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Optical and magneto-optical properties of Nd0.1La0.1Y1.8O3 transparent ceramics
Journal of Luminescence 209 (2019) 333–339
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Optical and magneto-optical properties of Nd0.1La0.1Y1.8O3 transparent ceramics
T
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Andrzej Kruka,d, , Tomasz Brylewskib, Mariusz Mrózekc a
University of Information Technology and Management, Faculty of Applied Computer Science, ul. Sucharskiego 2, 35-225 Rzeszów, Poland AGH University of Science and Technology, Faculty of Materials Science and Ceramics, al. Mickiewicza 30, 30-059 Kraków, Poland c Jagiellonian University, Faculty of Physics, Astronomy and Applied Computer Science, ul. Łojasiewicza 11, 30-348 Kraków, Poland d Pedagogical University of Cracow, Institute of Technology, Kraków, Poland b
A R T I C LE I N FO
A B S T R A C T
Keywords: Transparent ceramics Optical properties Microstructure Y2O3 Luminescence Verdet constant
Transparent ceramics in the form of yttrium oxide (Y2O3) doped with La and Nd and pure yttrium as a reference material were successfully prepared via modified EDTA gel process and hot isostatic pressure sintering. Their structure and microstructure were examined by means of scanning electron microscopy and X-ray diffraction, which revealed the high density and single-phase character of these materials. Their optical and magneto-optical properties were also analyzed. Emission spectra of Nd0.1La0.1Y1.8O3 after excitation performed with a continuous 808 nm laser beam showed strong and sharp emission lines. The luminescence lifetime was also investigated. The doped material exhibited a high Verdet constant of over 1000 deg/T m. The figure of merit was calculated in the VIS spectrum. The investigated transparent oxide ceramics can be considered a promising material for application in lasers and magneto-optical devices.
1. Introduction
methods such as the citrate process, EDTA gel processes, thermochemical reactions, and co-precipitation [1–9]. Chemical methods in particular are widely applied for this purpose, because they allow to obtain nanosized particles [7,9–12]. However, it is essential to choose the appropriate sintering process to achieve a high degree of consolidation. Advanced sintering techniques are required, including hot isostatic pressing (HIP), arc plasma melting (APM), spark plasma sintering (SPS), and sintering in vacuum or hydrogen [1–3,6,7,13–15]. By applying these techniques, it is possible to obtain a transparency of up to 80% vs. visible light and 90% vs. infrared light; these value are close to the theoretical values of approximately 83% and 75% respectively. For ultraviolet light, this ceramic is almost non-transparent because of large optical band gap (5.5 eV). Although pure Y2O3 exhibits no absorption bands, this drawback can be overcome by incorporating rare earth elements such as Nd, Yb, Eu, Er [3,7,8,13–15]. Another group of active dopants are elements that affect different chemical properties, e.g. Zr, La and LiF; these dopants can stabilize yttrium or lower its sintering temperature [1,7,12,16]. The magneto-optical properties of transparent materials significantly affect the performance of a Faraday rotator. For practical applications, materials need to exhibit desirable magneto-optical properties, i.e. a high angle of rotation, low-temperature sensibility, and low absorption [15].
The physical and chemical properties of solids have an impact on the quality and performance of devices that utilize them. In optics, significant parameters include transparency in visible and infrared light, and to the possibility of inducing the phenomenon of luminescence. Neodymium-doped yttrium oxides are a promising material for use in all types of high-power lasers thanks to their high thermal conductivity (13.6 W/mK), low thermal expansion coefficient, high dielectric constant (10−17), and relatively low phonon energy (591 cm−1) [1–3]. A very high melting point of about 2450 °C and good chemical stability are additional advantages of polycrystalline Y2O3. However, their mechanical properties are not sufficiently good [4]. Cubic Y2O3 is able to form trivalent bonds, which makes it suitable for doping with high concentrations of rare earth elements. The types of crystal structure in which Y2O3 may occur include cubic, C-bixbyite, monoclinic and hexagonal [1–4]. In the cubic type, each Y ion is located at the center of a cube. This structure contains 32 cations and 48 anions. Two kinds of cations exist – 8 Y3+ ions at b-sites with the C3i symmetry, and 24 Y3+ ions at d-sites with C2 point symmetry; the 48 oxygen anion are at e-sites with C1 point symmetry [5]. Transparent Y2O3 ceramics have so far been obtained by applying different methods, including the solid state reaction and chemical
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Correspondence to: University of Information Technology and Management, ul. Sucharskiego 2, 35-225 Rzeszów, Poland. E-mail address: [email protected] (A. Kruk).
https://doi.org/10.1016/j.jlumin.2019.01.027 Received 4 July 2018; Received in revised form 13 December 2018; Accepted 13 January 2019 Available online 14 January 2019 0022-2313/ © 2019 Elsevier B.V. All rights reserved.
Journal of Luminescence 209 (2019) 333–339
A. Kruk, et al.
The authors of the present study suggest that it is possible that the plane of light polarization in a transparent, polycrystalline yttria undergoes rotation in the UV-VIS-IR range [15]. Subsequent research indicated the possibility of achieving higher Verdet constants for sintered yttria modified with lanthanum and neodymium [15]. However, the results of the cited research were affected by a significant measurement error due to the presence of centers that scatter light in these sinters. This was caused by the microstructural defects in the sinters, which are associated with porosity and the inclusions consisting of secondary phases [15]. It was therefore necessary to conduct further studies aimed at obtaining sinters with the desired microstructure. Consequently, an EDTA gel process used to synthesize ceramic powders was modified in such a way as to limit the temperature increase that occurs during the thermal treatment of the gel precursor, as it is during this stage that grains crystallize and their phase and chemical composition is determined. This modified procedure yielded fine-crystalline powders with a narrow grain size distribution, which made it possible to obtain dense sinters with low total porosity and, subsequently, a fine-crystalline microstructure. In this paper, transparent ceramics in the form of Y2O3 co-doped with La and Nd was prepared via a modified EDTA gel process followed by HIP sintering. Magneto-optical properties, specific Faraday rotation angle, optical absorption coefficient in the UV-VIS-NIR and IR spectra, and magneto-optical figure of merit of yttria in the VIS wavelength range were determined. An additional objective was to investigate the luminescence and scintillation characteristics of doped yttria material.
