A Phosphosilicate Compound, NaCa3PSiO8: Structure Solution and

Dec 1, 2017 - The Rietveld refinement of the powder X-ray diffraction data, followed by the maximum-entropy method analysis, confirmed the obtained st...
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A Phosphosilicate Compound, NaCa3PSiO8: Structure Solution and Luminescence Properties Sanjith Unithrattil,† Paulraj Arunkumar,† Yoon Hwa Kim,† Ha Jun Kim,† Ngoc Hung Vu,† Jaeyeong Heo,† Woon Jin Chung,‡ and Won Bin Im*,† †

School of Materials Science and Engineering and Optoelectronics Convergence Research Center, Chonnam National University, 300 Yongbong-dong, Buk-gu, Gwangju 500-757, Republic of Korea ‡ Institute for Rare Metals & Division of Advanced Materials Engineering, Kongju National University, Cheonan, Chungnam 330-717, South Korea S Supporting Information *

ABSTRACT: NaCa3PSiO8 was synthesized in a microwaveassisted solid-state reaction. The crystal structure of the synthesized compound was solved using a least-squares method, followed by simulated annealing. The compound was crystallized in the orthorhombic space group Pna21, belonging to Laue class mmm. The structure consisted of two layers of cation planes, each of which contained three cation channels. The cation channels in each of the layers ran antiparallel to that of the adjacent layer. All the major cations together constituted four distinct crystallographic sites. The Rietveld refinement of the powder X-ray diffraction data, followed by the maximum-entropy method analysis, confirmed the obtained structure solutions. The electronic band structure of the compound was analyzed through density function theory calculations. Luminescence properties of the compound, upon activating with Eu2+ ions, were analyzed through photoluminescence measurements and decay profile analysis. The compound was found to exhibit green luminescence centered at ∼502 nm, with a typical broadband emission due to the transition from the crystal-field split 4f65d to 4f 7 levels. an opportunity for modification and tuning of the material.8,9 In framework-forming materials, the analogous structural orientations of the cages make it difficult to explore minute structural variations. Recently, several techniques have been developed to explore the structural details of such compounds.10−13 Of these techniques, simulated annealing is one of the most prominent. However, simulated annealing has an inherent drawback that an initial model is required to solve the structure; hence, it is often combined with other techniques, such as charge flipping and Fourier least-squares procedures. One, or a combination, of these methods are often capable of solving complex structures, provided that enough computational capability is reserved. In this work, we synthesized a new phosphosilicate compound, using a microwave-assisted solid-state synthesis method. The lattice parameters of the crystalline compound were estimated from first-principle calculations, and the space group was identified through a least-squares method. The detailed structure solution was carried out through a simulated annealing method, which estimated the agreement between the assumed model and the observed diffraction pattern and chemical restraints. Therefore, the crystal structure solved was verified through the Rietveld refinement and maximum-entropy

1. INTRODUCTION Materials with different frameworks and topologies have an important place in various scientific applications, and several such compounds have been developed over the past few years.1,2 The insights offered by such developments have altered the thought process behind material formation and their potential topologies. Materials with certain particular frameworks are often channelized toward specific applications, based on the understandings then; however, these materials may also offer a considerable number of topologies and are finding new applications that were considered unviable a few decades ago.3,4 Among the most prominent and well-understood materials that form such frameworks are aluminosilicates.5 Certain other frameworks, such as aluminophosphates, gallophosphates, gallosilicates, and phosphosilicates, are also extensively used in specific applications.6,7 Although most of these classes of compounds are natural in origin, specific applications based on these materials have only recently been developed, because of limitations in understanding their precise chemical and structural details. Precise determination of the chemical composition and identification of the exact crystalline structure are preliminary requirements for developing materials targeted to specific applications. This detailed understanding not only provides new material for some advanced applications, but also provides © XXXX American Chemical Society

Received: September 25, 2017

A

DOI: 10.1021/acs.inorgchem.7b02456 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry method (MEM) analysis.14 High-resolution transmission electron microscopy (HRTEM) was also used to confirm the lattice parameters of the solved crystal structure. Photoluminescence properties of the structure were also studied by activating it with the broadband-emitting activator (Eu2+), and the structural and luminescence correlation was also verified.

