Lanthanide Orthoantimonate Light Emitters: Structural, Vibrational

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Lanthanide Orthoantimonate Light Emitters: Structural, Vibrational and Optical Properties Kisla PF Siqueira, Patricia Pereira Lima, Rute A.S. Ferreira, Luis D. Carlos, Eduardo Bittar, Eduardo Granado, Juan C Gonzalez, Arturo Abelenda, Roberto Luiz Moreira, and Anderson Dias Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm502504b • Publication Date (Web): 23 Oct 2014 Downloaded from http://pubs.acs.org on October 29, 2014

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Chemistry of Materials

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Lanthanide Orthoantimonate Light Emitters: Structural, Vibrational and Optical Properties

Kisla P. F. Siqueira,† Patrícia P. Lima,‡ Rute A. S. Ferreira,‡ Luís D. Carlos,‡ Eduardo M. Bittar,# Eduardo Granado,§ Juan Carlos González,⊥ Arturo Abelenda,⊥ Roberto L. Moreira⊥ and Anderson Dias*,†

†

Departamento de Química, Universidade Federal de Ouro Preto, Campus Morro do

Cruzeiro, ICEB II, Ouro Preto-MG, 35400-000, Brazil ‡

Department of Physics and CICECO, Universidade de Aveiro, 3810-193 Aveiro, Portugal

#

Laboratório Nacional de Luz Síncrotron, C.P. 6192, Campinas-SP, 13083-970, Brazil

§

Instituto de Física “Gleb Wataghin”, UNICAMP, Campinas-SP, 13083-970, Brazil

⊥

Departamento de Física, ICEx, Universidade Federal de Minas Gerais, C.P. 702, Belo

Horizonte-MG, 30123-970, Brazil

ACS Paragon Plus Environment


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ABSTRACT: Lanthanide orthoantimonates represented by the chemical formula LnSbO4 (Ln are all the lanthanides elements with exception of cerium and promethium) were synthesized via solid-state reactions. The crystalline structures of LnSbO4 were determined by high-resolution Synchrotron X-ray diffraction and Rietveld method. The samples exhibited monoclinic structures with two different arrangements as a function of the ionic radius of the lanthanide metal. For compounds with largest ionic radii (Ln = La and Pr), the P21/n space group was determined, while compounds with intermediate and smallest ionic radii (Ln = Nd-Lu) were described under the P21/c setting. Raman spectroscopy was employed to study the vibrational features of all compounds, allowing us to determine the characteristic phonons for each structure and, consequently, to establish the relationship between chemical environment and vibrational properties. Optical features of typical SmSbO4 and TbSbO4 were thoroughly investigated, and the results indicate that the orthoantimonates are promising luminescent inorganic materials. The photoluminescence spectra (emission and excitation) of both compounds were obtained as a function of temperature under ultraviolet radiation, showing strong orange (Sm) and green (Tb) emissions, respectively. Chromaticity diagrams (CIE) were also determined for all LnSbO4 series, aiming to bring forward the color coordinates for these emitters tunned by the chemical environment and temperature.

KEYWORDS: orthocompounds; lanthanides; antimonate; crystal structure; Raman scattering; synchrotron X-ray diffraction; photoluminescence; emitters.


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INTRODUCTION Lanthanide-containing materials comprise a wide range of scientifically and technologically important compounds. These materials are chemically designed and further produced by using different routes depending on the final target: single crystals, glasses, organic-inorganic hybrids, and ceramics. A huge variety of properties can be obtained depending on the choice of the lanthanide, the host matrices and the crystalline structures in which they are inserted, either as a dopant or as self-activated element. Known for possessing rich luminescent properties, lanthanide-containing materials have been used in many technological applications, such as laser materials, flat panel displays, cathode ray tubes, up-conversion devices, white light emitting diodes (W-LEDs), X-ray scintillators, phosphors, and emitters.1-3 Reports on the production of lanthanide-containing materials with tailor-made properties have been published over the past 40 years and, in particular, in the last decade aiming to new technological applications, such as the use of lanthanidedoped crystals and glasses in quantum optical memories4,5 or markers for bio-imaging with low background.6 Also, lanthanide-containing materials are currently found in organicinorganic hybrids because of their potential applications as optically active components in photonic and bio-photonic devices.7,8 For up-conversion applications, whose concept involves infrared-to-visible light conversion and was firstly reported more than 50 years ago,9 lanthanide-containing materials represent the best choice for many appealing features in the display industry. A major benefit for using this technology is the ability to operate displays at very high brightness without deterioration of the emitting materials.10,11 Among these strategic lanthanide-based compounds, the broadest set is constituted by materials in ceramic form,



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being the most promising materials for use in spectrally selective emitters at infrared frequencies.12-14 Lanthanide orthophosphates (LnPO4) have been extensively applied in phosphors, laser hosts, biolabeling, and photo up-conversion materials because of their interesting polymorphism.15,16 Also, lanthanide-based apatites have been extensively studied due to their great interest as fluorescent lamp phosphors and as potential laser host materials.17 In view of the large chemical possibilities offered by the ceramic materials, the idea of engineering spectral emittance by properly combining chemical environment and structural properties of the synthesized materials has progressed, mainly on the basis of empirical approaches, aiming to produce novel lanthanide-containing materials by combining selective emission with structural properties for a variety of different applications ranging from aerospace to energy conversion.18 Lanthanide-containing ceramics have been studied by our research group in the last fifteen years. Within the scope of this research, various systems were synthesized by different chemical routes and investigated regarding their structural, vibrational and microwave response.19-27 In particular, single and complex perovskites belonging to different crystalline structures and exhibiting different microwave features were studied in view of their potential applications as dielectric resonators in telecommunication devices.1927

