Structure Transformation and Cerium-Substituted Optical Response

DOI: 10.1021/acs.inorgchem.7b02898. Publication Date (Web): December 20, 2017. Copyright © 2017 American Chemical Society. *E-mail: [email protected]...
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Article Cite This: Inorg. Chem. 2018, 57, 519−527

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Structure Transformation and Cerium-Substituted Optical Response across the Carbonitridosilicate Solid Solution (LaδY1−δ)2Si4N6C (δ = 0− 0.5) Anna C. Duke, Martin Hermus, and Jakoah Brgoch* Department of Chemistry, University of Houston, Houston, Texas 77204, United States

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ABSTRACT: Following an investigation proving La2Si4N6C crystallizes in a monoclinic space group, isostructural to Y2Si4N6C, the reportedly hexagonal (La0.5Y0.5)2Si4N6C was reinvestigated to examine the apparent crystal structure change across the solid solution. Initially, calculating the electronic structure and phonon density of states of (La0.5Y0.5)2Si4N6C in the P63mc space group revealed an imaginary phonon mode, which is indicative of a structural instability. Displacing the atoms along the pathway of the imaginary vibration led to a previously unreported space group for carbonitridosilicates, trigonal P31c. The assignment of the trigonal space group was subsequently confirmed by synthesizing (La0.5Y0.5)2Si4N6C using high-temperature, solid state synthesis and analyzing the crystal structure with high-resolution synchrotron X-ray powder diffraction. Preparing the solid solution, (LaδY1−δ)1.98Ce0.02Si4N6C (δ = 0−0.5), showed that the crystal structure changes from the monoclinic to the trigonal space group at δ ≈ 0.25. Finally, substituting Ce3+ in the crystal structure to investigate the optical response via steady-state luminescent and photoluminescent quantum yield measurements reveals severe luminescent quenching with increasing La3+ content, due to a combination of absorption of luminescence by the host structure and thermal quenching. These results display the virtue of combining computational and experimental techniques to solve inorganic crystal structures and assess potential phosphor hosts.

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INTRODUCTION LED lighting is an evolving technology poised to replace conventional lighting sources like incandescent or compact florescent bulbs. LEDs are ideal for the next generation of lighting because they have long operating lifetimes, high efficiencies, and contain environmentally benign materials.1,2 These devices typically produce white light by converting the emission from a blue or near-UV LED with at least one inorganic phosphor, which consists of a host structure like an oxide or nitride substituted with a rare-earth luminescent center such as Ce3+ or Eu2+.3 The LED emission is at least partially absorbed by the phosphor and down-converted to longer wavelengths; the combination of the LED and phosphor emission yields a broad spectrum white light.4 When searching for new, efficient phosphors, research has shown that materials with a high photoluminescent quantum yield (PLQY) tend to consist of crystallographically ordered, highly symmetric, and structurally rigid host structures.5,6 This reduces nonradiative (vibrational) relaxation pathways that can quench photoluminescence, thereby negatively impacting the PLQY.7 The best descriptor for identifying rigid inorganic host crystal structures is by comparing materials’ Debye temperatures (ΘD), which can be estimated using ab initio calculations.8,9 Although exceptions have been reported, a high ΘD often indicates that a © 2017 American Chemical Society

structure is rigid and expected to possess a high PLQY, whereas a low ΘD usually signifies the presence of nonradiative relaxation pathways and, therefore, a low PLQY.10 In the search for new phosphor hosts that have a high ΘD, one series of compounds that are prime candidates for investigation are nitridosilicates. These compounds possess covalently bonded, dense [SiN4] tetrahedral units resulting in exceptional mechanical response and optical properties.11−13 One specific nitridosilicate of interest follows the general composition AERESi4N7 (AE = Sr, Ba, Eu; RE = Y, Yb, Lu) and crystallizes in the hexagonal space group P63mc (Figure 1a). This compound is particularly unique because the crystal structure contains a four-coordinated nitrogen atom forming a star-shaped N(SiN3)4 unit, as illustrated in Figure 1b.14,15 The nitrogen at the center of the star-shaped structural unit possesses a formally positive charge, suggesting that the substitution of a more negative atom like carbon could result in a distinct series of compounds, carbonitridosilicates.16 Carbonitridosilicates can be obtained from the parent AERESi4N7 compound by substituting C4− for N3− along with exchanging the divalent cation for a trivalent cation to Received: November 13, 2017 Published: December 20, 2017 519

