Synthesis, structure, and luminescence properties of layered oxychloride Ba₃Y₂O₅Cl₂

Abstract

We report the synthesis, crystal structure, and optical properties of a new layered perovskite oxychloride, Ba₃Y₂O₅Cl₂. The structural and electronic properties of the compound were studied and compared with those of related compounds, Ba₃RE₂O₅Cl₂ (RE = Sc, Gd, and Lu). Furthermore, the luminescence properties due to Eu-doping were investigated. Polycrystalline Ba₃Y₂O₅Cl₂ samples were successfully synthesized by a solid-state reaction. The space group of the compound was I4/mmm and the lattice constants were a = 4.3971(8) Å and c = 24.848(5) Å. The Y atom in this compound shifted toward the apical oxygen site owing to the asymmetric coordination of O and Cl. Moreover, large anisotropic displacements were observed at the O sites. The direct bandgap of the compound was 5.4 eV. As Y³⁺ ions with a distorted octahedron have an asymmetric coordination environment, both magnetic and electric dipole transitions were observed by doping Eu³⁺ to Y³⁺ sites. Ba₃Y₂O₅Cl₂:Eu 4% also showed a high internal quantum yield (IQY) (>70%). Therefore, considering the photoluminescence spectra and high IQY, this compound is a potential host lattice for optical materials.

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Introduction

In recent years, mixed-anion compounds have garnered much attention owing to the various functionalities that originate from their special structural and electronic features.¹ The asymmetric local coordination environment of cations can be formed by utilizing two or more anions in the compound; this, in turn, causes a unique crystal field splitting and/or changes the transition probability of the luminescence centers. Controlling the coordinating environment of active cations in phosphor materials is a key method in inducing their luminescence properties as they are particularly sensitive to their coordinating anion environment.^2–5 For example, Eu³⁺ ions show strong luminescence due to the 4f–4f transition of ⁵D₀ → ⁷F_J (total angular momentum quantum number, J = 0, 1,…, 6). The magnetic dipole (MD) transitions are allowed in the case of J = 0, 1, 3, and 5, whereas the electric dipole (ED) transitions of J = 2, 4, and 6 are forbidden. Moreover, the ED transitions are allowed only when the Eu site has no inversion symmetry. Similarly, the charge transfer (CT) transitions, which occur upon the excitation of Eu³⁺ ions, are also affected by the local environment. Most highly efficient Eu³⁺ phosphors exhibit a narrow emission spectrum with a hyper-sensitive ED transition. However, considering the color rendering of white light illumination, a broad emission peak is suitable.

The coordination environment of the luminescence center is determined by the structure of the host lattice. For example, the Y sites in Y₂O₃:Eu³⁺ and Y₃Al₅O₁₂:Eu³⁺ are surrounded by six and eight oxygen ions, respectively, with rather high distortion. The asymmetric environment of rare-earth (RE) sites enables them to exhibit an ED transition. A greater variety of coordination environments can be realized for mixed anion compounds. For example, Y₂O₂S:Eu³⁺ is a well-known red phosphor, in which the Y site is surrounded by four oxygen and three sulfur ions. Strong ED transitions are observed because of their asymmetric environment. In addition, the excitation band appears at a longer wavelength than simple oxides, owing to the lower electronegativity of sulfur.⁶

In perovskite-related oxides, such as ABO₃ and A_n+1B_nO_3n+1, there are two cation sites, the A-site (coordination number (CN) = 12) and the B-site (CN = 6); depending on the cation combinations, Eu³⁺ can be substituted at either of these sites. The A- and B-sites have different coordination environments. The difference in coordination results in a variety of luminescence properties.^7,8 In the case of A-site substitution, the luminescence properties of Eu³⁺ can be fine-tuned by changing the local environment of the A-site.^9–11 However, for instances of B-site substitutions, MD transitions are dominant as the local environment of the B-site is almost symmetric and luminescence properties cannot be controlled as it is not easy to change the environment of B-sites.^12–14 In addition, if Eu³⁺ is located at a simple octahedral site, only the luminescence from MD transitions is observed.¹⁵ Furthermore, for mixed-anion compounds, there is a series of Ruddlesden–Popper-type perovskite compounds, with the chemical formula A_n+1B_nO_3n−1Cl₂ (A: alkaline earth metals (AE), B: transition metals or RE elements, and n: number of perovskite blocks), such as A₂BO₂Cl₂ (n = 1),^16–18 A₃B₂O₅Cl₂ (n = 2)^19–23 and A₄B₃O_7.5Cl₂ (n = 3).^21,24 In these compounds, the local environment of the B-site is asymmetric because it is surrounded by both oxygen and chlorine. In addition, relatively large cations, such as RE elements, can occupy the B-site of these compounds, for example, Ba₃Gd₂O₅Cl₂.²² However, the photoluminescence properties of these compounds have not been reported thus far.

