Engineering of LiTaO₃ Nanoparticles by Flame Spray Pyrolysis: Understanding In Situ Li-Incorporation into the Ta₂O₅ Lattice

Abstract

Lithium tantalate (LiTaO₃) perovskite finds wide use in pyroelectric detectors, optical waveguides and piezoelectric transducers, stemming from its good mechanical and chemical stability and optical transparency. Herein, we present a method for synthesis of LiTaO₃ nanoparticles using a scalable Flame Spray Pyrolysis (FSP) technology, that allows the formation of LiTaO₃ nanomaterials in a single step. Raman, XRD and TEM studies allow for comprehension of the formation mechanism of the LiTaO₃ nanophases, with particular emphasis on the penetration of Li atoms into the Ta-oxide lattice. We show that, control of the High-Temperature Particle Residence Time (HTPRT) in the FSP flame, is the key-parameter that allows successful penetration of the -otherwise amorphous- Li phase into the Ta₂O₅ nanophase. In this way, via control of the HTPRT in the FSP process, we synthesized a series of nanostructured LiTaO₃ particles of varying phase composition from {amorphous Li/Ta₂O₅/LiTaO₃} to {pure LiTaO₃, 15–25 nm}. Finally, the photophysical activity of the FSP-made LiTaO₃ was validated for photocatalytic H₂ production from H₂O. These data are discussed in conjunction with the role of the phase composition of the LiTaO₃ nanoparticles. More generally, the present work allows a better understanding of the mechanism of ABO₃ perovskite formation that requires the incorporation of two cations, A and B, into the nanolattice.

1. Introduction

Perovskites are efficient materials in electrocatalytic applications due to their low activation energy and high electron transfer kinetics; they also have the potential to be used as efficient photocatalysts due to their long-term stability and selectivity in catalytic reactions [1,2]. In the family of perovskites, titanates, tantalates, niobates, vanadates and ferrites have been used in many applications such as gas sensors, solid oxide fuel cells, dielectrics, ferroelectrics and photocatalysts for water splitting, to name a few [3,4,5]. Tantalum pentaoxide (Ta₂O₅) and its alkali tantalate perovskites, like lithium-tantalate (LiTaO₃), sodium-tantalate (NaTaO₃) and potassium-tantalate (KTaO₃), have been demonstrated as highly effective materials in photocatalytic water splitting and CO₂ reduction [5,6,7], making them promising candidates for solar energy conversion applications. Their photocatalytic activity is both attributed to, and determined by, the d-orbitals of Ta located at the conduction band, as well as to the efficient carrier delocalization promoted by small distortions in TaO₆ bonds [2,8].

Tantalate perovskites have been shown to be able to perform water splitting without the use of a cocatalyst [9,10] under proper UV photon excitation, i.e., their energy gap lies around 4.7 eV for LiTaO₃, 4 eV for NaTaO₃ and 3.6 eV for KTaO₃ [10,11]. Among the alkali tantalates, LiTaO₃ stands out as one of the most notable materials due to its remarkable physical properties, including piezoelectricity, pyroelectricity and ferroelectric characteristics [12]. Kato and Kudo investigated alkali tantalates and their photocatalytic water splitting ability, i.e., splitting water into H₂ and O₂ without the use of cocatalysts such as Pt or NiO [13,14].

So far, the methods used to synthesize perovskite are numerous, and all of them require high temperatures, i.e., sol–gel [15], hydrothermal [16], microwave [17,18] and flame aerosol synthesis [4,19,20]. The synthetic approach can have a great impact on the properties of the final material, affecting the morphology, shape, crystallite size, specific surface area (SSA) and structure of the perovskite. Compared to other synthetic methods, the fundamental advantage of flame spray pyrolysis (FSP), a flame aerosol synthesis route, stems from its straightforward scalability, enabling industrial production yields to reach kilograms per hour [21]. However, the synthesis of perovskite ABO₃ nanoparticles using FSP is rather challenging considering that we demand the concomitant insertion of two cations, A and B, into the lattice of ABO₃ at the correct stoichiometry of 1:1:3.

