Investigation on the physicochemical properties of La-doped Er0.05Y1.95O3 nanopowders

A series of high-purity Er0.05Y1.95O3 nanopowders with different lanthanum content was prepared by modification of the Pechini sol–gel method using citric acid and ethylene glycol as the chelating agent. The microstructure of the powders was studied by means of X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy. In order to evaluate the structural characteristics of the obtained gel, XRD measurements were carried out with calcination gels in selected temperatures. Simultaneous differential thermal analysis with thermal gravimeters indicates a decrease of calcination temperature with an increasing content of lanthanum ions. Morphological properties of the nano-sized powders were examined by scanning electron microscopy. Strong luminescence in near IR region was observed under 980 nm excitation at room temperature. By varying the concentration of La3+ ion, various intensities of upconversion luminescence can be easily achieved.


Introduction
Recently, new types of optical devices, such as high-energy laser, optical fiber amplifiers, and optical rotators, have attracted significant attention due to their unique physical properties. Y 2 O 3 , either pure or often doped with rare-earth ions, has been widely investigated as a host material for potential applications in lasers, three dimensional volumetric displays, fluorescent labels, and luminous pipes for high-intensity discharge lamps [1][2][3][4][5][6][7][8][9][10][11][12][13]. The performance of these devices is mainly dependent on the physicochemical properties of the powders that were sintered. The powders are useful due to their good chemical and physical stability, high corrosion resistance, and broad transparency range (0.2-10 lm) [1,2,5]. The recent research and reports have shown the relatively low phonon energy (430-550 cm -1 ) [1][2][3] and high thermal conductivity [13.6 W(mK) -1 ] [1][2][3][4]. Pure yttrium oxide exhibits C-bixbyite, cubic, hexagonal, or monoclinic crystal structure [5][6][7][8][9][10]. Many different chemical methods, such as sol-gel EDTA, hydrothermal, co-precipitation method, spray pyrolysis, thermochemical reactions, and solid state reaction, were widely used to synthesize nano-sized powders of pure and doped Y 2 O 3 [10][11][12][13][14][15]. Furthermore, citrate sol-gel process is one of wet chemical methods that receives attention due to its high efficiency and possibility to prepare high clean and nano-sized powders [4, 6-9, 13, 15]. Numerous studies conducted thus far indicate that the choice of preparation method influences the size, shape, and crystalline structure of powders, which determine optical, transmittance, and luminescence properties of sinters [1][2][3][4][5]. The rare-earth metal ions, e.g.: Nd, Er, Eu, and Tm, have been successfully used as an additive material in order to improve the performances of Y 2 O 3 materials [3][4][5][6][7][10][11][12]. Many papers have indicated that the doping of Y 2 O 3 with erbium ions allows for luminescence emission in the infrared range [3,14]. Additional substitution of La ions can also enhance luminescence in Er:Y 2 O 3 by tuning the symmetry of the crystal field around the rare-earth ions [15]. Notably, doped with rare-earth ions, Y 2 O 3 may be used as a magneto-optic ceramic potential to build a magneto-optic device [4,16]. This device is based on Faraday phenomena, and it uses external magnetic fields. Criteria for magneto-optical material are high transparence, chemical stability, and high Verdet constant. Nano-sized yttria meets the first two criteria. The Verdet constant V may be improved by doping with diamagnetic or/and paramagnetic ions, because it is the sum of the two types of component ions: V = V diamagnetic (k) ? V paramagnetic (k; T), where k is wavelength and T is temperature. [4,[16][17][18][19][20][21].
This paper focuses on the development of high clean La x :Er 0.5 Y 1.95-x O 3 nanopowders with doping concentration varied from 0 to 0.1 mol. The amount of La ions was varied to improve the sintering process and luminescence properties. In addition, lanthanum (paramagnetic) and erbium (paramagnetic) ions were used to improve the Verdet constant. The presented results show that varying the amount of lanthanum ions added to the powders has a significant effect on both microstructure and luminescence properties. The structure evolution and microstructure characterization of the powders were carried out by X-ray diffraction (XRD), Fourier-transform infrared (FT-IR) spectroscopy, and scanning electron microscopy (SEM). The surface of powders was analyzed by the Brunauer-Emmett-Teller (BET) method. Strong dependence of luminescence due to the La 3? concentration was observed. High yield upconversion emissions in Er-doped Y 2 O 3 powders were measured under the excitation of a 980 nm continuous wave diode laser.

