Аннотация:The magnetic subsystem of nabokoite, KCu7(TeO4)(SO4)5Cl, is constituted by buckled square kagome lattice of copper decorated by quasi-isolated Cu2+ ions. This combination determines peculiar physical properties of this compound evidenced in electron spin resonance (ESR) spectroscopy, dielectric permittivity ε, magnetization M and specific heat C_p measurements. At lowering temperature, the magnetic susceptibility χ = M/H passes through broad hump at about 150 K inherent for low-dimensional magnetic systems and evidences sharp peak at antiferromagnetic phase transition at T_N = 3.2 K. The C_p(T) curve also exhibits sharp peak at T_N readily suppressed by magnetic field and additional peak-like anomaly at T_peak = 5.7 K robust to magnetic field. The latter can be ascribed to low-lying singlet excitations filling the singlet-triplet gap in magnetic excitation spectrum of the square kagome lattice [J. Richter, O. Derzhko and J. Schnack, Phys. Rev. B 105, 144427 (2022)]. According to position of T_peak, the leading exchange interaction parameter J in nabokoite is estimated to be about 60 K. ESR spectroscopy provides indications that antiferromagnetic structure below TN is non-collinear. These complex thermodynamic and resonant properties signal the presence of two weakly coupled magnetic subsystems in nabokoite, namely spin-liquid with large singlet-triplet gap and antiferromagnet represented by decorating ions. Separate issue is the observation of antiferroelectric-type behavior in ε at low temperatures, which tentatively reduces the symmetry and partially lifts frustration of
magnetic interactions of decorating copper ions with buckled square kagome lattice.
