Increased Loading of Eu3+ Ions in Monazite LaVO4 Nanocrystals

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Increased Loading of Eu3+ Ions in Monazite LaVO4 Nanocrystals via Pressure-Driven Phase Transitions Pooja Gangwar,† Mohnish Pandey,† Sri Sivakumar,*,†,⊥,§ Raj Ganesh S. Pala,*,† and G. Parthasarathy*,‡ †

Department of Chemical Engineering, ⊥Material Science Programme, and §Centre for Environmental Science and Engineering, Indian Institute of Technology Kanpur, Kanpur, India ‡ CSIR-National Geophysical Research Institute, Hyderabad, 500007, India ABSTRACT: The concentration of Eu3+ ion/dopant in the LaVO4 monazite nanocrystal phase cannot be increased by the conventional synthetic procedures. We demonstrate a unique three-step methodology to increase the doping concentration of Eu3+ in the LaVO4 monazite nanocrystals. In the first step, Eu3+ is doped (10%) in the zircon LaVO4 nanocrystal phase, which does not have a limitation in terms of Eu3+ ion loading. In the second step, high pressure (∼5 GPa) is utilized to transform the zircon crystal phase to the monazite phase. In the third step, the pressure is brought back to the atmospheric level, wherein it is observed that the monazite crystal phase is retained in its metastable phase with the 10% Eu3+ ion doping concentration. The phase transitions have been characterized via electrical resistivity data, XRD, Raman spectroscopy, fluorescence spectroscopy, TEM, and density functional simulations. phase.10 A significant work via a hydrothermal method pursued by Sun and his co-workers has resulted in 20% Eu3+ loading of monazite LaVO4 nanocrystals.9 Further, the energy transfer from the VO4 group to the Eu3+ ion is also inefficient due to the large spacing between the Eu3+ ions and VO4 tetrahedrons in the monazite LaVO4, which reduces overlap of their wave functions.3 However, the lower symmetry around the Eu3+ ion in the monazite phase enhances the direct absorption by Eu3+, resulting in stronger photoluminescence than that of Eu3+ in the zircon phase, and this property can potentially be used in red phosphors excited with blue LEDs for white light generation. Also, in certain photoluminescent systems, the dopant is known to phase segregate if the crystal structure of the host lattice is transformed.11,12 Pressure has been extensively investigated as an approach to increase the stability of a specific crystal structure of a polymorphic system. While the phase transitions of the ABO4-type ternary oxides at high pressure have been explored extensively,13,14 only a few attempts have been made on investigating pressure-driven phase transformation in the lanthanide vanadate compounds. Also, the irreversibility of such phase transitions on removal of high pressure is also of interests as practical applications will usually be performed at atmospheric pressures. Zhang et al. have observed irreversible phase transition in GdVO4:Eu3+ microcrystals by applying high pressure from 7.4 to 16.4 GPa, where its zircon-type transformed to the scheelite-type crystal structure.15 When

1. INTRODUCTION The crystal structure of the host lanthanum orthovanadate plays an important role in a variety of properties and, specifically, in lanthanide ion photoluminescence.1−4 For example, the Eu3+-doped LaVO4 monazite crystal structure allows magnetic dipole transition (5D0 to 7F1) and the zircon phase of LaVO4 permits electric dipole transition (5D0 to 7F2) because of its low inversion symmetry with the D4h space group.5,6 Lanthanide orthovanadate is a dimorphic crystal, in which the relative stability of the monoclinic-monazite type (m) structure and the tetragonal-zircon type (t-) can be modulated by pH, pressure, temperature, and dopant concentration.2,7 It is anticipated that such variations control the relative contribution of the surface and subsurface energies that, in turn, determine the crystal structure.2,8 Generally, large LaVO4 crystals have the monazite structure and other orthovandates, including Y and Sc, crystallize in the zircon structure.3 The tendency of LaVO4 to crystallize in the monazite crystal structure has been qualitatively correlated to the large radii of the lanthanide ions enabling it to coordinate with nine oxygen ions in the monazite structure as opposed to eight ions in the zircon structure.3,4 The photoluminescence is also controlled by the amount of dopant that can be incorporated in the host crystal.1−4,6,9 The monazite phase is not considered as a good host as compared to the zircon phase because of the limited solubility of the Eu3+ ion in the monazite phase.3 While it has been possible to increase the solubility of the Eu3+ ion in the monazite phase in thin films using pulsed laser deposition,3 attempts to increase the dopant concentration (>3%) in the monazite phase nanocrystals results in the slow conversion of the crystal to the zircon phase because EuVO4 crystallizes as the zircon © XXXX American Chemical Society

