Rare-earth (RE)-doped nanomaterials possess extensive applications in lasers, three-dimensional (3D) display devices, optical temperature sensors, and optical amplifiers due to their excellent luminescence properties.^[1–3] Consequently, the design and application of RE-doped nanomaterial is currently a field which the researchers are greatly interested in, focusing on gaining a better understanding of their properties. Numerous methods have been successfully used to improve the optical properties of RE-doped nanomaterials.^[4–6] For example, ion doping has been demonstrated to be able to change the luminescence properties by modulating lattice fields and influencing the energy transfer behavior. For instance, Eu³⁺-doped Y₂O₃ nanomaterials are used widely as RE-doped nanomaterials in several applications, particularly in display fields.^[7–10] In terms of its optical property applications, slight changes in the crystal field environment have been shown to play an important role, with the exception of influences due to variations in dopant and doping amount.^[11–13] Doped Eu³⁺ ions exist in the yttrium oxide (Y₂O₃) host in small quantities, with the energy transfer between Eu³⁺ ions and Y₂O₃ host being sensitive to the surrounding local microenvironment in addition to the crystal environment.^[14–17]

High pressure has been demonstrated to be an effective method to induce significant changes in the structure and properties of materials. Under pressure, materials undergo lattice compression. This causes further potential structural transitions to occur, which can induce changes in the properties or lattice behavior of the material.^[18–21] In the case of Y₂O₃ (Eu³⁺) nanomaterial, pressure-induced microstructural deformation ahead of the lattice amorphous transition results in completely distinct microstructural states for doping Eu³⁺ ions.^[22] This clearly results in a change in crystal environment, which subsequently changes the local environment of Eu³⁺ ions. In addition, other doping ions cause a nonhomogeneous distortion of the microstructure, which is amplified under high pressure due to lattice compression. Thus, using high-pressure is a useful method of understanding the influence of the crystal environment and the local surroundings on luminescence behavior. Therefore, the studies of the pressure-dependent luminescence of Y₂O₃/Eu³⁺ nanomaterials assisted by doping ion modulation could provide valuable insights for the field.

Numerous studies have demonstrated the luminescence properties of Eu³⁺ ions under high pressure. However, few studies have focused on lattice microstructural behavior under induced pressure as a result of changes in crystal environments and local micro-surrounding with other doping ions.^[19,23–28] Investigations aiming to study the pressure-dependent photoluminescence (PL) spectra of both Y₂O₃/Eu³⁺ bulk and nanomaterials by using either 514.5-nm or 532-nm laser excitation reported red spectral shifts and a loss of PL peaks.^[20,22–28] In our recent study, we demonstrated the probe effect of up-conversion PL measurement in revealing slight changes of YO₆ octahedron in Y₂O₃/Eu³⁺ nanotubes under high pressure.^[19] However, currently, there are no studies to analyze the pressure behavior of the luminescence of Y₂O₃/Eu³⁺ nanomaterials by manipulating other auxiliary ions under high pressure. The study of microstructure-related luminescence properties would provide important insights into the understanding of the luminescence behavior of Eu³⁺ ions and ways to utilize Eu³⁺-doped Y₂O₃ nanomaterials.

In the present work, we carry out high-pressure experiments to investigate local microstructures through Raman and PL spectra of Y₂O₃/Eu³⁺ and Y₂O₃/Eu³⁺/Mg²⁺ nanorods by using 514-nm and 532-nm laser excitation. The amorphous transitions are found to occur at a pressure greater than 24 GPa for both Y₂O₃/Eu³⁺ and Y₂O₃/Eu³⁺/Mg²⁺ nanorods. Under high pressure (above 8 GPa), Y₂O₃/Eu³⁺ and Y₂O₃/Eu³⁺/Mg²⁺ nanorods exhibit different distorted states. In addition, Y₂O₃/Eu³⁺ and Y₂O₃/Eu³⁺/Mg²⁺ nanorods present different local micro-surrounding characteristics for Eu³⁺ ion in a pressure-modulated crystal field, based on the analyses of the following intensity ratios: I₀₋₂/I₀₋₁ of ⁵D₀–⁷F₂ to ⁵D₀–⁷F₁ and I₀₋₂A/B of ⁵D₀–⁷F₂ transitions. The doped Mg²⁺ ion is found to play a critical role in reducing the local microstructure ionicity when the YO₆ octahedron is distorted under high pressure. From this work, we show that doped ions can be utilized for detecting the tiny local microstructural changes upon high pressure by using Eu³⁺ luminescence.

