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Dec 14, 2016 - Undoped and 2.0 mol % Ln3+ (Ln = Sm, Eu) doped α-Zn2P2O7 samples ... The Energy-Scanning EXAFS beamline (BL-9) is a bending magnet ...
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Origin of Blue-Green Emission in α‑Zn2P2O7 and Local Structure of Ln3+ Ion in α‑Zn2P2O7:Ln3+ (Ln = Sm, Eu): Time-Resolved Photoluminescence, EXAFS, and DFT Measurements Santosh Kumar Gupta,*,† Partha Sarathi Ghosh,‡ Ashok Kumar Yadav,§ Shambhu Nath Jha,§ Dibyendu Bhattacharyya,§ and Ramakant Mahadeo Kadam† †

Radiochemistry Division, ‡Materials Science Division, and §Atomic and Molecular Physics Division, Bhabha Atomic Research CentreTrombay, Mumbai 400085, India S Supporting Information *

ABSTRACT: Considering the fact that pyrophosphate-based hosts are in high demand for making highly efficient luminescence materials, we doped two visible lanthanide ions, viz. Sm3+ and Eu3+, in Zn2P2O7. Interestingly, it was oberved that pure Zn2P2O7 displayed blue-green dual emission on irradiation with ultraviolet light. Emission and lifetime spectroscopy shows the presence of defects in pyrophosphate samples which are responsible for such emission. DFT calculations clearly pinpointed that the electronic transitions between defect states located at just below the conduction band minimum (arises due to VO1+ and VO2+ defects) and valence band maximum, as well as impurity states situated in the band gap, can lead to dual emission in the bluegreen region, as is also indicated by emission and lifetime spectra. X-ray absorption near edge spectroscopy (XANES) shows the stabilization of europium as well as samarium ion in the +3 oxidation state in α-Zn2P2O7. The fact that α-Zn2P2O7 has two different coordination numbers for zinc ions, i.e. five- and six-coordinate, the study of dopant ion distribution in this particular matrix will be an important step in realizing a highly efficient europium- and samarium-based red-emitting phosphor. Time resolved photoluminescence (TRPL) shows that both of these ions are heterogeneously distributed between five- and sixcoordinated Zn2+ sites and it is the six-coordinated Zn2+ site which is the most favorable for lanthanide ion doping. Extended Xray absorption fine structure (EXAFS) measurements also suggested that a six-coordinated zinc ion is the preferred site occupied by trivalent lanthanide ions, which is in complete agreement with TRPL results. It was observed that there is almost complete transfer of photon energy from Zn2P2O7 to Eu3+, whereas this transfer is inefficient and almost incomplete in case of Sm3+, which is indeed important information for the realization of pyrophosphate-based tunable phosphors.

1. INTRODUCTION

lanthanide-doped luminescence but also as strong candidates to be used in future generations of rare-earth-free luminescence materials either due to the presence of defects in the band gap or because of intrinsic charge transfer transitions. 6−12 Pyrophosphate hosts belong to the category of ideal oxidebased hosts and have been explored extensively from a luminescence point of view in the field of lanthanide-activated phosphors, persistent phosphors, dosimetry, UV-emitting

Lanthanide-doped inorganic materials with optimum chemical and thermal stability have attracted much attention because of their various scientific and technological applications such as luminescence materials,1 electrocatalysis,2 optical encoding,3 bioprobes for optical imaging,4 in vivo tumor-targeted imaging,5 etc. In this context the host plays a very important role and serves as a house for optically active ions. These materials need to have a sufficiently wide band gap, low phonon frequency, high thermochemical stability, easy availability, ease of synthesis, environmental benignness, etc. In fact, recently we have realized the potential of oxide-based hosts not only for © 2016 American Chemical Society

Received: August 1, 2016 Published: December 14, 2016 167

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diffractometer which operates in the Bragg−Brentano focusing geometry with Cu Kα radiation (λ= 1.5406 Å) as the X-ray source. Luminescence emission, excitation, and lifetime measurements were carried out on an Edinburgh Fluorescence unit with CD-920 controller equipped with a 150 W Xe flash lamp with 10−1000 Hz variable frequency as the excitation source. In the present study we have performed EXAFS measurements on Eu- and Sm-doped Zn2P2O7 powders to probe the local structure. The EXAFS measurements have been carried out at the Energy-Scanning EXAFS beamline (BL-9) at the Indus-2 Synchrotron Source (2.5 GeV, 150 mA) at Raja Ramanna Centre for Advanced Technology (RRCAT), Indore, India.26,27 The Indus-2 Synchrotron Source is a booster cum storage ring, operated in decay mode with a lifetime of ∼18 h. The Energy-Scanning EXAFS beamline (BL-9) is a bending magnet beamline at the Indus-2 synchrotron source dedicated to X-ray absorption spectroscopy (XAS) measurements. The beamline uses a double-crystal monochromator (DCM) which works in the photon energy range of 4−25 keV with a resolution of 104 at 10 keV. A 1.5 m horizontal premirror with meridional cylindrical curvature is used prior to the DCM for collimation of the beam and higher harmonic rejection. The second crystal of the DCM is a sagittal cylindrical crystal, which is used for horizontal focusing of the beam while another Rh/Pt-coated bendable post mirror facing down is used for vertical focusing of the beam at the sample position. In general, XAS spectra are recorded in transmission mode for reasonably concentrated samples; however, for low-concentration samples fluorescence mode measurements are preferred for better signal-to-noise ratio.28 Hence, XAS measurements on the samples at the Zn K edge have been performed in transmission mode, whereas the measurements at the dopant (Eu and Sm) L3 edges have been carried out in fluorescence mode due to their low concentration in the samples. Three ionization chambers (300 mm length each) have been used for data collection in transmission mode: one ionization chamber for measuring incident flux (I0), a second chamber for measuring transmitted flux (It), and the third ionization chamber for measuring the XAS spectrum of a reference metal foil for energy calibration. Appropriate gas pressures and gas mixtures have been chosen to achieve 10−20% absorption in the first ionization chamber and 70− 90% absorption in the second ionization chamber to improve the signal-to-noise ratio. In the case of fluorescence measurements, an Si drift detector (Vortex detector) was used in standard 45° geometry. Rejection of the higher harmonics content in the X-ray beam was performed by detuning the second crystal of DCM. Zn K edge measurements were done at 100 eV below the edge to 800 eV above the edge to cover a reasonable k range. However, L3 edge measurements were restricted by the presence of the L2 edges: the Sm L3 edge is at 6716 eV and L2 edge is at 7312 eV while the Eu L3 edge is at 6977 eV and L2 edge is at 7617 eV. Hence, measurements could only be done up to ∼600 eV above the L3 edges.

