Purpose: To study the thermal conductivity of single crystals of a Ba1–xLaxF2+x solid solution and a semi-empirical description of changes in thermal conductivity depending on the lanthanum content. Experimental: In the temperature range of 50–300 K, the thermal conductivity of single crystal Ba1–xLaxF2+x samples with lanthanum content from x = 0.001 to x = 0.300 was determined by the experimental method of long heat flow. Conclusions: A monotonic concentration dependence of thermal conductivity has been revealed. A semi-empirical expression has been proposed to approximate the experimental values of thermal conductivity

A series of Ca1-xYxF2+x solid solution x = 0.0005, 0.003, 0.007, 0.013, 0.02, 0.03, 0.04 single crystals were grown using the Bridgman method. The thermal conductivity of single crystals was measured using the absolute method of longitudinal heat flow in the range of 50 - 300 K. With an increase in the concentration of yttrium fluoride in the solid solution, a transition is observed from the temperature dependence characteristic of single crystals to a monotonically increasing one with increasing temperature, which is characteristic of disordered media. This behavior is associated with the scattering of phonons on nanosized clusters of defects present in the solid solution. Within the framework of a two-component model, including a superposition of thermal resistance coefficients from ordered and disordered media, a system of equations was obtained that provides a quantitative description of the experiment.

Photoluminescence quantum yield (ϕ) is a key parameter of any luminescent material. There are two main ways to determine this value: 1) absolute, which requires calculation of the number of emitted and absorbed photons; and 2) relative, which utilizes the emission of a reference sample with known ϕ. Both methods become more complicated in case of upconversion (UC) photoluminescence, due to its nonlinear nature. The main obstacle to employing the relative method is the lack of a suitable reference with known UC quantum yield (UCQY, ϕ UC). Herein, a new UCQY reference material is presented, based on SrF2:1%Yb3+,1%Er3+ single crystal, for the relative measurement of ϕ UC for near‐infrared (976 nm)‐to‐visible UC. When utilizing this reference material, the ϕ UC is determined to be 2.5% (at 100 W cm−2) for α‐NaYF4:18%Yb3+,2%Er3+@CaF2 nanocrystals (NCs). This result coincides very well with the value for the same NCs determined using the absolute method of ϕ UC = 2.4% (at 100 W cm−2). The intensity dependence of UCQY yield for the NCs determined using the SrF2:1%Yb3+,1%Er3+ reference exhibits good agreement with the results acquired with the absolute method. In addition, various effects that can have an impact on the measured UCQY using absolute and relative methods are discussed.

The cubic phase NaYF4:Pr3+, Yb3+ single crystals were grown in the resistive furnace by the BridgmanStockbarger technique with following milling in an agate mortar. The luminescence intensity ratio (LIR) between 3P1 - 3H5 and 3P0 - 3H5 emissions of Pr3+ was taken a temperature-dependent parameter. The calculated absolute (Sa) temperature sensitivity was 0.0075 at 320 K. The LIR between 3P0 - 1G4 (Pr3+) and 2F5/2 - 2F7/2 (Yb3+) emissions is also temperature-dependent due to phonon-assisted nature of energy transfer between 1G4 of Pr3+ and 2F5/2 of Yb3+. Sr and Sa demonstrated the highest values in the 100-220 K temperature range (Sa (max) = 0.0047 K-1 at 150 K).NaYF4:Pr3+, Yb3+ samples are useful in optical temperature sensing in the broad temperature range from 100 up to 320 K.

A series of single crystals of a Ca1-xYxF2+x solid solution with a fluorite structure containing 1- 19 mol.% YF3 (x =0.01-0.19) has been grown. Thermal analyzer STA 449 F3 Jupiter in DSC mode recorded the temperature dependences of the heat capacity Cp(T) in the temperature range from the room temperature to 1300 degrees C. A diffuse phase transition in the solid state for concentrations x =0.01-0.03 is fixed as an anomaly on the Cp(T) curves with a maximum at 1150 +/- 50 degrees C. With an increase in the content of YF3 (x =0.05-0.19), a very wide structured peak is recorded in the range of 650-1100 degrees C. The heat capacity anomaly is associated with the reversible rearrangement of defect nanoclusters, which affects the change in the anion sublattice.

Методом Бриджмена выращены монокристаллические образцы твердых растворов Ca x Sr y Ba z F 2 ( x = 0.31–0.4045, y = 0.31–0.50, z = 0.10–0.38) и Ca x Sr y Ba z Yb 0.005 F 2.005 ( x = 0.295–0.495, y = 0.30–0.50, z = 0.10–0.40) с флюоритовой структурой. Абсолютным стационарным методом продольного теплового потока в интервале 50–300 K исследована их теплопроводность. При комнатной температуре значения коэффициента теплопроводности всех исследованных образцов ниже 2.5 Вт/(м К). Теплопроводность убывает с увеличением содержания тяжелых компонентов в данных твердых растворах. Этот же фактор снижает негативное влияние на теплопроводность добавки гетеровалентной примеси YbF 3 .

Single crystals of Ba1 – x – yYbxRyF2 + x + y (R = Tm, Ho) solid solutions have been grown by the Bridgman technique in vacuum using a CF4 fluorination atmosphere. We have measured their thermal conductivity in the range 50–300 K and their refractive index from the visible to IR spectral region. As the ytterbium content increases from 2 to 14 mol

— Single crystals of Ca x Sr y Ba z F 2 ( x = 0.31–0.4045, y = 0.31–0.50, z = 0.10–0.38) and Ca x Sr y Ba z Yb 0.005 F 2.005 ( x = 0.295–0.495, y = 0.30–0.50, z = 0.10–0.40) fluorite solid solutions have been grown by the Bridgman technique, and their thermal conductivity has been measured in the range 50–300 K by an absolute steady-state axial heat flow technique. The room-temperature thermal conductivity of all the crystals studied is below 2.5 W/(m K). As the percentage of the heavy components of the solid solutions increases, their thermal conductivity decreases. In addition, this factor reduces the negative effect of the heterovalent dopant YbF 3 on the thermal conductivity of the crystals.

