Investigations of LiNb 1 − x Ta x O 3 Nanopowders Obtained with Mechanochemical Method

: Nanocrystalline compounds LiNb 1 − x Ta x O 3 of various compositions ( x = 0, 0.25, 0.5, 0.75, 1) were synthesized by high-energy ball milling of the initial materials (Li 2 CO 3 , Nb 2 O 5 , Ta 2 O 5 ) and subsequent high-temperature annealing of the resulting powders. Data on the phase composition of the nanopowders were obtained by X-ray diffraction methods, and the dependence of the structural parameters of LiNb 1 − x Ta x O 3 compounds on the value of x was established. As a result of the experiments, the optimal parameters of the milling and annealing runs were determined, which made it possible to obtain single-phase compounds. The Raman scattering spectra of LiNb 1 − x Ta x O 3 compounds ( x = 0, 0.25, 0.5, 0.75, 1) have been investigated. Preliminary experiments have been carried out to study the temperature dependences of their electrical conductivity. Author Contributions: Conceptualization, D.S.; Data curation, S.H. and U.Y.; Investigation, L.V., Y.S., D.W., S.H. and Y.Z.; Methodology, V.S. and A.L.; Resources, I.I.S. and I.S.; Writing—original draft, O.B.; Writing—review and editing, A.S. and H.F. authors


Introduction
Lithium niobate (LiNbO 3 , LN) and tantalate (LiTaO 3 , LT) are among the most studied oxide compounds in modern materials science. This interest is due to the widest application of these materials in functional electronics. Analytical studies of the global market for LN and LT sales performed by various marketing agencies [1][2][3][4] show that as of the end of 2020, their consumption in monetary terms exceeded $40 billion per year, and by 2027, it may reach more than $75 billion. Such impressive sales are due to the variety of electronics industry branches and devices that use LN and LT. Accordingly, the forms in which these materials are used in practice are also different-single crystals, thin films, micro-and nanopowders, ceramics. Moreover, they can have a different chemical compositioncongruent, stoichiometric, and contain various dopants of metal ions. It is obvious that such a variety of forms and applications again requires research to modify and optimize material properties. LiNb 1−x Ta x O 3 (LN-LT) solid solutions have recently been studied (see, for example, [5-8]), while they open up prospects for combining the advantages of both materials. synthesis of LT nanopowders. Furthermore, the synthesis and properties of nanopowders of LiNb 1−x Ta x O 3 solid solutions, with 0 < x <1, remain completely unexplored.
This work focuses on the preparation of LiNb 1−x Ta x O 3 nanopowders of different compositions (x = 0, 0.25, 0.5, 0.75, 1), using the mechanochemical synthesis method (high-energy ball milling of the starting materials Li 2 CO 3 , Nb 2 O 5 , Ta 2 O 5 and subsequent annealing). Furthermore, the crystal structure, Raman spectra, and electrophysical properties of obtained LN-LT compounds are studied.

Materials and Methods
The mixed lithium niobate-tantalate nanopowders with nominal compositions LiNb 1−x Ta x O 3 (x = 0, 0.25, 0.5, 0.75 and 1) were obtained by high-energy ball milling mixtures of Li 2 CO 3 , Nb 2 O 5 , and Ta 2 O 5 powders (manufactured by Alfa Aesar, purity 4N) taken in molar ratios corresponding to stoichiometric compositions. The masses of the components for obtaining compounds with a certain x value are given in Table 1. The synthesis was performed with the planetary ball mill machine Pulverisette-7. The rotation speed was equal to 600 rpm, and the duration of milling was about 10-15 h. A total of 134 balls of zirconium dioxide with a diameter of 5 mm and total weight of 91.5 g were used as working bodies. The mass ratio ball/sample was about 10. Milling was performed in 15 min cycles; subsequently, a reverse was carried out after each cycle. Based on the results of previous investigations (see, e.g., [21] where LiNbO 3 nanoparticles were synthesized by a mechanochemical technique), one can assume that the surfaces of particles are activated, but the synthesis exactly of an LN-LT compound was not fully completed after milling. This is confirmed by X-ray analysis of the powders performed after milling. As an example, Figure 1 shows a diffraction pattern of a powder with x = 0.5 compared to the reference pattern of a lithium niobate powder from the PDF database. The results of X-ray phase analysis indicate the formation of a predominantly amorphous precursor and partially a perovskite-like phase of lithium niobate.
