Structure and luminescence properties of Eu 2+ doped Lu x Sr 2 (cid:2) x SiN x O 4 (cid:2) x phosphors evolved from chemical unit cosubstitution

The design scheme of the chemical unit cosubstitution of [Lu 3+ –N 3 (cid:2) ] for [Sr 2+ –O 2 (cid:2) ] in Sr 2 SiO 4 :Eu 2+ has been put into practice to discover the new phosphor systems with tunable luminescence properties, and the structures and photoluminescence tuning of yellow-emitting Lu x Sr 2 (cid:2) x SiN x O 4 (cid:2) x :Eu 2+ phosphors have been investigated. Crystal structures of Lu x Sr 2 (cid:2) x (cid:2) y SiN x O 4 (cid:2) x : y Eu 2+ samples were resolved using the Rietveld method, suggesting that the as-prepared Sr 2 SiO 4 belonged to monoclinic symmetry ( P 2 1 / n ) of b -phase Sr 2 SiO 4 , while Sr 1.97 Eu 0.03 SiO 4 and Sr 1.965 Eu 0.03 Lu 0.005 SiO 3.995 N 0.005 belonged to orthorhombic symmetry ( Pnma ) of a -Sr 2 SiO 4 . The emission peaks of Lu x Sr 1.97 (cid:2) x SiN x O 4 (cid:2) x :0.03Eu 2+ phosphors were red-shifted from 563 to 583 nm upon increasing the [Lu 3+ –N 3 (cid:2) ] substitution content from x = 0 to x = 0.005, furthermore, the PL emission peaks of Lu 0.005 Sr 1.965 (cid:2) y SiN 0.005 O 3.995 : y Eu 2+ also showed a red-shift from 583 nm to 595 nm with increasing Eu 2+ concentration ( y = 0.03, 0.07, 0.10 and 0.15), and their corresponding red-shift mechanism has been discussed. The temperature dependent luminescence results further verified that the introduction of [Lu 3+ –N 3 (cid:2) ] for [Sr 2+ –O 2 (cid:2) ] in Sr 2 SiO 4 :Eu 2+ can improve the thermal stability of the photoluminescence, which indicated that the Lu x Sr 2 (cid:2) x (cid:2) y SiN x O 4 (cid:2) x : y Eu 2+ phosphors have potential applications in white light-emitting diodes (wLEDs).


Introduction
The discovery of new inorganic materials is always a hot issue in solid-state materials chemistry and is an important objective highlighted by the ''Materials Genome Initiative''. 1,2 To establish suitable strategies discovering new host materials for advanced optical materials, many conventional or modified approaches have been developed, such as cation/anion replacement in the single site of the inorganic phases, 3,4 prototype substitution from an original ''old'' phase to find out a ''new'' one, 5,6 combinatorial chemistry screening via the phase diagram, 7,8 the single-particlediagnosis approach along with the advanced measurement devices, 9 and our recently reported chemical unit cosubstitution strategy for photoluminescence tuning, 10 and so on. These strategies used for the discovery of the new phosphors demonstrated great success, and many important phosphors have been reported and used finally. Nevertheless, increasingly demanded by newly developed optical materials and devices, phosphors with tunable optical properties are continuously pursued by chemists or material scientists.