So far, there are few works in the literature regarding the FSP synthesis of ABO₃ nanostructures; for example, Angel et al. synthesized LaMO₃ (M = Mn, Fe, Co) nanomaterials using FSP and investigated the effect of dispersion gas flow on both the particle size and morphology [22]. In another work, Yuan et al. used the FSP method to deposit CuO quantum dots onto SrTiO₃ perovskite [23]. The authors explored the role of copper loading onto perovskites and optimized the catalytic efficiency for the complete oxidation of lean CO and CH₄, with the best-performing nanocomposite being 15 mol% CuO. Our group used FSP to synthesize BiFeO₃ perovskite or mullite-type Bi₂Fe₄O₉, which proved to be both efficient in the catalytic reduction of 4-nitrophenol to 4-aminophenol [24] and a highly efficient photocatalyst for O₂ production from H₂O [25].

In general, FSP is an effective method for the production of particles well-below 100 nm, a characteristic that might be difficult to achieve using wet synthesis routes. However, the crystallinity and phase transformation of flame-made perovskites are not straightforward. Kho et al. investigated the degree of crystallinity of as-prepared FSP-made BiVO₄ nanoparticles [26]. By varying the filter temperature (T_f) during FSP synthesis, the authors transitioned from the initially dominant amorphous to highly crystalline monoclinic-scheelite BiVO₄ when T_f was greater than 360 °C. The crystallization of BiVO₄ is highly dependent on T_f, suggesting that, the short residence time of the particles in the FSP flame, is inadequate to induce crystallization within the flame. The same trend was verified by our group in the case of FSP synthesis of W- and Zr-doped BiVO₄ [27]. Recently, we showed that FSP can achieve the synthesis of highly crystalline NaTaO₃ under the condition that a well-defined protocol is adopted [28]. Specifically, we found that in FSP-made NaTaO₃, the combustion profile in the FSP flame, i.e., the precursor/dispersion ratio (P/D), has a significant impact on both the crystallinity and the phase composition of the final material. A higher P/D ratio, where P corresponds to a higher solvent feed flow in the flame that boosts combustion enthalpy, favors the formation of 100% crystalline NaTaO₃ perovskite [28].

Herein, our aim was to develop an FSP process protocol to successfully synthesize crystalline LiTaO₃ nanomaterials. To do this, we systematically studied the thermodynamics of the FSP process that control the transition from {amorphous Li} + {Ta₂O₅} to crystalline LiTaO₃ perovskite. The underlying solid-state process involves the incorporation of Li atoms into the Ta₂O₅ lattice and thermodynamic stabilization to a stable LiTaO₃ crystal. Thus, our specific aims were [i] to use FSP to synthesize a series of amorphous Li/Ta₂O₅/LiTaO₃ nanomaterials with different percentages of the three components, [ii] to show that amorphous Li/Ta₂O₅ could be transformed to crystalline LiTaO₃ in a single post-FSP calcination step, [iii] to optimize the FSP process configuration in order to obtain high-purity (>95%) LiTaO₃ in one step and [iv] to validate the functional quality of the resulting LiTaO₃ in photocatalytic H₂ production from H₂O.

By an analysis of key data (XRD, Raman) with the aid of TEM, we provide a comprehensive physicochemical frame that allows an understanding of the Li/Ta₂O₅→LiTaO₃ transition in the FSP flame. We show that the HTPRT is the key parameter that must be optimized via control of the FSP reactor parameters.

