Phase diagram and phase transitions in ferroelectric tris-sarcosine calcium chloride and its brominated isomorphs

Tris-sarcosine calcium chloride [(TSCC), (CH₃NHCH ₂COOH)₃CaCl₂] is a uniaxial ferroelectric (FE) with a displacive second-order phase transition near T_c=130 K. A continuous range of solid solutions can be made by substituting Br for Cl, which lowers T_c 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 < Br < 40%): Order-disorder of the sarcosine methyl group at 185 K; displacive FE transition at 130 K (in pure TSCC); a second FE transition [previously hypothesized to be antiferroelectric (AFE) but probably not] at 64 K; and a new anomaly at ∼45 K which might be due to a phase transition or to Debye-like freezing of orientational disorder of some part of the sarcosine molecule. The probable sequence of structures is (upon cooling): Pnma with Z=4(D2h16) ambient 500 K > T > 185 K, disordered; Pnma with Z=4(D2h16)185K>T>130K (ordered); Pn2₁a with Z=4(C2v9)130K>T>64K (FE); P2₁a (C2h5) with Z =4, 64 K > T > 45 K (not AFE); T < 45 K, unknown structure. A sixth hexagonal structure at high temperatures (>500 K) is hypothesized to be D6h3(P6 ₃/mcm) with Z =2, but the samples decompose first at 503 K (230 °C).