Search

The photoluminescence properties of Cr 3+ -doped LaGaO 3 perovskites are investigated by high-pressure spectroscopy. The pressure-induced phase transition from orthorhombic ( Pbnm ) to rhombohedral ( R 3̅ c ) at around 2 GPa is confirmed by Raman spectroscopy. Cr 3+ -doped LaGaO 3 shows deep-red emission peaks around 730 nm due to the zero-phonon line ( R -line) and the phonon sidebands, which correspond to Cr 3+ : 2 E g → 4 A 2 g transitions in the ideal octahedral site and the Cr-Cr pair luminescence ( N -line) under ambient condition. Under a high pressure, the R -line shifts to a lower energy at a rate of -13 cm -1 /GPa. From the pressure dependence of photoluminescence excitation (PLE) spectra, it is suggested that the redshift of the R -line is caused by the decrease of Racah parameters B and C . Moreover, the N -line luminescence becomes stronger relative to the R -line with increasing pressure and the N -line/ R -line can be used to monitor the phase transition pressure. Under a high pressure, the tilt angle of the GaO 6 octahedral unit becomes smaller. It implies that the enhanced N -line luminescence is caused by the stronger superexchange interaction between Cr 3+ ions due to the increased Cr-O-Cr bond angle closer to 180°.

Bismuth-based (nano)materials have been attracting increasing interest due to appealing properties such as high refractive indexes, intrinsic opacity, and structural distortions due to the stereochemistry of 6s 2 lone pair electrons of Bi 3+ . However, the control over specific phases and strategies able to stabilize uniform bismuth-based (nano)materials is still a challenge. In this study, we employed the ability of bismuth to lower the melting point of silica to introduce a new synthetic approach able to confine the growth of bismuth-oxide-based materials into nanostructures. Combining in situ temperature-dependent synchrotron radiation X-ray powder diffraction (XRPD) with high-resolution transmission electron microscopy (HR-TEM) analyses, we demonstrate the evolution of a confined Bi 2 O 3 -SiO 2 nanosystem from Bi 2 SiO 5 to Bi 4 Si 3 O 12 through a melting process. The silica shell acts as both a nanoreactor and a silicon source for the stabilization of bismuth silicate glass-ceramic nanocrystals keeping the original spherical shape. The exciton peak of Bi 2 SiO 5 is measured for the first time allowing the estimation of its real energy gap. Moreover, based on a detailed spectroscopic investigation, we discuss the potential and the limitations of Nd 3+ -activated bismuth silicate systems as ratiometric thermometers. The synthetic strategy introduced here could be further explored to stabilize other bismuth-oxide-based materials, opening the way toward the growth of well-defined glass-ceramic nanoparticles.

Luminescence Boltzmann thermometry is one of the most reliable techniques used to locally probe temperature in a contactless mode. However, to date, there is no report on cryogenic thermometers based on the highly sensitive and reliable Boltzmann-based 4 T 2 → 4 A 2 / 2 E → 4 A 2 emission ratio of Cr 3+ . On the basis of structural information of the local HfO 6 octahedral site we demonstrated the potential of the CaHfO 3 :Cr 3+ system by combining deep theoretical and experimental investigation. The material exhibits simultaneous emission from both the 2 E and 4 T 2 excited states, following the Boltzmann law in a cryogenic temperature range of 40-150 K. The promising thermometric performance corroborates the potential of CaHfO 3 :Cr 3+ as a Boltzmann cryothermometer, being characterized by a high relative sensitivity (∼ 2%·K -1 at 40 K) and exceptional thermal resolution (0.045-0.77 K in the 40-150 K range). Moreover, by exploiting the flexibility of the 4 T 2 - 2 E energy gap controlled by the crystal field of the local octahedral site, the design proposed herein could be expanded to develop new Cr 3+ -doped cryogenic thermometers.

The development of noncontact thermometers with self-control to specific temperatures to be used as control markers with an additional degree of reliability is a challenge in the field of thermal sensors. Herein, a strategy exploiting the wide tunability of an intrinsic feature of oxide perovskites such as the phase-transition temperature to design a new class of ratiometric luminescent thermometers is introduced. The structural and optical response to the thermal stimuli of LaGaO 3 :Nd 3+ system is used as a prototype to show the unprecedented opportunity to combine the processes of two different regimes in the same compound, leading to a reliable optical thermal sensor with an intrinsic tell-tale sign at specific temperatures. High relative sensitivity, low temperature uncertainty, and good reproducibility, together with the need for a single calibration curve irrespective of the phase-transition temperature and the doping effects, attest the goodness of the thermometric performances. This work demonstrates the control of the phase-transition (orthorhombic ↔ rhombohedral) temperature, T c , of lanthanum gallate in the 400-700 K range by carefully doping the perovskite structure, as a proof of concept for the design of customized thermometers characterized by a spectral shape change acting as a self-fingerprint for the T c .