Powder characterization
In this paper, the synthesis of polycrystalline Er 0.05 Y 1.95 O 3 , La 0.01 Er 0.05 Y 1.94 O 3 , La 0.05 Er 0.05 Y 1.9 O 3, and La 0.1 Er 0.05 Y 1.85 O 3 nano-sized powders is reported. The powders were obtained using a modified citrate sol-gel process with ethylene glycol and citric acid as fuel. Thermal analysis and evolved gas analysis of the abovementioned precursor gels were carried out in the temperature range 25-700°C in a simultaneous thermogravimetry DTA/TG/EGA with a fast Quadrupole Mass Spectrometer (QMS) setup (Netzsch STA 449 F3 with a SDT 2960) with a heating rate 10°C min -1 in an ambient gas atmosphere and alumina as reference.
Structural characterization and phase identification of the obtained powders were carried out using a X'Pert PANalytical X-ray diffractometer, with CuK a (1.5405 Å ) radiation. XRD patterns of the gels and calcined powder were recorded over the angular range 10°-90°with step size 0.01°. Qualitative phase analysis was conducted by use of Highscore Plus software and database PCPDFWIN v.2.3.
The average crystallite size of the sample was calculated using the XRD data and the Scherrer formula: where K = 0.9, D XRD -denotes the crystallite diameter [nm], k-the wavelength of the incident X-rays, b-the corrected full width at half maximum (FWHM) width of the diffraction peak, and h-the diffraction angle. A pseudo-Voigt function was used to fit the XRD curve and to calculate the FHWM value, and b was determined using Eq. (2), where B obs is the FHWM that is related to the sample and B is the FHWM of the standard (corundum, a- The dislocation density of the particulate structure can be calculated using the relation: SEM microghaphs of the obtained powders were recorded using A FEI Nova NanoSEM 200. The powders were prepared by dispersing them in ethyl alcohol via ultrasonication for 0.5 h. Next, the solution was poured on a commercial carbon-coated copper grid and dried at 80°C for 3 h in a hot air oven. Fourier-transform infrared (FT-IR) spectra of the samples were conducted using an FT-IR spectrometer (Brucker 70 V) with the KBr pellet method in the wave number range 400-4000 cm -1 with a step 3 cm -1 . The distribution of the particle (agglomerate) size of the yttria powders was determined by the laser light diffraction method (Mastersizer 2000S, Malvern Instruments) with measurement error 5%.
The specific surface area of the prepared powders was estimated by the BET (Brunauer-Emmett-Teller) method using an ASAP 2010 v4.00 G instrument with tolerance under 10%.
Powders were milled in rotary mill (600 rpm) for 1 h in an ethanol medium with / = 1 mm zirconium balls and jars.
The luminescence spectroscopy of the powders was performed using a Hamamatsu NIR (0.1 nm resolution) spectrometer with continuous wave 980 nm laser diode (Spectra-Laser) as a light source.

Powder synthesis
The chemicals used were lanthanum (III) nitrate hexahydrate La(NO 3 ) 3 9 6H 2 O (Sigma-Aldrich; 99.9%), erbium (III) nitrate hexahydrate Er(NO 3 ) 3 9 6H 2 O (Sigma-Aldrich; C 99.9%), and yttrium nitrate hexahydrate Y(NO 3 ) 3 9 6H 2 O (Sigma-Aldrich; 99.9%). Glycerin alcohol (PEG, Sigma-Aldrich) and citric acid (CA, Sigma-Aldrich) were used as starting materials. First, metal hydrates were dissolved separately in minimum quantities of distilled water. The solutions of erbium hydroxide and lanthanum hydroxide were mixed homogeneously with PEG. At the same time, a yttrium hydroxide solution was mixed with CA using a magnetic stirrer maintained at a constant rotation speed of 300 rpm for 2 h at 80°C. After that, both solutions were mixed together by the magnetic sitter at a constant rotation speed of 300 rpm and heated at 80°C. The stoichiometric proportion of CA to PEG was 3:7, respectively. The solution was heated at 180°C for several hours until formation of gel took place. The precursor gel of La 0.05 Er 0.05 Y 1.9 O 3 was calcined at different temperatures, which were selected by differential thermal analysis with thermal gravimeters (DTA/TG) results: 180, 280, 350, 500°C, and 700°C for 2 h in ambient air. On the basis of studies of the decomposition of the gel precursors, the optimum conditions for production of yttrium oxide doped with erbium and lanthanum powder were determined. The optimum conditions for calcination were 