To obtain nanocrystalline LN-LT particles, subsequent annealing of powders was performed. To determine the range for selecting the optimal annealing temperature, thermal analysis curves of the powders after milling were recorded. These dependences for all samples had a similar character. As an example, Figure 2 shows the thermogravimetric curves for milled mixture corresponding to LiNb 0.5 Ta 0.5 O 3 powder. As can be seen, sharp mass loss is observed within 400-550 • C (TG and DTG curves). This event corresponds to the decomposition of lithium carbonate according to (1):  To obtain nanocrystalline LN-LT particles, subsequent an performed. To determine the range for selecting the optimal anne mal analysis curves of the powders after milling were recorded all samples had a similar character. As an example, Figure 2 show curves for milled mixture corresponding to LiNb0.5Та0.5O3 powd mass loss is observed within 400-550 °C (TG and DTG curves). T the decomposition of lithium carbonate according to (1):  To obtain nanocrystalline LN-LT particles, subseq performed. To determine the range for selecting the opt mal analysis curves of the powders after milling were all samples had a similar character. As an example, Figu curves for milled mixture corresponding to LiNb0.5Та0.5O mass loss is observed within 400-550 °C (TG and DTG c the decomposition of lithium carbonate according to (1) Li2CO3 = Li2O + CO2  At the same time, it is known that lithium carbonate decomposes above 800 • C [25]. Therefore, significant lowering of the temperature of this process is observed after milling the reaction mixture. In addition, the experimental value of mass loss in the temperature range 400-550 • C is about 20% of the theoretical value calculated by (1). The latter indicates that about 80% of lithium carbonate is decomposed at the stage of milling. Above 550 • C, the interaction of the formed lithium oxide with niobium (tantalum) oxides according to Reaction (2) occurs: Based on the obtained results, three annealing temperatures in air were chosen for the experiments-550, 700, and 800 • C.
Phase compositions of obtained nanoparticles were studied by X-ray phase analysis using the modernized DRON-3M diffractometer. Crystal structure parameters (unit cell dimensions, positional and displacement parameters of atoms) of both series of the materials were derived by full profile Rietveld refinement by using the WinCSD program package for structural analysis [26].
The micro-Raman spectra of LN-LT nanopowders were registered by confocal Raman microscope spectrometer MonoVista CRS+. The laser beam (λ = 532 nm) was focused in a 1 mm spot on the surface of pressured nanopowder.
Temperature dependencies of electrical conductivity in the range from 300 to 820 • C were obtained via impedance measurements in the frequency range from 1 Hz to 1 MHz using impedance gain-phase analyzer (Solartron 1260, Ametek Scientific Instruments, Hampshire, UK). For this experiment, pressed pellets with thickness varying from 1.25 to 1.7 mm and diameter of 10 mm were formed from LiNb x Ta 1−x O 3 nanopowders with x = 0, 0.25, 0.5, 0.75, and 1. Throughout the preparation process, the samples were heated up to 210 • C at the rate of 2 • C/min, while the pressure applied was 190 MPa at all times. Additionally, a constant voltage of 1 kV has been applied to the samples with an intention to electrically polarize them. The so-obtained pellets were subsequently annealed in air at 600 • C for 6 h. Platinum electrodes (5 mm in diameter) were deposited on both sides of each sample via screen printing (print ink: Ferro Corporation, No. 6412 0410). The samples were subsequently thermally treated for 1 h at 800 • C to ensure electrode adhesion.
Scanning electron microscopy (SEM) imaging of LiNb 0.5 Ta 0.5 O 3 sample has been performed after initial annealing at 600 • C ( Figure 3) and after impedance measurements ( Figure 4). The comparison of these two images reveals an increase in average grain size from approximately 100 to 200 nm due to temperature treatment during electrode preparation and the impedance studies. The SEM analysis after the impedance spectroscopy experiment, performed on a larger fragment of the sample, also demonstrates a high homogeneity of grain size distribution along the area.
Obtained impedance spectra of LiNb x Ta 1−x O 3 pressed samples are represented in form of Nyquist diagrams. Subsequently, an electrical equivalent-circuit model consisting of a constant phase element (CPE) connected in parallel with a bulk resistance R B is fitted to the measured data. The intercepts of semicircles in the range of low frequencies are interpreted as samples resistance and subsequently converted to conductivity using the relation σ = t(A × R B ) −1 , where t and A are the thickness of the sample and the electrode area, respectively.
