Erbium Orthoniobate-Tantalates: Structural, Luminescent and Mechanical Properties of ErNb_xTa_1−xO₄ Ceramics and Bactericidal Properties of ErNbO₄ Powder

Abstract

Fine powders of erbium niobate-tantalates ErNb_xTa_1−xO₄ (x = 0; 0.1; 0.3; 0.5; 0.7; 0.9; 1) have been synthesized by the liquid-phase method in this study. Ceramic samples have been prepared using conventional sintering from these powders. Rietveld refinement of XRD patterns of polycrystals determined the phase composition and clarified the parameters of the phase structure of ErNb_xTa_1−xO₄ solid solutions depending on the Nb/Ta ratio. The morphological features of the microstructure of erbium niobate-tantalate ceramics have been studied. Their mechanical properties, strength characteristics (Young’s modulus, microhardness) and critical stress intensity factor of the first kind K_IC have been estimated. The photoluminescent properties of ceramic solid solutions of erbium niobate-tantalates depending on the composition have been studied. Dark and photoinduced toxicity of finely dispersed ErNbO₄ powders have been studied in relation to Gram-positive, Gram-negative and spore-forming microorganisms. The best indicators of antibacterial activity of ErNbO₄ have been demonstrated in relation to Gram-positive cells of Micrococcus sp. The discovered properties open up the possibility of not only traditional use as functional materials, but also the use of these materials for disinfection of surfaces, water and biological tissues.

1. Introduction

The role of compounds and functional materials containing rare earth elements (REEs) in the development of modern technologies is constantly and inevitably increasing. The range of applications for such materials is expanding significantly. A huge amount of fundamental research is devoted to the design and characterization of luminescent materials that exhibit new photophysical effects [1,2,3]. The reason for this popularity is their role in modern technologies (display screens, optical communication amplifiers, LED lamps and solid-state lasers); the need for the materials is constantly growing.

Solid-state luminescent materials based on lanthanide niobate-tantalates REE(Nb_xTa_1−x)O₄ have the combined advantages of typical lanthanide photoluminescence (PL) and photophysical stability. Lanthanide PL is known for two types of regimes: the conversion of exciting radiation is increased in one case, and it is decreased in the other case. Up-conversion luminescence (UCL) refers to a nonlinear optical process [3,4,5]. What is characteristic of this process? Two or more near-infrared (IR) photons are sequentially absorbed into several real intermediate energy states, and luminescence is emitted at a wavelength shorter than the wavelength of the absorbed radiation.

Trivalent REE niobate-tantalates, such as Er and Yb, have attracted much attention because these PL materials can convert near-IR excitation into visible emission, that is, they are up-conversion phosphors [4,5,6]. UCL arises from 4f-4f orbital electronic transitions with accompanying wave functions. They are localized within a single lanthanide ion. These internal 4f electronic transitions of lanthanide ions are largely electric-dipole transitions. They are prohibited by the selection rules. However, the rules are relaxed due to the local mixing of f states with higher electronic configurations. The local mixing itself is caused by the splitting of the crystal field levels [7,8,9]. Variation in the Nb/Ta ratio in REE(Nb_xTa_1−x)O₄ increases energy transfer from the emission centers of the crystal host based on the TaO₄³⁻ and NbO₄³⁻ groups to the emission centers of REE cations.

Up-conversion phosphors are widely used in optical devices such as temperature sensors, IR quantum counters, and compact solid-state lasers [10]. However, converting near-IR to visible radiation eliminates background fluorescence and light scattering from biological materials. And this makes, in particular, micro-nano-grained up-conversion phosphors ideal for use in bioimaging with undeniable advantages, for example, the absence of autofluorescence of biological tissues and a large penetration depth [11]. However, the use of ABO₄-type compounds in medical practice requires preliminary biotesting and testing for toxicity, including in relation to various microorganisms [12].

It should also be noted that the phenomenon of resistance of pathogens to therapeutic drugs is the main reason for the negative effect on human health. This phenomenon is the basis for a sharp decrease in the effectiveness of etiotropic therapy for infectious diseases. The increasing resistance of microorganisms, on the one hand, to traditional antimicrobial drugs, and on the other hand, to traditional methods of chemical inactivation of bacteria makes the development of new effective methods for combating bacterial contamination and infections extremely relevant [13].

Finely dispersed powdered phosphors based on the compound ErNbO₄ emit over a wide range of the visible spectrum (~530–670 nm). They are almost insoluble in water in the temperature range −40–+100 °C, and are resistant to alkalis, acids and other aggressive environments. This markedly distinguishes them from organic and organometallic compounds. These properties also make it possible to use them in such conditions and situations where the chemical and temperature stability is important.

This significantly distinguishes them from organic and organometallic compounds used, for example, for antimicrobial photodynamic inactivation. And this expands the scope of its applications, for example, to the purification of industrial and agricultural wastewater, where the chemical and temperature stability of the bactericidal material is important [14,15,16,17,18,19,20,21].

Therefore, the urgent tasks for today are the following: first, the search for promising luminescent phosphors with UCL; secondly, the development of synthesis technology resulting in finely dispersed powders.

Such powders can be used in biology and medicine, including for antimicrobial photodynamic inactivation or bacteriotoxic light therapy (BLT) [14,15]. In addition, their use creates ceramic materials with increased hardness and strength, qualities necessary to produce a variety of sensors and counters, including those operating under conditions of increased mechanical loads.

