# Spectroscopy of low-phonon-energy Er:La2O2S oxysulfide

Marine Poitou, Simone Normani, Pavel Loiko, Alain Braud, Patrice Camy, Odile Merdrignac-Conanec

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Marine Poitou, Simone Normani, Pavel Loiko, Alain Braud, Patrice Camy, et al.. Spectroscopy of low-phonon-energy Er:La2O2S oxysulfide. EPJ Web of Conferences, 2025, 335, pp.08021. 10.1051/epj-conf/202533508021. hal-05494959

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Submitted on 5 Feb 2026

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# Spectroscopy of low-phonon-energy Er:La₂O₂S oxysulfide

Marine Poitou<fnref n="1" />, Simone Normani<fnref n="2" />,*, Pavel Loiko<fnref n="2" />, Alain Braud<fnref n="2" />, Patrice Camy<fnref n="2" />, and Odile Merdrignac-Conanec<fnref n="1" />

**Abstract.** Lanthanum oxysulfide powders doped with Erbium ions were fabricated by the combustion method followed by sulfurization at 1000 °C under H₂S + N₂ atmosphere. X-ray diffraction confirms their single-phase nature (sp. gr. *P*-3*m*1). Raman spectroscopy reveals a maximum phonon energy of 389 cm⁻<fnref n="1" />. The effect of Er³⁺ doping level (0.5 – 7 mol%) on the visible and mid-infrared emission properties of Er:La₂O₂S was studied.

## 1 Introduction

Mixed-anion materials like lanthanide oxysulfides attract attention for doping with trivalent rare-earth ions for phosphors with good chemical and thermal stability and high luminescence quantum yield. Lanthanum oxysulfide, La₂O₂S, also attracts attention due to its low-phonon energy behavior (hνph ~ 389 cm⁻<fnref n="1" />) unlocking mid-infrared emission. Recently, Le Coz et al. reported on the first infrared transparent La₂O₂S ceramic prepared by hot-pressing powders obtained by combustion synthesis and a sulfurization treatment [1]. This result highlights the interest in further studying the mid-infrared (MIR) emission properties of rare-earth ions in La₂O₂S. This work presents an investigation of the structural and spectroscopic properties of Er:La₂O₂S oxysulfide with various rare-earth doping levels seen as the first step towards the fabrication of infrared transparent ceramics emitting at ~2.8 µm.

## 2 Results and discussion

The crystalline Er:La₂O₂S powders were prepared by the solid combustion method followed by sulfurization. The starting reagents were lanthanum nitrate La(NO₃)₃, Er(NO₃)₃, and thioacetamide C₂H₅NS (dissolved in distilled water). The nominal Er³⁺ doping level was 0.5 - 7 mol%. The precursor powder was obtained by combustion synthesis at 500 °C during 30 min. Then, the precursors were subjected to sulfurization at 1000 °C during 30 min under an atmosphere of H₂S and N₂ gases, taken in the ratio of 1:9.

The X-ray powder diffraction (XRD) revealed the single-phase nature of powders (hexagonal class, sp. gr. D³₃d - *P*-3*m*1) being isostructural to La₂O₂S, Fig. 1(a). The Raman spectra of Er:La₂O₂S are shown in Fig. 1(b). In the structure of La₂O₂S, the La site is coordinated by

<footnote n="1">¹Institut des Sciences Chimiques de Rennes (ISCR), CNRS UMR 6226, Université de Rennes 1, F-35000 Rennes, France</footnote>

<footnote n="2">²Centre de Recherche sur les Ions, les Matériaux et la Photonique (CIMAP), UMR 6252 CEA-CNRS-ENSICAEN, Université de Caen Normandie, 6 Boulevard Maréchal Juin, 14050 Caen, France</footnote>

<footnote>* Corresponding author: simone.normani@ensicaen.fr</footnote>
three oxygens and four sulfurs in the nearest neighbor having a symmetry of $C_3v$. All four Raman-active modes are assigned in the spectra, at 99 (Eg), 191 (A1g), 358 (Eg) and 389 (A1g) $cm^{-1}$. This low phonon energy behavior is beneficial for targeting mid-infrared applications as it is expected to reduce the non-radiative relaxation from Er$^{3+}$ multiplets.

<figure><img src="image_2.png" /><figcaption>**Fig. 1.** Structure, vibronic and spectroscopic properties of Er:La₂O₂S powders: (a) XRD patterns; (b) Raman spectra; (c-f) visible and infrared emission of Er$^{3+}$ ions: (c) visible luminescence spectra, (d) red-to-green intensity ratio vs. Er$^{3+}$ doping level; (e) MIR luminescence at 2.8 µm; (f) luminescence lifetimes, $\tau_{\text{lum}}$, of the $^{4}\text{I}_{11/2}$ and $^{4}\text{I}_{13/2}$ levels vs. Er$^{3+}$ doping level. $\lambda_{\text{exc}}$ – excitation wavelength.</figcaption></figure>

The visible luminescence spectra are shown in Fig. 1(c). The emission bands in the green and red are assigned to the $^{2}\text{H}_{11/2}+^{4}\text{S}_{3/2} \rightarrow ^{4}\text{I}_{15/2}$ and $^{4}\text{F}_{9/2} \rightarrow ^{4}\text{I}_{15/2}$ transitions, respectively. With the Er$^{3+}$ doping, the relative emission intensity in the red (~660 nm) increases, as expressed by the red-to-green ratio which depends quadratically on the doping level, Fig. 1(d). This parameter is an indicator of the energy transfer efficiency between Er$^{3+}$ ions, leading to self-quenching of the green emission. The observed behavior is favorable for the design of color tunable (green-to-red) visible powder and ceramic phosphors based on Er:La₂O₂S.

The MIR luminescence related to the $^{4}\text{I}_{11/2} \rightarrow ^{4}\text{I}_{13/2}$ transition spans from 2.65 µm to 2.85 µm, Fig. 1(e). The luminescence lifetimes $\tau_{\text{lum}}$ of the $^{4}\text{I}_{11/2}$ and $^{4}\text{I}_{13/2}$ states of Er$^{3+}$ ion La₂O₂S powders are plotted as a function of the doping level in Fig. 1(f). For 0.5 mol% Er$^{3+}$ doping, they amount to 2.96 ms and 4.91 ms, respectively, while already at 7 mol% Er$^{3+}$ doping, the lifetime ratio ($\tau_{\text{lum}}(^{4}\text{I}_{11/2}) > \tau_{\text{lum}}(^{4}\text{I}_{13/2})$) becomes favorable for the mid-infrared emission.

## 3 Conclusion

To conclude, heavily-doped 7 mol% Er:La₂O₂S powders demonstrate broadband emission properties in the mid-infrared, around 2.8 µm, and a favorable ratio of the $^{4}\text{I}_{11/2}$ to $^{4}\text{I}_{13/2}$ luminescence lifetimes. Further work will focus on the fabrication of infrared transparent ceramics based on these materials.

## References

1. A. Le Coz, G. R. Durand, L. Cornet, N. Herbert, F. Gouttefangeas, L. Joanny, F. Célarié, F. Cheviré, O. Merdrignac-Conanec, J. Eur. Ceram. Soc. **43**(5), 2133-2142 (2023).