Fabrication and Magneto-Optical Properties of Yb2O3 Based Ceramics

Transparent ceramics based on ytterbium oxide have been successfully produced by vacuum sintering of self-propagating high-temperature synthesized powders with use of a La2O3 sintering aid. Phase composition and microstructure of the initial powders were studied by X-ray diffraction analysis and scanning electron microscopy. It was found that addition of 5 mol.% of La2O3 does not cause formation of secondary phases in the Yb2O3 powders. The 4% La:Yb2O3 ceramics showed the best in-line transmittance of 73% at a wavelength of 2 μm among the studied samples. Dependence of the Verdet constant on wavelength was investigated in the range of 0.4–2 μm. The most promising finding is use of the obtained material as Faraday isolators in the wavelength region of ~1.5 μm, where there are no absorption bands, and the Verdet constant is 8.6 rad/(T*m).


Introduction
The magnetic field causes a change in the dielectric tensor, which is responsible for the refractive index, absorption in the material, and various magneto-optical effects. One of these effects is the Faraday rotation of the polarization plane (circular birefringence). This effect is widely used for measuring the magnitude of the magnetic field, indirect determining the values of some parameters (for example, magneto-optical electric current sensors [1]), and various manipulations with the polarization of the transmitted radiation (Faraday devices, magneto-optical Q-switch modulators [2], choppers, polarization switches, deflectors [3], etc.).
Faraday isolators (FI) and rotators (FR) have found a wide application in laser physics. These devices are used to organize multi-pass amplifier schemes and schemes with a compensation for thermally induced birefringence of lasers active elements, for optical isolation of different parts of the laser scheme, and for ensuring propagation of laser radiation in a strictly specified direction.
Faraday isolator is an optical device, proposed in its traditional form by Lord Rayleigh. As shown in Figure 1, it represents a magneto-optical element (MOE) located between two polarizers (1,4) crossed relative to each other at the same angle and placed in an external magnetic field (3). MOE non-reciprocally rotates the polarization plane of the transmitted radiation by 45 • . A half-wave plate (2) is often included in the FI scheme, which is tuned to rotate polarization in the direction opposite to the Faraday rotation at an angle of 45 • , while the polarizers do not cross. This makes it possible to keep the direction of transmitted radiation polarization when the FI is introduced into it.
opposite to the Faraday rotation at an angle of 45°, while the polarizers do not cross. This makes it possible to keep the direction of transmitted radiation polarization when the FI is introduced into it. Due to the half-wave plate, in the absence of thermal effects polarization of radiation passing in the direction of the direct passage (from B to A) remains unchanged (for example, horizontal-parallel to the x-axis), and radiation passes through polarizer 4. On the backward passage (from A to B), polarization changes to vertical (parallel to the y-axis), and radiation is reflected by polarizer 1. A specific task is development of FI and FR for high average power lasers. Thermally induced effects arising from absorption in the magneto-optical element lead to inhomogeneous changes of transmitted radiation polarization. This worsens the main characteristics of Faraday devices-uniformity of polarization plane rotation (for FR) and isolation degree (for FI). Additionally, with increasing the power of transmitted radiation, the resulting thermal effects become stronger, worsening the performance of Faraday devices. At a certain power of laser radiation, the FI and FR no longer meet the requirements for isolation and uniformity of polarization rotation, and these devices need to be improved to advance into the high-power region. For this purpose, one could improve the magnetic system [4,5], use optical circuits with compensation for thermal effects [6][7][8][9], use optimal crystal orientations arising from dependence of thermal effects on the direction of crystallographic axes [10,11], and look for new magneto-optical materials.
