Sol-Gel Combustion Synthesis, Crystal Structure and Luminescence of Cr3+ and Mn4+ Ions in Nanocrystalline SrAl4O7

Stadnik, Vitalii; Hreb, Vasyl; Luchechko, Andriy; Zhydachevskyy, Yaroslav; Suchocki, Andrzej; Vasylechko, Leonid

doi:10.3390/inorganics9120089

Open AccessArticle

Sol-Gel Combustion Synthesis, Crystal Structure and Luminescence of Cr³⁺ and Mn⁴⁺ Ions in Nanocrystalline SrAl₄O₇

¹

Department of Physical, Analytical and General Chemistry, Lviv Polytechnic National University, 12 Bandera Str., 79013 Lviv, Ukraine

²

Semiconductor Electronics Department, Lviv Polytechnic National University, 12 Bandera Str., 79013 Lviv, Ukraine

³

Faculty of Electronics and Computer Technologies, Ivan Franko National University of Lviv, 1 Universytetska Str., 79000 Lviv, Ukraine

⁴

Institute of Physics, Polish Academy of Sciences, Al. Lotników 32/46, 02-668 Warsaw, Poland

^*

Authors to whom correspondence should be addressed.

Inorganics 2021, 9(12), 89; https://doi.org/10.3390/inorganics9120089

Submission received: 24 November 2021 / Revised: 11 December 2021 / Accepted: 13 December 2021 / Published: 16 December 2021

(This article belongs to the Section Inorganic Solid-State Chemistry)

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Abstract

:

A series of strontium dialuminate SrAl₄O₇ nanopowders with the grossite-type structure doped with chromium and manganese ions were synthesized by the combined sol–gel solution combustion method with use of two different strontium salts. The Cr³⁺ and Mn⁴⁺ ions concentrations were varied from 0.05 to 5 at.%. Evolution of phase composition, crystal structure, and microstructural parameters of the nanocrystalline materials depending on the synthesis conditions, temperature of thermal treatment, and dopant content were investigated by the X-ray powder diffraction and the scanning electron microscopy techniques. Photoluminescent properties of SrAl₄O₇ nanophosphors activated with Cr³⁺ and Mn⁴⁺ ions were studied at room temperature. The samples exhibit typical photoluminescence in the deep-red spectral region, corresponding to d-d transitions in Cr³⁺ or Mn⁴⁺ ions. The intensity of this deep-red emission is dependent on the dopant concentration and annealing temperature. Features of the formation of octahedral surroundings around Cr³⁺ or Mn⁴⁺ ions are discussed.

Keywords:

strontium dialuminate; Cr- and Mn-doping; combustion synthesis; crystal structure; luminescence

Graphical Abstract

1. Introduction

Aluminates (MAl₄O₇) and gallates (MGa₄O₇) of alkaline earth elements (M = Ca, Sr, Ba) with the monoclinic grossite-type structure are promising for applications in white light sources (WLEDs), fluorescent lamps, optical amplifiers, radiant displays, solid-state lasers, radiation dosimetry, sensors, and radiographs. Recently, a number of publications have appeared on the synthesis of MAl₄O₇-based materials doped with rare-earth elements (Eu, Dy, Nd, Sm), bismuth, as well as some transition elements such as Cr, Mn, and Cu [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15]. Phosphors activated with rare-earth (RE) ions have attracted great attention since they show high quantum efficiency, long persistence of phosphorescence, and suitable emitting color. On the other hand, phosphors doped with transition metal ions, particularly Mn⁴⁺ or Cr³⁺, which have broadband excitation in the UV and visible regions of the spectrum and deep red radiation can be an alternative to phosphors activated by RE ions.

Recently, there has been a growing interest in such inexpensive phosphors (without RE ions) for the deep-red and near-IR emission range. Red light with a wavelength in the range of 650–800 nm can control the growth process of plants by regulating the ratio of phytochrome [16]. Thus the aluminates of alkaline earth elements MAl₄O₇ with the grossite structure, doped with suitable activators emitting in the red region of the spectrum, can find a potential application for the manufacturing of LEDs for greenhouses illumination.

Luminescent studies of MAl₄O₇ and MGa₄O₇-based materials doped with Cr³⁺ or Mn⁴⁺ indicate that these activators are in an octahedral surrounding, while the structure of the host materials does not have intrinsic octahedral sites [1,6]. The mechanism of octahedron formation around these activators in the grossite-type structures is not fully understood.

