Phase Transitions in the Brominated Ferroelectric Tris‐Sarcosine Calcium Chloride
paraelectrics SrTiO 3 and KTaO 3 . [ 9 ] However, the ultra-weak nature of the dipole moment in TSCC referred to below may reduce the effects of such long-range dipole interactions to a narrow temperature region around the critical point. This would mean that more conventional classical or quantum critical behaviour would be expected over a wider range of temperatures and tuning parameters. TSCC is hydrogen bonded and was characterized in the past as order-disorder albeit with no direct supporting evidence. [ 16,17 ] Since the time of these studies TSCC has come to be a prototypical displacive ferroelectrics, [ 18 ] as demonstrated by the existence of an under-damped soft mode. This mode can be followed into the GHz frequency regime [ 19 ] from highT values of ca 630 GHz = 21 cm −1 . Deuteration [ 20 ] produces little change in T C , implying that the transition is not controlled by the N-Cl-hydrogen bonds, a result compatible with the soft-mode mechanism operating in TSCC. [ 2 ]
Phase diagram and phase transitions in ferroelectric tris-sarcosine calcium chloride and its brominated isomorphs
Tris-sarcosine calcium chloride [(TSCC), (CH3NHCH2COOH)3CaCl2] is a uniaxial ferroelectric (FE) with a displacive second-order phase transition near Tc=130 K. A continuous range of solid solutions can be made by substituting Br for Cl, which lowers Tc to 0 K at ∼72% Br. Such a quantum critical point differs from that in pseudocubic FEs, such as O-18 SrTiO3 or doped KTaO3. For many years, this system was thought to have only two phases, paraelectric and FE, at ambient pressure. However, we find from dielectric and resonant ultrasound spectroscopy that there are four phase transitions in TSCC and in TSCC:Br (for 0 T > 185 K, disordered; Pnma with Z=4(D2h16)185 K>T>130 K (ordered); Pn21a with Z=4(C2v9)130 K>T>64 K (FE); P21a (C2h5) with Z =4, 64 K > T > 45 K (not AFE); T 500 K) is hypothesized to be D6h3(P63/mcm) with Z =2, but the samples decompose first at 503 K (230 °C).
Pressure and Temperature Dependence of Dielectric Properties of Trissarcosine Calcium Chloride
The pressure (0–6 kbar), temperature (-200∼-70°C) and dc bias field (0–9×105 V/m) dependence of the dielectric properties (permittivity, spontaneous polarization, coercive field and phase transition temperatures) of trissarcosine calcium chloride (TSCC) were measured. The effect of pressure on the dielectric properties was remarkable. A remarkable decrease in the permittivity occurred at a pressure of about 5 kbar, where the ferroelectric state terminates. The new phase III appeared when pressure was applied and became more stable with increasing pressure. Judging from the dc bias field and temperature dependence of the permittivity and the non-occurrence of hysteresis loops and other considerations, the pressure-induced phase III is believed to be antiferroelectric. The dielectric properties associated with the paraelectric-ferroelectric transition in TSCC are also successfully interpreted in terms of a phenomenological theory.