Fig. 1. Schematic diagram of the device used to measure the Verdet constant, P – polarizer.
means of a modified citrate method with EDTA as an agent used to complexate metal cations in an aqueous solution, also known as the EDTA gel processes. The starting commercial solutions were lanthanum(III) nitrate hexahydrate – La(NO3)3 × 6H2O (Sigma-Aldrich, 99.9%), neodymium (III) nitrate hexahydrate –Nd(NO3)3 × 6H2O (Sigma-Aldrich, ≥99.9%), and yttrium(III) nitrate hexahydrate – Y(NO3)3 × 6H2O (SigmaAldrich, 99.8%), and ethylenediaminetetraacetic acid – C10H16N2O8 (Aldrich, 99.995%). The three nitrate salts with a known dry weight were dissolved in doubly distilled water to prepare nitrate solutions of neodymium (Nd(NO3)3), lanthanum (La(NO3)3) and yttrium (Y(NO3)3) with concentrations of 0.302, 0.8279 and 0.3861 mol/dm3, respectively. The above-mentioned solutions were mixed at a molar ratio required to obtain the nominal composition of La0.1Nd0.1Y1.8O3. Y2O3 served as a reference powder. To each of the solutions, 0.1 M EDTA was added by means of a peristaltic pump. The amount of the agent-complexing metal cations was selected so that the solution contained one mol of hydroxyl acid per each mol of metal cations. During intense mixing, the pH of 8 was maintained to prevent the precipitation of salt. The liquid precursor was evaporated and ignited to obtain powders, which were then ground in an agate mortar and calcinated in air at 700 °C for 10 h. In the final stage of powder synthesis, grain agglomerates were ground in a rotary-vibratory mill for 30 min in a propanol medium and with the use of zirconia beads, and then dried at room temperature. To obtain dense sinters, the powders obtained via calcination were ground with anhydrous ethanol with 3 wt% of oleic acid, added as a lubricant for pressing. After drying at ambient temperature, the obtained materials were formed into green bodies via biaxial pressing under a pressure of 100 MPa. The green bodies were then compacted by means of cold isostatic pressing (CIP) under 250 MPa, deparaffinated, and finally sintered. The green bodies were hot-pressed for 2 h in an argon atmosphere at 1400 °C and under a uniaxial pressure of 30 MPa, using a furnace manufactured by Thermal Technology Inc. (Astro Division). A graphite mold and graphite stamps were used to sinter the samples using the afore-mentioned method. To improve transparency, the obtained sinters underwent further thermal treatment that involved 10 h of annealing in air at 1000 °C, and then another 10 h of annealing, this time at 1100 °C. To allow their optical properties to be determined, the samples were polished to a mirror-like finish on both sides.
2. Experimental 2.1. Methodology A scanning electron microscope (FEI Nova NanoSEM 200) coupled with the EDAX Genesis XM X-ray microanalysis system with the Sapphire Si(Li) EDS detector was used to examine the morphology and chemical composition of the samples. In order to evaluate the grain size distribution and the corresponding average grain size of sintered pellets, SEM images after binarization were processed using the ImageJ (ver. 1.50) program. The investigated metallographic specimens were thermally etched for 0.5 h in air at 1000 °C. The X′Pert XRD diffractometer (Panalytical) was used for the phase analysis of the samples using the CuKα radiation at a scan rate of 0.008° per step. Highscore Plus software (Panalytical) with the standard data set of PCPDFWIN v.2.3. were used for data analysis. In-line UV/VIS/NIR (Specord 210VPlus, Analytik Jena) and IR transmittances (FT-IR Brucker 70 V) were also measured. Luminescence was excited by means of continuous wave 808 nm diode laser (Spectra Laser). The emission signal from the sample was recorded using a ThorLabs spectrometer. The Fluorolog Tau-3 Lifetime System (Horiba) was used to measure lifetimes. For the excitation sample was used a laser 480 nm. To measure the Verdet constant, a special spectrometer constructed at the Instytut Fotonowy was used. A system allowing Faraday rotation to be measured in an ac magnetic field was built to study the magnetooptical properties of materials over a wide spectral range (400–800 nm). This setup consisted of two coils in a Helmholtz system; the coils generated a stable ac magnetic field of 0.02 T, and featured a modulating technique with a "lock-in" amplifier and Wollaston prism with two detectors (balanced polarimeter). The device was used to measure Verdet constants as a function of wavelength. All components were isolated from any external field using a Faraday cage. Measurement error is above 5%. The setup is presented in Fig. 1. All measurements were conducting in room temperature.
3. Results and discussion 3.1. Microstructural study Fig. 2 shows representative SEM results obtained for the Y2O3 and La0.1Nd0.1Y1.8O3 sinters after HIP and thermal etching in air. Ceramics with very considerable density and an average crystallite size of 3–14 µm (Y2O3) and 7–20 µm (La0.1Nd0.1Y1.8O3) and single pores less than 0.3 µm in diameter inside grain boundaries can be seen. The
2.2. Sample preparation Two types of samples (La0.1Nd0.1Y1.8O3 and Y2O3) were prepared by 334
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Fig. 3. Grain size distribution in Y2O3 and La0.1Nd0.1Y1.8O3 bulk samples sintered.
3.2. XRD measurements During the structural analysis of the pure and doped Y2O3 samples, only the cubic crystal structure (JCPDS card # 98-008-1861) with the Ia-3 space group was found. The fitting employed a pseudo-Voigt function; the lattice parameters and crystallite size were calculated. The value of lattice spacing (dhkl) was calculated using Bragg's law. Lattice parameters (a) for both types of samples were calculated from the following formula:
Fig. 2. SEM micrograph of the polished etched surface of Y2O3 and La0.1Nd0.1Y1.8O3 bulk samples sintered for 2 h in argon at 1400 °C by means of HIP (magnification: 5000×).