( Figure 4). The comparison of these two images reveals an increas from approximately 100 to 200 nm due to temperature treatment d ration and the impedance studies. The SEM analysis after the im experiment, performed on a larger fragment of the sample, also de mogeneity of grain size distribution along the area.   Obtained impedance spectra of LiNbxTa1−xO3 pressed sample of Nyquist diagrams. Subsequently, an electrical equivalent-circu constant phase element (CPE) connected in parallel with a bulk the measured data. The intercepts of semicircles in the range of lo preted as samples resistance and subsequently converted to con tion σ = t(A × RB) −1 , where t and A are the thickness of the sampl respectively.   Table 2). Only the sample with x = 0 that corresponds to the 'pure' LiNbO 3 shows single phase composition as in work [21]. Observable broadening of the Bragg's peaks on the nanocrystalline character for the powders was revealed. Additional heat treatment of the materials at 800 • C led to the narrowing of the diffraction peaks and to the considerable change of the phase composition of the LiNb 1−x Ta x O 3 samples with x from 0.5 to 1, in which the increase of monoclinic LiNb 3 O 8 or Li(Nb,Ta) 3 O 8 phases and disappearing of individual Ta 2 O 5 and Nb 2 O 5 oxides were detected (see, e.g., Figure 5). No significant changes of the phase composition in the samples with x = 0 and 0.25 were observed after such annealing of the samples. als 2021, 11, x FOR PEER REVIEW LiNb3O8 or Li(Nb,Ta)3O8 phases and disappearing of individual were detected (see, e.g., Figure 5). No significant changes of the p samples with x = 0 and 0.25 were observed after such annealing o

X-ray Diffraction
As an example, Figure 6 demonstrates graphical results o LiNb0.75Ta0.25O3 material heat treated at 800 °C.
The WinCSD programme package was also used for the e tural parameters of the powders, as presented in Table 1. The av crostrains <ε> = <Δd>/d associated with the dispersion of interpla rived from the analysis of angular dependence of the Bragg's pe rection of instrumental broadening, the LaB6 external standard w that the average grain size of the LiNb1−xTaxO3 powders annealed and 66 nm. Increase of thermal annealing temperature led to e size, being especially pronounced for the nominally pure LiNbO3 ple. Corresponding changes for tantalum-reach materials are mu average grain size of LiNb1−xTaxO3 @800 °C powders with x from and 97 nm (see Table 2).     Analysis of the obtained structural parameters revealed that an in in both of the LiNb1−xTaxO3 specimens treated at 550 and 800 °C leads t The WinCSD programme package was also used for the evaluation of microstructural parameters of the powders, as presented in Table 1. The average grain size and microstrains <ε> = <∆d>/d associated with the dispersion of interplanar distances d were derived from the analysis of angular dependence of the Bragg's peaks profiles. For the correction of instrumental broadening, the LaB 6 external standard was used. It was revealed that the average grain size of the LiNb 1−x Ta x O 3 powders annealed at 550 • C is between 31 and 66 nm. Increase of thermal annealing temperature led to essential growth of grain size, being especially pronounced for the nominally pure LiNbO 3 and LiNb 0.75 Ta 0.25 O 3 sample. Corresponding changes for tantalum-reach materials are much less pronounced: the average grain size of LiNb 1−x Ta x O 3 at 800 • C powders with x from 0.5 to 1 lies between 80 and 97 nm (see Table 2).
Analysis of the obtained structural parameters revealed that an increase of Ta content in both of the LiNb 1−x Ta x O 3 specimens treated at 550 and 800 • C leads to the increase of the a-parameter and simultaneous decrease of the c-parameter (Figure 7a). As a result, a significant decrease of the c/a ratio and minor decrease of the unit cell volume in both LiNb 1−x Ta x O 3 series is observed. It is observed that similar to the compositional effect on the unit cell dimensions of LiNb 1−x Ta x O 3 materials, there is an increase of the heat treatment temperature from 550 to 800 • C, which led to increase of the a-parameter and simultaneous reduction of the c-parameter (see Figure 7).
The comparison of the obtained structural parameters of LN-LT samples with the corresponding structural data for nominally pure LN and LT [27][28][29][30][31][32][33] points to the formation of the continuous LiNb 1−x Ta x O 3 solid solution.
