Orbital arrangements and magnetic interactions in the quasi-one-dimensional cuprates A CuMoO₄(OH) (A = Na, K)

A new spin-1/2 quasi-one-dimensional antiferromagnet KCuMoO₄(OH) is prepared by the hydrothermal method. The crystal structures of KCuMoO₄(OH) and the already-known Na-analogue NaCuMoO₄(OH) are isotypic, comprising chains of Cu²⁺ ions in edge-sharing CuO₄(OH)₂ octahedra. Despite the structural similarity, their magnetic properties are quite different because of the different arrangements of d_x²-y² orbitals carrying spins. For NaCuMoO₄(OH), d_x²-y² orbitals are linked by superexchange couplings via two bridging oxide ions, which gives a ferromagnetic nearest-neighbor interaction J₁ of â51 K and an antiferromagnetic next-nearest-neighbor interaction J₂ of 36 K in the chain. In contrast, a staggered d_x²-y² orbital arrangement in KCuMoO₄(OH) results in superexchange couplings via only one bridging oxide ion, which makes J₁ antiferromagnetic as large as 238 K and J₂ negligible. This comparison between the two isotypic compounds demonstrates an important role of orbital arrangements in determining the magnetic properties of cuprates.