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Abstract

The low-temperature magnetic properties and magnetic structure of fully dehydrated black dioptase Cu6Si6O18 have been studied by single-crystal neutron diffraction and magnetization measurements in magnetic fields up to 30 T. The intrachain J(c) and interchain J(ab) interactions as well as the anisotropy of the exchange coupling J(c) have been determined using inelastic neutron-scattering techniques. Zero-field antiferromagnetic order at T-N = 6.7 K can be described by a commensurate propagation vector of k = (0, 0, 3/2) with respect to the hexagonal R (3) over bar :H unit cell. The Cu magnetic moments are aligned antiferromagnetically along the c axis with about 12 degrees tilt and are coupled ferromagnetically between the chains. The high-field magnetization measurements provide strong evidence for the presence of a spin-flop phase above 8 T. We found that J(c) in black dioptase is significantly increased, while J(ab) is much weaker compared to its counterpart, green dioptase Cu-6[Si6O18] 6H(2)O. We suggest that black dioptase behaves like a nearly ideal S = 1/2 antiferromagnetic Heisenberg spiral chain with enhanced quantum fluctuations and weak spinon confinement J(ab)/J(c) similar to 0.02.