Structural and luminescent properties of a Cr³⁺/Sm³⁺ doped GdAlO₃ orthorhombic perovskite for solid-state lighting applications

Abstract

The Cr³⁺ and Sm³⁺ doped GdAlO₃ perovskite with formula Gd_0.995Sm_0.005Al_0.995Cr_0.005O₃, was synthesized via a solid-state reaction method, and its structure, morphology, and photoluminescence properties were thoroughly investigated. The compound crystallizes in the orthorhombic Pbnm space group, with Cr³⁺ transition-metal ions substituting Al³⁺ in the octahedral symmetry site, and Sm³⁺ lanthanide (rare-earth) ions occupying the tetrahedral site. The material's morphology and chemical composition homogeneity were evaluated through Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray analysis. Photoluminescence excitation (PLE) and emission spectra (PL) were used to shed light on the electronic structure of the Cr³⁺ cations, through crystal field analysis in the O_h symmetry site. Theoretical studies enabled the precise assignment of the Cr³⁺ 3d–3d transitions. The intra-configurational 4f–4f transitions of Sm³⁺ resulted in a variety of excitation bands appearing in the high-wavelength range of the PLE spectrum. The photoluminescence studies supported the occurrence of energy transfer in the doped GdAlO₃ perovskite between Gd³⁺, Sm³⁺ and Cr³⁺ ions. The obtained results suggest the high potential of the synthesized material for solid state lighting applications.

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1. Introduction

The investigation of rare-earth-doped luminescent materials has unlocked new avenues of innovation, driving advancements across various high-tech industries.¹ Among these, white light-emitting devices (LEDs) stand out due to their exceptional luminescent properties. In the context of global energy challenges and the growing threat of climate change, the need for innovative and efficient phosphors tailored for solid-state lighting (SSL) is critical.^2,3 The demand for novel phosphors used in SSL displays and lamps continues to rise, driven by their prolonged lifespan, high luminescence efficiency, and contribution to sustainable environmental practices.^4–8

Aluminate-based host matrices have garnered significant attention as ideal candidates for phosphor development due to their superior crystallinity, excellent luminous characteristics, straightforward synthesis, favorable color parameters, and exceptional chemical and thermal stability. Their high heat resistance further enhances their suitability for doping with various ions, enabling the creation of materials optimized for diverse applications.^9,10 Consequently, aluminate-based materials have been extensively investigated and have found applications in energy harvesting for flat-panel displays,¹¹ SSL,¹² scintillators,¹³ optical lasers,¹⁴ light-emitting diodes,¹⁵ photonic storage devices, and remote sensing technologies.¹⁶

Among these materials, gadolinium aluminate (GdAlO₃), featuring an orthorhombic perovskite crystal structure (Pnma space group), is a particularly notable example. GdAlO₃ is uniquely suited for multifunctional applications due to its robust thermal and chemical stability, compatibility with rare-earth dopants, and excellent energy transfer characteristics. These properties make it an attractive host matrix for developing phosphors with tailored optical and magnetic functionalities. While gadolinium can be expensive, the strategic use of GdAlO₃ in doped compositions mitigates costs,¹⁷ and its performance advantages justify its use in niche applications such as magneto-optoelectronics and hybrid devices requiring simultaneous photoluminescent and magnetic properties.^18–20 Furthermore, GdAlO₃ based materials show promise for energy storage and conversion technologies, adding to their appeal as next-generation phosphor materials.

Cr³⁺ ions were selected as primary activators due to their well-established photoluminescent properties, including intense and tunable red emissions. To further enhance the material's spectral characteristics, Sm³⁺ was introduced as a codopant. Sm³⁺ was chosen for its ability to enhance red luminescence and its potential to act as an energy transfer mediator, facilitating a synergistic effect with Cr³⁺. This combination is particularly relevant for applications requiring optimized spectral output, such as full-spectrum lighting or display technologies. Moreover, the dopant combination leverages the perovskite structure of GdAlO₃ to enable efficient energy transfer and tailored luminescent properties.

In this investigation, we synthesized and characterized the structural and luminescence properties of a Cr³⁺ doped orthorhombic GdAlO₃ perovskite phosphor, codoped with Sm³⁺ to achieve the composition Gd_0.995Sm_0.005Al_0.995Cr_0.005O₃. This study aims to elucidate the fundamental mechanisms governing the optical properties of GdAlO₃ doped with Cr³⁺ and Sm³⁺, thereby fostering further exploration into the potential of complex oxides doped with transition and rare-earth elements for advanced technological applications. The findings presented here demonstrate the promise of this novel material for applications in solid-state lighting.

