Structures and Photophysical Characterization of the Samarium-doped Y2Mo3O12 Ceramics

Negative thermal expansion materials have been studied in various types of functional materials regardless of their intriguing physical properties. In this study, by introducing Sm 3+ ions into the Y 2 Mo 3 O 12 lattice, the hygroscopicity is reduced. Y 2 Mo 3 O 12 :xSm 3+ (x = 0.000 ~ 0.300) have been synthesized by using the solid-state reaction method, and their structures have been studied by using thermogravimetric analysis, X-ray diffraction, Raman spectrometry, FT-IR spectroscopy, and nano-SIMS. It is found that the Y 2 Mo 3 O 12 :Sm 3+ is formed in the orthorhombic [Y 2 Mo 3 O 12 ] phase for lower Sm 3+ concentrations and samarium molybdates [Sm 2 MoO 5 ] and [Sm 2 Mo 3 O 12 ] at higher Sm 3+ concentration samples. Elemental distribution images captured by using a nano-SIMS conrm the existence of [Sm 2 MoO 5 ] and [Sm 2 Mo 3 O 12 ] phases clearly. Additional phases due to the introducing of Sm 3+ ions conrm the reduction of atmospheric moisture. Photophysical properties obtained by using absorption and emission spectra reveal that the hygroscopicity-reduced Y 2 Mo 3 O 12 :Sm 3+ can be a possible orange-red-emitting phosphor with the near ultra-violet excitation.


Introduction
Molybdates with a general formula A 2 Mo 3 O 12 family, where A is a trivalent cation, are known to be hosts for transition metal ions and lanthanide ions [1][2][3]. Recently, these materials have attracted lots of attention because of negative thermal expansion (NTE), characterized by chemical exibility and a phase transition, from monoclinic to orthorhombic, depending on the A 3+ cation size [4][5][6][7][8]. These NTE materials are microporous and interstitial cation-free frameworks consisting of vertex-linked AO 6 octahedra and MO 4 tetrahedra, where each AO 6 octahedron joins MO 4 tetrahedra sharing oxygen of their corners [1,9].
The correlation between the crystal structure and the thermal expansion in A 2 Mo 3 O 12 is known [10]: Mo 3 O 12 with larger A 3+ cations shows more negative overall linear coe cient of thermal expansion than the ones with smaller A 3+ .
However, NTE materials have not been applied widely because of several disadvantages: hygroscopicity, high phase transition temperature, and metastable structure [11,12]. These hygroscopicity and high phase transition temperature in A 2 Mo 3 O 12 family have been studied widely and achieved progresses in practical applications [13][14][15]: the reduction of hygroscopicity in A 2 Mo 2 O 12 theoretically and experimentally [1,10,11]. Among these molybdates, Y 2 Mo 3 O 12 represents larger NTE because yttrium (Y) has larger ionic radius compared with other trivalent transition metals [9]. In order to understand the NTE mechanisms in Y 2 Mo 3 O 12 , several studies have been performed: the relation between AO 6 polyhedral distortion and NTE in orthorhombic Y 2 Mo 3 O 12 [16], and the effect of water species on the phonon modes [11,17]. There are many studies to improve the NTE properties in Y 2 Mo 3 O 12 [18,19], and to characterize various properties of Y 2 Mo 3 O 12 [20][21][22]. These studies have shown that the hygroscopicity is related to the sizes of cations A, and the reduction of hygroscopicity is achieved by substituting trivalent cations to Y 3+ sites [10,23]. There is another study about the reduction of hygroscopicity by adding Ce 3+ ions in Y 2 Mo 3 O 12 [18]. Also, there are many researches about the optical properties in another kind of yttrium molybdate Y 2 Mo 4 O 15 , as a phosphor material [24][25][26][27][28]. However, it is quite di cult to nd the optical studies in Y 2 Mo 3 O 12 material [29]. Hence, we start this research to expand the previous results to another rare-earth (RE) ion Sm 3+ ; i.e. reducing the hygroscopicity and studying the optical properties of Y 2 Mo 3 O 12 .
