3TeO6 (Ln = La

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Structural Characterization of B‑Site Ordered Ba2Ln2/3TeO6 (Ln = La, Pr, Nd, Sm, and Eu) Double Perovskites and Probing Its Luminescence as Eu3+ Phosphor Hosts Sariga C. Lal, Vidhya Lalan, and Subodh Ganesanpotti* Department of Physics, University of Kerala, Thiruvananthapuram, Kerala, India S Supporting Information *

ABSTRACT: Ba2Ln2/3TeO6 (Ln = La, Pr, Nd, Sm, and Eu) double perovskites were synthesized via solid-state ceramic route. Preliminary X-ray diffraction studies indicated a pseudocubic structure with lattice parameters ranging from 8.55 to 8.44 Å for the substitution of rare earths from La to Eu. Raman spectra show the frequency dependence of various Raman bands with respect to rare-earth substitution and exhibit a significant shift in peaks to higher wavenumber region, which was observed only for symmetric stretching modes of LnO6 and TeO6 octahedra. In accordance with observed number of bands and group theoretical predictions, the most likely symmetry of all compounds in the Ba2Ln2/3TeO6 system was found to be monoclinic with P21/n space group. Rietveld refinement of the XRD patterns further confirmed the P21/n space group and also the 1:1 rock salt ordering of the B-site cations. Diffuse reflectance spectra of Ba2Ln2/3TeO6 showed the optical bandgaps of these compounds between 3.9 and 4.8 eV, indicating the suitability as luminescent host material. The reduction in bandgap energy with lanthanide contraction of rare-earth ions is attributed to the widening of conduction band with octahedral tilting. Photoluminescence (PL) spectra and PL excitation spectra of Ba2La2/3−xEuxTeO6 (x = 0.025, 0.05, 0.075, 0.1, 0.125, 0.15) were investigated and found to exhibit bright orange-red emission under UV excitation. Chromaticity coordinates closely resemble those of commercial red phosphor Sr2Si5N8:Eu2+, which points toward the possible applicability of these new red phosphors in solid-state lighting industry. Finally, Judd−Ofelt intensity parameters Ωλ (λ = 2 and 4) were calculated, which indicate that Eu3+ ions occupy the symmetric octahedral B-site of the Ba2La2/3TeO6.

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INTRODUCTION Ideal cubic perovskites having general formula ABX3 are the most extensively studied compounds in materials science.1,2 Crystal structure as well as the interesting physical and chemical properties of these compounds have attracted researchers all over the world, persuading them to explore the different chemical combinations possible in the basic ABX3 structure. In recent years, there has been special interest on partial cation substitution in the B-site of perovskite compounds, that is, two different cations in the B-site that may remain ordered or disordered. In B-site ordered compounds (AB′B″X6), ordering can happen in three different waysrock salt, columnar, and layered. The most common type of ordering is the 1:1 rock salt ordering, where cations alternate in each octahedra in all the three dimensions.1 The types of elements that can be substituted in the B-site include the 3d, 4d, or 5d transition metals, lanthanides, actinides, and even some main-group elements.1,2 The properties such as electrical conductivity,3 magnetic ordering,4 etc. of this complex perovskites, also known as double perovskites, are mostly governed by the B-site cations. Hence the possibility of different combinations of cations in the B-site allows for the synthesis of new materials with interesting properties. © XXXX American Chemical Society

Another important fact regarding perovskite materials is that most of them do not possess aristotype cubic structure and are distorted. Octahedral tilting is the most common type of distortion present in perovskites. Since octahedral tilting modifies the coordination environment of the A-site cation while keeping octahedral coordination of B-site cations, the overall symmetry is lowered.1,5,6 Neutron diffraction is the ideal technique to understand such lowering of symmetry; however, neutron scattering facility is expensive and rather limited. Raman scattering is an alternative technique, which can be used to identify lowering of symmetry due to octahedral tilting. In the case of double perovskite systems like A2MWO6 (A = Ba, Sr, Ca, M = Mg, Zn, Ca, Cd),7 A2MgTeO6 (A = Ba, Sr, Ca),8 A(Ln1/2B1/2)O3 (A = Ba, Sr, B = Nb, Ta),9,10 A2CoBO6 (A = Sr, Ca, B = Te, W),11 ALaLiTeO6 (A = Ba, Sr),12 BaANaTeO6 (A = Bi, La)13 etc., Raman spectroscopy was used to identify the exact space group. In 2000, Woodward and Park14 reported a new 1:1 rock salt ordered cubic double perovskite Ba2Bi2/3TeO6, which was the result of their search for a material that can act as an efficient Received: December 7, 2017