2. Materials and methods

In this study, GdAlO₃ perovskite was synthesized via solid-state method. High-purity gadolinium oxide (Gd₂O₃) and aluminum oxide (Al₂O₃) powders were weighted in stoichiometric ratio to form the base material. To introduce chromium and samarium doping, appropriate amounts of samarium oxide (Sm₂O₃) and chromium oxide (Cr₂O₃) were added, ensuring the desired doping level of Gd_0.995Sm_0.005Al_0.995Cr_0.005O₃. The solid-state method was chosen for its robustness and ability to exert precise control over reaction conditions. Following pre-annealing at 700 °C for 24 hours to remove volatile inorganic materials, the mixture was ground using an agate mortar and pestle to ensure intimate mixing of the reactants. The resulting mixture was then sintered at 1200 °C for 24 hours, promoting atomic and ionic diffusion essential for the formation of the desired doped GdAlO₃ compound. The high temperature during sintering facilitated the incorporation of Cr³⁺ and Sm³⁺ ions into the crystal lattice, ensuring the development of the desired orthorhombic crystal structure. Finally, the sintered material was compacted into pellets with a diameter of 8 mm, which were used in the subsequent characterization and analyses experiments.

Characterization of the newly synthesized compound was undertaken using different techniques. The material was first analyzed by powder X-ray diffraction (X-ray Siemens D5000, with λ = 1.5406 Å CuK_α radiation). Data was collected within the 20–100° 2θ range, with a step size of 0.02°. Subsequently, photoluminescence emission (PL) and excitation (PLE) measurements were collected in a Horiba-Jobin-Yvon Fluorolog 322, operating in time-resolved mode (using a pulsed lamp) with a 0.05 ms delay after-flash. The morphology of the material and its chemical homogeneity were investigated by scanning electron microscopy (SEM), using a TESCAN VEGA3 SBH instrument equipped with an X-ray energy dispersive microscopy (EDS) detector. Image analysis was carried out using the ImageJ software,²¹ and structure analysis was undertaken using the Visualization Electronic and Structural Analysis (VESTA) software.^22–24

Raman spectra were collected (with 4 cm⁻¹ spectral resolution) in a LabRAM HR Evolution Raman microscope (Horiba Scientific) with a 600 g mm⁻¹ grating. The excitation was provided by a He–Ne laser (λ = 633 nm), which was focused on the sample to a ∼1 mm spot using a 100× objective. The applied laser power of ∼0.2 mW at the sample resulted in no detectable heating or degradation. The collection time was 10 s with 100 accumulations being averaged to generate the final spectra.

The chromaticity coordinates (x, y) of the synthesized Gd_0.995Sm_0.005Al_0.995Cr_0.005O₃ phosphor were determined using the CIE 1931 color space from its emission spectrum under 377 nm excitation. The correlated color temperature (CCT) and color purity (CP) values were calculated based on established methods.^25,26 The resulting parameters are presented in Table 1, demonstrating the material's potential for applications in warm white light-emitting diodes (LEDs).

Table 1 CIE parameters for the red luminescence color of the synthesized Cr³⁺/Sm³⁺ doped GdAlO₃ phosphor

x	y	CCT (K)	CP (%)
0.62157	0.37383	1892.46	87.08

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Fig. 1 shows the CIE 1931 chromaticity diagram for the Gd_0.995Sm_0.005Al_0.995Cr_0.005O₃ phosphor, generated from the emission spectrum under 377 nm excitation, where the coordinates are positioned in the reddish-orange region.

Fig. 1 The CIE graph for the studied Cr³⁺/Sm³⁺ doped GdAlO₃ phosphor.