The main efforts of researchers in the latter direction are aimed at developing new magneto-optical materials with better characteristics (high thermal conductivity and Verdet's constant, minimal absorption at operating wavelengths, etc.). Recently, ceramic technology for such material fabrication has been noted as one of the most promising developments, and on this basis a number of ceramics with unique characteristics have already been prepared [12][13][14][15][16]. Progress in development of large-size FIs, which are important for creation of high-power laser sources, can be related to use of rare-earth sesquioxides as a base material. Rare-earth oxides (REO) are very promising due to high concentrations of paramagnetic RE 3+ ions in comparison with other materials. REO have a wide transparency range and excellent mechanical and thermal properties. It has recently been shown that Tb2O3 [17][18][19], Dy2O3 [20,21], and Ho2O3 [22][23][24] ceramics have a higher Verdet constant of 3.5, 2.2 and 1.3 times, respectively, in comparison with one of the widely used materials, a single crystal of terbium gallium garnet (TGG). Other studied ceramics based on rare-earth cubic oxides include Er2O3 [25], which has a low value of the Verdet constant and is of limited interest for magneto-optical applications, and diamagnetic RE 3+ ion based ceramics (Y2O3, Sc2O3, Gd2O3, and Lu2O3), which have even lower values. There are some data in literature on preparation of Yb2O3 in the form of single crystals and study of its optical properties [26]. Information on production of Yb2O3 optical ceramics in the available literature is limited to a single patent [27], and magneto-optical characteristics of ceramics based on ytterbium oxide have not been studied. A specific task is development of FI and FR for high average power lasers. Thermally induced effects arising from absorption in the magneto-optical element lead to inhomogeneous changes of transmitted radiation polarization. This worsens the main characteristics of Faraday devices-uniformity of polarization plane rotation (for FR) and isolation degree (for FI). Additionally, with increasing the power of transmitted radiation, the resulting thermal effects become stronger, worsening the performance of Faraday devices. At a certain power of laser radiation, the FI and FR no longer meet the requirements for isolation and uniformity of polarization rotation, and these devices need to be improved to advance into the high-power region. For this purpose, one could improve the magnetic system [4,5], use optical circuits with compensation for thermal effects [6][7][8][9], use optimal crystal orientations arising from dependence of thermal effects on the direction of crystallographic axes [10,11], and look for new magneto-optical materials.
The main efforts of researchers in the latter direction are aimed at developing new magneto-optical materials with better characteristics (high thermal conductivity and Verdet's constant, minimal absorption at operating wavelengths, etc.). Recently, ceramic technology for such material fabrication has been noted as one of the most promising developments, and on this basis a number of ceramics with unique characteristics have already been prepared [12][13][14][15][16]. Progress in development of large-size FIs, which are important for creation of high-power laser sources, can be related to use of rare-earth sesquioxides as a base material. Rare-earth oxides (REO) are very promising due to high concentrations of paramagnetic RE 3+ ions in comparison with other materials. REO have a wide transparency range and excellent mechanical and thermal properties. It has recently been shown that Tb 2 O 3 [17][18][19], Dy 2 O 3 [20,21], and Ho 2 O 3 [22][23][24] ceramics have a higher Verdet constant of 3.5, 2.2 and 1.3 times, respectively, in comparison with one of the widely used materials, a single crystal of terbium gallium garnet (TGG). Other studied ceramics based on rare-earth cubic oxides include Er 2 O 3 [25], which has a low value of the Verdet constant and is of limited interest for magneto-optical applications, and diamagnetic RE 3+ ion based ceramics (Y 2 O 3 , Sc 2 O 3 , Gd 2 O 3 , and Lu 2 O 3 ), which have even lower values. There are some data in literature on preparation of Yb 2 O 3 in the form of single crystals and study of its optical properties [26]. Information on production of Yb 2 O 3 optical ceramics in the available literature is limited to a single patent [27], and magneto-optical characteristics of ceramics based on ytterbium oxide have not been studied. The aims of this work were to prepare transparent Yb 2 O 3 ceramics by vacuum sintering of nanodispersed powders and to evaluate optical and magneto-optical characteristics, which are of decisive importance for their use in Faraday isolators and rotators.

Experimental
Powders of ytterbium oxide and mixed ytterbium-lanthanum oxides were obtained by self-propagating high-temperature synthesis (SHS), previously successfully used for preparation of other rare-earth nanopowders [28][29][30][31][32][33]. Ytterbium and lanthanum nitrates were used as an oxidant, and glycine was used as a reducing agent.