Received: December 31, 2012 Revised: April 6, 2013

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Figure 1. Schematic diagram of phase transformation in the LaVO4:Eu3+ nanocrystal under high pressure.

the monazite structure do not revert back to the zircon phase when the pressure is reduced to atmospheric levels. (3) The dopant does not phase segregate when the Eu3+ ion doped LaVO4 monazite nanocrystals are at atmospheric pressure. (4) The density functional studies suggest that the transition of LaVO4 in the zircon structure to the monazite structure can be rationalized by enthalpy changes accompanied by negligible changes in the Bader charges structure(see Figure 1).

pressure is used for transforming the phase of AVO4-type crystals, the transformed phase is usually sustained at ambient pressure with the help of pressure quenching. Chen et al. have investigated irreversible phase transition in YVO4:Eu3+ under pressures up to 7−14 GPa through luminescence data.16 However, Zhang et al. reported a reversible phase transition from zircon to scheelite type in the tetragonal orthophosphates (YbPO4 and LuPO4) on the application of a pressure of ∼22 GPa for YbPO4 and 19 GPa for LuPO4.17 The extent of the high pressure required to achieve phase transition is often dependent on the nanostructure of the material as pressuredriven phases nucleate from the surface of the crystals.18,19 The size of these nanocrystals is such that it is of the order of the crystallite size in larger crystals, and these nanocrystals are free of single-point defects.18,19 In general, under high pressure, both large crystals and nanocrystals transform to a crystal phase with a lower volume to minimize their free energy. It has been observed that the pressure required for phase transformation of a nanocrystal increases as the size of the nanocrystal decreases.18−20 This observations can be correlated to the decrease in the “P*V” term in the free energy, whose contribution decreases in nanostructures.20,21 In addition to size, the kinetics of pressure-driven transitions is also dependent on the shape of the nanocrystals, and the nanocrystals show greater reversibility in phase transition.22 In light of the above considerations, the present work addresses (1) utilizing high-pressure transitions to increase the Eu3+ ion/dopant loading in the LaVO4 monazite nanocrystals without phase segregation of the dopant, (2) the irreversibility of high-pressure transitions when the pressure is decreased to atmospheric pressures, and (3) physico-chemical factors behind pressure-driven transitions in LaVO4. We have utilized in situ electrical resistivity, followed by XRD, Raman spectra, photoluminescence, TEM techniques, and density functional simulations, to address the above-mentioned issues. We find the following: (1) It is possible to synthesize LaVO4 doped with 10% Eu3+ in the zircon phase, then apply high pressures (∼5.3 GPa) to transform the crystal to monazite structure. (2) The Eu3+-doped LaVO4 nanocrystals formed in

2. EXPERIMENTAL SECTION Lanthanide(III) trinitrate hydrate (La(NO3)3·H2O, 99.9%, Aldrich), europium(III) trinitrate pentahydrate (Eu(NO3)3·5H2O, 99.9%, Aldrich), sodium orthovanadate (Na3VO4, Loba Chem.), and oleic acid (Qualigens) were purchased and used as received. 2.1. Synthesis of LaVO4:Eu3+ (10%) Nanocrystals. In a typical procedure,23 oleic acid (2 mL) was dissolved in an ethanol/water (1:1) mixture, followed by addition of a few drops of triethylamine to adjust the pH to ∼6. An aqueous solution (2 mL) of lanthanide salts (1.33 mM) La(NO3)3·6H2O and Eu(NO3)3·5H2O was added dropwise to the above mixture under constant stirring, followed by the addition of an aqueous solution of sodium orthovanadate (1.33 mM). The mixture was kept under constant stirring at 75 °C for 2 h and cooled to room temperature. The obtained nanocrystals were washed with ethanol several times to remove the excess oleic acid, followed by drying at room temperature. All the experiments were carried out with a 10 atom % Eu3+ concentration, which is confirmed by the XRF spectroscopy (Table 1). 2.2. High-Pressure Studies. High-pressure electrical resistivity measurements of the samples were carried out at room temperature by using a Bridgman opposed anvil cell technique up to 7 GPa, using steatite as a pressure-transmitting medium. The uncertainties in the pressure measurements are ±0.2 at 7 GPa . The pressure calibration

