Single-crystal growth, structural characterization, and physical properties of a decorated square-kagome antiferromagnet KCu$_7$TeO$_4$(SO$_4$)$_5$Cl

TL;DR

This work demonstrates the successful growth and comprehensive characterization of the decorated square-kagome antiferromagnet KCu$_7$TeO$_4$(SO$_4$)$_5$Cl. Using chemical vapor transport to grow single crystals and a suite of techniques including SC-XRD, magnetization, specific heat, dielectric, pyroelectric, and $^{35}$Cl NMR, the authors establish a distorted square-kagome Cu network decorated by interlayer Cu2 sites. They observe long-range AFM order below $T_ ext{N}\sim4.5$ K, nearly isotropic $\theta_ ext{CW}\approx-145$ K and $g\approx2.4$, alongside two ferroelectric transitions at $T_ ext{FE1}\sim30$ K and $T_ ext{FE2}\sim27$ K, driven by inversion-symmetry breaking, with NMR confirming static order and no metamagnetism up to 7 T in the single crystals. The results highlight the importance of interlayer decoration and support a three-dimensional modeling of magnetism and ferroelectricity in decorated square-kagome systems, offering new insights for frustrated quantum magnetism.

Abstract

The square-kagome lattice, composed of two-dimensional corner-sharing triangles, provides a novel platform for studying frustrated magnetism. However, material realizations of the square-kagome lattice remain scarce. Here, we report the single-crystal growth, structural characterization, magnetic and electric properties of KCu$_7$TeO$_4$(SO$_4$)$_5$Cl, a nabokoite-type compound featuring a distorted and decorated square-kagome lattice. Weak anomalies near 4 K are observed in both magnetization and specific heat, indicating the onset of a magnetic transition.The formation of a long-range antiferromagnetic state below 4.5 K is further confirmed by $^{35}$Cl nuclear magnetic resonance (NMR) measurements. Magnetic susceptibility data reveal nearly isotropic Curie-Weiss temperatures ($\sim-145$ K) and $g$-factors ($\sim2.4$) for both in-plane and out-of-plane magnetic fields. Moreover, we observe two successive ferroelectric transitions at $T_\mathrm{FE1}\sim30$ K and $T_\mathrm{FE2}\sim27$ K, driven by inversion-symmetry breaking, most likely associated with distortions in the Cu2O$_4$Cl$_1$ pyramids and the adjacent SO$_4$ tetrahedra. These results suggest that a three-dimensional model incorporating interlayer couplings via decorating sites is essential for capturing the magnetic and electric behaviors in KCu$_7$TeO$_4$(SO$_4$)$_5$Cl.

Figures (5)

• Figure 1: The square kagome lattice in KCu$_7$TeO$_4$(SO$_4$)$_5$Cl: (a) Idealized theoretical picture of the SKL lattice. (b) Perspective top-down view of the SKL plaquettes featuring Cu1 square and Cu1-Cu3-Cu1 triangular motifs, highlighting the coordination between copper atoms in the lattice. Bond distances $d_1$, $d_2$, and $d_3$ are outlined. The Cu2 atoms are positioned above and below the distorted SKL plaquettes. (c) Perspective side view, illustrating how the SKL Cu network is corrugated, with the Cu1-Cu3-Cu1 triangles alternating between pointing upwards and downwards, and the Cu2 atoms positioned above and below the Cu1 squares. Bond distances $d_4$, $d_5$, $d_6$, and $d_7$ are outlined, and the distance $d_8$ of Cu2 to the Cu1 atom situated in the neighboring corrugated SKL layer (not shown) is indicated schematically. (d) The complete KCu$_7$TeO$_4$(SO$_4$)$_5$Cl unit cell, including all present atoms and the most prominent coordination polyhedra. For clarity, the atomic positions in this figure have been shifted by $-\frac{1}{4}$, $\frac{1}{4}$, 0 relative to the refined ones in Tables \ref{['Table_XRD']} and \ref{['Table_XRD2']}. (e) Representative pale green, millimeter-sized single crystals. Structural properties are visualized using VESTAMommadb5098.
• Figure 2: (a) and (b) Temperature dependence of the magnetic susceptibility measured for a KCu$_7$TeO$_4$(SO$_4$)$_5$Cl single crystal with $H\parallel ab$ and $H\parallel c$, respectively. Red and black curves represent field-cooled (FC) and zero-field-cooled (ZFC) measurements. Insets in (a) and (b) show enlarged view at low temperatures near the transitions at $T^*$ and $T_\mathrm{N}$. (c) and (d) Theoretical fitting (red lines) of the magnetic susceptibility using the Husimi ansatz shown in Eq. \ref{['eq1']}. (e) and (f) Low-temperature magnetic susceptibility measured in various external magnetic fields. (g) and (h) Isothermal magnetization measured at selected temperatures. Insets in (g) and (h): Derivative of the magnetization obtained at 2 K.
• Figure 3: NMR spectra and spin-lattice relaxation rate $1/T_1$. (a,b) $^{35}$Cl spectra measured at various temperatures with fields of 5 T and 10 T applied along the $c$ axis. Data are shifted vertically for clarity. (c) $1/T_1$ as functions of temperatures measured under fields of 5 T and 10 T under two field orientations as labeled.
• Figure 4: (a) Specific heat of a KCu$_7$TeO$_4$(SO$_4$)$_5$Cl single crystal. The red line is a theoretical fitting to the phonon specific heat using the Debye Einstein model shown in Eq. \ref{['eq3']}. Inset in (a): Low-temperature specific heat near the transitions at $T^*$ and $T_\mathrm{N}$ in 0 and 14 T ($H\parallel c$). The curve for 14 T has been shift vertically for clarity. (b) Low-temperature specific heat divided by temperature $C_\mathrm{p}/T$, showing clear transitions at $T_\mathrm{FE1}$, $T_\mathrm{FE2}$ and $T^*$. (c) Excess specific heat ($C_\mathrm{ex}$) and the corresponding entropy ($S_\mathrm{ex}$). The excess specific heat is obtained by subtracting the total specific heat from the estimated phonon background shown in (a).
• Figure 5: (a) and (b) Temperature dependence of the real ($\varepsilon'$) and imaginary ($\varepsilon"$) parts of the relative dielectric constant measured using three frequencies. (c) and (d) Temperature-dependent pyroelectric current ($I_\mathrm{p}$) and electric polarization ($P$) measured using opposite electric fields ($E\parallel c=\pm10$ KV cm$^{-1}$). Background current measured in zero electric field is also presented for comparison. (e) $P-E$ hysteresis loops measured at 20 and 40 K.