# Fabrication and luminescent properties of erbium doped Y2O3-MgO and Gd2O3-MgO nanocomposite ceramics

Stanislav Balabanov, Dmitry Permin, Simone Normani, Alexander Belyaev, Vitaliy Koshkin, Pavel Andreev, Igor Ladenkov, Alain Braud, Patrice Camy, Rosa Maria Solé, et al.

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## To cite this version:

Stanislav Balabanov, Dmitry Permin, Simone Normani, Alexander Belyaev, Vitaliy Koshkin, et al. "Fabrication and luminescent properties of erbium doped Y2O3-MgO and Gd2O3-MgO nanocomposite ceramics." Journal of the American Ceramic Society, 2025, 108 (4), pp.e20330. 10.1111/jace.20330 hal-05371857

HAL Id: hal-05371857

https://hal.science/hal-05371857v1

Submitted on 19 Nov 2025

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# Fabrication and luminescent properties of erbium doped Y₂O₃-MgO and Gd₂O₃-MgO nanocomposite ceramics

Stanislav Balabanovª, Dmitry Perminª,ᵇ,*, Simone Normaniᶜ, Alexander Belyaevª, Vitaliy Koshkinª,ᵇ, Pavel Andreevᵇ, Igor Ladenkovf, Alain Braudᶜ, Patrice Camyc, Rosa Maria Soléd, Xavier Mateosd, and Pavel Loikoc

**Abstract:** We report on the fabrication and optical properties of erbium-doped Y₂O₃-MgO and Gd₂O₃-MgO nanocomposite ceramics, with a particular focus on their mid-infrared emission. The ceramics were fabricated by a self-propagating high temperature synthesis followed by hot pressing. The transmittance of 1.7 mm-thick 5 at.% Er:Y₂O₃-MgO and 7 at.% Er:Gd₂O₃-MgO ceramics at 3 µm amounted to 79.4% and 21.5%, respectively. This difference in transmission, as revealed by microstructural analysis, is due to variation in the distribution of residual pore sizes, while the average grain size is almost the same for both ceramics, being ~200 nm. The composites exhibited luminescence in the visible, near-infrared, and mid-infrared spectral ranges attributed to electronic transitions of Er³⁺ ions in the cubic sesquioxide phase. However, some peculiarities in the Raman and luminescence spectra were identified as compared to single-phase Er:Y₂O₃ and Er:Gd₂O₃ ceramics. Together with XRD studies and SEM/EDX elemental distribution maps, this may indicate certain solubility of MgO in the sesquioxide phase of the composites.

**Keywords:** composites; optical ceramics; sesquioxides; erbium ions; luminescence; mid-infrared.

<footnote>ªG.G. Devyatykh Institute of Chemistry of High-Purity Substances of the RAS, 49 Tropinin St., 603137 Nizhny Novgorod, Russia</footnote>

<footnote>ᵇN.I. Lobachevsky National Research University, 23a Gagarin Ave., 603022 Nizhny Novgorod, Russia</footnote>

<footnote>ᶜCentre de Recherche sur Les Ions, Les Matériaux et La Photonique (CIMAP), UMR 6252 CEA-CNRS-ENSICAEN, Université de Caen, 6 Boulevard Maréchal Juin, 14050 Caen, France</footnote>

<footnote>ᵈUniversitat Rovira i Virgili (URV), Física i Cristal·lografià de Materials i Nanomaterials (FiCMA-FiCNA)-EMaS, Marcel·lí Domingo 1, 43007 Tarragona, Spain</footnote>

<footnote>ᵉJoint-stock company Research and Production Enterprise “Salut”, 7 Larin St., 603950 Nizhny Novgorod, Russia</footnote>

<footnote>*Corresponding author, e-mail: permin@ihps-nnov.ru</footnote>
# 1. Introduction

The development of ceramic technology has led to significant advances in the production of solid-state laser materials [1,2]. Until recently, the range of potential polycrystalline optical media was limited to single-phase materials with a cubic crystal lattice, since the anisotropy of other types of structures leads to passive optical losses due to light scattering and / or birefringence. Several approaches have been developed to fabricate non-cubic optical (including laser) ceramics by orienting the grains [3] or limiting their size during sintering [4]. This reduces passive optical losses in ceramics, especially in the near and mid-infrared spectral ranges. For example, the ceramics alumina (Al₂O₃) [4], tetragonal yttria-stabilized zirconia (Y-TZP) [5], Yb³⁺-doped fluorapatite (FAP) [6], Nd³⁺- [7] or Er³⁺-doped [8] strontium fluorapatite (S-FAP), strontium fluoroarsenate (SFAs) [9] have been fabricated in this way, and laser emission has been achieved on fluorapatite samples doped with ytterbium [6] and neodymium [10] ions.

Another way to expand the range of optical materials is to create ceramics with a composite structure. By limiting the grain size to the nanometer scale, it becomes possible to create highly transparent materials in the infrared. A successful example of such ceramics are the composites made of cubic rare-earth sesquioxides and magnesium oxide. Y₂O₃-MgO (yttria-magnesia) has been extensively researched by U.S. defense contractors due to its potential as a protective infrared window material with improved mechanical strength and thermal shock resistance [11]. Other composites, such as Gd₂O₃-MgO (gadolinia - magnesia) [12], Dy₂O₃-MgO (dysprosia - magnesia) [13], Sc₂O₃-MgO (scandia - magnesia), and Lu₂O₃-MgO (lutetia - magnesia) [14], have also been developed.