As can be seen from the results presented in Table 2 and Figure 7, after annealing at T = 550 • C, the phase analysis indicates that niobium and/or tantalum oxides prevail among the traces of parasitic phases in the resulting nanopowders with the lithium niobate structure. At the same time, after annealing at T = 800 • C, the predominant parasitic phase is the compound Li(Nb,Ta) 3 O 8 , which, as compared to Li(Nb,Ta)O 3 , contains less lithium. The latter is due to the sublimation of lithium in the form of oxide [34]. This result indicates that the optimal annealing temperature of nanopowders lies in the range of 550-800 • C. In order to optimize the heat treatment conditions for obtaining single-phase a-parameter and simultaneous decrease of the c-parameter (Figure 7a). As a result, a significant decrease of the c/a ratio and minor decrease of the unit cell volume in both LiNb1−xTaxO3 series is observed. It is observed that similar to the compositional effect on the unit cell dimensions of LiNb1−xTaxO3 materials, there is an increase of the heat treatment temperature from 550 to 800 °C, which led to increase of the a-parameter and simultaneous reduction of the c-parameter (see Figure 7). The comparison of the obtained structural parameters of LN-LT samples with the corresponding structural data for nominally pure LN and LT [27][28][29][30][31][32][33] points to the formation of the continuous LiNb1−xTaxO3 solid solution.
As can be seen from the results presented in Table 2 and Figure 7, after annealing at T = 550 °C, the phase analysis indicates that niobium and/or tantalum oxides prevail among the traces of parasitic phases in the resulting nanopowders with the lithium niobate structure. At the same time, after annealing at T = 800 °C, the predominant parasitic phase is the compound Li(Nb,Ta)3O8, which, as compared to Li(Nb,Ta)O3, contains less lithium. The latter is due to the sublimation of lithium in the form of oxide [34]. This result indicates that the optimal annealing temperature of nanopowders lies in the range of 550-800 °C. In order to optimize the heat treatment conditions for obtaining single-phase nanocrystalline powders, a number of experiments were carried out on the milling and annealing of equiatomic lithium niobate-tantalate LiNb0.5Ta0.5O3 at different milling times and different annealing temperatures.
Based on the data of X-ray phase analysis of the synthesized compounds, it was found that the best results were achieved with a milling time of 12-15 h and a heat treatment temperature of 650-700 °C for 5 h. Such modes ensure the absence of parasitic phases (within the accuracy of the measurement method) and the absence of violation of stoichiometric ratio Li/Nb. Diffraction patterns of powders obtained in optimal conditions are shown in Figure 8. Based on the data of X-ray phase analysis of the synthesized compounds, it was found that the best results were achieved with a milling time of 12-15 h and a heat treatment temperature of 650-700 • C for 5 h. Such modes ensure the absence of parasitic phases (within the accuracy of the measurement method) and the absence of violation of stoichiometric ratio Li/Nb. Diffraction patterns of powders obtained in optimal conditions are shown in Figure 8.

Raman Spectra
The micro-Raman spectra of LN-LT nanopowders with different Nb and Ta content, annealed at 550 • C, are shown in Figure 9, and the positions of the observed bands are indicated in Table 3. As it is seen from Figure 9, the Raman spectra of LN-LT with different x are generally similar; however, some peculiarities are observed. Particularly, in the spectrum of pure LN, 15 Raman bands can be distinguished, 11 of which can be attributed to A1 and E vibrational modes. The A1 modes are polarized along the Z-axis, while the doubly degenerate E modes correspond to ionic motions along the X or Y-axis [35,36]. In the spectrum of LN-LT (x = 0.25), 25 Raman bands can be distinguished; for x = 0.5, the number of bands is equal to 20, for x = 0.75 to 15 and for pure LT to 17 (see Table 3). The differences in the number of distinguished bands could probably be caused by the overlapping of some bands with those with higher intensity. The observed differences are associated with different compositions of the nanopowders as well as with probably non-optimal technology regimes of nanopowders synthesis. Particularly, the following main specific features of the LN-LT nanopowders Raman spectra were revealed. 021, 11, x FOR PEER REVIEW Figure 8. Diffraction patterns of LiNb1−xTaxO3 nanopowders (x = anosynthesis in optimal modes (milling speed-600 rpm, mill for 5 h), as well as the reference diffractogram of LiNbO3.

