The problems of manufacturing nanoscale Gd2O3:Nd3+ phosphors using the liquid polymer-salt method have been considered. Within the framework of this method, the dual role of polyvinylpyrrolidone (PVP) as an organic solvent in the synthesis process has been determined. On the one hand, it stabilizes the formation of Gd2O3 crystals, preventing their uncontrolled growth and agglomeration, and on the other, it serves as a fuel during decomposition (combustion), contributing to an increase in the reaction temperature and thereby influencing the structural and emission properties of phosphors. It has been shown that successive drying of the initial homogeneous solution containing gadolinium and neodymium salts, as well as PVP, at room temperature for 24 h and heat treatment at 1000°C for 2 h make it possible to obtain high-luminescent near-infrared phosphors Gd2O3:Nd3+, whose crystals are characterized mainly by a cubic structure and an average size of about 40 nm. It has been experimentally confirmed that the developed method is suitable for modifying hollow-core antiresonant optical fibers made of silica glass with thin-film coatings based on the synthesized material and does not cause structural and phase transformation of the formed Gd2O3 crystals. It has been found out that the emission spectra of the nanoscale Gd2O3:Nd3+ phosphors obtained by the polymer-salt method at temperatures of 550 and 1000°C are identical, namely: (1) the shape of the luminescence peaks is the same for both specified heat treatment regimes regardless of intensity, (2) the main lum-inescence peak is located near 1064 nm wavelength and corresponds to the electron transition 4F3/2–4I11/2, and (3) additional luminescence peaks are located near 900 and 1340 nm wavelengths and correspond to the 4F3/2–4I9/2 and 4F3/2–4I13/2 electron transitions, respectively.

To select erbium and ytterbium doped germanate glasses and glass ceramics, which are most suitable as sensitive elements of fluorescent temperature sensors, a multivariate model of temperature calibration has been developed based on principal component analysis, cluster analysis and interval projection to latent structures of up-conversion green fluorescence spectra. The calibration model used 95 spectral variables for the GeO2-Na2O-Yb2O3-MgO-La2O3-Er2O3 glass-ceramic is characterized by the best quality parameters: the root-mean-square error is 0.37 K, the residual prediction deviation for the test subset is greater than 102, and the relative error does not exceed 0.20%.

Experimental results of polymer-salt synthesis of Yb:YAG nanopowders and analysis of their structure and luminescent properties are presented. Infrared absorption spectra of synthesized materials are presented. The results of XRD analysis show that Yb:YAG nanocrystals with sizes of 18-35 nm form at a temperature of 900-1100 °С. The study on photoluminescence spectra and decay curves shows that properties of synthesized powders are close to properties of macroscopic materials produced by traditional high-temperature methods.

Nanopowders of ytterbium-doped yttrium aluminum garnet are synthesized by the polymer–salt method and their crystal structure and spectral-luminescent properties are studied experimentally. The IR spectroscopy data of the materials synthesized are presented. X-ray diffraction analysis revealed that ytterbium-doped yttrium aluminum garnet nanocrystals 18–35 nm in size are formed at 900–1100°С. Study of the spectra and luminescence decay kinetics showed that the properties of the synthesized nanocrystals are close to the characteristics of macroscopic materials synthesized by traditional high-temperature methods.

A multivariate model of temperature calibration by the spectra of green up-conversion fluorescence based on the principal component analysis, cluster analysis, and the interval projection to latent structures is developed to select the best erbium- and ytterbium-doped germanate glasses and glass-ceramics for sensitive elements of fluorescent temperature sensors. The calibration model constructed for GeO 2 –Na 2 O–Yb 2 O 3 –MgO–La 2 O 3 –Er 2 O 3 glass ceramics using 95 spectral variables is characterized by the best quality parameters, namely, the root-mean-square error is 0.37 K, the residual predictive deviation for a test sampling is >102, and the relative error does not exceed 0.20%.

The article considers the aspects of nanoscale Gd2O3:Nd3+ phosphors synthesis using the liquid polymer-salt method. Within the framework of the method, the double role of polyvinylpyrrolidone (PVP) as an organic solvent in the process of synthesis was determined. On the one hand, PVP stabilizes the process of Gd2O3 crystals formation, preventing their uncontrolled growth and aggregation. On the other hand, PVP serves as a fuel during decomposition (combustion) increasing the temperature of reaction and thus influencing the structural and emission properties of phosphors. It is shown that drying of the initial homogeneous solution containing gadolinium and neodymium salts and PVP at the room temperature for 24 hours followed by thermal treatment at the temperature of 1000 °C for 2 hours allow formation of highly luminescent Gd2O3:Nd3+ near-infrared phosphors. The crystals grown are characterized mainly by the cubic structure and an average size of 40 nm. Experimental results confirmed that the developed method is appropriate for modification of a structure of silica hollow-core antiresonant fibers with thin-film coatings based on the synthesized material and does not result in structural and phase conversion of Gd2O3 crystals. It is found out that emission spectra of nanoscale Gd2O3:Nd3+ phosphors formed by the polymer-salt method at the temperatures of 550 °C and 1000 °C are identical, as follows: 1) the shape of the luminescence peaks is the same for the two mentioned regimes of thermal treatment regardless of intensity, 2) the main luminescence peak is located near the wavelength of 1064 nm and corresponds to a 4F3/2-4I11/2 electron transition, 3) additional luminescence peaks are located close to 900 nm and 1340 nm and refer to 4F3/2-4I9/2 and 4F3/2-4I13/2 electron transitions respectively.

We report data on the development of a polymer-salt method for the formation of aluminium yttrium garnet crystals doped with neodymium ions (YAG: Nd) inside the channels of a preform of microstructured fibre based on pure silica glass. The crystals are obtained by impregnating the channels with aqueous solutions of thermally decomposable salts (yttrium nitrate, aluminium nitrate, neodymium chloride) and an organic polymer, followed by drying and heat treatment at a temperature of 1100 degrees C. The resulting composite structure is drawn into the fibre at a temperature of 2000 degrees C. Using X-ray diffraction analysis, the presence of oriented YAG: Nd crystals ranging in size from 25 to 37 nm in the silica glass matrix of fibre is established. Measurements of the spectral dependence of optical losses in fibre show the presence of absorption bands of the optical signal, characteristic of Nd3+ ions. The shape of the luminescence spectra of nanocrystals is typical of YAG: Nd with a radiation peak at a wavelength of 1064 nm.