Abstract
Single-phase 
-doped 
red phosphor was obtained by nitridation of 
alloy powder under the high pressure of 
in a hot isostatic pressing apparatus at 
. The unit cell volumes of the single phase expanded linearly with the increase in value of 
from 0.2 to 0.8. A blueshift of the red emission peak of 
from 650 to 
was observed by weakening the crystal field strength around the 
ion. 
, with the optimum Eu concentration, showed high photoluminescence intensity over the whole range of 
value from 0.2 to 0.8. Unlike nitrides of alkaline-earth or rare-earth metals, the intermetallic alloy powder was stable under ambient conditions and was thus suitable as starting material for red phosphors for industrial production. In 
samples synthesized at 
, the same structure as 
, space group 36 
, was retained at unit cell volumes up to 
, exhibiting the maximum solubility of 
in the solid solution, 
.
White light-emitting diodes (LEDs) have attracted attention as a light source due to their many advantages in energy efficiency, long lifetime, reduced service costs, compactness, and environmentally sound features over conventional incandescent and fluorescent lamps. Recently, rapid improvements in 
-based blue LEDs have enabled the production of white LEDs which are competitive with fluorescent lamps in efficiency, evolving innovations in solid-state lighting. However, most conventional white LEDs consist of blue LEDs and yttrium aluminum garnet 
yellow-emitting phosphors1 and are not suitable for lighting that requires a higher color rendering index (CRI) because of the lack of red light.
Several approaches have been proposed to resolve this problem. First, in addition to a single yellow-emitting 
phosphor, a red-emitting phosphor is combined with blue LEDs.2 Second, green- and red-emitting phosphors instead of the yellow-emitting 
phosphor are further combined with blue LEDs.3 Third, blue-, green-, and red-emitting phosphors are combined with near-ultraviolet (nUV) LEDs.4 However, well-known red- and green-emitting phosphors do not provide viable solutions due to their lack of stability against moisture and the excitation band in the region of LED emission.
Recently, red-emitting nitride phosphors of 
and 
have been reported.5–7 Both have an unusual longer-wavelength emission of 
and wider absorption bands in the UV-visible range. In particular, 
, which has many advantages, including high resistance to most chemicals, temperature quenching, and mechanical strength attributable to its rigid crystal structure, seems most promising for applications in which saturated red is required. To improve the photoluminescence (PL) intensity, several approaches, including the use of alloys as precursors, have been reported.8, 9 For lighting applications with the aim of both high color rendering and efficiency, deep-red emission is desired to shift to shorter wavelengths, without loss of quantum output. For this purpose, to weaken the crystal field strength around 
, substitution of 
, which has a larger ionic radius than 
in the 
site of 
, seemed promising. However, sublimation of Sr occurred readily only by introducing Sr into the starting mixtures. As a result, the unit cell did not expand linearly with increasing Sr content of the starting mixtures, 
was formed as an impurity phase, and a decrease in red-emission intensity was observed. Therefore, we have proposed a synthesis method using alloys as starting materials under high pressure to produce a single phase of 
-doped 
phosphor, optimized for LED lighting with high color rendering and high efficiency. Here, we investigated the range of the solid solution, solubility of 
by substitution of 
to 
, and the influence of the substitution on the luminescence properties.
Experimental
Precursor preparation
Alloy method
Alloy powders, 
, were used as precursors for the preparation of 
phosphors. The alloys were prepared by arc-melting of strontium (99.5%), calcium (99.5%), aluminum (99.999%), silicon (99.999%), and europium (99.99%) metals to form uniform ingots under an Ar-gas atmosphere. The chemical composition of each alloy was confirmed by inductively coupled plasma (ICP). The alloy ingots were milled to a fine powder with an average particle diameter of 
under an atmosphere of 
gas.
Nitride mixture method
, 
, and 
are high-purity chemicals and were used without purification. Sr and Eu nitrides were prepared by firing of metallic Sr and Eu in 
, as described previously.5 The stoichiometric mixture of nitrides of Sr, Ca, Eu, 
, and 
was mixed in a glove box filled with purified 
.