More recently, lanthanide-based orthocompounds were synthesized by our group aiming

to develop an appropriate experimental methodology to produce novel luminescent materials suitable for superior optical applications.28-31 This class of materials attracts special attention of the scientific community and many papers on lanthanide-based orthophosphates,15,16 orthovanadates,32,33 orthoniobates,28 and orthotantalates29,34,35 have been published very recently. However, for orthocompounds containing antimony as


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lanthanide host, very few results were reported up to now. Gerlach et al.36 studied LnSbO4 (Ln = La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Lu) single-crystals obtained by endothermic chemical transport with TeCl4 as the transport agent but there are still many divergences about the structure exhibited by these compounds. At the best of our knowledge, there are no reports about the synthesis of the whole LnSbO4 series by solid-state reactions. In view of that, this work expands the previous knowledge of lanthanide-containing materials and reports on the synthesis of the complete series of LnSbO4 besides a detailed investigation of their crystalline structures and vibrational properties. Moreover, the optical properties were examined, allowing us to present a set of promising luminescent materials suitable for emitter applications. Recently, lanthanide-based materials emitting multiple visible colors have become an attractive research focus because of their important role in the field of light display systems, lasers and optoelectronic devices.37-40 In the present work, lanthanide-containing ceramics emitting various visible colors were achieved by tunning the chemical environment and temperature into the antimony matrices.

EXPERIMENTAL SECTION Synthesis. LnSbO4 materials were synthesized through solid-state reactions by using Ln2O3 (Ln = La, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, and Lu; >99.9% SigmaAldrich), Pr6O11, Tb4O7 (>99.9% Sigma-Aldrich) and Sb2O5 (>99.9% Sigma-Aldrich) as starting materials. Stoichiometric amounts were weighed and mixed with a mortar and pestle. The mixed powders were calcined with intermediate regrinding in the optimized conditions of temperature and time to yield single-phase compounds.



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XRD and SXRD. All samples were firstly characterized by conventional X-ray diffraction (XRD) using a Shimadzu D-6000 diffractometer with graphite monochromator and a nickel filter, in the range of 10-60°2θ (15 s/step of 0.02°2θ), operating with FeKα radiation (λ = 0.1936 nm), 40 kV and 20 mA (the results were automatically converted to CuKα radiation for data treatment and manipulation). High-resolution Synchrotron X-ray diffraction (SXRD) measurements were taken at room temperature in the superconducting wiggler XDS beamline of the Brazilian Synchrotron Light Laboratory, LNLS, with λ =0.65319 Å. The optical path of the beam upstream the sample position included a watercooled Pt-coated vertically collimating mirror, a LN2-cooled double Si(111) crystal monochromator with sagittal focusing and a Pt-coated vertically focusing mirror, yielding a highly monochromatic beam with dimensions of 2.0(H) x 0.17(V) mm2 full width at half maximum (FWHM) at the sample position. The samples were mounted in flat-plane θ/2θ geometry with a spinning axis perpendicular to the sample surface to optimize grain statistics. The diffraction profiles were taken in the vertical scattering plane using a 0.3 mm slit located 0.80 m away from the sample followed by a highly-oriented pyrolitic graphite analyzer and a high-throughput LaBr3 scintillator detector in the 2θ arm. This mounting resulted in high-resolution and low-background powder diffraction profiles, with typical Bragg peak widths of ~0.02° FWHM at low angles, and maximum peak heights of ~105 counts against a background level of ~102 counts. The measurements were performed in the range 4-90°2θ, covering ~2000 Bragg reflections, with a step size of 0.008°2θ. Rietveld refinements were performed using the GSAS+EXPGUI software package.41 Raman Spectroscopy. Raman scattering of the as-synthesized samples were collected in backscattering configuration by using two different instruments. An Horiba


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LABRAM-HR spectrometer was used with the 632.8 nm line of a helium-neon laser as excitation source (effective power of 6 mW at the sample surface). This spectrometer is equipped with diffraction gratings of 600 and 1800 grooves/mm, a Peltier-cooled charge coupled device (CCD) detector, and a confocal Olympus microscope (100× objective, N.A. 0.90), which guarantee an experimental resolution of typically 1 cm-1 for 10 accumulations of 30 s. Appropriate interference filter for rejecting laser plasma lines, and edge filters for stray light rejection were used. In addition, a Jobin-Yvon T64000 triple monochromator (50× objective, N.A. 0.75) equipped with the 760 nm line of a Ti-Sapphire laser and a LN2cooled CCD detector was also used. The spectral resolution was better than 2 cm-1 and the accumulation times were typically 5 collections of 10 s. All resulting spectra were corrected by Bose-Einstein thermal factor.42 Photoluminescence Spectroscopy. The low-temperature photoluminescence measurements were performed between 11 and 275 K using a He closed-cycle cryostat (Janis CT) managed by a Lakeshore 330 auto-tuning temperature controller. Photoluminescence spectra were recorded with a modular double-grating excitation spectrofluorimeter with a TRIAX 320 emission monochromator (Fluorolog-3, Horiba Scientific) coupled to a R928 Hamamatsu photomultiplier, using the front face acquisition mode. The excitation source was a 450 W Xe arc lamp. The emission spectra were corrected for detection and optical spectral response of the spectrofluorimeter and the excitation spectra were corrected for the spectral distribution of the lamp intensity using a photodiode reference detector. Room-temperature photoluminescence measurements were recorded with a Acton SP2500i spectrometer (Princeton Instruments) coupled to a Spec-10 LN2-cooled UV-



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enhanced Si CCD detector (Princeton Instruments). The samples were excited with the 325 nm line of a HeCd laser (Kimmon). The optical spectral response of the spectrometer was corrected by using a HL-2000-CAL (Ocean Optics) calibrated tungsten halogen light source.