Raman Spectra
The micro-Raman spectra of LN-LT nanopowders w annealed at 550 °C, are shown in Figure 9, and the posi indicated in Table 3. As it is seen from Figure 9, the Raman х are generally similar; however, some peculiarities are ob trum of pure LN, 15 Raman bands can be distinguished, A1 and E vibrational modes. The A1 modes are polarized bly degenerate E modes correspond to ionic motions alo spectrum of LN-LT (x = 0.25), 25 Raman bands can be disti of bands is equal to 20, for x = 0.75 to 15 and for pure LT to in the number of distinguished bands could probably b some bands with those with higher intensity. The observe different compositions of the nanopowders as well as wit ogy regimes of nanopowders synthesis. Particularly, the of the LN-LT nanopowders Raman spectra were revealed
In accordance with the results of [38], these bands could be induced by the presence of Li(Nb,Ta) 3 O 8 phase, as identified by X-ray diffraction technique (see Table 2).

2.
A more intensive band near 117-125 cm −1 is observed only for the LN-LT sample with x = 0.5. The presence of this band could be attributed to the contribution of Li(Nb,Ta) 3 O 8 and Ta 2 O 5 additional phases in this sample. Note that the authors in [38] observed the close bands at 116 and 136 cm −1 and attributed them to LiNb 3 O 8 . The bands near 100 cm −1 were attributed to Ta 2 O 5 by the authors of [39]. As it is followed from the XRD data (see Table 1), the simultaneous presence of Li(Nb,Ta) 3 O 8 and Ta 2 O 5 phases occurs only in the sample with x = 0.5, so the overlay of corresponding bands can result in a peculiar form of its spectrum. Furthermore, the sample with x = 0.5, i.e., with the composition intermediate between pure LN and pure LT, ought to essentially reveal the bands of both crystals, so it is no wonder that the spectrum of this sample has the most complex character. Furthermore, the sample with x = 0.5 reveals a significant increase of the bands' intensities near 260 and 630-670 cm −1 that visually looks like a widening of intensive neighboring peaks. This result is in good agreement with [40], where two intensive neighboring bands in the region of 600 cm −1 were also observed for the LN-LT sample with x = 0.553. Finally, it should be noted that the bands near 600 cm −1 are considerably broad for all investigated samples in comparison with the other observed bands. This is consistent with the results in [41] where it is concluded that the band at 600 cm −1 is broader for non-poled LN samples (particularly, nanopowders) than for polarized.

3.
The Raman spectra of LN and LT nano-and micropowders are shown in Figure 10 for comparison purposes. The latter were obtained by the crushing of LN and LT single crystals grown at SRC 'Electron-Carat'. As seen from Figure 10, the band observed at about 1008-1009 cm −1 for LT nanopowder is not pronounced for LT micropowder as well as for LN compounds. The similar band can be observed in Figure 9 for nanopowders with x = 0. As it is seen from Figure 9, the intensity of this band increases with increasing of x. Moreover, for x = 0.5, this band splits into two with the frequencies of 994 and 1008 cm −1 . As it is shown in [39], this band is absent in Ta  Contrary to the results in [42], we did not observe any remarkable effect of grain size reduction, i.e., decreasing of the intensities of all Raman bands caused by grain size decrease (see Figure 10).

5.
Increasing the spectral range up to 4000 cm −1 allows revealing the weak vibrations at 1600 and 3400 cm −1 (looking as low-intensive wide bands) that can be caused by the traces of OH-groups, which are always present in LN and LT as well as the traces of HCO 3 − groups (near 1750 and 2900 cm −1 ) present in synthesized compounds, which is probably due to the use of lithium carbonate as a component of the initial mixture.
Similarly to X-ray studies, we have measured the Raman spectra of nanopowder samples obtained under optimal conditions. These spectra are shown in Figure 11.

Raman Spectra
The micro-Raman spectra of LN-LT nanopowder annealed at 550 °C, are shown in Figure 9, and the p indicated in Table 3. As it is seen from Figure 9, the Ram х are generally similar; however, some peculiarities are trum of pure LN, 15 Raman bands can be distinguish A1 and E vibrational modes. The A1 modes are polariz bly degenerate E modes correspond to ionic motions spectrum of LN-LT (x = 0.25), 25 Raman bands can be d of bands is equal to 20, for x = 0.75 to 15 and for pure LT in the number of distinguished bands could probabl some bands with those with higher intensity. The obse different compositions of the nanopowders as well as w ogy regimes of nanopowders synthesis. Particularly, t of the LN-LT nanopowders Raman spectra were revea  1600 and 3400 cm −1 (looking as low-intensive wide b traces of ОН-groups, which are always present in L HCO3 − groups (near 1750 and 2900 cm −1 ) present in is probably due to the use of lithium carbonate as a c Similarly to X-ray studies, we have measured the Ram ples obtained under optimal conditions. These spectra ar  Figure 11. Raman spectra of LiNb1−xTaxO3 nanopowders obtained As seen from Figure 11, the spectra for the LiNb1−xTaxO those recorded for samples, which were obtained in non-op  As seen from Figure 11, the spectra for the LiNb 1−x Ta x O 3 are generally consistent with those recorded for samples, which were obtained in non-optimal conditions (see also Figure 9). The only difference is the band observed near 1000 cm −1 for the LiNb 1−x Ta x O 3 specimens with x = 0.25, 0.5, 0.75, and 1, obtained in a non-optimal regime (Figure 9). This band vanishes from the spectra of specimens, which are obtained in optimal regimes. The nature of this band remains not clear and requires additional studies.