2. Materials and Methods

2.1. Synthesis of LiTaO₃ Nanoparticles via Flame Spray Pyrolysis (FSP)

For the synthesis of LiTaO₃ nanoparticles (NPs), for convenience, we codenamed the FSP-made LiTaO₃ materials as LTO. A single-nozzle FSP reactor was used, schematically illustrated in Figure 1. Precursor solutions were prepared by dissolving 0.3 M Ta (V) n-Butoxide (99.9% Ta, STREM (Newburyport, MA, USA)) and 0.3 M Li-Butoxide (98%, STREM (Newburyport, MA, USA)) in xylene. Each precursor solution was then fed into the FSP system through a capillary with a steady precursor flow rate (P) of 3, 5, or 9 mL min⁻¹ and dispersed into fine droplets using an oxygen dispersion flow rate (D) of 9, 5, or 3 L min⁻¹ (Linde 99.999%). To initiate combustion, a premixed O₂/CH₄ pilot flame at flow rates of 4 L min⁻¹ and 2 L min⁻¹, respectively, was applied. Moreover, a 44 cm metal tube placed 2 cm above the burner (Burner–Tube Distance (BTD) = 2 cm) was used to enclose the flame compartment from the surrounding environment, prolonging the HTPRT and preventing cooling of the flame. Finally, the particle collection was assisted by an additional 10 L min⁻¹ O₂ sheath flow and a vacuum pump (BUSCH V40). The powder was then retrieved by scrubbing it off from a glass microfiber filter with a binder (Albet Labscience, Hahnemuehle, Dasen, Germany, GF 6 257) filter.

2.2. Structural Characterization Techniques

Raman spectroscopy: The structural features of the LTO NPs were evaluated by Raman spectroscopy using a HORIBA-Xplora Plus spectrometer (Kyoto, Japan), equipped with an Olympus BX41 microscope (Breinigsville, PA, USA). A 785 nm diode laser served as the excitation source, with the laser beam focused on the sample using the microscope. Raman spectra were obtained by performing 30 accumulations over 10 se at a consistent laser intensity, which was 100% of the laser’s total intensity.

Ultraviolet–Visible Diffuse Reflectance (UV–Vis DRS): DRS experiments were conducted using a Perkin Elmer Lambda-35 spectrophotometer (Waltham, MA, USA) using BaSO₄ powder as the background standard while operating at room temperature. The optical energy gap value (E_g) of each LTO semiconductor was determined through extrapolation of a straight line on the curve that was formed from the correlation between (αhv)^1/n and hv, called the Tauc plot method [28], and the following equation:

$${\left(ahv\right)}^{1/n}=C\left(hv-E_g\right)$$

(1)

where α is the absorption coefficient, hv is the photon energy, E_g is the energy gap and C is a constant. The exponent denotes the nature of the electronic transition; when n = 1/2, it is a direct allowed transition, and when n = 2, it is an indirect allowed transition.

Powder X-ray Diffraction (pXRD): In order to obtain the crystal structures of the nanoparticles, a D8 Advance Bruker diffractometer with a Cu source (Kα, λ = 1.5418 Å) and a secondary beam monochromator was used. All XRD patterns were collected at room temperature, operating at 36 kV and 36 mA. Using the Scherrer equation, the crystal size (d_XRD) of the nanoparticles was calculated [29]:

$$d_{XRD}={\displaystyle \frac{K \lambda }{\left(FWHM\right)\ast cos\theta }}$$

(2)

where K = 0.9, λ = 1.5418 Å and FWHM is the full width at half-maximum of the XRD peaks.

Transmission Electron Microscopy (TEM): The morphology and nanostructure of the nanoparticles were unveiled using an FEI Titan 80-300 S/TEM microscope at 300 kV accelerating voltage and a 21.5 mrad beam convergence angle. The powdered sample preparation involved sonicating the samples in ethanol, followed by depositing the resulting homogeneous suspension as a single droplet onto a TEM copper grid covered with lacey carbon film.