phosphors for biocidal applications, contrast agents for nuclear magnetic resonance imaging (MRI),13−17etc. Zinc pyrophosphate is considered to be an ideal luminescence host because of its various favorable properties such as ease of synthesis, low cost of the chemicals involved, high chemical and thermal stabilities, and environmental friendliness.18 α-zinc pyrophosphate (α-Zn2P2O7) exists as a thortveitite structure having monoclinic symmetry. Zinc exists in two different coordination polyhedra with and five and six oxygens.19 Recently zinc pyrophosphate has gained significant attention as a luminescence host, and a great deal of work has been done in the last 5 years or so.18,20,21 In fact, our group is also extensively involved in spectroscopic measurements on lanthanide and transition metal ion doped α-Zn2P2O7 for probing structural phase transitions, oxidation state of the dopant ion such as Mn, local structure, photoluminescence properties, and use as phosphor materials.22−25 In the case of europium- and samarium-doped samples,24,25 it has been observed that Eu3+/Sm3+ was distributed at both five- and six-coordinated Zn2+ sites; while complete host to dopant energy transfer takes place in the case of Eu3+, it is absent for Sm3+. Such statements have been made solely on the basis of photoluminescence (PL) emission and lifetime spectroscopy in our earlier works. In addition, the reason for emission from undoped α-Zn2P2O7 was not clear at that time, which is an important aspect from a photophysics perspective and needs to be understood thoroughly. To have a complete picture of whether both zinc sites are equally populated or if one site is more preferred than the other and so on, extended X-ray absorption fine structure (EXAFS) spectroscopy is the best technique. This is indeed an important step toward the realization of highly efficient luminescence materials because it is important to know the local site occupied by the activator ion in order to get optimum optical properties. In this work, we have synthesized the visible emitting lanthanide ions Sm3+ and Eu3+, which are probably the best spectroscopic probes, doped in α-Zn2P2O7 using a solid-state diffusion method. Detailed photoluminescence (PL) emission and lifetime spectroscopy measurements were carried out on the samples to reaffirm the results obtained in the previous individual studies.24,25 X-ray absorption spectroscopy, which comprises both EXAFS and Xray near edge structure (XANES) measurements, were used for the undoped and doped samples to establish the site occupancy of doped lanthanide ions at two zinc sites (five- and sixcoordinated Zn2+ site) in α-Zn2P2O7. Density functional theory based ab initio calculations were also performed in the presence of oxygen and zinc vacancies to propose a suitable mechanism which can qualitatively explain the origin of visible blue and green emission in pure Zn2P2O7. To the best of our knowledge, this is the first report on detailed photoluminescence (PL) properties of pure Zn2P2O7 using complementary experimental techniques and density functional theory (DFT) based calculations.

3. COMPUTATIONAL METHODOLOGY All electronic structure calculations were performed using spinpolarized plane wave based density functional theory (DFT) as implemented in the Vienna ab initio simulation package (VASP).29,30 The interaction between electrons and ions was described using the projector augmented wave (PAW) method31 including the valence states of Zn (3d, 4p; 12 valence electrons), P (3s, 3p; 5 valence electrons) and O (2s, 2p; 6 valence electrons). The Perdew−Burke− Ernzerhof (PBE)32 scheme to describe exchange-correlation potential for generalized gradient approximation (GGA) was used in this study. A Monkhorst−Pack33 k-space sampling of 5 × 5 × 5 in reciprocal space was used for the Brillouin zone integration, and a cutoff energy (Ecut) of 500 eV was used for the expansion of the plane wave basis set. For the ideal Zn2P2O7 (ZPO) unit cell, optimization of Ecut and kpoint meshes was carried out to ensure convergence of total energy to within a precision of 0.08 meV/atom. The total energies of ZPO were optimized for volume (or lattice parameter), unit-cell dimensions, and atomic positions. Conjugate gradient algorithms were used for the

2. EXPERIMENTAL SECTION Undoped and 2.0 mol % Ln3+ (Ln = Sm, Eu) doped α-Zn2P2O7 samples were prepared using the solid-state method described earlier.24 From now onward undoped α-Zn2P2O7 will be designated as ZPP, whereas Sm- and Eu-doped materials will be denoted ZSPP and ZEPP. X-ray diffraction (XRD) patterns of the powdered ZPP, ZSPP, and ZEPP samples were recorded using a Rigaku Miniflex-600 168

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Figure 1. (a) XRD pattern of undoped and 2.0 mol % Ln3+ (Ln = Sm, Eu) doped α-Zn2P2O7 (synthesis temperature 950 °C). (b) Unit cell of αZn2P2O7 with black, blue, and red atoms representing Zn, P, and O atoms, respectively.