Electrical Conductivity
Impedance data of the pressed LiNb x Ta 1−x O 3 samples are exemplarily shown for 400 • C ( Figure 12) and 600 • C (Figure 13) in the form of Nyquist diagrams. The representation of the resistivity is chosen to eliminate the geometrical factors of the samples (thickness and area). At 400 • C, the impedance data of all samples show a slightly depressed single arc semicircle. Such depression could be attributed to the non-ideal capacitance of the samples, which corresponds to low values of the CPE exponents obtained from fitting of an R b -CPE equivalent circuit model to experimental data. In particular, at 400 • C, exponent values vary between 0.74 (LiTaO 3 ) and 0.85 (LiNb 0.75 Ta 0.25 O 3 ) and slightly decrease with the tempera-ture increase. Another possible interpretation implies that the obtained impedance spectra may consist of two overlapping semicircles, with similar relaxation times, representing grain interior and grain boundary conduction mechanisms, respectively [43]. We note that in case of nanostructured materials, the grain boundary contributions may significantly overlap with that of the grain interiors, making it hard to distinguish between them [43,44]. Impedance spectra of all samples at temperatures above 500 °C reveal th of linear region of ρ''(ρ') dependence, which follows the semicircle intercept a quencies. This is exemplarily shown in Nyquist diagram at 600 °C (Figure temperature, the impedance of the samples has been additionally measured d Hz in order to examine the low-frequency region of the ρ''(ρ') dependence in m however, no changes in the line slope were observed. According to [45], such at low frequencies can be attributed to the electrode effect, which is typical f conductor: mobile charge carriers in form of ions are blocking the metal-samp On the other hand, a similar behavior of Z″(Z′) dependence at low frequencies, observed in [46] for polycrystalline lithium niobate samples at temperatures ab was associated with a grain boundaries conduction mechanism.  Impedance spectra of all samples at temperatures above 500 °C reveal the existe of linear region of ρ''(ρ') dependence, which follows the semicircle intercept at lower quencies. This is exemplarily shown in Nyquist diagram at 600 °C ( Figure 13). At temperature, the impedance of the samples has been additionally measured down to Hz in order to examine the low-frequency region of the ρ''(ρ') dependence in more de however, no changes in the line slope were observed. According to [45], such peculia at low frequencies can be attributed to the electrode effect, which is typical for an i conductor: mobile charge carriers in form of ions are blocking the metal-sample interf On the other hand, a similar behavior of Z″(Z′) dependence at low frequencies, which observed in [46] for polycrystalline lithium niobate samples at temperatures above 550 was associated with a grain boundaries conduction mechanism. The temperature dependencies of LiNbxTa1−xO3 conductivity are shown in Figur in form of Arrhenius plots. Generally, the behavior of conductivity is similar to tha single crystalline LiNbxTa1−xO3 reported in [47]. Furthermore, the conductivity tend correlate with Nb/Ta ratio, increasing with an increase of Ta content and reaching values at 820 °C of 0.022 S/m and 0.076 S/m for x = 0 and x = 1, respectively. However conductivity of LiNbxTa1−xO3 specimen with x = 0.5 deviates from this correlation, w Impedance spectra of all samples at temperatures above 500 • C reveal the existence of linear region of ρ"(ρ') dependence, which follows the semicircle intercept at lower frequencies. This is exemplarily shown in Nyquist diagram at 600 • C (Figure 13). At this temperature, the impedance of the samples has been additionally measured down to 0.01 Hz in order to examine the low-frequency region of the ρ"(ρ') dependence in more detail; however, no changes in the line slope were observed. According to [45], such peculiarity at low frequencies can be attributed to the electrode effect, which is typical for an ionic conductor: mobile charge carriers in form of ions are blocking the metal-sample interface. On the other hand, a similar behavior of Z"(Z ) dependence at low frequencies, which was observed in [46] for polycrystalline lithium niobate samples at temperatures above 550 • C, was associated with a grain boundaries conduction mechanism.
