Peculiar Crystal Structure and Strong-Rung Spin Ladders in KCu2BiO2(SO4)2
Larisa V Shvanskaya1,2, Tatiana D Bushneva1, Dmitry A Chareev3
1Moscow State University, Moscow 119991, Russia.
None:
KCu2BiO2(SO4)2 has been grown by chemical vapor transport and characterized by single-crystal X-ray diffraction. It crystallizes in the orthorhombic Pnma space group with a = 22.8761(8), b = 5.1961(2), c = 7.2083(2) Å, V = 856.83(5) Å3, Z = 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 Tmax ∼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 (J' = 264 K) and leg (J = 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 J' = 205 and J = 117 K, yielding an energy gap of Δ = 121 K. The ground state of KCu2BiO2(SO4)2 is a robust quantum spin liquid.
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