2.3. Photocatalytic Evaluation

Photocatalytic H₂ production from H₂O: Photocatalytic experiments were conducted in a double-walled photochemical reactor (Toption instrument Co., Ltd. (TaiBai Road, YanTa District, Xi’an, China)) with a total volume of 340 mL at room temperature (25 °C), regulated by tap circulation. A 250 W mercury lamp (UV irradiation) was used as the light source, positioned at the geometric center of the photoreactor within a quartz immersion well. The irradiation power, measured in situ with a power meter at an average experimental distance of 3 cm, was 0.34 W cm⁻². For each experiment, 69 mg of the catalyst was suspended in 275 mL of water/methanol mixture (methanol 20% v/v), where methanol served as a hole scavenger. To increase the photocatalytic production, 1% w/w Pt was photodeposited in situ in the reaction mixture. As the Pt precursor, dihydrogen hexachloroplatinate (IV) hydrate complex (H₂Pt₄C_l6. 6H₂O, 99.99%, Alfa Aesar (Ward Hill, MA, USA)) was used. In order to identify and quantify the produced gases, such as H₂, a Gas Chromatography system equipped with a Thermal Conductivity Detector (TCD) (Shimadzu GC-2014, Agilent Carboxen 1000 column (Stevens Creek Blvd., Santa Clara, CA, USA), Ar carrier gas) was employed.

3. Results

As shown in the XRD patterns in Figure 2 the diffraction peaks of the as-prepared LTO nanoparticles matched the structures of Ta₂O₅ (PDF 00-025-0922) and LiTaO₃ (PDF 00-029-0836) and exhibited different phase percentages. A detailed list of the so-produced nanomaterials can be found in

Table 1. FSP process parameters and structural characteristics of the as-prepared LiTaO₃ NPs.

Nanomaterial	P/D	[Li/Ta] Concentration (mM)	BTD (cm)	HTPRT (cm)	% Phase (±5%)		Crystallite Size (nm) (±0.5 nm) 1		CI (%) 2
					Ta2O5 (%)	LiTaO3 (%)	dXRD Ta2O5	dXRD LiTaO3
0.6-LTO_3_9	3/9	300:300	2	44	94	6	33.8	-	22.6
0.6-LTO_5_5	5/5	300:300	2	44	40	60	26.7	16.6	50.7
0.6-LTO_9_3	9/3	300:300	2	44	30	70	24	27.9	70.6
0.6-LTO_5_5	5/5	300:300	1	44	5	95	-	15.2	76.8
0.6-LTO_5_5	5/5	300:300	0.5	44	2	98	-	25	83.4

¹ The ±0.5 nm uncertainty stems from the uncertainty in the estimations of the FWHM of the XRD peaks. ² Crystallinity index (CI) = 100% − amorphous content (%). The amorphous content was determined by measuring the area under the curvature hump in the XRD baseline.

, where the materials are codenamed with respect to their total {Li + Ta} concentration, i.e., 0.6 M, and their corresponding P/D ratio. In all cases, the size of the crystalline domains (d_XRD) was estimated from the XRD patterns using the Scherrer equation and the peaks at 22.8° and 23.8°, which correspond to (0 0 1) and (0 1 2) of Ta₂O₅ and LiTaO₃, respectively; these are listed in Table 1. As such, d_XRD refers to the crystallite nanosize, while the size of the NPs can be further characterized using TEM.

With both the {Li + Ta} concentration and the BTD kept constant, adjusting the P/D ratio from 3/9 to 5/5 and finally to 9/3 led to an increase in the LiTaO₃ phase percentage at the expense of Ta₂O₅; see Table 1. This can be understood by taking into account that an increased P/D ratio means that more precursor is injected into/combusted in the flame; this results in longer and hotter flames and an enhanced HTPRT, which ultimately defines particle growth [30]. On the opposite side, a decrease in the P/D ratio leads to shorter flames, limiting the HTPRT. In the present case, for {0.6-LTO_3_9}, the P/D = 3/9 flame resulted in 94% Ta₂O₅ with only 6% LiTaO₃. As we show in Figure S1 and Table S1, in the {0.6-LTO_3_9} material, a short post-FSP treatment (480 °C, 30 min) boosted the LiTaO₃ formation to 74%. This demonstrates that in the as-prepared {0.6-LTO_3_9}, the Li atoms are present but not incorporated into any crystalline lattice; i.e., this material is [amorphous Li] + [Ta₂O₅] with little LiTaO₃. The short HTPRT for P/D = 3/9 did not allow the incorporation of Li into Ta₂O₅ to form LiTaO₃.