Recently, the exploration of new materials for white lightemitting diodes (wLEDs) has become a hot issue. 11,12 Among them, silicate phosphors represented by Eu 2+ or Ce 3+ doped orthosilicates A 2 SiO 4 (A = Ca, Sr, Ba) have drawn much attention owing to their versatile polymorphs and chemical compositions, broad excitation/emission bands and tunable optical properties. 13,14 As we know, crystal site engineering can be used to tune the luminescence properties by changing the coordination environment for phosphors employing Ce 3+ or Eu 2+ ions characterized by d-f transitions. Therefore, the nitridation of the orthosilicate phosphors has demonstrated great potential. [15][16][17][18][19][20][21][22][23] For example, Sohn reported the Sr 2 SiO 4Àx N 2x/3 :Eu 2+ phosphors. 15 Gu reported the N doped Sr 2 SiO 4 :Eu 2+ phosphors. 16 Lee reported the (Sr,M) 2 Si(O 1Àx N x ) 4 :Eu 2+ (M: Mg 2+ , Ca 2+ , Ba 2+ ) phosphors. 17 Zhao found the red-emitting oxonitridosilicate phosphors Sr 2 SiN z O 4À1.5z : Eu 2+ . 18 Park reported the effects of N 3À , Eu 2+ , and Ca 2+ substitutions on the structural and luminescence properties of Sr 2 SiO 4 :Eu 2+ . 19 Ju reported the modification of the coordination environment of Eu 2+ in Sr 2 SiO 4 :Eu 2+ phosphors to achieve full color emission. 20 Li reported the luminescence properties and the crystal structure of a-Sr 2 Si 3x/4 O 2 N x :Eu 2+ phosphors with different concentrations of N 3À ions. 21 Tian reported the optical spectrum adjustment of yellow-green Sr 1.99 SiO 4À3x/2 N x :0.01Eu 2+ phosphor. 22 Recently, Black found the new LaSrSiO 3 N and LaBaSiO 3 N phases, and these compounds activated with Eu 2+ showed orange-red emission. 23 Such an example is in accord with our previously reported chemical unit cosubstitution strategy, 10 so that the [La 3+ -N 3À ] chemical unit can be used to cosubstitute for the [Sr 2+ /Ba 2+ -O 2À ] couple in the (Sr,Ba) 2 SiO 4 :Eu 2+ phosphors. This is also the main topic highlighted in the present paper, and it is believed that such a strategy can be successfully used to discover the new phosphor materials and tune the luminescence properties. Therefore, the usage and verification of the design scheme of chemical unit cosubstitution will be significant in discovering other new solid state materials. Herein, the chemical unit cosubstitution of [Lu 3+ -N 3À ] for [Sr 2+ -O 2À ] in Sr 2 SiO 4 :Eu 2+ has been proposed in this paper, and the yellowemitting Lu x Sr 2Àx SiN x O 4Àx :Eu 2+ phosphors with tunable photoluminescence have been studied. As also mentioned above, the nitridation of the orthosilicate phosphors brings us some opportunities to search for new phosphors. In this work, we studied the phase structure evolution and luminescence properties of yellow-emitting Lu x Sr 2ÀxÀy SiN x O 4Àx :yEu 2+ phosphors, and the observed spectral red-shift with increasing [Lu 3+ -N 3À ] and Eu 2+ content has been discussed in detail.

Experimental
The designed Lu x Sr 1.97ÀxÀy SiN x O 4Àx :yEu 2+ phosphors were synthesized by a conventional high temperature solid-state reaction. The starting materials were as follows: SrCO 3 (A.R.), SiO 2 (A.R.), Lu 2 O 3 (A.R.), Si 3 N 4 (99.99%), and Eu 2 O 3 (99.99%). After mixing and grinding in an agate mortar for 20 min, the mixture was placed in a crucible and then sintered at 1500 1C for 4 h in a H 2 (10%)/N 2 (90%) reducing atmosphere to produce the final samples. Finally, the prepared phosphors were cooled to room temperature and reground for further measurements.
The powder X-ray diffraction (XRD) measurements were performed on a D8 Advance diffractometer (Bruker Corporation, Germany) operating at 40 kV and 40 mA with Cu Ka radiation (l = 1.5406 Å). The scanning rate for phase identification was fixed at 41 min À1 with 2y ranges from 101 to 701 and the data for Rietveld analysis were collected in a step-scanning mode with the step size of 0.021 and 10 s counting time per step over the 2y range from 101 to 1201. The morphology and chemical compositions have been detected by using the scanning electron microscopy (SEM, JEOL JSM-6510A) and the attached energy disperse spectroscopy (EDS) techniques. The PL and photoluminescence excitation (PLE) spectra were recorded using a Hitachi F-4600 spectrophotometer (HITACHI, Tokyo, Japan) equipped with a 150 W xenon lamp as the excitation source. The temperature-dependent luminescence properties were also measured on the same spectrophotometer, and it was combined with a self-made heating attachment and a computer-controlled electric furnace (Tianjin Orient KOJI Co., Ltd, TAP-02). The decay curves were recorded on an instrument (Edinburgh Instruments Ltd, UK) (FLSP920) with an F900 flash lamp as the excitation source.