Figure 2. (a) Photoluminescence excitation spectrum of Zn2P2O7. (b) Room-temperature emission spectrum of Zn2P2O7. (c) PL decay profile for Zn2P2O7 under pulsed diode laser excitation of 375 nm and emission of 520 nm.

the same and could be indexed to the monoclinic phase of αZn2P2O7 (JCPDS No. 08-0238). This indicates that Eu3+/Sm3+ ions occupy the Zn2+ sites in Zn2P2O7 lattice and have not distorted the basic pyrophosphate network. The trivalent oxidation state of doped lanthanide ion was confirmed using XANES measurements. Since no impurity traces of ZnO, Eu2O3, and Sm2O3 were observed, it is feasible to suggest that all the samples crystallize as α-Zn2P2O7. Figure 1b depicts the unit cell of α-Zn2P2O7. A detailed description of the crystal structure and the other high-temperature phase of α-Zn2P2O7 was reported in our earlier work.22,23

unit-cell optimizations, where relaxations were performed until the residual forces and stress in the equilibrium geometry were on the order of 0.005 eV/Å and 0.01 GPa, respectively. The final calculations of total electronic energy and density of states (DOS) were performed using the tetrahedron method with Blöchl corrections.34

4. RESULTS AND DISCUSSION 4.1. X-ray Diffraction (XRD). The XRD patterns of the undoped and 2.0 mol % Ln3+ (Ln = Sm, Eu) doped Zn2P2O7 samples are shown in Figure 1a. All of the patterns match well with that reported for JCPDS No. 08-0238. The diffraction patterns of all the samples (ZPP, ZSPP, and ZEPP) are almost 169

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Figure 3. GGA calculated total and angular momentum decomposed DOS of (a) ideal pyrophosphate Zn2P2O7 (ZPO) as well as ZPO with (b) a neutral oxygen vacancy (V0O), (c) an oxygen vacancy of charge 1+ (V1+O), (d) an oxygen vacancy of charge 2+ (V2+O), and (e) a neutral Zn vacancy.

At low temperature Zn2P2O7 crystallizes in a 6-fold superstructure (α polymorph) with I2/c space group. The PAW-GGA calculated equilibrium lattice parameters, mono-

clinic angles, and bond lengths are summarized in Table S1 in the Supporting Information along with experimentally determined values (by Rietveld refinements).35 Table S1 clearly 170

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The biexponential decay is an indication of the presence of two defect levels in Zn2P2O7. The lifetime value on the order 1−2 ns is typical of defect-related emission.7,8 4.2.2. DFT Calculations. We have calculated the electronic density of states (DOS) of ideal Zn2P2O7 and the change in electronic DOS in the presence of an oxygen vacancy (neutral and charged) and a neutral Zn vacancy. To generate defect structures, one oxygen and/or Zn atom is removed from the 132-atom ideal unit cell and the total energies of the structures comprised of oxygen (neutral and charged) and/or Zn vacancies were optimized with respect to volume, unit-cell dimensions (lattice parameter (a), b/a, c/a, and β) and atomic positions. Figure 3a presents the total and orbital angular momentum resolved density of states (DOS) for ideal Zn2P2O7, where the Fermi level is set to 0 eV. In the ideal form of Zn2P2O7, the lower part of the valence band (VB) is mainly contributed by O 2p orbitals hybridized with P 3p orbitals. The upper part of the VB is mainly contributed by the Zn 3d and O 2p states. The conduction band (CB) is mainly composed of Zn 4s states (in majority) and P 3p states. Finally, our PAW-GGA calculated electronic band gap of 3.83 eV demonstrates the insulating character of this material. Therefore, our PAW-GGA calculated results clearly reproduce the structural parameters and electronic properties of Zn2P2O7 quite well. Figure 3b presents the total and angular momentum decomposed DOS due to the presence of a neutral O vacancy. The spin-up and spin-down components are shown separately in the upper and lower panels, respectively. The overall nature of the VB remains unaltered, but an impurity band appears just at the end of the VB. Impurity states are mainly contributed by the zinc s, phosphorus p, and oxygen p states. The presence of a neutral O vacancy increases the equilibrium volume by 8.32 Å3, and the electronic band gap becomes 2.90 eV. In this case the Fermi level appears just below the CB. Figure 3c presents the total and angular momentum decomposed DOS due to the presence of an O vacancy with charge 1+ (VO1+). The spin-up and spin-down components are shown separately in the upper and lower panels, respectively. The overall nature of the VB remains unaltered, but two impurity bands appear in the band gap. In this case the Fermi level appears 1.03 eV above the VB in the band gap. The first impurity band appearing just above the Fermi level (in the minority spin component) is composed of d states of Zn and p states of P and O in the spin-down components. Impurity states generated due to spin-down components are not filled with electrons, as they are situated just above the Fermi energy. Impurity bands appearing just below the CB minimum are composed of s states of Zn and p states of P and O in both spin-up and spin-down components. The presence of defect states just below the CB minima reduces the band gap by 0.31 eV. For this case, the energy difference between the VB maximum and CB minimum (electronic band gap) is 3.52 eV. Figure 3d presents the total and angular momentum decomposed DOS due to presence of an O vacancy with charge 2+ (VO2+). The overall nature of the VB remains unaltered, but two impurity bands appear above the VB maximum in the band gap. The Fermi level is situated just above the VB maximum. The first impurity band appearing 0.73 eV above the Fermi level (in the minority spin component) is composed of s states of Zn and p states of P in the spin-down components. Another impurity band is also present just below the CB minimum, which is composed of s states of Zn and p