RESULTS AND DISCUSSION As a general trend, single-phase crystalline ceramics were obtained after processing. XRD results for the produced lanthanide series can be seen in Figure S1 (Supporting Information). The first step towards a detailed structural investigation of the synthesized compounds involved the high-resolution Synchrotron X-ray measurements. Representative SXRD profiles of LnSbO4 for Ln = La and Eu are given in Figures 1a and 1b, respectively. The profiles for these samples are very different, indicating distinct crystal arrangements. For both materials a good agreement between observed and calculated diffraction data was obtained using monoclinic structures. The La-based compound is isostructural to β-PrSbO4 5 (space group #14, P21/n setting, C2h with Z = 4), while the Eu one is similar to α-PrSbO4 5 (space group #14, P21/c setting, C2h with Z = 4).36 It is worth mentioning that in a previous

work we had indexed the LaSbO4 structure as P21/m.31 Instead, the present results correct that attribution, the correct structure being P21/n. Figure S2 (Supporting Information) displays a detailed view in a restricted angular interval (12.7-21.0° 2θ) for LaSbO4 (Fig. S2a) and EuSbO4 (Fig. S2b). We can observe in the Figure S2b an unindexed peak at 2θ ≈ 14°, which corresponds to the highest intensity peak of a spurious Eu3SbO7 phase, according to Siqueira et al.25 This spurious phase has a mass fraction of ~3%, which does not alter the discussed structural results hereafter.


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For orthoniobate and orthotantalate compounds previously reported by the literature, the presence of polymorphs is very common.28,29 In addition to these polymorphic phases, it was reported for LaSbO4 the existence of a specific temperature during solid-state processing in which the preferential crystallization of La3SbO7 occurs due to the loss of Sb2O5 by volatilization.31 Thus, it is necessary to emphasize the processing conditions employed in the present work, because they are intimately related with the resulting crystalline structures. For the orthoantimonates produced here, it was observed only two different XRD patterns, as previously proposed by Gerlach et al.36 For larger lanthanides, Ln = La and Pr, the samples crystallized in P21/n structure, while for both intermediate and small ionic radii (Nd-Lu) the P21/c structure is adopted. Table S1 (Supporting Information) shows the refined structural parameters for LaSbO4 and EuSbO4, which are representative of the two types of crystal structures exhibited by our LnSbO4 synthesized compounds.



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10

(a)

LaSbO4

Counts

50k

0k

(b)

EuSbO4 x10

50k

Counts

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0k

10

20

30

40

50

2θ (degrees)

Figure 1. SXRD patterns of LnSbO4 for (a) Ln = La and (b) Ln = Eu, with λ = 0.65319 Å. The cross symbols and solid lines represent observed and calculated patterns, respectively. The difference curves are shown at the bottom of each figure. Vertical bars indicate the expected Bragg peak positions according to the crystal structure models described in the text and refined lattice parameters given in Table S1 (Supporting Information).

The structural characteristics of the P21/n and P21/c phases could be more easily understood by analyzing the differences between the coordination numbers of the Ln atoms and their corresponding lattice parameters. The lanthanide coordination in the P21/n


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arrangement is nine, according to Golbs et al.,43 while for P21/c structure it is eight. The bond length between La―O (P21/n) has an average of 2.447 Å with distances among 2.397-2.541 Å. In this sample the average of Sb―O bond is 2.009 Å. On the other hand, for Eu―O (P21/c) we observed an average of 2.444 Å, with distances among 2.202-2.791 Å, and 1.997 Å to Sb―O bond. Another important difference between these structures is the spatial isolation of the [SbO4]3- units in tetrahedral for P21/n (La and Pr) against octahedral for P21/c structure (Nd-Lu).43 Regarding the lattice parameters, we can observe that the “b” parameter undergoes little variation during the structural change from P21/n to P21/c. On the other hand, “a” and “c” parameters are very distinct in P21/n arrangements,

while they are closer in the P21/c structure. The lattice parameters of all synthesized compounds were calculated from XRD data. Once the samples crystallized in a monoclinic structure their lattice parameters and diffraction planes are related by:

1 1 = 2 d sin 2 β

 h 2 k 2 sin 2 β l 2 2hl sin β  2 + + 2− b2 c ac a

  

,

(1)

where d is the distance between atomic layers in a crystal calculated with Bragg’s law ( 2d sin θ = nλ ), h, k and l are the Miller indices, and a, b, c, and β are the lattice parameters. Figure 2a contains the values of a, b and c as functions of lanthanide ionic radii,44 besides the resulting unit cell volumes along the La-Lu series (Fig. 2b). We observed a decrease of the cell volume as expected due the lanthanide contraction phenomena, which will be discussed below. During the phase change P21/n → P21/c the reduction in volume was about 11.5% from Pr to Nd, while along the Nd-Lu series (all belonging to P21/c space group) the reduction was just about 9.3%.