DOI: 10.1021/acs.inorgchem.7b02898 Inorg. Chem. 2018, 57, 519−527

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Inorganic Chemistry

composition, and the luminescent properties across the solution could prove very interesting. This contribution expands our investigation of carbonitridosilicates by focusing on the crystal chemistry of (LaδY1−δ)2Si4N6C (δ = 0−0.5). The recent discovery of the structural distortion in the La analogue (δ = 1) from the reported crystal structure prompted a closer investigation of the relationship between the composition and crystal structure in this composition space. As an ordered variant in the solid solution, this research first focused on examining the reported crystal structure of (La0.5Y0.5)Si4N6C using density functional theory (DFT) total energy calculations along with inspecting the phonon band structure to ascertain the most probable space group. These calculations were followed by validation of the space group by synthesizing (La0.5Y0.5)Si4N6C and analyzing the crystal structure using high-resolution synchrotron X-ray power diffraction data. The solid solution (LaδY1−δ)2Si4N6C (δ = 0−0.5) was then prepared with Ce3+ substitution, and the optical response was investigated using steady-state photoluminescence measurements. This work highlights the capability of using ab initio calculations in combination with highresolution diffraction data and photoluminescence spectroscopy to decipher the composition−structure−property relationship in carbonitridosilicates. Our results emphasize the potential of using computation and experiment in tandem to evaluate phosphor hosts.

Figure 1. (a) Structure of hexagonal P63mc AERESi4N7. (b) The starshaped N(SiN3)4 unit is highlighted.

maintain charge balance, culminating in a general formula RE2Si4N6C, where RE is a trivalent rare-earth. The first compositions reported in this class of compounds, Ho2Si4N6C and Tb2Si4N6C, crystallize in monoclinic P21/c and contain carbon at the center of a star-shaped unit, C(SiN4)4, as predicted.17−19 The site preference is directed by carbon’s proclivity for tetrahedral coordination and the higher charge density of the central site.19 Substituting the RE3+ ion in this phase for Y3+, Gd3+, and Lu3+ yields isotypic, isostructural compounds,20,21 whereas La2Si4N6C was initially reported to crystallize in orthorhombic Pnma, a supergroup to P21/c.18 However, ab initio calculations combined with synchrotron Xray powder diffraction and total neutron scattering proved that La2Si4N6C actually crystallizes in space group P21/c, with a subtle monoclinic distortion of only β = 90.13°, explaining the difficulty in determining the original space group assignment.19 These systems are also reported to form solid solutions between many of the end members; for example, (La0.5Y0.5)2Si4N6C is reported to crystallize in hexagonal space group P63mc.18 La3+, Gd3+, Y3+, and Lu3+ have also all been shown to form a solid solution with Sc3+ producing the general formula (M0.5Sc0.5)2Si4N6C with (Lu0.5Sc0.5)2Si4N6C crystallizing in a monoclinic space group, while the other scandium analogues are reported in the hexagonal space group P63mc, like the parent nitridosilicate AERESi4N7.22 Beyond the distinct crystal chemistry of carbonitridosilicates, these compounds also show promise as inorganic phosphors. In fact, the end members Gd2Si4N6C:Ce3+ and Lu2Si4N6C:Ce3+ have been reported as efficient phosphors with λem,max = 610 nm and λem,max = 540 nm, respectively.21 Y2Si4N6C:Ce3+ has also been reported to efficiently luminesce in the yellow-green region of the visible spectrum (λem,max = 538 nm) when excited by 384 nm light.21,23 The desire to tune the emission color through the substitution of the smaller Sc3+ led to an unexpected blue shift of luminescence to λem,max = 490 nm, whereas the substitution of La3+ led to an expected blue shift to λem,max = 442 nm.21−23 Nevertheless, the relationship between the optical properties and the expected change in the crystal structure from the monoclinic P21/c space group of both end members (La2Si4N6C and Y2Si4N6C) to the hexagonal P63mc space group across the solid solution has been largely overlooked. Thus, relating the crystal structure, chemical