shows that our PAW-GGA calculated values agree well with values experimentally determined by Stoger et al.35 within less than 1.75% deviations. Our PAW-GGA calculated equilibrium volume is overestimated by 6% in comparison to experimental values measured at 350 K. Table S2 in the Supporting Information compares PAW-GGA calculated atomic positions with previously reported experimental values, and these values are in good agreement. 4.2. Photoluminescence Study of Undoped Zn2P2O7. 4.2.1. Excitation and Emission Spectroscopy. Figure 2a shows the photoluminescence excitation (PLE) spectrum of undoped Zn2P2O7 under emission of 520 nm. The PLE spectrum consists various narrow peaks at around 230, 250, 259, 272, 287, 297, 310, 327, 335, 344, 362, 377, 388, and 423 nm. The presence of several PLE peaks is an indication of the fact that various kinds of defect states exist inside the band gap of ZPP at different energy positions. In fact, zinc pyrophosphate is reported to have an intrinsic negatively charged zinc vacancy (VZn′).36 In addition, high-temperature thermal treatment in air might also have invoked various kinds of oxygen-related defects. Figure 2b shows the emission spectrum of undoped Zn2P2O7 under excitation at 250 nm. There are two broad emission bands centered at around 450 and 520 nm. Such a multicolor emission profile is typical of a system where an excited electron relaxes via various pathways and there exist a number of defect states within the band gap of materials.37 The excitation wavelength used for recording the emission spectrum, 250 nm (∼4.85 eV), is less than that of the experimental band gap of Zn2P2O7; therefore, it is difficult for an electron in the valence band to be directly excited to the conduction. However, it is likely to be excited to the localized levels within the forbidden band gap. This indicates the presence of certain localized energy states within the band gap of the material because transition directly from the valence to the conduction band will be forbidden in this case. Therefore, the oberved visible emission in Zn2P2O7 cannot be attributed to free-exciton recombination. Normally free exciton emission has narrow characteristics and arises from band gap excitation. In this case this emission also cannot be attributed to the presence of impurities because the intensity and position of the emission band is independent of preparation conditions.38,39 An oxygen vacancy is known to be the most common defect and usually acts as a radiative center giving rise to PL. The blue emission band at 450 nm can be attributed to recombination of electrons trapped in a singly ionized oxygen vacancy with photoexcited holes in the valence band. The band at 520 nm is ascribed to the presence of a negatively charged zinc vacancy in the band gap of Zn2P2O7. To probe the exact nature of the defects, density functional theory based ab initio calculations will be done and discussed in a subsequent section. To exactly know the nature of the emissive center responsible for blue and green emission in the Zn2P2O7 sample, a corresponding luminescence decay profile was recorded and is shown in Figure 2c. The PL decay curve was fitted to a biexponential model using ⎛ t ⎞ ⎛ t⎞ I(t ) = A1 exp⎜ − ⎟ + A 2 exp⎜ − ⎟ ⎝ τ2 ⎠ ⎝ τ1 ⎠

(1)

where I(t) is the intensity, τ1 and τ2 are emission decay times, and A1 and A2 are their relative weights. The decay curve shows two different lifetime values, 0.51 and 2.59 ns, with magnitudes of 37 and 63%, respectively, and an average value of 1.37 ns. 171

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Figure 4. (a) Excitation spectra of Zn2P2O7:Sm3+ under 601 nm (λem). (b) Emission spectra of Zn2P2O7:Sm3+ under 250 nm (λex). (c) CIE diagram showing the coordinates and representing the color emitted by Zn2P2O7:Sm3+. (d) Luminescence decay time profile of Zn2P2O7:Sm3+ (λex 250 nm and λem 601 nm).

band gap, can lead to duel emission in the blue-green region as described in the PL emission spectrum. 4.3. Photoluminescence Study of Sm 3+ -Doped Zn2P2O7. In solids, lanthanide ion has the tendency to become stabilized in some of the unusual oxidation states other than the usual +3 oxidation state, such as 2+ and 4+. Eu, Sm, and Yb have the tendency to stabilize in 2+ oxidation states, whereas Tb sometime stabilizes as Tb4+. In these cases XANES measurements clearly show that samarium and europium ions stabilize in the +3 oxidation state (section 4.5, Figure 6). The photoluminescence excitation (PLE) spectrum of Zn2P2O7:Sm3+ at 601 nm emission wavelength (λmax) is shown in Figure 4a. A broad band in the range of 220−285 nm was assigned to the electronic charge transfer transition band from the filled 2p orbital of O2−to the empty 4f orbital of Sm3+ (CTB) with λmax at 250 nm. In the wavelength region 310−450 nm, various narrow excitation bands are also observed which are positioned at 347 nm (6H5/2 → 6H13/2), 369 nm (6H5/2 → 4D3/2), 377 nm (6H5/2 → 6P7/2), 406 nm (6H5/2 → 4 F7/2), 424 nm (6H5/2 → 6P5/2), and 441 nm (6H5/2 → 4G9/2), which are attributed to intra f−f transitions of Sm3+. The higher intensity of the CTB in comparison to the f−f band is due to the forbidden nature of intra-f−f transitions so that they have low molar extinction coefficients. An emission spectrum of Sm3+ ion doped Zn2P2O7 at an excitation wavelength of 250 nm (CTB) is also shown in Figure 4b. It is noted that spectral features remains similar on excitation with 250 nm (charge transfer) and 406 nm (f−f

states of P and O in the spin-up and spin-down components. For this case, the energy difference between the VB maximum and CB minimum (electronic band gap) is 3.51 eV. Figure 3e presents the total and angular momentum decomposed DOS due to the presence of a neutral Zn vacancy. The overall nature of the VB remains unaltered, but one small impurity band appears just above the VB maximum. The Fermi level is situated just above the VB maximum. The impurity band is composed of mainly p states of O in both spin-up and spin-down components. For this case, the energy difference between the VB maximum and CB minimum (electronic band gap) is 3.45 eV. We further calculated the vacancy formation energies of all these defects using a formula described in our previous study.7 The overall vacancy formation energy (Ef) follows the sequence Ef(VZn0) > Ef(VO0) > Ef(VO1+) > Ef(VO2+). The calculated vacancy formation energy values indicate that VO1+ and VO2+ defect formations are favored near the valence band in comparison to the neutral oxygen defect. The vacancy formation energy of Ef(VO1+) is greater than Ef(VO2+) by only 0.3 eV. Similarly, the vacancy formation energy Ef(VO2+) is smaller than Ef(VO0) and Ef(VZn0) by 0.90 and 2.96 eV, respectively. Therefore, the formation of VO2+ and VO1+ is favorable in comparison to other types of defects. Moreover, the electronic transitions between defect states located at just below the CB minimum (arising due to VO1+ and VO2+ defects) and VB maximum, as well as impurity states situated in the 172

