Incommensurately Modulated Crystal Structure and Photoluminescence Properties of Eu2O3- and P2O5-Doped Ca2SiO4 Phosphor

We prepared four types of Eu2O3- and P2O5-doped Ca2SiO4 phosphors with different phase compositions but identical chemical composition, the chemical formula of which was (Ca1.950Eu3+0.013☐0.037)(Si0.940P0.060)O4 (☐ denotes vacancies in Ca sites). One of the phosphors was composed exclusively of the incommensurate (IC) phase with superspace group Pnma(0β0)00s and basic unit-cell dimensions of a = 0.68004(2) nm, b = 0.54481(2) nm, and c = 0.93956(3) nm (Z = 4). The crystal structure was made up of four types of β-Ca2SiO4-related layers with an interlayer. The incommensurate modulation with wavelength of 4.110 × b was induced by the long-range stacking order of these layers. When increasing the relative amount of the IC-phase with respect to the coexisting β-phase, the red light emission intensity, under excitation at 394 nm, steadily decreased to reach the minimum, at which the specimen was composed exclusively of the IC-phase. The coordination environments of Eu3+ ion in the crystal structures of β- and IC-phases might be closely related to the photoluminescence intensities of the phosphors.


Introduction
Rare-earth doped dicalcium silicate (Ca 2 SiO 4 , C 2 S) polymorphs have been the subject of extensive study because of the promising applicability of stabilized high-temperature modifications to the phosphor materials of white light-emitting LEDs [1][2][3][4][5][6][7][8]. The red light emission is characteristic of the Eu 3+ -activated phosphors, due to the transition of the 5 D 0 -7 F 2 for Eu 3+ ion [3,4]. The photoluminescence (PL) originates from the 4f-4f dipole transitions and, hence, the emission wavelengths are nearly the same among the various types of phosphors with different host materials. However, the PL intensity has been tunable for Eu 3+ -activated lithium tantalite-based phosphors by controlling the coordination environments of the Eu 3+ ion [9,10]. Accordingly, we speculated that the PL intensities could be also tunable for the Eu 3+ -activated C 2 S phosphors, depending on the polymorphs stabilized at ambient temperature.
The C 2 S polymorphs established so far are, in order of increasing temperature, γ (orthorhombic), β (monoclinic), α' L (orthorhombic), α' H (orthorhombic), and α (trigonal) [11]. The transformation temperatures during the heating process are 963 K for β-to-α' L , 1433 K for α' L -to-α' H , and 1698 K for α' H -to-α. The phase change from α' L to β on cooling has been reported to be thermoelastic martensitic [12]. Thus, the two phases coexisted at temperatures between the transformation starting temperature (=M s ) and finishing temperature (=M f ), both of which steadily decreased, even below ambient temperature (=T a ), with increasing concentration of dopants in C 2 S. The stabilized phases at T a systematically changed from β (T a < M f ), β + α' L (M f < T a < M s ) to α' L (M s < T a ) with increasing concentrations of foreign ions such as Sr 2+ or P 5+ [13,14]. In the crystal structure of β-Ca 2 SiO 4 there are two types of Ca sites. One (Ca1 site) is surrounded by seven O atoms, and the other (Ca2 site) has an eight-fold coordination [15]. When activated by Eu 3+ ions, the ions enter both Ca1 and Ca2 sites to generate 5 D 0 -7 F 0 emission peaks at 270 nm excitation [16].
In addition to the five types of polymorphs previously mentioned, an incommensurate (IC) phase has been reported to occur for the P 2 O 5 -doped C 2 S [17][18][19][20]. The modulation wavevector, N −1 × b*, has been along the b-axis of the underlying orthorhombic basic structure, where N is a noninteger. Saalfeld and Klaska prepared the IC-phase with N = 3.75, and determined the atom arrangements within the hypothetical supercell of 4 × b based on the space group Pnm2 1 [17]. Recently, the incommensurately modulated crystal structure of (Ca 1.88 Eu 2+ 0.01 0.11 )(Si 0.78 P 0.22 )O 4 (N = 3.649) [20], where denotes a vacancy in Ca site, has been determined, using a (3 + 1)-dimensional description based on the superspace formalism [21,22].
In the present study we have, for the first time, clarified the PL properties of the Eu 3+ -activated IC-phase phosphor, the chemical formula of which was (Ca 1.950 Eu 3+ 0.013 0.037 )(Si 0.940 P 0.060 )O 4 . The incommensurately modulated crystal structure was determined from the X-ray powder diffraction data using a (3 + 1)-dimensional description based on the superspace group Pnma(0β0)00s. The sintered specimen was subject to different heat treatments in air at 1473-1773 K. We demonstrated a close relationship between the phase composition (consisting of β, α' L , and IC) and PL intensity.