RE-activated phosphors have been actively studied for the application in light-emitting diodes because of their chemical stability and relatively simple preparation conditions. Recently we have reported on the structures and optical properties of RE-doped apatites Ca 5 (PO 4 ) 3 Cl [30,31] and phosphates [32]. Trivalent samarium (Sm 3+ ) is applied as an activator for lots of phosphors that have red-orange emission arising from its 4 G 5/2 → 6 H J (J = 5/2, 7/2, and 9/2) transitions. Sm 3+ also determines the photophysical properties of phosphors such as their lifetimes and e ciencies operating under the near ultra-violet (NUV) excitation [33][34][35]. As a continuation of our work on the optical properties in Sm 3+ -doped inorganic materials, we investigate the photophysical properties of Y 2 Mo 3 O 12 .
In this work, we prepare Sm 3+ -doped Y 2 Mo 3 O 12 by using the solid-state reaction method in order to characterize the structures and photophysical properties. First, we report the synthesis procedures. We character the structures by using a thermogravimetric analyzer (TGA), an X-ray diffractometer (XRD), a Raman spectrometer, a Fourier-transform infrared (FT-IR) spectrometer, and an X-ray photoelectron spectrometer (XPS). Then we observe the morphology by using a scanning electron microscope (SEM).
Also we take elemental distribution images by using a high-resolution secondary ion mass spectrometer (nano-SIMS). Finally, we investigate photophysical properties of the Y 2   The elemental distribution is analyzed qualitatively by using a nano-SIMS (Cameca NS50). Nano-SIMS images are obtained by using a cesium primary ion beam with a diameter 100 nm, an impact energy 16 keV, and a beam current 0.4 pA. The raster size is 20 mm x 20 mm in all images with 256 pixels x 256 pixels for generating qualitative secondary ion images with 30 ms/pixel accumulation time. Samples are simultaneously imaged by taking the secondary ions that are detected by an electron multiplier.
The chemical behavior and the molecular bonding structure are characterized by using a FR-IR (Bruker, Vertex80v) in the range of 4,000 ~ 400 cm -1 . All IR spectra are collected with 256 scans and a spectral resolution of 4 cm -1 . Raman spectra are measured by using a Raman spectrometer (NanoBase XperRam200) in the range of 0 ~ 3000 cm -1 with a laser power of 3 mW (l = 532 nm) and 128 scans with 0.5 ms accumulation time. The valence states of the elements are studied by using an XPS (Thermo Fisher K-Alpha + ) with Al Ka (hn = 1486.6 eV) radiation. The XPS is operated at an accelerating voltage of 12 kV with energy 72 W. The measured area is an oval shape with short diameter of 400 mm, and energy step sizes are 1 eV and 0.1 eV for wide and narrow scans, respectively.
The synthesized powder is pressed into pellets with potassium bromide (KBr, Pike Tech. IR Grade) for the optical measurements. Absorption spectra are obtained by using an UV-Vis spectrometer (Agilent Cary 300) with 0.2 nm step and 1 s averaging time. Emission spectra are taken with a steady-state uorescence system with a 1000 W Xe-lamp. The excitation light from a Xe-lamp (Muller Electronik-Optik LAX-1000), selected by using a 320 mm focal length monochromator (Dongwoo DM320i), is focused onto the sample. Fluorescence from the sample is collimated and refocused into the emission monochromator (Dongwoo DM320i) with a 320 mm focal length. The collimated emission is detected by using a photomultiplier tube (PMT, Hamamatsu R955) after passing the cut-off lter (Edmund OG 515) and data are accumulated with a computer. All spectra are taken at room temperature. are observed along with [Y 2 Mo 3 O 12 ] phase. The decrease of the peak intensities of [Y 2 Mo 3 O 12 ] phase with an increase of Sm 3+ amount means that the crystal structure depends on the doped ion concentrations.