In this paper, we report the synthesis and crystal structure of a new layered oxychloride, Ba₃Y₂O₅Cl₂. It is interesting to study the luminescence properties of these compounds because of the specific local environment of the Y site. Optical properties such as absorption and photoluminescence on Eu-activation were studied and compared with those of related oxychlorides.

Experimental

Synthesis

Polycrystalline samples of Ba₃Y₂O₅Cl₂ and Ba(Y_1−xEu_x)₂O₅Cl₂ (Ba₃Y₂O₅Cl₂:Eu³⁺, x = 0–0.10) were synthesized via a conventional solid-state reaction with stoichiometric quantities of BaCO₃ (3N), BaCl₂ (3N), and Y₂O₃ (3N). Powder mixtures were pelletized and heated several times with intermittent grindings at 1173–1273 K in air for 24 h. The same procedure was followed for the synthesis of Ba₃RE₂O₅Cl₂ (RE = Gd, Ho, Er, and Lu). The chemical compositions were analyzed using energy-dispersive X-ray spectroscopy (EDX, Oxford SwiftED3000) equipped with a scanning electron microscope (SEM, Hitachi TM3000). Commercial Y₂O₃:Eu³⁺ powders (Nemoto & Co., Ltd) were used for the optical measurements.

X-ray diffraction and structural refinement

Sintered samples were measured by powder X-ray diffraction (PXRD) with an Ultima-IV diffractometer (Rigaku). Using Cu Kα radiation, data were collected from 5° ≤ 2θ ≤ 80° with a 0.02° step size. The lattice parameters were refined by PXRD patterns using an Si internal standard.

Transparent single crystals were found in the pellets synthesized by a normal solid-state reaction. Single crystal X-ray diffraction (SCXRD) data were collected using a Rigaku XtaLAB Pro diffractometer (Mo Kα radiation) at 300 K on a single crystal of 0.015 mm × 0.010 mm × 0.005 mm. The structural parameters were refined by the full-matrix least-squares method using SHELXL-2016.²⁵ The details of SCXRD are shown in Table S1 (ESI†). When the occupancy factors were refined, the factors of all atoms except for Cl exceeded 1. The factor for Cl was refined to 0.962(13). These factors approached unity within 3σ. Therefore, the occupancy of all ions was fixed at 1.

DFT calculations

The density functional theory (DFT)-based calculations were performed for Ba₃Y₂O₅Cl₂ by the Vienna ab initio simulation package (VASP) code²⁶ using the Perdew–Burke–Ernzerhof generalized gradient approximation (PBE-GGA) for the exchange and correlation functionals. Projector augmented-wave (PAW) potentials were used for Ba, Y, O, and Cl atoms.^27,28 A plane-wave basis set with a cutoff of 500 eV was used (k-point mesh 5 × 5 × 5). The self-consistent field (SCF) tolerance was 1.0 × 10⁻⁷ eV·atom⁻¹ and the cell parameters and atom positions were relaxed until the maximum force on each atom was less than 0.02 eV·Å⁻¹.

Optical properties

Diffuse reflectance was measured using a spectrophotometer (Shimadzu, UV-2600) equipped with an ISR-2600Plus integration sphere. The photoluminescence spectra and lifetime of luminescence were recorded using a spectrofluorometer (JASCO, FP-8500) and the internal quantum yield (IQY) was measured using an intensified multichannel spectrometer (Otsuka Electronics, MCPD-7000). Internal quantum yield (IQY) Φ_int is defined as follows:where N_abs and N_em are the numbers of absorbed and emitted photons, respectively. All optical measurements were performed at room temperature.