In general, the production of ABO₃ perovskites involves rather challenging thermodynamically driven ABO₃ phase formation, if we take into account the competing processes: the possibility of forming separate AO_n and BO_m oxides. Moreover, in the FSP process, this should be considered taking into account the speed of the process, i.e., milliseconds of exposure time for the particles in the high-temperature region [21] and temperature gradients ranging from ~3000 °C to 600 °C at the flame’s apex. In our case, the XRD data clearly showcase the dominant role of the flame profile (via the P/D) in LiTaO₃ perovskite formation. An increase in the P/D ratio has a profound effect on the phase composition. At P/D = 3/9, the incorporation of Li in the nanolattice failed, since we observed only the formation of Ta₂O₅. At a higher P/D ratio, LiTaO₃ was formed at a percentage of 70% in the case of 0.6-LTO_9_3 due to the prolongation of the time window in which the particles remained in the high-temperature region. However, we noticed that while the crystallinity of the final material increased as the P/D ratio increased from 22.6% up to 70.6%, the percentage of the LiTaO₃ phase had not yet reached a maximum (please see Table 1).

Thus, the present data for LiTaO₃, as well as our recent work on NaTaO₃ [28], clearly exemplify that in such ABO₃ perovskites, the P/D ratio serves a dual purpose, affecting both the crystallinity and the phase composition. However, further optimization was required for LiTaO₃.

To examine whether Li was present, and had not yet formed a crystal structure after FSP, the FSP process was followed by annealing at 480 °C for 30 min in atmospheric air. As shown in Figure S1, a quantitative analysis of the XRD patterns exhibited an increase in the LiTaO₃ percentages; therefore, we can conclude that even if the temperatures inside the flame region are more than enough for the formation of the LiTaO₃ phase, the effect of the short time-window of the FSP process is overwhelming, suppressing the formation of the perovskite structure.

In our endeavor to produce pure-phase LiTaO₃, we further adjusted the BTD parameter to regulate the air entrainment (AE). Process-wise, Pratsinis et al. quantitatively analyzed the role of AE during aerosol synthesis by computational fluid dynamics (CFD) and calculated the temperatures inside an enclosed FSP reactor [31]. Based on their findings, the overall temperature inside the tube decreases as AE increases due to cold gas (air) intrusion, plus dilution of the combustibles [32]. In the absence of AE, such as in a hermetically enclosed FSP burner, a recirculation vortex is expected to be formed [32], which leads to an extension of the high-temperature zone inside the reactor; therefore, the particles remain at high temperatures for a longer duration, which results in particle growth. In the intermediate case of AE = 50–100 L min⁻¹, the vortex is diminished and the overall temperature inside the tube decreases, leading to reduced particle growth. For higher AE and a longer BTD, the flow and temperature patterns are similar to those for the open FSP configuration [31].