The Y₂O₃/Eu³⁺ and Mg²⁺ co-doped Y₂O₃/Eu³⁺ nanorods used in this study were generated using a hydrothermal method as described previously.^[29] The Eu³⁺ doped Y_1.9Eu_0.1O₃ (YEO) and Mg²⁺ co-doped Y_1.8 Eu_0.1Mg_0.1O₃ (YEMO) samples were placed in a cubic structure (space group: Ia-3) and comprised of nanorods that were predominantly 50 nm–100 nm in diameter and 10 μm in length as depicted in Fig. 1. A diamond anvil cell (DAC) was used to carry out Raman and PL spectra experiments under high pressure. These experiments were done by using a 4:1 methanol–ethanol mixture as pressure medium. The pressure was calibrated by a shift in the ruby R₁ line. A Renishaw inVia Raman Microscope equipped with a charge coupled device (CCD) detector was used to collect Raman vibrational signals from 514.5-nm excitation. A spectrometer (focal length, 500 mm) combined with a liquid nitrogen-cooled CCD (Acton SP-2500 and PyLoN:100B, Princeton Instruments) was utilized to obtain high-pressure PL spectra. A 532-nm single-mode diode-pumped solid-state (DPSS) laser (power output, 50 mW) was used as the light source for experiments by using PL excitation. In PL experiments, all data were collected under identical experimental conditions for each pressure tested, including the same laser power, laser spot size, exposure time, and focus microscopic lens. The spectra resolutions are 1 cm⁻¹ and 0.1 nm, respectively for Raman and luminescence spectrometers in the high pressure experiments carried out in this study.

Fig. 1.

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	Fig. 1. (color online) X-ray diffraction (XRD) patterns (a) and SEM images of YEO (b) and YEMO (c) nanorods in ambient conditions.

Figure 2 illustrates the PL spectra in a range of 550 nm–675 nm (⁵D₀–⁷F_0,1,2) of YEO and YEMO nanorods under ambient conditions by using a laser excitation of 532 nm. With laser excitation, energy transfers to Eu³⁺ ions via the Y₂O₃ host lattice. Energy transfer is a critical determinant of the PL behavior of RE-doped material, and this energy transfer behavior is dependent on the structure and local microstructure of Eu³⁺ ions. Therefore, the study of pressure-dependent PL of Eu³⁺ ions could provide valuable insights into the changes in structural and local microstructural behavior, and also the changes in corresponding relationship between the Eu³⁺ ion PL and local microstructure.

Fig. 2.

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	Fig. 2. (color online) Photoluminescence spectra of Y1.9Eu0.1O3 (YEO) and Mg2+ co-doped Y1.8 Eu0.1Mg0.1O3 (YEMO) nanorods under ambient conditions.

