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Dopant Induced Stabilization of Metastable Zircon -Type Tetragonal LaVO Chandresh Kumar Rastogi, Shilendra Kumar Sharma, Akshay Patel, Gopalakrishnarao Parthasarathy, Raj Ganesh S Pala, Jitendra Kumar, and Sri Sivakumar J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b04508 • Publication Date (Web): 30 Jun 2017 Downloaded from http://pubs.acs.org on July 5, 2017

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Dopant Induced Stabilization of Metastable Zircon-Type Tetragonal LaVO4 Chandresh Kumar Rastogi,Ϯ Shilendra Kumar Sharma,Ϯ Akshay Patel,φ G. Parthasarathy,ζ,* Raj Ϯ, Ϯ, Ϯ, Ganesh S. Pala, φ,* Jitendra Kumar, * and Sri Sivakumar φ,* Ϯ

φ

Materials Science Programme, Department of Chemical Engineering, Indian Institute of Technology Kanpur, 208016, Kanpur, India ζ

CSIR- National Geophysical Research Institute, Hyderabad, 500007, India

ABSTRACT Zircon-type tetragonal LaVO4 phase is a suitable host for luminescent lanthanide ions, but exhibits poor stability and transforms to monazite-type monoclinic phase at elevated temperatures and pressures. Here, we have studied the effect of partial substitution of La3+ with Eu3+ ions on the phase stability of tetragonal LaVO4 as a function of doping concentration using the temperature and pressure induced phase transformation characteristics. Experimentally measured activation energy required for the initiation of phase transformation is found to increase with increasing europium content suggesting dopant induced stabilization of the tetragonal phase. The critical pressure (Pc) and critical temperature (Tc) required for tetragonal to monoclinic phase transition increases with increasing Eu3+ ions concentration; values being Tc = 300, 500 and 600 °C Pc = 2, 4, and 6.5 GPa for compositions x = 0, 0.025, and 0.05, respectively. The simulations based on density functional theory (DFT) support the Pc data and the decrease in the formation energy of tetragonal La1-xEuxVO4 (x = 0.0625 – 0.375) calculated with respect to monoclinic phase suggest improvement in the stability of tetragonal phase with increasing doping concentration. Further, the doped Eu3+ ions act as an optical probe and the observed variation in their luminescent characteristics is related to the structural transformation occurred due to pressure and heat treatment.

KEYWORDS: Europium, vanadate, phase transformation, energy transfer, photoluminescence, and DFT simulations

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INTRODUCTION Crystal structure plays a crucial role in determining the luminescence properties of lanthanidedoped nanomaterials.1 For example, the intensity ratio of red (5D0 → 7F2) to orange (5D0 → 7F1) color emissions of doped Eu3+ ions can be altered by changing the local symmetry.2 Several crystalline phases (e.g., zircon-type tetragonal LaVO43, low temperature monoclinic BiPO44, and tetragonal BiVO45) are important hosts for lanthanide ions but display limited stability. It is, therefore, essential to ensure their stability to maintain their desired luminescent properties. Lanthanum orthovanadate (LaVO4) has been extensively studied in recent years due to its structure dependent luminescent properties and finds potential application as phosphors for solid state lighting and displays.6-8 Generally, it crystallizes in two polymorphs i.e., monazite-type monoclinic and zircon-type tetragonal.2 The zircon-type tetragonal lanthanum orthovanadate (LaVO4) has proven to be a suitable host for other Ln3+ ions (Ln = Eu, Tb, Dy, Sm, Er, Yb, etc.) and exhibits superior luminescent properties. But this phase is unstable and transforms to monazite-type monoclinic at elevated temperatures and pressures.2,9 Various approaches available for stabilization of the metastable phase include precursor10-11 and ligand-based9 synthesis, crystal twinning,12 and impurity doping.13 The widely used precursor/ligand-based methods produce metastable phases exhibiting instability at elevated temperatures and pressures.2,