This work reports the results of precise structural investigations of Cr³⁺ and Mn⁴⁺ doped SrAl₄O₇ nanopowders synthesized by the sol–gel combustion route and the influence of the dopant concentrations and heat treatment temperature on their photoluminescent properties with the purpose to create new phosphors emitting in a deep red spectral region.

2. Results and Discussion

2.1. Evolution of Phase Composition, Crystal Structure, and Microstructural Parameters

Initially, the synthesis of SrAl₄O₇:Cr³⁺ was performed by using Sr(NO₃)₂·4H₂O as a strontium source. X-ray diffraction (XRD) examination of the as-obtained foamy product after the auto-combustion process at 800 °C revealed the formation of X-ray amorphous material (Figure 1, top panel).

Crystallization of SrAl₄O₇ occurs after extra annealing at 900 °C for 4 h. However, besides the desired grossite-type phase, the parasitic SrAl₂O₄ phase with a monoclinic structure is formed (Figure 1). Further heat treatment of the powders at 1000, 1100, 1200, 1300, and 1400 °C does not improve the phase composition of the materials. In contrast, the amount of secondary SrAl₂O₄ phase increased from 35–37 wt.% up to 43% after final firing at 1400 °C, as it was estimated from experimental XRD patterns by full quantitative profile Rietveld refinement (see Figure 2 as an example). Evidently, appearing of parasitic SrAl₂O₄ phase with higher Sr/Al ratio as in SrAl₄O₇ could be explained by deviation of Sr(NO₃)₂·4H₂O tetrahydroxonitrate precursor used from a stoichiometric composition.

Indeed, further usage of extra pure grade SrCO₃ as a strontium source for the next synthesis of two series of chromium and manganese doped SrAl₄O₇ materials allow obtaining almost single-phase products with the grossite structure. Both series of SrAl₄O₇:xCr (x = 0.0005, 0.005, 0.01, 0.02, 0.03, 0.04, 0.05) and SrAl₄O₇:xMn (x = 0.0005, 0.005, 0.05) powders were synthesized in order to study the effect of the concentration and type of activator ion on the luminescence properties of the phosphors. The chromium concentrations were chosen for easier comparison with the corresponding crystalline phosphors based on the isostructural strontium digallate SrGa₄O₇:xCr, recently described in [1]. XRD examination of new series of solution combustion synthesized SrAl₄O₇:xCr and SrAl₄O₇:xMn powders after their heat treatment at 1100 °C for 4 h revealed almost pure grossite-type structure for all dopant concentrations; only traces of foreign phases could be detected on some patterns (Figure 3).

The number of parasitic phases increased after extra annealing of the SrAl₄O₇:xCr³⁺ powders at 1200 °C. In the case of SrAl₄O₇:xMn⁴⁺ series, this effect is much more pronounced and the amount of the extraneous phases riches 20–25%. This observation is not surprising, because SrAl₄O₇ becomes metastable when the temperature is raised beyond 1100 °C [17].

Morphology of SrAl₄O₇:xCr powders, chemical composition, and the homogeneity of the element’s distribution was selectively controlled by scanning electron microscopy (SEM) and local X-ray spectral analysis on an example of the sample with x = 0.05 synthesized through solution combustion route and heat-treated at 1200 °C (Figure 4). SEM micrographs revealed a fluffy platelet-like structure of cirrus morphology with non-uniform distribution of crystallites. Nominal chemical composition and homogeneous distribution of elements through the powder analyzed was confirmed (Figure 4).

Microstructural parameters of the SrAl₄O₇:xCr powders, such as average crystallite size D_ave and microstrain values <ε> were evaluated from XRD patterns by full profile Rietveld fitting. It was established that D_ave varies in the limits of 65–110 nm independent of the activator concentration and the temperature of heat treatment (Figure 5, upper part). Evaluated lattice strain values <ε> show a minor tendency to a decreasing behavior with increasing x in SrAl₄O₇:xCr series (Figure 5, bottom part). One could also observe that the increase in the heat treatment temperature from 1100 to 1200 °C led to a detectable decrease of the microstress values for all concentrations of dopant.