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EXPERIMENTAL SECTION

Density Functional Theory Calculations. DFT calculations were conducted using the Vienna Ab initio Simulation Package (VASP). The calculations employed a plane-wave basis set with projector augmented wave (PAW) potentials.24 The Generalized Gradient Approximation (GGA) was used with exchange and correlation described by the Perdew−Burke−Ernzerhof (PBE) functional.25,26 The total energy calculations used a plane-wave cutoff energy of 500 eV, a 6 × 6 × 4 Γ-centered k-point mesh, and a convergence criteria of 1 × 10−8 and 1 × 10−6 eV for the electronic and structure relaxation, respectively. The structural model for P63mc (La0.5Y0.5)Si4N6C was constructed starting from the nitridosilicate, BaYbSi4N7, and substituting lanthanum on the Ba2+ site and yttrium on the Yb3+ site, while C4− was substituted onto the Wyckoff position 2a site to produce the carbonitridosilicate. This crystal structure model was optimized allowing the atomic positions, unit cell shape, and unit cell volume to change. The phonon density of states (phDOS) was subsequently calculated for (La0.5Y0.5)Si4N6C in the hexagonal space group (P63mc) using PHONOPY with a 2 × 2 × 1 supercell.27 Following the discovery of imaginary modes in the hexagonal space group phDOS at the Γ-point of the Brillouin zone, the atoms were displaced along the direction of the imaginary mode and the crystal structure was reoptimized. FINDSYM was used to determine the symmetry of the new, reoptimized space group and the phDOS was again calculated.27−29 The chemical bonding of (La0.5Y0.5)2Si4N6C was then evaluated based on the crystal orbital Hamilton population (−COHP) using the Local-Orbital Basis Suite toward ElectronicStructure Reconstruction (LOBSTER) code.30−33 Band gaps (Eg) were calculated using the HSE06 screened hybrid functional with a cutoff energy of 500 eV and a Γ-centered k-point mesh of 4 × 2 × 2 for Y2Si4N6C and 6 × 6 × 4 for (La0.5Y0.5)2Si4N6C.34 Synthesis and X-ray Diffraction. (LaδY1−δ)2Si4N6C (δ = 0, 0.0625, 0.125, 0.167, 0.25, 0.33, 0.417, 0.5) was prepared by grinding stoichiometric mixtures of the metal nitride and metal carbide powders, LaN (Materion Advanced Chemicals, 99.9%), YN (Materion Advanced Chemicals, 99.9%), α-Si3N4 (Thermo Fisher, 95%), and SiC (Alfa Aesar, 99.8%), with 2.5 wt % BaF2 and 0.5 wt % NH4Cl added as fluxes, in an argon atmosphere (O2 < 0.5 ppm; H2O < 0.5 ppm). To prepare the phosphor, Ce3+ was incorporated onto the RE site by adding a stoichiometric ratio of CeN (Materion Advanced Chemicals, 520

DOI: 10.1021/acs.inorgchem.7b02898 Inorg. Chem. 2018, 57, 519−527

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Inorganic Chemistry 99.9%), resulting in the general formula (LaδY1−δ)1.98Ce0.02Si4N6C. The reactants were heated in a graphite crucible at a rate of 20%/min to 80% power for 1 h in a 3 kW radio frequency furnace (≈1700 °C by optical pyrometer) under flowing, purified nitrogen then cooled at a rate of ≈18%/min to room temperature. This was followed by heating in air for 4 h at 700 °C to remove excess carbon.17,19,35 As there were no apparent single crystals present in the sample, the resulting products were ground into fine powders using an agate mortar and pestle and the phase purity was analyzed using a PanAnalytical X’Pert powder diffractometer with Cu Kα (λ = 1.54183 Å) radiation. To confirm the space group of (La0.5Y0.5)2Si4N6C identified by DFT, highresolution synchrotron X-ray powder diffraction (11-BM; Advanced Phonon Source)36,37 was obtained at 100 K with λ = 0.414174 Å. Rietveld refinements were conducted using the EXPGUI interface for the General Structure Analysis System (GSAS) with a shiftedChebyshev function to fit the background and a pseudo-Voigt function to fit the peaks.38,39 All structures drawings were created using VESTA.40 Optical Properties. Each pure phase Ce3+-substituted sample was mixed with silicon resin (GE Silicones, RTV615) and cast on a quartz (Chemglass) slide for optical property measurements. Steady-state luminescence measurements were conducted using a PTI QuantaMaster with a 75 W xenon arc lamp. A Janis cryostat (VPF-100) was used to obtain luminescent measurements between 80 and 500 K. The photoluminescent quantum yield was measured using a 150 mm Spectralon-coated integrating sphere and calculated using the method of de Mello et al.41 Diffuse reflectance spectra were obtained using an Agilent Technologies Cary 5000 with a diffuse reflectance attachment. A Kubelka−Munk transformation was used to convert the data to absorbance.42

Figure 2. (a) Calculated phonon density of states of (La0.5Y0.5)2Si4N6C in space group P63mc shows imaginary modes centered at ≈100i that can be attributed to the N atoms (shaded in gray) causing a structural instability and potential pathway for distortion. (b) The phonon band structure displaying the imaginary mode at the Γ-point of the first Brillouin zone that can be attributed to a displacement of the N atoms in the [SiN3C] polyhedra. (c) Calculated phonon density of states of (La0.5Y0.5)2Si4N6C in space group P31c shows no imaginary modes and is therefore dynamically stable.