The luminescent properties of such composite materials doped with optically active rare earth ions (RE³⁺) have recently gained significant attention. In particular, Safronova et al. [15] and later Wang et al. [16] demonstrated intense luminescence at ~1.93 and 2.12 µm from Ho:Y₂O₃-MgO ceramics. Blair et al. [17] and Ma et al. [18] performed optical spectroscopy measurements of the ⁴I₁₃/₂ → ⁴I₁₅/₂ transition (at ~1.55 µm) of Er³⁺ ions in Y₂O₃-MgO ceramics. Due to the fact that RE³⁺ ions are practically insoluble in MgO, the absorption and luminescence spectra of RE:Y₂O₃-MgO nanocomposites greatly resemble those of RE:Y₂O₃ ceramics.

In addition to the ⁴I₁₃/₂ → ⁴I₁₅/₂ transition at ~1.55 µm, Er³⁺ ions exhibit another laser transition of interest, namely ⁴I₁₁/₂ → ⁴I₁₃/₂, falling into the mid-infrared spectral range at ~2.80 µm [19]. The number of efficient laser materials in this range is rather limited, so the search for new ones is of great importance. Er³⁺-doped nanocomposites Y₂O₃-MgO and Gd₂O₃-MgO can be of particular interest for mid-infrared emission, since the light scattering in these materials decreases significantly with increasing the wavelength. Both yttria and gadolinia are attractive matrices for doping with Er³⁺ ions for several reasons, namely (i) they possess one of the lowest phonon energies among oxide materials, which reduces the rates of multiphonon non-radiative relaxation from the ⁴I₁₁/₂ excited-state; (ii) they allow for high Er³⁺ doping levels, which is particularly important for increasing the efficiency of energy-transfer upconversion from the metastable intermediate ⁴I₁₃/₂ state, avoiding the bottleneck effect for the self-terminating ⁴I₁₁/₂ → ⁴I₁₃/₂ transition; (iii) they have good thermal and mechanical properties and they are chemical inert.

The higher thermal conductivity of nanocomposites as compared to single-phase sesquioxide ceramics is considered as the main advantage of such materials and is relevant for potential laser applications. Indeed, at room temperature, the thermal conductivity of Er:Y₂O₃-MgO ceramics is two times higher than that of Er:Y₂O₃ [18]. However, the situation is likely to be reversed when the active element is cooled [20,21]. Another important factor is the weak dependence of thermal conductivity of the composites on the doping level which is
a commonly observed detrimental effect in single-phase cubic sesquioxide ceramics. This is owing to the fact that in the composite ceramics, the MgO grains are primarily responsible for this property.

The fabrication of bulk optical material Er:Gd₂O₃, in the form of both single-crystals and ceramics, is hindered by the low phase-transition temperature of ~1200 °C. In the case of the Gd₂O₃-MgO composite, however, the phase transition in Gd₂O₃ occurs without breaking the integrity of the sample. The annealing conditions can control the ratio of monoclinic and cubic phases in the composite. This is another way to expand the range of optical materials doped with Er³⁺ ions. From the technological standpoint, creating high-quality nano-sized composite ceramic may prove to be a more feasible alternative to single-phase sesquioxide ceramic materials.

Composites containing Y₂O₃ and Gd₂O₃ as host matrices for luminescent rare-earth ions offer advantages due to their relatively low phonon energies (among other sesquioxides), a property which is particularly demanded for mid-infrared emissions. MgO is a highly effective grain growth inhibitor that, unlike other commonly used sintering additives, is not expected to adversely affect the properties of the ceramic. For example, ZrO₂ is susceptible to photodarkening, La₂O₃ reduces thermal conductivity, and LiF promotes the grain growth [23]. Therefore, the RE₂O₃-MgO composites have the potential to enhance the material performance and improve the production process.

The aim of the present work was to fabricate Er:Y₂O₃-MgO and Er:Gd₂O₃-MgO composite ceramics and to study their optical properties, in particular focusing on their mid-infrared emission properties around ~2.8 µm.

## 2. Synthesis of ceramics

The starting materials used in this work were yttria Y₂O₃ (99.99% Polirit, Russia), gadolinia Gd₂O₃ (99.99% Polirit, Russia), erbia Er₂O₃ (99.99% Polirit, Russia), magnesia MgO (99.99% Unikhim, Russia), nitric acid HNO₃ (99.9999% Khimreaktiv, Russia), and aminoacetic acid NH₂CH₂COOH (99.9% Khimreaktiv, Russia).

Er:Y₂O₃-MgO and Er:Gd₂O₃-MgO starting powders were prepared by a self-propagating high-temperature synthesis (SHS) process described in detail previously [24,25]. In short, yttrium, gadolinium, erbium, and magnesium nitrates were prepared each by dissolving the corresponding oxide in nitric acid. The optimum volume ratio of yttria (gadolinia) to magnesia in infrared-transparent ceramics is 50:50 [11,26]. Er:Y₂O₃-MgO and Er:Gd₂O₃-MgO ceramics were obtained with the same ratio, assuming that erbia (Er₂O₃) dissolves only in yttria or gadolinia. The densities of Er:Y₂O₃ and Er:Gd₂O₃ solid solutions were calculated by the additive method. The nitrates of rare earths (RE) and magnesium were mixed in a given ratio, and then aminoacetic acid was added to the resulting solution according to the stoichiometry of the following reactions:

$$6\mathrm{RE}(\mathrm{NO}_3)_3 + 10\mathrm{NH}_2\mathrm{CH}_2\mathrm{COOH} \rightarrow 3\mathrm{RE}_2\mathrm{O}_3 + 20\mathrm{CO}_2 + 25\mathrm{H}_2\mathrm{O} + 14\mathrm{N}_2,$$

$$9\mathrm{Mg}(\mathrm{NO}_3)_2 + 10\mathrm{NH}_2\mathrm{CH}_2\mathrm{COOH} \rightarrow 9\mathrm{MgO} + 20\mathrm{CO}_2 + 25\mathrm{H}_2\mathrm{O} + 14\mathrm{N}_2.$$

The solutions were evaporated at 110 °C. Next, to initiate SHS, the quartz flask containing the precursors was placed in a furnace preheated to 500 °C. The combustion process resulted in the formation of a soft foam consisting of RE₂O₃-MgO nanopowders. After synthesis, the powders were annealed in air in a muffle furnace at a temperature of 800 °C for 5 hours (h).

