The crystal growth and properties of novel magnetic double molybdate RbFe5_{5}(MoO4_{4})7_{7} with mixed Fe3+^{3+}/Fe2+^{2+}states and 1D negative thermal expansion

Single crystals of new compound RbFe$_5$(MoO$_4$)$_7$ were successfully grown by the flux method, and their crystal structure was determined using the X-ray single-crystal diffraction technique. The XRD analysis showed that the compound crystallizes in the monoclinic space group P21/m, with unit cell parameters a = 6.8987(4), b = 21.2912(12) and c = 8.6833(5) Å, β = 102.1896(18)°, V = 1246.66(12) Å$^3$, Z (molecule number in the unit cell) = 2, R-factor (reliability factor) = 0.0166, and T = 293(2) K. Raman spectra were collected on the single crystal to show the local symmetry of MoO$_4$ tetrahedra, after the confirmation of crystal composition using energy dispersive X-ray spectroscopy (EDS). The polycrystalline samples were synthesized by a solid-state reaction in the Ar atmosphere; the particle size and thermal stability were investigated by scanning electron microscopy (SEM) and differential scanning calorimetry (DSC) analyses. The compound decomposes above 1073 K in an Ar atmosphere with the formation of Fe(III) molybdate. The thermal expansion coefficient along the c direction has the value α = −1.3 ppm K$^{−1}$ over the temperature range of 298–473 K. Magnetic measurements revealed two maxima in the magnetization below 20 K, and paramagnetic behavior above 50 K with the calculated paramagnetic moment of 12.7 μB per formula unit is in good agreement with the presence of $_3$Fe$^{3+}$ and $_2$Fe$^{3+}$ in the high-spin (HS) state. The electronic structure of RbFe5(MoO4)7 is comparatively evaluated using X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) calculations

Anti-Perovskite Li-Battery Cathode Materials

Through single-step solid-state reactions, a series of novel bichalcogenides with the general composition (Li2Fe)ChO (Ch = S, Se, Te) are successfully synthesized. (Li2Fe)ChO (Ch = S, Se) possess cubic anti-perovskite crystal structures, where Fe and Li are completely disordered on a common crystallographic site (3c). According to Goldschmidt calculations, Li+ and Fe2+ are too small for their common atomic position and exhibit large thermal displacements in the crystal structure models, implying high cation mobility. Both compounds (Li2Fe)ChO (Ch = S, Se) were tested as cathode materials against graphite anodes (single cells); They perform outstandingly at very high charge rates (270 mA g(-1), 80 cycles) and, at a charge rate of 30 mA g(-1), exhibit charge capacities of about 120 mA h g(-1). Compared to highly optimized Li1-xCoO2 cathode materials, these novel anti-perovskites are easily produced at cost reductions by up to 95% and, yet, possess a relative specific charge capacity of 75%. Moreover, these iron-based anti-perovskites are comparatively friendly to the environment and (Li2Fe)ChO (Ch = S, Se) melt congruently; the latter is advantageous for manufacturing pure materials in large amounts