Table 1. Elemental Composition of LaVO4:Eu (10%) Nanoparticles Determined by XRF sample name LaVO4 zircon phase (before pressurization) LaVO4 monazite phase (after pressurization) B

La (wt %) V (wt %) Eu (wt %) 25.02

10.61

2.30

26.60

10.24

2.57

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and methodology are described elsewhere.24−26 A Keithley electrometer (model No. 614), multimeter (model No. 199), and constant current source (model No.263) are used as measuring instruments. We used a four-probe method for resistivity measurements and pure copper wires as an electrode.26 Triplicate runs were made to check the reproducibility. Samples were subjected to 5.3 GPa high pressure, and the pressure was released to ambient pressure. These samples were collected and have been investigated by various characterization techniques. 2.3. Characterization. High-pressure measurements were carried out in an opposed anvil system using MgO as the pressuretransmitting medium. We used a four-probe method for resistivity measurements, and pure copper wires as an electrode. The crystalline nature and phase difference of the sample were examined by an X-ray diffraction apparatus (XPERT-PRO) with Cu Kα radiation (λ = 1.54 Å) operating at 45 kV and 40 mA. The photoluminescence measurements were done using a steady-state and time-resolved fluorescence spectrometer (Edinburgh instruments FLSP 920) attached with a red-sensitive Peltier element cooled Hamamatsu R928-P PMT detector. Steady-state emission spectra were recorded by exciting the samples with a steady-state Xe lamp. Decay curves were recorded by exciting the nanocrystals with a Nd:YAG laser, attached with an optical parametric oscillator (OPO) with an optical range of 210−2400 nm. These decay curves are well-fitted by a double exponential function

⎛ t ⎞ ⎛ t ⎞ y = y0 + A1 exp⎜ − 1 ⎟ + A 2 exp⎜ − 2 ⎟ ⎝ τ1 ⎠ ⎝ τ2 ⎠

Figure 2. Electrical resistivity of LaVO4:Eu3+ (10%) nanocrystals as a function of pressure.

normal sites. The sample showed a first-order discontinuous decrease in the electrical resistivity by a factor of 5 × 104 at 885 K and remains temperature invariant up to 1200 K. It has been interpreted as a thermally induced semiconductor-to-semimetal transition in LaVO4 at ambient pressure. In our studies, we observe a similar change at 5.0 GPa, where the electrical resistivity of nanocrystalline LaVO4:Eu3+ (10%) decreases discontinuously by an order of magnitude, indicating a firstorder phase transition. The electrical resistivity almost remained invariant during release of the pressure from 8 GPa to ambient pressures, indicating that the transformation is irreversible in nature. The samples recovered from the highpressure cell (subjected to 5.3 GPa pressure) were subjected to structural and morphological studies to identify the possible densification of the sample. Figure 3 shows XRD patterns of LaVO4:Eu3+ nanocrystals before and after the pressure treatment. XRD peak analysis of

(1)

where A1 and A2 are the relative components of the lifetime and τ is the lifetime of the luminescence. The average lifetime of the LaVO4:Eu3+ sample was calculated: τeff =

A1τ12 + A 2 τ22 A1τ1 + A 2 τ2

(2)

