A combined experimental and theoretical study on the photoluminescence (PL) properties of strontium zirconate (SZ) and Sm3+ doped SZ nanostructures is presented in this work.

Metal carbide ceramics offer potential as protective coatings for steels. Here we report a pseudopotential-based density functional ͑DFT͒ investigation of one such coating, wherein we predict the atomic structure, bonding, and the ideal work of adhesion (W ad ideal) of the interface between a TiC͑100͒ coating and a bcc Fe͑110͒ substrate. Calibration of the DFT approximations used yields TiC and Fe bulk properties in reasonable agreement with experiment. Subsequent characterization of the low-index TiC and Fe surfaces reveals that all surfaces retain near bulk termination, in agreement with experiment. Stabilities of both TiC and Fe surfaces increase with their packing densities, i.e., (110)Ͻ(111)Ͻ(100) for TiC and (111)Ͻ(100)Ͻ(110) for bcc Fe. We estimate that the minimum critical stress required for crack propagation in bcc Fe is 27% larger than that in TiC. The TiC͑100͒/Fe͑110͒ interface exhibits a lattice mismatch of ϳ2.1%, leading to a smooth interface with only a small structural relaxation, except for the ultrathin 1 monolayer ͑ML͒ coating. A mixture of metallic and covalent bonding dominates across the interface, due to significant C p-Fe d interaction and somewhat less pronounced Ti d-Fe d mixing; the latter is found to decrease with increasing coating thickness, but reaches a saturation value for 3-ML-thick coating. The asymptotic value of W ad ideal for the TiC͑100͒/Fe͑110͒ interface is predicted to be ϳ2.56 J/m 2 and is reached for a 3-ML-thick coating of TiC on Fe. This interface strength is considerably smaller than the energy required for cracking TiC or Fe, but may still be strong enough to survive as a coating for steel in extreme environments.

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Defect induced tunable emission in MgO is investigated using photoluminescence and DFT calculations.

The present work describes various defects-induced tunable emission behavior of MgAl2O4 compounds obtained after annealing at different temperatures through a sol–gel combustion route. Multiple defect centers, such as F, F2, F+, and F2 2+ and different shallow and deep defects were found to be present inside the band gap, as confirmed by the lifetime and time-resolved emission spectroscopy (TRES) studies. The tunable emission characteristic at different annealing temperatures could be linked with the phase behavior of the spinel. Excitation wavelength variation suggested that a photoconversion process of F to F+ centers was involved with λex = 250 nm, followed by a trapping–de-trapping mechanism of the released electrons within different trap states. An exchange mechanism of electrons in between conduction band and shallow states was also observed at room temperature, which was absent at low temperature, as indicated by the emission profile. These observations render it to be a potential optical-based thermal sensor material. DFT-based calculations were carried out for both pure and various oxygen-vacancy-introduced spinel phases in order to characterize the different defect states inside the band gap. Finally, on the basis of theoretical and experimental results, a model has been proposed to explain the mechanisms related to emission tunability.

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SrZrO3 perovskite (SZP) was synthesized using gel-combustion route and characterized systematically using X-ray diffraction and time-resolved photoluminescence techniques. A detailed analysis of the optical properties of Tb(3+) ions in SrZrO3 was performed to correlate them with the local environment of the lanthanide ions in this perovskite. Photoluminescence (PL) spectroscopy showed that emission spectrum consists of host as well as Tb(3+) emission indicating the absence of complete host-dopant energy transfer. On the basis of emission spectrum and PL decay study it was also observed that Tb(3+) is not homogeneously distributed in SrZrO3 perovskite; rather, it is occupying two different sites. It is corroborated using extended X-ray absorption fine structure studies that Tb(3+) is stabilized on both six-coordinated Zr(4+) and eight-coordinated Sr(2) site. The energies calculated using density functional theory (DFT) indicates that Tb occupation in Sr site is energetically more favorable than Zr site. The analysis of valence charge distribution also substantiated our structural stability analysis of site-selective Tb doping in SrZrO3. Time-resolved emission spectroscopy is employed to elucidate the difference in the spectral feature of Tb(3+) ion at Sr(2+) and Zr(4+) site. DFT-calculated density of states analysis showed that energy mismatch of Tb-d states with Zr-d and O-p states of SZP makes the energy transfer from host SZP to Tb(3+) ion difficult.

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