Colour, luminescence and bonding properties of tetrahedrally coordinated chromium(IV), manganese(V) and iron(VI) in various oxide ceramics

Color and Constitution of CrVI-Doped Bi2O3 Phases: The Structure of Bi14CrO24

“…EPR spectroscopy of the Cr4'-doped tetragonal solids yields a weak nearly isotropic signal, which further broadens on cooling. The g-value is slightly larger than 2.00, indicating unpaired electron density not bound to chromium, because otherwise g should be smaller than the spin-only value (11). Possibly low-lying impurity states are present, housing electrons, which are involved in a chargetransfer mechanism between Bi''' and Cr4'.…”

Valence Stabilization, Mixed Crystal Chemistry, and Electronic Transitions in Tetrahedral Oxo and Hydroxo Cr(IV), Mn(V), and Fe(VI) Clusters: A Theoretic Investigation

“…Energies of ligand-to-metal charge transfer transitions for tetra-oxo and tetra-hydroxo clusters of Cr'4, Mn4, and Fe4' are obtained using CI, A HF-SCF orbitals, and an [1e 2e] active space for the A (e) state and orbitals resulting from averaging the second-order density matrix (MCSCF runs) over 3t (¸)N2e(3d) and 1t (¸)N2e(3d) charge transfer excitations (active spaces [3t (¸)2e(3d)] and [1t (¸)2e(3d)], respectively) for metalto-ligand charge transfer states. Transition energies are listed and compared with experimental band maxima positions from literature (17) in Table 5. In order to study the effect of charge transfer states on multiplets of d type we have performed comparative CASSCF calculations extending the active spaces from [2e(3d)] to [1e(¸)2e(3d)] (for the A , E, and A states) and from [2e(3d) 4t…”

“…Such contributions are frequently neglected when treating vibronic problems, discussions usually being restricted to one-electronic operators and matrix elements (vibronic constants) (35). [17] with H(x) and S(x) being the mixing enthalpy and entropy given by [19] have to be modified correspondingly. According to Eqs.…”

“…According to Eqs. [17], and [19] (with S(x) being always positive and reaching a maximum at x"0.5), isomorphic mixing is tolerated by entropy because it increases the disorder in the system. In the following discussion we concentrate on H(x).…”

“…While orbital contributions from ligand (2s, 2p) and metal (3d) functions to the highest occupied molecular orbitals of e symmetry may vary in a wide range for a given electronic configuration, transfer of electrons from a nonbonding ligand, t and t , to antibonding metal e and t orbitals (usually encountered as charge transfer bands in the optical spectra for species in stable oxidation states) may take place in the ground state and break the valence form. Preparation and characterization of Cr'4, Mn4, and Fe4' valence forms in solids present a real challenge for inorganic chemists (17). It is therefore of fundamental interest to study the factors that affect the stabilization of these oxidation states when the respective cations isomorphically substitute, such as Ge'4, Si'4 (Cr'4), P4, As4, V4, (Mn4), and S4', Cr4', Se4' (Fe4'), and further how the character of metal-ligand bonding changes from left to right in the 3d transition metal isoelectronic (d) series.…”