We have investigated the spin and oxidation states of Co in ${\mathrm{Co}}_3{\mathrm{BO}}_5$ using x-ray magnetic circular dichroism (XMCD) and dc magnetic susceptibility measurements. At low temperatures, XMCD experiments have been performed at the Co $K$-edge in ${\mathrm{Co}}_3{\mathrm{BO}}_5$ and ${\mathrm{Co}}_2{\mathrm{FeBO}}_5$ single crystals in the fully ferrimagnetically ordered phase. The Co ($K$-edge) XMCD signal is found to be related to the ${\mathrm{Co}}^{2+}$ magnetic sublattices in both compounds, providing strong experimental support for the low-spin (LS) ${\mathrm{Co}}^{3+}$ scenario. The paramagnetic susceptibility is highly anisotropic. An estimation of the effective magnetic moment in the temperature range 100–250 K correlates well with two ${\mathrm{Co}}^{2+}$ ions in the high-spin (HS) state and some orbital contribution, while ${\mathrm{Co}}^{3+}$ remains in the LS state. The crystal structure of the ${\mathrm{Co}}_3{\mathrm{BO}}_5$ single crystal has been solved in detail at the $T$ range 296–703 K. The unit cell parameters and volume show anomalies at 500 and 700 K. The octahedral environment of the Co4 site strongly changes with heating. The generalized gradient approximation with Hubbard $U$ correction calculations have revealed that, at low-temperatures, the system is insulating with a band gap of 1.4 eV, and the ${\mathrm{Co}}^{2+}$ ions are in the HS state, while ${\mathrm{Co}}^{3+}$ are in the LS state. At high temperatures ($T$ > 700 K), the charge ordering disappears, and the system becomes metallic with all Co ions in $3d^7$ electronic configuration and HS state.

The kinetics of the solid-state reaction between nanolayers of polycrystalline copper and amorphous silicon (a-Si) has been studied in a Cu/a-Si thin-film system by the methods of electron diffraction and simultaneous thermal analysis (STA), including the methods of differential scanning calorimetry (DSC) and thermogravimetry (TG). It has been established that, in the solid-state reaction, two phases are formed in a sequence: Cu + Si → η″-Cu₃Si → γ-Cu₅Si. It has been shown that the estimated values of the kinetic parameters of the formation processes for the phases η″-Cu₃Si and γ-Cu₅Si, obtained using electron diffraction, are in good agreement with those obtained by DSC. The formation enthalpy of the phases η″-Cu₃Si and γ-Cu₅Si has been estimated to be: ΔH_η″-Cu3Si = −12.4 ± 0.2 kJ/mol; ΔH_γ-Cu5Si = −8.4 ± 0.4 kJ/mol. As a result of the model description of the thermo-analytical data, it has been found that the process of solid-state transformations in the Cu/a-Si thin-film system under study is best described by a four-stage kinetic model R3 → R3 → (Cn-X) → (Cn-X). The kinetic parameters of formation of the η″-Cu₃Si phase are the following: E_a = 199.9 kJ/mol, log(A, s⁻¹) = 20.5, n = 1.7; and for the γ-Cu₅Si phase: E_a = 149.7 kJ/mol, log(A, s⁻¹) = 10.4, n = 1.3, with the kinetic parameters of formation of the γ-Cu₅Si phase being determined for the first time.