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14

(a) a b c

a, b, c (Α)

12 10

P21/n

8 6

P21/c

4 374 3

Volume (Α )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

P21/n

(b)

Pr

352

P21/c

330

La

Nd

308 Lu

286 0.95

1.00

1.05

1.10

1.15

1.20

1.25

Ln ionic radius (Α)

Figure 2. (a) Lattice parameters a, b and c, and (b) unit cell volume of the LnSbO4 compounds as functions of Ln ionic radii.44

An extensive investigation of the room-temperature Raman phonons and vibrational properties for all LnSbO4 compounds was carried out for the first time and the results are discussed below. Group-theory calculations were made based on occupied Wyckoff positions and space groups for each set of compounds. For the LnSbO4 system, lanthanide, antimony and four oxygen atoms should occupy the 4e sites of symmetry C1, either for


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P21/n or P21/c structures. Due to these occupation sites, the Raman-active modes of this system can be decomposed according to the irreducible representation (i.r.) of the C2h point-group: 3Ag + 3Au + 3Bg + 3Bu

.

(2)

Then, using the site-group method of Rousseau et al.45 we can obtain the following distribution of the degrees of freedom at the Brillouin-zone center in terms of the i.r. of the C25h point group: ΓTOTAL = 18Ag + 18Bg + 18Au + 18Bu

.

(3)

Thus, we would expect 36 gerade Raman modes (18Ag + 18Bg) for the LnSbO4 system, which can be considered to be composed of two sublattices of Ln3+ and [SbO4]3units. Raman spectra were obtained at room temperature and the experimental results are displayed in Figure 3 (a-d) for decreasing ionic radius (La-Lu). Because of strong electronic transitions that appear under certain conditions, we accordingly used different laser lines. The final spectra showed in Figure 3 represent the best results obtained using infrared (760.0 nm) line for Ho and Er, and red (632.8 nm) line for La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Tm, Yb, and Lu. Some artifacts due to electronic transitions have also been found in Gd, Ho, Er and Yb spectra, which is quite usual in this kind of materials.26,29



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(b)

(a)

Gd Lu

Yb

Intensity (arb. units)


Eu

Sm

Nd

Tm

Er

Ho

Pr Dy

La

100

200

300

Tb

100

400

200

300

400

-1

-1

Wavenumber (cm )

Wavenumber (cm )

(c)

(d)

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Gd

Eu

Sm Nd

Yb



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Tm

Er

Ho

Dy

Pr

Tb

La

400

500

600

700

800

400

-1

Wavenumber (cm )

500

600

700

800

-1

Wavenumber (cm )

Figure 3. Raman spectra for all LnSbO4 materials obtained by solid-state reaction. The sequence plotted reproduces the variation (decreasing tendency) in the ionic radius44 and


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wavenumber (a) La-Gd between 50-400 cm-1; (b) Tb-Lu between 50-400 cm-1; (c) La-Gd between 400-800 cm-1; and (d) Tb-Lu between 400-800 cm-1.

Although the materials with P21/n and P21/c structures exhibited an equal number of Raman-active modes, they present different Raman patterns, as visualized in Figure 3. The results from Raman scattering indicated two different fingerprints in agreement with the crystal structures observed by SXRD. La and Pr samples show similar spectra (Group 1), while Nd-Lu exhibited comparable vibrational modes (Group 2). Because of the high number of active modes, a careful analysis was carried out by fitting the Raman experimental data with Lorentzian curves for all ceramics belonging to both structures (Groups 1 and 2), as it is exemplified in Figure S3 (Supporting Information) for typical samples, Pr (P21/n) and Dy (P21/c). Table S2 presents the depicted Raman modes obtained after experimental fitting procedures for the LnSbO4 samples. In a previous work,31 we could assign the Raman-active modes to Ag and Bg symmetries for a LaSbO4 ceramic, through polarized Raman measurements. Polarized Raman scattering is not currently used for studies of ceramic materials because of the intrinsic difficulties in discerning and assigning the active modes of unoriented samples, in contrast to oriented single-crystals; however, all predictions by group-theory remains valid when we are working with ceramic and not single crystals. In the present work, our LnSbO4 materials presented no suitable conditions to obtain reliable polarized Raman spectra (such as large grains or texture). Nevertheless, the assignment to PrSbO4 must be the same as for LaSbO4, reported previously,31 because both compounds have similar structural features.



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Let us now analyze the correlation between the observed Raman frequencies and the chemical environment of the lanthanide ions hosted by SbO4 sublattice, in order to determine the influence of Ln substitution along the lanthanide series. We can observe three different behaviors of the Raman active modes as we change the Ln in the LnSbO4 composition. For the majority of the vibrational modes, the wavenumbers decreased for increasing ionic radii, as expected. This behavior is because of the phenomenon of lanthanide contraction (crystal radii of the lanthanide ions decrease for increasing atomic number). This phenomenon lead to a closer packing of the [SbO4]3- units from La to Lu, which reduces the Sb―O distances to yield higher frequencies for smaller lanthanide ions. For some bands below 150 cm-1, an inverse phenomenon was observed (particularly the bands numbered #7 and #8 in Table S2). This behavior is likely due to a mass effect, in which the wavenumbers increase with increasing ionic radii.28 In this respect, larger ionic radius correspond to smaller mass, which results in the increasing of the frequency of the vibrations, shifting the bands for higher wavenumbers. These low-wavenumber bands are frequently interpreted as external modes, considering that LnSbO4 could be visualized as two sublattices: Ln3+ ions plus [SbO4]3- units. Thus, these two modes (#7 and #8) could be related primarily to the Ln3+ ions, if one considers that the harmonic approximation is valid. Other bands remain practically unchanged for varying lanthanide ions (the bands numbered #6 and #9 in Table S2, Supporting Information), which can be understood by considering a balance between the lanthanide contraction (tend to increase the wavenumbers for decreasing ionic radii) and the mass effect (dominated by the mass of [SbO4]3- unit, rather than by the lanthanide mass). This result was previously verified for LnVO4 compounds for B1g translational-type modes related to out-of-phase and in-phase movements of the Ln3+