In order to characterize structural changes, we carry out in-situ Raman scattering experiments on YEO and YEMO nanorods and show the pressure-dependent Raman spectra in Fig. 3. The representative Raman vibration of Y³⁺–O⁻² located at 378 cm⁻¹ in ambient conditions is observed to shift towards high wave numbers with increasing pressure. The intensity of the Raman vibration is observed to become notably weaker after 8 GPa–9 GPa, in the cases of YEO and YEMO samples. For YEO nanorods, the Raman vibration becomes very weak when the pressure reaches up to 18.9 GPa, and disappears completely at 24.2 GPa. Similar results were observed for YEMO nanorods, in which the Raman vibration begins to disappear when the pressure reaches up to 20 GPa. The disappearances of the Raman vibration for both YEO and YEMO nanorods indicate a phase transition from initial cubic structure to a different structure. With increasing pressure, no distinguishable Raman signal is detected. The phase transition behavior observed for Y₂O₃/Eu³⁺ nanomaterials doped with Eu³⁺ or Mg²⁺ ion(s) is similar to that of its Y₂O₃ host, with Y₂O₃ and Y₂O₃/Eu³⁺ nanomaterials reported to transform from an initial cubic phase into an amorphous phase.^{[14,15,20,24]} In addition to the disappearance of the Raman vibration, the Raman peak position is also observed to shift towards a higher wavenumber as deduced by pressure treatment. However, the Raman peak position is found to shift at different rates under pressure prior to the amorphous transition at 24 GPa. This observation is ascribed to the pressure-induced YO₆ octahedron distortion.^[22] We plot the pressure-dependent Raman peak position in Fig. 4. These plots show different variations of Raman vibration before and after 8 GPa of pressure: the peak shift becomes slower after 8 GPa and has an obvious change in the slope. This illustrates that the Raman peak shift that occurs with pressure induced by lattice compression becomes slower after 8 GPa. Below approximately 8 GPa, the variations in the Raman vibration are found to be consistent for YEO and YEMO nanorods. However, the changes in the extent of the slope become different after 8 GPa, with the slope changing less markedly in the case of YEMO nanorods. As previously reported, the different changes in behavior of the Raman vibration slope under pressure correspond to different lattice states that exist before structure transformation.^[22] The latter lattice state is thought of as a slightly distorted state that results from the YO₆ octahedron distortion. The weakened Raman intensity mentioned previously is caused by a local distortion of the Y₂O₃ lattice. In the present study, we show that the difference in distortion state between YEO and YEMO nanorods is mainly due to Mg²⁺ doping into the Y₂O₃ lattice, which generates a different pressure behavior for the YO₆ deformation.

Fig. 3.

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	Fig. 3. (color online) Pressure-dependent Raman spectra of YEO and YEMO nanorods.

Fig. 4.

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	Fig. 4. (color online) Raman peak positions of YEO and YEMO nanorods under high pressure.

The different lattice states of YEO and YEMO nanorods correspond to different crystal fields, which play a critical role in determining the luminescence properties of the doping Eu³⁺ ion. Thus, we successively analyze the pressure-dependent PL experiments of YEO and YEMO nanorods excited with a 532-nm laser. Given that ⁵D₀–⁷F_0,1,2 transitions exhibit comparatively strong PL intensities and reflect the information about crystal fields and local microstructural changes, we analyze the pressure-dependent PL spectra of ⁵D₀–⁷F_0,1,2 transitions for both YEO and YEMO nanorods (Fig. 5).

Fig. 5.

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	Fig. 5. (color online) Pressure-dependent PL spectra of YEO and YEMO nanorods.

With increasing pressure, all ⁵D₀–⁷F_0,1,2 transition peaks are found to shift towards higher wavelengths with a lattice compression. The strongest peak at 612 nm in ambient conditions becomes definitively weaker after 8 GPa, in the cases of both YEO and YEMO nanorods. When the pressure reaches up to 23 GPa–24 GPa, the group A peaks from the ⁵D₀–⁷F₂ transition (indicated in Fig. 2) are found to become weak, and therefore difficult to distinguish. This could be attributed to the amorphous transition of the Y₂O₃ host lattice that occurs under high pressure. Given the transition sensitivity of the Eu³⁺ ion to the local structure of the Y₂O₃ lattice, studies into factors that influence local structural changes on the PL spectra can provide valuable information. Corresponding changes in the local microstructure can be explained by the Judd–Ofelt theory.^[30,31] The Judd–Ofelt parameter Ω₂, defined as the intensity ratio I₀₋₂/I₀₋₁ of ⁵D₀–⁷F₂ to ⁵D₀–⁷F₁ transitions of Eu³⁺ ions, is related to short-range effect and reflects the properties of the local microstructure and surrounding Eu³⁺ ions. Meanwhile, the intensity ratio I₀₋₂A/B of ⁵D₀–⁷F₂ transitions also reflects a similar behavior of the lattice microstructural information.^[28]

