Ferroelectricity in the gapless quantum antiferromagnet ${\text{NH}}_4{\text{CuCl}}_3$

Ammonium trichlorocuprate, ${\mathrm{NH}}_4{\mathrm{CuCl}}_3$, a member of the family of quantum antiferromagnets ${\mathrm{M}}_3{\mathrm{CuCl}}_3(\text{M}=\text{K},\text{Tl},{\mathrm{NH}}_4^{+})$, well known for its gapless magnetization with unusual plateaus, is shown here to exhibit ferroelectricity below 67 K based on anomalies of the specific heat and dielectric constant, along with the appearance of spontaneous polarization and electric-field reversible pyroelectric current, making it a rare example of a Cu(II)-based ferroelectric. This suggests that the general ${\mathrm{M}}_3{\mathrm{CuCl}}_3$ family could be rendered multiferroic by alloying its alkali metals with ${\mathrm{NH}}_4^{+}$ ions.

Figure 1

Unit cell of ${\mathrm{NH}}_4{\mathrm{CuCl}}_3$. ${\mathrm{Cu}}^{2+}$ (red), ${\mathrm{Cl}}^{-}$ (green), and ${\mathrm{NH}}_4^{+}$ (turquoise) ions viewed along the $a$ axis. Dotted lines show likely $\mathrm{N}-\mathrm{H}\cdots \mathrm{Cl}$ hydrogen bonding.

Figure 2

(a) Temperature dependence of ${\epsilon }^{\prime }$ of ${\mathrm{NH}}_4{\mathrm{CuCl}}_3$ along different crystallographic axes. Plots are normalized from their values at 10 K for clarity. Inset shows a face index of ACC. (b) Temperature dependence of ${\epsilon }^{\prime }$ at 2 kHz for a pellet sample. Inset is the Curie-Weiss plot.

Figure 3

Close view of the temperature dependence of the normalized (10 K) dielectric constant of ACC along the polar $a$ axis. Here emphasis is placed on the presence of a shoulder, marked by the arrowhead, before the primary position of the transition around 68 K. For clarity, the inset shows the derivative of the primary curve in order to deduce the transition temperatures. The shoulder on the high temperature side suggests the presence of a double transition. The inset shows the derivative of the original points, and the maxima exposes two phase transitions at 69 and 67 K.

Figure 4

Spontaneous polarization of ACC along the $a$ axis with a number of applied electric fields. Not all fields are included for clarity.

Figure 5

Temperature dependence of the specific heat of ACC from 200 to 1.9 K. Transition: Note the two transitions $T_1$ and $T_2$, respectively, at 168 and $\approx 68$ K.

Figure 6

Expanded view of the the dielectric phase transition, which reveals the presence of a double peak. Upon estimation of the baseline via Origin software, deconvolution was attempted with Gaussian curves. Though the fits are not ideal, they reasonably match with the ${}^{14}\mathrm{N}$-NMR splittings found at 69 and 67 K. Recall that the ferroelectric transition occurred at 67 K, which is likely when the ACC had arrived at the centrosymmetric phase.

Figure 7

Normalized temperature dependent dielectric constant, both real and imaginary, as a function of temperature.

Figure 8

A comparison of the dielectric constant and derivative of heat capacity with respect to temperature. Note that the respective peak positions at 42 and 38 K almost match, indicating the correlation of the heat capacity with the dielectric phase transition.