1 h2 + k 2 + l 2 = 2 a2 dhkl
small grain size leads to a large density of grain boundaries. The average grain size of the ceramics increased together with neodymium and lanthanum content, as shown in Fig. 3. A more uniform distribution of grain size was achieved for the transparent ceramics modified with La and Nd. For both grain size distributions, a Gauss function was fitted. The La0.1Nd0.1Y1.8O3 surface was characterized by increased porosity inside grains. In addition, in the case of the La0.1Nd0.1Y1.8O3 sinter longitudinal precipitates formed at grain boundaries and in the grain interior. These precipitates can be clearly seen in Fig. 2. This may have been caused by carbon contamination during hot pressing in the graphite molds that are typically applied. To demonstrate a chemical composition shows the results of an EDS surface analysis performed for both samples (Fig. 2b). The molar ratios of chemical elements both samples corresponded to the assumed stoichiometric composition. In terms of the neodymium, lanthanum and yttrium ratios, the chemical composition was 0.011:0.01:1.79, respectively. Due to surface defects, i.e. porosity and the presence of the afore-mentioned precipitates, the material was characterized by relative low transparency. Fig. 4
(1)
where dhkl is the distance between adjacent planes, and h, k and l are Miller indices. Crystallite size was estimated from the full width at half maximum (FWHM) of the diffraction peaks using the well-known Scherrer equation [18]:
DXRD =
K ∙λ β∙cosθ
(2)
where K is the shape coefficient for the reciprocal lattice point (K= 0.89), λ is the wavelength of the X-rays (CuKα = 0.15406 nm), θ is the peak position, andβ is the width of specimen's peak corrected by an instrumental broad factor: 2 2 1/2 β = (Bobs −Bobs )
(3)
where Bobs is the FWHM that is related to the sample and Babs is the FWHM of the standard (Al2O3). The minimum dislocation density ρ in the crystal structure of the samples was estimated using the relation [19]:
ρ≈
335
1 Dxrd
2
(4)
Journal of Luminescence 209 (2019) 333–339
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Fig. 5. X-ray diffraction patterns recorded for Y2O3 and La0.1Nd0.1Y1.8O3 samples sintered for 2 h in argon at 1400 °C by means of HIP.
3.3. VIS-IR absorption spectra Fig. 6 shows the absorption spectra of pure Y2O3 (red line) and La0.1Nd0.1Y1.8O3 (black lines) in the wavelength range of 300–1100 nm. Adding neodymium and lanthanum ions causes a drop in transmission levels and the appearance of bands specific to neodymium. In total spectrum pure yttria shows transmittance ca. 50% and 30% for doped yttria. The peaks recorded for La0.1Nd0.1Y1.8O3 correspond to the following optical transitions in the Nd3+ ion energy level system: 4I9/2 → 4 G7/2; 4I9/2 → 4G5/2; 4I9/2 → 4F9/2; 4I9/2 → 4F7/2; 4I9/2 → 2H9/2 and 4I9/2 → 4F3/2 [21,22]. The change in absorbance observed for increased wavelengths may be explained by the Me theory and diffraction on grain boundaries [23]. 3.4. IR absorption spectra Fig. 7 shows the absorption spectra recorded for the examined samples in infrared wavelength range (2.5–25 µm). The presented of experimental data were not normalized for the sample thickness. The Y2O3 sample, which had a thickness of 0.4 mm, exhibited a much higher transparency in the wide IR spectrum than La0.1Nd0.1Y1.8O3, which was 0.47 mm thick. For the pure yttria sinter, no absorption bands were observed (one ca.1500 cm −1), in contrast to the sample doped with neodymium and lanthanum. The La0.1Nd0.1Y1.8O3 sinter exhibited a strong absorbent bands in the 1700–2800 cm−1 range. This difference in the distribution of relative intensities of emission lines can be attributed to the adsorption of atmospheric CO2 and H2O on the surface of the material [8,24]. The result of adsorption is the presence of CO32and OH- species on the surface of the samples; due to their high vibrational energies (peaks at ∼1500 and ∼2500 cm−1), the probability of multiphonon relaxation increased. In the infrared spectrum (Fig. 7), bands at 3700 and 1630 cm−1 were observed for La0.1Nd0.1Y1.8O3; these bands can be attributed to the stretching and bending of OHmolecules, respectively, while the band at 1520 cm−1 corresponds to adsorbed CO2 [8]. Incorporation yttrium by La and Nd ions shift absorbance band to higher wave number, and over 1000 cm−1 both sinters are no translucent.
Fig. 4. EDS surface analysis performed for areas for the Y2O3 a) and La0.1Nd0.1Y1.8O3 b) material sintered for 2 h in argon at 1400 °C by means of HIP.
The XRD patterns recorded for pure and doped Y2O3 are shown in Fig. 5. Sharp, well-defined peaks associated with the cubic structure without any secondary phases can be seen. The approximated value of the lattice spacing (dhkl), the lattice parameter (a), unit cell volume (V), the calculated crystallite size (Dxrd), and the dislocation density (ρ) of the investigated sinters are presented in Table 1. Doped Y2O3 exhibited a smaller average crystallite size than the pure one (Table 1), which is consistent with expectations and other results [12–15]. The lattice parameter of pure Y2O3 was very close to 10.604 Å – the theoretical value of the lattice parameter of yttria (JCPDS card # 98-008-1861). The larger volume for the doped sinter was consistent with the theoretical predictions [9–11], and can be attributed to the presence of La and Nd, which exhibit larger ionic radii. Inset Fig. 5 showed that, Bragg peaks moves towards lower positions with addition of La3+ ions, indicating change (stress) of the host lattice, because of the substitution of Y3+ (0.089 nm) ions with the larger La3+ (0.103 nm) ions. As a consequence, the local symmetry of the crystal field around Nd3+ reduces. Lanthanum contamination have also influence on increase of level luminescence [20].