We plot the intensity ratios I₀₋₂/I₀₋₁ and I₀₋₂A/B of ⁵D₀–⁷F₂ transitions of Eu³⁺ ions against pressure in Figs. 5 and 6. The intensity ratio I₀₋₂/I₀₋₁ exhibits different changes in behavior under pressures both below and above 8 GPa, while I₀₋₂(A/B) exhibits a sudden drop around 8 GPa. The I₀₋₂/I₀₋₁ ratio is observed to increase with pressure before 8 GPa, and decrease rapidly after 8 GPa. As discussed previously, an amorphous transition of the Y₂O₃ host lattice occurs at pressures higher than 20 GPa. Therefore, the sudden change observed in the I₀₋₂/I₀₋₁ ratio around 8 GPa is not caused naturally by the structural transition. This sudden change could be attributed to local structural changes of the YEO and YEMO nanorods. As Raman vibration under pressure indicates, the vibration position of the typical Raman mode Y³⁺–O⁻² is found to increase at different rates before and after 8 GPa. This demonstrates that the micro-surrounding of Eu³⁺ ions clearly changes. Before 8 GPa, the I₀₋₂/I₀₋₁ ratio is found to increase, indicating a crystal field strengthening for both YEO and YEMO nanorods. Indeed, the pressure causes a decrease in the distance between atoms and shortens bond lengths, which could ultimately result in the strengthening of the crystal field. After 8 GPa, the Y₂O₃ host lattice is found to transform into a distorted state in some local microstructures. However, the structure transformation is shown not to occur for the Y₂O₃ host lattice as illustrated by the Raman study. This transformation induces the crystal microstructure to change, followed by direct influence on the surrounding Eu³⁺ ions. In this disordered lattice state, the general lattice space is shown to be compressed continuously with pressure. However, the local microstructural disorder destroys the strengthening tendency of the Eu³⁺ local micro-surroundings. Under such circumstances, the covalency in the vicinity of Eu³⁺ ion-ligand increases, and the ionicity of Eu³⁺ local micro-surroundings exhibits a corresponding decrease.^[30,31] Therefore, the intensity ratio I₀₋₂/I₀₋₁ is found to decrease after 8 GPa, with the degree of decrease becoming larger with distortion increasing under pressure.

Fig. 6.

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	Fig. 6. (color online) PL intensity ratios I0−2/I0−1 of 5D0–7F2 and 5D0–7F1 transitions of YEO and YEMO nanorods with increasing pressure.

In addition to the sudden changes in intensity ratios of I₀₋₂/I₀₋₁ and I₀₋₂A/B around 8 GPa, we also observe that there is no typical Raman vibration position nor intensity ratio (I₀₋₂/I₀₋₁ and I₀₋₂A/B), which shows a different sudden change degree for each of YEO and YEMO nanorods. This suggests that both YEO and YEMO nanorods exhibit different crystal field properties and local micro-surroundings of Eu³⁺ ions after 8 GPa. As shown in Figs. 6 and 7, YEMO nanorods display higher intensity ratios of I₀₋₂/I₀₋₁ and I₀₋₂A/B at each pressure point measured after 8 GPa, which indicates that according to the Judd–Ofelt theory,^[30] the ionicity of the Eu³⁺ local micro-surroundings for YEMO nanorods is greater than that of YEO nanorods, although they are both in distorted lattice states. In the pressure-dependent Raman vibration spectra under high pressure, we also demonstrate that the vibration position variation differs after 8 GPa, with the larger bend observed with YEO nanorods, and the smaller bend observed with YEMO nanorods. Therefore, the different bend degrees of the Raman mode variety and intensity ratios (I₀₋₂/I₀₋₁ and I₀₋₂A/B) observed under high pressures appearing at 8 GPa are consistent and closely related to each other. As the pressure-dependent Raman studies illustrate, different bends indicate different distortion states in the local microstructure, with the smaller bend degree of Raman vibration exhibiting the weak distortion of the YO₆ octahedron for YEMO nanorods under high pressure. This leads to the higher ionicity of the local microstructure, corresponding to the higher ionicity of Eu³⁺ local micro-surroundings. Therefore, the intensity ratios, I₀₋₂/I₀₋₁ of ⁵D₀–⁷F₂ to ⁵D₀–⁷F₁ and I₀₋₂A/B of ⁵D₀–⁷F₂ transitions, exhibit higher values under high pressure in the case of Mg²⁺ co-doped YEMO nanorods.