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and [VO4]3- units.46 In addition, for La and Pr, it can be observed a non-linear increasing tendency exactly at the point where the phase transition occurs from P21/n and P21/c (between the ions Pr and Nd). Figure S4 (Supporting Information) shows plots of the wavenumbers listed in Table S2 versus ionic radii of lanthanide (according to Shannon44) for all LnSbO4 materials studied here. The optical properties of self-activated LnSbO4 materials were investigated aiming to look for new luminescent emitters. This study was based on the good performance already reported for lanthanide niobates and tantalates.47 It is known that luminescence can be obtained without the use of an activator and in this case we can use the term “selfactivated” for denominating such materials.48 For LnSbO4 compounds, luminescence is an inherent feature and this phenomenon occurs due to their chemical nature, i.e., no dopants are added and consequently extrinsic emission centers were not created. As representative examples of the photoluminescence properties of the LnSbO4 self-activated materials, we present and discuss below our results for SmSbO4 and TbSbO4 samples. In these systems, three types of energy transitions prevail in the visible range.49 The first one arises from the presence of the lanthanide cation and its unique intraconfigurational f-f transition (Sm3+ or Tb3+), which occurs as a sharp and intense emission line.49 Second, the 4fn → 4fn-15d interconfigurational transitions49 and, finally, the third type is constituted by the charge transfer transitions (also called Charge Transfer Band – CTB).49 For our samples, CTB originates from the charge transfer transition Sb5+―O2-, similarly to what is observed in niobates and tantalates.47,50 For the antimonate groups, we can consider that the conduction band is composed by Sb5+―4d orbital, while the valence band is formed by O2-―2p orbital, as verified for niobates through electronic structure calculations.51



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Figure 4 presents the emission spectra of SmSbO4 for different temperatures, under 275 nm excitation. The partial energy level diagram (right panel) shows the possible absorption and emission transitions from Sm3+ ions. The emission spectra displays a broad band centered around 450 nm, which is assigned to the CTB, and 4G5/2 → 6HJ (J = 5/2, 7/2, 9/2 and 11/2) emission lines from Sm3+ ion, which are located at 570 nm, 610 nm, 650 nm and 710 nm, respectively. Among them, the orange transition (4G5/2 → 6H7/2) exhibits the strongest emission. The emission bands for 4G5/2 → 6H5/2 and 4G5/2 → 6H11/2 are too weak and hard to be checked. We can observe that the relative intensities of the emission broad band (CTB) from SmSbO4 are more temperature dependent than the lines from Sm3+ ion. During temperature decreasing from 250 to 11 K, a strong intensification is observed for the broad band around 450 nm (maximum occurs at around 25 K). On the other hand, the intensity of the emission lines from the Sm3+ ion is slightly intensified within the same temperature range. The inset in Figure 4 shows the excitation spectra at 11 K and 200 K, monitored at the maximum intensity of the broad band, at 450 nm. The excitation spectra present a broad band (240-300 nm) with two main components around 245 and 275 nm. The broad band between 240 and 300 nm in the excitation spectra is attributed to the charge transfer transition of Sm3+, which occurs by electron delocalization from the filled 2p shell of the O2- to partially filled 4f shell of Sm3+, and also partly attributed to the low energy part of charge transition of SbO43-. It is well know that the interconfigurational (4fn-15d → 4fn) radiative transitions of trivalent lanthanide ions are always present in many host crystals.52-54 Undoubtedly, in LnSbO4 samples, the 4f8―4f75d1 transitions of Ln3+ can also play a great role on the luminescence behavior of the materials.


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4

G7/2

λexc 275 nm 11 K 25 K 50 K 75 K 100 K 125 K 150 K 175 K 200 K 250 K

4

λ em 450 nm

11 K 200 K

470

520

570

620

670

6

F5/2

H13/2

6

H11/2

6

H9/2

720

Wavelength (nm)

6

6

6

G5/2→ H11/2

8

4

4

420

6

H5/2

-1 3

10

6

375 F11/2

350

6

6

325 G5/2 → H9/2

300 G5/2 → H7/2

275 G5/2 → H5/2

250

energy (10 ) cm

12

6

370

18

14

4

320

20

G5/2

16

4

Intensity (a. u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

4 6

H7/2

2 0

Figure 4. Excitation (inset) and emission spectra of SmSbO4 with the partial energy diagram for Sm3+ ions.