Results and discussion

Ba₃Y₂O₅Cl₂ samples were successfully prepared by solid-state synthesis. Plate-like particles were found on the surface of the pellet (Fig. S1, ESI†). The grain sizes ranged from a few tens to a few hundred micrometers. The atomic ratio was evaluated as Ba : Y : Cl = 3 : 2.1 : 1.8 by EDX measurement, which is within the error range from the nominal composition of Ba₃Y₂O₅Cl₂. Fig. 1(a) shows the experimental and simulated PXRD patterns of Ba₃Y₂O₅Cl₂. The simulation pattern was determined from the results of the structural analysis. Except for a small amount of Y₂O₃ impurity, a single-phase was obtained. All peaks were assigned to the tetragonal cell with the space group I4/mmm. The lattice parameters obtained were a = 4.3971(8) Å and c = 24.848(5) Å. The intensities of the 00l reflections were larger than the simulation pattern because of the grain orientation. Furthermore, the phase was stable under air and no degradation was observed after a few days of exposure.

Fig. 1 (a) PXRD patterns of Ba₃Y₂O₅Cl₂. The top pattern (black) shows the experimental result and the bottom pattern (red) shows a simulated pattern. (b) Crystal structure of Ba₃Y₂O₅Cl₂. The orange, blue, red and green balls represent Ba, Y, O and Cl ions, respectively. The blue pyramids show the polyhedra of YO₅.

Table 1 presents the results of the single-crystal structure analysis. The lattice parameters evaluated from SCXRD were a = 4.4120(2) Å and c = 24.8781(15) Å, which are nearly consistent with the lattice parameters evaluated by PXRD. Moreover, the calculated profiles were in good agreement (within 1.5%) with those optimized by the DFT calculations.

Table 1 Refined structural parameters of Ba₃Y₂O₅Cl₂ single crystal

Atom	x	y	z	Occ.	U iso/Å^2	U 11/Å^2	U 22/Å^2	U 33/Å^2
I4/mmm (no. 139). a = 4.4120(2) Å, and c = 24.8781(15) Å. U12 = U13 = U23 = 0 for all sites. Rint = 0.059, and R1(I > 2σ(I)) = 0.020, wR2 = 0.040, and S = 1.26.
Ba1	0	0	0	1.0	0.0243(3)	0.0206(4)	0.0206(4)	0.0318(6)
Ba2	0	0	0.16244(2)	1.0	0.0100(2)	0.0079(2)	0.0079(2)	0.0143(3)
Y	1/2	1/2	0.08512(4)	1.0	0.0079(3)	0.0065(3)	0.0065(3)	0.0106(5)
Oeq	1/2	0	0.0969(2)	1.0	0.0226(13)	0.016(3)	0.007(3)	0.045(3)
Oap	1/2	1/2	0	1.0	0.051(5)	0.071(7)	0.071(7)	0.012(6)
Cl	1/2	1/2	0.20543(10)	1.0	0.0193(6)	0.0208(9)	0.0208(9)	0.0163(14)

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Fig. 1(b) illustrates the crystal structure of Ba₃Y₂O₅Cl₂ using the VESTA.²⁹ The structure was isostructural to that of Ba₃RE₂O₅Cl₂ (RE = Sc, and Gd–Lu) (RE = Sc, and Gd–Lu)^22,23 and comprised a double-perovskite-like Ba–Y–O layer and Ba–Cl rock-salt layer. In the perovskite layer, the double YO₅Cl octahedra were connected by corner-shared apical oxygen (O_ap) sites and the anion sites in the rock-salt layer were fully occupied by chlorine. Therefore, the structure obtained the stacking sequence, –(BaO)–(YO₂)–(BaCl)₂–(YO₂)–.