3. Results and discussion

3.1 Powder X-ray diffraction analyses

Powder X-ray diffraction (PXRD) is a powerful technique for elucidating crystal phases, lattice parameters, and overall structural characteristics of solid materials. To determine the phases present in the synthesized material, its PXRD diffraction pattern (Fig. 2) was obtained and compared with ICDD (International Center for Diffraction Data) database. The observed peaks could be successfully indexed to a primitive orthorhombic crystal lattice, within the Pbnm space group. The identification process used standard JCPDS data files [GdAlO₃ 46-0395; Gd₂O₃ 88-2165]. Rietveld refinement revealed the presence of two phases, the predominant one corresponding to the perovskite compound and the minor to Gd₂O₃. The presence of the Gd₂O₃ as impurity in the sample is consistent with previous studies,²⁷ where this impurity was also observed. Table 2 presents the refined crystallographic parameters, with the agreement between calculated and observed profiles being satisfactory, as indicated by a χ² value of 1.7. The fit quality between the observed and calculated diffraction patterns is reflected by R_p = 17.6, R_wp = 14.2, and R_e = 10.9. While, R_p (profile R-factor) is a measure of the agreement between the observed and calculated diffraction profiles, R_wp (weighted profile R-factor) is similar to R_p but takes into account the statistical weights of the observed intensities and R_e (expected R-factor) represents the ideal R-factor that would be obtained if the model perfectly fits the data, considering only statistical uncertainties. Fig. 3 depicts the structures of the GdAlO₃ perovskite, where Al³⁺ and Gd³⁺ cations are bonded to six and eight oxygen atoms, respectively. Al³⁺ ions occupy an octahedral (O_h) coordination site corresponding to corner-sharing AlO₆ octahedra with tilt angles of 24°, while Gd³⁺ stay in a coordination site whose symmetry is C_s. The compound presents four shorter and two longer Al–O bond lengths, along with two inequivalent O²⁻ sites. In the first site, O²⁻ is bonded in a 5-coordination geometry to three equivalent Gd³⁺ and two equivalent Al³⁺ cations; in the second, O²⁻ is bonded in a 4-coordination geometry to two equivalent Gd³⁺ and two equivalent Al³⁺ cations. Upon doping, Cr³⁺ and Sm³⁺ cations are hypothesized to replace Al³⁺ and Gd³⁺, respectively, due to the close match in their ionic radii. Cr³⁺ (0.615 Å in octahedral coordination) is similar in size to Al³⁺ (0.535 Å), making it a suitable substitute at the Al³⁺ site. Similarly, the ionic radius of Sm³⁺ (1.079 Å in nine-fold coordination) closely matches that of Gd³⁺ (1.053 Å), facilitating substitution at the Gd³⁺ site.

Fig. 2 Results of the performed Rietveld refinement for Gd_0.995Sm_0.005Al_0.995Cr_0.005O₃.

Table 2 Cell parameters and volume resulting from the Rietveld refinement for the synthesized compound, and statistical indicators

Formula	Gd0.995Sm0.005Al0.995Cr0.005O3	Rp	17.6
Space group	Pbnm	Rwp	14.2
a (Å)	5.2517	Re	10.9
b (Å)	5.2998	χ^2	1.7
c (Å)	7.4458	Gd2O3 (%)	3.0501
V (Å^3)	207.2401	SEM (nm)	177

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Fig. 3 Illustration (Vesta software images) of the structure of the GdAlO₃ perovskite, and of the 8-coordinated SmO₈ and 6-coordinated (octahedral) CrO₆ units.

3.2 EDS and particles morphology studies

Scanning electron microscopy (SEM) was employed to investigate the morphology of particles of the synthesized compound. Fig. 4 shows the presence of agglomerated and irregularly shaped grains, resulting from the substantial surface area of the particles and their interaction via weak van der Waals forces. The average particle size distribution, determined by Lorentz fitting using the ImageJ software, shows a peak centered at 177 nm.

Fig. 4 SEM image and particle size distribution histogram of the synthesized compound, and results of the Lorentz fit to the distribution data.

To confirm the chemical composition of the sample, its EDS spectrum was collected (Fig. 5). The spectrum exhibits characteristic peaks of all expected elements (Gd, Cr, Al, Sm, O) confirming the presence of these elements in the sample. The low-energy peaks correspond to lighter elements like Al and O, while the intermediate-energy peaks are attributed to Cr, Sm, and Gd, with distinct signals from their L and K edges. High-energy peaks, primarily from the K edges of Gd, also show some overlap from Sm, consistent with their atomic characteristics. All peaks are clearly labeled in the spectrum for clarity.

Fig. 5 EDS spectrum of Gd_0.995Sm_0.005Al_0.995Cr_0.005O₃ and elements distribution maps.

The elemental distribution maps, presented in the smaller images on the right of Fig. 5, provide a visual representation of the spatial distribution of the elements in the sample. While the main inset (colorful grain map) shows some inhomogeneity in the intensity of elemental signals, this variation is likely due to factors such as sample morphology, local concentration differences, or surface roughness, which can affect signal intensities. Despite these minor variations, the maps demonstrate significant co-localization of the elements, indicating a uniform distribution across the sample. This is further supported by the composite overlay of the elemental maps, where no regions are completely absent of any detected elements, confirming the overall homogeneity of the elemental distribution.

Table 3 provides the results of the quantitative analysis of the elements present in the sample, which show good agreement with the theoretical values for the expected composition. The observed differences between the nominal composition and EDS results, particularly for Cr and Sm, are attributed to the limitations of EDS in quantifying trace elements and matrix effects. Despite these minor discrepancies, the elemental maps confirm a homogeneous distribution of elements across the sample.