The emission spectra of the TbSbO4 (Figure 5) exhibits the typical sharp emission lines corresponding to the 5D4 → 7FJ (J = 6-0) transitions from Tb3+ ion under UV excitation at 280 nm. The dominant one is the 5D4 → 7F5 green transition at about 545 nm. The blue emission at wavelengths below 489 nm originating from 5D3 → 7FJ transitions of Tb3+ ions were not observed. This can be explained by the well-known cross-relaxation between the 5D3 and 5D4 of Tb3+ at higher Tb3+ concentrations.30 The emission spectra of the TbSbO4 also display a broad band attributed to the CTB. The temperature dependence of the emission intensity between 11 and 275 K can also be observed in Figure 5.



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Increasing the temperature from 11 to 275 K, a significant decreasing in the emission intensity of the CTB and Tb3+ emission lines occurs. The temperature dependence of the emission intensity for the TbSbO4 and SmSbO4 will be detailed below. Figure 5 (right panel) also presents a partial energy diagram for Tb3+ ion with its specific transitions. Excitation in the green and red regions is achieved by UV excitation of the 5D3 level at 280 nm. After that, non radiative decays to the 5D4 level (represented by the dashed arrow in the energy diagram), transitions to the 7FJ (J = 6, 5, 4, 3, 2) manifolds results in the emission lines centered around 490, 545, 583, 613 and 650 nm, as identified in the Figure 5. The inset in Figure 5 presents the excitation spectra at 11 K monitored at 450 and 542 nm. The excitation spectrum monitored at 450 nm presents a broad band between 240 and 300 nm, with two main components centered on 245 and 275 nm. While the excitation spectrum monitored at the 5D4 → 7F5 transition (542 nm) reveals a broad band (240-370 nm) with at least three main components at ca. 250, 275 and 315 nm. The excitation spectra consist of some evident broad bands, clustering in the range 240-260 nm and 260-295 nm. These bands could be assigned to the f―d transitions from Tb3+ ion, as well verified in SmSbO4 samples, discussed above. Generally, when one electron from Tb3+ is promoted from the ground state 4f8 to the 4f7―5d1 excited states, it can produce two kinds of 4f8―4f75d1 transitions: spin-allowed and spin-forbidden transitions.55 Thus, the bands around 250 and 275 nm are probably assigned to the spin-allowed and spin-forbidden 4f8―4f75d1 transitions from Tb3+. Similar assignment was found in other compounds activated with Tb3+ ions.56,57 Moreover, it was considered that these two excitation bands could also include the SbO43- group charge transfer state, i.e., the f―d transitions of Tb3+ overlapped with the CTB of SbO43- group resulting in the excitation bands.56 It is important


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to note that the differences in the intensities could result from the corresponding emission wavelengths. In our previous work concerning LaSbO4 samples, we found that the maximum CTB emission was registered at 428 nm.31 Here, for SmSbO4 and TbSbO4, the maximum intensities of the CTB are very similar, ca. 448 nm and 453 nm, respectively. Thus, the emission spectra in our lanthanide orthoantimonates showed ionic radius dependence for the CTB position, red shifting for decreasing lanthanide ionic radii.

5

250 275 300 325 350 375 7

D4→ F6

5

450

500

D3

550

25 5

5 7

D4→ F3

600

7

D4→ F2-0

650

Wavelength (nm)

700

20 -1

D4

3

15 10

7

D4→ F4 5

400

5

11 K 25 K 50 K 75 K 100 K 125 K 150 K 200 K 250 K 275 K

λ em 450 nm

5

30

λexc 280 nm

7

D4→ F5

energy (10 cm )

11K λ em 542 nm


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

7

FJ 5

7

0

F6

Figure 5. Excitation (inset) and emission spectra of TbSbO4 with the partial energy diagram for Tb3+ ions.

The temperature dependence on PL emission is now considered in detail. Figure 6 depicts the emission spectra for TbSbO4 samples in three different regions. The emission



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spectra were obtained under wavelength excitation of 320 nm and three transitions were analyzed: 5D4 → 7F6 (Fig. 6a), 5D4 → 7F5 (Fig. 6b) and 5D4 → 7F3 (Fig. 6c). These three typical emission peaks are split in different ways, which are a consequence of crystalline field effects whose extents are related to their structural characteristics. We can observe in Figure 6 similar emission spectra except for their relative intensity. The inset in this Figure presents the exponential decay of the intensity (integrated area for each transition) for increasing temperatures. Obviously, the low temperature favors the intensification of the emission lines from Tb3+ ions, as we can observe in Figure 6.

D4

7

F6

6

5

Integrated Intensity (x10 )

a)

Intensity (a. u.)

11K 25K 50K 75K 100K 125K 150K 200K 250K 275K

480

485

490

495

7 6 5 4 3 2 1 0

0

500

50 100 150 200 250 300 Temperature (K)

505

510

515

5

D4

7

F5

11K 25K 50K 75K 100K 125K 150K 200K 250K 275K

535

540

545

6

b)


Wavelength (nm)

Intensity (a. u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

12 9 6 3 0 0

50 100 150 200 250 300 Temperature (K)

550

555

560

565

Wavelength (nm)


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D4

7

F3

6

5


c) 11K 25K 50K 75K 100K 125K 150K 200K 250K 275K

Intensity (a. u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

605

610

615

620

3 2 1

0

50 100 150 200 250 300 Temperature (K)

625

630

635

640

Wavelength (nm)

Figure 6. Emission spectra of the (a) 5D4 → 7F6, (b) 5D4 → 7F5 and (c) 5D4 → 7F3 transitions for TbSbO4. The insets show the exponential decrease of emission intensity as a function of temperature.