The goal of this study is the synthesis of fine powders of erbium niobate-tantalates ErNb_xTa_1−xO₄ (x = 0; 0.1; 0.3; 0.5; 0.7; 0.9; 1); the preparation of ceramic samples and the study of their structural, mechanical and luminescent (up-conversion) characteristics; and also the study of the biological properties of ErNbO₄ powders, such as dark and photoinduced toxicity.

2. Methods

The synthesis of erbium niobate-tantalate powders ErNb_xTa_1−xO₄ (x = 0; 0.1; 0.3; 0.5; 0.7; 0.9; 1) was carried out using the liquid-phase method. The synthesis required high-purity fluoride Nb- and Ta-containing solutions or mixtures thereof. Their volumes provided the specified composition of niobate-tantalates. Solutions were prepared as follows: oxides Nb₂O₅ (99.9) and Ta₂O₅ (99.9) were dissolved in HF (99.9, Komponent-reaktiv Ltd., Moscow, Russia). Niobium, tantalum hydroxides or mixtures thereof were precipitated with 25% NH₄OH (99.9 (not more than 10⁻⁵ of 23 impurities), Sigma Tek, Khimki, Russia) from the initial solutions. Then, the sediment was washed three times using repulpation with deionized water to remove F⁻ ions at a ratio of solid (S) and liquid (L) phases S:V_L =1:3, and dried at 100 °C. Then, a solution of Er(NO₃)₃ with a given concentration of Er at a ratio of S:V_L =1:2 was added to the precipitate. Erbium nitrate solutions were prepared as follows: Er₂O₃ (oxide 99.9% LLC HimKraft, Moscow, Russia) was dissolved in HNO₃ (99.9, Vekton Ltd., Saint Petersburg, Russia). A 25% solution of NH₄OH (99, Sigma Tech, Khimki, Russia) at pH ~8–9 was introduced into the pulp formed during stirring. The precipitate was filtered and washed three times with deionized water at S:V_L = 1:4 to remove ammonium ions, and then it was dried. The temperature was ~140 °C. Then, the sediment was calcined in a resistance furnace at 700 °C for 4 h. Then, the sediment was ground in a chalcedony ball mill (grinder) MLW KM1 (MLW, Leipzig, Germany). Then, the powders were calcined for 5 h at 1200 °C. The gravimetric method determined the content of niobium and tantalum in fluoride Nb- and Ta-containing solutions. The potentiometric method determined the content of fluoride ions in fluoride Nb- and Ta-containing solutions. An EV-74 ion meter with an F-selective electrode EVL-1M3 (Zavod izmeritel’nych priborov, Belarus, Gomel’) was used as a research tool. Fluorine in the synthesized powders was analyzed by pyrohydrolysis. Erbium in filtrates and washing solutions was determined by inductively coupled plasma mass spectrometry (ICP-MS) ELAN 9000 DRC-e (PerkinElmer, Hopkinton, MA, USA). The elemental composition of the powders was evaluated by X-ray fluorescence spectrometry using a Spectroscan MAKC-GV (Spectron, Saint Petersburg, Russia).

The specific surface area of the powders was determined by the low-temperature nitrogen adsorption method (BET; FlowSorb II 2300; TriStar 3020 V1. 03, both from Micromeritics, Norcross, GA, USA). The average particle size of the powders d_av was determined from these data by the formula d_av = 6/S_sp·ρ, where S_sp is the specific surface area, m²/g; ρ is the density, g/cm³.

X-ray phase analysis of the synthesized ErNb_xTa_1−xO₄ samples was performed on a Rigaku XtaLAB Synergy-S X-ray diffractometer (Rigaku, Tokyo, Japan) with a new generation PhotonJet-S multifocal source and an HPC detector (Rigaku, Tokyo, Japan). A Rietveld refinement was applied including Full Pattern Decomposition (FPD). The cell parameters of the ceramic solid solutions ErNb_xTa_1−xO₄ (x = 0; 0.1; 0.3; 0.5; 0.7; 0.9; 1) were refined by the WPPF (Whole Powder Pattern Fitting).

From powders obtained using conventional sintering (CS), ceramic samples were prepared in the form of tablets with a diameter of 10 mm and a height of 2–3 mm. Polyvinyl alcohol was the binder. The tablets were pressed at a load of ~3.8 kg/cm². The tablets were sintered in a KEP 14/1400P electric furnace (MZVE LLC, Tryokhgorny, Russia). The temperature was 1400 °C, and the process lasted 3 h.

The multifunctional X-ray diffractometer Rigaku (RIGAKU, Japan, Tokyo) clarified the phase composition and structures of ceramic samples. The device is equipped with SmartLab Studio II software. The speed of the counter was 2 deg·min⁻¹ (CuKα radiation, scanning range 6–90°). The ICDD (International Diffraction Data Center) databases (PDF 4, relies 2022) were used to identify the phases. The WPPF helped clarify the structural characteristics of the phases. Atomic coordinates and crystallographic lattice characteristics were entered from cif files ICDD. The R-factor criteria were the values of the profile agreement factors R_p and R_wp, calculated using standard formulas.