9

As an alternative, dopant induced stabilization is a powerful

approach for structure stabilization with desired properties. For example, the high temperature hexagonal NaYF4 (β) phase has been stabilized with doping of Ti4+ ions at low temperatures by suppressing the formation of cubic phase.13 Farvid et al. have reported the formation of metastable corundum-type hexagonal In2O3 nanocrystals by adsorption of Mn3+ ions.14 Usai et al. have stabilized tetragonal phase of BiVO4 by introducing smaller size Y3+ ions (rY3+ = 1.04 Å) which require fewer oxygen ions than existing with Bi3+ ions (rBi3+ = 1.17 Å) in the monoclinic structure.15 Similarly, LaVO4 (rLa3+ = 1.22 Å) with coordination number (CN) of 9 prefers monoclinic structure while the other LnVO4 with smaller size Ln3+ ions (Ln = Y, Gd, and Eu ) prefers formation of the tetragonal LnVO4 with CN of eight.16 With these facts, europium (rEu3+ = 1.07 Å) has been chosen here as substitutional impurity to stabilize the zircon-type tetragonal La1-xEuxVO4 (x = 0 – 1.0) under certain processing conditions (temperature and pressure).


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In recent years, attempts have been made to synthesize zircon-type tetragonal LaVO4 by varying the processing conditions such as pH, reaction time, and ligands.6-9,17-24 Though the formation of zircon-type phase is achieved, however, its stability remains a concern at elevated temperature due to the removal of ligand which results in transformation to its thermodynamically stable monazite-type monoclinic phase. Although, such a phase transformation for LaVO4 system has been investigated at high temperatures9 and pressures2,21,25 conditions, yet, the stability of Eu3+doped LaVO4 as a function of Eu3+ ions concentration remains to be explored. Also, the exact role of Eu3+ ions in determining the stability of tetragonal is not well understood. The present work is an attempt to investigate the role of Eu3+ ions in the improvement in the phase stability of zircon-type tetragonal La1-xEuxVO4 (x = 0 – 0.375) using the temperature/pressure induced phase transformation characteristics and density functional theory (DFT) simulations. The increase in the experimentally obtained activation energy (Ea), critical annealing temperature (Tc), and critical pressure (Pc) with an increase in europium ions concentration are presented as evidence for the stability improvement of zircon-type tetragonal phase. Further, decrease in the formation energy of tetragonal La1-xEuxVO4 (x = 0.0625 – 0.375) calculated with respect to monoclinic phase using DFT simulations suggest improvement in the stability of tetragonal phase with increasing doping concentration.

EXPERIMENTAL Formation of Eu3+-doped LaVO4 Lanthanum (III) nitrate hydrate (La(NO3)2. xH2O, 99.9 %, Aldrich), europium (III) nitrate pentahydrate (Eu(NO3)2.5H2O, 99.9 %, Aldrich), sodium orthovanadate (Na3VO4, 99.9 %, Aldrich), and analytically pure oleic acid (Loba Chemie), triethyl amine (S.D. Fine Chemicals) and ethanol (Merck) were used for the synthesis. Eu3+-doped LaVO4 (La1-xEuxVO4; x = 0 – 1.0) samples were prepared at 75 ºC by precipitation method using oleic acid as a ligand. In a typical case, 0.5 ml oleic acid was added to a 70 ml mixture of de-ionized water and ethanol (1:1 v/v). The pH of the solution was adjusted to ~ 6 with triethyl amine. The aqueous solution of 1.33 mmol Ln(NO3)3.xH2O (Ln = La and Eu), prepared with stoichiometric amounts of lanthanum and europium salts, was then poured drop wise to the reaction medium at room temperature. Thereafter, another aqueous solution of 1.33 3 ACS Paragon Plus Environment


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mmol sodium orthovanadate was added at 75 °C for reaction to occur for 2h. Finally, the precipitate was collected and washed thrice with cyclohexane/acetone using a centrifuge at ~ 6000 rpm to obtain La1-xEuxVO4.2 In order to study phases and their stability, two sets of La1-xEuxVO4 samples were prepared with (x) in the range of (0 – 1.0) and (0 – 0.15) for pressure treatment at 2.0 – 6.5 GPa using a Bridgman anvil cell at room temperature (RT) and annealing purpose in the range 200 – 1200 °C for 3h, respectively. A heating rate of 5 °C/minute was used for attaining the calcination temperature.

