Investigations on dielectric and structural properties of ferroelectric betaine phosphite (BPI)

We have extended earlier measurements made on the phase diagram of betaine phosphite (BPI) under hydrostatic pressure by observing the anomalies in the real and the imaginary parts of the dielectric function ( epsilon ' and epsilon ") and the dissipation factor (tan delta ), especially in the ferroelectric (f) phase at various temperatures (23 K<T<300 K) and hydrostatic pressures (p<390 MPa) at frequencies nu =0.5, 1, 5, 10, 50, 100 kHz. Below the transition into the f phase at T c2 , tan delta displays a double-peak structure with a large dispersion in our frequency range and a large pressure dependence, both peaks merging for low pressures. The anomalous behaviour found in tan delta points to the existence of a distribution of relaxation times and competing dynamics in the 'ordered' phase. The relaxational behaviour of the low-temperature peak could indicate a transition into a glasslike phase in this pure compound. In order to clarify the nature of these new regions in the phase diagram we have studied BPI by means of Raman scattering between 20 and 3150 cm -1 and by far-infrared reflection between 15 and 600 cm -1 .