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DOI: 10.1021/acs.inorgchem.7b03049 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry photocatalyst under visible light. Recently, Kong and Cava15 reported crystal structure and magnetic properties of Ba2R2/3TeO6, where the Bi site of Ba2Bi2/3TeO6 is replaced by yttrium and the lanthanide ions (La, Pr, ..., Lu). In the present work, Bi was replaced by rare-earth ions La, Pr, Nd, Sm, and Eu, and investigates the structural and optical properties in detail. Here, in these double perovskites, the A-site is occupied by Ba2+, B′-site is occupied by the rare-earth ions, and B′′-site is occupied by Te6+. The cationic site B′, where the rare earth is substituted, is only two-thirds occupied to maintain the charge balance. This gives a one-third cation vacant site in these compounds. The change in ionic radii of the rare earths along with deficiency may result in structural change from the aristotype cubic to possible lower symmetries due to octahedral tilting. The cation deficiency influences the coordination of Asite cations, reducing it below 12, and this causes the octahedra to get tilted suitably to adjust the change. The physical as well as chemical properties of double perovskites depend mostly on its structural characteristics. Some of the distorted perovskites give better properties, such as electrical, magnetic, optical etc., compared to their cubic counterparts.16,17 The possible ordering of the B-site cations, the rare earths (La, Pr, Nd, Sm, and Eu) and Te, make these compounds interesting to probe their structural details. Double perovskites can also act as host materials for luminescent centers.18 The B′O6 and B″O6 groups can efficiently get excited through charge-transfer transitions by absorbing UV radiation and transfer the absorbed energy to the activator ions. A number of perovskite compounds such as vanadates, molybdates, tungstates, etc. were used as host materials, but luminescence properties of tellurates as host lattice were relatively unexplored.18,19 Nowadays, white light-emitting diodes (w-LEDs) play a major role in different arena of electronics industry that includes room lighting, monitors, displays, and other optical devices, owing to their advantages such as low power consumption, high luminous efficiency, and environmental friendliness.20 An emerging technique for the fabrication of wLEDs includes single-phase emission tunable phosphors pumped with UV or near-UV (n-UV) chips.21−23 In the past decades, searches for blue, yellow, and green phosphors had achieved great success, but fabrication of stable orange or red phosphors with appropriate luminous efficiency is still a challenge. This indicates the need for stable orange or red phosphors with high brightness in solid-state lighting industry. These phosphors can be realized by doping Eu3+ or Sm3+ on suitable host materials such as KLaSr3(PO4)3F, BaAl2Si2O8, Sr 2 CaWO 6 etc., since lanthanide ions can boost the luminescence activity due to their 5d−4f or 4f−4f transitions.24,25 In the present article, the luminescence properties of Eu3+-substituted Ba2La2/3TeO6 are examined in detail.

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and Ba2La2/3TeO6:Eu3+ (Eu3+ = 15, 12.5, 10, 7.5, 5, 2.5 mol %) were calcined at 1050 °C for 20 h. A slow heating rate was given to the samples for the oxidation of Te4+ to Te6+ (1 °C/min). Hereafter, Ba2Ln2/3TeO6 (Ln = La, Pr, Nd, Sm, and Eu) will be designated as BLTO, BPTO, BNTO, BSTO, and BETO, respectively. Characterization Techniques. The crystal structure and phase purity of the powdered samples were studied by X-ray diffraction technique (XRD) with Cu Kα radiation (λ = 1.5406 Å) using Bruker D8 Advance diffractometer (40 kV, 40 mA) having nickel filter and Lynx eye position sensitive detector in the 2θ range of 10−120 ° (step size 0.02°). Rietveld refinement was performed using TOPAS 4.2 software. The background was fitted using Chebyschev polynomial. The cell parameters, atomic coordinates, and occupancy were refined. Raman spectra of Ba2Ln2/3TeO6 (Ln = La, Pr, Nd, Sm, and Eu) were taken using LABRAM−HR 800 Raman spectrometer with a 514.5 nm argon ion laser (5 mW) having a resolution of 1 cm−1. The UV−vis diffuse reflectance spectra (DRS) were recorded using a JASCO V-550 UVvis spectrophotometer with photometric accuracy of ± 0.002 abs (0−0.5 abs). Photoluminescence excitation (PLE) and emission (PL) spectra were collected using a Fluoromax-4 spectrofluorometer with a 150 W ozone-free xenon arc lamp. Photoluminescence lifetime was measured using Jobin Yvon Fluorolog FL3-11 spectrofluorometer.

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RESULTS AND DISCUSSION 1. XRD Patterns of Ba2Ln2/3TeO6. Figure 1 shows the XRD patterns of Ba2Ln2/3TeO6 (Ln = La, Pr, Nd, Sm, and Eu),

Figure 1. XRD patterns of Ba2Ln2/3TeO6 (Ln = La, Pr, Nd, Sm, and Eu). The planes are indexed using psuedocubic (Fm3̅m) lattice planes. The superlattice reflection 111 marked with asterisk corresponds to the rock salt ordering of B-site cations. (inset) The splitting of 440 plane of Ba2La2/3TeO6 and Ba2Eu2/3TeO6.

which are quite similar, and indicates the phase purity and crystallinity of all the double perovskites. The patterns are indexed with psuedocubic lattice planes with Ba2Bi2/3TeO6 as a reference, and the lattice parameters of Ba2Ln2/3TeO6 (Ln = La, Pr, Nd, Sm, and Eu) are obtained as 8.55, 8.51, 8.50, 8.46, and 8.44 Å, respectively. As expected, the lattice parameters of the double perovskites decrease with the decrease in ionic radii of the rare earths. The presence of R point reflection corresponds to 111 lattice planes in addition with the sublattice reflections (fundamental perovskite reflections) denoting 1:1 rock salt ordering of these compounds. The expanded 111 reflection of Ba2Ln2/3TeO6 is shown in Figure SI 1. The possibility of partial ordering in the crystal lattice is not assumed in these systems.

EXPERIMENTAL SECTION

Synthesis Procedure. The samples Ba2Ln2/3TeO6 (Ln = La, Pr, Nd, Sm, and Eu) and Ba2La2/3TeO6:Eu3+ (Eu3+ = 15, 12.5, 10, 7.5, 5, 2.5 mol %) were synthesized by solid-state ceramic route. The starting materials were BaCO3 (Sigma Aldrich, 99%), La2O3 (Sigma Aldrich, 99.9%), Pr6O11 (Sigma Aldrich, 99.9%), Nd2O3 (Alfa Aesar, 99.5%), Sm2O3 (Alfa Aesar, 99.9%), Eu2O3 (Alfa Aesar, 99.9%), and TeO2 (Sigma Aldrich, 99%). Stoichiometric amounts of the powder mixtures were ball-milled in acetone medium using ceria-stabilized zirconia balls in a plastic container for 24 h. The slurries of Ba2La2/3TeO6, Ba2Pr2/3TeO6, Ba2Nd2/3TeO6, and Ba2Sm2/3TeO6 formed after ball milling were calcined at 1000 °C for 20 h, and those of Ba2Eu2/3TeO6 B