pixels per unit lattice parameter, using 515 reflections derived from the Rietveld refinement. 2.5. Density Functional Theory Calculations. Density functional theory (DFT) calculations were performed with a CASTEP program19 package on a 4 × 5 × 6 model super cell of the structure obtained through structural refinements. Structure optimization was performed until all the atoms tended to their fully relaxed positions. The electronic structure was calculated using the Perdew−Burke− Ernzerhof form of the Generalized Gradient Approximation (GGA). A plane-wave basis set was used with a kinetic energy cutoff for density mixing at 410 eV, and K-point sampling was chosen as the 2 × 2 × 2 Monkhorst−Pack grid (separation ∼0.04 Å−1). 2.6. Electron Microscopy and Luminescence Measurements. Fast Fourier transform (FFT) spectra obtained from HRTEM micrographs were analyzed using TECNAI F20. Luminescence properties were investigated using room-temperature photoluminescence (PL) spectra measured using a Hitachi F-4500 fluorescence spectrophotometer, over the wavelength range of 200−700 nm. Diffuse reflectance absorption spectra were recorded using a Thermo Scientific Evolution 220 ultraviolet−visible light (UV-vis) spectrophotometer in the wavelength range of 220−600 nm. Luminescence-decay measurements were performed using a 374-nm pulsed-laser diode and spectrometer at the Korea Advanced Institute of Science and Technology, South Korea.

2. MATERIALS AND METHODS 2.1. Synthesis and Crystallization. Powder samples of NaCa3PSiO8 (NCPS) and Eu2+-doped NCPS (NCPS:Eu2+) were prepared via a microwave-assisted solid-state reaction. Stoichiometric amounts of Na2CO3 (Aldrich, 99.9%), CaCO3 (Aldrich, 99.9%), NaPO3 (Aldrich, 99.99%), SiO2 (Aldrich, 99.9%), and Eu2O3 (Aldrich, 99.99%) were used as starting materials. The powder reagents were homogenized by grinding with a mortar and pestle, using acetone as the dispersing medium. The homogenized powder was then pelletized under an axial load of 1.5 tons and fired in an alumina crucible. Samples were first heated in a microwave furnace at 1300 °C for 10 min. The pellets were then ground and subsequently fired in a tube furnace at various temperatures in the range of 1300−1400 °C, under a reducing atmosphere of H2/N2 (5:95 (v/v)). The ramping time of the samples was set at 22 min and 5 h in the microwave and tube furnace, respectively. Samples were then naturally cooled down to room temperature before grinding to a fine powder. 2.2. Structure Solution. Powder X-ray diffraction (XRD) data were collected on a Philips X’Pert Pro diffractometer, using CuKα radiation (Philips X’Pert), over an angular range of 10° ≤ 2θ ≤ 100°, with a step size of 0.026°. Indexing of the obtained XRD peaks in the range of 10°−60° was carried out using N-TREOR,15 which is included in the EXPO package.16 The possible unit-cell dimensions and corresponding space groups were identified using the first 25 dvalues by applying systematic theta-zero point shifts. The plausible space group was identified from the systematic absence of indexed peaks. The identified structures were refined PIRUM in the package and the plausible space groups were chosen based on the figures of merit. A least-squares procedure was followed to obtain the cell dimensions, and atoms belonging to the compound were introduced in a stepwise manner. A reasonable structural model was selected by repeating the described steps. Structure solutions in the identified space groups were obtained through quantifying the feasibility of the cation and anion arrangements through a simulated annealing method performed in EXPO. 2.3. Rietveld Refinement. Refinement was carried out on the XRD data, using the General Structural Analysis System (GSAS).17 The structure solution obtained from the simulated annealing was used as the starting model. The profile shape was fitted with a split pseudo-Voigt function, while a shifted first-degree Chebyshev polynomial was used to fit the background. The atomic positions of the major cations were interchanged until a reasonable structure was obtained. The interchanged structure was again checked for consistency using simulated annealing after a reasonable convergence was achieved. After a few repetitive cycles, the final refinement was performed. The background coefficients, zero-point, half-width, pseudo-Voigt, and asymmetry parameters for the peak shape, scale factor, and unit-cell parameters were refined until convergence was achieved. 2.4. MEM Analysis. MEM analysis was performed with the PRIMA package,18 using the structural factors obtained from refinement. The calculations were performed using a limited-memory Broyden−Fletcher−Goldfarb−Shanno (BFGS) algorithm. The structural model obtained from the Rietveld refinement was used as the input model for PRIMA cycles. Subsequently, Rietveld refinement and MEM analyses were repeated alternately until a reasonable structural model was obtained. In the successive cycles of Rietveld refinement and MEM calculations, the structural factors were fixed, according to those obtained in the preceding MEM cycles, and the profile parameters were refined. The three-dimensional (3D) energy density of the structure was calculated with a resolution of 256 × 256 × 256