Transmission electron micrographs were obtained from an FEI Technai G2 U-Twin (200 KeV) instrument. X-ray fluorescence spectroscopy data were obtained from the ZSX Primus series. 2.4. Computational Methodology. We used density functional calculations for total energy studies within the plane-wave ultrasoft formalism as implemented in Quantum Espresso.27 A practical introduction toward the density functional calculation is provided in ref 28, and the basic theory and algorithm are presented in ref 29. Perdew−Burke−Ernzerhof functionals30 have been used for all the elements. Ionic cores of La, V, and O were allowed to contain 11, 13, and 6 “valence” electrons. Brillouin zone sampling has been performed with the Monkhorst−Pack scheme,31 and k-point grids of 6 × 6 × 7 and 5 × 5 × 6 have been used for the monazite and zircon phases, respectively. A kinetic energy cutoff of 50 Ry have been used for the kspace integration. A Gaussian broadening of 0.01 Ry was allowed for fractional occupancy of the bands. All structural optimizations have been done to reach the minimum energy until the forces on all the ions are ≤0.01 mRy/b.

3. RESULTS AND DISCUSSION 3.1. Experimental Studies. Figure 2 shows the pressure dependence of the electrical resistivity of the orthovanadate LaVO4:Eu3+ (10%) nanocrystal up to 8 GPa at room temperature. High-pressure electrical resistivity behavior of LaVO4:Eu3+ nanocrystals exhibits the positive slope of the electrical resistivity with the increase of pressure up to ∼5 GPa, which is typically observed in polycrystalline vanadate systems. Earlier studies on the electrical conductivity of LaVO4 at high temperatures by Gaur and Lal have shown that the LaVO4 is in a semiconducting state up to 855 K with a conductivity activation energy of 0.83 eV.32 It has been proposed that the conduction in this vanadate is due to hopping of holes from V4+ defect centers (present due to small oxygen deficiency) to V5+

Figure 3. XRD patterns of LaVO4:Eu3+ nanocrystals before pressure treatment and after high-pressure treatment (∼5.3 GPa). JCPDS data of zircon and monazite LaVO4 phases are also given.

LaVO4:Eu3+ nanocrystals before the pressure treatment matches with the tetragonal-zircon type. On the other hand, the XRD peaks of LaVO4:Eu3+ nanocrystals after the pressure treatment show the presence of the monazite phase along with a little amount of zircon-type phases. This clearly suggests that Eu3+-doped lanthanum orthovandate nanocrystals have been C

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transformed to monazite type from the tetragonal-zircon type phase (a ≠ c) on applying pressure up to ∼5.3 GPa. We note that the phase change is an irreversible transformation of LaVO4:Eu3+ to the monazite phase under pressure. A similar nature of transformation has been observed by Chen et al. in single-crystal YVO4:Eu3+ at pressures more than 7.5 GPa.16 We also note that there is no phase separation of EuVO4 after the pressure treatment, which is evident from the XRD. This is further supported by photoluminescence spectra (see below). Moreover, the XRF data show that the doping concentration of Eu3+ in zircon and monazite phases is 9.2% and 9.6%, respectively, which suggests that the doping concentration remains unchanged during pressure-driven phase transition. Figure 4 demonstrates the Raman spectra of LaVO4:Eu3+ nanocrystals before and after the pressure treatment. All the

Figure 5. (a) Emission spectra of LaVO4:Eu3+ nanocrystals before pressure treatment and after high-pressure treatment (∼5.3 GPa) under 395 nm excitation wavelength. (b) Excitation spectra of LaVO4:Eu3+ nanocrystals before pressure treatment and after highpressure treatment (∼5.3 GPa) by collecting 612 nm emission wavelength.

