X-ray study of tantalum nitrides

Abstract. Tantalum nitrides are characterized by a narrow energy gap, what makes these materials perspective in photocatalytic decomposition of water. Nitrides described in this paper were obtained by ammonolysis of tantalum powders with a specific surface. We evaluate the composition and structure of these nitrides. Obtained data describes all transformations during ammonolysis of tantalum powders, contributing to the directed synthesis of nitrides with specified properties.
Key words: magnesiothermic tantalum powders, ammonolysis, nitrides, X-ray methods, structure.

The subject of the study were tantalum nitrides obtained by ammonolysis from magnesiothermic powders with a large specific surface area. Such compounds are characterized by a narrow energy gap: 2.1 and 2.5 eV respectively [1]. This property makes these material perspective in photocatalytic decomposition of water under the influence of visible spectrum radiation. The traditional method of synthesizing these compounds until now has been ammonolysis of Ta2O5 powder in an NH3 current at high temperature [2-4]. We use as a precursor of tantalum powders with a large specific surface obtained by magnesium-thermal reduction of tantalite Mg4Ta2O9. Obtained powders are characterized by a porous structure and their surface size can reach 80 m2/g. The average pore size is about 7 nm, and the calculated particle size is 4-6 nm [5]. The calculations are based on the surface area.

The scientific novelty of the obtained results is that nitrides synthesized by this technique are the first time thoroughly studied by X-ray diffraction methods. The obtained data will contribute to the directed synthesis of tantalum nitrides with specified properties. In this regard, the studies carried out in this work are relevant.

We used the X-ray powder diffractometry method. Scattering curves were obtained from an automated X-ray diffractometer DRON-4. In this work we used copper radiation. The survey of each sample was carried out in the interval of angles 2Θ from 15° to 135°. Exposure time in the peak area was not less than 15 seconds per point.

The X-ray diffraction patterns of the crystalline samples were preliminarily processed in order to establish the exact position and intensity of the diffraction maxima. After that, a qualitative phase analysis was carried out. The refinement of the structure of the detected phases for several powders was carried out using the Rietveld method. The degree of conformity between the theoretical and experimental data was controlled by two factors: Rp (profile) and Rwp (weighed). The PDWin program complex was used for this case.

In this work, we carried out an X-ray study of five tantalum powders. The designations and synthesis conditions are presented in the table 1. Powders had initial surface of 56 m2/g. All samples were received from Institute of Chemistry and Technology of Rare Elements and Mineral Raw Materials of the Russian Academy of Sciences Kola Science Center. Table 1. Designations and conditions of powders.

The first powder is characterized by wide peaks with various intensity. The angular position of the strongest maxima shows us that one of the phases in original powder has body-centered cubic structure. However, the X-ray picture contains a number of additional maxima, mostly close to the main ones. The pictures of powders 2 and 3 are completely different. They have significant diffuse scattering. However, the position and intensity of main crystalline peaks are the same. Curves 4 and 5 are similar with each other, but absolutely do not look like previous ones. We can see a lot of strong peaks that located at different angles. It means that the phase composition changes with the raise of the temperature. Also, having estimated the latitude of the maxima, we can assume that the sizes of the crystallites of these powders are small.

The nature of the distribution of the intensity of scattering on the X-ray patterns of samples of nitrified tantalum allows us to state that they are crystalline (1, 4, 5) and amorphous-crystalline (2, 3) objects. Under conditions of low temperatures and a short exposure time, a phase transition with the formation of a new crystal lattice is difficult. The X-ray amorphous structure of the products of low-temperature ammonolysis of powders 2 and 3 is possibly a consequence of the beginning of the formation of an oxynitride lattice formed from a natural amorphous Ta2O5 oxide.

We established the phase composition of the powders. The results are presented in the table. Table 2. Phase composition of the powders.

According to the phase analysis results, original powder has 3 phases – hydride, nitride and pure tantalum. Powder 2 also has hydride and nitride, but there is less amount of hydrogen. When we raise the temperature up to 500 degrees, hydrogen comes out from the powder, and there is only nitride phase lefts here. Ammonolysis at higer temperatures transforms powders into completely different crystalline modification – tantalum oxynitride. Consequently, the reaction between nitrogen, hydrogen and tantalum finishes at these temperatures with the forming of this chemical compound.

We made a refinement of the structure of powders 1, 4 and 5 using Rietveld method. The nonconformity coefficients were Rwp = 5-12% and Rp = 4-9%. Table 3. Results of the structural refinement.

Speaking of initial powder, there is an increase of cell periods, and the biggest one is up to tantalum. A significant increase in the cell period of tantalum in is associated with the formation of solid solutions of hydrogen and nitrogen in tantalum. Tantalum oxynitride in powders 4 and 5 has a monoclinic syngony. It also can be seen from the table that the periods of the unit cell A and B in samples 4 and 5 decrease in comparison with the standard ones, the period C of sample 5 slightly increases. The monoclinic angle for the two powders almost does not change. The diffraction peaks are widened, which indicates the small size of the crystallites. The calculation showed that their size in powder is about 11 nm.

To sum up, we have established the phase composition of the powders and made a structural refinement. As it was mentioned before, knowledge about structure will contribute to the directed synthesis of tantalum nitrides. Since powders 2 and 3 has significant amorphous phase, which have not been thoroughly observed at this work, the short-range order parameters must be evaluated. That is our main objective in future studies.

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О. А. Яковлев