Materials and Methods
The materials were synthesized by solid-state reaction. A powder mixture with atom ratios of [Ca:Eu:Si:P] = [1.950:0.013:0.940:0.060] was prepared from reagent chemicals of CaCO 3 , SiO 2 , CaHPO 4 ·2H 2 O, and Eu 2 O 3 (>99.9% grade). Together with a small amount of acetone, it was thoroughly mixed in a planetary ball mill (Pulverisette P-6, Fritsch, Germany). The ZrO 2 balls, with 2 mm diameter, were used in the mill at a rotation speed of 400 rpm. The mixed specimens were pressed into pellets and heated at 498 K for 6 h, 973 K for 2 h, and then 1273 K for 8 h in air. The disc-shaped sintered specimens, the chemical formula of which corresponds to (Ca 1.950 Eu 3+ 0.013 0.037 )(Si 0.940 P 0.060 )O 4 , were subsequently annealed at four different temperatures (1473, 1573, 1673, and 1773 K) for 4 h in air, followed by quenching in water. The stable phases during the annealing processes were α at 1773 K and α' H at 1473-1673 K. We examined the specimens with a scanning electron microscope equipped with an energy-dispersive spectrometer to measure grain sizes and to confirm homogeneity.
Phase identification was made based on the X-ray diffraction (XRD) data (CuKα 1+2 ), which were obtained using an X-ray powder diffractometer (RINT 2500, Rigaku Co., Ltd., Japan) operated at 40 kV and 200 mA. The phase compositions, as well as the crystal structure of the IC-phase, were determined by the Rietveld method [23] from the X-ray profile intensity data (CuKα 1 ) collected at 298 K on another diffractometer (X'Pert PRO Alpha-1, PANalytical B.V., Almelo, the Netherlands), operated at 45 kV and 40 mA. We quantitatively determined the phase compositions of the samples using the phase-analysis method (2θ range of 25.0 • -43.0 • ) based on Brindley's procedure [24], the subroutine of which was implemented in the computer program RIETAN-FP [25] based on the structural parameters reported by Jost et al. [15] for the β-phase, Udagawa et al. [26] for the α' L -phase, and Saalfeld and Klaska [17] for the IC-phase having a supercell of 4 × b. We used the computer programs JANA2006 [27] for the detailed structural analysis of the IC-phase (2θ range of 5.010 • -147.981 • ), and VESTA [28] for the crystal structure drawing. Distortion parameters for the coordination polyhedra of the β-phase were determined using the computer program IVTON [29].
Excitation and emission spectra were obtained using a fluorescence spectrophotometer (F-7000, HITACHI, Japan). Figure 1 shows a series of XRD patterns of (Ca 1.950 Eu 3+ 0.013 0.037 )(Si 0.940 P 0.060 )O 4 phosphors annealed at different temperatures. For the specimen annealed at 1473 K, the phase composition was 70.7 mol % β and 29.3 mol % α' L ( Figure 2a). The specimen annealed at 1773 K was composed exclusively of the IC-phase, as evidenced from the satisfactory pattern fitting result (Figure 2b), although the structural model used was of the commensurate structure with the supercell of 4 × b. This implies that the modulation wavevector of the IC-phase should be close to 0.25 (=1/4) × b*. In fact, we found it to be 0.2433(2) × b* as discussed later, and hence there were slight positional shifts in Figure 2b from the proper positions for the weak reflections (e.g., 330 and 330 reflections with 2θ ≈ 31.05 • , and 530 and 530 reflections with 2θ ≈ 35.23 • ) ascribed to the superstructure. Based on the present structural model, the specimens annealed at 1573-1773 K were free from the α' L -phase. The relative amount of the IC-phase with respect to the β-phase steadily increased when the annealing temperature was increased from 1473 to 1773 K (Table 1).   We refined the incommensurately modulated structure starting from the initial structural model (superspace group Pnma(0β0)00s) that was equivalent to the crystal structure of (Ca 1.88 Eu 2+ 0.01 0.11 )(Si 0.78 P 0.22 )O 4 [20]. The refinement resulted in the satisfactory reliability indices of R(all) = 0.0326, Rw(all) = 0.0413, and S(all) = 2.99 ( Figure S1). The crystallographic data and the structural parameters are summarized in Tables 2 and 3, respectively. The basic structure contains two nonequivalent Ca sites, Ca1 and Ca2. In the average structure, the site occupation factors of both sites were fixed at 0.9750 for Ca, 0.0065 for Eu, and 0.0185 for vacancy. We tried to demonstrate the occupational modulation in all ranges of t (= x 4q × r, where x 4 is the fractional coordinate of the 4th direction in superspace description, q is the modulation wavevector, and r is the positional vector) at these sites to clarify the locations where the Eu 3+ ions concentrate. However, it remained unclear, because the bond valence sums are nearly the same between Ca 2+ and Eu 3+ . Figure 3 shows a partial structure of IC-(Ca 1 4 ] tetrahedra, with |Θ| < 5 • , is characteristic of the interlayer U. The atom arrangements of layer S and those of T, which are similar to the partial crystal structure of β-Ca 2 SiO 4 [15], are approximately related by a mirror plane perpendicular to the b-axis of Pnma(0β0)00s. The thicknesses of these layers correspond to one-and-a-half the d 100 -value of β-Ca 2 SiO 4 . Although it is very rare, there are S" and T" layers with a thickness almost twice the d 100 -value. A repetitive sequence of the bundled SUTU layer eventually constructs the commensurate structure having the supercell of 4 × b. The present incommensurately modulated structure with the wavelength of 4.110 × b is formed by the occasional replacements of S with S" and/or T with T" in the commensurate structure ( Figure S3). Since the modulated structures are definitely ordered ones, the connecting sequence of all these layers is strictly defined by the modulation function. In the crystal structure of IC-(Ca 1.88 Eu 2+ 0.01 0.11 )(Si 0.78 P 0.22 )O 4 with the modulation wavelength of 3.649 × b [20], there are two other layers of S' and T', the thicknesses of which are almost equal to the d 100 -value of β-Ca 2 SiO 4 . These layers have never been recognized in the present IC-phase.

