Distorted Kagome Lattice Generated by a Unique Orbital Arrangement in the Copper Mineral KCu3As2O7(OH)3

We study polycrystalline samples of KCu 3 As 2 O 7 (OH) 3 , a new candidate spin-1/2 kagome antiferromagnet, by magnetic susceptibility and heat capacity measurements above 2 K. The unique arrangement of the 3 z 2 - r 2 and x 2 - y 2 orbitals on the Cu 2+ kagome net is noted and compared to the orbital patterns found in other kagome minerals. It is suggested that this orbital arrangement gives rise to one antiferromagnetic and two ferromagnetic interactions on isosceles triangles forming a highly distorted kagome lattice. KCu 3 As 2 O 7 (OH) 3 is found to show an antiferromagnetic long-range order at T N =7.2 K. Remarkably, a spin entropy is more gradually released upon cooling below T N compared with a conventional magnetic long-range order, which may originate from the geometrical frustration still present in this highly distorted kagome lattice.