Synthesis, thermal expansion and optical properties of (1−x)NaAl(MoO4)2–xNaEr(MoO4)2 ceramics

Journal of Alloys and Compounds 564 (2013) 63–70

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Synthesis, thermal expansion and optical properties of (1 x)NaAl(MoO4)2–xNaEr(MoO4)2 ceramics Xian-Sheng Liu a, Bao-He Yuan a,b, Jun-Qiao Wang a, Wen-Bo Song a, Fu-Xing Cheng a, Er-Jun Liang a,⇑, Ming-Ju Chao a a b

School of Physical Science & Engineering, Key Laboratory of Materials Physics, Ministry of Education of China, Zhengzhou University, Zhengzhou 450052, China North China University of Water Resources and Electric Power, Zhengzhou 450011, China

a r t i c l e

i n f o

Article history: Received 14 September 2012 Received in revised form 17 February 2013 Accepted 19 February 2013 Available online 7 March 2013 Keywords: Ceramics Optical materials Optical properties Thermal expansion

a b s t r a c t (1 x)NaAl(MoO4)2–xNaEr(MoO4)2 ceramics with x = 0, 0.04, 0.08, 0.12, 0.16, 0.20 and 1.0 are synthesized by coprecipitation method, and their thermal expansion and room temperature optical properties are studied. It is found that the ceramics with two phases of monoclinic and tetragonal present large thermal expansion, and exhibit relatively strong near ultraviolet and green and weak red photoluminescence, which are also observed when excited with a 980 nm laser. With increasing photoluminescence of Er3+ ions in the ceramics, the intrinsic photoluminescence of the hosts is observed to decrease intensively. It could refer that excitation into the hosts is followed by an efficient energy transfer towards Er3+ ions. The relatively strong up-conversion green lights of the ceramics indicate the potential use as one of upconversion materials. Crown Copyright Ó 2013 Published by Elsevier B.V. All rights reserved.

1. Introduction Er3+-doped luminescence materials for efficient conversion of infrared/ultraviolet to visible light are very attractive for their potential applications in frequency up-conversion (UC) lasers, highdensity memories, solid-state color displays, thin film photovoltaic solar cells and other photonic devices [1–5]. The rare-earth-doped nanocrystals have already been investigated as hosts widely such as ZrO2 [6–9], Y2O3 [10–12], BaTiO3 [13], Gd2O3 [14], CaSnO3 [15] and NaY(WO4)2 [16]. Most rare-earth-doped crystals are singlephase hosts, whereas there are few reports on Er3+-doped hosts composed of two different symmetric crystal structures. Singlephase material as host for rare-earth doping to obtain luminescence is easy to fall off on heating because of different thermal expansion between luminescence materials and substrates. The mixed/composite materials with low thermal expansion could reduce the thermal stress. In addition, the intensity and symmetry of composite crystal fields could affect the photoluminescence (PL) of Re-ions [17].

⇑ Corresponding author at: School of Physical Science & Engineering and Key Laboratory of Materials Physics of Ministry of Education of China, Zhengzhou University, Zhengzhou 450052, China. Tel.: +86 371 67767838; fax: +86 371 67766629. E-mail address: [email protected] (E.-J. Liang).

Being inspired by thermal expansion properties of Al2(MoO4)3 (present positive thermal expansion) and Er2(MoO4)3 (present negative thermal expansion) [18,19], we consider that NaEr(MoO4)2 could has lower thermal expansion than that of NaAl(MoO4)2. Furthermore, the composition/mixture of NaAl(MoO4)2 and NaEr(MoO4)2 could possess lower thermal expansion than that of NaAl(MoO4)2. So far, Cr3+-doped NaAl(MoO4)2 as one of luminescence materials has been investigated [20,21], which shows a well defined broad band of luminescence between 660 and 840 nm. Whereas there are few reports on Er3+-doped NaAl(MoO4)2, and NaAl(MoO4)2 possesses high thermal expansion. Therefore, it is necessary to investigate the thermal and optical properties of (1 x)NaAl(MoO4)2–xNaEr(MoO4)2 ceramics by adjusting the value of x. In the present paper, we report the thermal expansion property, optical properties: Raman spectra, photoluminescence (PL) and frequency up-conversion of (1 x)NaAl(MoO4)2–xNaEr(MoO4)2 ceramics synthesized by coprecipitation method. It is shown that (1 x)NaAl(MoO4)2–xNaEr(MoO4)2 ceramics expand on heating and give rise to relatively strong near ultraviolet and green PL corresponding to 2H9/2 ? 4I15/2, 4H11/2 ? 4I15/2 and 4S3/2 ? 4I15/2, and weak red PL corresponding to 4F9/2 ? 4I15/2 transitions of Er3+ ions. It also exhibits relatively strong green and weak red emissions in the UC spectra when excited with a 980 nm laser. The results suggest that (1 x)NaAl(MoO4)2–xNaEr(MoO4)2 could be one of the promising candidates for green-light LEDs and UC emission applications.

