Multifunctional molecular magnets: Magnetocaloric effect in octacyanometallates

Abstract

Octacyanometallate-based compounds displaying a rich pallet of interesting physical and chemical properties, are key materials in the field of molecular magnetism. The [M(CN)8]n− complexes, (M = WV, MoV, NbIV), are universal building blocks as they lead to various spatial structures, depending on the surrounding ligands and the choice of the metal ion. One of the functionalities of the octacyanometallate-based coordination polymers or clusters is the magnetocaloric effect (MCE), consisting in a change of the material temperature upon the application of a magnetic field. In this review, we focus on different approaches to MCE investigation. We present examples of magnetic entropy change ∆Sm and adiabatic temperature change ∆Tad, determined using calorimetric measurements supplemented with the algebraic extrapolation of the data down to 0 K. At the field change of 5T, the compound built of high spin clusters Ni9[W(CN)8]6 showed a maximum value of −∆Sm equal to 18.38 J·K−1 mol−1 at 4.3 K, while the corresponding maximum ∆Tad = 4.6 K was attained at 2.2 K. These values revealed that this molecular material may be treated as a possible candidate for cryogenic magnetic cooling. Values obtained for ferrimagnetic polymers at temperatures close to their magnetic ordering temperatures, Tc, were lower, i.e., −∆Sm = 6.83 J·K−1 mol−1 (∆Tad = 1.42 K) and −∆Sm = 4.9 J·K−1 mol−1 (∆Tad = 2 K) for {[MnII (pyrazole)4]2[NbIV(CN)8]·4H2O}n and{[FeII (pyrazole)4]2[NbIV(CN)8]·4H2O}n, respectively. MCE results have been obtained also for other-[Nb(CN)8]-based manganese polymers, showing significant Tc dependence on pressure or the remarkable magnetic sponge behaviour. Using the data obtained for compounds with different Tc, due to dissimilar ligands or other phase of the material, the ∆Sm ~ Tc−2/3 relation stemming from the molecular field theory was confirmed. The characteristic index n in the ∆Sm ~ ∆ Hn dependence, and the critical exponents, related to n, were determined, pointing to the 3D Heisenberg model as the most adequate for the description of these particular compounds. At last, results of the rotating magnetocaloric effect (RMCE), which is a new technique efficient in the case of layered magnetic systems, are presented. Data have been obtained and discussed for single crystals of two 2D molecular magnets: ferrimagnetic {MnII (R-mpm)2]2[NbIV(CN)8]}·4H2O (mpm = α-methyl-2-pyridinemethanol) and a strongly anisotropic (tetren)Cu4[W(CN)8]4 bilayered magnet showing the topological Berezinskii-Kosterlitz-Thouless transition.