Ferrielectricity in the metal-organic ferroelectric tris-sarcosine calcium chloride

We report a paradigm of ferrielectricity in a single-phase crystal, tris-sarcosine calcium chloride [TSCC; ${\left(\mathrm{C}{\mathrm{H}}_3\mathrm{NHC}{\mathrm{H}}_2\mathrm{COOH}\right)}_3\mathrm{CaC}{\mathrm{l}}_2]$. Ferrielectricity is well known in smectic liquid crystals but almost unknown in true crystalline solids. C. F. Pulvari [Ferrielectricity, Phys. Rev. 120, 1670 (1960)] reported it in 1960 in mixtures of ferroelectrics and antiferroelectrics (AFEs), but only at high fields. Tris-sarcosine calcium chloride exhibits a second-order displacive phase transition near $T_{\mathrm{c}}=130\mathrm{K}$ that can be lowered to a quantum critical point at 0 K via Br or I substitution and phases previously predicted to be AFE at high pressure and low temperatures. Unusually, the size of the primitive unit cell does not increase in the so-called AFE phase. We measure hysteresis loops and polarization below $T=64\mathrm{K}$ and provide clear Raman evidence for this paraelectric-ferrielectric transition.

Figure 1

TSCC structure (modified from Ashida et al. [3]); the large arrows show the sarcosine molecule displacements below $T_{\mathrm{c}}$ from our neutron and x-ray data, discussed below.

Figure 2

(a) Left side. Raman data (intensity versus energy) in TSCC with 3% Br at various temperatures, showing anomalies in intensity and energy for low-frequency vibrations near 40 and $115\mathrm{c}{\mathrm{m}}^{-1}$ at $\sim T=75\mathrm{K}$. (b) Right side. Raman data (intensity versus energy) in TSCC with 3% Br at various temperatures, showing anomalies near $500\mathrm{c}{\mathrm{m}}^{-1}$ at $\sim T=50\mathrm{K}$.

Figure 3

Polarization data versus field $P$($E$) obtained at $T=50\mathrm{K}$ and 100 Hz for (a) undoped TSCC along the polar $+b$ axis; $E=4\mathrm{kV}/\mathrm{cm}$ along [0 1 0]. Switching is observed in the form of a single AFE-like loop (i.e. a FE loop with a very large bias field) from ∼8–12 kV/cm, in agreement with Ref. [9]. (b) For 30% Br-doped TSCC the bias voltage is larger (linearly proportional to Br%, approx. 10 kV/cm). However, this occurs only for positive voltages. For fields along the pseudohexagonal $c$ axis, no hysteresis is observed up to 100 kV/cm.

Figure 4

Pyroelectric current $J$ (blue curve) and spontaneous polarization $P$ (open circles) as a function of temperature for (a) undoped and (b) 30% Br-doped TSCC. Red curve denotes fit of $P$($T$) proportional to the mean-field tricritical dependence ${\left(T_{\mathrm{c}}-T\right)}^{\beta }$. Fitted parameters are $T_{\mathrm{c}}=124\mathrm{K},\beta =0.30$, and $T_{\mathrm{c}}=105\mathrm{K},\beta =0.25$ for 30% Br-doping. Note that the maximum spontaneous (zero-field) polarization $P$ measured in this way is comparable in magnitude to the switchable polarization observed in the $P$($E$) loops in Fig. 3 and that the lowest temperature phases are pyroelectric, and in fact that $P$ increases in these phases.

Figure 5

Polarization PUND data on 30% Br-doped TSCC at $T=50\mathrm{K}$. (a) Triangular and (b) trapezoid pulses of equivalent rise time were employed to discount transient current and/or charge injection effects. All pulses were separated by a wait time of 1 s. The current responses are identical for both pulse chains with clear switching events consistent with Fig. 3; a magnified view of the nonswitching current observed under negative bias is also shown (inset).

Figure 6

Specific heat data near 64 and 130 K in TSCC, modified from Haga et al. [23].

Figure 7

Preliminary evidence for a phase transition in TSCC near 48 K. Upper trace is thermal expansion coefficient, and lower trace is dielectric response. The latter show only a few data points at 48.0 K.

Figure 8

Change in Ca-O-C bond angle in pure TSCC as a function of temperature. X-ray data are open symbols; neutron data are shaded. This may be considered the microscopic order parameter, proportional to the macroscopic order parameter $\langle P\left(T\right)\rangle$. Here, 3AB and 4AB designate different oxygen sites. Dotted lines indicate extrapolated angles from the PE phase.

Figure 9

Resultant TSCC structure below $T_{\mathrm{c}}=\sim 120\mathrm{K}$.

Figure 10

Sarcosine molecule displacement with respect to Ca-ion (x-ray data); ordinate axis units are percent of the $b$ axis lattice constant. The temperature exponent is $\beta =0.27$. One outlying data point near $T=100\mathrm{K}$ has been removed.