Single-crystal growth, structural characterization, and physical properties of a decorated square-kagome antiferromagnet KCu$_{7}$TeO$_{4}$(SO$_{4}$)$_{5}$Cl

The square-kagome lattice (SKL) antiferromagnet, composed of two-dimensional corner-sharing triangles, provides a prominent platform for studying frustrated magnetism. However, material realizations of the SKL 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 compound featuring a distorted and decorated SKL. Weak anomalies at temperature near 4 K are observed in both the magnetization and the specific heat data, indicating the onset of a magnetic transition. The observation of the $^{35}$Cl nuclear magnetic resonance line splits below 4.5 K further confirms the formation of the long-range-order antiferromagnetism. Magnetic susceptibility data reveal nearly isotropic Curie-Weiss temperatures (∼−145K) and 𝑔 factors (∼2.4) for both in-plane and out-of-plane magnetic fields. Moreover, we observe two successive ferroelectric transitions at 𝑇$_{FE}$⁢1∼30K and 𝑇$_{FE}$⁢2∼27K, driven by inversion symmetry breaking, most likely associated with distortions in the Cu⁢$_2$⁢O$_4$⁢Cl$_1$ pyramids and the adjacent SO$_4$ tetrahedra. ... mehrThese results suggest that a three-dimensional model incorporating interlayer couplings via decorating Cu$_2$ sites is essential for capturing the magnetic and electric behaviors in KCu$_7$⁢TeO$_4$⁢(SO$_4$)$_5$⁢Cl.