According to the ionic radii reported by Shannon et al.,³⁰ the ionic radius of Y³⁺ (0.900 Å, CN = 6) was close to that of Ho³⁺ (0.901 Å, CN = 6) and Er³⁺ (0.890 Å, CN = 6). Therefore, the lattice parameters of Ba₃Y₂O₅Cl₂ were expected to be similar to those of Ba₃Ho₂O₅Cl₂ and Ba₃Er₂O₅Cl₂. This was because Besara et al. reported lattice constants that were measured at 200 K, synthesized Ba₃Ho₂O₅Cl₂ and Ba₃Er₂O₅Cl₂via solid-state synthesis and evaluated the lattice parameters by PXRD at room temperature.²²

The lattice constants of Ba₃Ho₂O₅Cl₂ were a = 4.408(11) Å, and c = 24.822(7) Å, and those of Ba₃Er₂O₅Cl₂ were a = 4.3969(7) Å, and c = 24.779(5) Å. Taking into account the difference between the lattice constants measured at 200 K and at room temperature, these parameters were quite consistent with the reported values.^20,21 Therefore, the lattice constants of Ba₃Y₂O₅Cl₂ were larger than those of the other two compounds. In chlorides, such as YCl₃ and HoCl₃, the Y–Cl distance is usually longer than the Ho–Cl or Er–Cl distance;³¹ therefore, the larger lattice constants in Ba₃Y₂O₅Cl₂, especially for the c-axis, can be explained by this tendency.

Fig. 2 shows a schematic view of the local coordination environments of Ba1, Ba2, and Y. The CNs of Ba1, Ba2, and Y were 12, 9, and 6, respectively. Ba1 was coordinated to eight equatorial oxygen (O_eq) atoms, whereas four O_ap and Ba2 atoms were coordinated by five Cl and four O_eq atoms. The Y site was coordinated by one O_ap, four O_eq, and one Cl, and the local structure of the Y site was symmetric along the ab plane and asymmetric along the c-axis. The bond lengths of Ba1–O_eq and Ba1–O_ap are 3.268(4) Å and 3.11976(15) Å, respectively. These bond lengths, especially Ba1–O_eq, were longer than the sum of the ionic radii of Ba²⁺ and O²⁻ (3.01 Å). Owing to the large bonding lengths, the bond valence sums (BVSs) of Ba1 were estimated to be 0.9809, which were much smaller than the expected value (∼2) approximated from the formal valence of Ba. The bond lengths of Ba2–Cl were 3.287(3) Å and 3.2980(9) Å, and that of Ba2–O_eq was 2.743(3) Å. The Ba2–Cl bond length was almost equal to the sum of the ionic radii (3.28 Å) but the bond length of Ba2–O_eq was slightly smaller than the expected value (2.87 Å). The BVS of Ba2 was estimated to be 2.132. Y–O_ap, Y–O_eq and the Y–Cl distances were 2.1176(11) Å, 2.2254(7) Å, and 2.993(3) Å, respectively. The Y–O_ap distance was ∼10% shorter, while the Y–Cl distance was ∼10% larger than the sum of the ionic radii and the position of Y shifted toward O_ap. This trend is explained by the difference in electronegativity between O²⁻ and Cl⁻ ions. Therefore, Y shifted from the center of the octahedron, creating an asymmetric coordination environment at the B-site.

Fig. 2 Schematic view of the crystal structure and local coordination environments of (a) Ba1, (b) Ba2 and (c) Y, respectively. The orange, blue, red and green ellipsoids show Ba, Y, O and Cl, respectively. Displacement ellipsoids are shown at the 99% level. Cation–anion bond length values are described on anions.

There were large anisotropic displacements at the O_eq and O_ap sites, as shown in Fig. 2. The thermal ellipsoids of O_eq were elongated along the c-axis and those of O_ap were elongated along the ab plane. Su et al. reported that such elongation of thermal ellipsoids occurred because of the mismatch in compensation of each layer.²³ The ideal sum of the radii of Y–O (2.30 Å) was greater than half the lattice constant (2.204 Å). Therefore, the Y–O_eq plane was compressed along the ab plane. In contrast, the bond length of Ba1–O_ap (equal to a/√2), 3.11976(15) Å, was much larger than the ideal sum of radii for Ba–O, 3.01 Å; hence, the Ba1–O_ap plane expanded along the ab plane.

