Observation of Haldane magnetism in organically templated vanadium phosphate ( en H 2 ) 0.5 VPO 4 OH.

We prepared an organically templated magnet, ( en H 2 ) 0.5 VPO 4 OH ( en H 2 = 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. ( en H 2 ) 0.5 VPO 4 OH consists of uniform chains of V 3+ (d 2 , S = 1) ions and exhibits Haldane magnetism with spin gap Δ = 59.3 K from the magnetic susceptibility χ ( T ) at μ 0 H = 0.1 T, which is reduced to 48.4 K at μ 0 H = 9 T according to the 31 P 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 ( en H 2 ) 0.5 VPO 4 OH than in the ammonium counterpart NH 4 VPO 4 OH. The single-ion anisotropy D and the interchain exchange J ' in ( en H 2 ) 0.5 VPO 4 OH and NH 4 VPO 4 OH were estimated in density functional calculations to find them very weak compared to the intrachain exchange J .