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Figure 5. (a) Excitation and (b) emission spectra of Zn2P2O7:Eu3+ under 593 nm (λem) and 250 nm (λex). (c) CIE diagram showing the coordinates and representing the color emitted by Zn2P2O7:Eu3+. (d) Luminescence decay time profile of Zn2P2O7:Eu3+ (λex 250 nm and λem 593 nm). The inset in (a) depicts a magnified picture of the excitation band from 300 to 500 nm.

from the previous one. Host emission completely dominates the emission profile, and there seems to be incomplete or no host−dopant energy transfer. To explain this anomaly, we have to understand that thermal treatment and synthesis methods play a very important role in defect chemistry. In case of samples prepared using complex polymerization,25 a low temperature was used for synthesis and perhaps defect density would have been less in that case in comparison to the solid state route, which involves repetitive heating and grinding at reasonably higher temperature. As discussed in an earlier section, the emission in the host Zn2P2O7 is induced by the presence of oxygen vacancies. The defect density is much higher in solid state synthesized phosphors in comparison to complex polymerization; host emission is highly intense in the Zn2P2O7:Sm3+ phosphor in this particular case. Of all these transitions, the transition at 601 nm (4G5/2 → 6 H7/2) has the maximum intensity. It is a well-known fact in Sm3+ luminescence spectroscopy that the transition 4G5/2 → 6 H7/2 at 601 nm (ΔJ = ± 1) is partially magnetic dipole (MD) and partially electric dipole (ED) in nature. The other transition at 562 nm (4G5/2 → 6H5/2) is purely MD in nature, whereas that at 643 nm (4G5/2 → 6H9/2) is purely ED in nature, which is sensitive to the crystal field.40 Generally, the integral intensity ratio of the electric dipole transition to the magnetic dipole transition, known as the asymmetry ratio, has been used

band) except for significant differences in their relative emission intensities. The intensities of emission bands with λex 406 nm (f−f band of Sm3+) were found to be quite low in comparison to that obtained with the charge transfer 250 nm band because of the fact that f−f transitions are Laporte-forbidden transitions and exhibit poor absorption coefficients in the UV region. The presence of intense defect induced host emission in the Sm3+doped sample indicates incomplete host to dopant energy transfer. This is an important reason that host−dopant energy transfer is inefficient in the case of Zn2P2O7:Sm3+. The emission spectrum of Zn2P2O7:Sm3+ displays (a) very strong defect-induced host emission and (b) relatively weaker sharp emission lines in the range between 555 and 700 nm characteristic of Sm3+ ions which are overlaid on a broad host emission. These weak Sm3+ peaks at 562, 601, 644, and 687 nm are attributed to the 4G5/2 → 6H5/2, 4G5/2 → 6H7/2, 4G5/2 → 6 H9/2, and 4G5/2 → 6H11/2 transitions of the Sm3+ ions, respectively. In previous work on Zn2P2O7:Sm3+ where we have synthesized the phosphor using a complex polymerization method,25 host emission was present along with an Sm3+ emission line but there was a substantial amount of energy transfer from the host to Sm3+. However, in this work we have used solid-state synthesis to synthesize the same phosphor and the spectral feature obtained in this case is entirely different 173

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Inorganic Chemistry to measure the symmetry of the local environment of the Ln3+ ion. In this particular phosphor, the presence of both an ED transition at 644 nm and MD transition at 562 nm in the emission spectrum is indicative of the fact that the Sm3+ ion occupies symmetric as well as asymmetric sites. To evaluate the material performance for phosphor applications, CIE chromaticity coordinates were evaluated for the Zn2P2O7:Sm3+ phosphor adopting standard procedures. This is represented as an asterisk in the CIE diagram shown in Figure 4c. It is clear from the values that Zn2P2O7:Sm3+ emits in the near-white region. This is an important step in the realization of white LEDs. To get an idea about the Sm3+ ion occupancy in these lattice sites, luminescence lifetime measurements were conducted. The decay profile (Figure 4d) was correctly fitted by using a biexponential temporal dependence using eq 1. The multiexponential decay (nonhomogenous distribution) behavior depends upon a number of factors such as varied nature of luminescent centers, energy transfer, and the presence of defects and impurities in the host.41 The two lifetime values are 68 μs (T1) and 1.06 ms (T2) with magnitudes of 38 and 62%, respectively. The presence of two decay times is indicative of the presence of two different environments for Sm3+ in α-Zn2P2O7, where six-coordinated Zn2+ has a greater population of trivalent samarium ions. We know that there are two types of Zn2+ ions in α-Zn2P2O7 with coordination numbers of 5 and 6. Cryptographically ZnO5 is less symmetric than ZnO6. If the concept of phonon energy is visualized (assuming the same host for the Ln3+), a relatively longer lifetime should be ascribed to a more symmetric site (presence of inversion symmetry) because in that particular case an f−f transition becomes more Laporte forbidden. On the other hand, a shorter lifetime is often associated with an asymmetric lattice site wherein selection rules become relatively relaxed. Thus, the above results indicate that samarium ions are distributed at both 5-Zn and 6-Zn sites and the shorter lived species (T1) is attributed to an Sm3+ ion occupying a fivecoordinated Zn2+ site whereas the longer lived T2 species is one that is localized at a more symmetric six-coordinated Zn2+ site. From coordination chemistry analogy, however, fivecoordinated Sm (SmO5) does not exist and all of the Sm3+ would have gone to a six-coordinated Zn2+ site. From the aforementioned emission studies it appears that Sm3+ occupies both five-coordinated Zn2+ as well as six-coordinated Zn2+. To confirm this, we have performed EXAFS measurements on the samples, which will be discussed in a subsequent section. 4.4. Photoluminescence Study of Eu 3+ -Doped Zn2P2O7. The photoluminescence excitation (PLE) spectrum of the Zn2P2O7:Eu3+ at 593 nm emission wavelength (λmax) is shown in Figure 5a. A broad band in the range of 220−270 nm was assigned to the electronic charge transfer transition band from the filled 2p orbital of O2− to the empty 4f orbital of Eu3+ (CTB) with λmax at 250 nm. If one carefully looks at the excitation spectra, there are other fine peaks in the region 350−500 nm (magnified image in the inset of Figure 5a), which are ascribed to intraconfigurational f−f transitions of Eu3+. Among these excitation bands, that at 395 nm is the relatively most intense in comparison to the others, which is attributed to the 7F0 → 5L6 f−f transition of the Eu3+ ion. Other relatively less intense excitation peaks at 318, 361, 381, 423, 460, and 486 nm were assigned to the electronic transitions 7F0 → 5H3, 5L9, 5L7, 5L6, 5D4, 5D3, and 5D2, respectively.