A full profile Rietveld refinement was also used for the evaluation of precise structural parameters of both SrAl₄O₇:Cr³⁺ and SrAl₄O₇:Mn⁴⁺ series. As the initial model for the refinement of the chromium- and manganese-doped SrAl₄O₇-based structures, atomic coordinates in nominally pure SrAl₄O₇ (CaO·2Al₂O₃) compound, derived from single-crystal data in Ref. [18] were used. In the refinement procedure, the lattice parameters, coordinates, and displacement parameters of all atoms were refined together with profile parameters and corrections for absorption and instrumental sample shift. In all cases, an excellent agreement between the calculated and experimental diffraction profiles was reached. Exemplary results of the Rietveld refinement of SrAl₄O₇:Cr³⁺ and SrAl₄O₇:Mn⁴⁺ structures are presented in Figure 6.

Obtained structural parameters for SrAl₄O₇:Cr³⁺ and SrAl₄O₇:Mn⁴⁺ powders, annealed at 1100 °C and corresponding interatomic distances calculated from these data are presented in Table 1 and Table 2, respectively.

Detailed analysis of obtained structural parameters of the SrAl₄O₇:xCr materials and their concentration-dependence on the chromium content revealed detectable deviations from the monotonic behavior in both series of the powders heat-treated at 1100 and 1200 °C (Figure 7). The discontinuations in the SrAl₄O₇:xCr series are manifested as detectable minima in all lattice parameters and a clear jump in the monoclinic angle at the value of the compositional parameter x = 0.02. As a result, clear minima in the unit cell volume of SrAl₄O₇:xCr structure is observed. Up to now, we have no explanation of such deviations in structural parameters observed for SrAl₄O₇:0.02Cr composition.

The crystal structure of SrAl₄O₇ can be represented as a framework of edge-shared SrO₇ polyhedra, which are connected with two types of apex-shared AlO₄ tetrahedra (see Figure 8 and insets on Figure 6).

In the SrAl₄O₇ structure, the Sr atoms are located in 4-fold positions (Table 1) and are surrounded by seven oxygen atoms located at the distances 2.53–2.83 Å (Table 2) and forming pentagonal bipyramids around the central atoms (see Figure 8). The next coordination sphere is formed with eight Al atoms and two oxygens located at the distances 3.26–3.76 Å (Table 2). Aluminum atoms in the SrAl₄O₇ structure are located in two non-equivalent 8-fold positions (Table 1), which are characterized by a tetrahedral environment (Figure 8) formed by four oxygen atoms located at the distanced 1.66–1.80 Å and 1.73–1.80 Å from Al(1) and Al(2) atoms, respectively (Table 2). In spite of there being no octahedral positions in the SrAl₄O₇ structure, further investigations of luminescence characteristics of SrAl₄O₇:Cr³⁺ and SrAl₄O₇:Mn⁴⁺ series (see next section) revealed clear features of octahedrally coordinated Cr³⁺ and Mn⁴⁺ ions. Similar observation (luminescence of transition metal ions in octahedral positions) was recently reported for the phosphors based on SrAl₄O₇ and the related isostructural compounds CaAl₄O₇ and SrGa₄O₇ [1,6,7]. It is necessary to note that there is no consensus in the literature on the scenario of cation substitution in the strontium and calcium aluminates and gallates (Sr/Ca)(Al/Ga)₄O₇ with transition metal ions. The authors [6] reported that red luminescence of CaAl₄O₇:Mn phosphors comes from distorted octahedral Mn⁴⁺ sites due to substitution for Ca sites in the CaAl₄O₇ structure. In contrast, Liu et al. [7] postulated that because the Mn⁴⁺ ionic radius is closely equal to that of Al³⁺, it is expected that Mn⁴⁺ ions can readily replace Al³⁺ ions, and are unlikely substituted for larger Ca²⁺ ions in the CaAl₄O₇ compound. Similarly, Lai et al. [1] suggested that red emission in SrGa₄O₇:Cr phosphors arises from Cr³⁺ ions in the distorted octahedra, which are generated by the transition from the GaO₄ tetrahedra to the octahedra, as Cr³⁺ ions replaced Ga³⁺ ions in SrGa₄O₇ structure.