Results and discussion
The full range XRD patterns and the magnified range between 251 and 351 of as-prepared Sr 2 Fig. 1a and b, respectively. The standard data for a-Sr 2 SiO 4 (JCPDS 39-1256) and b-Sr 2 SiO 4 (JCPDS 38-271) are also shown as a reference. From Fig. 1a and b, we can see that the as-obtained Sr 2 SiO 4 and Lu 0.005 Sr 1.995 SiN 0.005 O 3.995 samples can be indexed to the b-Sr 2 SiO 4 phase ( JCPDS 38-271) suggesting that the introduction of the [Lu 3+ -N 3À ] chemical unit in a small amount (x = 0.005) will not affect the phase transition. Moreover, we can find that diffraction peaks of the as-prepared Lu 0.005 Sr 1.995 SiN 0.005 O 3.995 sample shift to the high-angle direction compared to that of the as-obtained Sr 2 SiO 4 phase, which verified that the Lu-N dopant can enter the structural framework of the b-Sr 2 SiO 4 phase leading to the shrinkage of the unit cell. However, the impurities appear when the contents of the [Lu 3+ -N 3À ] chemical unit exceed x = 0.005 from our experimental results. So that we select the content of x = 0.005 for the following discussion. Furthermore, Fig. 1c and d displays the full range XRD patterns and the magnified range between 251 and 351 for the as-prepared Lu 0.005 Sr 1.995Ày SiN 0.005 O 3.995 :yEu 2+ (y = 0.03, 0.05, 0.07, 0.10, 0.15 and 0.20) phosphors. The standard data for a-Sr 2 SiO 4 (JCPDS 39-1256) are shown as a reference. The six samples exhibit the same diffraction peaks, which can be indexed to the a-Sr 2 SiO 4 phase (JCPDS 39-1256), and no other polymorphs, or other impurities were detected. It is believed that the introduction of Eu 2+ on the Sr 2+ sites will induce the phase transition from the b-Sr 2 SiO 4 phase to the a-Sr 2 SiO 4 phase, so that Eu 2+ will further stabilize the a-Sr 2 SiO 4 phase, which agree with the previous report. 24 In order to further compare the difference of the phase structure evolution depending on the introduction of the Eu 2+ and [Lu 3+ -N 3À ] chemical unit, the detailed Rietveld structure analysis of the selected Sr 2 SiO 4 , Sr 1 Table 2, which verified the proposed chemical unit cosubstitution model. Therefore, the above results verified that the [Lu 3+ -N 3À ] chemical unit and the Eu 2+ ions entered into the crystal lattice of Sr 2 SiO 4 can maintain the same phase structures. In the present study, the chemical unit cosubstitution takes place at the cation and coordination anion's sites of the polyhedra simultaneously, not the central tetrahedral sites, which are different from our previous study. 10 The substitution on such a site will have the direct effect on the crystal field environment of doped Eu 2+ , so that we can possibly find the tunable photoluminescence even if the content of the substituted [Lu 3+ -N 3À ] chemical unit is relatively low. In order to clearly demonstrate the proposed chemical unit cosubstitution, the possible atoms'    Fig. 4. As shown in Fig. 4a, the sample has irregular morphology, and when we focused on one minor aggregate for the EDS analysis, we can find that all the elements can be detected, and the contents in the same order of magnitude are in agreement with the designed ones in the chemical formula.  Fig. 5a and b, respectively. The emission spectra consist of an asymmetric broad band  centered at around 570 nm, which is ascribed to the electric dipole allowing the transition of the Eu 2+ ions from the lowest level of the 5d excited state to the 4f ground state. 