This indicates that this particular phosphor material can be effectively excited by 395 nm (near UV) as well as by 460 nm (blue LEDs). The emission spectrum of Eu3+ ion doped Zn2P2O7 with an excitation wavelength of 250 nm (charge transfer transition) is also shown in Figure 5b. In this case weaker host emission could be seen but europium emission lines dominate the spectrum, indicating very efficient photon energy transfer from zinc pyrophosphate (defect-induced emission) to Eu3+ in comparison to Sm3+, though both dopants have similar chemical and physical characteristics. To be very specific, the emission spectrum of Zn2P2O7:Eu3+ consisted of two three predominant features: (a) weaker defect-induced host emission and (b) relatively stronger europium emission lines in the range between 578 and 725 nm. There are four main peaks at 593, 615, 653, and 697 nm corresponding to 5D0 → 7F1, 5D0 → 7F2, 5 D0 → 7F3, and 5D0 → 7F4 respectively (J = 0, 1, 2, 3, 4). The strongest peak at 593 nm corresponds to a magnetically allowed dipole transition (MDT), which is not affected much by the chemical environment, whereas that at 615 nm is hypersensitive in nature as an electric dipole transition (EDT) and is very much perturbed by the local chemical environment, unlike the case for the MDT. Of all these transitions, the magnetic dipole transition at 593 nm (5D0 → 7F1) has the maximum intensity. The intensity of the MDT is greater than that of the EDT, indicating that the majority of Eu3+ occupies six-coordinated Zn2+ sites, which is also favorable from size and coordination chemistry factors: i.e., there is a small difference in size between six-coordinated Eu3+ and Zn2+ and five-coordinated Eu3+ and Zn2+ and also Eu prefers EuO6 over EuO5 energetically. However, the fact substantial intensity of EDT also exists along with MDT is indicative of the fact that the Eu3+ ion also occupies five-coordnated Zn2+ sites, though their fraction may differ substantially. To evaluate the material performance for phosphor applications, CIE chromaticity coordinates were evaluated for the Zn2P2O7:Eu3+ phosphor adopting standard procedures. This is represented as an asterisk in the CIE diagram shown in Figure 5c. It is clear from the values that Zn2P2O7:Eu3+ emits in the red region. This red phosphor could be used for efficient white LEDs. In this case also the lifetime decay profile exhibits biexponential behavior (Figure 5d), indicating a nonhomogenous environment around Eu3+ in Zn2P2O7. The two lifetime values are 135 μs (T1) and 477 μs (T2) in magnitudes of 23 and 77% respectively. The presence of decay times is indicative of the presence of two different environments for Eu3+ in α-Zn2P2O7, which could also be seen from the emission spectroscopic studies. The shorter lived species (T1) is attributed to Eu3+ ions occupying five-ccordinated Zn2+ sites, whereas the longer lived T2 species is that which is localized at the more symmetric six-coordinated Zn2+ site and the fraction of europium occupying six-coordinated Zn2+ sites is larger than that at five-coordianted sites. This is in agreement with our emission study, where MDT is intense than EDT. This has also been confirmed using EXAFS measurements described later. Eu3+ is a special spectroscopic probe, wherein on the basis of the number of stark splitting components seen in the emission lines from various transitions one could decipher the point group symmetry around the Eu3+ site. Among the transitions of the europium ion, 5D0 → 7F1 (pure magnetic dipole transition) and 5D0 → 7F2 (hypersensitive electric dipole transition) are the most important, whereas 5D0 → 7F3 and 5D0 → 7F4 is 174

DOI: 10.1021/acs.inorgchem.6b01788 Inorg. Chem. 2017, 56, 167−178

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Inorganic Chemistry

Figure 6. (a) Normalized XANES spectra of pure Zn2P2O7, Eu3+-doped Zn2P2O7, and Sm3+-doped Zn2P2O7 at the Zn K-edge along with the standards ZnO and Zn metal foil. (b) Normalized XANES spectra of Sm-doped Zn2P2O7 (top) at the Sm L3 edge and Eu-doped Zn2P2O7 (bottom) at the Eu L3-edge along with the respective standards.