Fig. 3 shows the band structure and partial density of state (PDOS) of Ba₃Y₂O₅Cl₂ obtained by the DFT calculations. The calculated band structure suggested that this compound had a direct bandgap of ∼3.6 eV, in contrast to the indirect bandgap of Ba₃Sc₂O₅Cl₂. The valence band maximum (VBM) was formed by the O_eq 2p orbital and the O_ap 2p orbital was located lower than the O_eq 2p orbital. Cl 3p was located at a lower energy than the two O 2p orbitals. While the 3p orbital of Cl occupies a higher energy than the O 2p band in usual oxychlorides, the Cl 3p orbital in this compound did not contribute to the VBM, which was similar to that of the related Ba₃RE₂O₅Cl₂ compounds.^22,23 The conduction band minimum (CBM) of Ba₃Y₂O₅Cl₂ was formed by the unoccupied 5d orbital of Ba1. Conversely, the Sc 3d orbital in Ba₃Sc₂O₅Cl₂ contributed to the CBM. This difference may explain the variations in the direct/indirect bandgap nature of these compounds. Ba2 5d and Y 4d orbitals were located at higher energies than the Ba1 5d orbitals. Therefore, the bandgap of Ba₃Y₂O₅Cl₂ consisted of Ba1 and O_eq in the perovskite-like layer and the Ba2 and Cl ions in the rock-salt layer did not contribute to the bandgap in the compound.

Fig. 3 (a) The band structure and (b) partial density of states (PDOS) of Ba₃Y₂O₅Cl₂.

Fig. 4 shows the Tauc plots of Ba₃Y₂O₅Cl₂, as well as those of the related compounds, Ba₃Lu₂O₅Cl₂ and Ba₃Gd₂O₅Cl₂. Bandgaps were estimated using the following equation:

(F(R)hν)² = A(hν − E_g), (2)

where F(R) is the Kubelka–Munk function, A is a constant, and E_g is the bandgap energy. The estimated bandgap of Ba₃Y₂O₅Cl₂ was 5.4 eV. The bandgaps of the related compounds were also estimated using the same procedure. By increasing the ionic radii of RE³⁺ from Lu³⁺ to Gd³⁺, the bandgap energy decreased as follows: Ba₃Lu₂O₅Cl₂ (5.6 eV) > Ba₃Y₂O₅Cl₂ (5.4 eV) > Ba₃Gd₂O₅Cl₂ (5.3 eV). According to this result, the bandgap energies of the system were mainly determined by the size of RE³⁺ and were not affected by the energy level of the d orbitals of the RE elements. This feature was consistent with the electronic structure obtained using DFT calculations.

Fig. 4 Tauc plots of Ba₃RE₂O₅Cl₂ (RE = Y, Gd and Lu) measured at room temperature. The solid lines show Kubelka–Munk transformed diffuse reflectance spectra and the dashed lines show the extrapolation of the linear portion at the band edge. The blue, red and orange colours represent Ba₃Lu₂O₅Cl₂, Ba₃Y₂O₅Cl₂, and Ba₃Gd₂O₅Cl₂, respectively.

Fig. 5 shows the excitation and emission spectra and an image of luminescence under a mercury lamp (254 nm excitation) of Ba₃Y₂O₅Cl₂:Eu³⁺ 1% and commercial Y₂O₃:Eu³⁺, which was used as a reference. In the excitation spectra, both samples showed excitation peaks that are characteristic of the CT transition of Eu³⁺. In the case of Y₂O₃:Eu³⁺, there were two peaks at ∼230 mm and ∼250 nm, whereas for Ba₃Y₂O₅Cl₂:Eu³⁺, there was only one peak at ∼250 nm. Unlike some mixed-anion compounds, such as Y₂O₂S, a large redshift of the excitation peak was not observed in Ba₃Y₂O₅Cl₂. This was in agreement with the DFT calculations, in which the Cl 3p orbital was located at a lower energy than the O 2p orbital in the compound.

Fig. 5 Photoluminescence of Y₂O₃:Eu³⁺ (black) and Ba₃Y₂O₅Cl₂:Eu³⁺ 1% (red) measured at room temperature. The dashed line shows the excitation spectrum, and the solid line shows the emission line. The inset image shows the emission of Ba₃Y₂O₅Cl₂:Eu³⁺ 1% under excitation.