Figure 4. Raman spectra of LaVO4:Eu3+ nanocrystals before pressure treatment and after high-pressure treatment (∼5.3 GPa).

simple energy level structure and nondegenerate ground (7F0) and emitting (5D0) states, and the relative intensity of emission transitions between 5D0 and 7F1 (magnetic dipole transition) and 7F2 levels (electric dipole transition) can be used to understand the variation in the symmetry around Eu3+ species in the lattice. The major emission bands of Eu3+ ions at 591 and 612 nm are assigned to the 5D0 to 7F1 and 5D0 to 7F2 transitions. Figure 5 demonstrates that the relative intensity of the 591 nm emission band to 612 nm is lesser for LaVO4:Eu3+ nanocrystals before the pressure treatment compared to the sample after the pressure treatment. This suggests that the Eu3+ ions in the LaVO4:Eu3+ nanocrystals before the pressure treatment are in the site with low inversion symmetry, the electric dipole transitions are allowed, and the 5 D0 to 7F2 transition is a dominant emission line.33 In contrast, the Eu3+ ions in the LaVO4:Eu3+ nanocrystals after the pressure treatment may be in an inversion symmetric environment, where the magnetic dipole transitions are allowed and the 5D0 to 7F1 transition is a dominant emission line. Moreover, the excitation spectrum (Figure 5b) of the zircon type proves that the energy transfer efficiency is more compared to that of the monazite type. This is due the lesser spacing between the Eu3+ ions and VO4 tetrahedrons in the zircon type compared to the monazite type, which increases the overlap of their wave functions. Additionally, there is a marginal decrease in the lifetime of Eu3+-doped LaVO4 nanocrystals from ∼1 to ∼0.97 ms against the application of high pressure, and the difference

major Raman peaks of LaVO4:Eu3+ nanocrystals before the pressure treatment match well with the zircon phase and are assigned to the internal stretching and vibrations of VO4 tetrahedra. The Raman peaks at lower vibrations (below ∼475 cm−1) correspond to La−O vibrations. Raman spectra of LaVO4:Eu3+ nanocrystals after the pressure treatment show the appearance of new Raman peaks in the range of 290−368 cm−1, which matches with the Ag and Bg nodes of the monazite phase. Additionally, the new Raman peaks in the range of 854 cm−1 also matches with that of the monazite LaVO4 phase. From the Raman spectra, we can conclude that the phase transition of zircon to monazite LaVO4:Eu3+ is irreversible. Additionally, the observed changes in the Raman spectra are consistent with the reported rare-earth vanadate microcrystals. We have further characterized the above samples by photoluminescence spectra as it is well-known that luminescence properties of lanthanide ions depend on their host crystalline nature and morphology. The photoluminescence spectra of LaVO4:Eu3+ nanocrystals before and after the pressure treatment synthesized by a precipitation method are shown in Figure 5a. It is noted that the monazite structure (monoclinic structure, α ≠ β = γ = 90°, a = 7.04 Å, b = 7.286 Å, c = 6.725 Å) has space group P21/n in which the Eu3+ ion occupies C1 site symmetry. On the other hand, the zircon structure (tetragonal structure, α = β = γ = 90°, a ≠ c, a = 7.49 Å, c = 6.59 Å) has space group symmetry I41/amd, where the Eu3+ ion can occupy at D2d sites. Also, the Eu3+ ion possesses a D

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pressure on resistivity and band gap, we performed pressure dependence calculation of the band gap (Figure 7). As can be

can be thought of arising from the variation in the environment of the Eu3+ ion. This further supports that the zircon phase has been transformed to the monazite phase after the pressure treatment. We also note that the lifetimes are in milliseconds, in contrast to EuVO4, which possesses the lifetime of ∼0.21 ms. This supports that EuVO4 has not been formed as a separate phase after the pressure treatment. The size and shape of the LaVO4:Eu3+ nanocrystals have been characterized by transmission electron microscopy (TEM). Figure 6 represents TEM images of LaVO4:Eu3+

Figure 7. DFT calculation of the variation of the band gap with pressure for zircon and monazite phases.

seen from Figure 7, the band gap varies linearly with pressure, which is similar to the empirical observation of variation of resistivity with pressure, when the LaVO4 is in the zircon phase. We also calculated the enthalpy of both the zircon and the monazite phases as a function of pressure. At the transition pressure, the enthalpy of both phases will be equal, and at higher values of pressures, the low volume, that is, the monazite phase, will dominate. The plot of enthalpy (per unit cell) versus pressure is shown in Figure 8. The calculated value of ∼5.42