The starting materials were ytterbium oxide Yb 2 O 3 (99.999% Yumeks, Russia), lanthanum oxide La 2 O 3 (99.99% Polirit, Russia), nitric acid HNO 3 (99.9999%, Khimreaktiv, Russia), and glycine NH 2 CH 2 COOH (99.9%, Khimreaktiv, Russia). Metal nitrates were obtained by dissolving~10 g of starting metal oxides in a nitric acid under stirring and heating. The solution concentration was determined by weighting a dry residue after evaporation and calcination at a temperature of 1100 • C. Then the solutions were mixed according to the desired stoichiometry of the ytterbium-lanthanum oxides, and glycine was added in a molar ratio of 1:1 with respect to the metal nitrates. The resulting precursor solution was evaporated at a temperature of~110 • C in a quartz flask, which was placed in a resistance furnace preheated to 300 • C. Heating of the precursor led to initiation of exothermic reactions and propagation of the SHS process over the entire volume of the charge. As a result, formation of highly dispersed white powders of ytterbium and ytterbium-lanthanum oxides occurred. The combustion process was accompanied by evolution of a large amount of gas-phase products. To ensure oxidation of organic by-products and desorption of water, the SHS-product was annealed at a temperature of 900 • C for 30 min. Powders of five compositions, Yb 2 O 3 , 1% La:Yb 2 O 3 , 2% La:Yb 2 O 3 , 3% La:Yb 2 O 3 , 4% La:Yb 2 O 3 , and 5%La:Yb 2 O 3 , were prepared.
To obtain the ceramics, the powders were uniaxially pressed in ∅ 15 mm disks under a pressure of 300 MPa. Sintering was carried out in a vacuum furnace at a temperature of 1750 • C for 3 h at a heating rate of 10 • C/min at a residual pressure of no more than 10 −2 Pa. Then the ceramic samples were calcined in air at 900 • C for 2 h and mirror polished on both sides.
XRD patterns of the prepared powders were recorded with a Shimadzu XRD-7000 (Shimadzu, Japan) with a graphite monochromator (Cu (Kα radiation λ = 1.54178 Å) in the 2θ range of 25-60 • with a scanning step of 0.04 • . Qualitative phase analysis was carried out using the "Diffrac.EVA" program. Presence of a particular phase in the samples was visually determined from the diffraction patterns obtained during the experiments using data from the PDF-2 powder diffraction bank. The average crystallite size of the powders was calculated from the peak broadening of the XRD pattern using Scherrer's Equation (1): where B is the full width at half maximum (in 2θ), θ is the angle of the reflex maximum (in 2θ), and λ is the average wavelength (1.54 Å). The powder morphology, ceramic microstructure, and average grain size were investigated using a JSM 6390 (JEOL, Japan) scanning electron microscope.
Density of the ceramic samples was calculated by hydrostatic weighing in water with an accuracy of 0.01 g/cm 3 .
Dependence of the Verdet constant on a wavelength was measured by two approaches: using probe laser sources at wavelengths of 1064, 1310, 1550, and 1940 nm and using the polarization-stepping method in the range of 400-1100 nm [34]. The Verdet constant was measured at a temperature of 25 • C. Magneto-optical element was fixed in the center of a magnetic system, which was placed between two polarizers: calcite wedge and Glan prism. The magnetic field changed less than for 0.2% of the value of 2.77 T over the length of the sample; thus, the magnetic field around the sample was uniform. Diode lasers at the wavelengths of 1310 and 1550 nm, and Nd:YAG (1064 nm) and Tm fiber (1940 nm) lasers were used as the probe laser sources. The Faraday rotation angle (θ) was determined by the angle of rotation of the Glan prism corresponding to the minimum signal on the camera in the absence and presence of a magnetic field.
Using the known values of the length of each studied sample L, the magnetic field B, and the Faraday rotation angle θ, the Verdet constant V was found at each probe wavelength. Avantes Avalight-hal-mini, generating in the 300-1400 nm wavelength range, was used as a broadband white light source of probe radiation for the polarization-stepping method. The Avantes AvaSpec Evo spectrometer was used as an analyzer of the transmitted radiation spectrum. The measurement range was determined by the white light source spectrum and the sensitivity of the scheme. The dependence of the Faraday rotation angle on wavelength was found by measuring the transmitted light spectrum intensity dependence on the Glan prism rotation in the presence and absence of a magnetic field.