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Inorganic Chemistry Figure SI 1 does not show any extra broadening of 111 planes, which is a clear indication of the absence of antiphase boundaries. The ordering of B-site is influenced by the charge and size difference of the cations. If the charge difference is 2 or greater than two and the size difference is greater than 0.1 Å, then rock salt ordering is favored.5,6 Here the lanthanides and tellurium have a charge difference of +3 and size difference of ∼0.4 Å. The splitting of 440 reflection in BLTO and BETO fitted using pseudo Voigt function is shown in inset of Figure 1, which indicates a lowering of symmetry for these compounds. Structure of perovskite compounds is strongly influenced by point defects like vacancies. It is essential to calculate the effective size of vacancies, since this should be needed while calculating the tolerance factor.26−29 Conventionally, the size of vacancies is assumed to be zero.29 In Ba2Ln2/3TeO6, the cation vacancy is in the B-site, and it is not a zero-dimensional defect. The vacancy concentration in these systems is ∼33%, which gives a positive vacancy size and hence cannot be neglected. The vacancy is due to the absence of the rare-earth ions in B′site, and thus we assumed the effective size of vacancy is same as that of rare-earth ions and calculated the tolerance factor. The tolerance factor is a measure of degree of distortion from the ideal cubic perovskite structure. Theoretically its value is 1 for cubic structure and deviates from unity as the symmetry lowers. The tolerance factor calculated for all these compounds (BLTO 0.9692, BPTO 0.9786, BNTO 0.9801, BSTO 0.9858, and BETO 0.9883) suggested a lower symmetry from cubic. This is in well agreement with the fact that the cation deficiency present in these double perovskites and the decrease in ionic radii of the rare earths causes the tilting of LnO6 and TeO6 octahedra. A plot of psuedocubic cell volume versus tolerance factor is shown in Supporting Information Figure SI 2. As expected, the figure shows an inverse relationship between cell volume and tolerance factor. In 1972, Glazer30 enumerated the possible tilt systems in double perovskites. For rock salt ordered double perovskites combined with simple octahedral tilting, there are a total of 12 possible tilt system corresponding to the 12 space groups. The tilting of octahedra is denoted using the symbol a#b#c#, where # stands for 0, +, and −, indicating no tilt, in-phase tilt, and outof-phase tilt around the 001 axes of the parent cubic perovskite (Pm3m ̅ ), respectively. It is difficult to identify the tilt system and hence the exact crystal structure from XRD, since the atomic scattering factor of oxygen is too low. The reflection splitting and extra reflections correspond to tilted octahedra comes mainly from the weak scatterers, that is, oxygen. Hence Raman studies were done to find the exact crystal structure, since Raman scattering is more sensitive to such kind of tilting, if present.8−13,31,32 2. Raman Spectra of Ba2Ln2/3TeO6 (Ln = La, Pr, Nd, Sm, and Eu). Room-temperature Raman spectra of all the samples were taken, and the combined Raman spectra are shown in Figure 2. It can be noted that all spectral profiles are nearly similar, which indicates the same symmetry for all the compounds. Group theoretical analysis predicts that there are only four Raman active bands for a double perovskite with aristotype cubic Fm3̅m symmetry.33,34 However, in the Raman spectra of Ba2Ln2/3TeO6 (Ln = La, Pr, Nd, Sm, and Eu), additional bands can be seen, which gave a strong indication of the lower symmetry. The correct structure can be predicted only if we know the number of bands present in each spectrum. The Raman spectra of Ba2Ln2/3TeO6 (Ln = La, Pr, Nd, Sm, and Eu) were deconvoluted using Lorentzian function, and the

Figure 2. Raman spectra of Ba2Ln2/3TeO6 (Ln = La, Pr, Nd, Sm, and Eu).

assigned bands are given in Table 1. The total number of bands present in Ba2Ln2/3TeO6 (Ln = La, Pr, Nd, Sm, and Eu) is more than 4. The deconvoluted Raman spectra of BLTO and BETO are shown in Figure 3a,b, respectively, and those of BPTO, BNTO, and BSTO are shown in the Supporting Information Figures SI 3−SI 5. Dominant bands located near 454 cm−1, near 766 cm−1, and between 100 and 200 cm−1 are previously observed in 1:1 ordered ceramics.8 The ν1 modes that appear in the region of 750−800 cm−1 are due to the symmetric oxygen stretching vibrations of the octahedra; the ν2 modes in the region of 550− 740 cm−1 account for the asymmetric oxygen stretching vibrations of the octahedra, and the vibrational bands assigned at 350−510 cm−1 are due to the oxygen bending motion of the octahedra. During ν1 and ν2 vibrations, all the cations were at rest, and the oxygen atoms are moving along the B′−O−B′′ axes. The external modes (translational and liberational modes) are in the region of 100−350 cm−1. The translational lattice modes correspond to the translation of “A” cation, whereas the liberational lattice modes are due to the rotation of the rigid octahedra. These external modes are sensitive to lowering of symmetry due to octahedral tilting.31 The presence of five lowenergy lattice modes designate octahedral distortions in all these compounds. Raman spectra presented no significant shift in frequency for bands below 600 cm−1, since these are associated to Ba (Ag⊕Bg cubic-like modes) and Ba−O (Ag, translational motions of Ba) vibrations. These modes are insensitive to the partial cation substitution in B-site. However, those bands below 300 cm−1 are sensitive to chemical substitution in B sites, and no change was observed, probably due to the invariance of B′′ cation (Te) in Ba2Ln2/3TeO6. For modes above 650 cm−1 (symmetric stretching of LnO6 and TeO6 octahedra), the change in composition of B-site affects the modes. The spectra present a significant shift in peaks to longer wavenumber region from BLTO to BETO (see Figure SI 6), which is a consequence of decreasing ionic radii, hence, a contracted unit cell and stronger ionic bonds. This indicates that more energy is required for the vibrational motion of molecules as the ions get massive from La3+ to Eu3+ as well as the ionic bonds get stronger with decrease in ionic radii. C