A ZEISS EVO 25 UltimMax 170 scanning electron microscope SEM (Carl Zeiss QEC GmbH, Oberkochen, Germany) examined morphology and sizes of powders, as well as the microstructure of ceramic samples. Then, based on the SEM scans and the technical capabilities of the ScanMaster program (National research nuclear university (MEPHI), Moscow Engineering Physics Institute, Russia), we analyzed the obtained data. ScanMaster was created to mathematically process such images and measure them. The program is able to select objects in an image and perform morphological operations on them. It can calculate object characteristics based on selected criteria, perform statistical processing of a set of selected objects based on selected criteria and generate diagrams and correlations. The mechanical properties of ceramic samples (microhardness and Young’s modulus) were studied by the contact method using a NanoSkan probe microscope-nanohardness tester (FSBI TISNCM, Troitsk, Russia). The methods were used from works [22,23,24,25]. The microhardness of ceramics was determined by comparative sclerometry [23], and Young’s modulus was determined from approach curves [22]. The crack resistance, which is the stress intensity factor for mode I, K_IC, was determined according to the model of Anstis et al. [24,25]. The load was 15 mN during the test. To reduce the standard error of the data, the obtained values of microhardness and crack resistance were averaged over ten measurements in 10 arbitrary areas measuring 60 × 60 μm for each sample.

PL spectra of the studied ceramics were obtained with an AvaSpec-3648 spectrofluorimeter (manufactured by Avantes, Apeldoorn, The Netherlands) when excited at a wavelength of 980 nm by a single-mode laser module KLM-H980 (PTI “Optronik”, St. Petersburg, Russia) in the IR range with a continuous radiation power of 120/200 mW. The operating wavelength range of the spectrofluorimeter AvaSpec-3648 (Avantes BV, Apeldoorn, The Netherlands) is 330–980 nm with a resolution of 0.025 nm. The measurement step was 0.17 nm.

A total of three bacterial strains were used to determine the photoinduced and dark toxicity of ErNbO₄ powders. These are Gram-positive—Micrococcus sp. and Gram-negative—Escherichia coli strains. Another object was the spore-forming bacterium Bacillus subtilis. The studied strains were subcultured onto freshly prepared agar nutrient medium for research purposes; they were incubated in a thermostat at 27 °C for three days. To prepare a bacterial suspension, cultures were treated with sterile saline. The initial number of bacterial suspension was more than ~10⁴ cells/mL. Photoinduced toxicity of ErNbO₄ powders was evaluated by the degree of inhibition of cell growth of three bacterial cultures under study in natural light. A sample of the material (2 or 5 mg/mL) was placed in a glass test tube containing 19 mL of saline solution, then 1 mL of bacterial suspension was added. A 2 mg sample was used for Gram-positive and Gram-negative bacteria, namely Micrococcus sp. and Escherichia coli, respectively; and a 5 mg sample was used for spore-forming bacteria Bacillus subtilis as they are more resistant to bactericidal substances. The test tube was tightly closed with a rubber sterile stopper and mounted on an orbital shaker LOIP LS-110 (LAB-PU-01) (JSC LOIP, Saint-Petersburg, Russia). The rotation speed was 200 rpm. The test tube was kept in natural light for 24 h. The tubes were exposed to light during natural daylight hours. Sowing was carried out after 4 and 24 h.

The source of natural light was sunlight. A lux meter (TKA-PKM, 06) and a UV Radiometer (TU 4215-003-16796024-16) monitored the illumination level during the experiments. Luxmeter TKA-PKM (06) was produced by the Research and Production Enterprise “TKA”, St. Petersburg, Russia. The illumination level of the samples in various spectral ranges is presented in

Table 1. Illumination in the spectral range from 380 to 980 nm and irradiance in the spectral range from 280 to 400 nm.

Lighting Type	Illumination in the Spectral Range from ~400 to 980 nm, Lux	Energy Illumination in the Spectral Range from 280 to 400 nm, mW/m2
Daylight	2617 ± 209	132 ± 13.2

.

Experiments in the dark were carried out to compare the results. They involved Gram-positive bacteria Micrococcus sp. Control experiments were carried out with the same lighting, but there were no luminescent phosphors in them. It has been found that the number of bacteria in the suspension does not significantly decrease over 4 and 24 h in natural light when there are no luminescent phosphors.

3. Results and Discussion

3.1. Dimensional Characteristics and Morphology of Powders

ICP-MS data indicate that, taking into account the volumes of solutions, the loss of erbium in filtrates and washing solutions is significantly less than 0.1 mg when synthesizing 30 g of powder for most of the studied samples. This means that gadolinium and europium are almost completely transferred from Er(NO₃)₃ solutions into the hydroxide precipitate. The data in

Table 2. Elemental composition and specific surface area of synthesized erbium niobate-tantalate powders.

Sample	Analysis Results, wt%			Specific Surface Area, Ssp, m2/g	Average Particle Size of Powders dav, µm
	Er	Nb	Ta
ErNbO4	50.40	29.62	-	1.04	0.78
ErNb0.9Ta0.1O4	47.68	26.37	5.75	1.39	0.64
ErNb0.7Ta0.3O4	46.22	19.40	15.89	1.28	0.56
ErNb0.5Ta0.5O4	44.50	13.04	24.95	1.15	0.57
ErNb0.3Ta0.7O4	41.65	7.53	34.07	1.54	0.42
ErNb0.1Ta0.9O4	40.86	2.43	40.55	1.65	0.39
ErTaO4	41.10	-	43.40	0.84	0.75

show that the concentrations of Er, Nb and Ta are close to the calculated values within the permissible error of the analysis method used (1%).

It has been found that the fluorine concentration in the powders is below the sensitivity limit of the analytical method used (less than 1 × 10^–3%).