0925-8388/$ - see front matter Crown Copyright Ó 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.02.140

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2. Crystal structure NaAl(MoO4)2 belongs to the group of layered crystals with the general formula MIMIII(MVIO4)2, where MI = Na, K, Rb, Cs, MIII = Al, Sc, In, etc. and MVI = Mo or W. It crystallizes in monoclinic structure (space group C2/c) with a = 9.621(2) Å, b = 5.3390(1) Å, c = 13.146(3) Å, b = 90.01(3)°, and has four formula units per unit cell (Z = 4) [22]. Mo and O atoms are in general positions while Na and Al atoms are in special positions on a 2-fold rotation axis and an inversion center, respectively. The temperature-dependent Raman studies on NaAl(MoO4)2 in the 173 to 657 °C range showed that the monoclinic structure was stable up to the melting point temperature [23]. The formula of NaEr(MoO4)2 is similar to that of NaAl(MoO4)2, however, it crystallizes in tetragonal phase, scheelite structure (space group I41/a) with a = 5.1618(8) Å, c = 11.288(3) Å. In the structure, Na and Er atoms are disordered over the same 4a site while Mo atoms reside on 4b site. The structure of NaEr(MoO4)2 may be regarded as composed of [MoO4]2 tetrahedra and of [(Na/Er)O8]14 polyhedra (each in the form of a distorted tetragonal antiprism) that share the oxygens. Each oxygen of the [MoO4]2 tetrahedron is shared by the different Na/Er polyhedra and each oxygen of the [(Na/Er)O8]14 polyhedron is shared by the different [MoO4]2 tetrahedra [24]. 3. Experimental All reagents used were analytical grade. NaAl(MoO4)2 powders were prepared by co-precipitation method using (NH4)6Mo7O24
4H2O, Na2MoO4
2H2O and Al(NO3)3
9H2O as raw materials. Three aqueous solutions (volumes are 20, 50 and 20 mL) with 2.5 mmol Na2MoO4
2H2O, 1.1 mmol (NH4)6Mo7O24
4H2O and 5.0 mmol Al(NO3)3
9H2O, respectively, were prepared and mixed according to the stoichiometric ratios of the destination material. The Na2MoO4
2H2O solution was first added into the (NH4)6Mo7O24
4H2O solution, and then Al(NO3)3
9H2O solution was added dropwise and white slurry produced rapidly. The pH value of the mixture was adjusted to 9 by adding dilute NH3
H2O/HNO3 (volume percentage 30%). The white slurry was aged for 12 h and then the upper clear solution was discarded. The remaining was dried at 80 °C followed by sintering at 600 °C for 4 h to obtain NaAl(MoO4)2. For the synthesis of (1 x)NaAl(MoO4)2–xNaEr(MoO4)2 powders with x = 0.04, 0.08, 0.12, 0.16, 0.20 and 1.00, the aqueous solution of Al(NO3)3
9H2O was added with a certain amount of Er(NO3)3
6H2O aqueous solution and the rest steps were the same as the preparation procedures above. Some of the powders were pressed into cylinders (6 mm in diameter and 16 mm in length) by uniaxial cold pressure at 300 MPa and then sintered (600 °C, 1 h) for linear thermal expansion and density measurements. And the others of the powders were pressed into pellets (10 mm in diameter and 3 mm in thickness) and sintered under the same conditions to test the optical properties. X-ray diffraction (XRD) measurements were carried out with an X-ray diffractometer (Model X’Pert PRO) to identify the crystalline phase. The linear thermal expansion coefficient was measured on the thermal expansion apparatus (LINSEIS DIL L76). The densities of the sintered ceramics were measured by using Archimedes’ technique. The microstructures were recorded using a scanning electron microscope (SEM, FEI Quanta 250). Raman and PL spectra were recorded by a Raman spectrometer (Model MR-2000, Renishaw) with laser excitation wavelengths of 532 and 633 nm. The excitation and emission spectra were performed at room temperature with a Fluoromax-4 spetrofluorometer (HORIBA Jobin Yvon). Frequency UC emission spectra were measured using a SBP500 luminescence spectrometer with a PMTH-S1-CR131 photomultiplier detector and a 980 nm laser as the excitation source. The diffuse reflectance spectrum was measured on a UV–VIR–NIR spectrophotometer (UV-3100) with BaSO4 as the reference and was converted to the absorbance data through the Kubelka–Munk method.

