UV-visible absorption and cathodoluminescence spectra of phase-pure epitaxial BiFeO3 thin films grown on SrTiO3(001) substrates by ultrahigh vacuum sputtering reveal a bandgap of 2.69–2.73eV for highly strained ∼70nm thick BiFeO3 films. This bandgap value agrees with theoretical calculations and recent experimental results of epitaxial BiFeO3 films, demonstrating only minimal bandgap change with lattice distortion. Both absorption and cathodoluminescence spectra show defect transitions at 2.20 and 2.45eV, of which the latter can be attributed to defect states due to oxygen vacancies.

General chemistry courses predominantly use expository experiments that shape student expectations of what a laboratory activity entails. Shifting within a semester to course-based undergraduate research activities that include greater decision-making, collaborative work, and “messy” real-world data necessitates a change in student expectations and epistemology. Design considerations for laboratories that transition students and teaching assistants from one format to another are described here along with experiences when implemented in the second semester of a large-enrollment general chemistry course.

Using a combination of UV-visible spectroscopy and electronic structure calculations, we have characterized the electronic structures and optical properties of AWO4 (A = Mn, Co, Ni, Cu, Zn, or Mg) tungstates with the wolframite structure. In MgWO4 and ZnWO4, the lowest energy optical excitation is a ligand to metal charge transfer (LMCT) excitation from oxygen 2p nonbonding orbitals to antibonding W 5d orbitals. The energy of the LMCT transition in these two compounds is 3.95 eV for ZnWO4 and 4.06 eV for MgWO4. The charge transfer energies observed for the other compounds are significantly smaller, falling in the visible region of the spectrum and ranging from 2.3 to 3.0 eV. In these compounds, the partially occupied 3d orbitals of the A(2+) ion act as the HOMO, rather than the O 2p orbitals. The lowest energy charge transfer excitation now becomes a metal-to-metal charge transfer (MMCT) excitation, where an electron is transferred from the occupied 3d orbitals of the A(2+) ion to unoccupied antibonding W 5d states. The MMCT value for CuWO4 of 2.31 eV is the lowest in this series due to distortions of the crystal structure driven by the d(9) configuration of the Cu(2+) ion that lower the crystal symmetry to triclinic. The results of this study have important implications for the application of these and related materials as photocatalysts, photoanodes, pigments, and phosphors.

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The structure and properties of Sr 1-x Ca x Ru 0.5 Mn 0.5 O 3 compositions have been investigated. Both bond distances and X-ray absorption measurements reveal Mn 3þ þ Ru 5þ T Mn 4þ þ Ru 4þ mixed valency across the entire series. Despite a complete lack of Ru/Mn chemical order, all samples magnetically order between 220 and 300 K. The characteristics and type of magnetic order are sensitive to the occupation and ordering of the Mn e g orbitals, which can be manipulated by changes in chemical pressure, via the Sr/Ca ratio. Sr-rich samples are tetragonally distorted by a cooperative Jahn-Teller distortion (CJTD) that leads to an elongation of the c-axis as well as antiphase rotations of the octahedra about the c-axis (a 0 a 0 ctilt system). The CJTD results from orbital ordering involving occupied d z 2 orbitals on Mn 3þ , which stabilize C-type antiferromagnetic order. For Sr-rich samples, the various oxidation states contribute in approximately equal proportions (i.e., Ru þ4.5 and Mn þ3.5 ). Substituting Ca 2þ for Sr 2þ triggers additional rotations of the octahedra (ab þ atilt system) that result in orthorhombic symmetry for Sr 1-x Ca x Mn 0.5 Ru 0.5 O 3 samples with x g 0.3. The crossover to orthorhombic symmetry is accompanied by the loss of orbital order and the emergence of an itinerant electron ferrimagnetic state. X-ray absorption near edge structure (XANES) measurements show that as the Ca 2þ content increases there is a shift in the valence degeneracy toward Mn 4þ þ Ru 4þ . This helps to explain the absence of orbital ordering in Ca-rich samples, as well as the crossover from antiferromagnetism to ferrimagnetism. Neutron diffraction and ac susceptibility measurements show that CaRu 0.5 Mn 0.5 O 3 undergoes magnetic phase separation into a disordered ferrimagnetic state (T C ≈ 230 K) and a G-type antiferromagnetic state (T N ≈ 95 K).