Figure 7 compares the emission spectra of the TbSbO4 excited at two different excitation wavelengths, 280 and 320 nm. Changing the excitation wavelength from 280 to 320 nm, a decrease in the emission intensity of the broad band (CTB) related to the Tb3+ emission lines is observed. In this case, the range from 375 to 530 nm was analyzed, which corresponds to charge transfer bands. In order to emphasize the strengthening of CTB under different excitation wavelengths, the spectrum under 320 nm was detached and investigated. It is important to note that the blue and black curves in Figure 7 were obtained at 11 K, and the relative emission intensities are showed as a function of the excitation wavelength only (the temperature dependence was already seen in previous Figures). The inset depicts a shift of the CTB during cooling (excitation line 280 nm), changing from 480.5 nm to 453.2 nm (blue shift) when the temperature decreased from 275 K to 11 K.



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300 Shift of CTB

250 200 150

4

100 5

50

3

455 460 465 470 475 480

λ exc = 280 nm λ exc = 320 nm

Temperature (K)

5

Intensity (x 10 )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

11K

0

Wavelength ( nm)

2

1 0 375

400

425

450

475

500

525

550

Wavelength (nm)

Figure 7. Emission spectra for TbSbO4 excited at 280 nm (black line) and 320 nm (blue line). The inset shows the shift of CTB as a function of temperature for the excitation line 280 nm.

It is well known that the thermal quenching behavior is an important feature on evaluating the characteristics of luminescent compounds.58 Thermal quenching is a reduction in luminescence intensity with increasing temperatures, due to opening up of competing non-radiative relaxation pathways.59 There are at least two explanations for this phenomenon. First, a decrease in the luminescence quantum efficiency with increasing temperatures (Mott-Seitz model) and, second, a reduction in the concentration of recombination centers during heating (Schön-Klasens model).59 In this work, the thermal


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quenching behavior of CTB for SmSbO4 (λexc=275 nm) and TbSbO4 (λexc=280 nm) were investigated in the temperature range 11-275 K and the results are showed in Figure S5 (Supporting Information). In this Figure, the emission intensities (relative integrated areas) were plotted against temperature for SmSbO4 (circles) and TbSbO4 (squares). As a general trend, the CTB of the investigated compounds exhibited a strong thermal quenching. For SmSbO4, we can observe a large increase (100 times) in the emission intensity from its initial value at 275 K (105) to its final value at 25 K (7x107). For TbSbO4, strong temperature quenching occurred below 150 K and the emission intensity increased of about 20 times (from 105 at 275 K to around 2x107 at 100 K). Color coordinates is another import merit factor for evaluating the performance of our emitters. The color coordinates for the series of lanthanide orthoantimonates were calculated using the intensity-calibrated emission spectra data, and are presented in 1931 CIE color space chromaticity diagrams.60 Figure 8 exhibits the 1931 CIE xy chromaticity space for SmSbO4 (Fig. 8a) and TbSbO4 (Fig. 8b). As we have seen previously, the luminescent emission is closely related to temperature. Thus, the coordinate emission color also depends on the temperature. Figure 8a shows the color coordinates of the SmSbO4 under excitation of 275 nm (see emission spectra in Figure 4). We can observe that for SmSbO4 compound the variation of the color happens in the one direction when the temperature change-over. For the SmSbO4 the relative intensity of CTB decreased with increasing of temperature (see Figure 4). This behavior is well illustrated by the calculus of the (x,y) CIE color coordinates, as well as displayed in Figure 8a. As the temperature increases from 11 to 250 K, the emission color coordinates change within the blue-light salmon pink region from (0.172, 0.169) to (0.376, 0.309) for the SmSbO4.



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Unlike what happens for SmSbO4, in the Figure 8b we can observe that for TbSbO4 compound the variation of the color occurs in two opposite directions. In this case, we will show the results of color coordinate using two different excitation wavelengths, i.e. 280 and 320 nm, the same displayed previously in the emission spectra to TbSbO4 (Figures 5-7). For the TbSbO4 the temperature dependence of the emission spectra is associated with the variations observed in the relative intensity of the CTB (see Figure 7) and Tb3+ emission lines under two different excitation (280 and 320 nm, see Figures 5 and 6). Nevertheless, the accurate differences among CTB and Tb3+ emission lines with the excitation energy are mainly on the intensities and positions of their transitions. These features justify the dependence of temperature, color coordinates and excitation wavelength, as well as help to understanding the changes in the color coordinates observed in the Figure 8b. Under excitation at 280 nm, as the temperature increases from 11 to 275 K the emission color coordinates change within the blue light-yellowish green from (0.221, 0.284) to (0.296, 0.377). On the other hand, under 320 nm excitation, the emission color coordinates change within the green light-blue light from (0.304, 0.438) to (0.232, 0.260). 0.9 520 nm 540 nm

510 nm

(a)

SmSbO4

530 nm

0.8

λexc = 275 nm

550 nm

0.7

560 nm 0.6 570 nm

500 nm 0.5

y

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

580 nm 590 nm 600 nm 610 nm 620 nm

0.4 490 nm

0.3

175 K 150 K

0.2

0.0 0.0

780 nm

11 K

480 nm 0.1

250 K

200 K

470 nm 380 nm 0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

x


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0.9 520 nm 540 nm

510 nm

λexc = 280 nm

550 nm

0.7

λexc = 320 nm

560 nm

0.6

570 nm

500 nm 0.5

580 nm 11 K

0.4

275 K 250 K 490 nm

0.3

11 K

590 nm 600 nm 610 nm 620 nm

50 K

780 nm

275 K

100 K

0.2 0.1

(b)

TbSbO4

530 nm

0.8

y

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

250 K 480 nm

0.0 0.0

470 nm 380 nm 0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

x

Figure 8. CIE chromaticity diagram showing the temperature dependence of the (x,y) color coordinates for (a) SmSbO4 and (b) TbSbO4.