Fig. 1. XRD patterns of (1 x)NaAl(MoO4)2–xNaEr(MoO4)2 particles with x = 0.00, 0.04, 0.08, 0.12, 0.16, 0.20 and 1.00, respectively.

doping effect of Er3+ (88.1 pm) into NaAl(MoO4)2 structure to substitute Al3+ (53.5 pm). With increasing value of x, the diffraction peaks at about 25.8°, 28.6°, 31.2°, 34.1°, 47.2°, 49.2° and 58.2° become strong corresponding to the tetragonal phase NaEr(MoO4)2 [24]. The result indicates that ratio of NaAl(MoO4)2 and NaEr(MoO4)2 in as-prepared ceramics are subsequently tuned by co-precipitation method, which could be used to explore moderate ratio for optimal properties. 4.2. Density and microstructure analysis The densities of (1 x)NaAl(MoO4)2–xNaEr(MoO4)2 ceramics with x = 0, 0.04, 0.08, 0.12, 0.16, 0.20 and 1.00 are measured to be about 2.64, 2.68, 2.71, 2.75, 2.69, 2.60 and 3.15 g/cm3, respectively (see Fig. 2a). It is obvious that the measured density of as-synthesized ceramics increases with the increase of x. The theoretical densities of the ceramics with x = 0.04, 0.08, 0.12, 0.16 and 0.20 are calculated to be about 3.708, 3.779, 3.851, 3.923 and 3.996 g/cm3, respectively, according to the reported theoretical density of 3.638 g/cm3 for NaAl(MoO4)2 [22] and 5.590 g/cm3 for NaEr(MoO4)2 [24]. This gives rise to relative densities (measured density/theoretical density) of the ceramics with x = 0, 0.04, 0.08, 0.12, 0.16, 0.20 and 1.00 being about 72.57%, 72.28%, 71.71%, 71.41%, 68.57%, 65.07%, 56.07%, and 56.35%, respectively. The monotonic decrease in relative density with increasing the contents of NaEr(MoO4)2 implies more porosities between the crystallites. Fig. 2b and c shows the SEM images of surface and cross section of (1 x)NaAl(MoO4)2–xNaEr(MoO4)2 ceramics with x = 0.00, 0.04, 0.08, 0.12, 0.16, 0.20 and 1.00, respectively. For pure NaAl(MoO4)2 (x = 0.00), small particles recombine into layers with least porosities, whereas NaEr(MoO4)2 (x = 1.00) ceramic exhibits a porous morphology. With increasing the contents of NaEr(MoO4)2, the 2phase ceramics exhibit more porosities between crystallites. This is in agreement with the relative density analysis.