3.5. Luminescence spectra Luminescence spectra of La0.1Nd0.1Y1.8O3 recorded in the NIR region after excitation with a 808 nm laser are shown in Fig. 8. As stated above, the Y2O3 lattice has two distinct crystallographic sites at which 336
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Table 1 Lattice spacing (dhkl), lattice parameter (a), and unit cell volume (V), crystallite size (Dxrd), and dislocation density (ρ) of pure Y2O3 and La0.1Nd0.1Y1.8O3. Y2O3
La0.1Nd0.1Y1.8O3
dhkl (222) [nm]
a [Å]
V [Å ]
Dxrd [nm]
ρ [10
3.0518
10.5718
1181.517
95.05
11.1
3
−14
m
−2
]
dhkl (222) [nm]
a [Å]
V [Å 3]
Dxrd [nm]
ρ [10−14 m−2]
3.0662
10.6216
1198.3214
69.05
21
Nd3+ ions can substitute yttrium ions: the ion can enter a site with either C2 or C3i point group symmetry. The latter site has a center of inversion, which implies selection rules that prevent all electric dipole (ED) transitions. This is only approximation, since ED transitions can occur via weak vibronic coupling. In nanocrystalline samples, the main contribution is from C3i sites due to magnetic dipole transitions, because the extent of vibronic coupling is much lower than in bulk crystals. Consequently, the spectra presented in Fig. 8 contain predominantly ED transitions of Nd3+ ions from C2 sites and magnetic dipole transitions from both sites. These spectra consist of several excitation bands which can be attributed to different transitions [21,22]. The peaks in the wavelength range of 900–1000 nm correspond to the following optical transitions in the Nd3+ ion energy level system: 4F3/2 → 4I9/2, between 1000 and 1150 nm: 4F3/2 → 4I11/2 and between 1300 and 1400 nm 4F3/2 → 4I13/2. No luminescence signal riginating from La ions was detected. La ions contamination increase a luminescence level of Nd ions doped yttria, as was described in others papers [18,21,22].
Fig. 6. Vis-NIR absorption spectra recorded for Y2O3 and La0.1Nd0.1Y1.8O3 samples sintered for 2 h in argon at 1400 °C by means of HIP.
3.6. Luminescence lifetime Fig. 9 shows the luminescence decay curves for La0.1Nd0.1Y1.8O3. The luminescence decay profile was adjusted by means of a single-exponential function. The decay curves were fitted according to Eq. (5)
t y = y0 + Aexp ⎛ ⎞ ⎝ T1 ⎠ ⎜
⎟
(5)
where t represents time, A, y0 stands for free parameters, and T1 is relaxation decay. The specific time of relaxation calculated from Eq. (5) was 315 ns and this level is relatively long [6,25]. 3.7. Verdet constant measurements
Fig. 7. Wide IR absorption spectra recorded for Y2O3 and La0.1Nd0.1Y1.8O3 samples sintered for 2 h in argon at 1400 °C by means of HIP.
Fig. 10 shows the results of Verdet constant measurements for La0.1Nd0.1Y1.8O3, performed between 400 and 800 nm. In general, the dependence between the absorption coefficient and the refractive index
Fig. 8. Spectrum recorded for a La0.1Nd0.1Y1.8O3 sample sintered for 2 h in argon at 1400 °C by means of HIP after excitation with a 808 nm laser.
Fig. 9. Luminescence lifetime for a La0.1Nd0.1Y1.8O3 sample sintered for 2 h in argon at 400 °C by means of HIP after excitation with a 808 nm laser. 337
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Fig. 10. Verdet constant of La0.1Nd0.1Y1.8O3 in the spectral range of 400–800 nm.
Fig. 12. The CIE chromaticity diagram of La0.1Nd0.1Y1.8O3 sample sintered for 2 h in argon at 1400 °C by means of HIP.
of a given material is defined by the Kramers–Kronig relation, which predicts strong spectral variations of the refractive index around the absorption resonance (525, 600, 750 nm in Fig. 5) of the sample under consideration [24,25]. The Verdet constant, however, is the quantity which reflects not just the sample’s refraction, but rather its magnetic/ polarization anisotropy (birefringence). The results are ca. 10 times lower than previous data reported for La0.1Nd0.1Y1.8O3 in paper [15,16], which indicated values of 180, 225, 450, 120 deg/T m at 532, 633, 780, 808 nm, respectively. The previous measurements were carried out using a simpler device with a much higher (ca. 40%) measurement error. Another reason for these discrepancies may be an inverse correlation between the transparency level and the Verdet constant [17]. Nevertheless, the Verdet constant is still relatively high, with values typical of doped chalogenic glass and other materials [26–29]. What should be highlight, in contrast to garnet crystals the Verdet constant is independent of measurement point. Based on the performed spectrophotometric measurements (Fig. 6) and the values of the Verdet constant (Fig. 10), the possibility of applying sintered La0.1Nd0.1Y1.8O3 as an optical isolator was evaluated. A parameter which can be used to help assess the usefulness of the material for practical applications such as optical isolators or circulators is the figure of merit (FOM) (Fig. 11), as defined by the following equation:
FOM =
V α
coefficient (m−1). Absorption coefficient was calculated from Lambert-Beer law:
A = log
I0 = αl I
(7)
where, A – absorbance (a.u.), l – length of sample (m), I0- Intensity of light before sample, I- Intensity of light after sample As can be seen in Fig. 11, the FOM of the investigated La0.1Nd0.1Y1.8O3 sample was strongly affected by the wavelength. Despite the impact of division by the absorbance coefficient, the highest values of FOM correlated with the occurrence of absorbance peaks.
3.8. The CIE chromaticity diagram To visualize the color of emitted light from the La0.1Nd0.1Y1.8O3 sample, color coordinates were calculated for three different types of standard illuminant: A, C and D65. They are collectively presented in a CIE1931 chromaticity diagram in Fig. 12. The computed color coordinates show different color coordinates – (X = 0.4583, Y = 0.4106), (X = 0.3218, Y = 0.3282) and (X = 0.3242, Y = 0.3406) for A, C, and D65, respectively. Points are present in the different region of chromaticity diagram.
(6)
where V represents the Verdet constant (deg/T m) and α the absorption
4. Conclusions A modified EDTA gel process was used to synthesize Y2O3 and La0.1Nd0.1Y1.8O3 micro-powders. Spectroscopic studies showed that the samples had a single-phase composition, indicating the complete incorporation of both neodymium and lanthanum ions into the crystal lattice of Y2O3. Sinters were successfully fabricated by means of the HIP method. The absorbance levels determined via spectrometric measurements performed over the wavelength range of 350–1100 nm. The absorbance of the for Y2O3 and La0.1Nd0.1Y1.8O3 samples in the IR range reached a minimum at 1250 cm−1 and 1400 cm−1, respectively. La0.1Nd0.1Y1.8O3 was characterized by high Verdet constant (over 1000 deg/Tm) with maximum peaks corresponding to absorption peaks. The prepared materials therefore appear suitable for a wide range of applications in electro-optics.
Acknowledgements Fig. 11. Dependence of the figure of merit (FOM) of La0.1Nd0.1Y1.8O3 on wavelength.
