Tris-sarcosine calcium chloride [(TSCC), (CH 3 NHCH 2 COOH) 3 CaCl 2 ] 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 SrTiO 3 or doped KTaO 3 . For many years, this system was thought to have only two phases, paraelectric and FE, at ambient pressure.

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... ever, 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(D 16 2h ) ambient 500 K > T > 185 K, disordered; Pnma with Z = 4(D 16 2h )185 K > T > 130 K (ordered); P n2 1 a with Z = 4(C 9 2v )130 K > T > 64 K (FE); P 2 1 a (C 5 2h ) 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 D 3 6h (P6 3 /mcm) with Z = 2, but the samples decompose first at 503 K (230 • C).