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Inorganic Chemistry Table 1. Raman Band Assignments of Ba2Ln2/3TeO6a internal

a

symmetry and identified the space groups. In 1993, Anderson et al.5 reported that the most commonly occurring structures for double perovskites having B-site cation ordering are those in space groups Fm3̅m (tilt system a0a0a0) and P21/n (tilt system a−a−c+), based on their review of more than 160 B-site ordered double perovskites. In 2006, Fujiko et al.35 reported tetragonal P42/n for B-site ordered Sr2MnWO6. Group theoretical calculations predict 35 Raman active modes, among which 17 are low-energy lattice modes. Another possibility is tetragonal P4/mnc, with only 14 Raman active modes. According to group theory, there are 27 allowed bands for monoclinic structure belonging to C2/c space group. The reported examples for this space group are very few. Nonetheless, Maria et al.36 described a structural phase transition for Pb2MnTeO6 double perovskite from I2/m to C2/c when temperature is lowered to 120 K. A2MgTeO6 (A = Ba, Sr, Ca),8 previously reported, are tetragonal (I4/m) for Ba- and Sr-based materials and monoclinic (P21/n) for Ca-based material. In tetragonal I4/m (C4h5) space group, there are two molecules per unit cell. With factor group analysis, the first-order Raman modes near the Brillouin zone center in terms of irreducible representations of C4h point group is ΓRaman = 3Ag + 3Bg + 3Eg, which gives only nine Raman active modes. For monoclinic C2/m (C2h3), there are four molecules per unit cell. Group theoretical analysis predicts nine Raman active modes, and the irreducible representation is given by ΓRaman = 3Ag + 6Bg. Being primitive, monoclinic P21/n (C2h5) has two molecules per unit cell. The first-order Raman modes in terms of irreducible representation of C2h point group is given by ΓRaman = 12Ag + 12Bg, which gives 24 Raman active modes. Thus, Howard’s predictions and the reported evidence strongly suggest the possible symmetries be P42/n and I4/m (tetragonal) and C2/c and P21/n (monoclinic). The Raman spectra of Ba2Ln2/3TeO6 (Ln = La, Pr, Nd, Sm, and Eu) presented 16 to 21 bands, which reduce the possibilities to three, eliminating I4/m symmetry. The total number of modes 27 and 35 for C2/c and P42/n space groups, respectively, suggest the possibility of these space groups for Ba2Ln2/3TeO6. P42/n space group can be excluded from the list, as the number of T and L modes are much less than 17 as per the group theoretical predictions.35 Moreover, P42/n is often ascribed to compounds with ordered perovskite structure and a Jahn−Teller ion (Mn3+, Ni 3+, or Cu 3+), for example, SrMnWO6,37 Sr2MnMoO6,38 etc. However, these space groups

external

compound

ν1, cm−1

ν2, cm−1

ν5, cm−1

T or L, cm−1

Ba2La2/3TeO6

755 764

379 397 426 457

103 134 160 185 285 342

Ba2Pr2/3TeO6

756 766

380 397 454

104 141 163 192 284

Ba2Nd2/3TeO6

747 764

380 398 454

104 135 163 188 282

Ba2Sm2/3TeO6

759 770

382 399 455

105 138 168 191 284

Ba2Eu2/3TeO6

758 771

562 584 595 619 643 676 694 715 734 587 603 646 685 696 724 587 598 648 685 694 724 588 598 647 690 699 721 572 584 649 692 703 726

381 398 456 509

105 138 175 193 285

Ln = La, Pr, Nd, Sm, and Eu.

Howard et al.33 reported that there were 12 different space groups possible for double perovskites A2B′B′′X6 that arise from rock salt ordering combined with simple octahedral tilting. They obtained the group−subgroup relations for each

Figure 3. Deconvoluted Raman spectra of (a) BLTO and (b) BETO. Experimental data are in solid lines, while the fitting curve is the green balls. Red lines represent the phonon modes adjusted by Lorentzian curves. D


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Inorganic Chemistry

Figure 4. Refined XRD patterns of (a) BLTO calcined at 1000 °C and (b) BETO calcined at 1050 °C. (insets) The fitting of the super lattice reflection 011.

Table 2. Refined Crystallographic Parameters of Ba2Ln2/3TeO6 (Ln = La and Eu) space group: P21/n

Ba2La2/3TeO6 a = 6.0538(5) Å

b = 6.0612(7) Å

c = 8.5723(4) Å

Rexp = 2.30%

Rwp = 5.15%

Rp = 3.83%

β = 90.031(1)° GOF = 2.24

ions

Wyckoff sites

x

y

z

Ba La Te O1 O2 O3 Ba2Eu2/3TeO6

4e 2d 2c 4e 4e 4e

0.465(3) 0.5 0 0.215(8) 0.200(5) 0.534(1)

0.531(8) 0 0.5 0.249(2) 0.771(6) 0.016(6)

0.260(8) 0 0 0.042(1) 0.025(1) 0.264(3)

a = 5.9732(0) Å

b = 5.9899(8) Å

c = 8.4482(0) Å

Rexp = 1.99%

Rwp = 3.48%

Rp = 2.61%

occupancy

Beq (Å2)

1 0.75(5) 0.667 0.38(9) 1 0.38(9) 1 0.98(9) 1 0.98(1) 1 0.97(4) space group: P21/n β = 90.016(1)° GOF = 1.75

ions

Wyckoff sites

x

y

z

occupancy

Beq (Å2)

Ba Eu Te O1 O2 O3

4e 2d 2c 4e 4e 4e

0.492(4) 0.5 0 0.212(5) 0.200(5) 0.539(2)

0.542(2) 0 0.5 0.246(5) 0.771(6) 0.016(6)

0.249(7) 0 0 0.022(1) 0.005(1) 0.264(3)

