Inelastic neutron scattering on three mixed-valence dodecanuclear polyoxovanadate clusters

The magnetic exchange interactions in the mixed-valence dodecanuclear polyoxovanadate compounds Na-4[V(8)(IV)V(4)(V)A(8)(III)O(40)(H2O)].23H(2)O, Na-4[(V8V4As8O40)-V-IV-As-V-O-III(D2O)].16.5D(2)O, and (NHEt3)(4)[(V8V4As8O40)-V-IV-As-V-O-III(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: (S) over cap (ex) = -2J(12)(xy)((S) over cap (1x)(S) over cap (2x) + (S) over cap (3x)(S) over cap (4x) + (S) over cap (1y)(S) over cap (2y) + (S) over bar (3y)(S) over bar (4y)) - 2J(12)(z)((S) over bar (1z)(S) over bar (2z) + (S) over bar (3z)(S) over bar (4z)) - 2J(23)(xy)((S) over bar (2x)(S) over bar (3x) + (S) over bar (1x)(S) over cap (4x)+ (S) over cap (2y)(S) over cap (3y)+ (S) over cap (1y)(S) over cap (4y)) - 2J(23)(z)((S) over cap (2z)(S) over cap (3z) + (S) over cap (1z)(S) over cap (4z)). The following sets of parameters were derived: for Na-4[V12As8O40(H2O)].23H(2)O, J(12)(xy) = J(12)(z) = -0.80 meV, J(23)(xy) = J(23)(z) = -0.72 meV; for Na-4[V12As8O40(D2O)].16.5D(2)O, J(12)(xy) = J(12)(z) = J(23)(xy) = J(23)(z) = -0.78 meV; for (NHEt3)(4)[V12As8O40(H2O)].H2O, J(12)(xy) = -0.80 meV, J(12)(z) = -0.82 meV, J(23)(xy) = -0.67 meV, J(23)(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.