Antiferroelectric antiferromagnetic type-I multiferroic Cu₉O₂(SeO₃)₄Cl₆

We report that Cu₉O₂(SeO₃)₄Cl₆ is a new multiferroic compound. Comprehensive studies of this compound have been carried out on single crystals as well as polycrystalline samples. Magnetic susceptibility c and specific heat C with H || c–axis both reveal an anomaly at T_N = 37 K associated with the long-range antiferromagnetic order. However, no signature of this phase transition is seen with the H || b-axis, providing evidence of the anisotropic nature of its magnetic properties. The results of c measurements are consistent with the temperature evolution of the intensity of magnetic reflections from the neutron diffraction experiments. The magnetic structure derived from neutron scattering measurements shows that half of the Cu(5) ions carrying no moments. Furthermore, an anomaly in the dielectric constant near T_N and observable magnetoelectric coupling below T_N are found. At high temperatures, a dielectric peak at T_E = 267 K is identified with no measurable spontaneous polarization below T_E, thereby suggesting an antiferroelectric order. A step-like anomaly seen in c(T) at T_E hints at weak spin-lattice coupling. Concomitantly, high-resolution temperature-dependent synchrotron x-ray diffraction experiments elucidate a local structural distortion at T_E.