1 0.667 1 1 1 1

0.84(5) 0.39(5) 0.39(5) 0.99(3) 0.98(7) 0.97(8)

were eliminated, as the Rietveld refinement does not converge compared to that of P21/n space group. The fitting of the crystallographic structure based on the experimental data of Ba2Ln2/3TeO6 in space group C2/c resulted in R-factors Rp = 7.62%, Rwp = 11.87%, Rexp = 1.29%, and χ2 = 9.20. Fitting with the space group P42/n gave better R-factors compared to C2/c but not better than the P21/n. This gives the conclusion that the most likely symmetry for Ba2Ln2/3TeO6 (Ln = La, Pr, Nd, Sm, and Eu) is monoclinic P21/n. The monoclinic (P21/n) structure results from the combination of in-phase and antiphase tilts of LnO6 and TeO6 octahedra around the 001 axes of the ideal cubic perovskite.7,31,39−41 In Ba2Ln2/3TeO6, Ba2+ and O2− occupy 4e sites of C1 symmetry, whereas Ln3+ (La, Pr, Nd, Sm, and Eu) and Te6+ ions occupy 2d and 2c sites of Ci symmetry, respectively. In terms of the internal and external Raman active modes, the irreducible representation for a P21/n space group (tilt system a−a−c+) is given as

Γ = T (3A g + 3Bg ) + L(3A g + 3Bg ) + v1(A g + Bg ) + v 2(2A g + 2Bg ) + v5(3A g + 3Bg )

(1)

The reasons for the absence of extra bands predicted by group theory might be due to (i) the polycrystallinity of samples, (ii) band may be so weak that they could not be identified among the background noises, (iii) the inability to resolve the splitting of bands that may overlap, or (iv) the bands may fall outside the range of spectra.7 The Raman modes are thus identified and arrived at the result that Ba2Ln2/3TeO6 (Ln = La, Pr, Nd, Sm, and Eu) belongs to monoclinic P21/n (C2h5) space group. The number of vibrational modes are found to decrease with the lanthanide contraction of the B′-site ions. This can be accounted for by the fact that the structure of the double perovskites are approaching cubic with the decrease in ionic radii of the lanthanide ions. The presence of symmetry elements containing translational components such as glide planes, screw axes, etc. lead to the systematic absences/ extinctions of some hkl reflections in the XRD patterns. The systematic absences of hkl reflections h0l with h + l = 2n + 1 E


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Inorganic Chemistry

Figure 5. Crystal structure of Ba2La2/3TeO6. (green) Octahedrally coordinated La. (pink octahedra) Te. (blue sphere) Ba. (red sphere) Oxygen.

Table 3. Selected Bond Distances of Ba2Ln2/3TeO6a

a

bonds

bond lengths (Å)

bonds

bond lengths (Å)

bonds

bond lengths (Å)

bonds

bond lengths (Å)

bonds

bond lengths (Å)

La−O1 La−O2 La−O3 Te−O1 Te−O2 Te−O3

2.318 2.291 2.277 2.036 2.056 2.034

Pr−O1 Pr−O2 Pr−O3 Te−O1 Te−O2 Te−O3

2.316 2.276 2.262 2.004 2.043 2.021

Nd−O1 Nd−O2 Nd−O3 Te−O1 Te−O2 Te−O3

2.294 2.269 2.263 2.007 2.033 2.022

SmO1 SmO2 SmO3 Te−O1 Te−O2 Te−O3

2.276 2.256 2.249 1.991 2.023 2.009

Eu−O1 Eu−O2 Eu−O3 Te−O1 Te−O2 Te−O3

2.272 2.253 2.247 1.988 2.021 2.007

Ln = La, Pr, Nd, Sm, and Eu.

which confirm the P21/n space group. Occupancy of the atoms at site 4e and 2c was fixed to unity, and that of 2d was fixed to 0.667. The lowering of symmetry from ideal cubic structure in Ba2Ln2/3TeO6 (Ln = La, Pr, Nd, Sm, and Eu) was supposed to be due to the octahedral tilting and is confirmed from the refinement of XRD patterns. The octahedral tilting (a−a−c+) of these compounds is illustrated in Figure 5. The bond lengths in the metal oxide octahedra are given in Table 3, which is an indication of the size of distortion in these double perovskites. The average bond angle of the B′−O−B′′ bond is 162.40° (La−O−Te), 162.89 ° (Pr−O−Te), 163.75 ° (Nd−O−Te), 164.34 ° (Sm−O−Te), and 165.35 ° (Eu−O−Te). The bond angle is advancing toward 180°, which explains the decrease in distortion of symmetry as the B-site cation changes from La to Eu, which is also in agreement with the results obtained from the Raman spectra. 3. UV−Visible Diffuse Reflectance Spectra. Five novel double perovskites were synthesized, and the structural characterizations of them were performed. Now, we want to investigate its application potential as host material for luminescent active centers. Figure 6 shows the DRS (Kubelka−Munk method) of Ba2Ln2/3TeO6 (Ln = La, Pr, Nd, Sm, and Eu). The optical bandgaps of all the compounds were calculated from Kubelka−Munk function using the following expression for direct bandgap.47