RESULTS AND DISCUSSION Electronic Structure of (La0.5Y0.5)2Si4N6C. It is remarkable that (La0.5Y0.5)2Si4N6C is reported to crystallize in hexagonal P63mc considering both end members La2Si4N6C and Y2Si4N6C crystallize in monoclinic space group P21/c. In fact, P63mc is not related to P21/c through any group−subgroup relationship. Therefore, we first sought to confirm the space group of (La0.5Y0.5)2Si4N6C considering the La analogue was originally reported with the incorrect space group.18 DFT was first employed to calculate the phonon dispersion curve for (La0.5Y0.5)2Si4N6C in the P63mc space group. As illustrated in Figure 2a, the resulting phonon density of states surprisingly indicates the presence of imaginary phonon modes, which are generally attributed to a structural instability, suggesting that (La0.5Y0.5)2Si4N6C in P63mc is dynamically unstable and prone to distort.43,44 Inspecting the phonon band structure (Figure 2b) reveals the imaginary modes lie at the Γ-point of the Brillouin zone, allowing the visualization of the vibrational modes in real space. Figure 2b demonstrates that nitrogen is the primary contributor to the imaginary modes, also indicated by the partial phDOS, causing a twist of the star-shaped structural unit. Displacing the nitrogen atoms along the path of this imaginary phonon mode and reoptimizing the crystal structure is a method to alleviate the instability and obtain a dynamically stable phase. The resulting distorted crystal structure has lower symmetry, forming space group P31c, and the phonon dispersion curve shows an absence of any imaginary modes, illustrated in Figure 2c. Comparing the total internal (DFT) energy of the two crystal structures shows that the structure in space group P31c is 12.7 meV/atom lower in energy compared to hexagonal P63mc following the removal of the imaginary phonon modes. Comparing the DFT optimized crystal structures indicates small, but meaningful, changes. Most notably, the calculated N1−Si2−C1 angles between the silicon tetrahedra decrease

from 111.44° (P63mc) to 106.56° (P31c), while the N2−Si1− C1 angle increases from 111.44° (P63mc) to 116.62° (P31c). This slight tilt of the C(SiN3)2 tetrahedra breaks the mirror plane and screw axis, breaking the inversion symmetry present in hexagonal P63mc, leading to the lower symmetry space group, trigonal P31c. The differences in the chemical bonding between the hexagonal and the trigonal space groups, as probed by decomposing the structures into their pairwise chemical bonding interactions based on a crystal orbital Hamilton population (−COHP) analysis, reveal one possible origin of the structure preference. The −COHP curves show that the Y−N, Si−N, and Si−C interactions remain essentially the same regardless of crystal structure. The difference arises when comparing the interactions between La3+ and N3−, which are plotted in Figure 3 along with their associated integrated −COHP (ICOHP) values at the Fermi level. The ICOHP for the three distinct La−N bonds shows a decrease by 46%, 37%, and 18% from P63mc to P31c. The more negative values for P31c signify the La−N bonding in the trigonal space group is optimized with fewer antibonding interactions occupied below the Fermi level. The inferred consequence of these changes, considering ICOHP tends to scale with bond strength, is a stronger, more covalent network. Indeed, this is supported by changes in the La−N bond lengths within (La0.5Y0.5)2Si4N6C, which decrease by an average of 3.0555 Å in the calculated hexagonal P63mc crystal structure to 3.0422 Å in the calculated trigonal P31c crystal structure. Experimental Confirmation of the Trigonal Space Group in (La0.5Y0.5)2Si4N6C. To experimentally verify that (La0.5Y0.5)2Si4N6C crystallizes in the trigonal space group P31c, the phase was prepared by reacting the starting metal nitrides and carbides in a radio frequency furnace. Examining the resulting phase pure powder X-ray diffraction pattern indicates that (La0.5Y0.5)2Si4N6C indexes as either hexagonal P63mc,

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DOI: 10.1021/acs.inorgchem.7b02898 Inorg. Chem. 2018, 57, 519−527

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Inorganic Chemistry

Table 1. Structure Refinement Data for (La0.5Y0.5)2Si4N6C Obtained via Rietveld Refinement of Synchrotron X-ray Powder Diffraction Data refined composition radiation type; λ (Å) 2Θ range (deg) T (K) crystal system space group; Z lattice parameters (Å) V (Å3) Rp Rwp Rf 2 χ2

Figure 3. (a) Calculated −COHP curves for the La−N bonds in (La0.5Y0.5)2Si4N6C P63mc show large antibonding peaks below the Fermi level. The inset displays the La−N bonds, using color to relate the bonds to the −COHP curve and the ICOHP. (b) Calculated −COHP curve for the La−N bonds of (La0.5Y0.5)2Si4N6C in P31c displays a significant decrease in the occupied antibonding orbitals while the associated ICOHP values are more negative.

