Decorated Honeycomb Lattice and Successive Magnetic Transitions of the Delafossite Derivative K₄Ni₅Te₃O₁₆

We report the synthesis and characterization of the delafossite derivative K₄Ni₅Te₃O₁₆, which hosts a decorated honeycomb Ni²⁺ lattice. Single-crystal X-ray diffraction, electron microscopy, and neutron diffraction establish a noncentrosymmetric Pmm2 crystal structure that combines regular honeycomb units with elongated motifs formed by Ni²⁺ trimers along extended edges. Magnetic susceptibility reveals strong antiferromagnetic interactions, while heat capacity measurements identify two successive magnetic transitions at 30 and 12 K. Neutron diffraction shows that these transitions correspond to a progression from partial to full long-range magnetic order on distinct Ni sites, reflecting competition among the various exchange interactions. The resulting magnetic point group, mm2.1′, inherits the lattice polarity and permits high-order magnetoelectric coupling. Consistent with this symmetry, a quadratic magnetic-field-induced polarization is experimentally observed below the magnetic ordering temperature, likely arising from exchange-driven magnetostriction coupled to the polar lattice. These results establish K₄Ni₅Te₃O₁₆ as an interesting platform for engineering B-site ordering in delafossite derivatives to realize complex quantum magnetism and functional responses.