Low-temperature magnetocaloric effect of the polyoxovanadate molecular magnet {VIV/V12As8}: An experimental study

•Experimental study of polyoxovanadate molecular magnet is reported.•Magnetic data analysis reveals a contribution from the off-center V4 units.•Isothermal magnetic entropy change is determined for high field-change values.•Rigorous estimation of the systematic errors in entropy change is performed. Magnetic measurements have been performed for the compound (NH(C2H5)3)4[VIV8VV4AsIII8 O40(H2O)]·H2O compound in order to determine its magnetocaloric effect in terms of the isothermal entropy change and compare it to the theoretical prediction based on the previously developed model. A closer inspection of the experimental magnetization and susceptibility data revealed that this model is incomplete neglecting a contribution from the off-center V4 subsystems, where the vanadium ions are in the mixed valence state. The probability that two valence electrons residing within the two off-center V4 subsystems are magnetically decoupled was estimated to be equal to 2 – 3 %. For most of the time they turn out to be magnetically correlated by the high antiferromagnetic superexchange interaction. The magnetocaloric effect calculated with the full model accounting for the off-center contribution was found to agree satisfactorily with the values derived from the measured magnetization data. A valuable byproduct of the analysis is the rigorous estimation of the errors in the isothermal entropy change due to the necessary numerical approximations.