Characterizations An X-ray diffractometer (X’ Pert PRO), an UV-Vis-NIR spectrophotometer (Varian model Carry 5000), a transmission electron microscope (FEI Technai G2) were used for evaluation of phase(s), optical absorption measurements, and observation of microstructure, respectively. A thermogravimetric analyzer (Perkin Elmer TGA 8000TM) and a differential scanning calorimeter (Perkin Elmer DSC 8500) were used for thermal analysis. Further, a fluorescence spectrophotometer (Edinburgh instruments FLSP 920), equipped with a double monochromator, 450 W Xenon lamp as an excitation source, and Peltier element cooled Hamamatsu R928-P PMT detector was employed for obtaining the luminescence spectra. The luminescence decay curves were recording using a 100W micro-flash lamp (µF920H) as the excitation source. All the characterizations were performed on the powder samples at room temperature while the digital image displaying emission from La1-xEuxVO4 (x = 0.025) sample was taken by dispersing it in cyclohexane.

High-pressure methods High-pressure experiments were carried out at room temperature in a Bridgman anvil cell using WC opposed anvils, with pyrophillite gasket, and no pressure transmitting medium is used as the La1-xEuxVO4 samples are very soft as a solid transmitting medium. The actual pressures on the samples were determined through the well-known solid-solid phase transitions in bismuth, ytterbium, tellurium, and thallium. The details of the equipment, calibration, uncertainties involved, and the procedure followed those described elsewhere.26 As the size of the samples 4 ACS Paragon Plus Environment

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used in the high pressure chamber is typically 1-2 mm, three to four independent high-pressure experiments were performed on each sample, in order to collect more pressurized samples as the pressure induced transformation in these samples are irreversible in nature.

COMPUTATIONAL METHODS Density functional theory (DFT) have been performed to obtain trends in phase transition pressure (Pc) with respect to doping concentration for La1-xEuxVO4. Electronic band structures and partial density of states (PDOS) have been obtained to study the effect of Eu3+ ions doping and crystal phase on the band gap of La1-xEuxVO4. Perdew–Burke–Ernzerhof (PBE27) functional and projector augmented wave (PAW28) method have been used in the framework of plane wave density functional theory implemented in Vienna Ab initio Simulation Package (VASP29). All the calculations were performed using a plane wave basis set with energy cut off 400 eV. With fshell configuration of Eu3+ ions, the spin polarized PBE+U30 approach has been utilized in the current study with optimized effective U value of 8 eV. The pseudo-potential of La, V, O and Eu contain 11 (5s2 5p6 5d1 6s2), 11 (3p6 3d4 4s1), 6 (2s2 2p4) and 17 (4f7 5s2 5p6 6s2) valence electrons, respectively. Brillouin zone sampling has been performed using gamma centered mesh with k-points 2×2×3 for both the tetragonal (zircon-type) and monoclinic (monazite-type) LaVO4. The geometry has been relaxed by optimizing all structural parameters until the force on each ion is smaller than 0.01 eV/Å.