(La0.465(4)Y0.535(4))2Si4N6C Synchrotron (11-BM) X-ray; 0.414174 0.5−49.994 100 trigonal P31c; 2 a = 6.005393(7) c = 9.91450(2) 309.659(1) 9.35 12.71 2.92 6.533

forming a trimer that lies in the horizontal ab plane. These trimers are alternatingly corner connected through the C4− atom and the N3− anions by the Si2 tetrahedra (Wyckoff position 2a), forming a chain along the c-axis, isostructural to αSi3N4. Placed in the holes in between the C(SiN3) tetrahedra are two crystallographically independent RE3+ sites occupied by La3+ and Y3+. The first site, located at Wyckoff position 2b, produces a large [REN12] icosahedron with a polyhedral volume of 66.019 Å3 and an average RE-N bond length of 3.022 Å, whereas the second site, also at Wyckoff position 2b, forms a much smaller [REN6] octahedron with a polyhedral volume of only 16.771 Å3 and a shorter average RE−N bond length of 2.331 Å. The larger polyhedron is primarily occupied by the larger La3+ ion (r9‑coord = 1.216 Å), with a refined La3+:Y3+ occupancy ratio of approximately 8:1, whereas the smaller site is predominantly occupied by Y3+ (r9‑coord = 1.075 Å) over La3+ in about a 22:1 ratio.45 Solid-Solution of (LaδY1−δ)2Si4N6C:Ce3+ (δ = 0−0.5) and Corresponding Optical Response. To study the relationship between the structure change and the luminescent properties, the solid solution was prepared including 1% Ce substitution on the RE3+ site as the luminescent center, yielding the general formula, (LaδY1−δ)1.98Ce0.02Si4N6C (δ = 0−0.5). It is clear from the DFT calculations and refinement that (La0.5Y0.5)2Si4N6C crystallizes in P31c. Thus, Y2Si4N6C:Ce3+, (La0.0625Y0.9375)2Si4N6C, (La0.125Y0.875)2Si4N6C:Ce3+, (La0.167-

which contained the imaginary phonon modes, or trigonal P31c. A Rietveld refinement was therefore performed considering both space groups, the reported space group (P63mc), and the predicted space group (P31c). The refinement of (La0.5Y0.5)2Si4N6C in P31c, shown in Figure 4a with the refinement results provided Table 1, produced marked improvement compared to the refinement in the P63mc space group, with a 10% improvement of Rp and Rwp, a 50% improvement of Rf 2, and an 18% improvement of χ2. The refi ne d at o m i c p o s i t i o ns f o r t h e r e fin e m e n t o f (La0.5Y0.5)2Si4N6C in trigonal P31c are provided in Table 2. The refinement for (La0.5Y0.5)2Si4N6C in hexagonal P63mc along with the structure refinement data are supplied in Figure S1 and Table S1, respectively. Following the computational and experimental results, it is undeniable that (La0.5Y0.5)2Si4N6C crystallizes in the noncentrosymmetric, trigonal space group P31c (No. 159). The crystal structure, shown in Figure 4b, contains two crystallographically distinct C(SiN3) tetrahedra. The Si1 site, located at Wyckoff position 6c, produces a tetrahedra that is corner connected through nitrogen atoms to two other Si1 tetrahedra,

Figure 4. (a) Rietveld refinement of high-resolution synchrotron X-ray powder diffraction data of (La0.5Y0.5)2Si4N6C in space group P31c. The measured data are shown in black, the fit by the yellow line, and the difference between the data and the fit by the blue line. (b) Structure of (La0.5Y0.5)2Si4N6C featuring a corner-connected network of C(SiN3) tetrahedra with La3+ and Y3+ alternating in the voids. 522

DOI: 10.1021/acs.inorgchem.7b02898 Inorg. Chem. 2018, 57, 519−527

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Inorganic Chemistry

Table 2. Refined Atomic Positions for (La0.5Y0.5)2Si4N6C in P31c Obtained from Rietveld Refinement of 11-BM Synchrotron Xray Powder Diffraction Data atom

Wyck. position

x

y

z

site occ.