The annealed powders were compacted in a 15 mm diameter stainless steel mold at 40 MPa. The green bodies obtained in this way were consolidated by hot pressing (HP) under
≤10 Pa vacuum in a graphite mold at temperatures of 1450°C (for Er:Y₂O₃-MgO) and 1400 °C (for Er:Gd₂O₃-MgO) and a uniaxial pressure of 50 MPa, using home-made equipment. The green bodies were isolated with graphite paper to reduce the interaction between the mold material and the ceramic samples. The heating schedule included an isothermal hold at 800 °C for 20 min to desorb moisture and carbon dioxide, heating to sintering temperature at a rate of 20 °C/min, holding for 60 min, and free cooling. The initial uniaxial pressure applied to the green bodies was 3 MPa. After the temperature reached 900 °C, the uniaxial pressure was increased to its maximum at a rate of 1 MPa/min. The samples were further annealed in air at 1100 °C for 5 h in a muffle furnace to remove oxygen vacancies and carbon impurities. The thickness of the polished ceramic samples was 1.7 mm. Ceramics with the compositions of 1 at.%, 3 at.%, 5 at.%, and 7 at.% Er:Y₂O₃-MgO and 7 at.% Er:Gd₂O₃-MgO were fabricated. Here, the percentages represent the number of yttrium/gadolinium atoms replaced by erbium.

### 3. Characterization

The X-ray diffraction (XRD) measurements for the powders and ceramics were performed using an XRD-7000 diffractometer (Shimadzu, Kyotocity, Japan), CuKα radiation, 30 kV/40 mA. The scans were performed in the Bragg-Brentano geometry in the range of diffraction angles 2θ of 20°–80° with the step of 0.04° and an exposure time of 1-3 s. The qualitative phase analysis was carried out using the PDF-2 database (ICDD, 2012). The cell parameters refinement was carried out by the Rietveld method using the Diffrac.TOPAS software (Bruker, Karlsruhecity, Germany) with the following cif-files from the ICSD database (2016): MgO (# 64930), Y₂O₃ (# 82420), cubic-Gd₂O₃ (# 41270), and monoclinic-Gd₂O₃ (# 636103).

The specific surface areas ($S_{BET}$) of the SHS powders were measured by nitrogen adsorption according to the Brunauer-Emmett-Teller (BET) method using Meta Sorbi-MS equipment. The average BET-equivalent particle diameter ($d_{BET}$, in nm) was calculated assuming their spherical shape according to the following equation: $d_{BET} = 6000/(ρ×S_{BET})$, where ρ is the theoretical density of the material calculated by the additive method. The densities of the sintered samples were determined by hydrostatic weighing in distilled water on a Sartorius CPA balance (Sartorius, Germany) with an accuracy of 0.005 g/cm³, which is ~0.1% of the theoretical density of the composites. The microstructure of the composites was examined on an Auriga CrossBeam scanning electron microscope (SEM) (Carl Zeiss, Germany) operated at an accelerating voltage of 3 keV using a secondary electron detector. The average grain size was determined by measuring the width of a minimum of 200 grains. No shape factor was applied. The Energy Dispersive X-ray (EDX) based element mapping was studied with a FEI Scios 2 Field Emission Scanning Electron Microscope with a focused Gallium ion beam (FESEM-FIB).

The infrared spectra of the RE₂O₃-MgO composite ceramics were recorded on an FT-801 Fourier transform IR spectrometer (Simex, Russia). The Raman spectra were measured using a confocal Raman microscope (inVia, Renishaw) equipped with a ×50 objective (Leica) and an Ar⁺ ion laser (457.0 nm).

The spectra of optical absorption in the near-IR were measured using a Perkin-Elmer LAMBDA 1050 spectrophotometer with a resolution of 1 nm.

The luminescence spectra in the mid-infrared were measured using a home-made set-up. It included a Ti:Sapphire laser (Mira, Coherent) tuned to ~0.96 µm, a large mode area ZrF₄ fiber for light collection and an optical spectrum analyzer (OSA, Yokogawa AQ6376) purged with N₂ gas to eliminate the structured water vapor absorption in the air. The effect of the
residual water absorption was further removed by calibrating the OSA spectral response with a 20 W quartz lamp.

The luminescence decay was studied under excitation by a ns optical parametric oscillator (Horizon, Continuum), and the decay curves were measured employing a 1/4 m monochromator (Oriel 77200), an InGaAs detector with a preamplifier and an 8 GHz oscilloscope (DSA70804B, Tektronix).

## 4. Results and discussion

### 4.1. Er:Y₂O₃-MgO and Er:Gd₂O₃-MgO powders

The SHS powders were calcined in air at 800 °C for 5 h immediately prior to hot pressing. This mode was previously developed for undoped Y₂O₃-MgO powders [24] and allows the removal of residual carbon-containing substances as much as possible while maintaining nm-sized particles within the powders and, consequently, preserving their high sinterability. The specific surface areas $S_{BET}$ of the powders after calcination were 27 and 25 m²/g for Er:Y₂O₃-MgO and Er:Gd₂O₃-MgO, respectively. This is not much different from the values for undoped powders, amounting to 34 and 23 m²/g, respectively, and is probably due to variations in the SHS synthesis conditions rather than the effect of erbium oxide. The corresponding average BET-equivalent particle diameters $d_{BET}$ were 50 nm for Er:Y₂O₃-MgO and 42 nm for Er:Gd₂O₃-MgO powders.

