Local and macroscopic order parameter variations associated with low temperature phase transitions in lawsonite, CaAl2Si2O7(OH)2·H2O

Abstract

The temperature dependence of lattice parameters and dilatation data of natural lawsonite shows anomalies at 265 K and 130 K. The spontaneous strains are relatively small (a few per mill) and the components of the strain tensor allowed in the respective point groups show almost tricritical behaviour. At temperatures below ∼100 K order parameter saturation is observed. IR spectra of lawsonite were investigated at temperatures between 50 and 500 K in the range from 150–4500 cm −1 . The autocorrelation function has been used to quantify the decrease in line widths, which occurs with falling temperature due to phase transitions. Changes in frequency, Δω, due to ordering and changes of the line width have been used to characterise the state of local order over the temperature range. The IR data reveal an almost tricritical behaviour for the square of the effective line width of (δΔcorr) 3800 . (δΔcorr) 2 3800 , (Δω) 2 3650 and (δω) 2 900 correlate to the square of the e 1 strain component. (δΔcorr) 2 900 , and (Δω) 2 3550 seem to follow a more complex behaviour of the strain components e 2 2 and e 3 2 . The findings from IR and strain analysis allow the transitions to be understood as strain mediated ordering processes of protons in a channel parallel to the crystallographic c-axis. Close adherence to a Landau pattern of order parameter evolution suggests that the main driving force may be a displacive lattice distortion with a soft mode origin, however.