Table 2 demonstrates that the average particle sizes of the powders in the series of studied compositions are quite similar. Therefore, we present a typical shape of the powders using a sample with the largest particle size ErNbO₄ and with the smallest particle size ErNb_0.1Ta_0.9O₄ as an example (Figure 1).

Figure 1 shows that the powders consist of nearly spherical particles of similar morphology. The particle size of the ErNbO₄ powders ranges from 0.1 to 1.25 μm, as seen in Figure 1a. The average size of the ErNb_0.1Ta_0.9O₄ powders is ~0.1 to 0.7 μm, as seen in Figure 1b. It should be noted that the powder particles tend to stick together and form larger agglomerates.

3.2. Structure of Ceramic Solid Solutions

Figure 2 shows XRD patterns of ErNb_xTa_1−xO₄ samples (x = 0; 0.1; 0.3; 0.5; 0.7; 0.9; 1).

Since the ionic radii of niobium and tantalum atoms in mixed orthoniobates-tantalates in the oxidation state of +5 and six-fold coordination are the same and equal to 0.64 Å [26], in accordance with the Goldschmidt tolerance factor, one can expect the formation of a continuous series of solid solutions REENb_xTa_1−xO₄ at x = 0–1. However, it was established that in the ErNb_xTa_1−xO₄ series, near x = 0.5, a structural transition occurs from the monoclinic symmetry of the C2/c cell to the monoclinic P2/a cell at the stage of FPD. The studied solid solutions ErNb_xTa_1−xO₄ at x = 0.5–1 crystallize in the structure of β-fergusonite with a monoclinic cell of erbium orthoniobate (ICDD, card 04-006-1916, space group (SPGR): C2/c (15) Z = 4), and at x ≤ 0.5 in the form of M-fergusonite erbium orthotantalate (ICDD, card 04-012-5293, SPGR: P2/a (13), Z = 2). The cell parameters of ErNb_xTa_1−xO₄ (x = 0; 0.1; 0.3; 0.5; 0.7; 0.9; 1) refined by the WPPF method are presented in

Table 3. Refined values of the periods of unit cells of the main phases of ceramic samples ErNb_xTa_1−xO₄ (x = 0; 0.1; 0.3; 0.5; 0.7; 0.9; 1).

Sample Composition	Phase in the Sample	a, Å	b, Å	c, Å	β, °	V	Rwp, %	Rp, %
ErNbO4	04-006-1916 SPGR: C2/c (15) ErNbO4	7.02557(8)	10.91922(14)	5.06648(7)	131.4512(6)	291.315	7.49	5.67
ErNb0.9 Ta0.1O4	04-006-1916 SPGR: C2/c (15) ErNbO4	7.0183(3)	10.9163(4)	5.0642(2)	131.390(2)	291.077	10.19	5.96
ErNb0.7 Ta0.3O4	04-006-1916 SPGR: C2/c (15) ErNbO4	7.0040(3)	10.9126(4)	5.0592(2)	131.1898(19)	290.992	8.68	5.32
ErNb0.5 Ta0.5O4	04-006-1916 SPGR: C2/c (15) ErNbO4 Phase weight 82%	6.9911(2)	10.9080(4)	5.05474(19)	131.0188(18)	290.836	6.08	4.01
	04-012-5293; ErTaO4 P2/a (13) Phase weight 18%	5.3035(4)	5.4538(4)	5.1333(6)	96.100(8)	147.639	6.08	4.01
ErNb0.3 Ta0.7O4	04-012-5293, ErTaO4 P2/a (13)	5.2996(3)	5.4523(3)	5.0550(3)	95.276(2)	145.446	10.00	6.72
ErNb0.1 Ta0.9O4	04-012-5293, ErTaO4 P2/a (13) M	5.28439(11)	5.44368(11)	5.09790(10)	96.3053(8)	145.762	4.63	3.35
ErTaO4	04-012-5293, ErTaO4 P2/a (13) M	5.2827(3)	5.4402(3)	5.0981(3)	96.352(3)	145.614	6.42	4.73

. It should be noted that in the ErNb_xTa_1−xO₄ sample, at x = 0.5, two phases were identified simultaneously: about 82% monoclinic β-fergusonite with the SPGR C2/c and 18% monoclinic M-fergusonite with P2/a. The concentration region around the point Nb/Ta = 1 in the series of solid solutions ErNb_xTa_1−xO₄ is probably a morphotropic region of coexistence of two phases of fergusonite.

Figure 3 shows the structures of the monoclinic cells P2/a and C2/c and their corresponding coordination polyhedra. The differences lie in the anionic composition.

It is believed that the coordination polyhedron around the Ta atoms for the P2/a cell can be fairly interpreted as a tetrahedron (semi-dodecahedron) (4 + 2), and for the C2/c cell and Nb atoms, it can be fairly interpreted as a distorted octahedron [27]. Moreover, the additional O atoms in the case of C2/c (β-fergusonite) are closer to the atoms of the cation sublattice A⁵⁺ than in M-fergusonite with the P2/a cell (Figure 3,

Table 4. Interatomic distances between A⁵⁺ and O²⁻ ions in ErNb_xTa_1−xO₄ (x = 0; 0.1; 0.3; 0.5; 0.7; 0.9; 1).