RESULTS AND DISCUSSION Phase identification In order to study the effect of Eu3+ ions doping on the stability of zircon-type tetragonal La1xEuxVO4,

x-ray diffraction (XRD) patterns of as-synthesized product as such and after i)

annealing at different temperatures (200 - 1200 °C) for 3h, and ii) subjecting to pressures (2 – 6.5 GPa) have been analyzed. XRD patterns of as-prepared samples shown in Figure 1 match well with zircon-type tetragonal LaVO4 structure (PDF No. 32-0504, a = 7.49 Å, c = 6.59 Å, Z = 4 and Space group I41/amd) alone or with traces of monoclinic phase (PDF No. 25-0427, a = 7.07 Å b = 7.29 Å c = 6.77 Å, β = 105º Z = 4; and Space group P21/m) in case of compositions x ≤ 0.05. A progressive shift in the diffraction peaks towards higher angle side for La1-xEuxVO4 (x = 0 – 0.15) with increasing ‘x’ (Figure S1 of the supporting information) suggest a decrease in cell 5 ACS Paragon Plus Environment


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parameters which may be due to the substitution of La3+ with smaller size Eu3+ ions (rEu3+ = 1.07 Å and rLa3+ = 1.16 Å) in LaVO4. The estimated values of lattice parameters and cell volume are mentioned in Table S1 of the supporting information. The oleic acid is used as a ligand in the synthesis and facilitates the formation of metastable zircon-type LaVO4 phase. As evident from Figure 1, La1-xEuxVO4 (x = 0.10) prepared without oleic acid contains monoclinic phase as well along with tetragonal phase. In contrast, pure zircon-type tetragonal phase results with the use of oleic acid as a ligand. This clearly indicates vitality of ligand in yielding a pure tetragonal phase perhaps via capping and inhibiting the emergence of stable monoclinic LaVO4. Moreover, the amount of monoclinic phase is less in the case of x = 0, i.e., sample with no Eu3+ species when prepared using a ligand. Thus, both the oleic acid as a ligand and europium content in La1-xEuxVO4 are crucial and coupled as well (playing a similar role) in the formation of the zircon-type tetragonal phase.

Phase stability Effect of temperature Figure 2 a-e shows XRD patterns of La1-xEuxVO4 (x = 0, 0.025, 0.05, 0.10 and 0.15) powder samples after annealing at different temperatures. These clearly demonstrate the presence of tetragonal and/or monoclinic phase(s), similar to LaVO4, depending upon the annealing temperature and europium content. The tetragonal phase transforms to monoclinic phase after subjecting to heat treatment at a temperature determined by europium content (x). As discussed above, the as-prepared samples having x ≤ 0.05 exhibit some traces of monoclinic phase; however, the emphasis is on identifying the temperature for initiation of phase transformation (i.e. as synthesized zircon-type tetragonal phase should transform to monazite-type monoclinic phase), whose signature will be on the increase of monoclinic phase beyond the as synthesized amount. The dopant concentration, annealing temperature and annealing time are coupled variables. If the dopant concentration increases, at fixed annealing temperature, the time required for the appearance of monazite/monoclinic phase will increase. As a corollary, if the dopant concentration increases, at fixed annealing time, the temperature required for the appearance of monazite/monoclinic phase will increase. Having synthesized La1-xEuxVO4 predominantly possessing tetragonal phase, the central objective of annealing experiments is to identify critical 6 ACS Paragon Plus Environment

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temperature (Tc, temperature at which the phase transformation is initiated), whereas equilibrium requires the completion of phase transition which is not the focus of the present study, especially since, the kinetics of phase transition is slow preventing the determination of the time required to attain equilibrium. Using, this criterion, the critical temperature required for the initiation of phase transformation (Tc) is found to be 300, 500, 600, 700, and 800 ºC for composition x = 0, 0.025, 0.05, 0.10, and 0.15, respectively (Figure 2 f). The complete phase transformation occurs at 600, 800, 900, 1200 ºC for compositions x = 0, 0.025, 0.05, and 0.10, respectively. In addition, the tetragonal phase is maintained for composition x = 0.30 even after heat treatment at an elevated temperature of 1200 °C for 3h (Figure S2 of the supporting information). Accordingly, it is predicted that a temperature of above 1200 ºC is needed for the complete phase transition for the compositions x ≥ 0.15. The increase in Tc with more loading of europium content suggests that the presence of europium ions inhibits the tetragonal to monoclinic phase transition in La1-xEuxVO4 system. The activation energy (Ea) required for tetragonal to monoclinic phase transformation has also been determined for La1-xEuxVO4 (x = 0.01 – 0.10) systems. Fixed amount (50 mg) of each sample was heated for 3h at a specified temperature depending upon the composition. The amount of tetragonal and monoclinic phases present were deduced from the room temperature xray diffraction patterns using an X’Pert HighScore Plus software. The model used for phase quantifications are described elsewhere.31 The fractional amounts of both the phases present in each sample are mentioned in Table 1. The number of moles of tetragonal remained is given by: =

