Microscopic dynamic and macroscopic thermodynamic character of the I\bar 1 - P\bar 1 phase transition in anorthite

The dynamic character of the phase transition in anorthite from Monte Somma has been studied by high-temperature hard mode infrared spectroscopy. The mean local order parameter, as revealed by the temperature evolution of the frequencies of absorption bands between 540 and 620 cm_-1, follows classical second-order Landau behaviour with a critical exponent β = 1.. There is no observable first-order step. Anomalous line broadening of the 582 cm_-1 band indicates that dynamic fluctuations with a relaxation time τ ≈ 10⁻¹⁰ s exist over a limited temperature interval of approximately 150 K about T_c, and decay rapidly as T becomes greater than T_c. Previous order-disorder models of the high-temperature phase are not supported by these results. The Ca-flip motions, which we link to the line broadening, stabilize the driving soft mode of the transition. These flip motion fluctuations do not give rise to departures from the classical Landau theory because of the essential co-elastic nature of the phase transition. In the light of these results the structural instability can be described as an essentially displacive transition.