In our case, the formation of high-purity LiTaO₃ nanopowders is closely related to the BTD parameter, in addition to the P/D ratio. In all cases, the {Li + Ta} concentration, HTPRT and P/D ratio were kept constant and a second series of LTO materials was prepared by adjusting only the BTD (see Figure 3A). By decreasing the BTD to 1 cm, with lower AE, we observed a dramatic effect on the Ta₂O₅ phase, which was reduced to 5% from 40%, while the LiTaO₃ phase increased from 60% to 95%. A further decrease in the BTD induced small changes in the phase composition, reaching 2% and 98% for Ta₂O₅ and LiTaO₃, respectively (see Figure 3B). Based on the CFD analysis by Pratsinis [31] and the research by Waser [32] regarding the effect of AE, altered via lifting-off the enclosing tube, during CuO synthesis, a clear correlation between the BTD and AE emerges. In the case of CuO, enclosure of the FSP reactor (AE = 0 L min⁻¹) resulted in larger nanoparticles i.e. due to their prolonged exposure to high temperatures [33]. Increasing the BTD results in higher AE flows, smaller nanoparticles, and reduced vortex recirculation inside the tube, which subsequently leads to a decrease in the temperature flow field [31,32]. In the present case of LiTaO₃, Figure 3A,B show that this increase in the temperature flow-field inside the FSP reactor, as induced by the smaller BTD values, resulted in NPs with higher LiTaO₃ phase percentages. Thus, in this way we achieved the synthesis of the desired LiTaO₃ perovskite phase in a single step, eliminating the need for post-FSP treatment.

An analysis of TEM images using open-access software ImageJ v.1.52a, showed a size distribution ranging from 10 to 90 nm (Figure 3C), where the majority of the particles fell between 10 and 40 nm, with a mean size of d_TEM = 23.6 ± 0.4 nm. Moreover, TEM images (Figure 3D) of the highest-percentage LTO material, i.e., 0.6-LTO_5_5, BTD: 0.5, depicted the formation of quasi-spherical LiTaO₃ particles forming neck-sintered aggregates, typical of FSP-produced nanoparticles. Additionally, Figure 3E shows the formation of crystalline Miller planes with spacing of d₀₁₂ = 3.76 Å corresponding to the LiTaO₃ phase. Energy-dispersive X-ray (EDX) analysis confirmed the existence of Ta but did not detect Li due to its low energy of characteristic radiation (Figure 3H).

The LTO materials were further studied using UV–Vis DRS and Raman spectroscopy. The energy gap values (E_g) of the materials were estimated via the Tauc plot (see Figure 4A,B), using the Kubelka–Munk method. As anticipated, none of the samples exhibited absorption in the visible region. Their spectra consisted of a single absorption band below 400 nm, which is characteristic of this group of oxides. From the Tauc plot in Figure 4B, it is evident that materials 0.6-LTO_3_9_BTD_2 and 0.6-LTO_5_5_BTD_2 exhibited E_g values of 4 eV and 3.92 eV, respectively. These values are in accordance with literature values for Ta₂O₅ E_g equal to 4 eV [33]. At P/D = 9/3, where the LiTaO₃ percentage was higher, we observed an increment in E_g up to 4.5 eV, a value close to the literature E_g of 4.7–4.84 eV for LiTaO₃ [10,11].

Figure S2 in the Supporting Information shows the Raman spectra of the as-prepared LTO materials. In all cases, the absence of well-defined Raman peaks witnesses the formation of an amorphous/semi-crystalline surface on the LTO flame-made perovskites, which is also evident from the TEM images (see Figure 3E). Post-FSP annealing at 480 °C for 30 min of the LTO materials produced Raman spectra of enhanced intensity and sharpness, shown in Figure 4C. According to the literature [34,35], the Raman bands at <250 cm⁻¹ are attributed to the internal deformation of O-2Ta and O-3Ta, while the bands between 300 and 500 cm⁻¹ correspond to Li-O stretching and O-Li-O bending modes. Furthermore, the bands shown at the regions between 550 and 1000 cm⁻¹ are ascribed to the stretching modes of O-3Ta and O-2Ta [34,35]. Overall, taking into account the XRD and Raman spectra, the data in

Table 2. Summarized Raman band frequencies of the calcined materials and the vibrational modes in the crystal lattice that correspond to these frequencies.