27 As shown in Fig. 5b (with increasing) [Lu 3+ -N 3À ] content, the emission peaks give a red-shift from 563 nm to 583 nm. Fig. 5c comparatively gives the typical images of Lu x Sr 1.97Àx SiN x O 4Àx :0.03Eu 2+ (x = 0, 0.0025, 0.00375 and 0.005) phosphors under a 365 nm UV lamp and natural light, respectively. We can clearly find the emission color evolution from yellow green to yellow with increasing [Lu 3+ -N 3À ] content for the cosubstitution of [Sr 2+ -O 2À ]. As we know, the N 3À is less electronegative and more polarizable than O 2À and its introduction in Sr 1.97 Eu 0.03 SiO 4 increases the covalent character of the bonds with the Sr 2+ /Eu 2+ , and we can infer that some of the Eu 2+ ions in the Lu x Sr 1.97Àx SiN x O 4Àx : 0.03Eu 2+ phosphor are coordinated with nitrogen, so that we can find the obvious red-shift behavior. Fig. 6a shows the as-measured PL and PLE spectra of We can see that the emission color evolves from light yellow to deep yellow with increasing Eu 2+ doping content. The main reason for this red-shift is an increase of crystal field splitting, which in turn decreases the 5d-4f transition energy. 28 The temperature dependence of the emission spectra and the relative PL intensity of Sr 1.97 SiO 4 :0.03Eu 2+ and Lu 0.005 Sr 1.965 -SiN 0.005 O 3.995 :0.03Eu 2+ were comparatively investigated and are given in Fig. 7a and b. Furthermore, as shown in Fig. 7c, the emission intensity at 100 1C is about 30% of that measured at room temperature for the Sr 1.97 SiO 4 :0.03Eu 2+ phosphor while this value is about 51% for Lu 0.005 Sr 1.965 SiN 0.005 O 3.995 :0.03Eu 2+ phosphor. The improved thermal stability depending on the introduced [Lu 3+ -N 3À ] chemical unit can be clearly found, which should be ascribed to the enhanced chemical bond effect with a rigid structure originating from the incorporated N atoms and the charge balance originating from such a cosubstitution. In order to better understand the temperature dependence of the luminescence, the activation energy DE can be calculated using the following equation: 29 where T is the temperature and c is a constant, k is the Boltzmann constant (8.629 Â 10 À5 eV K À1 ). I 0 and I T are the initial PL intensity of the phosphor at room temperature and Fig. 7 The temperature dependent emission spectra of Sr  Fig. 8. As depicted in Fig. 8a and where t is the time, I(t) is the luminescence intensity at time t, A 2 and A 1 are constants, and t 1 and t 2 are rapid and slow time for the exponential components. Then we can calculate the average lifetime t* by using the formula as follows: The calculated average decay lifetime values of Eu 2+ ions in Lu x Sr 1.97Àx SiN x O 4Àx :0.03Eu 2+ (x = 0, 0.00125, 0.0025, 0.00375, 0.005) are determined to be 6.98, 6.84, 6.66, 6.48 and 6.24 ms, respectively, depending on the different [Lu 3+ -N 3À ] contents. From Fig. 8b we can find that the lifetime values of Eu 2+ ions are determined to be 6.24, 5.23, 4.27, 3.31 and 1.67 ms for the Lu 0.005 Sr 1.965Ày SiN 0.005 O 3.995 :yEu 2+ phosphors with y = 0.03, 0.05, 0.07, 0.10 and 0.15, respectively. Obviously the decay time decreases gradually with increasing Eu 2+ concentration. It is proposed that the nonradiative and self-absorption rate of the internal doped ions evidently increases when activators cross the critical separation between the acceptor (quenching site) and the donor (activator ion) with increasing Eu 2+ concentration. 31,32 Fig . 9 shows the CIE chromaticity diagram of the select Lu x Sr 1.97Àx SiN x O 4Àx :0.03Eu 2+ and Lu 0.005 Sr 1.895 SiN 0.005 O 3.995 :0.10Eu 2+ phosphors (l ex = 365 nm). The CIE results are listed in Table 3. From Fig. 9, one can see that the emission colors of Lu x -Sr 2ÀxÀy SiN x O 4Àx :yEu 2+ phosphors can be shifted from yellowish green to deep yellow by the chemical unit cosubstitution of [Lu 3+ -N 3À ] and the increasing Eu 2+ doping concentration