Figure 6. TEM images of LaVO4:Eu3+ nanocrystals (a) before pressure treatment and (b) after high-pressure treatment (∼5.3 GPa).

nanocrystals before and after pressure treatment synthesized by a precipitation method. TEM images clearly suggest that there is no change in the size and shape of LaVO4:Eu3+ nanocrystals. Both the TEM images demonstrate that the size of both the particles are in the range of 15−20 nm, and significant aggregation is observed after the pressure treatment. 3.2. Density Functional Theoretical Simulations. Simulations were performed within density functional theory (DFT) to obtain insight into two issues: (1) electrical resistivity measurement (Figure 2) and (2) pressure dependence of total enthalpy. The electrical resistivity increases monotonously with increasing pressure, and a sudden decrease in electrical resistivity is observed at around 5 GPa. Electrical resistivity is a function of concentration of free electronic carriers, which is, in turn, a function of band gap for a given dopant concentration, pressure, and temperature. The monotonous increase in resistivity increase indicates a monotonous increase in band gap, and the sudden decrease in band gap is suggestive of changes in band gap due to changes in the host-crystal structure. This physical picture was further quantitatively analyzed via DFT. Simulating LaVO4 with doped Eu is practically unfeasible at the present level of computational resources accessible to our group. However, introducing doping of Eu3+ has a negligible change in the lattice constant of LaVO4 due to the similarities in sizes of Eu3+ and La3+.6 Taking this into account, we explored the response of the host lattice LaVO4 to pressure. The equilibrium lattice constant computed in the present study agrees well with the equilibrium lattice constant computed at zero external pressure in a previous computational study performed for exploring the electronic, optical, and vibrational properties of LaVO4 polymorphs.7 As found in many other semiconductors, resistivity depends on the band gap. In most cases, as thermal excitation is the cause for the increase in the concentration of free-electronic carriers, the resistivity has the exponential dependence on the band gap, and hence, the logarithm of the resitivity will show a linear behavior versus the band gap. To correlate the effect of

Figure 8. DFT calculation of the enthalpy (eV per unit cell) and volume of zircon (nm3) and monazite phases versus pressure. The intersection of the two lines gives the transition pressure.

GPa for the transition pressure, computed for the host lattice, is comparable to the experimental value of ∼5.14 GPa measured for LaVO4:Eu3+ nanocrystals. Thus, the physicochemical factors behind both the experimental observations of a monotonous increase in band gap and the sudden decrease in resistivity with pressure is well-captured within the DFT simulations. In some instances, changes in coordination structure due to external pressure is also accompanied by changes in the ionicity of the bonds, as correlated by their Bader charges.34,35 However, in the present case, Bader charges35 in both the monazite and the zircon phases do not differ much, and the Bader charges of La, V, and O are ∼+2.3, +2.1, and −1.1, respectively. E