4. Results and discussion 4.3. Thermal expansion property 4.1. Crystal phase analysis Fig. 1 shows the XRD patterns of (1 x)NaAl(MoO4)2– xNaEr(MoO4)2 particles with x = 0, 0.04, 0.08, 0.12, 0.16, 0.20 and 1.00, respectively. It is obvious that the diffraction pattern of NaAl(MoO4)2 agrees well with that of a monoclinic structure (ICDD-PDF No. 00-054-0243). There is a clear peak shift to lower angles observed between the x = 0 and x = 0.04 samples, which implies an increase in crystal lattice of NaAl(MoO4)2 resulted from a

Fig. 3 shows the relative linear length change with temperature of (1 x)NaAl(MoO4)2–xNaEr(MoO4)2 ceramics with x = 0, 0.04, 0.08, 0.12, 0.16, 0.20 and 1.00. It is found that the ceramics present positive thermal expansion. The coefficient of thermal expansion (CTE) of pure NaAl(MoO4)2 is about 15.09  10 6 °C 1 larger than those of the others, which is consistent with the report of a high anisotropic thermal expansion in cell volume by XRD results from 20 to 300 °C [21]. While the CTE of pure NaEr(MoO4)2 is the least

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Fig. 2. Densities (a), surface (b) and cross sections (c) SEM images of (1 x)NaAl(MoO4)2–xNaEr(MoO4)2 ceramics with x = 0.00, 0.04, 0.08, 0.12, 0.16, 0.20 and 1.00, respectively.

about 11.73  10 6 °C 1 in the ceramics, which is reported hardly. The CTEs of the (1 x)NaAl(MoO4)2–xNaEr(MoO4)2 ceramics de-

crease with increasing the value of x. It is well known that the CTE of materials is related with the anharmonic effect of

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Fig. 3. Relative linear length change (dL/L0) with temperature of the ceramics of (1 x)NaAl(MoO4)2–xNaEr(MoO4)2 with x = 0.00, 0.04, 0.08, 0.12, 0.16, 0.20 and 1.00: dL is the magnitude of linear length change, and L0 is the magnitude of linear length at room temperature.

crystal, therefore the grain boundary and crystal lattice in (1 x)NaAl(MoO4)2–xNaEr(MoO4)2 ceramics could change with the ratio of the components. Meanwhile, the CTE decrease could relate to the difference in ions radius (Er3+: 0.88 Å and Al3+: 0.53 Å) and crystal structures (monoclinic and tetragonal) [25]. The results indicate that the CTEs of the ceramics could be decreased slightly to reduce the crack in the materials on heating. Furthermore, the results imply that it is possible to synthesize the low thermal expansion optical ceramics made from large positive and low/negative thermal expansion materials [25,26]. In addition, the nonlinear part of composite with x = 0.20 is observed at about 480 °C lower than that of two components, which could relate to the combination of monoclinic and tetragonal phases of NaAl(MoO4)2 and NaEr(MoO4)2 to change the grain boundary.

4.4. Optical properties Fig. 4 shows the Raman spectra of (1 x)NaAl(MoO4)2– xNaEr(MoO4)2 ceramics with x = 0, 0.04, 0.08, 0.12, 0.16, 0.20 and 1.00 with 633 nm excitation. The group theory analysis predicts 33 Raman-active modes for the NaAl(MoO4)2 crystal in the C 62h structure. 18 modes are internal of the [MoO4]2 tetrahedra,

Fig. 4. Raman spectra of (1 x)NaAl(MoO4)2–xNaEr(MoO4)2 ceramics with x = 0.00, 0.04, 0.08, 0.12, 0.16, 0.20 and 1.0 at 633 nm excitation.