Ueq

La1 Y1 La2 Y2 Si1 Si2 N1 N2 C1

2b 2b 2b 2b 6c 2a 6c 6c 2a

1/3 1/3 1/3 1/3 0.3382(2) 0 0.3040(4) 0.520(1) 0

2/3 2/3 2/3 2/3 0.1552(3) 0 0.0969(4) 0.0185(5) 0

0.0699(4) 0.0699(4) 0.4319(4) 0.4319(4) 0.2425(4) 0 0.0672(5) 0.2875(3) 0.3145(7)

0.886(6) 0.114(6) 0.044(5) 0.956(5) 1 1 1 1 1

0.00323(4) 0.00323(4) 0.00323(4) 0.00323(4) 0.0025(2) 0.0025(2) 0.0035(4) 0.0035(4) 0.0036(8)

Y0.833)2Si4N6C:Ce3+, (La0.25Y0.75)2Si4N6C:Ce3+, (La0.33Y0.67)2Si4N6C:Ce3+, and (La0.417Y0.583)2Si4N6C:Ce3+ were also prepared to ascertain where the crystal structure changes from monoclinic P21/c observed for Y2Si4N6C:Ce3+. Figure 5

The primary interest in carbonitridosilicates has centered on their potential as inorganic phosphors when substituted with Ce3+. Evaluating the luminescence of the solid solution (Figure 6a) shows a broad excitation band with Y2Si4N6C:Ce3+ possessing two individual peaks centered at λmax = 385 nm and λmax = 425 nm. Substituting La3+ for Y3+ in the crystal structure does not change the location of the longer wavelength excitation peak; however, the intensity decreases until it disappears proportionally with the concentration of La3+ substitution. The shorter excitation peak blue shifts to λmax = 380 nm by δ = 0.167. Interestingly, the addition of more La3+ (δ > 0.25), which also results in the structure change to P31c, leads to the presence of only one excitation peak that continues to blue shift to λmax = 370 nm for (La0.5Y0.5)2Si4N6C:Ce3+. Exciting these materials at 365 nm results in bright yellow luminescence with λmax = 556 nm for Y2Si4N6C:Ce3+, which is slightly different than a previous report (λmax = 538 nm) that was synthesized starting from rare-earth oxides.23 However, considering oxygen is a weaker field ligand than nitrogen, and would therefore induce a smaller crystal field splitting, the previously observed blue-shifted luminescence is likely due to the presence of residual oxygen in the final product.47 Substitution of the larger La3+ causes a blue shift of the luminescence for the solid solution (Figure 6b). Though (La0.0625Y0.9375)2Si4N6C:Ce3+ maintains the yellow luminescence, (La0.125Y0.875)2Si4N6C:Ce3+ luminesces yellow-green. Further addition of La3+ leads to (La0.25Y0.75)2Si4N6C:Ce3+, (La0.33Y0.67)2Si4N6C:Ce3+, and (La0.417Y0.583)2Si4N6C:Ce3+ luminescing green and blue-green luminescence with λmax = 503 nm for the ordered variant, (La0.5Y0.5)2Si4N6C:Ce3+. This blue shift

Figure 5. Plot of volume per Z as a function of δ in (LaδY1−δ)2Si4N6C:Ce3+ displaying a linear increase with increasing La3+ content until δ ≈ 0.25 above which the deviation from linearity indicates the change in symmetry.

shows that the calculated volume per Z increases linearly with increasing La3+ content, following Zen’s law until a deviation in linearity occurs at δ ≈ 0.25.46 At this point, the twist of the N3− atoms leads to the observation of the trigonal P31c until at least δ = 0.5.

Figure 6. (a) Excitation and emission spectra for (LaδY1−δ)2Si4N6C:Ce3+ (δ = 0−0.5). All compounds are excited at 365 nm. (b) λem,max of (LaδY1−δ)2Si4N6C:Ce3+ (δ = 0−0.5) plotted vs La3+ concentration displays blue shifting luminescence with increasing La3+. (c) The FWHM of the emission peaks of (LaδY1−δ)2Si4N6C:Ce3+ (δ = 0−0.5) shows that increasing La3+ content decreases the emission peak width. 523