We would like to emphasize that due to the specific characteristics of each lanthanide and the infeasibility of reporting all of them in just one work, detailed photoluminescence results for the other compounds synthesized here will be presented in future works. However, for sake of completeness, the room temperature 1931 CIE chromaticity diagram for all synthesized compounds was constructed and it is presented in Figure 9. Here, the goal of this investigation was to compare the color exhibited by all these new emitter materials synthesized in our laboratory, beyond to highlight the different specific colors obtained by each lanthanide present in a same orthoantimonate matrix at room temperature. These results reveal that it is possible to tune the emission color of the lanthanide-based emitters changing the chemical environment of the SbO4 matrix. Figure 9 also presents the images (photographs) for EuSbO4, DySbO4 and ErSbO4 ceramics in the



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moment that they are excited and emitting their respective colors as indicated at the chromatic diagram. Table 1 shows all the LnSbO4 compounds and their respective color coordinates obtained with the arrangement described above.

0.9 520 nm 530 nm

0.8

540 nm

510 nm

0.7

550 nm 560 nm

0.6

570 nm

500 nm

0.5

y

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

580 nm 590 nm

0.4 0.3

490 nm

Pr

0.2 0.1 0.0 0.0

480 nm

Yb

600 nm 610 nm 620 nm 780 nm

Tm Dy Lu Ho Gd Tb Eu Nd Er Sm

470 nm 380 nm

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

x

Figure 9. CIE chromaticity diagram coordinates of all LnSbO4 compounds synthesized in this work.


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Table 1. Color coordinates (x,y) obtained by 325 nm ultraviolet excitation at room temperature for all the LnSbO4 compounds. Compounds PrSbO4 NdSbO4 SmSbO4 EuSbO4 GdSbO4 TbSbO4 DySbO4 HoSbO4 ErSbO4 TmSbO4 YbSbO4 LuSbO4

(x,y) color coordinates (0.239, 0.183) (0.380,0.207) (0.334, 0.181) (0.462, 0.217) (0.384, 0.281) (0.302, 0.238) (0.318, 0.286) (0.371, 0.286) (0.254, 0.160) (0.309, 0.290) (0.224, 0.145) (0.303, 0.262)

Emitted color (CIE) Bluish purple Purplish pink Red purple Purplish red Purplish pink White White Purplish pink Purple White Bluish purple White

CONCLUSIONS In this work, LnSbO4 (Ln are all the lanthanides elements with exception of cerium and promethium) have been successfully synthesized at optimized conditions of temperature and time by using solid-state reactions. Their crystalline structures were resolved through conventional XRD and high-resolution Synchrotron X-ray diffraction. Two different arrangements belonging to the monoclinic structure were identified and related to the ionic radius of the lanthanide metal. Orthoantimonates with largest lanthanide ions (La and Pr) belong to the P21/n space group, while compositions involving intermediate and smallest lanthanides (Nd-Lu) were better described under the P21/c setting. Vibrational features were investigated by Raman spectroscopy, which allow us to determine the characteristic phonons for each structure and, consequently, to establish the relationship between chemical environment and vibrational modes. Representative



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compositions for this orthoantimonate series (SmSbO4 and TbSbO4) were thoroughly explored in terms of their optical properties as a function of temperature under ultraviolet radiation. The results showed that these materials exhibit strong orange (Sm) and green (Tb) emissions. Also, by constructing the chromaticity diagrams, it was possible to obtain the color coordinates for these promising emitters, which imply that the visible colors could be adjusted by the chemical environment of the lanthanide-containing ceramics LnSbO4

ASSOCIATED CONTENT Supporting Information Available Figures S1-S5 and Tables S1-S2. This material is available free of charge via the Internet at http://pubs.acs.org

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; Fax: +55 31 35591707; Tel: +55 31 35591716 Notes The authors declare no competing financial interest.


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ACKNOWLEDGEMENTS The authors acknowledge the financial support from CAPES, CNPq, FINEP and FAPEMIG (Brazil), and FCT (Portugal). Special thanks to Prof. Franklin M. Matinaga (UFMG) for Raman measurements in Ho- and Er-containing ceramics. Thanks are also due to Dr. Marcus Vinícius Baeta Moreira for the sample’s photographs taken during PL measurements.

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TOC GRAPHIC

0.9 520 nm 530 nm

0.8

540 nm

510 nm

0.7

550 nm 560 nm

0.6

LnSbO4

500 nm

570 nm

0.5

y

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

580 nm 590 nm

0.4 0.3

490 nm

Pr

0.2 0.1 0.0 0.0

480 nm

Yb

600 nm 610 nm 620 nm 780 nm

Tm Dy Lu Ho Gd Tb Eu Nd Er Sm

470 nm 380 nm

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

x


Lanthanide Orthoantimonate Light Emitters: Structural, Vibrational

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