The XRD patterns for Er:Y₂O₃-MgO, Er:Gd₂O₃-MgO and undoped powders are shown in Fig. 1, and the unit-cell parameters are summarized in Table 1. Only cubic phases are observed in the XRD patterns: cubic RE₂O₃ with a space group *I*2̄3 adopting the body-centered bixbyte (C-type) structure and cubic MgO with a space group *Fm*-3m with a NaCl-type structure. The proximity between the ionic radii of Er³⁺ (0.89 Å) dopant ions and those of Y³⁺ (0.90 Å) and Gd³⁺ (0.938 Å) host-forming cations (all values correspond to VI-fold coordination by oxygen), causes very small changes in the unit-cell parameters as a result of Er³⁺ doping. Although the cell size of doped sesquioxides slightly decreased as expected, these changes are within the measurement error interval. Due to the nanoscale nature of the powders, the diffraction peaks are low in intensity and significantly broadened.

<figure><img src="image_3.png" /><figcaption>**Figure 1.** XRD patterns of Er:Y₂O₃-MgO and Er:Gd₂O₃-MgO nanopowders obtained by self-propagating high-temperature synthesis. *Vertical dashes* - standard reflections for Y₂O₃ (PDF # 00-041-1105), Gd₂O₃ (PDF # 00-012-0797) and MgO (PDF # 01-071-1176).</figcaption></figure>

<table><caption>**Table 1.** XRD analysis of Er:Y₂O₃-MgO and Er:Gd₂O₃-MgO nanopowders.</caption><thead><tr><th rowspan="2">Powder</th><th colspan="3">Unit-cell parameter a, Å</th></tr><tr><th>Y₂O₃</th><th>MgO</th><th>Gd₂O₃</th></tr></thead><tbody><tr><td>Y₂O₃-MgO</td><td>10.612(21)</td><td>4.221(9)</td><td>-</td></tr><tr><td>Er:Y₂O₃-MgO</td><td>10.597(3)</td><td>4.215(1)</td><td>-</td></tr></tbody></table>
<table><tr><td>Gd<sub>2</sub>O<sub>3</sub>-MgO</td><td>4.217(16)</td><td>10.81(4)</td></tr><tr><td>Er:Gd<sub>2</sub>O<sub>3</sub>-MgO</td><td>4.219(5)</td><td>10.797(12)</td></tr></table>

## 4.2. Microstructure of Er:Y<sub>2</sub>O<sub>3</sub>-MgO and Er:Gd<sub>2</sub>O<sub>3</sub>-MgO ceramics

The XRD powder analysis of the Er:Y<sub>2</sub>O<sub>3</sub>-MgO and Er:Gd<sub>2</sub>O<sub>3</sub>-MgO ceramics is shown in Fig. 2. There are no changes in the phase composition of the Er:Y<sub>2</sub>O<sub>3</sub>-MgO ceramics compared to the starting powders. It remains biphasic, based on cubic Y<sub>2</sub>O<sub>3</sub> and cubic MgO. According to Vegard's rule, the unit-cell parameter for a solid-solution of 7 mol.% Er<sub>2</sub>O<sub>3</sub> in Y<sub>2</sub>O<sub>3</sub> should be a = 10.606 Å, while our measured value is a = 10.579(2) Å; for magnesia, the standard value is a = 4.217 Å, and that measured for ceramics is a = 4.215(2) Å. A noticeable distortion of the Er:Y<sub>2</sub>O<sub>3</sub> unit cell can be caused by partial dissolution of magnesia in yttria, which is also supported by the SEM/EDX element distribution maps discussed below.

The ceramic Er:Gd<sub>2</sub>O<sub>3</sub>-MgO is also biphasic, but if MgO remains cubic, then Er:Gd<sub>2</sub>O<sub>3</sub> is completely (within the error of the XRD analysis) in the monoclinic phase with a space group of C2/m with Sm<sub>2</sub>O<sub>3</sub>-type (B-type) structure. Table 2 lists the unit-cell parameters of this phase in comparison with the standard ones for monoclinic erbia and gadolinia. A stronger decrease of the cell volume than that expected for 7 mol.% Er<sub>2</sub>O<sub>3</sub> in Gd<sub>2</sub>O<sub>3</sub> solid solution can be explained, as in the case of Er:Y<sub>2</sub>O<sub>3</sub>-MgO, by the partial dissolution of magnesia in gadolinia. The unit-cell parameter of magnesia is a = 4.213(2) Å, which is only slightly different from the standard one.

<figure><img src="image_4.png" /><figcaption>**Figure 2.** X-ray powder diffraction patterns of (a) Er:Y<sub>2</sub>O<sub>3</sub>-MgO and (b) Er:Gd<sub>2</sub>O<sub>3</sub>-MgO ceramics. Vertical dashes - standard reflections for cubic Y<sub>2</sub>O<sub>3</sub> (PDF # 00-041-1105), monoclinic Gd<sub>2</sub>O<sub>3</sub> (PDF # 01-074-8036) and MgO (PDF # 01-071-1176).</figcaption></figure>

Figure 3 shows the SEM/EDX element distribution maps and EDX spectra measured for the Er:Y<sub>2</sub>O<sub>3</sub>-MgO and Er:Gd<sub>2</sub>O<sub>3</sub>-MgO ceramics. Although the small size of the grains makes the picture of the element distribution blurred, there are some regularities that agree with the XRD analysis of the ceramics, as well as with literature data. For example, rare-earth atoms are predominantly found in localized regions (separate grains), while magnesium ones are
more homogeneously distributed. This may indicate some solubility of magnesium in sesquioxide phases and the fact that the composites consist of phases of (i) MgO and (ii) solid solutions of MgO and Er₂O₃ in Y₂O₃ (or Gd₂O₃). We suggest that the magnesium content in the solid solutions is rather small. It is known that the maximum solubility of ~5 mol.% MgO in Er₂O₃, ~7 mol.% in Y₂O₃ and ~2.5% in Gd₂O₃ is reached at high temperatures around ~2100 °C [27,28] and rapidly decreases with decreasing temperature.

