Reto Basler, Grégory Chaboussant, Andreas Sieber, Hanspeter Andres, Mark Murrie, Paul Kögerler, Hartmut Bögge, Debbie C. Crans, Erich Krickemeyer, Stefan Janssen, Hannu Mutka, Achim Müller and H. U. Güdel
Inelastic Neutron Scattering on Three Mixed-Valence Dodecanuclear Polyoxovanadate Clusters
Inorg. Chem. 41, 5675-5685 (2002)                 

Abstract: The magnetic exchange interactions in the mixed-valence dodecanuclear polyoxovanadate compounds Na₄[V^IV₈V^V₄As^III₈O₄₀(H₂O)]·23H₂O, Na₄[V^IV₈V^V₄As^III₈O₄₀(D₂O)]·16.5D₂O, and (NHEt₃)₄[V^IV₈V^V₄As^III₈O₄₀(H₂O)]·H₂O were investigated by an inelastic neutron scattering (INS) study using cold neutrons. In addition, the synthesis procedures and the single-crystal X-ray structures of these compounds have been investigated together with the temperature dependence of their magnetic susceptibilities. The magnetic properties below 100 K can be described by simply taking into account an antiferromagnetically exchange coupled tetramer, consisting of four vanadium(IV) ions. Up to four magnetic transitions between the cluster S = 0 ground state and excited states could be observed by INS. The transition energies and the relative INS intensities could be modeled on the basis of the following exchange Hamiltonian: _ex = -2J₁₂^xy (_1x2x + _3x4x + _1y2y + _3y4y) - 2J₁₂^z (_1z2z + _3z4z) - 2J₂₃^xy (_2x3x + _1x4x + _2y3y + _1y4y) - 2J₂₃^z (_2z3z + _1z4z). The following sets of parameters were derived: for Na₄[V₁₂As₈O₄₀(H₂O)]·23H₂O, J₁₂^xy = J₁₂^z = -0.80 meV, J₂₃^xy = J₂₃^z = -0.72 meV; for Na₄[V₁₂As₈O₄₀(D₂O)]·16.5D₂O, J₁₂^xy = J₁₂^z = J₂₃^xy = J₂₃^z = -0.78 meV; for (NHEt₃)₄[V₁₂As₈O₄₀(H₂O)]·H₂O, J₁₂^xy = -0.80 meV, J₁₂^z = -0.82 meV, J₂₃^xy = -0.67 meV, J₂₃^z = -0.69 meV. This study of the same {V₁₂As₈}-type cluster in three different crystal environments allows us to draw some conclusions concerning the applicability on INS in the area of nondeuterated molecular spin clusters. In addition, the effects of using nondeuterated samples and different sample container shapes for INS were evaluated.