Codoped
(Tb,Eu) ZnO films grown by magnetron sputtering on a silicon
substrate and annealed up to 1200 °C showed intense photoluminescence
(PL) emission from Eu3+ ions. The high-temperature annealing
led to diffusion and segregation of rare earth (RE) elements toward
the bottom of the film, which induced the formation of nanometric
Zn-free inclusions responsible for remarkable PL emission intensity.
Combined electron diffraction, chemical contrast imaging, and optical
studies of these nanometric phases have been carried out. Large inclusions
of zinc silicates and RE silicates a few hundreds of nanometers in
size were observed, embedded in a silica phase. The structural determination
of these RE-rich inclusions was carried out by combining atomic Z contrast imaging (high-angle annular dark-field imaging)
and precession electron diffraction data. Upon annealing at 1200 °C,
it appeared that the structure was related to an F-type disilicate
structure. Energy-dispersive X-ray spectroscopy and electron energy
loss spectroscopy experiments were carried out to determine the ratios
between elements and the oxidation states of the RE elements in the
abovementioned inclusions. A (Tb,Eu)2Si2O7 formulation is proposed from dynamical precession electron
diffraction tomography refinements, leading to a +III valence state
for the RE species in agreement with spectroscopic results. PL modeling
is also in good agreement with the experimental data. These results
complete those obtained at 1100 °C for which the inclusions were
identified as some RE10–x
(SiO4)6O2–x
oxyapatite
structures and pointed out a combined structural and PL properties’
evolution between 1100 and 1200 °C annealing temperatures.
We investigate the band structure of Fe-based superconductors using the first-principle method of density-functional theory. We calculated the band structure and the density of states at the Fermi level for ReFeAsO (Re=Sm, Er) superconductors. Our calculations indicate that the maximum critical superconducting transition temperature Tc will be observed for compounds with Sm and Er at 55 and 46K, respectively.
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