3. RESULTS AND DISCUSSION 3.1. Compound Structure Solution. Indexing of the powder diffraction pattern of NCPS provided an orthorhombic phase with a satisfactory figure of merit. The possible space groups were selected based on systematic absence in the indexed peak profiles. Most of the chosen space groups failed to converge, providing Rp > 40%. In the subsequent cycles, the number of peaks were reduced by avoiding ∼15 minor peaks in the selected range. The integrated intensities were used to estimate the prominent reflections and the corresponding lattice parameters were derived through an ab initio leastsquares procedure. All the space groups compatible with the k + l = 2n reflection condition were tested for convergence. The search in the space groups belonging to the Pna_ extinction symbol yielded reasonable convergence and, from the plausible model, orthorhombic Pna21 was selected as the starting model. Each of the major cationsnamely, Na and Caalong with four O atoms were used to derive the primary structure through a direct method. The number of atoms was increased in the subsequent cycles so that the volume per atom matched the estimated value of ∼12. The initial model obtained from the random-model-based method (RAMM) procedure20 was extracted and used as an input model for subsequent simulated annealing to derive the approximate structure. In the process, ∼1084 configurations were calculated; from these, the best configurations were selected and used for Rietveld refinement. After reasonable convergence, satisfying the major peaks of the XRD profile, the positions of the elements were manually interchanged to obtain a reasonable structure. The structural model thus obtained was used as the starting model for the subsequent cycles of simulated annealing. Upon repeating this process for many cycles, a structural model with reasonable bond lengths was obtained. The unit-cell content of the structure along the (010) plane is shown in Figure 1a. The converged model of NCPS displays orthorhombic symmetry (space group Pna21), with all the atoms in the general position 4a. The lattice parameters are obtained as 13.5585(4), 5.4720(2), and 9.2572(3) Å. The Rietveld refinement is shown in Figure 2, and the inset of Figure 2 shows the ORTEP21 image of the NCPS molecule. B

DOI: 10.1021/acs.inorgchem.7b02456 Inorg. Chem. XXXX, XXX, XXX−XXX

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Table 1. Crystal Data of NaCa3PSiO8 Obtained from Rietveld Refinement, along with the Refinement Parametersa parameter

value/remark NaCa3PSiO8 330.28 orthorhombic Pna21 293 K

chemical formula molar weight crystal system space group temperature unit-cell parameters a b c V Z radiation type Kα1 Kα2

Figure 1. (a) Unit-cell representation of NaCa3PSiO8. The green, orange, red, blue, and brown spheres indicate Na, Ca, P, Si, and O atoms, respectively. (b) Polyhedral representation of the major cations in the unit cell.

13.5585(4) Å 5.4720(2) Å 9.2572(3) Å 686.82(3) Å3 4 λ = 1.540500 Å λ = 1.544300 Å Data Collection

diffractometer data collection mode 2θ values 2θmin 2θmax 2θstep

Philips X’Pert Pro reflection 10.03° 119.96° 0.026° Refinement

R factors and goodness of fit Rp Rwp Rexp R(F2) χ2 No. of data points No. of parameters No. of restraints

0.047 0.064 0.051 0.06225 2.86 4229 26 0

a

In each case, the number in parentheses is the estimated standard deviation of the least significant figure.

