Temperature-pressure phase diagram of deuterated tetramethyl-ammonium tetrachlorozincate

The (P, T) phase diagram of deuterated tetramethylammonium tetrachlorozincate (d-TMA ZnCl 4 ) has been obtained by measuring the value of the wave vector of the distorsion wave → q = (qδ a*,0,0) as function of pressure(0-1.5 Kbars) and temperature (-5 to 40°C). This phase diagram realizes a nice illustration of the Devil's stair case. It contains different commensurate stairs at qδ = 1/3, 2/5 (ferroelectric phase), 3/7 and 1/2. In the high temperature part of the diagram it is possible to follow a continuous path in an incommensurate phase, going from qδ =0.43 to qδ = 1/3. For temperatures below 35°C the 2/5 stair separates the diagram in two distinct incommensurate phases (qδ > 0.40 and qδ < 0.40). In the low temperature region important hysteresis effects were observed when passing through the different commensurate phases. On contrary, in other regions of the phase diagram, the qδ variations (and too the intensity of the first order satellites) was found to be quite reversible.