The increasing interest in the development of ratiometric optical thermal sensors has led to a wide variety of new systems with promising properties. Among them, singly-doped ratiometric thermometers were recently demonstrated to be particularly reliable. With the aim to discuss the development of an ideal optical thermal sensor, a combined experimental and theoretical insight into the spectroscopy of the Bi 2 Ga 4 O 9 :Cr 3+ system is reported showing the importance of an insightful analysis in a wide temperature range. Low-temperature photoluminescence analysis (from 10 K) and the temperature dependence of the lifetime investigation, together with the crystal field analysis and the modeling of the thermal quenching process, allow the estimation of key parameters such as the Debye temperature (cutoff frequency), the Huang-Rhys parameter, and the energy barrier between 2 E g and 4 T 2g . Additionally, by considering the reliable class of singly-doped ratiometric thermometers based on a couple of excited states obeying the Boltzmann law, the important role played by the absolute sensitivity was discussed and the great potential of Cr 3+ singly-activated systems was demonstrated. The results may provide new guidelines for the design of reliable optical thermometers with outstanding and robust performances.

The application of nanosized inorganic UV filters in cosmetic field is limited by their high photocatalytic properties that could induce the degradation or dangerous transformation of the organic molecules in sunscreen formulations. To overcome this problem and simultaneously enlarge the window of filter's absorption, we propose the growth of bismuth titanates Bi x Ti y O z into mesoporous silica nanoparticles (MSN). We investigated the chemical-physical properties by means of XRPD, TEM, UV-vis spectroscopy, N 2 physisorption, XPS, and SF-ICP-MS analysis, while the influence on the environment was evaluated through photocatalytic tests. The growing process of this new nanosystem is discussed underlining the key role of the Bi 3+ ion that, acting as a low-melting point agent for the silica framework, led to a self-sealing mechanism. The excellent UV shielding properties combined with a radical suppression of the photocatalytic activity make the proposed nanosystem a perfect candidate for the development of the next generation nanomaterials for sunscreen formulations.

Systematic assessments of cathodoluminescence (CL) spectroscopy, Raman spectroscopy (RS), and X-ray diffraction (XRD) are presented for pure zirconia and for a series of Y-doped zirconia powders (henceforth, simply referred to as undoped ZrO2 and YSZ powders, respectively) synthesized according to a coprecipitation method of Zr and Y chlorides. Emphasis is placed here on spectral emissions related to oxygen-vacancy sites (i.e., oxygen hole states) equally detected from undoped and Y-doped ZrO2 samples, either as intrinsic defects or, extrinsically induced, by means of cathodoluminescence. Most counterintuitively, the undoped ZrO2 sample (i.e., the one with presumably the lowest amount of oxygen vacancies) experienced the strongest CL emission. A progressive "quenching" effect on CL emission with increasing the fraction of Y(3+) dopant could also be observed because the intrinsic vacancies present in the undoped lattice are the most efficient since they can trap two electrons to gain electrical neutrality. However, as soon as Y(3+) ions are introduced in the system, those intrinsic vacancies migrate to Y-sites in next-nearest-neighbor locations, namely in a less efficient lattice location. This phenomenon is tentatively referred to as "delocalization" of vacancy sites. Moreover, the fact that Y-doped zirconia series presents quite similar CL spectra compared to the undoped zirconia could be an evidence that the radiative centers of undoped and Y-doped ZrO2 are basically the same. A fitting procedure has been made aiming to give a rational description of the variation of the spectra morphology, and a parameter able to describe the monoclinic to tetragonal phase transformation has been found. This parameter and the overall set of CL data enabled us to quantitatively assess polymorphic phase fractions by CL spectroscopy in the scanning electron microscope.

The reaction kinetics between alkaline lignite fly ashes and CO2 (pCO2 = 0.01--0.03 MPa)were studied in a laboratory CO2 flow-through reactor at 25--75 degrees C. The reaction is characterized by three phases that can be separated according to the predominating buffering systems and the rates of CO2 uptake. Phase I (pH > 12, < 30 min) is characterized by the dissolution of lime, the onset of calcite precipitation and a maximum uptake, the rate of which seems to be limited by dissolution of CO2. Phase II (pH < 10.5, 10--60 min) is dominated by the carbonation reaction. CO2 uptake in phase III (pH < 8.3) is controlled by the dissolution of periclase (MgO) leading to the formation of dissolved magnesium-bicarbonate. Phase I could be significantly extended by increasing the solid-liquid ratios and temperature, respectively. At 75 degrees C the rate of calcite precipitation was doubled leading to the neutralization of approximately 0.23 kg CO2 per kg fly ash within 4.5 h, which corresponds to nearly 90% of the total acid neutralizing capacity.