The temperature dependencies of LiNb x Ta 1−x O 3 conductivity are shown in Figure 14 in form of Arrhenius plots. Generally, the behavior of conductivity is similar to that of single crystalline LiNb x Ta 1−x O 3 reported in [47]. Furthermore, the conductivity tends to correlate with Nb/Ta ratio, increasing with an increase of Ta content and reaching the values at 820 • C of 0.022 S/m and 0.076 S/m for x = 0 and x = 1, respectively. However, the conductivity of LiNb x Ta 1−x O 3 specimen with x = 0.5 deviates from this correlation, which could not be solely attributed to experiment uncertainty. The latter, according to our estimations, is approximately 6% at 800 • C. This issue requires additional studies and will be a subject of subsequent investigations.
Crystals 2021, 11, x FOR PEER REVIEW 14 could not be solely attributed to experiment uncertainty. The latter, according to our mations, is approximately 6% at 800 °C. This issue requires additional studies and wi a subject of subsequent investigations.
The absolute values of conductivity ( Figure 14) are generally higher, comparin the obtained previously for single crystalline and polycrystalline LiNbO3 and LiTaO3 52]. As reported in [51], such a difference is associated with a greater number of free in polycrystalline LiNbO3 and LiTaO3 due to a developed system of grain boundaries much more defective structure in general [51]. Consequently, the variation of conduc ity, reported for different polycrystalline samples, can be attributed to differences in average grain size in particular. The conductivity of all samples increases linearly in Arrhenius presentation u around 620 °C. After a transition temperature range of 620-670 °C, we observed the ductivity increase as linear again; however, the slope changes (see mark in Figure 14). T indicates that the conductivity is governed by a different process than that at lower t peratures. The obtained results enable the determination of activation energy, EA u the relation: where σ0, T, EA, and k represent the pre-exponential constant, absolute temperature, vation energy, and the Boltzmann constant, respectively. Activation energies and pre ponential factors, obtained by fitting the Arrhenius equation to the measured conducti data, as well as the temperature ranges for fitting are summarized in Table 4. The co sponding fits for low and high-temperature regions are exemplarily shown for the LiT specimen in Figure 15. The absolute values of conductivity ( Figure 14) are generally higher, comparing to the obtained previously for single crystalline and polycrystalline LiNbO 3 and LiTaO 3 [45][46][47][48][49][50][51][52]. As reported in [51], such a difference is associated with a greater number of free ions in polycrystalline LiNbO 3 and LiTaO 3 due to a developed system of grain boundaries and much more defective structure in general [51]. Consequently, the variation of conductivity, reported for different polycrystalline samples, can be attributed to differences in the average grain size in particular.
The conductivity of all samples increases linearly in Arrhenius presentation up to around 620 • C. After a transition temperature range of 620-670 • C, we observed the conductivity increase as linear again; however, the slope changes (see mark in Figure 14). This indicates that the conductivity is governed by a different process than that at lower temperatures. The obtained results enable the determination of activation energy, E A using the relation: where σ 0 , T, E A , and k represent the pre-exponential constant, absolute temperature, activation energy, and the Boltzmann constant, respectively. Activation energies and pre-exponential factors, obtained by fitting the Arrhenius equation to the measured conductivity data, as well as the temperature ranges for fitting are summarized in Table 4. The corresponding fits for low and high-temperature regions are exemplarily shown for the LiTaO 3 specimen in Figure 15.   Generally, the activation energies obtained in our study (Table 4) are consistent w the values obtained previously for polycrystalline lithium niobate and/or lithium tanta Generally, the activation energies obtained in our study (Table 4) are consistent with the values obtained previously for polycrystalline lithium niobate and/or lithium tantalate [43,46,[50][51][52][53], varying from 0.63 to 1.25 eV. It should be noted that the wide range of activation energies reported [43,46,[50][51][52][53] is associated with their strong dependence on samples grain size, which is in particular shown in a comparative impedance spectroscopy study of single-, micro-, nanocrystalline, and amorphous lithium niobate [43]. Similar to our study, the authors in [43] observed slightly depressed semicircles on ρ"(ρ') dependence of nanocrystalline LiNbO 3 and attributed such depression to the superposition of two conduction mechanisms, arising from the grain interior and grain boundary, respectively. The nanopowders studied in [43] were obtained by high-energy ball milling with the grains size of about 20 nm.