<figure><img src="image_5.png" /><figcaption>**Figure 3.** SEM/EDX elemental distribution maps in (a-c) Er:Y₂O₃-MgO and (d-f) Er:Gd₂O₃-MgO ceramics: (a,d) Mg K; (b,e) Gd L and (c,f) Er L. EDX spectra of (g) Er:Y₂O₃-MgO and (h) Er:Gd₂O₃-MgO ceramics.</figcaption></figure>

Figure 4 shows the typical SEM images of the fractured surfaces of the Er:Y₂O₃-MgO and Er:Gd₂O₃-MgO ceramics. The pores in the Er:Y₂O₃-MgO ceramic, Fig. 4(a), are homogeneously distributed, and there are almost no pores larger than the grains. This conclusion is based on the analysis of multiple images, as only one pore is usually visible in a given image. In the Er:Gd₂O₃-MgO ceramic, Fig. 4(b), despite similar total porosity, the pores are homogeneously distributed, but there are voids as large as several grains. As will be shown below, this results in increased light scattering, especially at short wavelengths. It can be assumed that the composites have a density close to 100%. However, the SEM analysis typically allows for investigation of only a small area of the material and may not be the most suitable method for accurately determining the integral density.

In the fracture structure of the Er:Y₂O₃-MgO ceramic, as in undoped samples [24], intragranular fracture is observed on the Er:Y₂O₃ grains. However, a rather large number of intragranular fractures on MgO grains is atypical. Usually, the strength of MgO grains is much higher than that of the rare-earth sesquioxides, so they remain intact. The average grain size is ~206 nm, and the grain size distribution is narrow for both phases.
The fracture of the Er:Gd₂O₃-MgO ceramic is also slightly different from that of the undoped samples [25]. As expected, intergranular fractures of magnesia grains are observed, but the destruction of the Er:Gd₂O₃ solid solution occurs through the volume of the grain. This is probably due to stresses in the monoclinic Er:Gd₂O₃ solid solution, as shown by the XRD analysis. Although the hot-pressing temperature was 50 °C lower, the average grain size of ~199 nm is almost the same as that of the yttria-based composite.

<figure><img src="image_6.png" /><figcaption>**Figure 4.** SEM images of the fracture surfaces of hot-pressed (a,c) Er:Y₂O₃-MgO and (b,d) Er:Gd₂O₃-MgO ceramics.</figcaption></figure>

### 4.3. Spectroscopy of Er:Y₂O₃-MgO and Er:Gd₂O₃-MgO ceramics

Figure 5 shows the IR transmission spectra of Er:Y₂O₃-MgO and Er:Gd₂O₃-MgO ceramics. The addition of erbia did not lead to any peculiarities in the transmission spectrum compared to the undoped composites, except for the absorption band of Er³⁺ ions at ~1.5 µm associated with the ⁴I₁₅/₂ → ⁴I₁₁/₂ transition. This band is not visible for the Er:Gd₂O₃-MgO ceramic due to higher light scattering in this spectral range. The absorption lines observed at 4.94 µm are associated with carbon (II) oxide trapped in the pores, and those in the region of 7 µm - with magnesium and/or rare-earth metals carbonates [29]. The noticeably stronger light scattering in the Er:Gd₂O₃-MgO ceramic, as demonstrated above, is due to a wider pore-size distribution compared to the Er:Y₂O₃-MgO composites. The transparency range of both composites extends up to 9 µm. Around 3 µm, the transmittance of the 5 at.% and 7 at.% Er:Y₂O₃-MgO ceramics reaches 79.4% and 78.0%, respectively.

For laser applications of Er:RE₂O₃-MgO composites, it is essential to ensure high transparency at the envisioned laser and pump wavelengths. Mid-infrared Er lasers operating
on the $^4I_{11/2} \rightarrow ^4I_{13/2}$ transition can be pumped at 0.98 µm (to the upper laser level, $^4I_{11/2}$) or at 1.5 µm (to the terminal laser manifold, $^4I_{13/2}$, employing an efficient energy-transfer upconversion process, $^4I_{13/2} + ^4I_{13/2} \rightarrow ^4I_{15/2} + ^4I_{9/2}$). For the developed materials, the former pumping scheme looks more promising as it is expected to correspond to much lower light scattering for the pump radiation. Still, the level of scattering losses around 1.5 µm has to be lowered by optimizing the synthesis conditions. To achieve acceptable losses, it is necessary to limit the grain sizes to approximately 150 nm, considering the difference in the refractive index between the cubic sesquioxide and magnesia phases, which is about $\Delta n = 0.18$ in the spectral range from 0.98 to 3 µm [30,31]. This limit for the grain size seems feasible.