× (1) ℎ

where f is the fraction of tetragonal phase present and m is the mass of the sample. Also, the rate of tetragonal to monoclinic phase transformation can be written as −

$% = ! = !" #&' (2)



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where k0 is pre-exponent factor, Ea is the activation energy for phase transformation per mole, R is the gas constant and T is the absolute temperature. The activation energy for different compositions, deduced from the slope of ln[-dN/dt] versus 1/T plots (Figure 3a) found to increase with increase in europium content, values being 22.2, 26.2, 27.3, and 32.1 kJ/mole for compositions x = 0, 0.025, 0.05, and 0.10, respectively (Figure 3b). Such a trend suggests suppression of tetragonal to monoclinic transformation with increasing ‘x’ in La1-xEuxVO4. Thus, europium ion insertion in LaVO4 facilitates stability of zircon-type tetragonal phase. Thermal Analysis TG curve recorded for La1-x EuxVO4 (x = 0) sample in the temperature range 25 – 1000 °C with a scan rate of 3 °C/min is shown in Figure S3a of the supporting information. It depict weight loss in three regimes, viz., ~ 6%, 24%, and 4% in the temperature ranges 125 – 330 °C, 330 – 460 °C, 690 – 770 °C, respectively. While the first two weight losses are associated with the removal of moisture and oleic acid, the third one arises due to oxidation of the residual carbon species in La1-xEuxVO4. Similar weight losses are observed in oleic acid based synthesis of Fe3O4 nanoparticles.32 The weight loss profiles for other La1-xEuxVO4 compositions (x = 0.025 and 0.05) are somewhat similar in nature with marginal differences (Figure S3 b, c of the supporting information). DSC curves for La1-xEuxVO4 (x = 0, 0.025 and 0.05) samples depict broad exothermic bands because of (i) ligand removal and (ii) tetragonal-monoclinic transformation. Hence, the transition temperature (Tc) cannot be ascertained precisely due to the prevailing overlap of the two phenomena involving different heat flow but occurring simultaneously. Also, the phase transformation process is quite slow. The variation in heat evolution observed in samples with increasing europium content (x) points to suggest progressive wearing of the transformation. Effect of pressure Figure 4 depicts XRD patterns of La1-xEuxVO4 (x= 0 – 1.0) after subjecting to hydraulic pressure in the range 2.4 – 6.5 GPa. Notice that the samples with the composition in the range x = 0 – 0.025 exhibit single monazite-type monoclinic phase similar to LaVO4 (PDF No. 25-0427). In contrast, both the tetragonal and monoclinic phases appear for compositions range of x = 0.05 – 0.20 with a progressive decrease in the content of the later. This suggests europium substitution inhibits tetragonal to monoclinic phase transformation as progressively higher pressure is 8 ACS Paragon Plus Environment

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required in case of doped samples with more europium content. Nevertheless, a pressure as high as 6.5 GPa fails to induce phase transitions for compositions x = 0.30 – 1.0, leaving the sole presence of the tetragonal phase. The presence of a single tetragonal phase in both the pressure subjected and the heat treated samples indicates its thermodynamic stability in La1-xEuxVO4 system for compositions having x ≥ 0.30. The above findings further support the enhancement in the stability of zircon-type tetragonal phase with increasing europium content in La1-xEuxVO4 which is further substantiated by DFT based simulations (discussed below).

