Modeling of the Charge-Transfer-Induced Spin Transition in the Tetranuclear Cyanide-Bridged [Co2Fe2] Complex

The course of the charge-transfer-induced spin transition demonstrated by the cyanide-bridged tetranuclear [Co2Fe2(bpy*)4(CN)6(tp*)2](PF6)2·2CP·8BN complex has been followed by DFT calculations of the single-point energies for different total spin values of the complex in a wide temperature range. With the aid of these calculations, the picture of spin conversion, that the compound undergoes, has been restored. It has been demonstrated that at 100 K the two crystallographically unique tetranuclear Fe2Co2 subunits A and B present in the structure contain diamagnetic low-spin FeII and low-spin CoIII ions. From the three subunits A, A', and B detected crystallographically in the compound at 200 K, only the A ones contain paramagnetic low-spin FeIII and high-spin CoII ions, while at 260 K, a half of all clusters contained in the crystal are in this state. From the DFT calculations, it also follows that at 320 K in the crystal only paramagnetic units are present. The results obtained are in accordance with the experimental data on the magnetic susceptibility. A possibility to predict new materials exhibiting spin transitions on the basis of DFT calculations of single-point energies as functions of temperature is discussed.

Iridates from the molecular side

New exotic phenomena have recently been discovered in oxides of paramagnetic Ir⁴⁺ ions, widely known as ‘iridates'. Their remarkable properties originate from concerted effects of the crystal field, magnetic interactions and strong spin-orbit coupling, characteristic of 5d metal ions. Despite numerous experimental reports, the electronic structure of these materials is still challenging to elucidate, and not attainable in the isolated, but chemically inaccessible, [IrO₆]^8– species (the simplest molecular analogue of the elementary {IrO₆}⁸⁻ fragment present in all iridates). Here, we introduce an alternative approach to circumvent this problem by substituting the oxide ions in [IrO₆]⁸⁻ by isoelectronic fluorides to form the fluorido-iridate: [IrF₆]²⁻. This molecular species has the same electronic ground state as the {IrO₆}⁸⁻ fragment, and thus emerges as an ideal model for iridates. These results may open perspectives for using fluorido-iridates as building-blocks for electronic and magnetic quantum materials synthesized by soft chemistry routes.

Iridates are known to exhibit a range of exotic electronic and magnetic behaviours but it is difficult to prepare isolated [IrO₆]⁸⁻ species via soft chemical routes. Here, the authors isolate the isoelectronic [IrF₆]²⁻ complex, and assess it as a model and for iridate analogues.

Conductive hybrid carbon nanotube (CNT)–polythiophene coatings for innovative auditory neuron-multi-electrode array interfacing

Surface modification of platinum electrodes to improve neuron-electrode interface and electrode conductive properties in cochlear implants.

Microscopic theory of cooperative spin crossover: Interaction of molecular modes with phonons

In this article, we present a new microscopic theoretical approach to the description of spin crossover in molecular crystals. The spin crossover crystals under consideration are composed of molecular fragments formed by the spin-crossover metal ion and its nearest ligand surrounding and exhibiting well defined localized (molecular) vibrations. As distinguished from the previous models of this phenomenon, the developed approach takes into account the interaction of spin-crossover ions not only with the phonons but also a strong coupling of the electronic shells with molecular modes. This leads to an effective coupling of the local modes with phonons which is shown to be responsible for the cooperative spin transition accompanied by the structural reorganization. The transition is characterized by the two order parameters representing the mean values of the products of electronic diagonal matrices and the coordinates of the local modes for the high- and low-spin states of the spin crossover complex. Finally, we demonstrate that the approach provides a reasonable explanation of the observed spin transition in the [Fe(ptz)6](BF4)2 crystal. The theory well reproduces the observed abrupt low-spin → high-spin transition and the temperature dependence of the high-spin fraction in a wide temperature range as well as the pronounced hysteresis loop. At the same time within the limiting approximations adopted in the developed model, the evaluated high-spin fraction vs. T shows that the cooperative spin-lattice transition proves to be incomplete in the sense that the high-spin fraction does not reach its maximum value at high temperature.