Previous studies revealed the possibility to attain relatively high transparency (~70%) in the short-wave infrared for undoped 1 mm-thick Y₂O₃-MgO [32] and Gd₂O₃-MgO [33] composites. Comparable levels of transmittance have been achieved for Er:Y₂O₃-MgO ceramics with varying levels of doping [18] (without specifying the thickness of the ceramic samples). This omission is significant because in materials prone to strong light scattering, even small increases in sample thickness can significantly reduce the transmittance. Given the absorption characteristics of Er³⁺ ions, it is plausible that the ceramics were much thinner than 1 mm. For potential laser applications, the thickness of the active element is crucial for reaching sufficiently high pump absorption. Efficient lasing at 2.8 µm has been achieved in 1.2 mm-thick sesquioxide ceramics doped with 7 at.% Er³⁺ [34]. In the case of Er:RE₂O₃-MgO composites, where the cubic sesquioxide phase comprises only half of the volume, it is reasonable to infer that the thickness of the ceramic elements should be at least comparable, if not greater, to target similar laser performances.

Increasing the proportion of Er:RE₂O₃ in the composite can reduce the required thickness of the active material. An equal volume ratio of RE₂O₃ and MgO is used in the present work to minimize the grain growth due to the mutual pinning effect [32]. However, since the diffusion rates of RE³⁺ and Mg²⁺ ions are different, increasing the proportion of the sesquioxide phase in the composite results in less grain growth than in the case of adding magnesia. For example, in the Y₂O₃-MgO composite, reducing the magnesia fraction to 30% resulted in a slight increase in the average grain size from 173 to 185 nm, which had minimal impact on the transmission spectrum [26]. While this approach seems viable, it is crucial to consider at least three factors when implementing it in practice. A positive factor is the reduced required sample thickness, while two negative factors are decreased thermal conductivity and increased light scattering at short wavelengths.

It can be speculated whether Er³⁺ ions are an interfering factor in achieving transparency of composites in the short wavelength spectral range. On the one hand, the presence of Er³⁺ ions contributes to a certain degree of lattice disorder due to the formation of a sesquioxide solid solution, (Y₁-xErx)₂O₃ or (Gd₁-xErx)₂O₃. This can enhance diffusion during sintering which is usually beneficial for pore healing. At the same time, diffusion of Mg²⁺ ions may also be enhanced, resulting in more pronounced grain growth. This increases Fresnel reflection losses at phase boundaries, potentially outweighing the benefits of reduced porosity. For example, when comparing the 1% Er:Sc₂O₃-MgO composite sintered at a temperature 100 °C lower than its counterpart, 1% Er:Lu₂O₃-MgO, it was found that the former material exhibited both a larger average grain size and reduced transmittance throughout the transparency window [14]. This difference is likely due to the greater solubility of MgO in Sc₂O₃ than in Lu₂O₃.

To achieve high optical quality of composites, it is essential to control not only the average grain size and porosity but also larger defects such as pore clusters, local composition
variations, and inclusions. These defects, even at low concentrations, can significantly
contribute to light scattering as their sizes approach the wavelengths of interest. The
emergence of these defects can be caused by various factors at different stages of the ceramic
technology. While optical microscopy is an effective method for analyzing such defects in
single-phase sesquioxide or magnesia ceramics, it is not suitable for composites. To overcome
this challenge, alternative methods for defect detection and monitoring in composites must
be explored.

<figure><img src="image_7.png" /><figcaption>Figure 5. Infrared transmission spectra of Er:Y₂O₃-MgO and Er:Gd₂O₃-MgO composite ceramics.</figcaption></figure>

The Raman spectra of the composites doped with 7 at.% Er<sup>3+</sup> are reported in Fig. 6, each in direct comparison with that of its single-phase 7 at.% Er:RE<sub>2</sub>O<sub>3</sub> ceramic counterpart, showing a maximum phonon energy of 573 cm<sup>-1</sup> for Er:Gd<sub>2</sub>O<sub>3</sub>-MgO and 595 cm<sup>-1</sup> in the case of Er:Y<sub>2</sub>O<sub>3</sub>-MgO. For the body-centered cubic (C-type) RE<sub>2</sub>O<sub>3</sub> structure, the factor group analysis predicts the irreducible representations for the optical (Γ<sub>op</sub>) modes at the Γ-point (k = 0) as follows: Γ<sub>op</sub> = 4A<sub>g</sub> + 4E<sub>g</sub> + 14F<sub>g</sub> + 5A<sub>2u</sub> + 5E<sub>u</sub> + 16F<sub>u</sub>. Out of these, 22 modes (A<sub>g</sub>, E<sub>g</sub> and F<sub>g</sub>) are Raman-active [35,36]. The measured Raman spectra clearly identify the presence of the cubic sesquioxide phase [37]. For the Er:Y<sub>2</sub>O<sub>3</sub>-MgO and Er:Gd<sub>2</sub>O<sub>3</sub>-MgO ceramics, a total of 12 and 16 modes are observed, respectively, and the most intense modes (at 378 cm<sup>-1</sup> and 363 cm<sup>-1</sup>, respectively) are assigned to A<sub>g</sub> + F<sub>g</sub> vibrations. The low-phonon-energy behavior of the Er:Gd<sub>2</sub>O<sub>3</sub>-MgO composite is favorable for reducing the multiphonon non-radiative relaxation from Er<sup>3+</sup> excited-states responsible for near- and mid-infrared luminescence [38].

Magnesium oxide has a weak Raman response with a strong effect of the crystal size on the intensity of lattice vibrations. Microcrystals of MgO, if Raman-active, are expected to give rise to two peaks, one at ~280 cm⁻¹, and another at ~446 cm⁻¹ [39]. The comparison of the measured Raman spectra of the Er:RE₂O₃-MgO composites with those for single-phase ceramics of Gd₂O₃ and Y₂O₃, Fig. 6(a,b), reveals no appearance of any prominent additional peaks for the composite materials with respect to those of magnesia-free ceramics. This suggests that the Raman contributions of the MgO fraction are probably silent. However, the Raman spectra of the cubic sesquioxide phases within the composites are slightly different from those for the single-phase ceramics. For both Er:RE₂O₃-MgO composites, the Raman peaks are shifted to higher energies and broadened. For example, for Er:Y₂O₃-MgO, the dominant Raman mode appears at 378.1 cm⁻¹ (bandwidth: 9.3 cm⁻¹) while for Er:Y₂O₃, it is found at 376.2 cm⁻¹ and it is more narrow (bandwidth: 7.4 cm⁻¹). This can be explained by the suggested dissolution of Mg within the sesquioxide grains. Also in Fig. 6(a,b), variations in the
intensities of the Raman peaks of composites and single-phase ceramics are visible. These variations may be caused by several factors, such as differences in the average grain size or surface roughness. Therefore, comparing them alone does not provide conclusive information about differences in the studied materials.