Results And Discussion
In order to show that these phases are related to the hygroscopicity, we take TGA data. Figure 1(c) shows the weight variations of the samples according to the temperature. A gradual decrease of weight corresponding to the release of adsorbed water and/or crystal water with increasing temperature is observed, which means the reduction of hygroscopicity. In other words, the newly obtained phases look responsible for the reduction of hygroscopicity of the synthesized powders. Nano-SIMS is utilized in elemental analysis, with high spatial resolution, by tracing a small amount of elements and isotopes. With high lateral resolution and sensitivity, the possibility of imaging components based on the elemental and isotopic compositions of atomic and molecular ion fragments can be offered by using a nano-SIMS. The obtained information through nano-SIMS complements other analytical results obtained by using other types of SIMS experiments [36]. Also we can compare elemental distributions clearly at a xed position of the sample because nano-SIMS can record ve atomic mass images simultaneously with a high-resolution of 50 nm. Thus, we have applied this equipment to studying phosphors and reported the element compositional analysis results in phosphor material [37]. In this study, we apply nano-SIMS to our samples rather than energy-dispersive spectrometer (EDS).
We take the elemental acquisition images of oxygen, yttrium, molybdenum, and samarium in the image has different scales depending on the element. We observe that elements are distributed throughout the samples, although the obtained images provide only qualitative information. Sm 3+ is observed even in a lower amount-doped sample. At rst, when we prepare the samples, we think that Sm 3+ ions are substituted into Y 3+ sites through the sintering procedure as in Ref. [18] and in Y 2 Mo 3 O 12 ] in higher concentration powders. Thus, the images obtained by using a nano-SIMS strongly support the discussion of new phases in XRD patterns for the higher Sm 3+ concentration samples.
For the structural aspect, we take the Raman and the FT-IR spectra of the samples and represent them in   [9][10][11]. The bands at about 867 cm -1 and 936 cm -1 (represented by blue vertical dotted lines) correspond to symmetric and asymmetric stretching vibrations of the MoO 4 tetrahedra, respectively [11,18]. These show clear peak position shifts and their relative intensities, respectively, with the doped amount, which means that the in uences of atmospheric moisture on the stretching vibrations of the MoO 4 tetrahedra become weaker [11]. As mentioned earlier, we can think that these shifts are related to the phase variations of the materials with the Sm 3+ amounts. The small peaks around 600 ~ 400 cm -1 are correspond to the vibrations of polyhedra MoO 4 and YO 6 [2].
From the Raman and the FT-IR spectra shown in Fig. 4, we observe the structural variations as in XRD patterns with the doped Sm 3+ concentrations. At room temperature Y 2 Mo 3 O 12 forms in orthorhombic structure with a space group Pba2 and undergoes a phase transition, ferroelectric-ferroelastic to paraelectric-paraelastic, at higher temperature [40]. Although we have synthesized the Y 2 Mo 3 O 12 samples at higher temperature, we have taken the data at room temperature. Thus, we need more study about Y 2 Mo 3 O 12 samples whether the reduction of hygroscopicity is directly related to the new phases appeared at higher concentrations.
To see the chemical states of the elements in the prepared powders, we take XPS spectra. Figure 5 represents the wide scan XPS spectra of the Y 2 Mo 3 O 12 :xSm 3+ samples with concentrations of 0.300, 0.150, 0.050, and 0.000 mol, respectively, from top to bottom. In XPS spectra, the position on the kinetic energy scale means the difference between the photon excitation energy, and the spectrometer work function corresponds to a binding energy of 0 eV with reference to the Fermi level [41]. All the peak positions are calibrated with respect to carbon 1s at 284.6 eV. Peaks of elements such as Y, Mo, O, and Sm including KLL (oxygen Auger peak) are clearly identi ed [41]. Other small peaks such as Y 4p/O 2s around 23~24 eV, Mo 4p around 36 eV, Mo 4s around 63 eV, and Mo 3s around 506 eV are not designated in the gure for clarity, even though they are identi ed. The starting oxides are burned in the atmosphere during sintering process, which means that some of the oxygen atoms from the starting materials are used during the sintering process inside of the furnace. Also, we take XPS spectra in high vacuum without etching the samples. Hence, the observed carbon peaks are from atmospheric carbon dioxide attached to the sample powders during the pre-treatment procedure for the measurements.