Figure 7. (A) Fourier-transformed EXAFS spectra of (a) pure Zn2P2O7, (b) Eu-doped Zn2P2O7, and (c) Sm-doped Zn2P2O7 samples at the Zn Kedge. (B) Fourier-transformed EXAFS spectra of (a) Sm-doped Zn2P2O7 at Sm L3-edge and (b) Eu-doped Zn2P2O7 at Eu L3-edge. The left panel is for k-space spectra, and the right panel is for R-space spectra. In both parts the R-space spectra shown here are phase-uncorrected which show peaks at R values lower than actual distances.

neither of pure magnetic nor of pure electrical origin.42 The magnetic dipole transition is not perturbed much by the local chemical environment, as discussed earlier, whereas 5D0 → 7F2 is strongly perturbed by the immediate surroundings. We have considered the emission spectral pattern of 5D0 → 7F1 and 5D0 → 7F2 to arrive upon the point group symmetry of Eu3+ in Zn2P2O7. Figure S1 in the Supporting Information shows the slow scan recording of the emission spectrum of selective 5D0 → 7F1 and 5D0 → 7F2 regions. The substitution of Zn2+ with Eu3+ may result in significant lattice distortion because of different ionic sizes and charges. From the Stark splitting pattern shown in Figure S1 in the Supporting Information, three and four peaks for 5D0 → 7F1 (ΔJ = ± 1) and 5D0 → 7F2 (hypersensitive, ΔJ = ± 2) transition of Eu3+ could be resolved. According to the

branching rules of various point groups,43 this study imples that the actual site symmetry of Eu3+ in Zn2P2O7 reduces from the original Oh/C4v for 6-Zn/5-Zn to C2v. It is interesting to observe that Sm3+ has a 4f5 and Eu3+ has a 6 4f valence shell configuration. The fact that a difference of just one electron in the valence shell leads to an enormous difference in the photophysical characteristics is something which needs to be understood. Such a difference is due to their different energy states as well as the location of those states relative to the host. Such differences are dictated by host referred binding energy (HRBE), which is the difference in kinetic energy of an electron ejected from the valence band with that from an impurity state (dopant ion).42 One can understand the physics behind the reason for HRBE differences from one lanthanide to another lanthanide ion in the same host 175

DOI: 10.1021/acs.inorgchem.6b01788 Inorg. Chem. 2017, 56, 167−178

Article

Inorganic Chemistry through the classical work by Dorenbos.44 If one sees the DOS plot of Zn2P2O7, the lower part of the valence bands (VB) is mainly contributed by O 2p orbitals hybridized with P 3p orbitals. The upper part of the VB is mainly contributed by the Zn 3d and O 2p states. The conduction band (CB) is mainly composed of Zn 4s states (in majority) and P 3p states. Figure S2 in the Supporting Information qualititatively explains the energy transfer dynamics in Zn2P2O7:Sm3+ and Zn2P2O7:Eu3+. We believe the energy mismatch of Sm d states with Zn 3d and O p states of Zn2P2O7 qualitatively explains why the energy transfer from host to Sm ion is very difficult. In addition, the difference in ionic size between the six-coordinated Zn2+ ion and six-coordinated Eu3+ ion is less than that with the sixcoordinated Sm3+ ion. 4.5. Probing the Local Structure around Ln3+ (Ln = Sm, Eu) in Zn2P2O7: EXAFS and XANES Studies. The XANES spectra of pure Zn2P2O7 and doped samples are shown in Figure 6a at the Zn K-edge. The Zn metal foil and ZnO standards are also plotted with the sample, which indicates the 2+ oxidation states of Zn ions. A small increase in the white line intensity is observed for Sm- and Eu-doped samples. The XANES spectra of the samples measured at Sm and Eu L3 edges are shown in Figure 6b along with those of Eu2O3 and Sm2O3 standards, which indicate that both dopant ions are present in +3 oxidation states in the samples. The data reduction to obtain k3χ(k) vs k plots and subseqently the radial distribution function χ(R) versus R plots from the measured μ(E) versus E spectra has been carried out following the standard procedures28,45 using the IFEFFIT software package.46 The k3χ(k) vs k plots and Fouriertransformed EXAFS spectra or χ(R) versus R plots of pure Zn2P2O7, Eu-doped Zn2P2O7, and Sm-doped Zn2P2O7 samples at the Zn K-edge and dopant edges (Sm and Eu) are shown in parts A and B of Figure 7, respectively. The first peak at 1.5 Å in phase-uncorrected spectra shown in Figure 7A is due to contributions of oxygen atoms, and the second peak at 2.8 Å is the combined contributions of Zn and P atoms. The theoretical EXAFS spectra were simulated using the structural parameters (atomic coordination and lattice parameters) of Zn2P2O7 from ref 24. Zn2P2O7 has a thortveitite structure, in which [P2O7]4− groups are in a staggered configuration and belong to the monoclinic space group I2/c.24,25 The oxygen coordination number of two Zn atoms in a unit cell is 5, and that of the other Zn atom is 6. The first peak in the FT-EXAFS spectra of the samples at the Zn K-edge has contributions from both a five oxygen atom coordination shell and a six oxygen atom coordination shell. The five-atom coordination shell has four oxygens at 2.07 Å and one oxygen at 1.84 Å. The six-atom coordination shell has four oxygen atoms at 1.96 Å and two oxygen atoms at 2.0 Å. To fit the first coordination peak, two feff calculations were generated with two different Zn sites and their ratio was kept fixed according to the theoretical value. The bond distances, coordination numbers, and disorder (Debye− Waller) factors (σ2), which give the mean square fluctuations in the distances, have been used as fitting parameters. The best fit results are summarized in Table 1. It should be noted here that the R-space spectra shown in Figure 7 are phase-uncorrected, which show peaks at R values lower than the actual distances, while the actual bond distances obtained from fitting are given in Tables 1 and 2. The second peaks at 2.8 Å in the phase-uncorrected Fourier transform spectra shown in Figure 7 are fitted with the path taken from feff calculations since the scattering path lengths are

Table 1. Bond Length, Coordination Number, and Disorder Factor Obtained by EXAFS Fitting at Zn Edge path Zn−O