SPGR	Distance	Nb Content, x
		1	0.9	0.7	0.5	0.3	0.1	0
C2/c (15), Z = 4	Nb–O(1)a[O(1)b]	1.76588	1.76454	1.76171	1.75912
	Nb–O(2)a[O(2)b]	2.42890	2.42703	2.42326	2.41979
	Nb–O(2)c[O(2)d]	1.82574	1.82646	1.82990	1.83271
P2/a (13), Z = 2	Ta–O(1)a[O(1)b]				1.92793	1.92689	1.91520	1.91407
	Ta–O(1)a[O(1)b]				2.48018	2.45572	2.47252	2.47237
	Ta–O(2)c[O(2)d]				1.99687	1.97556	1.98987	1.98973

). In the structure of β-fergusonite, the atoms of the crystalline host (tantalum or niobium) are in a strongly distorted octahedral coordination with six Ta(Nb)–O bonds whereas in M-fergusonite the tantalum (niobium) atoms are in a tetrahedral coordination; see Figure 3.

The cell parameters increase with increasing Nb content in the host (see Table 3) due to the increase in distances between A⁵⁺ and oxygen ions (Table 4).

This pattern is probably due to the different electronegativity of Ta (1.5 on the Pauling scale) and Nb (1.6 on the Pauling scale) and, as a consequence, the different effective charges on the Ta⁵⁺, Nb⁵⁺ and O²⁻ ions in compounds with a covalent bond [28]. In the case of TaO₆⁻ or NbO₆⁻ groups, a small increase in the electronegativity of the cation from Ta⁵⁺ to Nb⁵⁺ is accompanied by an increase in the ionic radius of O²⁻. These changes probably lead to an increase in the length of the A⁵⁺–O²⁻ bonds.

Deviations in the lattice constants from Vegard’s law are explained, as in work [29], by the formation of inhomogeneities (clusters) enriched with one of the substituted atoms.

In addition, the ErTaO₄ sample contains about 6 wt% Er₃TaO₇ cubic system with Fm-3m (225) (ICDD, card 00-024-0406), and the ErNb_0.9Ta_0.1O₄ ceramic sample contains about 10 wt% cubic Er₃NbO₇ (ICDD, card 00-024-1080, SPGR: Fm-3m) (Figure 2, Table 3).

3.3. Morphological and Mechanical Characteristics of Ceramic Solid Solutions ErNb_xTa_1−xO₄ (x = 0; 0.1; 0.3; 0.5; 0.7; 0.9; 1)

A series of SEM images were performed to characterize the morphologies and estimate the grain sizes of ErNb_xTa_1−xO₄ (x = 0; 0.1; 0.3; 0.5; 0.7; 0.9; 1). The microstructure of ceramic samples and their size composition are shown in Figure 4.

The data in Figure 4 show that ErNb_xTa_1−xO₄ ceramic samples consist of grains of similar morphology with pronounced boundaries. The largest crystallite size is in the ErNbO₄ sample, from ~0.7 to ~10 μm, as seen in Figure 4a. It is likely that the large grain size of ErNbO₄ ceramics may be due to the sintering temperature of the ceramics. It is close to the melting point.

If Ta in the ErNb_xTa_1−xO₄ composition increases, respectively, and the melting temperature increases, the ceramic grain size decreases. The range of crystallites from ~ 0.2 to ~ 2.6 μm is typical for the ErNb_0.9Ta_0.1O₄ and ErNb_0.7Ta_0.3O₄ samples, as seen in Figure 4b,c. The size of the main fraction of crystallites is from ~0.18 to ~2.2 μm for ErNb_xTa_1−xO₄ ceramics, where x ≤ 0.5, as seen in Figure 4d–g. Ceramic samples ErNb_0.9Ta_0.1O₄ and ErTaO₄ containing cubic phases with Fm-3m (225) Er₃NbO₇ and Er₃TaO₇, respectively, (Figure 4b,g) are the most heterogeneous and contain individual grains larger than 3.0 µm. In general, ErNb_xTa_1−xO₄ ceramics (x = 0; 0.1; 0.3; 0.5; 0.7; 0.9; 1) prepared by CS from erbium niobate-tantalate powders synthesized by the liquid-phase method have a finer grain size than those obtained by solid-phase synthesis with crystallite sizes about or more than 5.0 µm [30,31,32,33].

The contact method helped to study the mechanical properties of ceramic samples.

Table 5. Mechanical characteristics of ErNb_xTa_1−xO₄ ceramic samples.

Sample Composition	Microhardness, H, GPa	Young’s Modulus, E, GPa	Crack Resistance, KIC, MPa m0.5
ErNbO4	6.11 ± 0.86	297.6 ± 10.8	1.44 ± 0.2
ErNb0.9Ta0.1O4	2.86 ± 0.31	84.8 ± 1.8	0.7 ± 0.1
ErNb0.7Ta0.3O4	5.68 ± 1.0	162.4 ± 6.9	1.20 ± 0.1
ErNb0.5Ta0.5O4	3.82 ± 0.33	137.7 ± 4.3	1.10 ± 0.1
ErNb0.3Ta0.7O4	5.12 ± 1.1	223.1 ± 5.9	1.16 ± 0.15
ErNb0.1Ta0.9O4	3.4 ± 0.361	98.8 ± 2.7	0.94 ± 0.1
ErTaO4	4.2 ± 0.9	139.3 ± 6.8	1.06 ± 0.1

shows the results of measurements and calculations of the mechanical characteristics of ceramic samples.

