Light emission under MeV hydrogen and oxygen ions in stoichiometric SrTiO ₃ are identified at temperatures of 100 K, 170 K and room-temperature. MeV ions predominately deposit their energies to electrons in SrTiO ₃ with energy densities orders of magnitude higher than from UV or x-ray sources but comparable to femtosecond lasers. The ionoluminescence (IL) spectra can be resolved into three main Gaussian bands at 2.0 eV, 2.5 eV and 2.8 eV, whose relative contributions strongly depend on irradiation temperature, electronic energy loss and irradiation fluence. Two main bands, observed at 2.5 eV and 2.8 eV, are intrinsic and associated with electron–hole recombination in the perfect SrTiO ₃ lattice. The 2.8 eV band is attributed to recombination of free (conduction) electrons with an in-gap level, possibly related to self-trapped holes. Self-trapped excitons (STEs) are considered suitable candidates for the 2.5 eV emission band, which implies a large energy relaxation in comparison to the intrinsic edge transition. The dynamics of electronic excitation, governs a rapid initial rise of the intensity; whereas, accumulated irradiation damage (competing non-radiative recombination channels) accounts for a subsequent intensity decrease. The previously invoked role of isolated oxygen vacancies for the blue luminescence (2.8 eV) does not appear consistent with the data. An increasing well-resolved band at 2.0 eV dominates at 170 K and below. It has been only previously observed in heavily strained and amorphous SrTiO ₃ , and is, here, attributed to transitions from d ( t _2g ) conduction band levels to d ( e _g ) levels below the gap. In accordance with ab initio theoretical calculations they are associated to trapped electron states in relaxed Ti ³⁺ centers at an oxygen vacancy within distorted TiO ₆ octahedra. The mechanism of defect evolution monitored during real-time IL experiments is presented. In conclusion, the light emission data confirm that IL is a useful tool to investigate lattice disorder in irradiated SrTiO ₃ .

An up-to-date review on recent results for self-trapping of free electrons and holes, as well as excitons, in strontium titanate (STO), which gives rise to small polarons and self-trapped excitons (STEs) is presented. Special attention is paid to the role of carrier and exciton self-trapping on the luminescence emissions under a variety of excitation sources with special emphasis on experiments with laser pulses and energetic ion-beams. In spite of the extensive research effort, a definitive identification of such localized states, as well as a suitable understanding of their operative light emission mechanisms, has remained lacking or controversial. However, promising advances have been recently achieved and are the objective of the present review. In particular, significant theoretical advances in the understanding of electron and hole self-trapping are discussed. Also, relevant experimental advances in the kinetics of light emission associated with electron-hole recombination have been obtained through time-resolved experiments using picosecond (ps) laser pulses. The luminescence emission mechanisms and the light decay processes from the self-trapped excitons are also reviewed. Recent results suggest that the blue emission at 2.8 eV, often associated with oxygen vacancies, is related to a transition from unbound conduction levels to the ground singlet state of the STE. The stabilization of small electron polarons by oxygen vacancies and its connection with luminescence emission are discussed in detail. Through ion-beam irradiation experiments, it has recently been established that the electrons associated with the vacancy constitute electron polaron states (Ti³⁺) trapped in the close vicinity of the empty oxygen sites. These experimental results have allowed for the optical identification of the oxygen vacancy center through a red luminescence emission centered at 2.0 eV. Ab-initio calculations have provided strong support for those experimental findings. Finally, the use of Cr-doped STO has offered a way to monitor the interplay between the chromium centers and oxygen vacancies as trapping sites for the electron and hole partners resulting from the electronic excitation.

A nanoscale hierarchical dual‐phase structure is reported to form in a nanocrystalline NiFeCoCrCu high‐entropy‐alloy (HEA) film via ion irradiation. Under the extreme energy deposition and consequent thermal energy dissipation induced by energetic particles, a fundamentally new phenomenon is revealed, in which the original single‐phase face‐centered‐cubic (FCC) structure partially transforms into alternating nanometer layers of a body‐centered‐cubic (BCC) structure. The orientation relationship follows the Nishiyama–Wasser‐man relationship, that is, (011)_BCC || ( 1¯1¯1)_FCC and [100]_BCC || [ 11¯0]_FCC. Simulation results indicate that Cr, as a BCC stabilizing element, exhibits a tendency to segregate to the stacking faults (SFs). Furthermore, the high densities of SFs and twin boundaries in each nanocrystalline grain serve to accelerate the nucleation and growth of the BCC phase during irradiation. By adjusting the irradiation parameters, desired thicknesses of the FCC and BCC phases in the laminates can be achieved. This work demonstrates the controlled formation of an attractive dual‐phase nanolaminate structure under ion irradiation and provides a strategy for designing new derivate structures of HEAs.

The origin of the blue emission in SrTiO₃ has been investigated as a function of irradiation fluence, electronic excitation density, and temperature using a range of ion energies and masses. The emission clearly does not show correlation with the concentration of vacancies generated by irradiation but is greatly enhanced under heavy-ion irradiation. The intensity ratio of the 2.8 and 2.5 eV bands is independent of fluence at all temperatures, but it increases with excitation rate. The 2.8 eV emission is proposed to correspond to a transition from conduction band states to the ground state level of the self-trapped exciton center. GRAPHICAL ABSTRACT IMPACT STATEMENT A novel mechanism attributes the origin of the intriguing blue band in SrTiO₃ to a non-localized transition of the self-trapped exciton center from conduction band states to the ground level.

Oxygen vacancies are known to play a central role in the optoelectronic properties of oxide perovskites. A detailed description of the exact mechanisms by which oxygen vacancies govern such properties, however, is still quite incomplete. The unambiguous identification of oxygen vacancies has been a subject of intense discussion. Interest in oxygen vacancies is not purely academic. Precise control of oxygen vacancies has potential technological benefits in optoelectronic devices. In this review paper, we focus our attention on the generation of oxygen vacancies by irradiation with high energy particles. Irradiation constitutes an efficient and reliable strategy to introduce, monitor, and characterize oxygen vacancies. Unfortunately, this technique has been underexploited despite its demonstrated advantages. This review revisits the main experimental results that have been obtained for oxygen vacancy centers (a) under high energy electron irradiation (100 keV–1 MeV) in LiNbO₃, and (b) during irradiation with high-energy heavy (1–20 MeV) ions in SrTiO₃. In both cases, the experiments have used real-time and in situ optical detection. Moreover, the present paper discusses the obtained results in relation to present knowledge from both the experimental and theoretical perspectives. Our view is that a consistent picture is now emerging on the structure and relevant optical features (absorption and emission spectra) of these centers. One key aspect of the topic pertains to the generation of self-trapped electrons as small polarons by irradiation of the crystal lattice and their stabilization by oxygen vacancies. What has been learned by observing the interplay between polarons and vacancies has inspired new models for color centers in dielectric crystals, models which represent an advancement from the early models of color centers in alkali halides and simple oxides. The topic discussed in this review is particularly useful to better understand the complex effects of different types of radiation on the defect structure of those materials, therefore providing relevant clues for nuclear engineering applications.