Zn−O

Zn−Zn

Zn−P

Zn−O

Zn−O

param

Zn2P2O7

Zn2P2O7:Sm

R (Å) 1.83 1.83 N 1 1 σ2 0.0051 0.0032 R (Å) 2.07 2.07 N 4 4 σ2 0.0053 0.0072 R (Å) 3.12 3.14 N 2 2 σ2 0.0098 0.0091 R (Å) 3.16 3.18 N 4 4 σ2 0.0103 0.0074 Contribution from Six-Coordinated Structure R (Å) 1.96 1.97 N 4 4 σ2 0.0033 0.0027 R (Å) 2.00 2.01 N 2 2 σ2 0.0018 0.0018

Zn2P2O7:Eu 1.82 1 0.0052 2.02 4 0.0144 3.14 2 0.0094 3.17 4 0.0093 2.05 4 0.0033 2.01 2 0.0017

Table 2. Bond Length, Coordination Number, and Disorder Factor Obtained by EXAFS Fitting at Sm and Eu L3 Edge path (M = Sm, Eu)

param

Zn2P2O7:Sm

R (Å) 2.29 N 5 σ2 0.0023 M−Zn R (Å) 3.18 N 2 σ2 0.0028 M−P R (Å) 3.15 N 3 σ2 0.0241 Contribution from Six-Coordinated Structure M−O R (Å) 2.49 N 6 σ2 0.0036 M−O

Zn2P2O7:Eu 2.39 5 0.0082 3.35 2 0.0154 3.26 3 0.0220 2.47 6 0.0061

the same for Zn sites coordinated to both five and six oxygens. The second peak has contributions of two Zn atoms at 3.12 Å and four P atoms at 3.16 Å. The bond lengths and coordination numbers remain the same for Sm-doped samples; however, the Zn−O bond length is decreased (2.02 Å) slightly for the fivecoordinated sphere and increased (2.05 Å) for the sixcoordinated sphere. The best fit results for the dopant edge data are summarized in Table 2. The first peak observed at ∼2.0 Å in phaseuncorrected Fourier transform spectra (Figure 7B) for both Sm and Eu edges are fitted with the combined contribution of the five-coordinated sphere and the six-coordinated sphere and their ratio was kept variable during the fitting. In case of the Eudoped sample, this first peak has contributions of five Eu−O bonds at 2.39 Å and six coordinated Eu−O bonds at 2.47 Å with a weighted ratio of 42:58 of the two coordination shells. If the Eu atoms replace the Zn atoms in an equal ratio, then the coordination contribution of five-coordinated and six-coordinated sites should be 66:33. The ratio obtained here indicates that more Eu atoms are located at six-coordinated sites. This is in complete agreement with our emission and lifetime 176

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spectroscopy, where MDT predominates over EDT and 66% of Eu3+ resides at six-coordinated Zn2+. The first peak in the Sm-doped sample has contributions of five coordinated Sm−O bonds at 2.40 Å and six coordinated Sm−O bonds at 2.49 Å with a weighted ratio of the two coordination shells of 27:73. This indicates that more Sm atoms are replacing six-coordination sites in comparison to fivecoordination sites. The above results are in complete agreement with luminescence data.

AUTHOR INFORMATION

Corresponding Author

*S.K.G.: e-mail, [email protected] and santufrnd@gmail. com; tel, +91-22-25590636; fax, +91-22-25505151. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Kaushik Sanyal (Fuel Chemistry Division, BARC) for XRD measurements.

5. CONCLUSIONS Pure and lanthanide ion (Sm3+ and Eu3+) doped Zn2P2O7 phosphors have been synthesized using a solid-state route and characterized systematically using XRD, PL, and EXAFS techniques. A XANES study clearly shows that these lanthanide ions are stabilized as trivalent species. It was oberved from structural studies that zinc pyrophosphate crystallizes in a monoclinic geometry with two different coordinations for the zinc ion: ZnO5 and ZnO6. Photoluminescence measurements on the undoped Zn2P2O7 sample depicted an interesting gamut of blue-green color emission on excitation with ultraviolet light. The lifetime value predicted that such emission is probably related to the presence of defects in the pyrophosphate compound. DFT pinpointed the presence of an oxygen vacancy below conduction band through density of state calculations. The mechanism involved is transition from the defect states located at just below the CB minimum (arises due to singly and doubly ionized VO1+ and VO2+ oxygen vacancies) and VB maximum, as well as impurity states present in the band gap giving rise to a dual blue-green band under ultraviolet excitation. The photoluminescence investigations on doped sample show that the host completely transfers its energy to the europium ion, whereas there is inefficient/incomplete energy transfer on samarium doping. This is clearly depicted in their color coordinate diagrams, which show that Zn2P2O7:Sm3+ emits in the near-white region whereas Zn2P2O7:Eu3+ displays red emission. To optimize their performance for highly efficient luminescence materials, the local sites around samarium and europium ions must be known. From a comparison of relative intensities of electric and magnetic dipole transitions and relative population analysis from lifetime data, europium and samarium ion are distributed at both five- and six-coordinated zinc ion sites but the relative population at six-coordinated sites is higher. EXAFS analysis shows complete agreement with the above luminescence results.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01788. . Stark splitting patterns of magnetic (ΔJ = ± 1) and hypersensitive electric dipole transitions (ΔJ = ± 2) of Eu3+ in Zn2P2O7, qualititative depiction of schematic energy transfer dynamics in Zn 2 P 2 O 7 :Sm 3+ and Zn2P2O7:Eu3+, PAW-GGA calculated equilibrium lattice parameters, atomic positions, bond lengths, and band gaps along with previous experimental measurements and PAW-GGA calculated equilibrium atomic positions compared with experimentally reported atomic positions in a Zn2P2O7 unit cell (PDF) 177

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Inorganic Chemistry

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DOI: 10.1021/acs.inorgchem.6b01788 Inorg. Chem. 2017, 56, 167−178