Support from the 2016/23/D/ST8/00014 grant (National Science Centre, Poland) is gratefully acknowledged. 338
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Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Optical and magneto-optical properties of Nd0.1La0.1Y1.8O3 transparent ceramics
T
⁎
Andrzej Kruka,d, , Tomasz Brylewskib, Mariusz Mrózekc a
University of Information Technology and Management, Faculty of Applied Computer Science, ul. Sucharskiego 2, 35-225 Rzeszów, Poland AGH University of Science and Technology, Faculty of Materials Science and Ceramics, al. Mickiewicza 30, 30-059 Kraków, Poland c Jagiellonian University, Faculty of Physics, Astronomy and Applied Computer Science, ul. Łojasiewicza 11, 30-348 Kraków, Poland d Pedagogical University of Cracow, Institute of Technology, Kraków, Poland b
A R T I C LE I N FO
A B S T R A C T
Keywords: Transparent ceramics Optical properties Microstructure Y2O3 Luminescence Verdet constant
Transparent ceramics in the form of yttrium oxide (Y2O3) doped with La and Nd and pure yttrium as a reference material were successfully prepared via modified EDTA gel process and hot isostatic pressure sintering. Their structure and microstructure were examined by means of scanning electron microscopy and X-ray diffraction, which revealed the high density and single-phase character of these materials. Their optical and magneto-optical properties were also analyzed. Emission spectra of Nd0.1La0.1Y1.8O3 after excitation performed with a continuous 808 nm laser beam showed strong and sharp emission lines. The luminescence lifetime was also investigated. The doped material exhibited a high Verdet constant of over 1000 deg/T m. The figure of merit was calculated in the VIS spectrum. The investigated transparent oxide ceramics can be considered a promising material for application in lasers and magneto-optical devices.
1. Introduction
methods such as the citrate process, EDTA gel processes, thermochemical reactions, and co-precipitation [1–9]. Chemical methods in particular are widely applied for this purpose, because they allow to obtain nanosized particles [7,9–12]. However, it is essential to choose the appropriate sintering process to achieve a high degree of consolidation. Advanced sintering techniques are required, including hot isostatic pressing (HIP), arc plasma melting (APM), spark plasma sintering (SPS), and sintering in vacuum or hydrogen [1–3,6,7,13–15]. By applying these techniques, it is possible to obtain a transparency of up to 80% vs. visible light and 90% vs. infrared light; these value are close to the theoretical values of approximately 83% and 75% respectively. For ultraviolet light, this ceramic is almost non-transparent because of large optical band gap (5.5 eV). Although pure Y2O3 exhibits no absorption bands, this drawback can be overcome by incorporating rare earth elements such as Nd, Yb, Eu, Er [3,7,8,13–15]. Another group of active dopants are elements that affect different chemical properties, e.g. Zr, La and LiF; these dopants can stabilize yttrium or lower its sintering temperature [1,7,12,16]. The magneto-optical properties of transparent materials significantly affect the performance of a Faraday rotator. For practical applications, materials need to exhibit desirable magneto-optical properties, i.e. a high angle of rotation, low-temperature sensibility, and low absorption [15].
The physical and chemical properties of solids have an impact on the quality and performance of devices that utilize them. In optics, significant parameters include transparency in visible and infrared light, and to the possibility of inducing the phenomenon of luminescence. Neodymium-doped yttrium oxides are a promising material for use in all types of high-power lasers thanks to their high thermal conductivity (13.6 W/mK), low thermal expansion coefficient, high dielectric constant (10−17), and relatively low phonon energy (591 cm−1) [1–3]. A very high melting point of about 2450 °C and good chemical stability are additional advantages of polycrystalline Y2O3. However, their mechanical properties are not sufficiently good [4]. Cubic Y2O3 is able to form trivalent bonds, which makes it suitable for doping with high concentrations of rare earth elements. The types of crystal structure in which Y2O3 may occur include cubic, C-bixbyite, monoclinic and hexagonal [1–4]. In the cubic type, each Y ion is located at the center of a cube. This structure contains 32 cations and 48 anions. Two kinds of cations exist – 8 Y3+ ions at b-sites with the C3i symmetry, and 24 Y3+ ions at d-sites with C2 point symmetry; the 48 oxygen anion are at e-sites with C1 point symmetry [5]. Transparent Y2O3 ceramics have so far been obtained by applying different methods, including the solid state reaction and chemical
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Correspondence to: University of Information Technology and Management, ul. Sucharskiego 2, 35-225 Rzeszów, Poland. E-mail address: [email protected] (A. Kruk).
https://doi.org/10.1016/j.jlumin.2019.01.027 Received 4 July 2018; Received in revised form 13 December 2018; Accepted 13 January 2019 Available online 14 January 2019 0022-2313/ © 2019 Elsevier B.V. All rights reserved.
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The authors of the present study suggest that it is possible that the plane of light polarization in a transparent, polycrystalline yttria undergoes rotation in the UV-VIS-IR range [15]. Subsequent research indicated the possibility of achieving higher Verdet constants for sintered yttria modified with lanthanum and neodymium [15]. However, the results of the cited research were affected by a significant measurement error due to the presence of centers that scatter light in these sinters. This was caused by the microstructural defects in the sinters, which are associated with porosity and the inclusions consisting of secondary phases [15]. It was therefore necessary to conduct further studies aimed at obtaining sinters with the desired microstructure. Consequently, an EDTA gel process used to synthesize ceramic powders was modified in such a way as to limit the temperature increase that occurs during the thermal treatment of the gel precursor, as it is during this stage that grains crystallize and their phase and chemical composition is determined. This modified procedure yielded fine-crystalline powders with a narrow grain size distribution, which made it possible to obtain dense sinters with low total porosity and, subsequently, a fine-crystalline microstructure. In this paper, transparent ceramics in the form of Y2O3 co-doped with La and Nd was prepared via a modified EDTA gel process followed by HIP sintering. Magneto-optical properties, specific Faraday rotation angle, optical absorption coefficient in the UV-VIS-NIR and IR spectra, and magneto-optical figure of merit of yttria in the VIS wavelength range were determined. An additional objective was to investigate the luminescence and scintillation characteristics of doped yttria material.