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(14) Bose, D. N.; Parthasarathy, G.; Mazumdar, D.; Gopal, E. S. R. Phys. Rev. Lett. 1984, 53, 1368. (15) Zhang, C. C.; Zhang, Z. M.; Dai, R. C.; Wang, Z. P.; Zhang, J. W.; Ding, Z. J. J. Phys. Chem. C 2010, 114, 18279. (16) Chen, G.; Stump, N. A.; Haire, R. G.; Peterson, J. R.; Abraham, M. M. J. Phys. Chem. Solids 1992, 53, 1253. (17) Zhang, F. X.; Lang, M.; Ewing, R. C.; Lian, J.; Wang, Z. W.; Hu, J.; Boatner, L. A. J. Solid State Chem. 2008, 181, 2633. (18) Chen, C. C.; Herhold, A. B.; Johnson, C. S.; Alivisatos, A. P. Science 1997, 276, 398. (19) Tolbert, S. H.; Alivisatos, A. P. Science 1994, 265, 373. (20) Grunwald, M.; Dellago, C. Nano Lett. 2009, 9, 2099. (21) Tolbert, S. H.; Alivisatos, A. P. J. Chem. Phys. 1995, 102, 4642. (22) Wickham, J. N.; Herhold, A. B.; Alivisatos, A. P. Phys. Rev. Lett. 2000, 84, 923. (23) Singh, S.; Tripathi, A.; Rastogi, C. K.; Sivakumar, S. RSC Adv. 2012, 2, 12231. (24) Parthasarathy, G. J. Appl. Geophys. 2006, 58, 321. (25) Parthasarathy, G. Mater. Lett. 2007, 61, 4329. (26) Chandra, U.; Sharma, P.; Parthasarathy, G.; Sreedhar, B. Am. Mineral. 2010, 95, 870. (27) Giannozzi, P.; Baroni, S.; Bonini, N.; Calandra, M.; Car, R.; Cavazzoni, C.; Ceresoli, D.; Chiarotti, G. L.; Cococcioni, M.; Dabo, I.; Dal Corso, A.; de Gironcoli, S.; Fabris, S.; Fratesi, G.; Gebauer, R.; Gerstmann, U.; Gougoussis, C.; Kokalj, A.; Lazzeri, M.; Martin-Samos, L.; Marzari, N.; Mauri, F.; Mazzarello, R.; Paolini, S.; Pasquarello, A.; Paulatto, L.; Sbraccia, C.; Scandolo, S.; Sclauzero, G.; Seitsonen, A. P.; Smogunov, A.; Umari, P.; Wentzcovitch, R. M. J. Phys.: Condens. Matter 2009, 21, 395502. (28) Sholl, D. S.; Steckel, J. A. Density Functional Theory: A Practical Introduction; John Wiley & Sons, Inc.: Hoboken, NJ, 2009. (29) Martin, R. M. Electronic Structure: Basic Theory and Practical Methods; Cambridge University Press: Cambridge, U.K., 2004. (30) Perdew, J. P.; Burke, K.; Ernzenhof, M. Phys. Rev. Lett. 1996, 77, 3865. (31) Monkhorst, H. J.; Pack, J. D. Phys. Rev. B 1976, 13, 5188. (32) Gaur, K.; Lal, H. B. J. Mater. Sci. 1984, 19, 3325. (33) Kirby, A. F.; Foster, D.; Richardson, F. S. Chem. Phys. Lett. 1983, 95, 507. (34) Oganov, A. R.; Solozhenko, V. L.; Gatti, C.; Kurakevych, O. O.; Le Godec, Y. J. Superhard Mater. 2011, 33, 363. (35) Bader, R. F. W. An Introduction to the Electronic Structure of Atoms and Molecules; Clarke, Irwin & Co. Ltd.: Toronto, Canada, 1970.

4. CONCLUSION In this study, we demonstrate that it is possible to dope monazite LaVO4 nanocrytals with 10% Eu3+ by first preparing the LaVO4:Eu3+ nanocrystal in the zircon crystal phase and then utilizing external pressure to transform it to the monazite phase. When the external pressure is removed, neither does LaVO4:Eu3+ transform to the zircon phase nor does the Eu dopant segregate out of the host matrix. A sudden drop in the electrical resistivity data around a high pressure of ∼5.3 GPa confirms the change in the phase transition from the zircon to the monazite phase. Additionally, XRD, Raman spectra, and photoluminescence of LaVO4:Eu3+ nanocrystals confirm the presence of the monazite phase after the pressure treatment. Furthermore, all the above characterizations clearly suggest that the phase transition is irreversible. The density functional simulations have also been employed to explore the enthalpy changes, trends in band gap, and ionicity of the host LaVO4 monazite and zircon lattices.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (S.S.), [email protected] (R.G.S.P.), [email protected] (G.P.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.S. greatly acknowledges the funding from the Department of Science and Technology (DST/CHE/200900220) and DST Thematic Unit of Excellence @ IIT Kanpur. M.P. and R.G.S.P. thank the Department of Science and Technology, Government of India, for supporting part of this work via the grants DST/CHE/20110263 and DST/ME/20110310. G.P. thanks the Director, Council of Scientific and Industrial Research− NGRI, SHORE and Planex, Space Application Center, Department of Space, Government of India, for financial support.

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REFERENCES

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