Table 3 Elemental composition obtained from the EDS data of the studied sample and theoretical values for Gd_0.995Sm_0.005Al_0.995Cr_0.005O₃

Element	Series	Experimental (weight%)	Error (%)	Theoretical (weight%)	Experimental (atomic%)
Al	K	16.19	1.61	14.57	17.56
Cr	K	0.30	0.09	0.14	0.23
Sm	L	2.92	0.18	0.41	1.13
Gd	L	80.59	3.92	84.88	81.08
Total		100.00		100.00	100.00

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3.3 Raman spectroscopy analyses

The symmetry analyses of Pbnm space group symmetry perovskites of general formula ABO₃ has been detailed by Chopelas, who has performed a systematic study of the Raman spectra of 30 different aluminate orthorhombic perovskites with emphasis on YAlO₃ and GdAlO₃.²⁸ In the Brillouin zone center, the 24 Raman active optical modes span the symmetry species 7A_g + 7B_1g + 5B_2g + 5B_3g. Chopelas has observed and assigned twenty of these modes for GdAlO₃ using single crystal polarized Raman spectroscopy with excitation at 488.0 or 457.9 nm (Ar⁺ laser).²⁸

The Raman spectra of GdAlO₃ and of the synthesized Gd_0.995Sm_0.005Al_0.995Cr_0.005O₃ compound obtained in the present study using 633 nm excitation are shown in Fig. 6, together with the difference spectrum obtained by subtracting the spectrum of the undoped GdAlO₃ perovskite from that of the newly prepared compound. The difference spectrum allows to better notice the spectral changes due to the presence of the dopant atoms, which for most of the bands correspond to small shifts to lower frequency. These changes could be anticipated considering the much larger mass of chromium compared to aluminum (51.996 vs. 26.9815). On the other hand, the mass difference between gadolinium and samarium is small (157.2 vs. 150.4) so that the effects of the substitution of Gd³⁺ ions by Sm³⁺ at the level of substitution in the studied compound could be expected to lead to experimentally non-detectable perturbations in the frequencies of the modes where the movements of these atoms play a major role.

Fig. 6 Raman spectra of the prepared compound (a) compared to that of undoped GdAlO₃ (b). Plot (c) is the difference spectrum obtained by subtracting the spectrum of the undoped GdAlO₃ perovskite from that of the prepared compound.

The collected spectrum of the undoped GdAlO₃ compound follows closely those reported before.^28,29 Assignments are given in Table 4, which includes also the data for the studied Sm³⁺/Cr³⁺ doped compound. Compared to the previous investigations,^28,29 the present results have allowed to propose assignments for the 4 modes not yet firmly assigned before: the assignment of the A_g mode 6 proposed by Gouadec et al.²⁹ was confirmed to the band now located at 445.0 cm⁻¹ (440 cm⁻¹ as reported before²⁹), and the B_1g modes 4 and 6 are assigned to bands observed at 280.0 and 570.0 cm⁻¹, respectively, at frequencies in the ranges observed for other orthorhombic Pbnm aluminium oxide perovskites;²⁹ the B_2g mode 1 that had been previously ascribed to the band at 146 cm⁻¹, is now reassigned to the band observed at 119.0 cm⁻¹, while the band at 146 cm⁻¹ is reassigned to the B_2g mode 2 (this band is also assigned to the A_g mode 2, following the assignments proposed before;^28,29 see Table 4). The assignment of all other modes follows those previously suggested.^28,29

Table 4 Assignment of the Raman spectra of GdAlO₃ and of Gd_0.995Sm_0.005Al_0.995Cr_0.005O₃

Symmetry	Mode	GdAlO3			Gd0.995Sm0.005Al0.995Cr0.005O3
		This study	28	29	This study
a Assigned in ^28 to the B2g 1 mode.b This band is among all bands the one undergoing the largest frequency shift and broadening, indicating that the movement of the Al^3+/Cr^3+ ions have a large contribution to the corresponding vibration.c The appearance of the additional low-frequency band is, with all probability, a consequence of the Al^3+ by Cr^3+ substitution.
Ag	1	94.5	95	93.5	93.0
	2	146.5	146	146	145.0
	3	233.0	232	233	233.0
	4	314.0	313	315	313.0
	5	362.0	368	366	361.0^b
	6	445.0	—	440	427.0/445.0^c
	7	535.0	536	534	533.5
B1g	1	112.0	111	—	111.0
	2	162.0	160	—	159.0
	3	222.5	222	223	221.0
	4	280.0	—	—	278.0
	5	414.0	414	405	412.0
	6	512.0	512	—	508.0
	7	570.0	—	—	566.0
B2g	1	119.0	—	—	117.5
	2	146.5	146^a	146	145.0
	3	400.0	398	399	399.0
	4	468.0	475	470	468.0
	5	522.0	523	522	521.0
B3g	1	174.0	174	172	172.0
	2	217.0	217	215	217.0
	3	334.0	323	—	334.0
	4	400.0	400	399	399.0
	5	551.0	551	—	547.0

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The analysis of the Raman spectra provided new data for the undoped material that allowed for fine tuning of the assignments for that material, and are completely new for the synthesized doped material.