DOI: 10.1021/acs.inorgchem.7b02898 Inorg. Chem. 2018, 57, 519−527

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Inorganic Chemistry by approximately 50 nm is expected and due to a change in the crystal field splitting of the Ce3+ 5d orbitals. As the larger La3+ is incorporated into the structure, the [RE−O6] bonds expand. resulting in weaker splitting of the Ce3+ 5d orbitals, leading to blue-shifting of the luminescence.48 Examining the emission spectra closer shows that the substitution of La3+ causes other subtle changes in the optical properties. As the lanthanum content increases, the full width at half-maximum (FWHM) of the emission peak decreases from 3970 cm − 1 for Y 2 Si 4 N 6 C:Ce 3 + to 3577 cm − 1 for (La0.5Y0.5)2Si4N6C:Ce3+ (Figure 6c). This can be explained by inspecting the local environment around the RE sites that contain the luminescent center. In monoclinic P21/c, there are two Y3+ sites ideal for Ce3+ substitution. The first site, Y1, centers an eight-coordinated polyhedra with a bond length distortion index of 0.13349, while the Y2 site occupies a squareface monocapped trigonal prism with a bond length distortion index of 0.09034.19 The structure change to P31c, induced by the addition of La3+, leads to more symmetric RE sites, indicated by a lower Wyckoff multiplicity and bond length distortion. According to Rietveld refinement, La3+ primarily occupies the Y1 site, which, in trigonal P31c, is located at Wyckoff position 2b and produces an icosahedron with a 25% decrease in bond length distortion index. Also, at Wyckoff position 2b, the Y2 site is transformed into an octahedron with a 96% improvement in bond length distortion. This increasingly symmetric environment surrounding the luminescent center, Ce3+, as the structure transitions from monoclinic P21/c to the more symmetric trigonal P31c leads to the observed decrease in FWHM.49 The high degree of structural connectivity stemming from the star-shaped motif follows recent phosphor design rules that suggest that this should produce an efficient inorganic phosphor.5,6,50 This high degree of connectivity is further reflected in the calculated Debye temperatures (ΘD). The end members, Y2Si4N6C and La2Si4N6C. have a calculated ΘD of 718 and 614 K, respectively, while the ΘD of the ordered solid solution (La0.5Y0.5)2Si4N6C lies in between with a value of 693 K. Therefore, to evaluate the luminescent efficiency, the photoluminescent quantum yield (PLQY) of the series was measured. Figure 7a shows that incorporation of La3+ into the structure leads to a drastic decrease in PLQY from 37(1)% for Y2Si4N6C:Ce3+. Adding in just a small amount of lanthanum (δ = 0.0625) leads to a sharp decline in efficiency to a PLQY of 24.1(6)%. By 50% substitution of La3+, (La0.5Y0.5)2Si4N6C:Ce3+, the luminescence is essentially quenched with a PLQY of only 3.8(8)%. The measured PLQY for this series is drastically lower than expected based on the high calculated ΘD. This surprisingly low PLQY may arise from the presence of vacancies, which are common in nitrides and carbides, that can lead to additional nonradiative (vibrational) relaxation pathways.51−55 Further, it was also observed that, across the solid solution, the color of the unsubstituted powder, which is normally white for phosphors, changed from a pale yellow to brown with increasing La3+ (inset Figure 7b). To evaluate this color change, the diffuse reflectance of the unsubstituted host compounds, i.e., (LaδY1−δ)2Si4N6C (δ = 0, 0.25, 0.5), was measured and converted to absorbance. From the spectra plotted in Figure 7b, it is clear that the substitution of La3+ for Y3+ causes a broad absorption peak at approximately 550 nm to appear. As this is approximately the location of the observed luminescence, it is likely that the inclusion of La3+ into the host

Figure 7. (a) Photoluminescent quantum yield (PLQY) measurements of (LaδY1−δ)2Si4N6C: Ce3+ (δ = 0−0.5) excited with 365 nm light shows that increasing La3+ content drastically decreases PLQY. (b) Absorbance spectra for unsubstituted Y 2 Si 4 N 6 C (red), (La0.25Y0.75)2Si4N6C (light blue), and (La0.5Y0.5)2Si4N6C (navy). with insets displaying the color of the unsubstituted powder, show a peak (λmax = 550 nm) arising upon addition of La3+.

structure (LaδY1−δ)2Si4N6C results in reabsorption of the Ce3+ luminescence, further contributing to luminescence quenching. This drop in PLQY was additionally evaluated by determining the effect of temperature on the luminescent intensity. Steady state photoluminescent spectra were obtained from 80 to 500 K, and the thermal quenching temperature (T50), the temperature at which the luminescence decreases to 50% of the low temperature value, was determined. Figure 8

Figure 8. Relative integrated intensity of temperature dependent photoluminescent spectra from 80 to 500 K was plotted against temperature for (a) Y2Si4N6C, (b) (La0.25Y0.75)2Si4N6C, and (c) (La0.5Y0.5)2Si4N6C.

