Les hypovanadates de magnésium MgVO3 et MgV2O5. Structure cristalline de MgVO3

Alkaline-Earth Metal Effects on Physical Properties of Ferromagnetic AVO3 (A = Ba, Sr, Ca, and Mg): Density Functional Theory Insights

The effects of alkaline-earth metals on electronic, optical, thermodynamic, and physical properties of ferromagnetic AVO₃ (A = Ba, Sr, Ca, and Mg) have been investigated by first-principles calculations within the GGA+U formalism based on density functional theory. The optimized structural parameters are in good agreement with the available experimental results that evaluate the reliability of our calculations. The cell and mechanical stability is discussed using the formation energy and Born stability criteria, respectively. The mechanical behaviors of AVO₃ are discussed on the basis of the results of elastic constants, elastic moduli, Peierls stress, and Vickers hardness. The nature of the ductile-brittle transition of AVO₃ compounds was confirmed by the values of Pugh's ratio, Poisson's ratio, and Cauchy pressure. The electronic band structures, as well as density of states, reveal the half-metallic behavior of BaVO₃ and SrVO₃. However, CaVO₃ and MgVO₃ exhibit spin-gapless and magnetic semiconductor characteristics, respectively. The microscopic origin of the transition from the half-metallic to semiconductor nature of AVO₃ is rationalized using electronic properties. The presence of covalent, ionic, and metallic bonds in AVO₃ compounds is found by the analysis of bonding properties. The single-band nature of half-metallic AVO₃ is seen by observing hole-like Fermi surfaces in this study. Furthermore, the various thermodynamic and optical properties are calculated and analyzed. The refractive index suggests that AVO₃ could be a potential candidate for applications to high-density optical data storage devices.

Synthesis and Characterization of Magnesium Vanadates as Potential Mg-ion Cathode Materials Through an Ab Initio Guided Carbothermal Reduction Approach

Many technologically relevant transition metal oxides for advanced energy storage and catalysis feature reduced transition metal (TM) oxides and are often nontrivial to prepare because of the need to control the reducing nature of the atmosphere in which they are synthesized. In this work, we show that an ab initio predictive synthesis strategy can be used to produce multiple gram-scale products of various MgV x O y -type phases (δ-MgV 2 O 5 , spinel MgV 2 O 4 , and MgVO 3 ) containing V 3+ or V 4+ relevant for Mg-ion battery cathodes. Characterization of these phases using 25 Mg solid-state NMR spectroscopy illustrates the potential of 25 Mg NMR for studying reversible magnesiation and local charge distributions. Rotor-Assisted Population Transfer is used as a much needed signal-to-noise enhancement technique. The ab initio guided synthesis approach is seen as a step forward towards a predictive synthesis strategy for targeting specific complex TM oxides with variable oxidation states of technological importance beyond Mg-ion (and indeed Li-ion) chemistry.

Spin-Gap Formation and Thermal Structural Studies in Reduced Hybrid Layered Vanadates

Reduced layered M(C4H4N2)V4O10 ((I, M = Co; II, M = Ni; III, M = Zn); C4H4N2 = pyrazine, pyz) hybrid solids were synthesized via hydrothermal reactions at 200-230 degrees C, and their structures determined by single-crystal X-ray diffraction (Cmcm, No. 63, Z = 4; a = 14.311(2), 14.2372(4), 14.425(1) A; b = 6.997(1), 6.9008(2), 6.9702(6) A; and c = 11.4990(8), 11.5102(3), 11.479(1) A; for I, II, and III, respectively). All three solids are isostructural and contain V4O102- layers condensed from edge- and corner-shared VO5 square pyramids. A single symmetry-unique V atom is reduced by 1/2 electron (on average) and bonds via its apical oxygen atom to interlayer Mpyz2+ chains. Magnetic susceptibility measurements show a strong temperature dependence and a Curie constant that is consistent with two randomly localized spins per V4O10(2-) formula for III. Further, the unusual discovery of a remarkably well-defined transition to a singlet ground state, as well as formation of a spin gap, is found for III at 22(1) K. The temperature-dependent electrical conductivities show apparent activation energies of 0.36 (I), 0.46 (II), and 0.59 eV (III). During heating cycles in flowing N2, the samples exhibit weight losses corresponding to the removal of predominantly pyrazine, pyrazine fragments, and CO2 via reaction of pyrazine with the vanadate layer. The complete removal of pyrazine without loss of crystallinity is found for well-ground samples of I and III. The SEM images of I and II after heating at 400-500 degrees C show relatively intact crystals, but at 600 degrees C further structural collapse results in the formation of macropores at the surfaces.