SrCu3V2O8(OH)2 – dynamic Jahn–Teller distortions and orbital frustration in a new S = ½ kagome antiferromagnet

In this article we report the synthesis of a new model S = ½ KAFM, SrCu3V2O8(OH)2, the Sr2+ analogue of vesignieite created by substitution of Ba2+ by isomagnetic Sr2+, hereafter referred to as ‘Sr-vesignieite’. Like many other kagome magnets, the magnetic moments in ‘Sr-vesignieite’ are hosted by Jahn–Teller distorted Cu2+ ions, which possess orbital degeneracy. Rather than causing a static and coherent distortion to the crystal structure, as might have been predicted, our studies of the crystallographic and bulk magnetic properties of ‘Sr-vesignieite’ evidence orbital fluctuations and orbital-ordering transitions. The mediating role of orbitals in superexchange leads to a natural coupling of the spin and orbital degrees-of-freedom, which introduces orbital-driven Ising-type modulations to the magnetic Hamiltonian. Further, we find the crystallographic signature of the dynamic Jahn–Teller effect in the other model KAFMs, indicating that the orbital fluctuations and phase transitions are more common than previously thought. It follows that behavior currently ascribed to the quantum behavior of spins may in part be due to spin–orbital fluctuations.