Fig. 7.

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	Fig. 7. (color online) PL intensity ratios I0−2A/B of 5D0–7F2 transitions of YEO and YEMO nanorods with increasing pressure.

The doped Mg²⁺ ions act as modifiers of the Y₂O₃ lattice and YO₆ octahedron. These doped Mg²⁺ ions are believed to play a critical role in modifying the microstructure and local micro-surroundings of Eu³⁺.^[32] Compared with the radii of Y³⁺ (0.089 nm) and Eu³⁺ (0.198 nm) ions, the ionic radius of Mg²⁺ is small (0.072 nm). Following doping, the doped Mg²⁺ ions partially substitute the Y³⁺. The Mg²⁺ ion modification shortens the Mg–O bond length, compared with the undoped Y–O bond length. Due to the microscaled doping of Mg²⁺, the Y₂O₃ host lattice does not exhibit general structural changes in ambient conditions in the lower pressure range. However, when the pressure is greater than 8 GPa, the Y₂O₃ (Y₂O₃/Eu³⁺) host lattice transforms into a distorted state, deforming the YO₆ octahedron. Consequently, the doping effects of Mg²⁺ ions on the Y₂O₃ crystal lattice become clear. According to the results from a previous study, the two different kinds of YO₆ octahedrons of Y₂O₃ (Y₂O₃/Eu³⁺) nanomaterials exhibit different deformation properties.^[14] The YO₆ octahedron with C₂ symmetry is demonstrated to be easily distorted by pressure compared with the octahedron with S₆ symmetry. Due to the fact that the YO₆ octahedron becomes distorted under pressures greater than 8 GPa, ideal Y₂O₃ (Y₂O₃/Eu³⁺, the lattice arrangement breaks, inducing a decrease in the local microstructure ionicity. Thus, the intensity ratios, I₀₋₂/I₀₋₁ of ⁵D₀–⁷F₂ to ⁵D₀–⁷F₁ and I₀₋₂A/B of ⁵D₀–⁷F₂ transitions of Eu³⁺ are found to exhibit a sudden drop with pressure increasing. In the case of the Mg²⁺-ion-doped Y₂O₃(Y₂O₃/Eu³⁺, the decrease in the Y₂O₃(Y₂O₃/Eu³⁺ microstructure ionicity is found to be attenuated due to its shorter Mg–O bond length, which is found to strengthen the crystal field. Therefore, the YEMO nanorods show higher intensity ratios of I₀₋₂/I₀₋₁ of ⁵D₀–⁷F₂ to ⁵D₀–⁷F₁ and I₀₋₂A/B of ⁵D₀–⁷F₂ transitions. This suggests that the doped ions can be used as a lattice ion modifier to study tiny local microstructural changes induced through Eu³⁺ luminescence under high pressure.

In this work, we perform pressure-induced microstructural analyses of Y₂O₃/Eu³⁺ and Y₂O₃/Eu³⁺/Mg²⁺ nanorods using Raman and photoluminescence spectra. The Y₂O₃/Eu³⁺ and Y₂O₃/Eu³⁺/Mg²⁺ nanorods are shown to transform into an amorphous phase at a pressure of 24 GPa. Pressure-dependent Raman peak positions exhibit different distorted states after 8 GPa, in the cases of Y₂O₃/Eu³⁺ and Y₂O₃/Eu³⁺/Mg²⁺ nanorods. The Y₂O₃/Eu³⁺/Mg²⁺ nanorods are demonstrated to exhibit a stronger crystal field around Eu³⁺ ions when the intensity ratios of I₀₋₂/I₀₋₁ of ⁵D₀–⁷F₂ to ⁵D₀–⁷F₁ and I₀₋₂A/B of ⁵D₀–⁷F₂ transitions in the distorted lattice state are investigated. The doped Mg²⁺ ion is found to play a critical role in reducing the decrease in crystal ionicity under high pressure. Additionally, these studies show that the doped ions can be used as a lattice ion modifier to investigate tiny local microstructural changes with the help of Eu³⁺ luminescence under high pressure.