Phonon Branch	Assignment	Raman Shift (cm−1)	Raman Shift (cm−1) (Literature)
E (TO1)	O-3Ta and O-2Ta deformation	145	143 [34], 144 [35]
A1 (TO1)		209	208 [34], 209 [35]
E (TO4)	Li-O stretching and O-Li-O bending	317	315 [34,35]
A1 (LO2)		357	356 [34,35]
E (LO5)		381	383 [34], 380 [35]
E (TO7)		466	465 [34], 463 [35]
A1 (TO4)	O-3Ta stretching	597	596 [34,35]
E (LO8)		659	661 [34,35]
A1 (LO4)	O-2Ta stretching	863	861 [34], 863 [35]

allow us to safely conclude that, in these cases, Li atoms were successfully incorporated into the Ta₂O₅ lattice to form a stable LiTaO₃ lattice.

Figure 4D shows the photocatalytic activity of the as-prepared and calcined materials, with the highest percentage of LiTaO₃, i.e., 0.6_LTO_5_5_BTD_0.5 and 0.6_LTO_5_5_BTD_0.5_480 °C_30 min, respectively. Compared to the photocatalytic activity of pristine Ta₂O₅, both LTO perovskites exhibited lower H₂ production under UV irradiation i.e. despite the improved crystallinity induced by the refinement of the BTD. In all cases, Pt⁰ was used as a co-catalyst due to its high work function of Φ_Pt = 5.65 eV, compared to those of other noble metal co-catalysts (Φ_Pd = 5.30 eV, Φ_Au = 5.21 eV and Φ_Ag = 4.26 eV) [36], which leads to a high energy difference between the conduction band of the semiconductor and the noble metal co-catalyst. This energy difference favors the formation of a stronger Schottky barrier and the trapping of electrons in the conduction band of the semiconductor.

Moreover, the slow electron accumulation by the cocatalyst, in the case of LTO materials, leads to retardation of the H₂ production rate, up to 45 min, compared to 15 min delay, observed with Ta₂O₅. However, these results do not come as a surprise, due to the difference in the energy gap values of Ta₂O₅ and LiTaO₃, as we have already stated. Besides this, all ATaO₃ compounds (A being Li, Na or K) feature corner-sharing TaO₆ octahedra forming a perovskite-like structure. The photocatalytic activities of these materials depend on the A-site cation of the structure. It was also reported that the Ta-O-Ta bond angles have an intrinsic role in photocatalytic activity and aid in energy delocalization [37,38]. LiTaO₃ exhibits a bond angle of 143°, while NaTaO₃ and KTaO₃ form bond angles of 163° and 180°, respectively. This makes LiTaO₃ a tantalate perovskite with high E_g, high distortions and low energy delocalization compared to the aforementioned Ta-based materials.

4. Conclusions

In the present work, we extended the versatility of FSP in the family of Ta perovskites through the successful synthesis of LiTaO₃. By adjusting the P/D ratio, which affects both the combustion enthalpy and the HTPRT, simultaneous crystallization and phase transformation from {amorphous Li/Ta₂O₅} to {Ta₂O₅-LiTaO₃} was achieved. However, post-calcination of the flame-made Ta₂O₅-LiTaO₃ sample enhanced the LiTaO₃ phase percentage. Further optimization involved control of the AE by adjusting the BTD, which affected the overall HTPRT profile. These findings imply that the flame synthesis of LiTaO₃ requires finetuning of the exposure of the NPs to the HTPRT zone to achieve high crystallinity and the incorporation of Li into the Ta₂O₅ nanolattice.

However, despite the bold advancements in FSP-made perovskite synthesis, some critical problems emerge. Technology-wise, lithium and tantalum are considered critical elements in Europe due to their vital roles in emerging technologies and their limited supply. Lithium is a key component in rechargeable batteries, particularly for electric vehicles and renewable energy storage systems, which are vital for the transition to a low-carbon economy. Tantalum is widely used in electronic components, such as capacitors and semiconductors, which are crucial for modern electronics and communication systems. Intensive use of Li and Ta will lead to resource scarcity, environmental impacts, supply chain vulnerabilities and economic pressures. To mitigate these challenges, there will be an increased need for technological innovation. This includes developing alternative materials, advancing recycling technologies and improving the efficiency and lifespan of products that utilize lithium and tantalum.