and 0k0 with k = 2n + 1 in the XRD patterns suggest the monoclinic space group with P21/n symmetry.42−46 The occurrence of reflection (011) also proves that the unit cell is monoclinic. The reflections that indicate A cation displacement (ddo), in phase distortion (ood), out of phase tilting distortion, and cation ordering (ooo) in the XRD patterns correspond to P21/n symmetry. The existence of (330) reflection corresponds to atomic displacement, (204) and (404) correspond to inphase tilting, and (011) and (013) correspond to cation ordering, which lead us to infer the simultaneous occurrence of in-phase and out-of-phase tilting. Thus we concluded the monoclinic (P21/n) symmetry of these double perovskites.44,46 The crystal structure derived from the Raman spectra of Ba2Ln2/3TeO6 (Ln = La, Pr, Nd, Sm, and Eu) is thus confirmed by Rietveld refinement of the XRD patterns. All the patterns were fitted using the monoclinically distorted unit cell with P21/n symmetry. The lattice parameters of the monoclinic unit cell were determined using the relation a ≈ 2 a p , b ≈ 2 a p , c ≈ 2a p and β ≈ 900, where ap is the lattice parameter of the primitive cubic unit cell. The refined XRD patterns of BLTO and BETO are shown in Figure 4a,b, respectively, and those of BPTO, BNTO, and BSTO are shown in Figures SI 7−SI 9. In this structure, the rare-earth ions having atomic coordinates (0.5,0,0) are situated at the 2d site, and Te (0,0.5,0) is situated at the 2c Wyckoff sites, making the LnO6 and TeO6 octahedra alternatively arranged in two interleaved face-centered cubic (fcc) sublattices. The octahedra are linked by sharing a single oxygen atom, and the O1, O2, and O3 anions are localized in the corner of the octahedra at Wyckoff position 4e. The A-site cation Ba is situated at the 4e Wyckoff position. The refined parameters are tabulated in Table 2 (the refinement results of Ba2Ln2/3TeO6 (Ln = Pr, Nd, and Sm) are given in Table SI 1). The refinement gives satisfactory agreement factors, Rwp, RP, Rexp, and goodness of fit (GOF),

[F(R ∞)hv]2 ∝ (hv − Eg )

(2)

Here F(R∞) is the remission or Kubelka−Munk function given by F(R ∞) =

(1 − R ∞)2 k = 2R ∞ s

(3)

where R∞ is the reflection coefficient (R∞ = Rsample/Rreference), k is the absorption coefficient, and s is the scattering coefficient. F


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Inorganic Chemistry

Figure 7. Rietveld refinement of XRD pattern of Ba2La0.567Eu0.1TeO6. Figure 6. DRS of Ba2Ln2/3TeO6 (Ln = La, Pr, Nd, Sm, and Eu). (inset) The bandgaps of Ba2Ln2/3TeO6.

spectrum when substituted with Eu3+ in the Bi3+ site. The B′O6 and B′′O6 groups can be excited using UV light through charge-transfer transitions and transfer the absorbed energy to the Eu3+ activator. Eu3+ is a popular red-emitting activator due to its 5D0−7FJ (J = 0, 1, 2, 3, 4) transitions.24,25,49 Here we describe the PLE and PL spectra of Ba2La2/3−xEuxTeO6 (x = 0.15, 0.125, 0.1, 0.075, 0.05, and 0.025) for six different concentrations. Figure 8a shows PLE spectrum, and Figure 8b shows PL spectrum. As can be seen from the figure, the photoluminescence excitation and emission intensities increase with increase in concentration of Eu3+ ions but decrease when the Eu3+ concentration reaches 12.5 mol %. It is well-known that the concentration of doping ion influences the phosphor performance. When the doping concentration increases, the concentration quenching may occur, as the exciting energy migrates about the activator ions before emitting; that is, a nonradiative energy transfer happens. This energy migration occurs due to the electric multipolar interaction between the Eu3+ ions and can be quantitatively explained using Van Uitert’s model given by50 I = k[1 + β(x)θ /3 ]−1 (4) x

From the plot of [F(R∞)hν]2 versus hν, the bandgap Eg in electronvolts can be evaluated by extrapolating the straight line to [F(R∞)hν]2 = 0. The optical bandgaps calculated are given in the inset of Figure 6. The compounds show energy of absorption in nearUV region (250−320 nm). Thus Ba2Ln2/3TeO6 can be efficiently excited using UV radiation and can act as host materials for luminescent centers. The local coordination of the B-site cations plays an important role in determining the bandgap. The detailed results can only be explained by doing band structure calculations. However, it can be seen that the bandgap decreases with decrease in ionic radii of the rare-earth ions, which can be interpreted as the increase in width of the conduction band. Eng et al. obtained the correlation between the linearity of B′−O−B′′ and bandgap, which says that when the bond angle becomes less than 180° with octahedral tilting, an increase in bandgap is observed due to the narrowing of conduction band.48 4. Photoluminescence Spectra of Ba2La2/3−xEuxTeO6. Since Ba2La2/3TeO6 can efficiently be excited using UV radiation and hence can act as a good host material for optically active centers, it would be interesting to investigate the luminescence properties by doping with suitable rare-earth ions. XRD patterns of Eu3+ substituted Ba2La2/3TeO6 for six different concentrations (x = 0.15, 0.125, 0.1, 0.075, 0.05, and 0.025) are given in Figure SI 10. Figure 7 shows the Rietveld refinement of the XRD pattern of Ba2La0.567Eu0.1TeO6, and the refined parameters are listed in Table 4. The XRD pattern is indexed with monoclinic unit cell (space group = P21/n). All structural characters are similar to that of the host material. The ionic radius of Ba2+ and La3+ for 12-fold coordination is 1.61 and 1.36 Å, respectively. The difference in the ionic radii of Ba2+ and La3+ provides a least probability that site swapping of these ions actually occurs. Here, Eu3+ is substituted at the La3+ site. However, Eu3+ in 12-fold coordination is least examined, and for sixfold coordination its ionic radius is 0.947 Å. The ionic radius of Te6+ for sixfold coordination is 0.56 Å, and this incompatibility reduces the possibility of site swapping between Te6+ and Eu3+. Nguyen et al.18 reported that Ba2Bi2/3TeO6 exhibits a weak luminescence in the orange-red region of the electromagnetic

where x is the activator ion concentration, k and β are constants for a particular system, I is the luminescence intensity, and θ represents the type of interaction between the rare-earth ions. θ = 3, 6, 8, or 10 corresponding to exchange interaction, electric dipole−dipole (D−D), electric dipole−quadrupole (D−Q), and electric quadrupole−quadrupole (Q−Q) interactions, respectively. Figure 9 shows the dependence of luminescence intensity on Eu3+ ion concentration. θ was deduced to be 7.29 using equation 4, and this implies that electric dipole−dipole interactions occur among Eu3+ ions, which is responsible for the fluorescence quenching of the 5D0−7FJ transitions. Another quantity that can be estimated to get an idea about the energy-transfer mechanism among the Eu3+ ions is the critical distance Rc. Concentration quenching happens as the amount of Eu3+ ions increases, which is due to the radiationless energy transfer among the Eu3+ ions. Critical distance of energy transfer from an excited ion to another ion is defined as the distance for which the probability of transfer equals the probability of radiative emission from the excited ion. For the cases where both the excited ion and the ion to which the energy is transferred are equal, like in this case, it is possible to obtain Rc from experimental data, more precisely from the G