namely stretching m1(Ag + Bg), m3(3Ag + 3Bg) and bending m2(2Ag + 2Bg), m4(3Ag + 3Bg). The remaining modes stand for translational motions (Ag + 2Bg) of the Na+ ions, librations (3Ag + 3Bg) and translations (3Ag + 3Bg) of the [MoO4]2 ions. The translations of the Al3+ ions have Au and Bu symmetry and they are Raman inactive [27]. The Raman spectrum of NaAl(MoO4)2 displays clearly the symmetric and asymmetric stretching vibrational modes (985, 960, 931, 810 and 768 cm 1), the bending vibrations of [MoO4]2 units (from 420 to 340 cm 1) and translations of Na+ ions, translations and librations of [MoO4]2 (below 300 cm 1). These modes appear also in the spectra of (1 x)NaAl(MoO4)2–xNaEr(MoO4)2 ceramics with x = 0.04, 0.08, 0.12, 0.16 and 0.20, respectively. Besides the sharp Raman bands assigned to the NaAl(MoO4)2, there appear two groups of pronounced bands from 450 to 650 cm 1 (corresponding peak wavelengths: 653 nm, 657 nm and 658 nm) and from 700 to 950 cm 1 (corresponding peak wavelengths: 665 nm, 668 nm, and 671 nm) in the (1 x)NaAl(MoO4)2– xNaEr(MoO4)2 ceramics with x = 0.04, 0.08, 0.12, 0.16, 0.20 and 1.00. The wavelengths of these bands correspond well to the energy difference of 4F9/2 ? 4I15/2 transition of Er3+ [28–30], they could be the PL of Er3+ ions excited by laser in the composite crystal fields of (1 x)NaAl(MoO4)2–xNaEr(MoO4)2. In order to discriminate the PL from the Raman scattered light, we measured the Raman spectra of (1 x)NaAl(MoO4)2– xNaEr(MoO4)2 ceramics (x = 0, 0.04, 0.08, 0.12, 0.16, 0.20 and 1.00) with 532 nm excitation. The results are shown in Fig. 5. Comparison of Figs. 4 and 5 shows clearly that the two groups of the pronounced bands are shifted obviously in wavenumbers but keep their wavelength unchanged when excited with different wavelength lasers (as shown in the insert of Fig. 5). The fixed emission wavelength excited by different excitation wavelengths is the characteristic of PL spectrum, because the energy levels are invariable in general. While the Raman shift in energy gives information about the vibrational modes in the crystal structure, which should be fixed even using different lasers. Therefore, the bands can be attributed unambiguously to the PL of 4F9/2 ? 4I15/2 transitions of Er3+ [28–30]. It is also found that (1 x)NaAl(MoO4)2–xNaEr(MoO4)2 ceramics with x = 0.04, 0.08, 0.12, 0.16, 0.20 and 1.00 give rise to relatively strong green emissions at about 542, 545 and 554 nm in addition to the red emissions when excited with 532 nm laser light. Due to the intense PL, the weak Raman signal in the ceramics cannot be observed. Considering the energy levels of Er3+, the relatively

Fig. 5. Raman spectra of (1 x)NaAl(MoO4)2–xNaEr(MoO4)2 ceramics with x = 0.00, 0.04, 0.08, 0.12, 0.16, 0.20 and 1.0 at 532 nm excitation.

X.-S. Liu et al. / Journal of Alloys and Compounds 564 (2013) 63–70

strong green emissions at about 542, 545 and 554 nm are attributed to 4S3/2 ? 4I15/2 transitions of Er3+ [28–30]. In order to further clarify the mechanisms of the relatively intense green emissions in the ceramics, we measured the excitation spectrum of (1 x)NaAl(MoO4)2–xNaEr(MoO4)2 ceramics with x = 0.12 at fixed emission wavelength 550 nm, as shown in Fig. 6. The 532 nm laser excitation (as indicated with the dotted line) corresponds to the resonance transition from the ground state 4I15/2 to the excited state 4S3/2. Since the difference between the exciting and emitting light wavelengths is small, the green emissions at about 542, 545 and 554 nm are therefore the transitions from the excited state 4S3/2 to the sublevels of the ground state. Due to the double resonance nature, the green emissions are very intense. Fig. 6 shows also that the most efficient excitation wavelength for the green light emissions is around 379 nm, corresponding to the excitation from ground state to excited state 4G11/2. In Fig. 7a we show the PL spectra of (1 x)NaAl(MoO4)2–xNaEr(MoO4)2 ceramics with x = 0.12 and 1.00 with 379 nm excitation, which show similar PL bands with 379 nm excitation. However, the sample of x = 0.12 gives rise to much stronger PL intensity than that of x = 1.00. The results suggest that the strong near ultraviolet emission band at about 405 nm and green emission bands at about 522, 541 and 553 nm result from the transitions of Er3+ in the (1 x)NaAl(MoO4)2–xNaEr(MoO4)2 ceramics rather than the single-phase of NaEr(MoO4)2. The relatively strong near ultraviolet emission band at about 405 nm matches the level difference of excited state 2H9/2 and ground state 4I15/2, which is therefore attributed to the transition 2H9/2 ? 4I15/2 of Er3+ ions. To ascertain the mechanism of near ultraviolet light 405 nm and if existing energy transfer from host composite crystals to Er3+ ions, we performed the excitation spectrum of (1 x)NaAl(MoO4)2– xNaEr(MoO4)2 ceramics with x = 0.12 at fixed emission wavelength 405 nm (see Fig. 7b). It is found that the most efficient excitation wavelength for the near ultraviolet emission 405 nm is around 336 nm, however, which is not typical level of Er3+ ions and then is considered as a certain state of (1 x)NaAl(MoO4)2–xNaEr(MoO4)2 ceramics. Meanwhile, the spectrum line below 280 nm rises up obviously, as implies that there could exist some contributions of transition electrons absorbing larger-than-bandgap photons, i.e. there could be energy transfer from host composite crystals to Er3+ ions. Therefore the bandgap estimations are performed depending on the absorption spectra of NaAl(MoO4)2, the ceramics with x = 0.12 and NaEr(MoO4)2 (see Fig. 7c–e). Their bandgaps are estimated to be 4.50 eV (276 nm), 3.44 eV (360 nm), 3.69 eV (336 nm), respectively. For bandgap of the ceramics, it becomes