Our analysis of the structural peculiarities of the grossite-type MAl₄O₇ and MGa₄O₇ host materials [19], as well as SrAl₄O₇:Cr³⁺ and SrAl₄O₇:Mn⁴⁺ series investigated (see, e.g., interatomic distances distribution around the Sr²⁺ and Al³⁺ cations in corresponding structures presented in Table 2) favor a first above-mentioned scenario of cation substitution, namely, replacing with transition metal ions for the Sr sites. Indeed, in the case of Cr³⁺/Mn⁴⁺ substitution for Sr²⁺ ions, which are surrounded with 7 oxygens located at the distances 2.53–2.83 Å (Figure 8, Table 2), the transition metal ions can easily coordinate around themselves six near oxygen atoms to form CrO₆ or MnO₆ octahedra. In contrast, it is difficult to imagine the entry of Cr³⁺ or Mn⁴⁺ ions into both tetrahedral positions of Al³⁺ ions in the SrAl₄O₇ structure and their subsequent conversion into the octahedral ones, given the great distance to the next two oxygen atoms located at 3.44–3.38 Å from Al1 atoms and at 2.90 Å and 3.16 Å from Al2 atoms (see Table 2), which are required for octahedra formation. Unfortunately, very low concentrations of Cr³⁺ and Mn⁴⁺ ions in the materials studied do not allow refining their positions in SrAl₄O₇ structure from X-ray powder diffraction data. To shed light on the mechanism of transition of metal cation substitution in MAl₄O₇ and MGa₄O₇ materials, a precise investigation of local structures is required.

2.2. Luminescent Properties

Characteristic photoluminescence excitation and emission spectra of SrAl₄O₇:0.0005Cr³⁺ annealed at 1300 °C are shown in Figure 9. The luminescence excitation spectra show excitation in the wavelength range from 250 to 650 nm, with three intense maxima at approximately 330, 420, and 560 nm. Two broad maxima in the visible spectral region corresponded to the intracenter 4A₂→4T1(⁴F) and ⁴A₂→⁴T₂(⁴F) transitions in Cr³⁺ ions.

The UV excitation band at about 330 nm (see Figure 9a) is most probably associated with a charge-transfer O^2–→Cr³⁺ or/and with ⁴A₂→⁴T₁ (⁴P) transitions. The photoluminescence is observed in the spectral region of 640–760 nm at room temperature. The dominant narrow luminescence lines (R-lines) observed in SrAl₄O₇:Cr at about 691.7 and 693.2 nm at room temperature corresponds to the transitions from the split (3.9 meV) metastable ²E level to the ⁴A₂ ground state and are inherent for the Cr³⁺ ions in an octahedral environment. In the isostructural SrGa₄O₇ compound [1], similar luminescent features were also shown to arise from Cr³⁺ ions in the distorted octahedra. It should be noted that such excitation and emission bands are typical for Cr³⁺ ions in strong crystal field strength of the octahedral surrounding for different oxide materials (see [20,21,22], for instance).

The photoluminescence efficiency of phosphors can be improved by changing the annealing temperature and/or the concentration of the doping impurity. That is why these parameters are essential in determining the optimal conditions for obtaining suitable phosphors.

The concentration dependence of integral photoluminescence intensity of SrAl₄O₇ powders for different levels of Cr³⁺ doping in the range of x = 0.0005–0.05 after samples annealing at 1200 °C for 4 h are presented in the inset of Figure 9b. It was established that with the increasing of Cr³⁺ ions content, there were no significant changes in the shape of the luminescence spectra. However, the integral photoluminescence intensity increases until the concentration of Cr³⁺ ions reaches 0.01. Further, the intensity begins to sharply decrease with an increase in the Cr³⁺ concentration. Probably, this behavior is coupled with the above-mentioned discontinuations in concentration dependencies of structural parameters observed in the SrAl₄O₇:xCr series between x = 0.01 and 0.02 (see Figure 7). The decrease in the photoluminescence intensity at a concentration of Cr³⁺ ions over 0.01 is due to concentration quenching, which is explained by the processes of cross-relaxation between neighboring Cr³⁺ ions, i.e., with increasing Cr³⁺ content, the distance between adjacent Cr³⁺ ions decreases. Therefore, there is a more effective non-radiative energy transfer, which leads to a decrease in the photoluminescence intensity with the Cr³⁺ dopant concentration increasing.

