The molecular coordination complex ${\mathrm{NiI}}_2{(3,5-\mathrm{lut})}_4$ [where (3,5-lut) $=$ (3,5-lutidine) $=({\mathrm{C}}_7{\mathrm{H}}_9\mathrm{N}$)] has been synthesized and characterized by several techniques including synchrotron x-ray diffraction, electron-spin resonance, superconducting quantum interference device magnetometry, pulsed-field magnetization, inelastic neutron scattering, and muon spin relaxation. Templated by the configuration of 3,5-lut ligands the molecules pack in-registry with the Ni–$\mathrm{I}\cdots \mathrm{I}$–Ni chains aligned along the $c$ axis. This arrangement leads to an uncommon through-space $\mathrm{I}\cdots \mathrm{I}$ magnetic coupling which is directly measured in this work. The net result is a near-ideal realization of the $S=1$ Haldane chain with $J=17.5\mathrm{K}$ and energy gaps of ${\mathrm{\Delta }}^{\parallel }=5.3\mathrm{K} {\mathrm{\Delta }}^{\perp }=7.7\mathrm{K}$, split by the easy-axis single-ion anisotropy $D=-1.2\mathrm{K}$. The ratio $D/J=-0.07$ affords one of the most isotropic Haldane systems yet discovered, while the ratio ${\mathrm{\Delta }}_0/J=0.40\left(1\right)$ (where ${\mathrm{\Delta }}_0$ is the average gap size) is close to its ideal theoretical value, suggesting a very high degree of magnetic isolation of the spin chains in this material. The Haldane gap is closed by orientation-dependent critical fields ${\mu }_0{H}_{\mathrm{c}}^{\parallel }=5.3\mathrm{T}$ and ${\mu }_0{H}_{\mathrm{c}}^{\perp }=4.3\mathrm{T}$, which are readily accessible experimentally and permit investigations across the entirety of the Haldane phase, with the fully polarized state occurring at ${\mu }_0{H}_{\mathrm{s}}^{\parallel }=46.0\mathrm{T}$ and ${\mu }_0{H}_{\mathrm{s}}^{\perp }=50.7\mathrm{T}$. The results are explicable within the so-called fermion model, in contrast to other reported easy-axis Haldane systems. Zero-field magnetic order is absent down to $20\mathrm{mK}$ and emergent end-chain effects are observed in the gapped state, as evidenced by detailed low-temperature measurements.

• Received 30 August 2019
• Revised 18 November 2019

DOI:https://doi.org/10.1103/PhysRevResearch.2.013082

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Published by the American Physical Society