700°C for 10 h in air. Figure 1 illustrates the DTA/TG analysis of the four dried gels, Er 0.05 Y 1 La 0.05 Er 0.05 Y 1.9 O 3 gel result from higher amounts of crystalline water molecules and organic groups, as confirmed by TG analysis (higher total mass loss than in the case of the other investigated gels). The type of the gaseous decomposition products confirms the oxidation reactions of a gel, and consequently, the total decomposition of the materials studied. The QMS analysis allows a more precise description of the type of the volatile decomposition products emitted under oxidation. The QMS spectra of the volatile decomposition products emitted under heating of selected gel are presented Fig. 2. According to the QMS results, one can clearly see the presence of difference components: OH (m/z = 17), H 2 O (m/z = 18), CO (m/z = 28), NO (m/z = 30), O 2 (m/z = 32), CO 2 (m/z = 44), and NO 2 (m/z = 46) when the gel was calcined from room temperature to 700°C in ambient air. At the beginning, there was a release of fragments of the water molecules (H 2 O, OH) and NO 2 ; next, below 200°C, there is an indication that desorption of chemically and physically absorbed water also existed at temperatures lower than 470°C. However as the temperature grows above 250°C, all signals drastically increased, reaching a maximum at 290°C. Further heating of the gel above 290°C decreased emission of CO and NO gases and other signals were not observed. In the second temperature range above 300°C, there was a release of all gases with a maximum at 374°C, which can also be confirmed by the first endothermic peaks at 374°C. At this temperature, the TG measurement indicates a strong of mass loss, as shown Fig. 1b. The temperature at the beginning of crystallization varies according to the La ion content. Finally at temperatures higher than 400°C, the gel residue undergoes oxidation processes and thus the emission of OH, O 2 , H 2 O, CO 2, and rest of the CO. Above temperatures higher than 500°C, no gaseous decomposition products were detected. Figure 3 shows the XRD diagrams of La 0.  Fig. 4. The investigated samples contained different amorphous structure, which could be explained by a higher amount of lanthanum that favors the formation of a liquid phase during heating [4,19]. Figure 5 presents XRD patterns of a series of powders: Er 0.05 Y 1.95 O 3 , La 0.01 Er 0.05 Y 1.94 O 3 , La 0.05 Er 0.05 Y 1.9 O 3, and La 0.1 Er 0.05 Y 1.85 O 3 calcinated at 700°C in ambient air. The identification of the crystalline phase of polycrystalline ceramic powders was performed using software X'pert HighScore (JCPDS card no. 98-008-1861). The theoretical model (Ical) was fitted to the experimental XRD data (Iexp). According to the Rietveld refinement results, the obtained ceramics are single phase with Ia-3 space group.

Experimental
No secondary phases were detected in the ceramics. The lattice parameters, such as the lattice spacing d hkl , the lattice parameter a, unit cell volume V, crystallite size D xrd, and dislocation density q of the investigated sinters, are shown in Table 1. The microstructural parameters depend on the amount of La ions, suggesting that both Er and La cations have been successfully diffused into the crystal lattice of yttria. In this type of lattice, there are two distinct sites available for the substitution of trivalent lanthanide ions. Two kinds of cations exist: 8 Y 3? ions are on b-sites with symmetry C 3i and 24 Y 3? ions on d-sites with point symmetry C 2, while 48 oxygens are on e-sites with point symmetry C 1 [20][21][22][23][24]. Hence, the effect of Er, La on lattice parameters could be correlated to a bigger ionic radii than the ionic radius of Y 3? (Y 3? -0.90 Å , La 3? -1.3 Å and Er 3? -0.95 Å [4,19]). Thus, the slight adjustment of Y 2 O 3 lattice by the doped La 3? ions was observed. The position of the Bragg peaks [222] moves toward lower angles with an increase in La 3? ions (inset in Fig. 3), as shown in Table 2, indicating a change of the host lattice. With increase of lanthanum ions, the lattice parameter and unit cell volume of investigated powders increase. This agrees with FT-IR results (Fig. 8). As a consequence, the local symmetry of the crystal field around Er 3? reduces. The obtained results agree with other investigations [1-6, 8-12, 14-16].