Constituent Phases and Crystal Structure of IC-Phase
In previous studies, a negative linear correlation has been reported between the Nand P/(P+Si)-values for the P 2 O 5 -doped C 2 S [14,19].
The larger N-value for (Ca 1.950 Eu 3+ 0.013 0.037 )(Si 0.940 P 0.060 )O 4 (N = 4.110) than for (Ca 1.88 Eu 2+ 0.01 0.11 )(Si 0.78 P 0.22 )O 4 (N = 3.649) would be principally induced by the smaller P/(P+Si)-value of the former material as compared with the latter. Figure 4 shows the emission and excitation spectra of the four types of Eu 3+ -activated phosphors annealed at 1473-1773 K. Under excitation at 394 nm, the red light was emitted due to the 7 F 0 -5 L 6 intraconfigurational transition of Eu 3+ . The sharp and strong emission peaks appeared at around 594 nm ( 5 D 0 -7 F 1 , magnetic dipole), 625 nm ( 5 D 0 -7 F 2 , electric dipole), and 706 nm ( 5 D 0 -7 F 4 , electric dipole). In this paper, we controlled the phase compositions by annealing at different temperatures in order to investigate the relationship between the crystal structures and PL intensities. The PL intensity steadily decreased with increasing annealing temperature (Figure 4). This implied that the phase compositions of the Eu 3+ -activated phosphors readily affected the PL intensities. Actually, the intensity was highest for the specimen annealed at 1473 K, the constituent phases of which were β and α' L , as described previously. With increasing relative amount of the IC-phase with respect to the coexisting β-phase (Table 1), the PL intensity of the phosphor steadily decreased to reach the minimum, at which the specimen was composed exclusively of the IC-phase. As reported previously [8], the PL intensity has also been related to the relative amount of the α' L phase with respect to the β phase.

