Phase transitions and polarization switching in quasi-one-dimensional organic ferroelectrics of phenyltetrazole family

TL;DR

Using Ising-like pseudospin models, the paper addresses phase transitions and polarization switching in APHTZ and MPHTZ crystals. It builds mean-field Hamiltonians with long-range dipole interactions and couplings to longitudinal and transverse fields $E_a$ and $E_b$, predicting a second-order ferroelectric transition at $T_c=\frac{J_{11}+J_{12}}{4k_B}$ for APHTZ and a 90° rotation of net polarization under transverse bias with a tricritical point at $T^*$; MPHTZ shows a uniaxial AFE with a similar topology and a tricritical point related to $T_N=\frac{J_{11}-2J_{12}}{4k_B}$. Fitted parameters yield $J_{11}/k_B\approx 1.6\times 10^3$ K and $J_{12}/k_B\approx -4.16\times 10^1$ K (MPHTZ) or $+2.4\times 10^1$ K (APHTZ), with dipole moments that reproduce experimental $P_a(E_a)$ and switching fields; the predicted energy density $W$ is around 1 J/cm$^3$, consistent with observations. The work provides a predictive, minimal Ising-like framework for proton-tautomer-driven FE/AFE order in hydrogen-bonded organic crystals and suggests experimental tests and refinements, including electrocaloric considerations and beyond-mean-field effects.

Abstract

Pseudospin models are proposed for description of the phase transitions, dielectric characteristics, and polarization switching in two crystals of the phenyltetrazole family. One of them, APHTZ, is a canted ferroelectric, whereas the other, MPHTZ is a simple antiferroelectric. In APHTZ the electric field, applied perpendicularly to the axis of spontaneous polarization, flips the polarization in one of the two sublattices, effectively rotating the non-zero net polarization by 90$^\circ$ and switching the system between two different ferroelectric configurations. The temperature-electric field phase diagrams are constructed. The diagram topology appears to be typical for the Ising-like antiferroelectric systems.

Figures (11)

• Figure 1: Two fully ordered hydrogen bonded 5-(4-R-Phenyl)-1H-tetrazole chains with opposite polarizations, indicated by large green arrows. Switchable and non-switchable $\pi$-bonds of tetrazole groups are shown in red and black, respectively.
• Figure 2: Two projections of the crystal structure of APHTZ, visualized Mercury using the X-ray data of Ref. Horiuchi:2023. Hydrogen bonds, linking the tetrazole groups, are shown in green. The off-center displacements of protons along these bonds are exaggerated. The FE-I configuration is depicted (see text).
• Figure 3: Fully ordered ferroelectric configurations FE-I and FE-II of the APHTZ model (see text and Ref. Horiuchi:2023). Two layers of parallel chains of dipoles oriented along [110] (blue arrows, sublattice $s=1$) and [1$\bar{1}$0] (red arrows, sublattice $s=2$) are shown for each configuration. Full and open green circles are the occupied and vacant proton sites on each bond; each dipole always points towards the occupied site. Large green arrows indicate the net polarization directions. The dipole sizes and relative distances between the sites, chains, etc are schematic only and do not reflect the real ones.
• Figure 4: The temperature dependences of the spontaneous polarization $P_a= \mu_a\eta_0/v$, inverse dielectric permittivities $\varepsilon_a^{-1}$ (left) and $\varepsilon_b^{-1}$ (right) of APHTZ. Dashed lines: $P_a$ calculated with $\eta_0$ approximated according to Eqs. (\ref{['sqrt']}) and (\ref{['etaapproxsat']}).
• Figure 5: Dependences of the polarization components $P_a$ (left) and $P_b$ (center) of APHTZ on the field $E_b$ at different temperatures. The arrows indicates the bends in the $P_b(E_b)$ curves. The inset: the permittivity $\varepsilon_b=\varepsilon_0^{-1}dP_b/dE_b$ obtained by numerical differentiation of $P_b$ as a function of $E_b$. (right) The $T$-$E_b$ color contour plot of the angle $\varphi=\arctan P_b/P_a$ between the net polarization vector and the $a$ axis. Solid and dashed lines: first and second order phase transitions, respectively; : the tricritical point $(T^*,E_b^*)$.