Fig. 1. Schematic diagram of the device used to measure the Verdet constant, P – polarizer.
means of a modified citrate method with EDTA as an agent used to complexate metal cations in an aqueous solution, also known as the EDTA gel processes. The starting commercial solutions were lanthanum(III) nitrate hexahydrate – La(NO3)3 × 6H2O (Sigma-Aldrich, 99.9%), neodymium (III) nitrate hexahydrate –Nd(NO3)3 × 6H2O (Sigma-Aldrich, ≥99.9%), and yttrium(III) nitrate hexahydrate – Y(NO3)3 × 6H2O (SigmaAldrich, 99.8%), and ethylenediaminetetraacetic acid – C10H16N2O8 (Aldrich, 99.995%). The three nitrate salts with a known dry weight were dissolved in doubly distilled water to prepare nitrate solutions of neodymium (Nd(NO3)3), lanthanum (La(NO3)3) and yttrium (Y(NO3)3) with concentrations of 0.302, 0.8279 and 0.3861 mol/dm3, respectively. The above-mentioned solutions were mixed at a molar ratio required to obtain the nominal composition of La0.1Nd0.1Y1.8O3. Y2O3 served as a reference powder. To each of the solutions, 0.1 M EDTA was added by means of a peristaltic pump. The amount of the agent-complexing metal cations was selected so that the solution contained one mol of hydroxyl acid per each mol of metal cations. During intense mixing, the pH of 8 was maintained to prevent the precipitation of salt. The liquid precursor was evaporated and ignited to obtain powders, which were then ground in an agate mortar and calcinated in air at 700 °C for 10 h. In the final stage of powder synthesis, grain agglomerates were ground in a rotary-vibratory mill for 30 min in a propanol medium and with the use of zirconia beads, and then dried at room temperature. To obtain dense sinters, the powders obtained via calcination were ground with anhydrous ethanol with 3 wt% of oleic acid, added as a lubricant for pressing. After drying at ambient temperature, the obtained materials were formed into green bodies via biaxial pressing under a pressure of 100 MPa. The green bodies were then compacted by means of cold isostatic pressing (CIP) under 250 MPa, deparaffinated, and finally sintered. The green bodies were hot-pressed for 2 h in an argon atmosphere at 1400 °C and under a uniaxial pressure of 30 MPa, using a furnace manufactured by Thermal Technology Inc. (Astro Division). A graphite mold and graphite stamps were used to sinter the samples using the afore-mentioned method. To improve transparency, the obtained sinters underwent further thermal treatment that involved 10 h of annealing in air at 1000 °C, and then another 10 h of annealing, this time at 1100 °C. To allow their optical properties to be determined, the samples were polished to a mirror-like finish on both sides.
2. Experimental 2.1. Methodology A scanning electron microscope (FEI Nova NanoSEM 200) coupled with the EDAX Genesis XM X-ray microanalysis system with the Sapphire Si(Li) EDS detector was used to examine the morphology and chemical composition of the samples. In order to evaluate the grain size distribution and the corresponding average grain size of sintered pellets, SEM images after binarization were processed using the ImageJ (ver. 1.50) program. The investigated metallographic specimens were thermally etched for 0.5 h in air at 1000 °C. The X′Pert XRD diffractometer (Panalytical) was used for the phase analysis of the samples using the CuKα radiation at a scan rate of 0.008° per step. Highscore Plus software (Panalytical) with the standard data set of PCPDFWIN v.2.3. were used for data analysis. In-line UV/VIS/NIR (Specord 210VPlus, Analytik Jena) and IR transmittances (FT-IR Brucker 70 V) were also measured. Luminescence was excited by means of continuous wave 808 nm diode laser (Spectra Laser). The emission signal from the sample was recorded using a ThorLabs spectrometer. The Fluorolog Tau-3 Lifetime System (Horiba) was used to measure lifetimes. For the excitation sample was used a laser 480 nm. To measure the Verdet constant, a special spectrometer constructed at the Instytut Fotonowy was used. A system allowing Faraday rotation to be measured in an ac magnetic field was built to study the magnetooptical properties of materials over a wide spectral range (400–800 nm). This setup consisted of two coils in a Helmholtz system; the coils generated a stable ac magnetic field of 0.02 T, and featured a modulating technique with a "lock-in" amplifier and Wollaston prism with two detectors (balanced polarimeter). The device was used to measure Verdet constants as a function of wavelength. All components were isolated from any external field using a Faraday cage. Measurement error is above 5%. The setup is presented in Fig. 1. All measurements were conducting in room temperature.
3. Results and discussion 3.1. Microstructural study Fig. 2 shows representative SEM results obtained for the Y2O3 and La0.1Nd0.1Y1.8O3 sinters after HIP and thermal etching in air. Ceramics with very considerable density and an average crystallite size of 3–14 µm (Y2O3) and 7–20 µm (La0.1Nd0.1Y1.8O3) and single pores less than 0.3 µm in diameter inside grain boundaries can be seen. The
2.2. Sample preparation Two types of samples (La0.1Nd0.1Y1.8O3 and Y2O3) were prepared by 334
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Fig. 3. Grain size distribution in Y2O3 and La0.1Nd0.1Y1.8O3 bulk samples sintered.
3.2. XRD measurements During the structural analysis of the pure and doped Y2O3 samples, only the cubic crystal structure (JCPDS card # 98-008-1861) with the Ia-3 space group was found. The fitting employed a pseudo-Voigt function; the lattice parameters and crystallite size were calculated. The value of lattice spacing (dhkl) was calculated using Bragg's law. Lattice parameters (a) for both types of samples were calculated from the following formula:
Fig. 2. SEM micrograph of the polished etched surface of Y2O3 and La0.1Nd0.1Y1.8O3 bulk samples sintered for 2 h in argon at 1400 °C by means of HIP (magnification: 5000×).