Results and Discussion
Vacuum sintering of nanopowders with use of lanthanum oxide as a sintering aid is one of the most effective approaches in production of optical rare-earth oxides ceramics. For example, Y 2 O 3 [28,29], Dy 2 O 3 [20,21], Ho 2 O 3 [23], Lu 2 O 3 [30], and Er 2 O 3 [25] ceramics were previously synthesized based on this approach. The action of lanthanum oxide in improvement of REO sinterability consists in increasing the number of point defects in the material, which accelerates mass transfer and causes rapid healing of pores. However, La 2 O 3 has limited solubility in cubic REOs and tends to segregate along grain boundaries and form secondary phases. This leads to fluctuations in the refractive index in the sample volume and scattering of the transmitted radiation. Thus, it is important to select the concentration of lanthanum oxide in the material in order to prevent formation of secondary phases, but at the same time to provide sufficient improvement in sinterability to obtain pore-free material. In this regard, experiments on Yb 2 O 3 sintering were carried out on a series of samples containing 0-5 mol.% La 2 O 3 . Magnetochemistry 2020, 6, 63 4 of 10 changed less than for 0.2% of the value of 2.77 T over the length of the sample; thus, the magnetic field around the sample was uniform. Diode lasers at the wavelengths of 1310 and 1550 nm, and Nd:YAG (1064 nm) and Tm fiber (1940 nm) lasers were used as the probe laser sources. The Faraday rotation angle (θ) was determined by the angle of rotation of the Glan prism corresponding to the minimum signal on the camera in the absence and presence of a magnetic field.

Properties of Yb 2 O 3 Based Powders
Using the known values of the length of each studied sample L, the magnetic field B, and the Faraday rotation angle θ, the Verdet constant V was found at each probe wavelength. Avantes Avalight-hal-mini, generating in the 300-1400 nm wavelength range, was used as a broadband white light source of probe radiation for the polarization-stepping method. The Avantes AvaSpec Evo spectrometer was used as an analyzer of the transmitted radiation spectrum. The measurement range was determined by the white light source spectrum and the sensitivity of the scheme. The dependence of the Faraday rotation angle on wavelength was found by measuring the transmitted light spectrum intensity dependence on the Glan prism rotation in the presence and absence of a magnetic field.

Results and Discussion
Vacuum sintering of nanopowders with use of lanthanum oxide as a sintering aid is one of the most effective approaches in production of optical rare-earth oxides ceramics. For example, Y2O3 [28,29], Dy2O3 [20,21], Ho2O3 [23], Lu2O3 [30], and Er2O3 [25] ceramics were previously synthesized based on this approach. The action of lanthanum oxide in improvement of REO sinterability consists in increasing the number of point defects in the material, which accelerates mass transfer and causes rapid healing of pores. However, La2O3 has limited solubility in cubic REOs and tends to segregate along grain boundaries and form secondary phases. This leads to fluctuations in the refractive index in the sample volume and scattering of the transmitted radiation. Thus, it is important to select the concentration of lanthanum oxide in the material in order to prevent formation of secondary phases, but at the same time to provide sufficient improvement in sinterability to obtain pore-free material. In this regard, experiments on Yb2O3 sintering were carried out on a series of samples containing 0-5 mol.% La2O3.   The morphology of SHS powders is caused by the action of gaseous substances that disperse the batch and lead to production of crumbly white Yb 2 O 3 . The corresponding equation of chemical reactions can be written as follows:

Properties of Yb2O3 Based Powders
As revealed from XRD analysis (Figure 3a), the as-synthesized powders are a single-phase cubic Yb 2 O 3 S.G.: Ia − 3 (206) (PDF # 00-041-1106); impurities of other phases were not detected. The XRD crystallite size of the powders decreases with increasing the lanthanum oxide content. The concentration of La 2 O 3 is close to the solubility limit in Yb 2 O 3 , which probably explains why it interferes with powder crystallization. The lattice parameter linearly increases (Figure 3b) with the La 2 O 3 concentration and corresponds to Vegard's law, but due to the high dispersity of the powders, errors in determining this value are significant. Therefore, it is difficult to affirm substitutional solid solutions of ytterbium and lanthanum oxide, which are formed in the powders. However, in Ho 2 O 3 and Er 2 O 3 powders, obtained under similar conditions, a significantly higher solubility of La 2 O 3 was found in comparison with the literature data. This was explained by both the nanodispersed disordered structure of the powders and kinetic difficulties in formation of the second phase, since segregation of lanthanum ions does not have time to occur during the SHS process. The morphology of SHS powders is caused by the action of gaseous substances that disperse the batch and lead to production of crumbly white Yb2O3. The corresponding equation of chemical reactions can be written as follows: As revealed from XRD analysis (Figure 3a), the as-synthesized powders are a single-phase cubic Yb2O3 S.G.: Ia -3 (206) (PDF # 00-041-1106); impurities of other phases were not detected. The XRD crystallite size of the powders decreases with increasing the lanthanum oxide content. The concentration of La2O3 is close to the solubility limit in Yb2O3, which probably explains why it interferes with powder crystallization. The lattice parameter linearly increases (Figure 3b) with the La2O3 concentration and corresponds to Vegard's law, but due to the high dispersity of the powders, errors in determining this value are significant. Therefore, it is difficult to affirm substitutional solid solutions of ytterbium and lanthanum oxide, which are formed in the powders. However, in Ho2O3 and Er2O3 powders, obtained under similar conditions, a significantly higher solubility of La2O3 was found in comparison with the literature data. This was explained by both the nanodispersed disordered structure of the powders and kinetic difficulties in formation of the second phase, since segregation of lanthanum ions does not have time to occur during the SHS process.

