Peculiar Crystal Structure and Strong-Rung Spin Ladders in KCu ₂ BiO ₂ (SO ₄ ) ₂

KCu₂BiO₂(SO₄)₂ has been grown by chemical vapor transport and characterized by single-crystal X-ray diffraction. It crystallizes in the orthorhombic <i>Pnma</i> space group with <i>a</i> = 22.8761(8), <i>b</i> = 5.1961(2), <i>c</i> = 7.2083(2) Å, <i>V</i> = 856.83(5) ų, <i>Z</i> = 4. Its peculiar crystal structure is organized by heteropolyhedral layers built by edge- and vertex-sharing CuO₅ and BiO₈ polyhedra. Potassium cations are embedded between the layers, which are additionally reinforced by the SO₄ tetrahedra. The temperature dependence of magnetic susceptibility evidences a broad hump at <i>T</i>_max ∼122 K, accompanied by the Curie tail at lowest temperatures, while the temperature dependence of specific heat features no anomalies ascribable to the phase transition in the whole temperature range. Ab initio calculations demonstrate that the exchange interaction pattern in this material is highly nontrivial. A specific orbital ordering results in the strongest coupling being that of the third-nearest neighbors. This leads to the formation of a spin ladder, a structure that is not directly apparent from crystallographic considerations. There are strong antiferromagnetic interactions on both the rung (<i>J</i>' = 264 K) and leg (<i>J</i> = 140 K) of the ladder, which result in an energy gap of Δ = 161 K in the magnetic excitation spectrum. The Pade approximation of the experimental data provides somewhat lower values of <i>J</i>' = 205 and <i>J</i> = 117 K, yielding an energy gap of Δ = 121 K. The ground state of KCu₂BiO₂(SO₄)₂ is a robust quantum spin liquid.