Microstructure and elemental analysis The transmission electron micrographs of as prepared, pressure treated (6.5 GPa) and annealed (800 °C for 3h) La1-xEuxVO4 (x = 0) samples are shown in Figure S4 (a, b and c) of the supporting information. Notice that the crystallites in the size range of ~15 – 20 nm flattened after the pressure treatment and cover region of ~50 nm. Further, the heat treatment results in aggregation of the particles into size ranges ~ 75 – 200 nm. The elemental composition of asprepared La1-xEuxVO4 (x = 0.05) determined by energy dispersive X-ray spectroscopy confirmed the appropriate presence of lanthanum, vanadium, and europium (data not shown).

Spectroscopic characterizations UV-VIS absorption spectra of the as-prepared and pressure treated La1-xEuxVO4 (x = 0 – 0.10) samples are shown in Figure 5a. These contain absorption bands in the wavelength range of 250 – 550 nm with the peaks centered at ~ 275 and 267 nm for as-prepared and pressurized samples, respectively. The blue shift observed in the absorption band for the pressure treated (6.5 GPa) sample is possibly because of an increase in the bandgap energy resulting due to tetragonal to monoclinic phase transformation which is evident from band-structure plot obtained using density functional theory simulations (discussed below). Moreover, increase in the absorption intensity in the wavelength range 325 – 500 nm is noticed with increase in doping concentration which is attributed to the formation of shallow impurity states near the valence and conduction bands on Eu3+ ions doping as confirmed by band structure obtained by DFT simulation (discussed below). 9 ACS Paragon Plus Environment


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Figure 5b depicts Raman spectra of La1-xEuxVO4 (x = 0.025) sample before and after pressure treatment. Eight Raman modes are assigned as per reported data for tetragonal LaVO42,21. Accordingly, the peak shifts at lower frequencies correspond to La–O vibrations and the most pronounced signal at ~ 851 cm-1 represents V–O bond stretching mode ν1(Ag). The marked changes in the peak shift occurring after pressure treatment include new modes in the range 300 – 368 cm-1 and displacement of ν1(Ag) mode to ~ 853 cm-1 matching with the known data of monoclinic phase.2 The above studies clearly suggest structural transformation from zircon-type tetragonal La1-xEuxVO4 (x = 0.025) to monazite-type monoclinic phase upon pressure treatment. In order to study changes in photoluminescence of La1-xEuxVO4 (x = 0 – 0.20), emission, excitation, and lifetime data were collected for the as-prepared, annealed (500 – 1200 °C) and pressure treated (~ 6.5 GPa) samples. Figure 6a shows the emission spectra of as-prepared La1xEuxVO4

(x = 0.01 – 0.20) observed with an excitation wavelength of 310 nm. These spectra

were normalized with respect to emission peak at ~ 618 nm. The sharp peaks appear at ~ 593, 618, 653, 698 nm arise due to Eu3+ ions 5D0 → 7FJ, (J = 1, 2, 3, and 4) transitions, respectively.33 Since all the spectra are nearly similar, emission characteristics emission seems to be unaltered with an increase in europium content. The prominent emission peaks centered at ~ 618 nm and ~ 698 nm occur due to electric dipole transitions with pronounced splitting which can be attributed to crystal field effect.3 The other emissions occurring at ~ 593 and ~ 653 nm are weak and arise due to forbidden magnetic dipole transitions.3 The 5D0 → 7F2 transitions are the hyper sensitive electric dipole transitions and therefore influenced by the structural changes whereas 5D0 → 7F1 being magnetic dipole transitions are unaffected.34 Therefore the variation in the emission intensity at ~ 618 nm (5D0 → 7F2) vis-à-vis ~ 593 nm (5D0 → 7F1) is indicative of structural changes. The strong emission at ~ 618 nm suggests a lack of inversion symmetry existing around Eu3+ ions in as-prepared tetragonal La1-xEuxVO4 (x = 0 – 0.15).