The integral photoluminescence intensity in the spectral region 640–800 nm depending on the annealing temperature during 4 h was investigated for SrAl₄O₇:0.0005Cr³⁺ powder to establish the optimal annealing temperature of powders. The intensity of photoluminescence increases with increasing annealing temperature, in the range 1200–1400 °C. In particular, a change in calcination temperature of samples from 1200 to 1300 °C leads to a ~6 times increase in emission intensity. For the sample annealed at 1400 °C, the emission is more than an order higher than for the sample annealed at 1300 °C. We can assume that at higher temperatures, the intensity of photoluminescence increases significantly due to the improving crystallinity of SrAl₄O₇ and/or increase in the concentration of Cr³⁺ ions in a regular octahedral surrounding.

The SrAl₄O₇:Mn⁴⁺ samples also show red photoluminescence, but it is slightly shifted towards higher energies relative to the luminescence of the Cr³⁺ activator (Figure 10). In the case of SrAl₄O₇:0.0005Mn⁴⁺ under excitation with UV or blue lights, all the samples exhibit typical Mn⁴⁺ red photoluminescence (Figure 10a). The excitation spectrum of this luminescence consists of two broad peaks at 313 and 455 nm in the spectral range between 250 to 500 nm. The intensity of the blue peak is at about 20% of the UV peak intensity. The photoluminescence spectrum of Mn⁴⁺ (Figure 10b) extends from 600 to 750 nm with a sharp maximum at about 653 nm, attributed to ²E→⁴A₂ transitions in Mn⁴⁺ ions. It should be noted that only Mn⁴⁺ in an octahedral surrounding can emit such a spectrum in the red spectral range. As was shown for the CaAl₄O₇, when Mn⁴⁺ replaces Ca(1), the local field is distorted due to increased Coulomb interaction, and it turns into a typical octahedral field eventually [6].

It should be noted that Mn⁴⁺ ions, as well as Cr³⁺ ions, have an electronic configuration of 3d³. The emission bands on both sides of the narrow intense maximum at 653 nm correspond to the anti-Stokes and Stokes sidebands, which are caused by oscillations of 3d³ electrons when the activator ions are located in the octahedral positions of the structure.

3. Materials and Methods

3.1. Samples Preparation

A combined sol–gel solution combustion method was used to synthesize a series of nanocrystalline SrAl₄O₇ powders doped with Cr³⁺ and Mn⁴⁺ ions. In this method, the following substances were used: Sr(NO₃)₂·4H₂O or SrCO₃ (Sfera Sim, Lviv, Ukraine), Al(NO₃)₃·9H₂O (Alfa Aesar, Haverhill, MA, USA), Cr(NO₃)₃·9H₂O (Alfa Aesar, Haverhill, MA, USA), and Mn(NO₃)₂·4H₂O (Sfera Sim, Lviv, Ukraine) as metal sources, citric acid C₆H₈O₇ (CA) and ethylene glycol (EG) (Sfera Sim, Lviv, Ukraine) as a chelating agent and fuel. A 25% NH₄OH (Sfera Sim, Lviv, Ukraine) solution was used to achieve the desired pH of the solutions. All substances were analytical grade of purity. Molar ratio of sum of metal cations, CA and EG was kept as follow: n(Ʃ(Meⁿ⁺): n(CA): n(EG) = 1:2:8. The concentration of Cr or Mn dopant varied from 0.05 to 5.0 at.%. The amounts of reagents were calculated according to their nominal composition, considering the desired mass of final products (typically from 0.75 to 2.0 g) and mixed in ceramic vessels with a volume of up to 200 mL. Obtained mixtures of components with a minimal amount of distilled water were placed in a preheated to 80–90 °C oven for 30–60 min to ensure complete dissolution of the components, followed by neutralization of the resulting solutions to pH 7 in the case of Cr-doped series or by achieving pH 9.5 for Mn-doped samples. After correcting the pH, the solutions were placed again in an oven heated to 125–150 °C for 2–4 h to remove residual water and ensure completeness of the esterification process. Visually, the completion of the process was monitored by the appearance of the gel, which from transparent began to acquire a brownish-black color. After finishing of gelation process, the plates with the formed gel were cooled and placed in a muffle furnace, preheated to the temperature of 800 °C necessary to initiate the processes of components decomposition and spontaneous combustion of the mixture. Due to the rapid thermal decomposition and self-combustion of the components of the reaction mixture, the formation of a bulky foamy product (Figure 11, inset) was observed.

After furnace cooling, the obtained foamy products were ground in a porcelain mortar and subsequently annealed at the temperatures from 900 to 1400 °C to study the effect of heat treatment temperature on the phase composition, crystal structure, morphology, and luminescent characteristics of the materials prepared.