<figure><img src="image_8.png" /><figcaption>**Figure 6.** Raman spectra of 7 at.% Er:RE<sub>2</sub>O<sub>3</sub>-MgO composites, each compared with the respective single-phase 7 at.% Er:RE<sub>2</sub>O<sub>3</sub> sesquioxide ceramics: (a) RE = Gd, and (b) RE = Y. *Numbers* indicate the Raman peak frequencies in cm<sup>-1</sup>, λ<sub>exc</sub> = 457 nm.</figcaption></figure>

Owing to the high enough transparency of the Er:R<sub>2</sub>O<sub>3</sub>-MgO composite in the near-infrared spectral range, we were able to clearly resolve the absorption bands of Er<sup>3+</sup> ions related to transitions from the ground state to both the <sup>4</sup>I<sub>13/2</sub> and the <sup>4</sup>I<sub>11/2</sub> excited manifolds. Figure 7 shows the absorption spectrum of Er<sup>3+</sup> ions in Y<sub>2</sub>O<sub>3</sub>-MgO, expressed in absorption coefficient, α<sub>abs</sub>. For the <sup>4</sup>I<sub>15/2</sub> → <sup>4</sup>I<sub>11/2</sub> transition, the maximum absorption coefficient amounts to 1.36 cm<sup>-1</sup> at 971 nm and for the <sup>4</sup>I<sub>15/2</sub> → <sup>4</sup>I<sub>13/2</sub> one, the absorption is much stronger, expressed by α<sub>abs</sub> = 8.45 cm<sup>-1</sup> at 1536 nm.

<figure><img src="image_9.png" /><figcaption>**Figure 7.** Er<sup>3+</sup> absorption bands (the <sup>4</sup>I<sub>15/2</sub> → <sup>4</sup>I<sub>11/2</sub> and <sup>4</sup>I<sub>15/2</sub> → <sup>4</sup>I<sub>13/2</sub> transitions) in the 7 at.% Er:Y<sub>2</sub>O<sub>3</sub>-MgO composite.</figcaption></figure>

The normalized luminescence spectra of Er<sup>3+</sup> ions in both the Gd<sub>2</sub>O<sub>3</sub>-MgO and Y<sub>2</sub>O<sub>3</sub>-MgO composites are presented in Fig. 8. In the visible, Fig. 8(a), the two prominent emissions are in the green at ~550 nm, and in the red at ~670 nm. They are assigned respectively to the radiative transitions to the ground state from the thermally coupled <sup>2</sup>S<sub>3/2</sub> + <sup>2</sup>H<sub>11/2</sub> multiplets and from the <sup>4</sup>F<sub>9/2</sub> manifold. The spectra of visible luminescence of the two composites are rather similar but exhibit a blue-shift for gadolinia-base composite which is ascribed to the different crystal-field strengths for cubic yttria and gadolinia. The luminescence spectra in the
near-IR, covering the ~0.98 µm emission from the $^4I_{11/2}$ manifold and the ~1.5 µm emission from the $^4I_{13/2}$ one, Fig. 8(b), show a similar behavior.

<figure><img src="image_10.png" /><figcaption>**Figure 8.** Luminescence spectra of Er$^{3+}$ ions in 7 at.% Er:RE$_2$O$_3$-MgO composites: (a) visible emissions, $\lambda_{exc}$ = 457 nm; (b) near-infrared emissions, $\lambda_{exc}$ = 804 nm; (c) mid-infrared emission, $\lambda_{exc}$ = 974 nm.</figcaption></figure>

The spectra of mid-infrared luminescence, Fig. 8(c), are broad, spanning from 2.58 to 3.04 µm; this luminescence is assigned to the $^4I_{11/2} \rightarrow ^4I_{13/2}$ Er$^{3+}$ transition[19]. The spectra contain several narrow intense peaks at 2710, 2717, 2726, and 2741 nm (for both composites), and a broad and less intense band at longer wavelengths, at 2837 nm (Er:Y$_2$O$_3$-MgO, bandwidth: 13 nm) or 2842 nm (Er:Gd$_2$O$_3$-MgO, bandwidth: 15 nm), which is the most usual set of wavelengths for achieving laser operation with Er$^{3+}$-doped cubic sesquioxides, given the reabsorption from the terminal laser level and the structured water vapor absorption in air within this spectral range.

Figure 9 shows the comparison between the mid-infrared emission spectra of erbium ions for the single-phase 7 at.% Er:Y$_2$O$_3$ ceramic and the 7 at.% Er:Y$_2$O$_3$-MgO composite. The spectral features of the two ceramics are visibly very similar, which suggests that the possible
contribution of magnesia to the change in the local environment of active ions, if any, is relatively weak. This, in turn, seems to confirm that all emitting erbium ions reside in the cubic sesquioxide phase.

