STRUCTURAL AND MAGNETIC-PROPERTIES OF (CH3)4NNI(NO2)3 - A HALDANE-GAP SYSTEM

We report investigations on the structure and the magnetic properties of (CH3)4NNi(NO2)3. From extended x-ray-absorption fine structure and x-ray-diffraction experiments, we show that (CH3)4NNi(NO2)3 has a chainlike structure of Ni(II) ions coupled via three NO2 anions. High-temperature susceptibility is consistent with one-dimensional antiferromagnetism with J/k = -12 K and g = 2.25. Below 10 K, a strong decrease of susceptibility and proton relaxation rate is observed, in agreement with the Haldane prediction of a singlet ground state for the S = 1, one-dimensional Heisenberg antiferromagnet. From the magnetization-versus-field data at low temperature, the Haldane gap is estimated to 4.5 K, close to the expected value 0.41 (J) for the S = 1, one-dimensional Heisenberg antiferromagnet.