The data in Table 5 show that the mechanical characteristics of ceramics strongly depend on the phase composition of the samples. The best mechanical characteristics, microhardness (H = 6.11 ± 0.86 GPa), Young’s modulus (E = 297.6 ± 10.8 GPa) and crack resistance (K_IC = 1.44 ± 0.2, MPa m^0.5) are found in the ErNbO₄ ceramic sample and the ErNb_0.7Ta_0.3O₄ single-phase sample, as seen in Figure 2, Table 3. Low mechanical characteristics of the ErNb_0.5Ta_0.5O₄ sample are due to the coexistence of two fergusonite phases. The low mechanical characteristics of the ErNb_0.9Ta_0.1O₄ sample are naturally due to the presence in the sample of about 10 wt% cubic Er₃NbO₇ phase (ICDD, card 00-024-1080, SPGR: Fm-3m). The key parameters in the difference in the mechanical characteristics of the compared orthoniobate-tantalates are the method of obtaining the materials, the properties and concentration of doping REEs and the ratio of crystalline phases in the sample. Nevertheless, the values of the mechanical characteristics of ErNb_1−xTa_xO₄ ceramic samples synthesized by the liquid-phase method are consistent with the results presented in the literature [30,31,32,33]. The authors of [31,32] show the range of Young’s modulus for REE-niobates (RENbO₄) (RE = Y, La, Nd, Sm, Gd, Dy, Yb)—60-170 GPa and hardness up to 11.48 GPa. The values of Young’s modulus ∼120 GPa for YbTaO₄ and LuTaO₄, ∼270 GPa for ScTaO₄ are presented in [33]. Taking into account the differences in measurement methods (nanoindentation and sclerometry), our results of measuring these quantities are in a good agreement with the data in the literature. For example, the Young’s modulus of RENbO₄ ceramics decreases linearly as the ionic radius of RE³⁺ increases [31,32]. The ionic radius of Er³⁺ is smaller than the ionic radii of Sm³⁺, Gd³⁺, Dy³⁺; therefore, it is natural that the single-phase ceramic sample ErNbO₄ has higher mechanical characteristics than orthoniobates Sm³⁺, Gd³⁺ and Dy³⁺. The latter are considered by the authors of [31,32].

3.4. Study of PL of Ceramic Solid Solutions ErNb_xTa_1−xO₄ (x = 0; 0.1; 0.3; 0.5; 0.7; 0.9; 1)

Figure 5 shows the PL spectra of ErNb_xTa_1−xO₄ ceramics when laser radiation excites the near-IR range (λ_exc = 980 nm). The PL spectra of the samples under study consist of bands of different intensities which depend on the composition of the ceramics. The PL bands correspond to intraconfigurational transitions of the Er³⁺ ion in the host of ceramic samples in the spectral range from 500 to 700 nm. For example, the main radiative transitions of the Er³⁺ ion correspond to (²H_11/2, ⁴S_3/2) → ⁴I_15/2 in the region of 500–560 nm and the main radiative transitions of the Er³⁺ ion correspond to ⁴F_9/2 → ⁴I_15/2 in the region of 640–680 nm [34,35,36], as seen in Figure 5. PL peaks at 585 and 594 nm and, as far as we know, have not been previously observed in ceramic and single-crystalline niobate-tantalates of alkali and REEs [35,36]. The reason for the appearance of these bands can presumably be associated with the radiative transition ⁴G_9/2 → ⁴I_11/2 of the Er³⁺ ion during three-photon absorption according to the refined Dieke’s diagram [34].

The mechanism of UCL of the Er³⁺ ion consists of sequential absorption of IR radiation and subsequent emission in the visible region. Initially, the first low-energy quantum with energy E_exc = 1.27 eV is absorbed by the Er³⁺ ion and transferred to the excited state (transition ⁴I_15/2 → ⁴I_11/2). Subsequent absorption of the second quantum transfers the Er³⁺ ion to a higher energy state. This increases the population of the ⁴F_7/2 levels.

Further nonradiative recombination with the participation of lattice phonons transfers the Er³⁺ ion to the ⁴S_3/2 state. The radiative transition ⁴S_3/2 →⁴I_15/2 produces UCL in the visible region. The radiative transition ²H_11/2 → ⁴I_15/2 (Figure 5 appears in the PL spectra near 535 nm due to the small energy difference between the ²H_11/2 and ⁴S_3/2 levels and the influence of the crystal field of the ceramic host [34,35,36,37,38].

The transition of the Er³⁺ ion to the ⁴F_9/2 state is possible in two ways. The first is phonon relaxation from the ⁴S_3/2 level to the ⁴F_9/2 level. However, this transition has a low probability as the intensity of the radiative transition ⁴F_9/2 → ⁴I_15/2 is extremely low when the UV region is excited, compared to that when the near-IR region is excited [37]. The second way involves nonradiative relaxation from the ⁴I_11/2 level to ⁴I_13/2 and subsequent absorption of exciting radiation (⁴I_13/2 → ⁴F_9/2 transition). The latter provides intense PL at 675 nm, as seen in Figure 5. The transfer of the Er³⁺ ion to the excited state ⁴F_7/2 is also possible due to the resonant energy transfer between two REEs where one element is a sensitizer, the second is an activator [39].