2

The CIE chromaticity

coordinates as deduced for La1-xEuxVO4 (x = 0.05) sample are (0.63, 0.33). Its digital image showing the strong red color emission from the sample is displayed in the inset (Figure 6a). The excitation spectra of as-prepared La1-xEuxVO4 (x = 0.01 – 0.20) samples recorded for Eu3+ ions emission at ~ 618 nm are shown in Figure 6b. These contain intense broad bands in the wavelength range of ~ 250 – 350 nm, arising due to charge transfer from [VO4]3- group to Eu3+ ions 23, and a few sharp lines at ~363, 382, 395, 417, and 465 nm corresponding to Eu3+ ions 7F0 10 ACS Paragon Plus Environment

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→ 5D4, 7F0 → 5L7, 7F0 → 5L6, 7F0 → 5D3 and 7F0 → 5D2 transitions, respectively.35 The observed splitting in the excitation band with peak maxima at ~ 270 and 310 nm is due to a doublet of [VO4]3- group. In LaVO4, the crystal field reduces the original Td symmetry of [VO4]3− to D2d and causes splitting of its degenerate excited level into 1A1 (1E) and 1E(1T2) states.36-37 As a consequence, the excitation from the ground level 1A2 (1T1) to the above states results in two bands at ~ 270 and 310 nm, respectively. Similar splitting in the excitation band has been observed in LaVO4: Sm3+ system.36 The emission spectra of La1-xEuxVO4 (x = 0.025) sample heat treated for 3h at different temperatures (500 – 1200 °C) are recorded with an excitation wavelength of 310 nm as shown in Figure 6c. The spectra consist of sharp characteristic peaks of Eu3+ ions similar to those observed in as-prepared samples. However, the variation in the relative intensity of emission peaks at ~ 593 nm (5D0 → 7F1) and ~ 618 nm (5D0 → 7F2) is observed with an increase in the annealing temperature. It may be pointed out that Eu3+ ions occupy D2d and C1 site symmetry positions in tetragonal and monoclinic LaVO4, respectively.2 An asymmetry parameter (A21) defined as the red-to-orange luminescence intensity ratio is useful in determining the emitter site occupancy. A21 can be deduced by integrating the emission peaks due to electric dipole (5D0 → 7F2) and magnetic dipole (5D0 → 7F1) transitions using the relation:38 )* =

+." ,* . λ ./"

+010 , . λ .""

(3)

The A21 value comes out to be 9.8 for the as-prepared La1-xEuxVO4 (x = 0.025) sample. After annealing for 3h each at 500, 600, 800, and 1200 °C, A21 values become 9.2, 8.7, 4.3, and 4.1, respectively. The decrease in A21 parameter with an increase in annealing temperature can be due to structural transformation in La1-xEuxVO4 (x = 0.025) upon heat treatment (discussed below). The excitation spectra (λem = 618 nm) of La1-xEuxVO4 (x = 0.025) samples after annealing at different temperatures in the range 500, 600, 800, and 1200 °C are presented in Figure 6d (spectra are normalized with respect to the band at ~310 nm). These exhibits broad excitation band in the wavelength range 250 – 350 nm and sharp peaks at ~ 363, 382, 395, 417, and 465 nm similar to those observed for as-prepared samples (Figure 6b), but, with the difference in relative



