A new thermographic nanocrystalline Sr4Al14O25 :Mn4+, Tb3+ phosphor was developed including the optimization of the concentrations of both dopants and of the synthesis conditions. The combination of the thermally quenched luminescence from the Mn4+ ions to the almost temperature independent emission from Tb3+ provides a sensitive luminescent thermometer (SR=2.8%/oC at 150oC) with strong emission colour variability. In addition, a figure of merit for this luminescence thermochromism was proposed, as the relative sensitivities of the x and y CIE coordinates, which for this phosphor reaches at 150°C SR(x)=0.6%/oC and SR(y)=0.4%/oC, respectively. Non-contact thermal imaging was demonstrated with this phosphor using a single consumer digital camera and exploiting the ratio of red (R) and green (G) channels of the RGB images thereby confirming the high applicative potential of Sr4Al14O25 :Mn4+, Tb3+ nanocrystals for thermal sensing and mapping.

The near-infrared luminescence of Ca₆Ba(PO₄)₄O:Mn⁵⁺ is demonstrated and explained. When excited into the broad and strong absorption band that spans the 500–1000 nm spectral range, this phosphor provides an ultranarrow (FWHM = 5 nm) emission centered at 1140 nm that originates from a spin-forbidden ¹E → ³A₂ transition with a 37.5% internal quantum efficiency and an excited-state lifetime of about 350 μs. We derived the crystal field and Racah parameters and calculated the appropriate Tanabe–Sugano diagram for this phosphor. We found that ¹E emission quenches due to the thermally-assisted cross-over with the ³T₂ state and that the relatively high Debye temperature of 783 K of Ca₆Ba(PO₄)₄O facilitates efficient emission. Since Ca₆Ba(PO₄)₄O also provides efficient yellow emission of the Eu²⁺ dopant, we calculated and explained its electronic band structure, the partial and total density of states, effective Mulliken charges of all ions, elastic constants, Debye temperature, and vibrational spectra. Finally, we demonstrated the application of phosphor in a luminescence intensity ratio thermometry and obtained a relative sensitivity of 1.92%K⁻¹ and a temperature resolution of 0.2 K in the range of physiological temperatures.

The influence of the crystal field strength and host material composition on the spectral position of the emission band and thermal stability of Fe³⁺luminescence was investigated in order to develop a highly sensitive luminescence thermometer.

From quencher to potent activator – multimodal luminescence thermometry with Fe³⁺ in the oxides MAl₄O₇ (M = Ca, Sr, Ba).

In response to the ongoing quest for new, highly sensitive upconverting luminescent thermometers, this article introduces, for the first time, upconverting luminescent thermometers based on thermally induced structured phase transitions. As demonstrated, the transition from the low-temperature monoclinic to the high-temperature tetragonal structures of LiYO₂:Yb³⁺,Er³⁺ induces multifaceted modification in the spectroscopic properties of the examined material, influencing the spectral positions of luminescence bands, energy gap values between thermally coupled energy levels, and the red-to-green emission intensities ratio. Moreover, as illustrated, both the color of the emitted light and the phase transition temperature (from 265 K, for LiYO₂:Er³⁺, 1%Yb³⁺, to 180 K, for 10%Yb³⁺), and consequently, the thermometric parameters of the luminescent thermometer can be modulated by the concentration of Yb³⁺ sensitizer ions. Establishing a correlation between the phase transition temperature and the mismatch of ion radii between the host material and dopant ions allows for smooth adjustment of the thermometric performance of such a thermometer following specific application requirements. Three different thermometric approaches were investigated using thermally coupled levels (S_R = 1.8%/K at 180 K for 1%Yb³⁺), green to red emission intensities ratio (S_R = 1.5%/K at 305 K for 2%Yb³⁺), and single band ratiometric approach (S_R = 2.5%/K at 240 K for 10%Yb³⁺). The thermally induced structural phase transition in LiYO₂:Er³⁺,Yb³⁺ has enabled the development of multiple upconverting luminescent thermometers. This innovative approach opens avenues for advancing the field of luminescence thermometry, offering enhanced relative thermal sensitivity and adaptability for various applications.