Fig. 6. Excitation spectrum of (1 x)NaAl(MoO4)2–xNaEr(MoO4)2 ceramics with x = 0.12: emission wavelength is fixed at 550 nm.

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narrow less than that of the two components. The wavelength 336 nm (vs. bandgap 3.69 eV) is coincident with the peak at 336 nm in the Fig. 6 and Fig. 7b, which indicates 336 nm is corresponding to the conduction band NaEr(MoO4)2. It could be concluded that, the electrons absorbing larger-than-bandgap photons transfer to conduction band of NaAl(MoO4)2 and then non-radiation transfer to the conduction band of NaEr(MoO4)2 and the lower states of Er3+ (2G7/2, 2K15/2, 2G9/2, 4G11/2 and 2H9/2), and finally transfer to the ground state of Er3+ emitting violet blue light of 372, 379 and 405 nm (see Fig. 7d). The more PL spectra of (1 x)NaAl(MoO4)2–xNaEr(MoO4)2 ceramics with x = 0.12 (Fig. 8a) with different excitation wavelengths could confirm the suggestion above. One can find that, besides the emission band corresponding to the conduction band about 336 nm of NaEr(MoO4)2, there are also emission bands at about 372, 379, and 405 nm from transitions of Er3+ ions. It is a fact that the relative intensity of near ultraviolet light 405 nm is much lower than those of 372 and 379 nm, which could relate to electron non-radiation relaxation in favor of the smaller energy difference. The similar phenomena about smaller level difference inducing stronger emission have been observed also in the emissions of 405 nm (excited by 379 nm) and 542, 545 and 554 nm (excited by 532 nm) mentioned above. For the excitation wavelengths increase from 321 to 336 nm, the relative intensities of emission bands at about 372 and 379 nm become stronger and the intense peaks of emission bands shift to longer wavelengths. It shows that the emissions from host composite crystals become weak while those from Er3+ ions get strong, i.e. more electrons transfer from 336 nm (host ceramics) to states of Er3+ [31,32]. Fig. 8b and c about excitation spectra indicates the mechanisms of emission bands of 372 and 379 nm. The obvious rise of the curve from 276 nm is considered to relate to the contribution of conduction band electron transitions, which could also be consistent with the suggestion of energy transfer from host ceramics to Er3+ ions. The UC emission spectra of (1 x)NaAl(MoO4)2–xNaEr(MoO4)2 ceramics with x = 0.04, 0.08, 0.12, 0.16 and 0.20 excited with 980 nm laser wavelength are illustrated in Fig. 9, where the insert shows the dependence of the UC emission intensity at about 553 nm on the value of x. It is seen that there are relatively strong green emissions from 2H11/2 ? 4I15/2 and 4S3/2 ? 4I15/2 transitions and weak red emissions from 4F9/2 ? 4I15/2 of Er3+ ions [28–30] when excited with a 980 nm laser. The photon energy of a 980 nm laser matches the absorption transitions from the ground state 4I15/2 to the excited state 4I11/2 and from 4I11/2 to higher state 4F7/2. Er3+ ions will be first pumped to 4I11/2 and then to 4F7/2 by successive absorption of pump photons. The excited Er3+ ions at 4F7/2 by two-photon absorption relax to lower states 2H11/2 and 4S3/2, from where they transit back to the sublevels of the ground state by emitting photons. The transitions 2H11/2 ? 4I15/2 and 4S3/2 ? 4I15/2 correspond to the green emissions around 525 nm and 550 nm. The relaxation of the excited Er3+ ions at 4F7/2 to the energy level 4F9/2 is also possible, and then the Er3+ ions transit back to the ground state and give rise to the red emissions around 650 nm and 670 nm. However, the probability for the Er3+ ions to relax from 4F7/2 to the 4 F9/2 level is very low because direct transition from 4F7/2 to 4 F9/2 is forbidden (dissatisfy Laporte selection rule about g-u or u-g transition). It is noticed that the intensities of UC emissions of the ceramics increase gradually with increasing the value of x till 0.12 and then decrease with further increasing the value of x. Similar phenomenon is observed for the PL spectra as shown in Fig. 5. On the one hand, the phenomenon is related to the concentration of Er3+ ions. In the lower concentrations of Er3+ ions, Er3+–Er3+ distances are far apart and excitation-emission process is predominant. However, at