Qualitatively similar dependencies were observed for polycrystalline samples of nonstoichiometric LiNbO 3 in [53]. Here, the authors have obtained the LiNbO 3 micropowders by the ball milling technique with the grain sizes varying from 2 to 3 m. Obtained powders were subsequently annealed and isostatically pressed in pellets at 2500 bars [53]. It is assumed in [53] that the conductivity of LiNbO 3 in low and high-temperature regions is governed by the polaronic and ionic conduction mechanisms, respectively. The activation energies obtained in [53] vary from 0.78 to 0.88 eV for the low-temperature region and from 1.07 to 1.20 eV for the high-temperature region, which is in good correlation with the values obtained in our study.
Furthermore, the authors in [51] studied the electrical properties of polycrystalline LiNbO 3 up to 800 K and obtained the activation energy of 0.88 eV, which is very close to the corresponding value in our work. The sol-gel method was applied for the synthesis of nanopowders. Subsequently, the powders were annealed at 1200 • C for 3 h and pressed in pellets [51]. The average grain size was between 0.2 and 2 m. Since the measurements in [51] did not extend to higher temperatures, no evidence of a second conducting mechanism was observed.

Conclusions
In summary, it is shown in the current study that the mechanosynthesis is the most simple and most accessible method of lithium niobate and tantalate nanoparticles production. This method allows obtaining LiNb 1−x Ta x O 3 nanopowders in only two stages: high-energy ball milling of the raw powders (Li 2 CO 3 , Nb 2 O 5 , Ta 2 O 5 ) and subsequent annealing of the obtained precursors. Furthermore, this method allows completely excluding usually undesirable wet chemistry procedures from the technological process.
A number of LiNb 1−x Ta x O 3 nanopowders (x = 0, 0.25, 0.5, 0.75, 1) is obtained by means of the mechanosynthesis technique for the first time, to the best of our knowledge.
The X-ray analysis of LiNb 1−x Ta x O 3 nanopowders obtained at different conditions allowed for determination of the optimal parameters for milling and annealing runs: milling at 600 rpm for 12-15 h and subsequent annealing of powders in air at temperatures 650-700 • C for 5 h. This allowed obtaining single-phase LiNb 1−x Ta x O 3 nanopowders for all the studied values of x.
The crystal structure of the samples was determined by the X-ray diffraction technique. The full-profile Ritveld refinement was used for determination of the crystal structure parameters and micro-structure parameters of LiNb 1−x Ta x O 3 nanopowders. It is shown that the replacement of niobium by tantalum in the LiNb 1−x Ta x O 3 structure asymmetrically influences the parameters of the unit cell: increasing the Ta content x leads to increasing the lattice parameter a and the simultaneous decreasing of parameter c accompanied by a slight decreasing of the unit cell volume. It is shown that the average size of crystallites varies from 31 nm (treatment only at 550 • C) to 206 nm (additional treatment at 800 • C) for different samples.
Raman scattering in LiNb 1−x Ta x O 3 nanopowders (x = 0, 0.25, 0.5, 0.75, 1) obtained by mechanosynthesis was studied, to the best of our knowledge, for the first time. It is shown that the obtained Raman spectra are generally similar. Some features (band shifts, changes in their intensity, the formation of new bands) are attributed to the different values of x. For samples obtained in non-optimal milling and annealing runs, bands typical for the parasitic phases LiNb(Ta) 3 O 8 , Nb 2 O 5 , and/or Ta 2 O 5 are observed in the spectra.
The measured temperature dependence of electrical conductivity in air up to 820 • C shows similar behavior for all studied LiNb 1−x Ta x O 3 samples and tends to correlate with the Nb/Ta ratio. Two linear regions are observed in the Arrhenius presentation, which is attributed to different conductivity mechanisms. Activation energies vary from 0.86 ± 0.015 eV to 0.91 ± 0.015 eV for different compositions in the low-temperature region and from 0.99 ± 0.069 eV to 1.09 ± 0.072 eV at high temperatures. Data Availability Statement: All relevant data presented in the article are stored according to institutional requirements and as such are not available online. However, all data used in this manuscript can be made available upon request to the authors.