RESUMO
This paper reports an experimental high-pressure study of natural mineral ferberite (FeWO4) up to 20 GPa using diamond-anvil cells. First-principles calculations have been used to support and complement the results of the experimental techniques. X-ray diffraction patterns show that FeWO4 crystallizes in the wolframite structure at ambient pressure and is stable over a wide pressure range, as is the case for other wolframite AWO4 (A = Mg, Mn, Co, Ni, Zn, or Cd) compounds. No structural phase transitions were observed for FeWO4, in the pressure range investigated. The bulk modulus (B0 = 136(3) GPa) obtained from the equation of state is very close to the recently reported value for CoWO4 (131(3) GPa). According to our optical absorption measurements, FeWO4 has an indirect band gap that decreases from 2.00(5) eV at ambient pressure to 1.56(5) eV at 16 GPa. First-principles simulations yield three infrared-active phonons, which soften with pressure, in contrast to the Raman-active phonons. These results agree with Raman spectroscopy experiments on FeWO4 and are similar to those previously reported for MgWO4. Our results on FeWO4 are also compared to previous results on other wolframite-type compounds.
RESUMO
High pressure X-ray diffraction, Raman scattering, and electrical measurements, together with theoretical calculations, which include the analysis of the topological electron density and electronic localization function, evidence the presence of an isostructural phase transition around 2 GPa, a Fermi resonance around 3.5 GPa, and a pressure-induced decomposition of SnSb2Te4 into the high-pressure phases of its parent binary compounds (α-Sb2Te3 and SnTe) above 7 GPa. The internal polyhedral compressibility, the behavior of the Raman-active modes, the electrical behavior, and the nature of its different bonds under compression have been discussed and compared with their parent binary compounds and with related ternary materials. In this context, the Raman spectrum of SnSb2Te4 exhibits vibrational modes that are associated but forbidden in rocksalt-type SnTe; thus showing a novel way to experimentally observe the forbidden vibrational modes of some compounds. Here, some of the bonds are identified with metavalent bonding, which were already observed in their parent binary compounds. The behavior of SnSb2Te4 is framed within the extended orbital radii map of BA2Te4 compounds, so our results pave the way to understand the pressure behavior and stability ranges of other "natural van der Waals" compounds with similar stoichiometry.
RESUMO
The first pressure-induced solid-phase synthesis of a zeolite has been found through compression of a common zeolite, ITQ-29 (see scheme, Si yellow, O red). The new microporous structure, ITQ-50, has a unique structure and improved performance for propene/propane separation with respect the parent material ITQ-29.
RESUMO
We explore the potential of Tb- and Yb-doped InVO4, InTaO4, and InNbO4 for applications as phosphors for light-emitting sources. Doping below 0.2% barely change the crystal structure and Raman spectrum but provide optical excitation and emission properties in the visible and near-infrared (NIR) spectral regions. From optical measurements, the energy of the first/second direct band gaps was determined to be 3.7/4.1 eV in InVO4, 4.7/5.3 in InNbO4, and 5.6/6.1 eV in InTaO4. In the last two cases, these band gaps are larger than the fundamental band gap (being indirect gap materials), while for InVO4, a direct band gap semiconductor, the fundamental band gap is at 3.7 eV. As a consequence, this material shows a strong self-activated photoluminescence centered at 2.2 eV. The other two materials have a weak self-activated signal at 2.2 and 2.9 eV. We provide an explanation for the origin of these signals taking into account the analysis of the polyhedral coordination around the pentavalent cations (V, Nb, and Ta). Finally, the characteristic green (5D4 â 7F J ) and NIR (2F5/2 â 2F7/2) emissions of Tb3+ and Yb3+ have been analyzed and explained.