Single-phase samples of the Ba1−xCexF2+x solid solution (x = 0.3–0.4) were synthesized by directional crystallization in the form of single crystals and by co-precipitation from aqueous nitrate solutions using potassium fluoride as a fluorinating agent in the form of nanopowders. The cathodoluminescence of the pressed powder samples was studied in comparison with the BaF2: Ce single crystals in 250–460 nm (2.7–5 eV) spectral range upon excitation by an electron accelerator. The cathodoluminescence spectra of the samples revealed a wide band in the range of 3.0–4.0 eV, which consists of two typical components of Ce3+ with decay time 23 ns in the case of single crystals and three decay times 27 ns, 140–170 ns, and ~600 ns in the case of pressed powders. The decay time of the short-wavelength component (27 ns) in the case of pressed powders is close to the lifetime of the excited state of the Ce3+ ion. The developed X-ray phosphors can be applied for embedding in diamonds for diamond–nanoparticle composite preparation.

Single crystals of Ca1 – хBaхF2 (x = 0.001–0.05 and 0.85–0.99) solid solutions have been grown by the Bridgman method, and their thermal conductivity has been measured in the range 50–300 K by an absolute steady-state axial heat flow technique. The thermal conductivity of all samples has been shown to decrease with increasing temperature. At 300 K, the lowest thermal conductivity of the samples with x ≤ 0.05 exceeds 6 W/(m K), and that of the samples with x ≥ 0.85 is a factor of 2 lower.

We present the measurements of up-conversion luminescence quantum yields of $\text{MF}_{2}:\text{Yb}:\mathrm{R}(\mathrm{M}=\text{Ca},\ \text{Sr},\ \text{Ba},\ \text{Pb}; \mathrm{R}=\text{Er},\ \text{Tm},\ \text{Ho})$ single crystals at $0.1-490.0\ \mathrm{W}/\text{cm}^{2}$ pumping (976 nm). We revealed the solid solution compositions, which demonstrated the highest quantum yields.

We present the luminescence radiative lifetime of of Er 3 + in single crystals of BaF2 -SrF2 -ErF3 solid solutions at room (300 K) temperature. The results show that the lifetime of Er 3+ depends significantly on content of Er 3+ ion in the single crystals. Increasing Er 3 + from 0.1 to 20% leads to decreasing the lifetimes from 39.3(3) to 10.8(1) ms for 4 $I$ 13/2 that can be explained by concentration quenching of luminescence due to higher content of the activating ion.

Lanthanide-based upconversion (UC) allows harvesting subbandgap near-infrared photons in photovoltaics. In this work, we investigate UC in perovskite solar cells by implementing UC single crystal BaF2:Yb3+, Er3+ at the rear of the solar cell. Upon illumination with high-intensity sub-bandgap photons at 980 nm, the BaF2:Yb3+, Er3+ crystal emits upconverted photons in the spectral range between 520 and 700 nm. When tested under terrestrial sunlight representing one sun above the perovskite's bandgap and sub-bandgap illumination at 980 nm, upconverted photons contribute a 0.38 mA/cm(2) enhancement in the short-circuit current density at lower intensity. The current enhancement scales non-linearly with the incident intensity of subbandgap illumination, and at higher intensity, 2.09 mA/cm(2) enhancement in current was observed. Hence, our study shows that using a fluoride single crystal like BaF2:Yb3+, Er3+ for UC is a suitable method to extend the response of perovskite solar cells to near-infrared illumination at 980 nm with a subsequent enhancement in current for very high incident intensity.

Optical properties of BaF2 single crystals doped with Yb3+ and Er ions are studied. The highest ϕUC value of 10.0% was demonstrated for the BaF2:Er3+ (2 mol%) /Yb3+ (3 mol%) sample under 490 W cm−2 of 976 nm excitation.

— The thermal conductivity κ of single crystals of the Sr 1– x Ba x F 2 fluorite solid solution series has been measured in the temperature range 50–300 K by an absolute steady-state axial heat flow technique. The thermal conductivity has been shown to decrease with increasing temperature. Its composition dependences have a minimum at intermediate compositions. The congruently melting composition Sr 0.34 Ba 0.66 F 2 , corresponding to the minimum in liquidus curves, has κ = 15.2 W/(m K) at 50 K and 2.95 W/(m K) at 300 K.

The thulium (Tm 3+ ) doped solid state lasers, operating on the transition 3 F 4 → 3 H 6 are sources of broadly tunable "eye-safe" laser radiation in the 1.8 to 2.1 µm region where are located important absorption bands of different molecular gases such as H 2 O or CO 2 . This spectral region is interesting for applications such as coherent radar systems, atmospheric remote sensing, high-resolution spectroscopy, and medical surgery. This broad range of applications stimulates research on thulium solid state lasers and search for new host materials. [1] - [2] The basic spectroscopic and laser properties of novel Tm:LuF 3 -CaF 2 mixed fluoride crystal were investigated in previous paper [3] . In this paper we focus on the laser wavelength tuning properties. The crystal was grown in A. M. Prokhorov General Physics institute by the Bridgmann-Stockbauer technique in fluorinating atmosphere. The composition of the melt was (TmF 3 ) 1 (CaF 2 ) 97 (LuF 3 ) 2 . The sample was cut from the grown crystal in the form of 7.1 mm thick block with plan-parallel polished faces without any anti-reflection coating.