In the emission spectra, photoluminescence from the Eu³⁺ 4f–4f transitions was observed in both samples. A strong single peak of ED transition was observed for Y₂O₃:Eu³⁺ at 611 nm, which is in good agreement with the reported value. For Ba₃Y₂O₅Cl₂:Eu³⁺, emission from both ED and MD transition peaks was observed at 575–583 nm, 597–600 nm, 614–625 nm, 653–673 nm, and 693–725 nm. The emission color was orange, as shown in the inset image in Fig. 5. The aforementioned small amount of Y₂O₃ impurity present in the sample had no clear contribution to the luminescence. The intensities of the ED and MD transition peaks in this compound were almost similar, which has not been reported thus far. Owing to this, the compounds show a broad spectra in the red region. Moreover, this is contrary to the ED transition peak of a simple oxide perovskite with a B-site Eu³⁺, which is much lower than the MD transition peaks.^12–14 As described above, the characteristic ED/MD transition ratio of the compound originates from the local structure of the Y site.

The lifetime of the luminescence at the ⁵D₀ → ⁷F₂ transition in Ba₃Y₂O₅Cl₂:Eu 4% is shown in Fig. 6. All emission peaks show almost similar decay curves with CT-state excitation. The decay curve I(t) was fitted by the following function:

where I₀ is the initial intensity and τ is the decay constant. The decay curve was well fitted to one-component exponential decay. The decay constant τ was evaluated as 1.32 ms. Similar results were obtained by other transition peaks or concentrations of Eu³⁺ (Fig. S2, ESI†).

Fig. 6 The luminescence decay of Ba₃Y₂O₅Cl₂:Eu 4%. The excitation wavelength is 252 nm and monitoring wavelength is 621 nm.

The IQYs of Ba₃Y₂O₅Cl₂:Eu³⁺ with different Eu concentrations are shown in Fig. 7. The highest quantum yield was achieved at x = 0.04, with IQY of over 70%; this is larger than that of YOF:Eu³⁺.³² The IQY is in good agreement with the calculated quantum yield from the lifetime (Table S2, ESI†). Considering the high IQY, characteristic ED/MD transition ratio, and stability in air, Ba₃Y₂O₅Cl₂ can be a promising candidate for several phosphor materials, i.e., not only for Eu³⁺ but also for other RE elements.

Fig. 7 Internal quantum efficiencies (IQYs) of Ba₃Y₂O₅Cl₂ with different Eu concentrations.

Conclusions

We successfully synthesized new layered oxychlorides, Ba₃Y₂O₅Cl₂, by a solid-state reaction. The space group of the compound was I4/mmm, and the lattice constants were a = 4.3971(8) Å and c = 24.848(5) Å. Y ions were located toward O_ap in the YO₅Cl octahedron because of the difference in electronegativity between O²⁻ and Cl⁻ ions. A direct bandgap was suggested by the DFT calculations, and VBM and CBM were formed by O_eq 2p and Ba 5d orbitals, respectively. The bandgap of Ba₃Y₂O₅Cl₂ was estimated to be 5.4 eV by diffuse reflectance measurement. In Ba₃Y₂O₅Cl₂:Eu³⁺, both MD and ED transitions were observed with similar intensities, and for 4% Eu-doping, IQY was as high as 70%.

Author contributions

M. T., K. K., and H. O. designed and explored the new compound. Y. I., Y. S., Y. Tsuc., F. T., T. N., Y. Tsuj., and H. O. synthesized and measured the optical properties. Y. I., Y. S., T. Y., and H. O. measured luminescence properties. K. F. and M. Y. analyzed the crystal structure and performed the DFT calculations. All the authors have approved the final version of the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by JSPS Grant-in-Aid for Scientific Research on Innovative Areas “Mixed Anion” (Grant Number JP16H06439, 16H06440, and 19H04711) and Grant-in-Aid No. 18J01627 and 20K15032. We thank Prof. T. Takeda and N. Hirosaki in NIMS for assistance with the IQY measurements.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0tc04415f

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