3.2. Physical–Chemical Characterization

The evolution of phase composition, crystal structure, and microstructural parameters of nanophosphors within the synthesis process and further thermal treatment was investigated by X-ray powder diffraction using a modified DRON-3M powder diffractometer (Burevesnik, Leningrad, former USSR). XRD patterns were collected by using Cu Kα-radiation (λ = 1.54456 Å) in the 2θ range of 10–110° with the step of 0.02° Crystal structure parameters of SrAl₄O₇:Cr³⁺ and SrAl₄O₇:Mn⁴⁺ materials were derived from experimental XRD data by full profile Rietveld refinement by using WinCSD program package [23]. The same method was used for the evaluation of average crystallite sizes and microstrain values from an analysis of angular dependence of Bragg’s diffraction maxima. For a correction on instrumental peak broadening LaB₆ external standard was used. Scanning electron microscopy (Tescan Vega 3 LMU, Brno, Czech Republic) and X-ray spectral analysis (Aztec ONE with X-MaxN 20 detector, Oxford Instruments, Abingdon, UK) were used for spot-check examination of the morphology of the powders, chemical composition, and homogeneity of cation distribution in the materials synthesized.

Photoluminescence and photoluminescence excitation spectra were measured using a Horiba/Jobin-Yvon Fluorolog-3 spectrofluorometer (HORIBA Instruments, Edison, NJ, USA) with a 450 W continuous spectrum xenon lamp for excitation and optical detection with a Hamamatsu R928P photomultiplier operating in the photon counting mode. The measured photoluminescence excitation spectra were corrected with the xenon lamp emission spectrum. The photoluminescence spectra were corrected for the spectral response of the spectrometer system used.

4. Conclusions

Series of Cr³⁺ and Mn⁴⁺ doped strontium dialuminate SrAl₄O₇ nanophosphors with an average crystallite size of 65–115 nm were synthesized for the first time by the combined sol–gel solution combustion method. Precise structural characterization of the SrAl₄O₇:Cr³⁺ and SrAl₄O₇:Mn⁴⁺ materials with different dopant concentrations heat-treated at 1100 °C and 1200 °C revealed monoclinic grossite-type structure.

After visible and UV excitations, SrAl₄O₇ phosphors exhibit an intense deep-red and NIR emission. This strong red emission is peaked at about 695 nm in Cr³⁺ and at about 653 nm in Mn⁴⁺ doped phosphors is due to ²E→⁴A₂ transitions. This luminescence occurs from distorted octahedral Cr³⁺/Mn⁴⁺ sites, which arise due to substitution for Sr sites in the strontium aluminate. The latter suggestion was made based on an analysis of peculiarities of the interatomic distances distribution in the host SrAl₄O₇ material and the related MAl₄O₇ and MGa₄O₇ compounds.

It was found that the optimal concentration of impurities Cr³⁺ and Mn⁴⁺ is close to 0.01 and 0.0005, respectively. Despite the fact that the crystallinity of the structure is formed after annealing of samples at 1000 °C, the maximum of manganese and chromium ions photoluminescence intensity is observed only after annealing at 1400 °C, which may indicate the easier formation of octahedra around these transition ions at higher temperatures.

Author Contributions

Conceptualization, L.V. and Y.Z.; validation, L.V. and A.S.; investigation, V.S., V.H., A.L. and Y.Z.; writing—original draft preparation, V.S.; writing—review and editing, L.V., Y.Z. and A.S.; visualization, V.S. and V.H.; supervision, L.V.; project administration, L.V.; funding acquisition, L.V. and Y.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Research Foundation of Ukraine, grant no. 2020.02/0373, and by Polish National Science Centre, projects no. 2018/31/B/ST8/00774 and 2019/33/B/ST8/02142.

Acknowledgments

V.S., V.H., A.L. and L.V. acknowledge the support of the National Research Foundation of Ukraine under grant no. 2020.02/0373 “Crystalline phosphors’ engineering for biomedical applications, energy-saving lighting and contactless thermometry”. Y.Z. and A.S. acknowledge the support of the Polish National Science Centre (projects no. 2018/31/B/ST8/00774 and 2019/33/B/ST8/02142).