The two types of Si and P tetrahedra in the structure are noninteracting and independently oriented. The Si tetrahedra formed by the atoms at Si sites share one of their faces with adjacent Na polyhedra, while no other tetrahedra share their faces with large cation polyhedra. All the O atoms in the crystal belong to either SiO44− or PO43− tetrahedra, whose average bond lengths are 1.544(4), 1.566(11), and 1.588(1) Å for P and Si tetrahedra arranged in the respective order parallel and antiparallel along the a-axis of the crystal. The relevant bond lengths of the atoms in the structure are listed in Table S1. The two cation layers of the unit cell observed from the (100) plane can be well-classified as two antiparallel arrangements of a layer that consists of three channels of cations. One of these channels consists of a sequence of Si or P polyhedra and alternate Na or Ca atoms. The second channel, adjacent to the latter, consists of an antiparallel arrangement of the latter channel. The third channel is a slightly zigzag arrangement of Ca atoms, which together run linearly along the a-axis. Figures S1(a) and S1(b) in the Supporting Information illustrates these sequences. The local compositional structure was analyzed through MEM analysis performed on the XRD profile obtained at room temperature. The obtained structural model from the Rietveld refinement was used as is, without further modification, in the initial cycle. Subsequently, Rietveld refinement and MEM

Figure 2. Rietveld refinement of the powder X-ray diffraction (XRD) profile of NaCa3PSiO8. Data (points) and fit (lines), difference profile, and expected reflection positions are displayed. The inset shows the coordination environment of the major cations in the structure. The inset of the figure shows the ORTEP image of the molecule with an ellipsoid probability of 50.

The relevant structural and refinement parameters are provided in Table 1, and the fractional coordinates of lattice points are listed in Table 2. The structure has one Na site, three Ca sites, one P site, one Si site and eight O sites. The inset of Figure 1b shows the polyhedral representation of the Na and Ca sites, along with Si and P tetrahedra in the unit cell. In the structure, Na occupies a site with 9-fold coordination and Ca occupies three crystallographically distinct sites, with 8-fold coordination for Ca1 and Ca2 sites and 9-fold coordination for Ca3 sites. The geometric parameters of the atomic bonds in the structure are provided in Table S1 in the Supporting Information. C

DOI: 10.1021/acs.inorgchem.7b02456 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 2. Fractional Atomic Coordinates of Atoms in NaCa3PSiO8 Obtained from Rietveld Refinementa Atomic Parameters

a

atom

site

x

y

z

occupancy

Na Ca1 Ca2 Ca3 P Si O1 O2 O3 O4 O5 O6 O7 O8

4a 4a 4a 4a 4a 4a 4a 4a 4a 4a 4a 4a 4a 4a

0.7972(9) 0.5378(5) 0.3792(6) 0.3707(7) 0.7618(7) 0.5150(9) 0.6204(17) 0.7478(15) 0.7384(16) 0.7110(18) 0.4674(16) 0.4565(15) 0.6247(12) 0.5166(21)

0.7449(32) 0.7643(25) 0.2571(26) 0.7446(26) 0.7725(26) 0.7600(4) 0.3220(31) 0.5410(5) 0.0030(5) 0.6680(4) 0.5200(5) 0.7650(6) 0.7420(5) 0.0310(6)

0.5625(12) 0.2119(9) 0.3266(9) 0.9201(9) 0.2024(11) 0.5334(11) 0.6695(22) 0.3188(26) 0.3238(25) 0.0786(23) 0.4884(23) 0.6878(23) 0.5498(20) 0.9575(24)

1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0

U [Å2] 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0.

01907 01864 01386 02414 01468 02406 03803 02289 02501 03191 01378 02377 01940 00685

The number in parentheses indicates the standard deviation of the least significant figures.

analyses were repeated alternately until a reasonable structural model was obtained. In the successive REMEDY cycles22 and Rietveld refinement, the structural factors were fixed as per those obtained in the preceding MEM cycles, and the profile parameters were refined. The iterative process was repeated until all the constraints set in the model were satisfied with a reasonable convergence of R values. The three-dimensional (3D) energy density of the structure was calculated with a resolution of 256 × 256 × 256 pixels per unit lattice parameter, using 515 reflections derived from the Rietveld refinement. The final reliable R factors of MEM electron density are defined by ∑ R WF =

