Production of nanopowder of cerium (III) fluoride obtained by pulsed electron beam evaporation in vacuum

The method of pulsed electron beam evaporation in vacuum was first used to obtain CeF3 nanopowder (NP). During NP production, a high evaporation rate of the target (~ 7 g/h) and a higher percentage of NP collection (> 72%) were observed, both for fluoride and the previously obtained CeO2 oxide. It was found that the produced NP contains two crystalline phases: hexagonal CeF3 (95 wt.%, coherent scattering region ≈ 8 nm and [Ce-O-F] or [Ce-F]. The magnetic susceptibility of CeF3 nanoparticles (NPles) coincides with the susceptibility of micron particles, indicating the potential for using such NPles as a contrast agent for tomography. High specific surface area (CeO2-270 m2/g, CeF3 – 62 m2/g), large pore volume (0.35-0.11 cm3/g) allow the use of NPles as nanocontainers for drug delivery.


Introduction
Various methods for the production of nanofluorides are constantly proposed, promising for the creation of luminescent materials, catalysts, biomedical applications, etc. [1][2][3]. CeF 3 is one of the least stable rare earth trifluorides from pyrohydrolysis point of view [4,5], and it also easily undergoes Ce(III) → Ce(IV) oxidation in the presence of oxygen. When the solid-state target is evaporated from the CeF 3 by pulsed electron beam evaporation (PEBE) under vacuum, the above-mentioned undesirable factors (pyrohydrolysis and oxidation) are eliminated.
The purpose of the work was to test the method of PEBE in vacuum [6] for the synthesis of trifluoride CeF 3 nanoparticles (NPles), to establish the main physicochemical characteristics of the obtained particles and to compare them with the corresponding characteristics of oxide NPles CeO 2 .

Materials
Cerium(III) fluoride micron powder (CeF 3 , 99.9%, free of water, China) was used for producing nanopowder (NP). The X-ray diffractogram was taken on the D8 DISCOVER diffractometer. Nitrogen adsorption and desorption isotherms at 77 K were obtained using Micromeritics TriStar 3000 V 6.03 A. Thermal analysis of the samples was carried out on thermoanalytic complex NETZSCH STA-409. Magnetic measurements were carried out using a Faraday balance at a RT.

Synthesis of CeF 3 NP
The general scheme of the CeF 3 NP synthesis process from the raw material CeF 3 micron powder using the PEBE method in vacuum is shown in figure 1. The CeF 3 micron powder (figure1 (a)) targets were pressed on a manual press in a titanium mold ( figure 1 (b)). Target evaporation mode (figure 1 (c)): accelerating voltage 38 kV, beam current 0.3 A, pulse duration 100 μs, repetition rate 100 Hz, evaporation time 45 min, amount of vaporized target material 4.4 g, NP collection (excluding losses) 3.1 g, NP collection percentage 72.3%. The resulting NP had very little adhesion to glass substrates. NP color is beige (figure 1 (d)).

Thermal analysis
The thermal stability of the obtained CeF 3 NP was determined using the synchronous differential scanning calorimetry and thermogravimetry (DSC-TG) method and mass spectral analysis. Figure 4 shows the DSC-TG synchronous heating/cooling curves and H 2 O, CO 2 mass spectra of the sample CeF 3 NP in the temperature range 40-1400°C in argon atmosphere. temperature of 730°C, no thermal changes were observed on the DSC curve, however, on the TG curve, starting at the temperature 400°C, a constant increase in sample weight was observed, lasting up to the 1400°C temperature. Considering that the heating of the sample took place in an argon atmosphere, the increase in the mass of the sample can be explained by the physical adsorption of argon atoms in the pores of mesoporous CeF 3 NP. The thermal stability of the CeF 3 sample after heating to the temperature 730°C was possibly disturbed, as indicated simultaneously by an exothermic peak on the DSC curve (730-1170°C). It is most probable that the exothermic peak 2 could be caused by evaporation from the surface and from the pores of the sample impurity -cerium tricarbonate. A large exothermic peak 3 in the temperature range from 1170 to 1400°C (figure 4) is associated with the phase transformation of the metastable hexagonal phase CeF 3 into a more stable at high temperature x-CeF 3 phase, the structural type of which will be determined in the future. The adsorption of the inert gas Ar and the chemically inert gas N 2 on the mesoporous CeF 3 NP is of undeniable interest and requires further investigation. Figure 5 (a, b) shows the magnetization curves of fluoride and cerium oxide at room temperature.  [11].

Magnetic properties of CeF 3 NP
Curves of magnetization of CeF 3 NP are linear functions of the field, and the susceptibility size determined by an inclination of curves corresponds to tabular value at the room temperature of 1.1×10 -6 cm 3 /g, i.e. transition to a nanostate didn't affect magnetic behavior of CeF 3 in any way. In turn, the NP CeO 2 we obtained earlier by the PEBE method [11] showed a noticeable ferromagnetism (FM) difference ( figure 5 (b)), which is consistent with the data on the magnetic behavior of CeO 2 NPles at RT given in the review [12] Note the recent work [13] in which a significant increase in FM at room temperature was observed in composite NPles of CeO 2 /CeF 3 . The saturation magnetization of CeO 2 /CeF 3 composites (NPles size 100 nm) was half the magnetization of the initial LF CeO 2 (NPles size 30 nm). NPles of CeO 2 /CeF 3 were synthesized by means of CeO 2 NP fluoration, therefore, presumably, NPles with the structure of CeF 3 were formed on CeO 2 NPles' surface. An increase in the saturation magnetization of CeO 2 /CeF 3 NPles compared to the initial ones, as well as those annealed in vacuum and air, CeO 2 NPles, in [13] was linked to the interface of the fluorinated nanocomposites of CeO 2 /CeF 3 . Obviously, in order to obtain oxide-fluoride NPles, it is possible to use the reverse synthesis pathway to produce air annealing of paramagnetic CeF 3 NPles obtained by the PEBE method in vacuum at different temperatures, in order to obtain a FM controlled response in the NPles with