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. The evolution of XRD patterns of SrAl₄O₇:Cr³⁺ powders, successively annealed at different temperatures. Arrows indicate extra peaks of the parasitic SrAl₂O₄ phase.

Figure 2. Graphical results of simultaneous two-phase Rietveld refinement illustrating coexistence of two monoclinic phases—SrAl₄O₇ (blue) and SrAl₂O₄ (red) in the SrAl₄O₇:Cr³⁺ sample, annealed at 1400 °C.

Figure 3. Experimental XRD patterns of SrAl₄O₇:xCr³⁺ and SrAl₄O₇:xMn⁴⁺ powders with different chromium and manganese content, annealed at 1100 °C. Arrows indicate the traces of the foreign phase(s).

Figure 4. SEM images, results of local X-ray spectral analysis and elements distribution in the SrAl₄O₇:0.05Cr powder heat-treated at 1200 °C.

Figure 5. Average crystallite size D_ave and lattice strain values <ε> of SrAl₄O₇:Cr³⁺ powders, depending on the Cr³⁺ content and annealing temperature. The dashed B-Spline lines are a visual guide.

Figure 6. Graphical results of Rietveld refinement of SrAl₄O₇:Cr³⁺ (top) and SrAl₄O₇:Mn⁴⁺ (bottom) structures with x = 0.005. Experimental XRD patterns (black circles) are shown in comparison with calculated ones. Short vertical bars indicate positions of Bragg’s maxima in grossite-type C2/c structure. Insets show polyhedral views of SrAl₄O₇ structure in two different projections. The a, b, and c indicate corresponding crystallographic axes.

Figure 7. Dependencies of the lattice parameters and volume of the SrAl₄O₇:xCr³⁺ unit cell on nominal content of chromium in samples.

Figure 8. Polyhedral representation of SrAl₄O₇ structure. Two types of AlO₄ tetrahedra (blue Al1O₄ and pink Al2O₄ ones) and SrO₇ polyhedra (yellow) are shown. The a, b, and c indicate corresponding crystallographic axes.

Figure 9. The luminescence excitation (a) and photoluminescence (b) spectra of Cr³⁺ ions in SrAl₄O₇:0.0005Cr³⁺ powders. The inset of (b) presents the concentration dependence of Cr³⁺ integral photoluminescence.

Figure 10. The luminescence excitation (a) and photoluminescence (b) spectra of Mn⁴⁺ ions in SrAl₄O₇:0.0005Mn⁴⁺ powders. The inset of (b) presents the concentration dependence of Mn⁴⁺ integral photoluminescence.

Figure 11. Technological scheme of sol–gel combustion synthesis of Cr³⁺ and Mn⁴⁺ doped SrAl₄O₇. Inset shows self-combustion products of precursors for SrAl₄O₇:xMn materials with x = 0.0005, 0.005, and 0.05.

Table 1. Lattice parameters, coordinates, and displacement parameters of atoms in SrAl₄O₇:xCr³⁺ and SrAl₄O₇:xMn⁴⁺ (x = 0.005) materials (SG C2/c, Z = 4) after heat treatment at 1100 °C.

Lattice Parameters, Residuals	Atom, Sites	x/a	y/b	z/c	B_iso/eq, Å²
SrAl₄O₇:xCr³⁺
a = 13.0462(9) Å b = 9.0221(7) Å c = 5.5439(4) Å β = 106.209(2)° V = 626.6(2) Å³	Sr, 4e	0	0.8113(4)	1/4	1.00(6)
	Al1, 8f	0.1663(6)	0.0857(8)	0.2940(15)	0.7(2)
	Al2, 8f	0.1259(6)	0.0857(8)	0.2655(15)	0.6(2)
	O1, 4e	0	0.516(2)	1/4	0.7(5)
	O2, 8f	0.1252(9	0.0376(12)	0.540(3)	1.6(4)
	O3, 8f	0.1281(8)	0.2554(13)	0.179(2)	0.7(3)
	O4, 8f	0.1936(11)	0.4433(11)	0.595(2)	1.5(4)
R_I = 0.056; R_P = 0.190
SrAl₄O₇:xMn⁴⁺
a = 13.0469(8) Å b = 9.0227(6) Å c = 5.5443(3) Å β = 106.197(2)° V = 626.8(1) Å³	Sr, 4e	0	0.8115(4)	1/4	0.99(6)
	Al1, 8f	0.1672(6)	0.0853(7)	0.2974(13)	0.9(2)
	Al2, 8f	0.1246(6)	0.4390(8)	0.2660(13)	1.0(2)
	O1, 4e	0	0.5218(15)	1/4	1.7(5)
	O2, 8f	0.1243(8)	0.0374(11)	0.544(2)	1.2(3)
	O3, 8f	0.1260(7)	0.2567(12)	0.170(2)	0.8(3)
	O4, 8f	0.1904(9)	0.4434(10)	0.597(2)	1.3(4)
R_I = 0.057; R_P = 0.172