clearly shows that increasing the La3+ content in the solid solution leads to a drastic decrease in thermal stability. There are two possible mechanisms to explain this quenching, photoionization and phonon-assisted relaxation.56 The first, photoionization, occurs when the band gap between the host conduction and valence band is so low as to allow the thermal excitation of excited state Ce3+ electrons into the host valence band where they are lost as a photocurrent. To ascertain if a decreasing band gap is the source of the observed quenching behavior, the band gaps of both Y2Si4N6C and (La0.5Y0.5)2Si4N6C were calculated. DFT calculations reveal that Y2Si4N6C has a HSE06 calculated band gap of 4.00 eV that increases to 524

DOI: 10.1021/acs.inorgchem.7b02898 Inorg. Chem. 2018, 57, 519−527

Article

Inorganic Chemistry 4.61 eV for (La0.5Y0.5)2Si4N6C, which is incompatible with the photoionization mechanism. Therefore, the most probable cause of this thermal quenching upon increasing La 3+ concentration is phonon-assisted relaxation in which there is an intersection between the ground and excited state potential energy surfaces. In Ce3+ phosphors, exciting an electron ideally leads to relaxation back to the ground state with the subsequent emission of a photon. However, in phosphors that experience phonon-assisted relaxation, the intersection between the ground state and excited state potential energy surfaces results in a nonradiative relaxation pathway that suppresses photon emission. It has been shown that materials containing more rigid bonds generally experience less phonon-assisted relaxation.56 Therefore, it can be concluded that substitution of the larger, softer La3+ in place of the smaller Y3+ decreases the rigidity of the structure, leading to increased thermal quenching behavior. This loss of rigidity is further reflected by the decreasing ΘD going from Y2Si4N6C to La2Si4N6C.

Rietveld refinement and accompanying structure refinement data of (La0.5Y0.5)2Si4N6C in space group P63mc (PDF) Accession Codes

CCDC 1585380 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, UK; fax: +44 1223 336033.

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Corresponding Author

*E-mail: [email protected]. ORCID

Anna C. Duke: 0000-0001-5949-391X Martin Hermus: 0000-0003-2857-1457 Jakoah Brgoch: 0000-0002-1406-1352

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Notes

CONCLUSION During an investigation of the electronic structure of (La0.5Y0.5)2Si4N6C, the phDOS was calculated in the reported space group P6 3 mc. The calculation revealed that (La0.5Y0.5)2Si4N6C in P63mc contains imaginary modes, indicating an instability due to the nitrogen atoms. The nitrogen atoms were displaced and the structure relaxed to reveal a new space group, trigonal P31c. The phDOS of (La0.5Y0.5)2Si4N6C crystallizing in P31c was calculated and no imaginary modes were found. (La0.5Y0.5)2Si4N6C was then synthesized using a high-temperature solid state reaction and a Rietveld refinement conducted using both P63mc and P31c space groups. The refinement of (La0.5Y0.5)2Si4N6C in P31c led to significantly improved refinement parameters, signaling that is indeed the correct space group. The structure preference is driven by an apparent difference in the chemical bonding, most notably the La−N interactions. Lastly, the solid solution from δ = 0 to δ = 0.5 was prepared substituted with 1% Ce3+ on the RE3+ site. Y2Si4N6C:Ce3+ luminesces bright yellow, while incorporation of La3+ leads to blue-shifted, weak blue-green luminescence for (La0.5Y0.5)2Si4N6C:Ce3+. Along with a generally low observed PLQY for the entire solid solution, possibly due to anion vacancies within the host structure, the addition of La3+ also leads to a drastic decrease of PLQY. Diffuse reflectance measurements of the unsubstituted compound reveals that (La0.5Y0.5)2Si4N6C:Ce3+ possesses an absorption peak at 550 nm, approximately the location where emission occurs. It can be concluded that the host structure absorbs the luminescence produced by substituting Ce3+ into the structure, quenching luminescence. The thermal stability also decreases upon the inclusion of La3+ as a result of increased phonon-assisted relaxation. The work presented here provides further evidence of the effectiveness of using ab initio computation in combination with high-resolution diffraction to definitively solve inorganic crystal structures.

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AUTHOR INFORMATION

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank the department of Chemistry and the Division of Research at the University of Houston for providing generous startup funds. M.H. is thankful for financial support provided by the Elby Nell McElrath Postdoctoral Fellowship. Use of the Advanced Photon Source at Argonne National Laboratory was facilitated by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, through Contract No. DE-AC02-06CH11357.

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