1 h2 + k 2 + l 2 = 2 a2 dhkl
small grain size leads to a large density of grain boundaries. The average grain size of the ceramics increased together with neodymium and lanthanum content, as shown in Fig. 3. A more uniform distribution of grain size was achieved for the transparent ceramics modified with La and Nd. For both grain size distributions, a Gauss function was fitted. The La0.1Nd0.1Y1.8O3 surface was characterized by increased porosity inside grains. In addition, in the case of the La0.1Nd0.1Y1.8O3 sinter longitudinal precipitates formed at grain boundaries and in the grain interior. These precipitates can be clearly seen in Fig. 2. This may have been caused by carbon contamination during hot pressing in the graphite molds that are typically applied. To demonstrate a chemical composition shows the results of an EDS surface analysis performed for both samples (Fig. 2b). The molar ratios of chemical elements both samples corresponded to the assumed stoichiometric composition. In terms of the neodymium, lanthanum and yttrium ratios, the chemical composition was 0.011:0.01:1.79, respectively. Due to surface defects, i.e. porosity and the presence of the afore-mentioned precipitates, the material was characterized by relative low transparency. Fig. 4
(1)
where dhkl is the distance between adjacent planes, and h, k and l are Miller indices. Crystallite size was estimated from the full width at half maximum (FWHM) of the diffraction peaks using the well-known Scherrer equation [18]:
DXRD =
K ∙λ β∙cosθ
(2)
where K is the shape coefficient for the reciprocal lattice point (K= 0.89), λ is the wavelength of the X-rays (CuKα = 0.15406 nm), θ is the peak position, andβ is the width of specimen's peak corrected by an instrumental broad factor: 2 2 1/2 β = (Bobs −Bobs )
(3)
where Bobs is the FWHM that is related to the sample and Babs is the FWHM of the standard (Al2O3). The minimum dislocation density ρ in the crystal structure of the samples was estimated using the relation [19]:
ρ≈
335
1 Dxrd
2
(4)
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Fig. 5. X-ray diffraction patterns recorded for Y2O3 and La0.1Nd0.1Y1.8O3 samples sintered for 2 h in argon at 1400 °C by means of HIP.
3.3. VIS-IR absorption spectra Fig. 6 shows the absorption spectra of pure Y2O3 (red line) and La0.1Nd0.1Y1.8O3 (black lines) in the wavelength range of 300–1100 nm. Adding neodymium and lanthanum ions causes a drop in transmission levels and the appearance of bands specific to neodymium. In total spectrum pure yttria shows transmittance ca. 50% and 30% for doped yttria. The peaks recorded for La0.1Nd0.1Y1.8O3 correspond to the following optical transitions in the Nd3+ ion energy level system: 4I9/2 → 4 G7/2; 4I9/2 → 4G5/2; 4I9/2 → 4F9/2; 4I9/2 → 4F7/2; 4I9/2 → 2H9/2 and 4I9/2 → 4F3/2 [21,22]. The change in absorbance observed for increased wavelengths may be explained by the Me theory and diffraction on grain boundaries [23]. 3.4. IR absorption spectra Fig. 7 shows the absorption spectra recorded for the examined samples in infrared wavelength range (2.5–25 µm). The presented of experimental data were not normalized for the sample thickness. The Y2O3 sample, which had a thickness of 0.4 mm, exhibited a much higher transparency in the wide IR spectrum than La0.1Nd0.1Y1.8O3, which was 0.47 mm thick. For the pure yttria sinter, no absorption bands were observed (one ca.1500 cm −1), in contrast to the sample doped with neodymium and lanthanum. The La0.1Nd0.1Y1.8O3 sinter exhibited a strong absorbent bands in the 1700–2800 cm−1 range. This difference in the distribution of relative intensities of emission lines can be attributed to the adsorption of atmospheric CO2 and H2O on the surface of the material [8,24]. The result of adsorption is the presence of CO32and OH- species on the surface of the samples; due to their high vibrational energies (peaks at ∼1500 and ∼2500 cm−1), the probability of multiphonon relaxation increased. In the infrared spectrum (Fig. 7), bands at 3700 and 1630 cm−1 were observed for La0.1Nd0.1Y1.8O3; these bands can be attributed to the stretching and bending of OHmolecules, respectively, while the band at 1520 cm−1 corresponds to adsorbed CO2 [8]. Incorporation yttrium by La and Nd ions shift absorbance band to higher wave number, and over 1000 cm−1 both sinters are no translucent.
Fig. 4. EDS surface analysis performed for areas for the Y2O3 a) and La0.1Nd0.1Y1.8O3 b) material sintered for 2 h in argon at 1400 °C by means of HIP.
The XRD patterns recorded for pure and doped Y2O3 are shown in Fig. 5. Sharp, well-defined peaks associated with the cubic structure without any secondary phases can be seen. The approximated value of the lattice spacing (dhkl), the lattice parameter (a), unit cell volume (V), the calculated crystallite size (Dxrd), and the dislocation density (ρ) of the investigated sinters are presented in Table 1. Doped Y2O3 exhibited a smaller average crystallite size than the pure one (Table 1), which is consistent with expectations and other results [12–15]. The lattice parameter of pure Y2O3 was very close to 10.604 Å – the theoretical value of the lattice parameter of yttria (JCPDS card # 98-008-1861). The larger volume for the doped sinter was consistent with the theoretical predictions [9–11], and can be attributed to the presence of La and Nd, which exhibit larger ionic radii. Inset Fig. 5 showed that, Bragg peaks moves towards lower positions with addition of La3+ ions, indicating change (stress) of the host lattice, because of the substitution of Y3+ (0.089 nm) ions with the larger La3+ (0.103 nm) ions. As a consequence, the local symmetry of the crystal field around Nd3+ reduces. Lanthanum contamination have also influence on increase of level luminescence [20].
3.5. Luminescence spectra Luminescence spectra of La0.1Nd0.1Y1.8O3 recorded in the NIR region after excitation with a 808 nm laser are shown in Fig. 8. As stated above, the Y2O3 lattice has two distinct crystallographic sites at which 336
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Table 1 Lattice spacing (dhkl), lattice parameter (a), and unit cell volume (V), crystallite size (Dxrd), and dislocation density (ρ) of pure Y2O3 and La0.1Nd0.1Y1.8O3. Y2O3
La0.1Nd0.1Y1.8O3
dhkl (222) [nm]
a [Å]
V [Å ]
Dxrd [nm]
ρ [10
3.0518
10.5718
1181.517
95.05
11.1
3
−14
m
−2
]
dhkl (222) [nm]
a [Å]
V [Å 3]
Dxrd [nm]
ρ [10−14 m−2]
3.0662
10.6216
1198.3214
69.05
21
Nd3+ ions can substitute yttrium ions: the ion can enter a site with either C2 or C3i point group symmetry. The latter site has a center of inversion, which implies selection rules that prevent all electric dipole (ED) transitions. This is only approximation, since ED transitions can occur via weak vibronic coupling. In nanocrystalline samples, the main contribution is from C3i sites due to magnetic dipole transitions, because the extent of vibronic coupling is much lower than in bulk crystals. Consequently, the spectra presented in Fig. 8 contain predominantly ED transitions of Nd3+ ions from C2 sites and magnetic dipole transitions from both sites. These spectra consist of several excitation bands which can be attributed to different transitions [21,22]. The peaks in the wavelength range of 900–1000 nm correspond to the following optical transitions in the Nd3+ ion energy level system: 4F3/2 → 4I9/2, between 1000 and 1150 nm: 4F3/2 → 4I11/2 and between 1300 and 1400 nm 4F3/2 → 4I13/2. No luminescence signal riginating from La ions was detected. La ions contamination increase a luminescence level of Nd ions doped yttria, as was described in others papers [18,21,22].
