Luminescence probes that facilitate multimodal non-contact measurements of temperature are of particular interest due to the possibility of cross-referencing results across different readout techniques. This intrinsic referencing is an essential addition that enhances accuracy and reliability of the technique. A further enhancement of sensor performance can be achieved by using two luminescent ions acting as independent emitters, thereby adding in-built redundancy to non-contact temperature sensing, using a single readout technique. In this study we combine both approaches by engineering a material with two luminescent ions that can be independently probed through different readout modes of non-contact temperature sensing. The approach was tested using Al₂O₃ co-doped with Cr³⁺ and Mn⁴⁺, exhibiting sharp emission lines due to ²E → ⁴A₂ transitions. The temperature sensing performance was examined by measuring three characteristics: temperature-induced changes of the intensity ratio of the emission lines, their spectral position, and the luminescence decay time constant. The processes responsible for the changes with temperature of the measured luminescence characteristics are discussed in terms of relevant models. By comparing temperature resolutions achievable by different modes of temperature sensing it is established that in Al₂O₃-Cr,Mn spectroscopic methods provide the best measurement accuracy over a broad temperature range. A temperature resolution better than ±2.8 K can be achieved by monitoring the luminescence intensity ratio (40–145 K) and the spectral shift of the R-line of Mn⁴⁺ (145–300 K range).

The tuning of second (SHG) and third (THG) harmonic emission is studied in the model system LiNb${ }_{1-x}$Ta${ }_x$O${ }_3$ ($0\le x\le 1$, LNT) between the established edge compositions lithium niobate (LiNbO${ }_3$, $x=0$, LN) and lithium tantalate (LiTaO${ }_3$, $x=1$, LT). Thus, the existence of optical nonlinearities of the second and third order is demonstrated in the ferroelectric solid solution system, and the question about the suitability of LNT in the field of nonlinear and quantum optics, in particular as a promising nonlinear optical material for frequency conversion with tunable composition, is addressed. For this purpose, harmonic generation is studied in nanosized crystallites of mechanochemically synthesized LNT using nonlinear diffuse reflectometry with wavelength-tunable fundamental femtosecond laser pulses from 1200 nm to 2000 nm. As a result, a gap-free harmonic emission is validated that accords with the theoretically expected energy relations, dependencies on intensity and wavelength, as well as spectral bandwidths for harmonic generation. The SHG/THG harmonic ratio ≫1 is characteristic of the ferroelectric bulk nature of the LNT nanocrystallites. We can conclude that LNT is particularly attractive for applications in nonlinear optics that benefit from the possibility of the composition-dependent control of mechanical, electrical, and/or optical properties.

Calcium dialuminate, CaAl₄O₇, nanopowders with a grossite-type structure, doped with chromium ions, were synthesized via the combined sol–gel solution combustion method. The evolution of phase composition, crystal structure, and microstructural parameters of the nanocrystalline materials depending on the temperature of the thermal treatment was investigated via X-ray powder diffraction and applying the Rietveld refinement technique. The photoluminescent properties of CaAl₄O₇ nanophosphors activated with Cr³⁺ ions were studied over the temperature range of 4.5–325 K. The samples show deep red and near-infrared luminescence due to the ²E → ⁴A₂ and ⁴T₂ → ⁴A₂ energy level transitions of Cr³⁺ ions under excitation in the two broad emission bands in the visible spectral region. The R lines emission reveals a strong temperature dependence. The feasibility of the material for non-contact luminescence sensing is investigated, and good sensitivity is obtained based on the (R2/R1) luminescence intensity ratio and the lifetime of the emission.

The results are reported on a precise crystal structure and microstructure determination of new mixed YVO₄-based orthovanadates of Y_0.5R_0.5VO₄ (R = Sm, Tb, Dy, Ho, Tm, Yb, Lu) as well as some Bi³⁺-doped RVO₄ (R = La, Gd, Y, Lu) nano- (submicro-) materials. The formation of continuous solid solutions in the YVO₄–RVO₄ pseudo-binary systems (R = Sm, Tb, Dy, Ho, Tm, Yb, Lu) has been proved. The lattice constants and unit cell volumes of the new mixed orthovanadates were analyzed as a function of R³⁺ cation radius. The impact of crystal structure parameters on the energy band gap of the materials was studied by means of photoluminescence studies of the Bi³⁺-doped compounds.

A series of strontium dialuminate SrAl₄O₇ nanopowders with the grossite-type structure doped with chromium and manganese ions were synthesized by the combined sol–gel solution combustion method with use of two different strontium salts. The Cr³⁺ and Mn⁴⁺ ions concentrations were varied from 0.05 to 5 at.%. Evolution of phase composition, crystal structure, and microstructural parameters of the nanocrystalline materials depending on the synthesis conditions, temperature of thermal treatment, and dopant content were investigated by the X-ray powder diffraction and the scanning electron microscopy techniques. Photoluminescent properties of SrAl₄O₇ nanophosphors activated with Cr³⁺ and Mn⁴⁺ ions were studied at room temperature. The samples exhibit typical photoluminescence in the deep-red spectral region, corresponding to d-d transitions in Cr³⁺ or Mn⁴⁺ ions. The intensity of this deep-red emission is dependent on the dopant concentration and annealing temperature. Features of the formation of octahedral surroundings around Cr³⁺ or Mn⁴⁺ ions are discussed.

The sensitivity of luminescence properties in materials doped with transition metal (TM) ions to changes of temperature makes them particularly promising for thermometric applications. Designing and optimizing such materials requires a deep understanding of their structure, local environment of emission centres, and luminescence processes. In this work, we investigate the potential of Li₂SnO₃ doped with Cr³⁺ and Mn⁴⁺ as a dual-emitting luminescence temperature sensor. Li₂SnO₃ was chosen as the host material due to it being able to host both Cr³⁺ and Mn⁴⁺ at Sn octahedral positions. As a result, Mn⁴⁺ ions exhibit a distinctive ²E → ⁴A₂ line emission due to a strong crystal field, and Cr³⁺ ions experience an intermediate crystal field strength resulting in both, ²E → ⁴A₂ and ⁴T₂ → ⁴A₂ emissions. Through thorough examination, using powder X-ray diffraction (XRD), electron paramagnetic resonance (EPR) and photoluminescence techniques we identified two distinct types of [SnO₆] octahedral centers that correspond to two types of slightly different Cr³⁺ and Mn⁴⁺ emission centers in the Li₂SnO₃ structure. The high sensitivity of the decay time constant for the ²E → ⁴A₂ emission of Li₂SnO₃–Cr³⁺, Mn⁴⁺ to temperature changes (2.0%/K at 190 K and 5.8%/K at 220 K for Cr³⁺ and Mn⁴⁺, respectively) positions the material as an attractive non-contact temperature sensor. Furthermore, application of such a dual-emitter luminescence material as a temperature sensor expands its sensitivity across a broader temperature range and offers the additional advantage of cross-checking measurements compared to materials solely doped with Cr³⁺ or Mn⁴⁺.