ions [19; 20]. One of them (usually Yb\({}^{3+}\)) acts as a donor, absorbs exciting radiation, and transfers it to two other acceptor ions. Acceptor ions are selected in such way that one of the energy transfers is resonant, its probability does not depend on temperature and this signal is used as a reference. The second transfer is non-resonant and with increasing temperature, its probability increases (phonon-assisted energy transfer). This approach was realized for Yb\({}^{3+}\)-Er\({}^{3+}\)-Ho\({}^{3+}\)-doped NaYF\({}_{4}\) nanoparticles and allows thermometry in the SWIR region.  
Pumping was carried out into the absorption band Yb\({}^{3+}\) (\({}^{7}\)F\({}_{7/2}\rightarrow\)\({}^{5}\)F\({}_{5/2}\)), followed by energy redistribution between Ho\({}^{3+}\) and Er\({}^{3+}\) ions as a result of non-radiative energy transfer. In this case, the energy transfer of Yb\({}^{3+}\rightarrow\) Ho\({}^{3+}\) is phonon assisted, as a result of which the luminescence intensity of Ho\({}^{3+}\) ions at the transition \({}^{5}\)I\({}_{6}\)\(\rightarrow\)\({}^{5}\)I\({}_{8}\) will depend on temperature, and the energy transfer of Yb\({}^{3+}\rightarrow\) Er\({}^{3+}\) is resonant, as a result of which the luminescence of Er\({}^{3+}\) ions at the transition \({}^{4}\)I\({}_{13/2}\rightarrow\)\({}^{4}\)I\({}_{15/2}\) depends on temperature weakly [21]. This makes it possible to measure the relative coefficient of temperature sensitivity by the ratio of the integral intensities of the luminescence bands. The obtained thermal sensitivity was in the range of 1.34-1.87%-\({}^{\circ}\)C in the physiological temperature range.  
To date, the active search for new thermal sensors continues. The main tasks are to expand the range of operating temperatures and the range of excitation and emission wavelengths, as well as to increase thermal sensitivity. The sensitivity between 0.5 and 1.5% K\({}^{-1}\) at around 300 K is typically for Er\({}^{3+}\)[22]. In the paper [23], it was proposed to use silica coating to expand the thermometry range to 900 \({}^{\circ}\)C; the relative sensitivity was from 1.02% K\({}^{-1}\) at 300 K to 0.13% K\({}^{-1}\) at 900 \({}^{\circ}\)C. The use of Nd\({}^{3+}\) ions allows a shift of the operating wavelength range into NIR and obtains a relative sensitivity of 1.1% K\({}^{-1}\) for NaYF\({}_{4}\):0.1%Nd\({}^{3+}\)[24]. For Nd\({}^{3+}\)/Tm\({}^{3+}\)/Yb\({}^{3+}\)/Gd\({}^{3+}\) four-doped NaYF\({}_{4}\) nanoparticles the relative sensitivity was significantly higher and reached 2.89% K\({}^{-1}\) in the temperature range of 333-553 K [25].  
Thus, using thermally coupled levels, taking into account resonant and non-resonant energy transfer channels from the sensitier, it is possible to expand the range of wavelengths in which thermometry can be performed, as well as increase the thermal sensitivity.  
Fluoride matrices such as NaYF\({}_{4}\)[26], NaGdF\({}_{4}\)[27], and MF\({}_{2}\), where M = Ca, Sr, Ba [28], have proven to be effective photonics materials [29]. It is known that an increase in matrix molecular weight leads to a decrease in multi-phonon relaxation and usually increases the luminescence energy yield [30; 31]. In this connection, the purpose of the study is to search for new \"heavy\" matrices based on fluorides and recognize their spectral and luminescent characteristics for luminescent nanothermometers. As part of the study, a method was developed for the synthesis of a new \"heavy\" matrix Ba\({}_{0.5-\mathrm{x}}\)Ln\({}_{0.5}\)Na\({}_{\mathrm{x}}\)F\({}_{2.5-\mathrm{x}}\) (x = 0.08) and phosphors based on it for testing as a dual-band luminescent thermometer.