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intensities. The broad band becomes less prominent vis-à-vis sharp peaks with an increase in annealing temperature. The variation in relative intensity is in consonance with the increased tetragonal to monoclinic phase transformation occurring at higher annealing temperature as evident from x-ray diffraction data (Figure 2b). This further provides credence to the above conjecture, i.e., correlative with the relative amount of tetragonal and monoclinic present.2 Obviously, the energy transfer process responsible for the origin of excitation band (250 – 350) nm, is more efficient in tetragonal phase vis-à-vis monoclinic structure which can be understood as follows.2 The arrangements of atomic species in the optimized structure (based on DFT data discussed below) of tetragonal and monoclinic La1-xEuxVO4 (x = 0.25) were visualized using the graphics shown in Figure 7. The graphics were drawn with the help of visual molecular dynamics39 (VMD) and Visualization for Electronic and Structural Analysis40 (VESTA) graphical visualization packages. As evident from graphics, Eu3+ ions occupy La3+ sites and are asymmetrically surrounded by [VO4]3- groups in monoclinic LaVO4. Further, the lengths of LaO, Eu-O, V-O bonds and bond angles between La-O-V in both the phases as estimated for the optimized structures of La1-xEuxVO4 (x = 0.25) are listed in Table 2. Accordingly, the spacing between [VO4]3- group and Eu3+ ions is larger in monoclinic vis-à-vis tetragonal phase; values being 3.38 Å and 3.26 Å, respectively. This causes poor energy transfer from [VO4]3- to Eu3+ ions in monoclinic phase. In contrast, Eu3+ ions are symmetrically surrounded by [VO4]3- group at relatively smaller distances in tetragonal phase with a better overlap of wave-functions resulting in efficient energy transfer. This explains the observed difference in intensities of charge transfer band at 310 nm and direct excitation peak at ~395 nm in two phases, i.e., the weak band at 310 nm in monoclinic as compared to tetragonal phase. The emission spectra of La1-xEuxVO4 (x = 0.01 – 0.20) samples after pressure treatment at ~ 6.5 GPa, are recorded using excitation wavelength of 310 nm as shown in Figure 6e. These contain characteristics emission peaks of Eu3+ ions at ~ 593, 618, 653, 698 nm. Note that the emission at 615 nm invariably dominates in all the pressure treated samples similar to as-prepared and annealed La1-xEuxVO4 samples. However, the intensity ratio (I615/ I593) [termed as an asymmetric parameter (A21)] in samples undergone pressure and heat treatments is lesser compared to untreated samples. It is reported that Eu3+ ions occupy crystallographic sites with C1 and D2d symmetry configuration in monoclinic and tetragonal La1-xEuxVO4 phases, respectively. The lack of inversion symmetry at both these sites leads to strong emission at ~615 nm vis-à-vis ~ 593 nm 12 ACS Paragon Plus Environment

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because of hypersensitive nature of Eu3+ ion 5D07F2 transitions. We note that the sample after the pressure and heat treatment possess both the phases and increase in the particle size. Thus, the variation in the hypersensitive emission (5D07F2) may not be only due to the phase change but also due to an increase in the size of the particles.2, 6 The excitation spectra of La1-xEuxVO4 (x = 0.01 – 0.2) samples after pressure treatment at 6.5 GPa are shown in Figure 6f. Notice that the relative intensity of host excitation band (250 – 350 nm) vis-à-vis sharp peaks at ~ 363, 382, 395, 417, and 465 nm (due to Eu3+ ions direct excitation) depends upon the composition. As evident from XRD data, tetragonal La1-xEuxVO4 (x ≤ 0.05) completely transforms to monoclinic with increased pressure. The larger spacing between [VO4]3- tetrahedral and Eu3+ ions reduce the overlap between their wave functions and, in turn, makes the ([VO4]3- → Eu3+) energy transfer process less efficient. The changes observed in the excitation spectra of pressure treated samples La1-xEuxVO4 (x = 0 – 0.20) samples are associated with tetragonal to monoclinic phase transformation. The luminescence lifetime of Eu3+ ions 5D0 level of as-prepared and pressure treated samples has been estimated from the decay curves of 618 nm emission (Figure S5 of the supporting information) by fitting using a bi-exponential function as follows:33 y = y" + A exp(−t⁄τ ) + A* exp(−t * ⁄τ* )

(4)

where A1 and A2 are the pre-exponents while τ1 and τ2 are the time periods in which luminescence intensity falls to 1/e of its original value. The effective luminescence lifetime τeff is then estimated by following formula:33 τ

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