3.4 Photoluminescence measurements

3.4.1 Photoluminescence emission spectrum (PL). The room temperature PL spectra of Gd_0.995Sm_0.005Al_0.995Cr_0.005O₃ and GdAlO₃ compounds are shown in Fig. 7. The spectra were collected in time-resolved mode (using a pulsed lamp) with excitation at 377 nm and with a 0.05 ms delay after flash. The intense and well-resolved line observed at 726 nm is ascribed to a Cr³⁺ 3d–3d transition from the excited ²E_g(²G) to the ground ⁴A_2g(⁴F) state.³⁰ This emission, with its remarkable tissue-penetrating capability and minimal tissue damage, makes the synthesized compound a promising candidate for in vivo imaging applications and optoelectronic devices.

Fig. 7 Comparison between the GdAlO₃ and Gd_0.995Sm_0.005Al_0.995Cr_0.005O₃ PL spectra (collected with excitation at 377 nm).

Additionally, an emission line at 735 nm, commonly observed in systems containing high concentrations of Cr³⁺ ions (e.g., GaAlO₃:Cr³⁺, LaAlO₃:Cr³⁺ (ref. 31)), is likely due to energy transfer between exchange-coupled Cr³⁺ pairs and individual Cr³⁺ ions. Emission bands observed at 720, 704, 697, and 680 nm are attributed to the Gd³⁺ 4f–4f transitions (⁶P_j → ⁸S_7/2, j = 3/2, 5/2 and 7/2). Bands at 660 nm (⁴G_5/2 → ⁶H_11/2), 647 nm (⁴G_5/2 → ⁶H_9/2), 600 nm (⁴G_5/2 → ⁶H_7/2), and 564 nm (⁴G_5/2 → ⁶H_5/2) are assigned to Sm³⁺ 4f–4f transitions.³²

3.4.2 Photoluminescence excitation spectra (PLE). Fig. 8 presents the PLE spectra of the studied doped material at room temperature, collected with emission at 600 and 697 nm. The spectrum monitored at λ_em = 697 nm spectra present a narrow UV excitation band corresponding to Gd³⁺ 4f–4f transition, located at 275 nm and assigned to the absorption ⁸S_7/2 → ⁶I_11/2, and a broad band located at 326 nm, due to the ⁸S_7/2 → ⁶P_j transition.³³ Two additional broad bands in the visible region correspond to Cr³⁺ 3d–3d transitions: 564 nm (⁴A_2g(⁴F) → ⁴T_2g(⁴F)) and 418 nm (⁴A_2g(⁴F) → ⁴T_1g(⁴F)).

Fig. 8 PLE spectra of the studied doped perovskite monitored at (a) 697 and (b) 600 nm.

In contrast, the PLE spectrum monitored at λ_em = 600 nm reveals multiple excitation bands above 340 nm, arising from intra-configurational 4f–4f transitions of Sm³⁺.³⁴ These bands, detailed in Table 5, correspond to transitions from the ground state ⁶H_5/2 to various excited states (e.g., ⁴K_7/2, ⁴H_7/2, ⁶P_7/2, ⁴F_7/2, ⁶P_5/2, ⁴G_9/2, ⁴F_5/2, ⁴G_7/2, ⁴G_5/2).^35,36 Weak excitation peaks at 275 nm (⁸S_7/2 → ⁶I_11/2) and 318 nm (⁸S_7/2 → ⁶P_7/2) further confirm the occurrence of energy transfer from Gd³⁺ to Cr³⁺ ions.

Table 5 Excitation transitions of Gd_0.995Sm_0.005Al_0.995Cr_0.005O₃

Transition for Sm^3+ (ion)	Wavelength (nm)
^6H5/2 → ^4K7/2	346
^6H5/2 → ^4H7/2	363
^6H5/2 → ^6P7/2	377
^6H5/2 → ^4F7/2	405
^6H5/2 → ^6P5/2	418
^6H5/2 → ^4G9/2	441
^6H5/2 → ^4F5/2	464
^6H5/2 → ^4G7/2	480
^6H5/2 → ^4G5/2	561

