Abstract
Tris-sarcosine calcium chloride [(TSCC), (] is a uniaxial ferroelectric (FE) with a displacive second-order phase transition near K. A continuous range of solid solutions can be made by substituting Br for Cl, which lowers to 0 K at ∼72% Br. Such a quantum critical point differs from that in pseudocubic FEs, such as O-18 SrTiO or doped KTaO. 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 ambient 500 K > > 185 K, disordered; Pnma with (ordered); with (FE); with 4, 64 K > > 45 K (not AFE); < 45 K, unknown structure. A sixth hexagonal structure at high temperatures (>500 K) is hypothesized to be (P6/mcm) with 2, but the samples decompose first at 503 K (230 °C).
4 More- Received 24 August 2010
DOI:https://doi.org/10.1103/PhysRevB.83.094102
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