PL Properties Associated with Phase Compositions
In previous studies for the Eu 3+ -activated lithium tantalite-based phosphors, the centroid-to-cation distance (=eccentricity) of [(Li, Eu)O 12 ] polyhedra was associated with increased efficiency of the red light emission [9,10]. The larger the magnitude of eccentricity became, the stronger the PL intensity of the emission became. In a similar manner, the eccentricity of the Eu 3+ position in the crystal structure of P 2 O 5 -doped C 2 S phosphors could also be closely related to the PL intensity. We hypothesized that the eccentricity values of [(Ca, Eu)O n ] polyhedra (n is the coordination number) in IC-(Ca 1.950 Eu 3+ 0.013 0.037 )(Si 0.940 P 0.060 )O 4 might be smaller compared to those of the Eu 3+ -activated β-phase, which would eventually cause a much weaker PL intensity for the former than for the latter.
With β-Ca 2 SiO 4 , the Eu 3+ ions occupied both Ca1 and Ca2 sites [15].  Table 4. With Ca1 sites, the eccentricity values were larger for the β-phase (=0.044 nm) than for the IC-phase at t = 0.689 (=0.031 nm). Furthermore, the eccentricity value of the IC-phase at t = 0.189 (=0.009 nm) was much smaller than that of the β-phase (=0.022 nm) for the Ca2 sites. Accordingly, the Eu 3+ ions that contribute to the relatively weak red light emission intensity of the IC-phase could preferentially occupy those Ca sites. Phase parameter t is defined by t = x 4 − q × r, where x 4 is the fractional coordinate of the 4th direction in (3 + 1)-dimensional superspace description, q is the modulation wavevector, and r is the positional vector.

Conclusions
Four types of red-light-emitting phosphors, with different phase compositions but identical chemical composition, were prepared. A close relationship was suggested between the coordination environments of the Eu 3+ ion in the crystal structures of the βand IC-phases and their PL properties.

1.
The βand α' L -phases coexisted for the specimen annealed at 1473 K. With increasing annealing temperature, the relative amount of the IC-phase with respect to the β-phase steadily increased.
The specimen annealed at 1773 K was composed exclusively of the IC-phase.

2.
The incommensurately modulated crystal structure was determined using a (3 + 1)-dimensional description based on the superspace group Pnma(0β0)00s. It was composed of β-phase like layers (S, S", T, and T") and the interlayer (U). The S and S" layers included the [MO 4 ] tetrahedra (M = Si or P) tilted clockwise when viewed along [100]. Those of T and T" contained the tetrahedra tilted counterclockwise. The interlayer U was characterized by a slightly tilted tetrahedra. The incommensurate modulation, with modulation wavevector of 0.2433(2) × b*, was induced by the long-range stacking order of these layers.

3.
The PL intensity was the highest for the specimen consisting of both βand α' L -phases. As the relative amount of the IC-phase increased with respect to the coexisting β-phase, the PL intensity steadily decreased. The eccentricity of the Eu 3+ position in the crystal structures of the βand IC-phases could be closely related to the PL intensities.
Supplementary Materials: The following are available online at http://www.mdpi.com/1996-1944/13/1/58/s1, Figure S1: Comparison of the observed diffraction pattern of (Ca 1.950 Eu 3+ 0.013 0.037 )(Si 0.940 P 0.060 )O4 (symbol: +) with the corresponding calculated pattern (upper solid line). The difference curve is shown in the lower part of the diagram. Vertical bars indicate the positions of Bragg reflections. Figure S2: Tilting of the [MO 4 ] tetrahedron evaluated by the angle Θ. (a) The tilting angles Θ 1 (red line) and Θ 2 (blue line) with t. Because the differences between Θ 1 and Θ 2 in the whole regions of 0 ≤ t ≤ 1 are within 0.1 degree, the corresponding two curves are almost overlapped with each other. (b) Two types of tetrahedra showing the tilting angles Θ 1 and Θ 2 . (c) An average of the two angles (Θ 1 + Θ 2 )/2 with t. The tilting angles Θ 1 and Θ 2 in (a) are defined for two types of tetrahedra as illustrated in (b). As Θ 1 and Θ 2 are slightly different, an average of the two angles, Θ = (Θ 1 + Θ 2 )/2 in (c), is used as the tilting angle in discussion for simplicity. Phase parameter t is defined by t = x 4 − q × r, where x 4 is the fractional coordinate of the 4th direction in (3 + 1)-dimensional superspace description, q is the modulation wavevector, and r is the positional vector. Figure S3: (a) Projection of a partial structure along the a-axis. (b) Enlargement of the structure, as indicated by an arrow in (a), showing the replacement of layer S with layer S". (c) An example of the replacement of layer T with layer T.