Table 2. Selected interatomic distances in SrAl₄O₇:xCr³⁺ and SrAl₄O₇:xMn⁴⁺ (x = 0.005) structures annealed at 1100 °C.

Atoms	Distances [Å]	Atoms	Distances [Å]	Atoms	Distances [Å]
SrAl₄O₇:xCr³⁺
Sr-O3 × 2	2.570(11)	Al1-O2	1.659(14)	Al2-O3	1.731(14)
Sr-O2 × 2	2.634(12)	Al1-O3	1.679(13)	Al2-O1	1.763(10)
Sr-O1	2.660(15)	Al1-O2	1.755(14)	Al2-O4	1.794(14)
Sr-O2 × 2	2.821(12)	Al1-O4	1.78(2)	Al2-O4	1.805(15)
Sr-Al1 × 2	3.256(8)	Al1-Al2	3.109(11)	Al2-O1	2.899(8)
Sr-Al1 × 2	3.319(8)	Al1-Al2	3.114(12)	Al2-Al2 × 2	2.979(11)
Sr-Al2 × 2	3.656(8)	Al1-Sr	3.256(8)	Al2-Al1	3.109(11)
Sr-O4 × 2	3.688(13)	Al1-Sr	3.319(8)	Al2-Al1	3.114(12)
Sr-Al2 × 2	3.725(8)	Al1-O4	3.441(15)	Al2-O2	3.130(15)
SrAl₄O₇:xMn⁴⁺
Sr-O3 × 2	2.524(9)	Al1-O2	1.671(13)	Al2-O3	1.731(12)
Sr-O2 × 2	2.614(10)	Al1-O3	1.722(12)	Al2-O1	1.769(9)
Sr-O1	2.614(14)	Al1-O2	1.754(12)	Al2-O4	1.784(13)
Sr-O2 × 2	2.822(10)	Al1-O4	1.805(14)	Al2-O4	1.798(13)
Sr-Al1 × 2	3.257(7)	Al1-Al2	3.101(10)	Al2-O1	2.891(7)
Sr-Al1 × 2	3.338(7)	Al1-Al2	3.127(10)	Al2-Al2 × 2	2.983(10)
Sr-Al2 × 2	3.653(7)	Al1-Sr	3.257(7)	Al2-Al1	3.101(10)
Sr-O4 × 2	3.653(11)	Al1-Sr	3.338(7)	Al2-Al1	3.127(10)
Sr-Al2 × 2	3.724(8)	Al1-O4	3.486(13)	Al2-O2	3.157(13)

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Stadnik, V.; Hreb, V.; Luchechko, A.; Zhydachevskyy, Y.; Suchocki, A.; Vasylechko, L. Sol-Gel Combustion Synthesis, Crystal Structure and Luminescence of Cr³⁺ and Mn⁴⁺ Ions in Nanocrystalline SrAl₄O₇. Inorganics 2021, 9, 89. https://doi.org/10.3390/inorganics9120089

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Stadnik V, Hreb V, Luchechko A, Zhydachevskyy Y, Suchocki A, Vasylechko L. Sol-Gel Combustion Synthesis, Crystal Structure and Luminescence of Cr³⁺ and Mn⁴⁺ Ions in Nanocrystalline SrAl₄O₇. Inorganics. 2021; 9(12):89. https://doi.org/10.3390/inorganics9120089

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Stadnik, Vitalii, Vasyl Hreb, Andriy Luchechko, Yaroslav Zhydachevskyy, Andrzej Suchocki, and Leonid Vasylechko. 2021. "Sol-Gel Combustion Synthesis, Crystal Structure and Luminescence of Cr³⁺ and Mn⁴⁺ Ions in Nanocrystalline SrAl₄O₇" Inorganics 9, no. 12: 89. https://doi.org/10.3390/inorganics9120089

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