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3.4.3 Energy transfer mechanism. The luminescence of Cr³⁺ can be further enhanced by the presence of lanthanides due to the energy transfer (ET) from Gd³⁺ and Sm³⁺ to Cr³⁺. The photoluminescence (PL) spectrum of GdAlO₃:Cr³⁺ and Sm³⁺ shows a decrease in the emission intensity of the Gd³⁺ ion at 697 nm and 720 nm (⁶P_J → ⁸S_7/2) compared to the emission intensity in the undoped GdAlO₃ spectrum (Fig. 7). We also notice that the red line at 726 nm, corresponding to the main luminescence ²E_g(²G) → ⁴A_2g(⁴F) from GdAlO₃:Cr³⁺ and Sm³⁺, is intense. This observation confirms the significant energy transfer from Gd³⁺ and Sm³⁺ ions to the Cr³⁺ dopant ions, which further enhances the luminescence of the Cr³⁺ ions. Energy transfer occurs from the Gd³⁺ emissions at 697 nm and 720 nm (⁶P_J → ⁸S_7/2) to the emission of Cr³⁺ ions. Additionally, energy transfer from Sm³⁺ emissions at 564 nm (⁴G_5/2 → ⁶H_5/2; green), 600 nm (⁴G_5/2 → ⁶H_7/2; yellow), 647 nm (⁴G_5/2 → ⁶H_9/2; orange), and 660 nm (⁴G_5/2 → ⁶H_11/2; red) further enhances the ²E_g(²G) → ⁴A_2g(⁴F) emission as shown in Fig. 9. The higher excited states ⁴T_1g(⁴P), ⁴T_1g(⁴F), ⁴T_2g(⁴F) relax non-radiatively to the lowest excited state ²E_g(²G) of Cr³⁺, which then undergoes radiative relaxation from the ²E_g(²G) state to the ground state ⁴A_2g(⁴F).

Fig. 9 Energy transfer between Gd³⁺, Sm³⁺ and Cr³⁺ ions.

3.4.4 Potential energy surface. In transition ion complexes, electronic transitions are essentially electric dipolar in nature. The d–d transitions for transition metal complexes are in principle prohibited by Laporte's rule.³⁷ Nevertheless, observation of these transitions can be explained by vibronic coupling (coupling between vibrational and electronic wave functions). For example, for an octahedral complex, it can be assumed that some of the vibrations distort the octahedron in such a way that the center of symmetry is destroyed. The states of the configuration dⁿ then no longer strictly preserve their character g and the transitions, normally prohibited by Laporte's rule, become “slightly permitted”.³⁷ Vibronic origins are mainly associated with odd-parity vibrational modes. Table 6 gives the selection rules for the Cr³⁺ ion occupying an O_h site in the studied doped perovskite. As shown in the table, the Cr³⁺ 3d–3d transitions observed in the PLE (Fig. 10) are governed by the vibronic mechanism that allow non-graded parity modes (a_1u, a_2u, t_1u, t_2u and e_u).

Table 6 Selection rules for electronic transitions of Cr³⁺ occupying an O_h site symmetry in the studied doped perovskite

d–d transitions	Electric dipole transition (odd vibrational mode that makes the transition permitted) 〈Γi|T1u|Γf〉Γu = A1g
^4A2g(^4F) → ^4T2g(^4F)	a1u, t1u, t2u, eu
^4A2g(^4F) → ^4T1g(^4F)	a2u, t1u, t2u, eu

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Fig. 10 Adiabatic potential energy curves for ²E_g(²G), ⁴T_2g(⁴F), ⁴T_1g(⁴F) and ⁴T_1g(⁴P) states.

To better understand the absorptions and emissions observed in the PLE and PL spectra of the studied compound, the coupled potential energy surface model was used. To simplify the model, a single normal coordination coordinate Q was considered. Due to the characteristics of the electronic transition, which is an intra-configuration d–d excitation, the potential energy minima of the doublet ²E_g(²G) and fundamental ⁴A_2g(⁴F) states are considered to be at the same configuration (Q = 0 Å) and have the same vibrational frequency. As the metal–ligand antibonding molecular orbitals are populated by d–d excitation, the minimum potential energy of the states ⁴T_2g(⁴F), ⁴T_1g(⁴F) and ⁴T_1g(⁴P) is shifted by the quantity ΔQ relative to the minimum of the ²E_g(²G) state.