1 |F σ2 0

∑

− FMEM|

1 |F | σ2 0

(1)

where F0 is the observed structure factor, σ is their estimated standard deviation, FMEM is a structure factor estimated by the MEM analysis, and the summation was conducted over the reflections analyzed by MEM. RWF was determined to have a value of 1.92%.14 Figure 3a shows the electron-density distribution of the cation polyhedra along the (002) plane and the corresponding MEM isosurface. The electron-density distribution around the major cation, Ca, occupying the Ca1 site, is found to be more distorted from spherical symmetry, compared to the rest of the major cations. The charge-density distribution of Na is one of the most spherical among the major cation positions. This could be taken as an indicator of antisite disordering, where Ca1 positions may be occupied by the Na ions. The possibility of Ca occupying Na sites appears to be negligible. These differently charged units will have different polarizabilities, determining the availability of electrons for bond formation. This leads to nonspherical charge-density distribution around the major cation sites. In the electron-density distribution map, the covalent nature of the Si−O and P−O bonds can also be seen. The electronic band structure and density of states of the compound calculated from the structural data are shown in Figure 3b. Partial density of states for the individual elements are shown in Figure S2 in the Supporting Information. The structure has a direct band gap of 4.45 eV. The minimum of the conduction band is formed from Ca 3d states at G, and a

Figure 3. (a) Electron-density distribution in NaCa3PSiO8 calculated using the MEM and shown alongside the unit cell. The spheres at the center of the isosurface indicate the corresponding atoms. (b) Electronic band structure of NaCa3PSiO8 and (c) the density of states.

subsidiary minimum also exists at Y. The conduction band is predominantly formed from Ca 3d states, with additional contributions from Na 3s and Na 3p, Ca 4s, P 3s and P 3p, Si 3s and Si 3p, and O 2p states. The width of the conduction band is ∼7.2 eV. The upper valence band of NCPS is mostly formed from O 2p states, with contributions from P 3p and Si 2p states, with a bandwidth of ∼7.4 eV. The lower part of the valence band between −5.1 eV and −6.1 eV is mostly formed from P 3p and O 2s states, with contributions from Na 2p, Si D

DOI: 10.1021/acs.inorgchem.7b02456 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 4. (a) HRTEM image and (b) SAED pattern of the NaCa3PSiO8 crystal. The (hkl) planes are indexed based on the structural data obtained from Rietveld refinement of the XRD profile. The inset of panel (b) shows the reciprocal lattice drawn by interconnecting the diffraction spots.

Figure 5. (a) PL excitation and emission spectra of NaCa3PSiO8:xEu2+ as a function of activator concentration x. (b) Dependence of activator concentration on phosphor emission intensity. (c) Log−log plot of activator-ion concentration and intensity per activator ion of NaCa3PSiO8:Eu2+.

3s, P 3s, and P 3p states. A third band positioned between −7.4 and −7.9 eV is formed by contributions from P 2s, O 2s, and 2p states. The band structure below −16 eV mostly consists of several narrow bands. The narrow bands, at around −16 eV and −17 eV, are mostly formed from O 2s and Si 2p states, and P 2s, Si 2s and O 2s states, respectively. The bands below this are predominantly formed from Ca 2p and Na 2s states, with contributions from P 2s and O 2s states. Figure 4a shows the HRTEM image of the NCPS powder samples. The well-defined lattice fringes, with d-spacing corresponding to the (013) lattice planes of the orthorhombic NCPS, are clearly observed. The measured d-spacing is found to be in good agreement with the d013 spacing of the corresponding plane obtained from the Rietveld refinement. This plane also corresponds to the major peak of the XRD profile. Figure 4b shows the recorded selected-area electron diffraction (SAED) pattern of the sample. The SAED pattern displays regularly arranged diffraction spots, which are indexed to the major planes of the structure. The indexed planes are found to be consistent with the XRD pattern of the structure and interconnection of the diffraction spots yields a reciprocal lattice with lattice parameters corresponding to that of the calculated values, as shown in the inset of Figure 4b. 3.2. Luminescence Properties. The optical properties of the compound were analyzed by activating the structure with Eu2+ ions. PL excitation and emission spectra of NCPS:xEu2+, for x values in the range of 0.0025−0.03, are shown in Figure 5a. With near-UV excitation at 324 nm, the phosphor shows a