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(e)

0.045 0.040

NaEr (MoO4)2

0.030

0.010 0.005

2H9/2 405nm 4G11/2 379nm

0.015

4F3/2 4F5/2 450nm

0.020

4S3/2 551nm 2H11/2 525nm 4F7/2 490nm

0.025 2F9/2 660nm

α 2 hυ (eV)

0.035

2.4

2.8

3.2

336nm 3.69eV

0.000 1.6

2.0

3.6

4.0

Photon energy (eV) Fig. 7. PL spectra (a) of (1 x)NaAl(MoO4)2–xNaEr(MoO4)2 ceramics with x = 0.12 and 1.00 with 379 nm excitation. Excitation spectrum (b) of the ceramics with x = 0.12: emission wavelength is fixed at 405 nm. The bandgap estimations (c–e) depending of absorption spectra of NaAl(MoO4)2, the ceramics (x = 0.12) and NaEr(MoO4)2. The schematic diagram (f) of electron transitions in the ceramics and Er3+ ions to emit violet blue light. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

higher concentrations, the distances between two Er3+ ions become shorter, thus leading to the formation of Er3+ clusters [33]. As a result, non-radiation decay from concentration-quenching processes at higher concentrations may become predominant, resulting in a significant drop in the emission intensity. On the other hand, the phenomenon could have something to do with the symmetry degree of the host crystals. NaAl(MoO4)2 is monoclinic while NaEr(MoO4)2 is tetragonal structure. With increasing the value of x, the asymmetry degree decreases, while for the lower value of x, the effective number of Er3+ ions increases to give out more emission also. For the larger value of x, the effective number of Er3+ ions decreases to give out less emission. It suggests that Er3+ emissions have lower efficiency in symmetry crystal fields than in asymmetry crystal fields [34]. A comparison of the PL and UC emission spectra (see Figs. 4, 5 and 7–9) of Er3+ ions in the crystal fields of (1 x)NaAl(MoO4)2–

xNaEr(MoO4)2 ceramics with x = 0.04, 0.08, 0.12, 0.16, 0.20 and 1.00 shows similar emission spectra, though the excitation wavelengths are different (379, 532, 633 and 980 nm). All these measurements indicate that the starting meta-stable levels for photon emission are 4F9/2, 4S3/2, 2H11/2 and 2H9/2. For the excitation wavelength of 321 nm, only emissions about 372 and 379 nm are relatively more intense, which show their meta-stable levels for the emissions are 2G7/2, 2G9/2, 2K15/2 and 4G11/2. From these experiments, it is possible for us to draw the absorption of photon and excitations, non-radiation relaxation, and emission transitions of Er3+ ions in the ceramics by 321, 379, 532, 633 and 980 nm, which is shown in Fig. 10, similar with the emission transitions of Er3+ ions in single-phase crystal field reported previously [6]. However, for simplicity the transitions from the state of host lattice to that of Er3+ ions are not presented (because it has been mentioned above). The transition 4F9/2 ? 4I15/2 could involve

