Observation of Haldane magnetism in organically templated vanadium phosphate (enH₂)_0.5VPO₄OH

We prepared an organically templated magnet, (enH₂)_0.5VPO₄OH (enH₂ = diprotonated ethylenediamine), hydrothermally and characterized its crystal structure by powder X-ray diffraction and Fourier-transform infrared spectroscopy, and its physical properties by magnetization, specific heat and nuclear magnetic resonance measurements and density functional theory calculations. (enH₂)_0.5VPO₄OH consists of uniform chains of V³⁺ (d², S = 1) ions and exhibits Haldane magnetism with spin gap Δ = 59.3 K from the magnetic susceptibility χ(T) at μ₀H = 0.1 T, which is reduced to 48.4 K at μ₀H = 9 T according to the ³¹P shift. The NMR data evidence the formation of a spin-glass state of unpaired S = 1/2 spins at T_S-G ≈ 3 K and indicate that the Haldane S = 1 spin chain segments are much longer in the organically templated magnet (enH₂)_0.5VPO₄OH than in the ammonium counterpart NH₄VPO₄OH. The single-ion anisotropy D and the interchain exchange J' in (enH₂)_0.5VPO₄OH and NH₄VPO₄OH were estimated in density functional calculations to find them very weak compared to the intrachain exchange J.