A scanning electron microscope analysis was used to investigate the particle size and morphology of powders with chemical composition: Er 0.05 Y 1 (Fig. 6). The typical morphological images, presented in Fig. 6, show that all of these powders have irregularly shaped individual particles with clear-cut edges, which indicates excellent [211] Intensity/a·u. crystallinity. Nano-sized particles produced micron-sized agglomerates as can be seen in Fig. 6. The surface of the agglomerates, especially of Er 0.05 Y 1.95 O 3 , has many cracks and pores. The SEMs of studied ceramic powders show the presence of large agglomerates in the irregular block form of approximately 2 lm and separately agglomerate over 5 lm. Figure 7 shows the particle-size distribution of La 0.01-Er 0.05 Y 1.94 O 3 and La 0.1 Er 0.05 Y 1.85 O 3 ceramic powders (as an example) after milling for 1 h in ethanol. Dispex (BASF) medium was used as a dispersion material. The ceramic powders exhibited a mono-modal agglomerate distribution with the wide size between 0.1 and 10 lm before milling procedure and decreased 10 times from 0.01 to 1 lm after milling, which is consistence with SEM morphology (Fig. 6). The highest yield of milling process was observed for La 0.1 Er 0.05 Y 1.84 O 3 powder. Increase amount of lanthanum ions led to increased size of agglomerates but they are easily broken during ball milling. BET parameter of investigated powders is presented in Table 3. Before milling procedure, all powders have similar BET parameter ca. 0.7, and then after milling, BET parameter is change randomly. In order to obtain a large number of a nano-sized powders, a milling procedure has to be continuous.  Figure 8 shows the FT-IR spectra of the ceramic powders. The identification of functional bands referred to previous and other studies [14,17,19]. The FT-IR spectrum of the ceramic powder presents the characteristic bands of a Y 2 O 3 cubic network. Three bands at 560 cm -1 , 460 cm -1 , and 414 cm -1 are attributed to the symmetric stretching Y-O vibrations. Bands over 750 cm -1 are assigned to the different organic stretching from KBr and are therefore not shown in Fig. 8. Detected shift of specific bands is ca. 3 cm -1 , so comparable to the resolution of FT-IR device; however, this shift was confirmed by XRD measurement. It is consequence of different lattice constant. Increased quantities of lanthanum added to the lattice of Y 2 O 3 are associated with dependences of the specific band shifts. The larger ionic radii than matrix induce stress in the powders, which increase in the Y-O bond length manifested by a corresponding change in the frequency of vibrations. The irreducible representations for the optical and acoustical modes are as follows:  Figure 9 presents the luminescence spectra of the samples in the NIR region excited by the 980 nm laser diode. Luminescence spectra are typical for Er ions doped into Y 2 O 3 ceramic or YAG crystal [14,15,17]. According to Lei transition of 4 I 13/2 to 4 I 15/2 , Er 3? ions can emit large band of lights from 1450 nm to 1650 nm because of energy level Stark splitting [15]. The luminescence of 1531 nm is obviously dominant in the IR spectrum and is considered as first harmonic in laser. It can be seen on Fig. 9 that the intensities of the infrared emissions increase along with the more addition of La ions (inset Fig. 9). Because of small difference of the crystallite size between the samples, its influence on the luminescent property can be neglected. To absorption) process, non-radiatively relax to 4 I 13/2 state, and next radiate to 4 I 15/2 state for the 1.5 lm emission [15].
As it is commonly known, although the f-f transitions of electric dipole are parity forbidden, they become partially allowed when the rare-earth ion is situated at a low symmetry site due to the intermixing of the intra-4f states with higher electronic configuration [25]. Hence, the probability of f-f transitions increases, leading to the raise of oscillator intensity [26]. Since the contribution of oscillator intensity f to the GSA cross section r can be determined by: where e electron charge, m mass, Dv, f frequency, it may seem obvious that the raise off causes the increase of GSA cross section r, which contributes to the 1.5 lm enhancement. As a consequence, the main mechanism for the luminescence enhancement begins to occur due to the fact that the co-doping of La 3? causes tailoring the local environment around the erbium ions. It results in the increase of the GSA cross section and increase of IR luminescence.

Conclusions
The novel ceramic powders: Er 0  could be sintering in order to obtain a transparence bulk sample and exanimate magneto-optical properties.