To see the effect of Sm 3+ introducing, we take the narrow scan XPS spectra of the Y 2 Mo 3 O 12 :xSm 3+ samples and represent them in Fig. 6. Figure 6 Fig. 6(e). The values of the line positions re ect the energies of the elemental peaks observed.
We are interested in the actual intensities of the strongest emission band, 6 Fig. 8(b) by following the calculated energy levels [43]. From this gure, we can nd that Sm 3+ ions are excited from the ground state 6 H 5/2 to the excited state 4 F 7/2 with an excitation of 405 nm, then the ions at 4 F 7/2 level relax to the 4 G 5/2 level through the non-radiative relaxation process. Thus, the emission bands around 566 nm, 603 nm, and 649 nm arising from the transitions 4 G 5/2 to 6 H 5/2 , 6 H 7/2 , and 6 H 9/2 , respectively, are observed.
The intensity ratio between electric and magnetic dipole transition gives a measure for the symmetry of local environment of the trivalent 4f ions [37]. Magnetic dipole transitions obey the selection rule of DJ = 0 and +1, where J is the angular momentum, and electric dipole transitions obey the selection rule of DJ ≤ 6 unless J or J' = 0 when DJ = 2,3,6. Thus, the transition 4 G 5/2 → 6 H 5/2 is a magnetic dipole transition because this satis es the selection rule of DJ = +1, while the transition 4 G 5/2 → 6 H 9/2 is an electric dipole transition. The transition 4 G 5/2 → 6 H 7/2 is a partly magnetic and partly forced electric dipole transition [41]. The transition 4 G 5/2 → 6 H 11/2 is not observed in this study because of its forbidden feature DJ = 3.
Consider the ratio between the intensities of the magnetic dipole transition 4 G 5/2 → 6 H 7/2 to the electric dipole transition 4 G 5/2 → 6 H 9/2 , which is a measure of the local symmetry of the coordination polyhedron of the Sm 3+ ions. Hence, the larger value of this ratio means more distortion from the inversion symmetry [37]. The obtained values are almost constant between 1.33 and 1.48, which means that the Sm 3+ ions are embedded in samarium molybdates [Sm 2 MoO 5 ] and [Sm 2 Mo 3 O 12 ] structures rather than in the distorted cation environment. In our previous result for CaTiO 3 :Sm 3+ phosphors [37], the calculated values are from 1.40 to 1.53, which implies that the environment of Sm 3+ ions are substituted into the host material and remain in symmetric state. Also the observed spectral widths remain similar values varying from 8.55 ~ 9.23 nm except the lower concentration samples. Therefore there is no signi cant structural environment change around Sm 3+ ions with increase of the concentration, except the formation of new phases.
As mentioned in Introductory section, Y 2 Mo 3 O 12 is known as a NTE material having disadvantage such as hygroscopicity. From the previous discussion, we have observed a phase transition for the samples of the concentrations x ≥ 0.100 and a concentration quenching for the samples of the higher concentrations. We can think that around 0.100 mol of Sm 3+ -doped Y 2 Mo 3 O 12 is a hygroscopicityreduced sample with highly luminescent feature. There are many studies in reducing the hygroscopicity by using the ion-substitution methods and the coating methods. As shown in this study, RE 3+ -doped Y 2 Mo 3 O 12 can be a candidate material for reduced hygroscopicity as well as photonic application. We need more studies for the newly formed samarium molybdates whether these can also be host materials for photonic applications.

Summary
In this work, structural and optical properties of RE 3+ -doped Y 2 Mo 3 O 12 ] appeared at higher Sm 3+ concentration samples according to the XRD patterns. TGA results con rmed the reduction of hygroscopicity. Elemental distribution images obtained by using nano-SIMS strongly supported the existence of Sm 2 MoO 5 and Sm 2 Mo 3 O 12 as in XRD patterns. Raman spectra and FT-IR spectra showed phase variations, modes due to the symmetric and asymmetric vibrations of MoO 4 tetrahedron, and translational and librational vibrations of the polyhedra YO 6 and MoO 4 . XPS study showed the effect caused from Sm 3+ -doping. Photophysical properties revealed that the appropriated amount of Sm 3+ -doped Y 2