to T = 740°C from synthetic feldspars along the join CaAl2Si2O8-SrAl2Si2O8 (An-SrF). Three samples of composition An95SrF5, An90SrF10 and An85SrF15 were investigated, and the evolution of cell parameters with T was determined by Rietveld analysis of powder X-ray diffraction patterns. The high-temperature P - I phase transition, previously observed with Tc = 241°C in anorthite, was found in An95SrF5, An90SrF10 and An85SrF15 feldspars at Tc = 233(5)°, 195(2)° and 174(2)°C respectively. The transition was revealed by the disappearance of critical reflections and variations in the rate of change of cell parameters with temperature. A significant, although small (between 0.0025 and 0.0012 at room temperature), spontaneous strain could be measured, allowing the thermodynamic behaviour of the transition to be modelled. A second-order trend for An90SrF10 and An85SrF15 [beta = 0.504(7) and 0.505(7) respectively] or nearly second-order for An95SrF5 [beta = 0.458(4)] was observed in contrast with tricritical behaviour of end member anorthite. An extrapolation of the Tc vs composition to room temperature indicates that the critical composition for the P phase is An60SrF40.