<figure><img src="image_11.png" /><figcaption>**Figure 9.** Comparison of mid-infrared luminescence spectra of Er³⁺ ions in the 7 at.% Er³⁺-doped Y₂O₃ ceramic and its composite counterpart Y₂O₃-MgO, λexc = 974 nm.</figcaption></figure>

The luminescence dynamics from the $^4I_{11/2}$ and $^4I_{13/2}$ Er³⁺ manifolds (the upper and lower laser levels for the mid-infrared laser transition) in the fabricated composites were studied under resonant excitation. The ceramic samples were finely powdered to reduce the effect of radiation trapping. The luminescence decay curves for 7 at.% Er³⁺-doped RE₂O₃-MgO ceramics are reported in Fig. 10. One can immediately notice that, considering the same Er³⁺ doping level, while the mean luminescence lifetime of the $^4I_{11/2}$ level is consistently lower than that of the $^4I_{13/2}$ manifold, the two lifetimes are much closer in the case of Gd₂O₃-MgO. This is due to the weaker non-radiative path from the $^4I_{11/2}$ level in cubic gadolinia related to its lower-energy phonon spectrum. This makes Er:RE₂O₃-MgO composites appealing for laser emission in the mid-infrared.
<figure><img src="image_12.png" /><figcaption>**Figure 10.** Luminescence decay curves for the $^4I_{13/2}$ and $^4I_{11/2}$ Er$^{3+}$ states in the 7 at.% Er:RE$_2$O$_3$-MgO composites measured under resonant excitation, $<\tau_{lum}>$ - mean luminescence lifetime.</figcaption></figure>

Table 3 reports the average luminescence lifetimes for all the studied composites, showing how the lifetime ratio between the $^4I_{11/2}$ and $^4I_{13/2}$ Er$^{3+}$ manifolds in Er:Y$_2$O$_3$-MgO gradually increases with the erbium doping increasing from 1 at.% (lifetime ratio = 0.33) to 7 at.% (lifetime ratio = 0.54). For the 7 at.% Er:Y$_2$O$_3$-MgO composite, the mean luminescence lifetimes of the $^4I_{11/2}$ and $^4I_{13/2}$ Er$^{3+}$ states are 1.76 ms and 3.27 ms, respectively.

For a single-phase 7 at.% Er:Y$_2$O$_3$ ceramic studied for comparison, the following mean luminescence lifetimes were obtained: 2.74 ms ($^4I_{11/2}$) and 5.88 ms ($^4I_{13/2}$) yielding a lifetime ratio of 0.47. Thus, the composite yttria-magnesia ceramic exhibits a certain lifetime shortening as compared to the single-phase cubic sesquioxide one. This quenching is mostly assigned to defects present in the composite material and we believe that it can be reduced by improving the optical quality of the composite ceramics. Note that for the 7 at.% Er:Gd$_2$O$_3$ ceramic studied for comparison, the measured luminescence lifetimes were <1 ms for both considered states, indicating the prospects of the nanocomposite technology for creating gadolinia-based materials emitting in the mid-infrared.

<table><caption>**Table 3.** Luminescence lifetimes of the $^4I_{13/2}$ and $^4I_{11/2}$ Er$^{3+}$ states in the Er:RE$_2$O$_3$-MgO composites.</caption><thead><tr><th rowspan="2">Composite</th><th rowspan="2">Er<sup>3+</sup> doping,<br>at.%</th><th colspan="2">Luminescence lifetime, ms</th></tr><tr><th><sup>4</sup>I<sub>13/2</sub></th><th><sup>4</sup>I<sub>11/2</sub></th></tr></thead><tbody><tr><td rowspan="2">Gd<sub>2</sub>O<sub>3</sub>-MgO</td><td>7</td><td>2.74</td><td>2.18</td></tr><tr><td>1</td><td>7.23</td><td>2.36</td></tr><tr><td rowspan="3">Y<sub>2</sub>O<sub>3</sub>-MgO</td><td>3</td><td>6.02</td><td>2.34</td></tr><tr><td>5</td><td>4.85</td><td>2.12</td></tr><tr><td>7</td><td>3.27</td><td>1.76</td></tr></tbody></table>

**5. Conclusions**
This study has demonstrated the potential of Er:Y₂O₃-MgO and Er:Gd₂O₃-MgO nanocomposite ceramics as optical materials with appealing emission properties in the mid-infrared spectral range. The luminescence spectra of Er³⁺ ions in both composites show similarities in the visible, near-, and mid-IR emissions, with a notable blue-shift for the gadolinia-based ceramic. The broad spectrum of mid-IR luminescence spanning from ~2.6 to 3.0 µm is consistent with the ⁴I₁₁/₂ → ⁴I₁₃/₂ Er³⁺ transition in the cubic sesquioxide phase. Mid-IR Er lasers operating on this transition are preferably pumped at 1.5 µm to the terminal laser manifold, ⁴I₁₃/₂, employing an energy-transfer upconversion process, ⁴I₁₃/₂ + ⁴I₁₃/₂ → ⁴I₁₅/₂ + ⁴I₉/₂. The efficiency of this process increases with higher concentrations of Er³⁺ ions in the composite.

Raman peaks of the erbium-doped cubic sesquioxide phase in both composites are shifted to higher energies and broadened compared to single-phase Er:Y₂O₃ and Er:Gd₂O₃ ceramics. Together with XRD data and SEM/EDX elemental distribution maps, this may indicate some solubility of MgO in the sesquioxide phase of the composites. At the same time, the luminescence spectrum of Er³⁺ ions in the composites shows only minor changes compared to the spectra for single phase ceramics.

## Acknowledgements

The study was funded by the Russian Science Foundation (Research Project No. 22-73-10084), French Agence Nationale de la Recherche (ANR) (project SPLENDID2, ANR-19-CE08-0028), "RELANCE" Chair of Excellence project funded by the Normandy Region, Région Normandie, France (Contrat de plan État-Région (CPER)).
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