Fig. 6. Vis-NIR absorption spectra recorded for Y2O3 and La0.1Nd0.1Y1.8O3 samples sintered for 2 h in argon at 1400 °C by means of HIP.
3.6. Luminescence lifetime Fig. 9 shows the luminescence decay curves for La0.1Nd0.1Y1.8O3. The luminescence decay profile was adjusted by means of a single-exponential function. The decay curves were fitted according to Eq. (5)
t y = y0 + Aexp ⎛ ⎞ ⎝ T1 ⎠ ⎜
⎟
(5)
where t represents time, A, y0 stands for free parameters, and T1 is relaxation decay. The specific time of relaxation calculated from Eq. (5) was 315 ns and this level is relatively long [6,25]. 3.7. Verdet constant measurements
Fig. 7. Wide IR absorption spectra recorded for Y2O3 and La0.1Nd0.1Y1.8O3 samples sintered for 2 h in argon at 1400 °C by means of HIP.
Fig. 10 shows the results of Verdet constant measurements for La0.1Nd0.1Y1.8O3, performed between 400 and 800 nm. In general, the dependence between the absorption coefficient and the refractive index
Fig. 8. Spectrum recorded for a La0.1Nd0.1Y1.8O3 sample sintered for 2 h in argon at 1400 °C by means of HIP after excitation with a 808 nm laser.
Fig. 9. Luminescence lifetime for a La0.1Nd0.1Y1.8O3 sample sintered for 2 h in argon at 400 °C by means of HIP after excitation with a 808 nm laser. 337
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Fig. 10. Verdet constant of La0.1Nd0.1Y1.8O3 in the spectral range of 400–800 nm.
Fig. 12. The CIE chromaticity diagram of La0.1Nd0.1Y1.8O3 sample sintered for 2 h in argon at 1400 °C by means of HIP.
of a given material is defined by the Kramers–Kronig relation, which predicts strong spectral variations of the refractive index around the absorption resonance (525, 600, 750 nm in Fig. 5) of the sample under consideration [24,25]. The Verdet constant, however, is the quantity which reflects not just the sample’s refraction, but rather its magnetic/ polarization anisotropy (birefringence). The results are ca. 10 times lower than previous data reported for La0.1Nd0.1Y1.8O3 in paper [15,16], which indicated values of 180, 225, 450, 120 deg/T m at 532, 633, 780, 808 nm, respectively. The previous measurements were carried out using a simpler device with a much higher (ca. 40%) measurement error. Another reason for these discrepancies may be an inverse correlation between the transparency level and the Verdet constant [17]. Nevertheless, the Verdet constant is still relatively high, with values typical of doped chalogenic glass and other materials [26–29]. What should be highlight, in contrast to garnet crystals the Verdet constant is independent of measurement point. Based on the performed spectrophotometric measurements (Fig. 6) and the values of the Verdet constant (Fig. 10), the possibility of applying sintered La0.1Nd0.1Y1.8O3 as an optical isolator was evaluated. A parameter which can be used to help assess the usefulness of the material for practical applications such as optical isolators or circulators is the figure of merit (FOM) (Fig. 11), as defined by the following equation:
FOM =
V α
coefficient (m−1). Absorption coefficient was calculated from Lambert-Beer law:
A = log
I0 = αl I
(7)
where, A – absorbance (a.u.), l – length of sample (m), I0- Intensity of light before sample, I- Intensity of light after sample As can be seen in Fig. 11, the FOM of the investigated La0.1Nd0.1Y1.8O3 sample was strongly affected by the wavelength. Despite the impact of division by the absorbance coefficient, the highest values of FOM correlated with the occurrence of absorbance peaks.
3.8. The CIE chromaticity diagram To visualize the color of emitted light from the La0.1Nd0.1Y1.8O3 sample, color coordinates were calculated for three different types of standard illuminant: A, C and D65. They are collectively presented in a CIE1931 chromaticity diagram in Fig. 12. The computed color coordinates show different color coordinates – (X = 0.4583, Y = 0.4106), (X = 0.3218, Y = 0.3282) and (X = 0.3242, Y = 0.3406) for A, C, and D65, respectively. Points are present in the different region of chromaticity diagram.
(6)
where V represents the Verdet constant (deg/T m) and α the absorption
4. Conclusions A modified EDTA gel process was used to synthesize Y2O3 and La0.1Nd0.1Y1.8O3 micro-powders. Spectroscopic studies showed that the samples had a single-phase composition, indicating the complete incorporation of both neodymium and lanthanum ions into the crystal lattice of Y2O3. Sinters were successfully fabricated by means of the HIP method. The absorbance levels determined via spectrometric measurements performed over the wavelength range of 350–1100 nm. The absorbance of the for Y2O3 and La0.1Nd0.1Y1.8O3 samples in the IR range reached a minimum at 1250 cm−1 and 1400 cm−1, respectively. La0.1Nd0.1Y1.8O3 was characterized by high Verdet constant (over 1000 deg/Tm) with maximum peaks corresponding to absorption peaks. The prepared materials therefore appear suitable for a wide range of applications in electro-optics.
Acknowledgements Fig. 11. Dependence of the figure of merit (FOM) of La0.1Nd0.1Y1.8O3 on wavelength.
Support from the 2016/23/D/ST8/00014 grant (National Science Centre, Poland) is gratefully acknowledged. 338
Journal of Luminescence 209 (2019) 333–339
A. Kruk, et al.
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