broad emission band ranging from 400 nm to 650 nm. The broad band, with a full width at half maximum (fwhm) of ∼102 nm, is due to the closely spaced 4f 0−4f65d transition bands of Eu2+ from multiple sites in the host lattice that can accommodate divalent dopant ions. In addition to the multicomponent emission spectra of Eu2+, narrow peaks due to Eu3+ transitions are also observed in all the samples, especially those with higher dopant concentrations. However, neither the excitation peaks corresponding to the Eu3+ transitions, nor the charge-transfer band or host absorption band, are observed in the excitation bands of the moderately doped samples. In heavily doped samples, faint peaks corresponding to the Eu3+ transitions are observable. Figure 5b shows the dependence of emission intensity on Eu2+ concentration. Concentration quenching is observed above a nominal doping level of 0.025 mol. This is probably due to the presence of multiple sites accommodating Eu2+ ions in the host lattice. The critical distance for concentration quenching was calculated using Blasse’s relation, which is given as ⎡ 3V ⎤1/3 R c ≈ 2⎢ ⎥ ⎣ 4πXcN ⎦

(2)

where V is the volume of the unit cell, N is the number of host cations in the unit cell, and Xc is the critical concentration. The critical distance for concentration quenching was found to be 27.94 Å, which is greater than the typical separation between E

DOI: 10.1021/acs.inorgchem.7b02456 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 6. (a) Deconvolution of the emission band of NaCa3PSiO8:Eu2+ into its component Gaussian peaks. (b) Diffuse reflectance spectra of NaCa3PSiO8 with and without introducing activator ions. (c) The luminescence decay profile of NaCa3PSiO8:Eu2+ fitted with a three-component exponential decay model.

Ca3+ ions, and 1.23 Å for nine-coordinated Na+ ions, while Ea (regardless of coordination environment) was defined to have a value of 1.62 eV. The calculated Eu2+ emission-peak positions in the nine-coordinated Na+ site, nine-coordinated Ca2+ site, and eight-coordinated Ca2+ sites are 20 451 cm−1 (489 nm), 19 347 cm−1 (517 nm), and 18 121 cm−1 (552 nm), respectively. The calculated energy of each emission band matches approximately with the deconvoluted Gaussian peaks, indicating that Eu2+ ions substitute at three of the possible lattice sites. To understand the intrinsic absorbance of the host and the doped phosphor, diffuse reflectance spectra were measured. While the host compound shows a noticeable absorption band at ∼245 nm, it shows high reflectance across the entire visible region (Figure 6b). In contrast, the doped compound shows significant absorption in the UV and near-UV regions, which extends into the visible region, to 420 nm. The intrinsic absorbance of the host compound is strongest at 245 nm, while the dopant absorption is strongest at ∼350 nm. The latter prominent absorption band, which is only observed in the doped compound, is assigned to the 4f−5d transitions of Eu2+ ions substituted at various sites in the structure. This is in good agreement with the excitation spectrum of the Eu2+-doped compound monitored at 502 nm. From the absorption edge of the compound, the optical band energy gap can be estimated by extrapolating the reflectance curve near the absorption edge. The estimated value of the optical band gap (Eg) is 4.84 eV. The proposed multiband emission from three Eu2+ sites and the possibility of energy-transfer mechanisms between the Eu2+ levels that are only separated by small energy differences were analyzed from the luminescence decay of the phosphor. Decay profiles of the activator ions in the NCPS host lattice were measured by excitation with a pulsed laser. The decay profile monitored at 505 nm was found to have a multicomponent decay mechanism. The decay profile shown in Figure 6c is fitted with a three-component exponential decay model of the form,25

the possible activator sites in the crystal lattice. The energytransfer mechanism that contributes toward the concentration quenching was analyzed using the relation I k = x 1 + β(x)θ /3

(3)