The potentials for the excited states ²E_g(²G), ⁴T_2g(⁴F), ⁴T_1g(⁴F) and ⁴T_1g(⁴P) are as follows:

V(²E_g(²G)) = 1/2(kQ²) + E_ZPL(²E_g(²G)) (1a)

V(⁴T_2g(⁴F)) = 1/2(k(Q − ΔQ₁)²) + E_ZPL(⁴T_2g(⁴F)) (1b)

V(⁴T_1g(⁴F)) = 1/2(k(Q − ΔQ₁)²) + E_ZPL(⁴T_1g(⁴F)) (1c)

V(⁴T_1g(⁴P)) = 1/2(k(Q − ΔQ₁)²) + E_ZPL(⁴T_1g(⁴P)) (1d)

where E_ZPL(²E_g(²G)), E_ZPL(⁴T_2g(⁴F)), E_ZPL(⁴T_1g(⁴F)), and E_ZPL(⁴T_1g(⁴P)) are the energies of the minima for the ²E_g(²G), ⁴T_2g(⁴F), ⁴T_1g(⁴F) and ⁴T_1g(⁴P) states, respectively, and k is the Raman frequency of GdAlO₃ assigned to the ground and excited states. The adiabatic potential state surfaces, obtained from eqn (1) are depicted in Fig. 10.

3.4.5 Crystal field analysis of Cr³⁺ in GdAlO₃ and relevance to PLE and PL spectra. The electronic structure of Cr³⁺ in GdAlO₃ can be ascertained by employing the Crystal Field Theory (CFT). In this work, we expand on previous studies by providing a more detailed analysis of the crystal field effects and their influence on the photoluminescence (PL) behavior of Cr³⁺ in this host material. The Cr³⁺ ion energy levels are assessed using a complete Hamiltonian, and the crystal field strength (D_q), as well as the Racah parameters (B and C), are found by analyzing the optical PLE and PL spectra.

The Cr³⁺ ion energy levels are assessed using the complete Hamiltonian:^38–50

H = H₀ + H_ee(B, C) + H_Trees(α_Trees) + H_CF(D_q) + H_SO(ζ) (2)

where H₀ is the configuration Hamiltonian, H_ee is the electron–electron repulsion Hamiltonian,^51–53 H_SO is the spin–orbit coupling Hamiltonian, the H_Trees Hamiltonian included the Trees correction,^54,55 and H_CF is the crystal field Hamiltonian for octahedral symmetry (O_h):^56–58In eqn (2) and (3) the parameter D_q represents the ligand field splitting, while numerical techniques are used to specifically calculate the matrix elements for the C_q(k) operators, as described in ref. 59. The α_Trees and ζ parameters are calculated using the following equations:^54–56

α_Trees = N⁴α₀

and

ζ = N²ζ₀ (4)

with:where, N is the reduction factor.⁶⁰

We derived a stable crystal-field and Racah parameters for Cr³⁺ ions, which allow to calculate the electronic energy levels of Cr³⁺(3d³) ions in the O_h symmetry site of a Cr³⁺ doped GdAlO₃ perovskite (GdAlO₃:Cr³⁺). The free ion Cr³⁺ parameters B₀, C₀, α₀ and ζ₀ are listed in Table 7.^41–43 The crystal-field strength parameter D_q was computed using the energy of the quartet excited state ⁴T_2g(⁴F), and the Racah parameters B and C were calculated based on the observed excited states of the ⁴T_1g(⁴F) quartet and the ²E_g(²G) doublet, decreasing in relation to their values for the free ion due to the covalency effect. The values of α and ζ were obtained using eqn (3)–(5). The calculations were done using a custom code that was developed in our laboratory, by S. Kammoun, using the Maple 2024 software. The calculated energies (Table 8) are in excellent agreement with the experimental findings of Yeung et al.,⁶¹ thus validating our computational method.

Table 7 Optical parameter values for the theoretical study of GdAlO₃:Cr³⁺

Parameter	Value [this study]	Parameter	Value^43,51
Dq (cm^−1)	1773	B0 (cm^−1)	918
B (cm^−1)	617	C0 (cm^−1)	4133
C (cm^−1)	3045	α0	30
α (cm^−1)	21.12	ζ0	275
ζ (cm^−1)	230.75
Dq/B	2.87
C/B	4.94

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Table 8 Experimental and calculated energies (cm⁻¹) of GdAlO₃:Cr³⁺