Sintering and Microstructure of (YbxLa1-x)2O3 Ceramics
After vacuum sintering, partial reduction of Yb 3+ ions occurred, which manifested in dark color. After oxidative annealing in air at 900 °C for 5 h, the samples became almost colorless.
Lanthanum oxide notably affects sintering of the ceramics, which is reflected in the ceramic microstructure. Figure 4 shows micrographs of the surface of the (YbxLa1−x)2O3 ceramic samples. The Yb2O3 sample contains a number of pores at grain boundaries and at triple junctions. The density of individual Yb2O3 ceramics is about 97% of the theoretical value. A large number of pores is also observed in ceramics containing 1% of the La2O3. With an increase in the lanthanum oxide content, a decrease in the porosity of the samples (2-3% La2O3) is observed. Samples with a sintering aid content of 4-5% are of almost the theoretical density; no pores could be seen on the ceramic surfaces. Lanthanum oxide does not cause significant grain growth in comparison with individual ytterbium oxide, as was in the case of sintering of lutetium-yttrium-lanthanum oxides solid solutions [30]. Mostly intragranular fracture was observed in ceramics regardless of lanthanum oxide content. This indicates the absence of displacement of any impurities from the grain volume, which weakens the grain boundaries. As seen on fractograms, segregations and other inhomogeneities, except for pores, are not found in ceramics.
The appearance of the obtained ceramics based on Yb2O3 is shown in Figure 5. The 4% La:Yb2O3 ceramics show the highest level of transparency. When the sintering aid content is less than 4%, the light transmission of the ceramics is limited by scattering caused by the residual porosity, which

Sintering and Microstructure of (Yb x La 1−x ) 2 O 3 Ceramics
After vacuum sintering, partial reduction of Yb 3+ ions occurred, which manifested in dark color. After oxidative annealing in air at 900 • C for 5 h, the samples became almost colorless.
Lanthanum oxide notably affects sintering of the ceramics, which is reflected in the ceramic microstructure. Figure 4 shows micrographs of the surface of the (Yb x La 1−x ) 2 O 3 ceramic samples. The Yb 2 O 3 sample contains a number of pores at grain boundaries and at triple junctions. The density of individual Yb 2 O 3 ceramics is about 97% of the theoretical value. A large number of pores is also observed in ceramics containing 1% of the La 2 O 3 . With an increase in the lanthanum oxide content, a decrease in the porosity of the samples (2-3% La 2 O 3 ) is observed. Samples with a sintering aid content of 4-5% are of almost the theoretical density; no pores could be seen on the ceramic surfaces. Lanthanum oxide does not cause significant grain growth in comparison with individual ytterbium oxide, as was in the case of sintering of lutetium-yttrium-lanthanum oxides solid solutions [30]. Mostly intragranular fracture was observed in ceramics regardless of lanthanum oxide content. This indicates the absence of displacement of any impurities from the grain volume, which weakens the grain boundaries. As seen on fractograms, segregations and other inhomogeneities, except for pores, are not found in ceramics.