Article

Inorganic Chemistry Table 4. Refined Crystallographic Parameters of Ba2La0.567Eu0.1TeO6 space group: P21/n

ion Ba La Eu Te O1 O2 O3

a = 6.0391(0) Å Rexp = 2.33% site 4e 2d 2d 2c 4e 4e 4e

b = 6.0418(0) Å Rwp = 4.32% x 0.474(3) 0.5 0.5 0 0.212(5) 0.200(5) 0.539(2)

c = 8.5542(0) Å Rp = 3.12% z 0.258(6) 0 0 0 0.042(4) 0.025(7) 0.264(3)

y 0.501(4) 0 0 0.5 0.246(5) 0.771(6) 0.016(6)

occupancy 1 0.567 0.1 1 1 1 1

β = 90.029(9)° GOF = 1.85 Beq (Å2) 0.75(7) 0.39(1) 0.39(2) 0.39(1) 0.98(9) 0.98(1) 0.97(1)

Figure 8. (a) PLE and (b) PL emission spectra of Ba2La2/3TeO6:xEu3+ (x = 0.025, 0.05, 0.075, 0.1, 0.125, 0.15).

ions in the unit cell (the activator is introduced on Z ion sites). We have N = 6 and xc = 0.1; the cell volume was estimated to be 314.5 Å3. The critical distance Rc was calculated to be 10.01 Å. When the critical distance between the activator and sensitizer ions is less than 4 Å, then exchange interaction takes place, and when it is greater than 4 Å, then electric multipolar interaction takes place between the ions. The critical distance also confirms the dipole−dipole interaction among the Eu3+ ions. The excitation spectra were obtained by using an emission wavelength of 592 nm. The excitation spectra exhibited a broad charge-transition band at 272 nm and narrow peaks at 394 and 464 nm. The latter appear due to the f−f electronic transitions of the Eu3+ ion and can be assigned as 7F0−5L6 and 7F0−5D2 transitions.24,25,49 The presence of broad band in the excitation spectra suggested energy overlap of the Te6+−O2‑ and Eu3+− O2− charge-transition bands. This indicates that energy transfer actually happens between the octahedral moiety and Eu3+ ion. The luminescence spectra presented four emission lines at 592 nm (5D0−7F1: magnetic dipole transition), 611 nm (5D0−7F2: electric dipole transition), 633 nm (5D0−7F3), and 710 nm (5D0−7F4) when excited using 272 nm. The local structural changes in the crystal lattice can be identified from the relative intensity of electric dipole and magnetic dipole transitions. The f−f transitions of the Eu3+ are forbidden according to Laporte’s selection rule (spin and parity forbidden). The transitions are efficient only when the host lattice environment changes to a lower symmetry. The hypersensitivity of the electric dipole transition, that is, the strong dependence of intensity on the chemical environment, can be used to probe the local structural changes in the crystal lattice. To have the electric dipole transition occur, the crystal field must be non-centrosymmetric. Otherwise the Laporte’s selection rule is violated, and hence the electric dipole radiation

Figure 9. Relation between integrated emission intensities and Eu3+ concentrations on Ba2La2/3TeO6.

concentration quenching data. This requires the critical concentration of the activator ion above which the concentration quenching occurs. In this case, critical distance is assumed to be the average shortest distance between the neighboring activator ions for the critical concentration. On the basis of the above idea, Blasse proposed an expression for critical distance given by51

⎡ 3V ⎤1/3 R C = 2⎢ ⎥ ⎣ 4πxcN ⎦

(5)

in which V is the volume of the unit cell, xc is the critical concentration of the activator ion, and N is the number of Z H


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Inorganic Chemistry

that the lanthanide ion occupies a highly symmetric environment.53−55 Judd−Ofelt theory gives the magnetic dipole transition rate (A01) of 5D0−7F1 transition of Eu3+ion as

would be strictly forbidden. The symmetry of the crystalline field is determined by the chemical composition of the host. If the electric dipole transition of the Eu3+ ions is dominant, then it means that Eu3+ ion occupies a site with no inversion symmetry.18 From the PL spectrum, clearly the magnetic dipole transition is predominant; that is, the Eu3+ ions occupy the symmetric octahedral B-site, which was confirmed from the Rietveld refinement of the XRD pattern. The CIE diagram displayed in inset of Figure 10 implies that BLTO:Eu3+

A 01 =

64π 4 ϑ13n3Smd 3h(2J + 1)

(8)

where υ1 is the wavenumber of the corresponding transition, n is the effective refractive index of the phosphor, Smd = 7.83 × 10−42 units, which is a constant and independent of the medium, h is Planck’s constant, and 2J + 1 = 1 for 5D0 transitions. The electric dipole transition rates (5D0−7FJ, J = 2, 4, and 6) is expressed as A 0J =

64π 4 ϑ3J 3h(2J + 1)

e2

n(n2 + 2)2 9

∑

7

Ωλ⟨5D0 U (λ) FJ ⟩2

λ = 2,4,6

(9)

where A0J is the coefficient of spontaneous emission, e is the charge of electron, Ωλ is the Judd−Ofelt parameters, and ⟨5D0|| U(λ)||7FJ⟩2 is the squared reduced matrix element. The transition rate of each energy level is proportional to the integral intensity of the emission spectrum. 3 ∫ I J d ϑ A 0J e 2 ϑJ (n2 + 2)2 = = Ωλ⟨5D0||U (λ)||7 FJ ⟩2 ∫ I1d ϑ A 01 Smd ϑ13 9n2

(10)

Figure 10. PL Decay curve of BLTO: Eu3+ (10 mol %). (inset) The chromaticity diagram showing the CIE coordinates for BLTO:Eu3+ and Sr2Si5N8:Eu2+.

