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

Abstract

Tris-sarcosine calcium chloride [(TSCC), (${\mathrm{CH}}_{3}{\mathrm{NHCH}}_{2}\mathrm{COOH}){}_{3}{\mathrm{CaCl}}_{2}$] 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 \ensuremath{\sim}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. 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 \ensuremath{\sim}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 $Z=4({D}_{2h}^{16})$ ambient 500 K > $T$ > 185 K, disordered; Pnma with $Z=4({D}_{2h}^{16})185 \mathrm{K}T130 \mathrm{K}$ (ordered); $\mathit{Pn}{2}_{1}a$ with $Z=4({C}_{2v}^{9})130 \mathrm{K}T64 \mathrm{K}$ (FE); $P{2}_{1}a$ $({C}_{2h}^{5})$ 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 ${D}_{6h}^{3}$(P6${}_{3}$/mcm) with $Z$ $=$2, but the samples decompose first at 503 K (230 \ifmmode^\circ\else\textdegree\fi{}C).