Magnetochemistry 2020, 6, 63 6 of 10 presumably, a perovskite LaYbO3 phase is formed in ceramics in addition to the cubic one, which causes scattering, especially in the region of short wavelengths.   The appearance of the obtained ceramics based on Yb 2 O 3 is shown in Figure 5. The 4% La:Yb 2 O 3 ceramics show the highest level of transparency. When the sintering aid content is less than 4%, the light transmission of the ceramics is limited by scattering caused by the residual porosity, which follows from the sample microstructure. With an increase in the La 2 O 3 content up to 5%, presumably, a perovskite LaYbO 3 phase is formed in ceramics in addition to the cubic one, which causes scattering, especially in the region of short wavelengths.   A sample with the highest transmission (4% La:Yb 2 O 3 ) was chosen to study the optical properties. Figure 6 shows the transmission spectrum of ceramics after vacuum sintering. In-line transmittance for the 4% La sample is maximum in the wavelength range of 5-6 µm and reaches 82.8%. The short-wavelength transmission edge of the La:Yb 2 O 3 ceramics is in the region of 270 nm, and the long-wavelength edge is at 8.7 µm. In the near-infrared range there are absorption bands characteristic of the Yb 3+ ion in the wavelength range of 0.9-1.03 µm.
Magnetochemistry 2020, 6, 63 7 of 10 A sample with the highest transmission (4% La:Yb2O3) was chosen to study the optical properties. Figure 6 shows the transmission spectrum of ceramics after vacuum sintering. In-line transmittance for the 4% La sample is maximum in the wavelength range of 5-6 μm and reaches 82.8%. The short-wavelength transmission edge of the La:Yb2O3 ceramics is in the region of 270 nm, and the long-wavelength edge is at 8.7 μm. In the near-infrared range there are absorption bands characteristic of the Yb 3+ ion in the wavelength range of 0.9-1.03 μm. The results of measuring the Verdet constant for 4% La:Yb2O3 ceramics are shown in Figure 7 in comparison with the widely used TGG material (see Figure 7a) and with some other known magneto-optical materials which are transparent in the spectral range beyond 1.4 μm (see Figure 7b). The dependences of the Verdet constant on the wavelength are obtained by approximating experimental data for a TGG single crystal [32], Dy2O3 ceramics [20] using the approximation formula from [33], Er2O3 ceramics [25], Ho2O3 ceramics [24,35], EuF2, NHF single crystals [36], and for some types of magneto-optical glasses: ternary phosphate glass [37] and TeO2 [38].
Dependence of the Verdet constant on the wavelength in the transparency region is well described by the equation V = A/(λ 2 − λ0 2 ), where A = 2 × 10 6 rad•nm 2 /(T•m), λ0 = 245 nm (blue dotted line), and this dependence is very different in the region of absorption bands (0.9-1.1 μm), where it is impossible to use this material as a magneto-optical one. The results of measuring the Verdet constant for 4% La:Yb 2 O 3 ceramics are shown in Figure 7 in comparison with the widely used TGG material (see Figure 7a) and with some other known magneto-optical materials which are transparent in the spectral range beyond 1.4 µm (see Figure 7b). The dependences of the Verdet constant on the wavelength are obtained by approximating experimental data for a TGG single crystal [32], Dy 2 O 3 ceramics [20] using the approximation formula from [33], Er 2 O 3 ceramics [25], Ho 2 O 3 ceramics [24,35], EuF 2 , NHF single crystals [36], and for some types of magneto-optical glasses: ternary phosphate glass [37] and TeO 2 [38]. NHF has a transparency window in the range of 1.25-1.69 µm and a lower value of the Verdet constant compared to Yb 2 O 3 . Thus, use of Yb 2 O 3 ceramics as a magneto-optical material is promising in the range of 1.5-1.9 µm due to the fact that the absorption bands are far from this range and the value of the Verdet constant is relatively high. Zinc selenide (ZnSe) has no absorption bands in this range and the value of the Verdet constant is~1.4 times higher; however, to objectively compare the prospects of these materials for use as Faraday isolators for laser radiation with high average power, it is necessary to compare the mechanical and thermo-optical properties of these materials, which are planned to be studied in the near future.

Conclusions
La:Yb 2 O 3 powders and optical ceramic samples were obtained for the first time by self-propagating high-temperature synthesis and vacuum sintering. It was shown that addition of 4 mol.% of La 2 O 3 enhances the sinterability of Yb 2 O 3 , improving optical transmission of the ceramics. The prepared ytterbium oxide ceramics are transparent in the 270-900 nm and 1.1-8.7 µm spectral ranges. The resulting 4% La:Yb 2 O 3 ceramics have an optical transmission 73% at a wavelength of 2 µm, limited by residual porosity and probably secondary phase enriched with La 2 O 3 . Due to the distant location of the absorption bands and the relatively high value of the Verdet constant, the use of Yb 2 O 3 ceramics as a magneto-optical media in the wavelength range of 1.5-1.9 µm is the most promising at the moment.