From this equation the Judd−Ofelt intensity parameters can be calculated.53−56 Judd−Ofelt intensity parameters and radiative parameters obtained from the emission spectrum and lifetime data are assigned in Table.5

phosphor exhibits orange-red color with chromaticity coordinates of (0.629, 0.369) for BLTO:Eu3+ (0.1) that are close to those of commercial red phosphor Sr2Si5N8:Eu2+ (0.62, 0.37). The decay time studies were performed at excitation wavelength of 272 nm with λem = 592 nm. Figure 10 shows the decay curve of BLTO:Eu3+ excited at 272 nm. The curve was well-fitted using exponential decay equation24,25,49 I(t ) = A × exp( −t /τ ) + I0

Table 5. Judd−Ofelt Intensity Parameters and Radiative Parameters Obtained from the Emission Spectrum and Lifetime Data Judd−Ofelt intensity parameters Ω2 (1 × 10−20 cm2) Ω4 (1 × 10−20 cm2) Radiative parameters radiative transition rate, AR (s−1) nonradiative transition rate, ANR (s−1) total transition rate, AT (s−1) measured lifetime, τobs (ms) calculated radiative lifetime, τrad(ms) quantum efficiency, η (%) branching ratio of 5D0−7F1 transition, β01 (%) branching ratio of 5D0−7F2 transition, β02 (%) branching ratio of 5D0−7F4 transition, β04 (%)

(6)

where A is a scalar, and τ is the decay time constant. The lifetime is calculated to be 3.55 ms. The decay curve of Ba2La0.667−xEuxTeO6 (x = 0.025, 0.075, 0.1, and 0.15) is shown in Supporting Information Figure SI 11. Correlated color temperature (CCT) was calculated using McCamy method.52 CCT = − 449n3 + 3525n2 − 6823.3n + 5520.33

(7)

where n = (x − xe)/(y − ye), (x, y) are the chromaticity coordinates, and (xe, ye) are (0.3320, 0.1858), the coordinates corresponding to the epicenter of convergence of the isotemperature lines of the CIE 1931 chromaticity diagram. The CCT obtained was 1810 K, and this low value indicates warmer (more yellow-red) light. The site symmetry of Eu3+ ion in the crystal Ba2La2/3TeO6 is further confirmed by the calculation of Judd−Ofelt intensity parameters Ωλ (λ = 2, 4, 6). Judd−Ofelt theory helps to predict the site symmetry of the luminescence active center as well as the luminescence behavior of the phosphor in which the theory is applied. The theory is actually used for characterizing the optical transitions of rare-earth ions in specific coordination environment. When the Ω2 parameter is low, then it implies

1.11 0.04 76.57 205.12 281.69 3.55 13.06 27.18 41.06 36.03 22.94

In the emission spectrum of Ba2La2/3TeO6:Eu3+ phosphor, the magnetic dipole transition is dominant. This results in a considerably higher value for the integrated emission intensity of 5D0−7F1 transition and hence a lower value for the Ω2 intensity parameter. This indicates that the Eu3+ ion occupies a highly symmetric site in the Ba2La2/3TeO6 host matrix, which is in well agreement with the results obtained from the XRD pattern and PL spectrum. The higher value of the branching ratio of the 5D0−7F1 transition shows that the relative contribution of this transition to the total radiative decay rate of the excited state is high I


Inorganic Chemistry

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compared to the other transitions, which is in good agreement with the experimental observation. The quantum efficiency is comparatively less than the commercially available sulphidic red phosphor Y2O2S:Eu3+ (η = 35%).

CONCLUSIONS In summary, Ba2Ln2/3TeO6 (Ln = La, Pr, Nd, Sm, and Eu) were synthesized as single-phase materials, and their crystal structure and optical properties were investigated for the first time. Raman spectra of all the compounds presented additional bands, which led us to conclude that the structure is not cubic but changes to a lower symmetry. Fitting of Raman lines gives the most probable space group of P21/n for Ba2Ln2/3TeO6, which is further confirmed by Rietveld refinement of XRD patterns. Obtained optical bandgaps from DRS can be correlated with the average Ln−O−Te bond angle, and the bandgap increases as the bond angle decreases from 180°. Eu3+substituted Ba2La2/3TeO6 shows photoluminescence, and concentration quenching is obtained for 12.5 mol % of Eu3+. Chromaticity coordinates indicate that the phosphor is suitable for potential applications. Furthermore Judd−Ofelt parameters of Eu3+ ions in Ba2La2/3TeO6 are calculated and confirm that Eu3+ ions occupy the symmetric octahedral B-site of the host lattice. ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b03049. Plot of cell volume vs tolerance factor. Deconvoluted Raman spectra of BPTO, BNTO, and BSTO. Peak shift in Raman spectra. Rietveld refinement of XRD patterns of BPTO, BNTO, and BSTO. Combined XRD pattern of BLTO:Eu3+, PL decay curves of Ba2La0.667−xEuxTeO6, Table con ta in in g re finem en t para meter s o f Ba2Ln2/3TeO6 (Ln = Pr, Nd, Sm) (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: gsubodh @gmail.com ORCID

Subodh Ganesanpotti: 0000-0002-6784-094X Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. S.G. designed the project; S.C.L. and V.L. executed the experimental work. S.C.L. wrote manuscript in discussion with S.G. and V.L. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge International Centre for Powder Diffraction Data for granting financial aid to perform the project for synthesizing five novel compounds and Science and Engineering Research Board, India. Authors also acknowledge DST PURSE programme of Univ. of Kerala and Dept. of Optoelectronics, Univ. of Kerala for experimental support. J


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K


3TeO6 (Ln = La

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