The highest quantum yield of PL is observed in ErNbO₄ ceramics in comparison with ErTaO₄ in individual compounds. The excess in intensity of the ⁴S_3/2 → ⁴I_15/2 transition is about 60%, as seen in Figure 5. One of the reasons for the weak intensity of UCL of ErTaO₄ ceramics may be due to the presence of an additional phase in the form of Er₃NbO₇ (cubic system, Figure 2, Table 3). It leads to the creation of structural defects of various sizes at the boundaries of two phases. The latter can lead to an increase in the proportion of nonradiative processes and weak PL intensity of Er³⁺ ions in the ErTaO₄ host. The PL intensity in the green and red regions of the spectrum depends on the Nb/Ta ratio in mixed ceramic matrices.

Changing the Nb/Ta ratio changes the average distance between the atoms of the crystal host (A⁵⁺) and O²⁻ (Table 4). This can cause PL quenching in the tantalate host [40,41]. Substitution of Nb⁵⁺/Ta⁵⁺ in the solid solution also leads to changes in the crystal field strength due to a change in the lengths of the A⁵⁺–O²⁻ bonds. The atom substitution affects the symmetry of the local environment of the luminescence center. Both of these quantities affect the host element of the transition between the ground and excited states of the ion and the shape of the vibronic potential and change the PL intensity.

Figure 6 shows the change in the intensity of the peaks at 555 and 672 nm in the samples depending on the composition. If there is a gradual decrease in Nb from x = 1 to x = 0.5, the PL intensity at 555 and 672 nm increases in ErNb_x-1Ta_xO₄ ceramics, with the exception of ceramics with the composition ErNb_0.9Ta_0.1O₄. According to X-ray phase analysis, an additional phase Er₃NbO₇ (see Figure 2, Table 3) is present in the last sample. This phase reduces the PL intensity to the greatest extent at 555 nm. When the amounts of Nb and Ta in the ErNb_0.5Ta_0.5O₄ ceramic structure are equal, the PL intensity of both maxima reaches its maximum value. And this intensity prevails over the PL intensity in individual compounds. This may be promising for creating temperature sensors based on this material [35,37,38].

These results correlate well with the presence of two phases in the sample with the composition ErNb_0.5Ta_0.5O₄—about 82% monoclinic β-fergusonite with C2/c and 18% monoclinic M-fergusonite with P2/a. An increase in the PL intensity is observed in the region around Nb/Ta = 1. This may be due to the structural rearrangement of one cell into another. This in turn affects the efficiency of Er PL due to anisotropic changes in the distance between the ErO₈ polyhedron and neighboring AO₆.

The intensity of the PL bands at 585 and 594 nm monotonically increases in the series of ErNbO₄ →ErNb_0.7Ta_0.3O₄ →ErNb_0.5Ta_0.5O₄ ceramics, which are monophasic with a fergusonite-β type structure, as seen in Figure 2.

The main energy process in the studied ErNb_xTa_1−xO₄ ceramics is the energy transfer from the host. At the same time, there is a general tendency of increasing PL intensity in the blue region of the spectrum in ErNb_xTa_1−xO₄ with increasing niobium concentration [42]. If there were no disorder processes in the niobium-tantalum sublattice, the growth of PL intensity (I) with increasing niobium concentration (x) would proceed without anomalies and the dependence I(x) would be close to linear. However, due to the structural transition from the monoclinic symmetry of the C2/c cell to the monoclinic P2/a cell and the positional disorder of the niobium and tantalum cations near the concentration x = 0.5, a pronounced maximum is observed in the I(x) dependence.

When the niobium concentration increases in the range x = 0.5–0.7, the positional disorder in the B-sublattice decreases and the polarization of the oxygen octahedron AO₆ changes (Table 4); a regular decrease in the PL intensity is also observed.

At further increase in niobium concentration (x = 0.7–1.0), the tendency of the intrinsic PL intensity growth in ErNb_xTa_1−xO₄ with increasing niobium concentration comes into force [42]. That is, the energy transfer from the ErNb_xTa_1−xO₄ host to Er³⁺ increases and, accordingly, the PL intensity at the intracenter transitions of Er³⁺ ion grows.

3.5. Bacteriological Properties of Powder ErNbO₄

To test the photobactericidal properties, a simple composition, ErNbO₄, which has moderate luminescent properties, was chosen,. In experiments made to test photoinduced toxicity under natural light, luminescent powder ErNbO₄ showed good bactericidal action against bacterial cells of Escherichia coli, Bacillus brevis and Micrococcus sp. The residual number of bacteria after exposure for 24 h was ~20.6, 16.94 and 11.67%, respectively, of the initial value, as seen in

Table 6. Change in the number of bacterial cells in the sample depending on the exposure time under natural light in the presence of ErNbO₄ powder. ErNbO₄ concentration—2 and 5 (Bacillus brevis) mg/mL.

Sample	Bacterial Cell Type	Number of Bacterial Cells
		Initial		in 4 h		in 24 h
		×104 cells/mL	%	×104 cells/mL	%	×104 cells/mL	%
1	Escherichia coli	6 ± 0.4	100	1.2 ± 0.5	20.3	1.2 ± 0.018	20.6
2	Bacillus brevis	6 ± 0.4	100	1.5 ± 0.19	25.0	1 ± 0.2	16.94
3	Micrococcus sp.	6 ± 0.4	100	1.25 ± 0.75	20.8	0.7 ± 0.01	11.67

.

