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

The temperature dependence of lattice parameters and dilatation data of nat ural 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 tens or allowed in the respective point groups show almost tricritical behaviour . At temperatures below similar to 100 K order parameter saturation is obse rved. IR spectra of lawsonite were investigated at temperatures between 50 and 50 0 K in the range from 150-4500 cm(-1). The autocorrelation function has bee n used to quantify the decrease in line widths, which occurs with falling t emperature due to phase transitions. Changes in frequency, Delta omega, due to ordering and changes of the line width have been used to characterise t he state of local order over the temperature range. The IR data reveal an a lmost tricritical behaviour for the square of the effective line width of ( delta Delta corr)(3800). (delta Delta corr)(2)(3800), (Delta omega)(2)(3650 ) and (delta omega)(2)(900) correlate to the square of the e(1) strain comp onent. (delta Delta corr)(2)(900) and (Delta omega)(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 understoo d 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 displaciv e lattice distortion with a soft mode origin, however.