Inelastic Neutron Scattering on Three Mixed-Valence Dodecanuclear Polyoxovanadate Clusters^†

The magnetic exchange interactions in the mixed-valence dodecanuclear polyoxovanadate compounds Na4[VIV8VV4AsIII8O40(H2O)]·23H2O, Na4[VIV8VV4AsIII8O40(D2O)]·16.5D2O, and (NHEt3)4[VIV8VV4AsIII8O40(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: Ĥex = −2J12xy(Ŝ1xŜ2x + Ŝ3xŜ4x + Ŝ1yŜ2y + Ŝ3yŜ4y) − 2J12z(Ŝ1zŜ2z + Ŝ3zŜ4z) − 2J23xy(Ŝ2xŜ3x + Ŝ1xŜ4x + Ŝ2yŜ3y + Ŝ1yŜ4y) − 2J23z(Ŝ2zŜ3z + Ŝ1zŜ4z). The following sets of parameters were derived: for Na4[V12As8O40(H2O)]·23H2O, J12xy = J12z = −0.80 meV, J23xy = J23z = −0.72 meV; for Na4[V12As8O40(D2O)]·16.5D2O, J12xy = J12z = J23xy = J23z = −0.78 meV; for (NHEt3)4[V12As8O40(H2O)]·H2O, J12xy = −0.80 meV, J12z = −0.82 meV, J23xy = −0.67 meV, J23z = −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.