The most resistant to the effect were Gram-negative bacterial cells Escherichia coli, as seen in Table 6. This is due to the more complex structure of their cell wall compared to Gram-positive Micrococcus sp. It is generally known that the main structure responsible for the general resistance of Gram-negative bacteria to various external agents is the outer membrane. It is part of the cell wall [43]. The outer membrane is based on a two-layer asymmetric structure. Their outer layer consists mainly of liposaccharides (LPS) and protein complexes embedded between LPS [44]. The total negative charge of LPS is associated with a high content of negatively charged groups in the central part of these macromolecules.

Probably, under the influence of the polycationic compound ErNbO₄, LPS are released, disintegration of the outer membrane occurs and its permeability increases [45,46].

Gram-positive bacterial cells Micrococcus sp. participated in experiments without lighting. Experiments have shown that the number of bacterial cells Micrococcus sp. decreases only by half when they are kept in the dark. This is apparently due to the insignificant bactericidal activity of ErNbO₄ itself,

Table 7. Change in the number of bacterial cells of Micrococcus sp. under natural light and in the dark, in the presence of ErNbO₄ powder. The concentration of ErNbO₄ is 2 mg/mL.

Bacterial Cell Type	Experimental Conditions	Number of Bacterial Cells
		Initial		in 4 h		in 24 h
		×105 cells/mL	%	×105 cells/mL	%	×105 cells/mL	%
Micrococcus sp.	light	1.41 ± 0.5	100	0.29 ± 0.18	20.6	0.16 ± 0.02	11.35
	dark	1.41 ± 0.5	100	0.7 ± 0.3	49.65	0.7 ± 0.01	49.65

.

Table 7 shows that in the dark, the number of bacterial cells Micrococcus sp. in the presence of ErNbO₄ phosphor powder decreases significantly less and more slowly than in natural light.

Photoinduced toxicity of ErNbO₄ applied to bacterial cells of Micrococcus sp. is higher than dark toxicity by ~5 times. Photoinduced toxicity of the ErNbO₄ luminophore is most effective for inactivating bacterial cells Micrococcus sp. Their distinctive feature is the simple structure of the cell wall. It should also be noted that the experimental results are very reproducible. Experiments in light with bacterial cells Micrococcus sp. were repeated in a comparative series of experiments in light and in the dark. The residual amount of Micrococcus sp. after exposure for 24 h in the first experiment was 11.67%, and it was 11.35% under the same conditions in the second, as seen in Table 6 and Table 7. Thus, the results coincide within the experimental error.

4. Conclusions

The liquid-phase synthesis method using erbium nitrate solutions and co-precipitated niobium and tantalum hydroxides helped to obtain fine powders of erbium niobate-tantalates ErNb_xTa_1−xO₄ (x = 0; 0.1; 0.3; 0.5; 0.7; 0.9; 1)

Ceramic samples were prepared from the resulting fine powders, according to CS. The phase structures of ErNb_1−xTa_xO₄ ceramic solid solutions depending on the Nb/Ta ratio in the composition of the samples were obtained by Rietveld refinement of X-ray diffraction patterns of polycrystals. It has been shown that the structure of the ErNb_xTa_1−xO₄ solid solution from x = 1 to x = 0.5 corresponds to the structure of fergusonite-β with a monoclinic cell of erbium orthoniobate (ICDD, card 04-006-1916 SPGR: C2/c (15), Z = 4).

It has been established that in the ErNb_xTa_1−xO₄ series, at about x = 0.5, a structural transition occurs from the monoclinic symmetry of the C2/c cell to the monoclinic P2/a cell. When Ta in ErNb_xTa_1−xO₄ increases (at x = 0.3 or less), the solid solution crystallizes in the structure of M-fergusonite, ICDD, card 04-012-5293 SPGR: P2/a (13), Z = 2. The crystal matrix atoms (tantalum or niobium) are in a distorted octahedral coordination with six Ta(Nb)–O bonds in the β-fergusonite structure, and the tantalum (niobium) atoms are in a tetrahedral coordination in M-fergusonite.

The mechanical characteristics of ceramic solid ErNb_xTa_1−xO₄ (x = 0; 0.1; 0.3; 0.5; 0.7; 0.9; 1) solutions were determined. It was shown that the best mechanical characteristics, microhardness (H = 6.11 ± 0.86 GPa), Young’s modulus (E = 297.6 ± 10.8 GPa) and crack resistance (K_IC = 1.44 ± 0.2, MPa m^0.5) were possessed by the ErNbO₄ ceramic sample and the single-phase niobate-tantalate sample ErNb_0.7Ta_0.3O₄ with microhardness (H = 5.68 ± 1.0 GPa), Young’s modulus (E = 162.4 ± 6.9 GPa) and crack resistance (K_IC = 1.20 ± 0.1 MPa m^0.5).

The luminescent properties of ceramic samples were studied. It was shown that when laser radiation excites in the near-IR range (λ_exc = 980 nm), solid solutions of ErNb_xTa_1−xO₄ (x = 0; 0.1; 0.3; 0.5; 0.7; 0.9; 1) irradiate PL with increasing conversion of the exciting radiation, and maximum value of PL was in ErNb_0.5Ta_0.5O₄.

The photoinduced and dark toxicity of the up-conversion phosphor ErNbO₄ was studied in relation to Gram-positive, Gram-negative and spore-forming microorganisms. It was shown that inactivation of Gram-positive bacteria Micrococcus sp. by the up-conversion phosphor ErNbO₄ is most effective in the light. This is apparently due to the structure of their cell wall.