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Fig. 8. PL spectra (a) with excitation wavelengths of 254, 273, 276, 321 and 336 nm and excitation spectra (b and c) with emission wavelengths of 379 and 372 nm of (1 x)NaAl(MoO4)2–xNaEr(MoO4)2 ceramics with x = 0.12 and 1.00.

Fig. 10. Schematic diagram of energy levels and transition of excited Er3+ ions in the composite crystal fields of (1 x)NaAl(MoO4)2–xNaEr(MoO4)2. Fig. 9. The UC emission spectra of (1 x)NaAl(MoO4)2–xNaEr(MoO4)2 ceramics with x = 0.04, 0.08, 0.12, 0.16, 0.20 corresponding to a, b, c, d and e, respectively, excited with a 980 nm laser. The insert shows the intensity of UC emission at 553 nm with different magnitudes of x.

two groups of broad PL bands (around 650 and 680 nm), because each of those consists of multiplet emissions in PL and UC emission spectra. The same phenomenon has been observed for the transition 4S3/2 ? 4I15/2 (multiplet emissions around 545 and 553 nm). These could relate to sublevels of the ground state 4I15/2 and excited states 4F9/2 and 4S3/2.

5. Conclusions The synthesis, densities, thermal expansion and optical properties of (1 x)NaAl(MoO4)2–xNaEr(MoO4)2 ceramics with x = 0, 0.04, 0.08, 0.12, 0.16, 0.20 and 1.00 are studied to realize relatively strong emissions. It is found that the ceramics can efficiently emit near ultraviolet light (around 379 and 405 nm), green light (510– 560 nm) and relatively weak red light (630–680 nm). The energy transfer from host ceramics to Er3+ could also be concluded. It presents relatively strong up-conversion green light emissions and

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weak red light emissions as well when excited with a 980 nm laser. The properties make the ceramics interesting in applications of near ultraviolet and green light LEDs, UC materials. Acknowledgments This work was supported by the National Science Foundation of China (No. 10974183) and by the Education Commission of China (No. 20114101110003) and the fund for Science & Technology innovation team of Zhengzhou (2011-03). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jallcom.2013. 02.140. References [1] V. Mahalingam, F. Mangiarini, F. Vetrone, V. Venkatramu, M. Bettinelli, A. Speghini, J.A. Capobianco, J. Phys. Chem. B 112 (2008) 17745–17749. [2] P. Ghosh, J. Oliva, E. Rosa, K. Haldar, D. Solis, A. Patra, J. Phys. Chem. C 112 (2008) 9650–9658. [3] A. Luis, L. Menezes, C. Araújo, R. Gonçalves, S. Ribeiro, Y. Messaddeq, J. Appl. Phys. 107 (2010) 113508. [4] G. Tripathi, V.K. Rai, D.K. Rai, S.B. Rai, Spectrochim. Acta, Part A 66 (2007) 1307–1311. [5] R. Plugaru, J. Piqueras, E. Nogales, B. Méndez, J.A. García, T.J. Tate, J. Optoelectron. Adv. Mater. 4 (4) (2002) 883–892. [6] A. Patra, C. Friend, R. Kapoor, P. Prasad, J. Phys. Chem. B 106 (8) (2002) 1909– 1912. [7] I. Hyppänen, J. Hölsä, J. Kankare, M. Lastusaari, L. Pihlgren, T. Soukka, J. Fluoresc. 18 (2008) 1029–1034. [8] L. Díaz-Torres, E. Rosa-Cruz, P. Salas, C. Angeles-Chavez, J. Phys. D: Appl. Phys. 37 (18) (2004) 2489–2495.

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