where k and β are constants, and θ assumes values of 6, 8, and 10 for dipole−dipole, dipole−quadrupole, and quadrupole− quadrupole interactions, respectively. Figure 5c shows a plot of the log (I/x) versus log(x) relationship, and the slope of the linear fit is found to be −2.02 for NCPS:Eu2+. Generally, the energy transfer of a forbidden transition occurs through exchange interaction. In the case of NCPS:Eu2+, the overlap between excitation and emission spectra is minimal; therefore, exchange interaction could be ruled out as a concentrationquenching mechanism. The asymmetric emission band of NCPS:Eu2+ can be deconvoluted into three Gaussian components, with peak positions at 476, 520, and 572 nm, along with trivalent Eu3+ emission peaks, as shown in Figure 6a. This assumption is based on three types of possible sites in the crystal lattice; namely, one nine-coordinated Na+ site, one nine-coordinated Ca2+ site, and two eight-coordinated Ca2+ sites, which have matching radii, because of the asymmetric peak, which is not characteristic of the single component Eu2+ emission. In addition, the extremely low value of the critical concentration suggests that the lattice contains closely spaced centers with similar energy levels, enabling easy and efficient transfer of excited-state energy to the quenching centers. To estimate the number of possible sites for Eu2+ emission and their corresponding energies, an empirical equation proposed by Van Uitert was employed.23 According to the relation24 ⎡ ⎤ ⎛ V ⎞1/ V × 10−(nEar)/80⎥ E (cm−1) = Q ⎢1 − ⎜ ⎟ ⎝4⎠ ⎢⎣ ⎥⎦

(4)

2+

where E is the Eu emission-peak position, Q is the energy of the lower d-band edge for the free Eu2+ ion (Q = 34 000 cm−1), V is the valence of the Eu2+ ion (V = 2), n is the number of anions in the immediate shell about the Eu2+ ion, Ea (eV) is the electron affinity of the anion atom, and r (Å) is the radius of the host cation replaced by the Eu2+ ion. Here, r was defined as 1.12 for eight-coordinated Ca3+ ions, 1.18 for nine-coordinated

⎛ t ⎞ ⎛ t ⎞ ⎛ t⎞ I(t ) = I1 exp⎜ − ⎟ + I2 exp⎜ − ⎟ + I3 exp⎜ − ⎟ ⎝ τ2 ⎠ ⎝ τ1 ⎠ ⎝ τ3 ⎠

(5)

where I1, I2, and I3 are constants and τ1, τ2, and τ3 are the decay times of the constituent components. The effective lifetimes of F

DOI: 10.1021/acs.inorgchem.7b02456 Inorg. Chem. XXXX, XXX, XXX−XXX

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the Eu2+ ions in the excited states of the host structure can be calculated using the equation τ* =

I1τ12 + I2τ2 2 + I3τ32 I1τ1 + I2τ2 + I3τ3

ACKNOWLEDGMENTS This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Science, ICT and Future Planning (No. 2017R1A2B3011967).

(6)

■

and were found to be 0.40 and 0.26 μs for activator concentrations of 0.0125 and 0.025 mol, respectively. The constituent decay times are typical of Eu2+ emission and the values of the constants I1, I2, and I3 (provided in Table S3) confirm the assumption of Eu2+ substitutability at three possible types of sites in the structure. From the decay constants and from the relative strength of the higher wavelength emission in the emission spectra, it can be concluded that the activator ions, although occupying all the major cation sites, is preferentially occupied at sites with higher coordination environment.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02456. Listings of Rietveld refinement results, bond lengths, luminescence decay parameters, unit cell, and partial density of states (PDF) Accession Codes

CCDC 1576433 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, U.K.; fax: +44 1223 336033.

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REFERENCES

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4. CONCLUSIONS The crystal structure of the new composition NaCa3PSiO8 has been successfully. The least-squares method combined with simulated annealing was used to analyze different solutions and reach a reasonable solution. The compound was identified as crystallized in an orthorhombic space group, Pna21. The obtained structure was analyzed with MEM analysis and DFT calculations and the results confirmed the structure solutions. HRTEM analysis was used to confirm the lattice parameters obtained in the structure solution and the lattice planes have been indexed. The photoluminescence properties of the compound were analyzed by activating with Eu2+ ions. The compound was found to have a broad emission due to 5d → 4f transitions in Eu2+ ions. Activator ions were identified as occupying three crystallographically distinct sites and were verified by analyzing the decay profile of the Eu2+ emission.

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*Tel.: +82-62-530-1715. Fax: +82-62-530-1699. E-mail: [email protected]. ORCID

Sanjith Unithrattil: 0000-0001-9072-7163 Paulraj Arunkumar: 0000-0002-9308-085X Jaeyeong Heo: 0000-0002-2602-6538 Won Bin Im: 0000-0003-2473-4714 Notes

The authors declare no competing financial interest. G

DOI: 10.1021/acs.inorgchem.7b02456 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/acs.inorgchem.7b02456 Inorg. Chem. XXXX, XXX, XXX−XXX