Oh	Eobs
^4A2g(^4F)	0	0	0
^2Eg(^2G)	—	13 767	13 896 (4)
^2T1g(^2G)		14 282	14 404 (4)
			14 461 (2)
^4T2g(^4F)	17 730	17 730	17 637 (2)
			17 692 (4)
			17 802 (2)
			17 806 (4)
^2T2g(^2G)	—	20 897	20 966 (4)
			21 072 (2)
^4T1g(^4F)	23 923	24 097	23 988 (4)
			23 997 (2)
			24 002 (2)
			24 006 (4)
^2A1g(^2G)	—	29 333	29 514 (2)
^2T1g(^2P)	—	31 342	31 541 (2)
			31 587 (4)
^2T1g(^2H)	—	31 653	31 744 (2)
			31 918 (4)
^2Eg(^2H)	—	33 266	33 397 (4)
^2T1g(^2H)	—	36 101	36 161 (2)
			36 196 (4)
^4T1g(^4P)	—	38 348	38 208 (2)
			38 223 (4)
			38 326 (4)
			38 361 (2)
^2T2g(^2H)	—	40 349	40 252 (2)
			40 355 (4)
^2A2g(^2F)	—	41 673	41 683 (2)
^2T2g(^2aD)	—	49 008	49 258 (2)
			49 369 (4)
^2T2g(^2F)	—	50 418	50 335 (4)
			50 455 (2)
^2Eg(^2aD)	—	50 334	50 564 (4)
^2T1g(^2F)	—	54 944	54 841 (2)
			54 966 (4)
^2T2g(^2bD)	—	68 763	68 605 (4)
			68 804 (2)
^2Eg(^2bD)	—	72 704	72 761 (4)

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Electrons are usually transferred from one atom to another during ionic bonding, producing positively and negatively charged ions. Eqn (6) allows to evaluate the type of bonding for Cr³⁺ in GdAlO₃:Cr³⁺.⁶² In this equation, h is a measure of the degree of electron delocalization between the cation to the ligands. For the considered system, the constant k is equal to 0.21.^63,64

The calculated large value for h (1.44) indicates a high degree of d-electron delocalization, revealing the high ionic character of the bonds established between Cr³⁺ ions and their ligands.

The basic optical characteristics of 3dⁿ transition-metal ions doped into crystalline hosts can be explained by the crystal-field model. The Tanabe–Sugano energy-level diagram presented in Fig. 11 shows how the ratio D_q/B affects the electronic energy levels of 3dⁿ trivalent ionic species in the octahedral site of the GdAlO₃ host. The vertical line in Fig. 11 for D_q/B = 2.87 corresponds to Cr³⁺ doped into the GdAlO₃ perovskite in an octahedral site symmetry.

Fig. 11 Tanabe–Sugano energy-levels diagram highlighting the case of Cr³⁺(3d³) ion doped into the octahedral symmetry site of GdAlO₃, as expressed by the vertical line for D_q/B = 2.87.

The theoretical study for determining the electronic structure of Cr³⁺ ion in GdAlO₃:Cr,Sm using crystal field theory confirms that this ion replaces Al³⁺ at an octahedral site. Indeed, there is a satisfactory agreement between the experimental and theoretical levels, considering that Cr³⁺ replaces Al³⁺ at an octahedral site. The importance of the theoretical study using crystal field theory lies in the fact that it confirms the oxidation state and the site occupied by the Cr³⁺ ion. Also, it is only through this theoretical study that the d–d transitions can be accurately assigned, as these transitions vary for the same ion depending on the matrix (intermediate crystal field).

4. Conclusion

In conclusion, the orthorhombic Gd_0.995Sm_0.005Al_0.995Cr_0.005O₃ sample, synthesized through the solid-state reaction method, underwent comprehensive investigation of its structure, morphology, and photoluminescence properties. The material crystallized in the orthorhombic Pbnm space group, with Cr³⁺ ions substituting for Al³⁺ ions in octahedral site symmetry within the GdAlO₃:Sm³⁺, Cr³⁺ matrix. Examination via Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray analysis confirmed the morphology and homogeneity of the sample's chemical composition. Photoluminescence excitation (PLE) and photoluminescence (PL) spectra revealed the electronic structure of Cr³⁺ ions in the O_h symmetry site, with theoretical studies allowing for precise attribution of their 3d–3d transitions. Additionally, intra-configurational 4f–4f transitions of Sm³⁺ resulted in various excitation bands observed in the PLE spectrum at higher wavelengths. Moreover, the photoluminescence studies give additional support to the occurrence of energy transfer among Gd³⁺, Sm³⁺, and Cr³⁺ ions. Based on these findings, the synthesized nanophosphor samples demonstrate strong potential for applications in solid-state lighting.

Data availability

Data are available upon request to the authors.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The CQC-IMS is supported by FCT through projects UI0313B/QUI/2020, UI0313P/QUI/2020 and LA/P/0056/2020 (national funds). This work was also supported by FCT – Fundação para a Ciência e Tecnologia, I. P. through the projects UIDB/04564/2020 and UIDP/04564/2020, with DOI identifiers 10.54499/UIDB/04564/2020 and 10.54499/UIDP/04564/2020, respectively. Access to TAIL-UC facility funded under QREN-Mais Centro Project No. ICT_2009_02_012_1890 is gratefully acknowledged.

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