Nitridation
The alloy powder was transferred to a boron-nitride crucible and loaded into a hot isostatic pressing (HIP) apparatus. The 
pressure was increased to 
at room temperature after filling the apparatus with 
, followed by evacuation, and then raised to 
by the following heating process. The alloy powder was heated to 
and held at that temperature for 
, and then the temperature was reduced to around 
. The pressure was simultaneously decreased to around 
by the cooling process and then reduced to atmospheric pressure by venting. Finally, the sample was cooled to room temperature. After nitridation, the sample was finely ground. 
was prepared at 
(gauge basis) from the conventional mixture of nitrides as reported previously.7 Postsynthesis treatments, for example, milling, classification, and washing, were performed to obtain the dispersed phosphors with an average size of 
.
In order to investigate the effects of 
pressure in the nitridation process, the precursors were calcined at 0.92 and 
(gauge basis) under a stream of nitrogen gas.
Analysis and evaluation
Powder X-ray diffraction (XRD) patterns were measured with PW1700 (PANanalytical, 
) at 
and 
for phase identification in continuous-scan mode (
in 
). Lattice-parameter determination was performed by a nonlinear, least-squares method from XRD data collected with an X'Pert Pro MPD (PANanalytical, 
) at 
and 
between 5 and 
.
Elemental analysis of the precursor alloy and phosphors was performed using ICP and an O, N analyzer (LECO). The compositions of the particles, both of alloy and phosphor, were confirmed by scanning electron microscopy (SEM) energy-dispersive X-ray analysis (EDX) (model S-3400N equipped with EMAX ENERGY; Hitachi and Horiba, respectively).
PL spectra at room temperature were measured with an optical multichannel analyzer (model C7041; Hamamatsu Photonics) equipped with a Xe lamp. The wavelength resolution was 
. CIE color coordinates were calculated by multiplication of the relative luminescence with the spectral tristimulus values at each wavelength between 480 and 
. Excitation spectra were measured with a spectrofluorometer (model F-4500; Hitachi).
The temperature dependence of PL was measured with an MCPD7000 (Otsuka Electronics) equipped with temperature-controlled sample holders and a Xe lamp.
Results and Discussion
Phase formation of alloy precursors
Figure 1 shows the XRD patterns of the 
alloy samples.
Figure 1. XRD patterns of 
alloy precursors 
.
The measured patterns indicated that all samples were single phase and had the same structure as 
with 
-type structure.10 The shift of diffraction peaks toward lower angles with increasing Sr content exhibited unit-cell expansion. As shown in Fig. 2, the unit-cell volume increased with increasing occupancy of Sr in the alkaline-earth site, indicating the atomic-level dispersion of the elements. The homogeneity of the alloy precursors was confirmed by SEM-EDX of the alloy powder, as well as ICP elemental analysis and XRD phase identification of several parts of the ingots. All components could be dispersed at the atomic level using the alloy precursors.
Figure 2. Dependence of the unit-cell volume of alloy precursors on Sr content 
.
The alloy ingots were stable in air even after milling under an atmosphere of 
. The oxygen contents of the fine powder (average diameter: 
) were below 
after exposure to air and 
even after exposure to 100% relative humidity air at room temperature for 
. In contrast, fine powders of alkaline-earth nitrides, the source materials for the conventional synthesis method, decomposed completely within a few minutes to hydroxides under ambient conditions. High stability under ambient conditions is one of the advantages of the use of alloy precursors for synthesis of pure nitrides.
Optimization of 
synthetic method
The effects of 
pressure in the calcination process were examined using the mixture of nitrides as a precursor. As shown in Fig. 3, a near-single phase of 
(the calculated pattern is shown in Fig. 4d) was obtained at 
, whereas substantial amounts of 
and 
were observed at 
. The PL intensity of 
synthesized at 
was 83% of optimized 
(0.8%), while the PL intensity of the phosphor synthesized at 
was 70%.
Figure 3. XRD patterns of 
phosphors synthesized at 190 and 
from nitride mixtures.
Figure 4. XRD patterns of 
doped 
phosphors synthesized from alloy precursors at 
: (a) 
, (b) 0.5, and (c) 0.8 observed; (d) 0.8 calculated.
To elucidate the effects of 
pressure on phase formation, it seems helpful to present a discussion regarding thermodynamics. The nominal decomposition reaction of 
to 
is expressed as shown in Eq. 1
However, in the reaction conditions, at 
, nitrides of alkaline-earth elements are readily decomposed into elemental Sr and 
at low pressure, because all strontium nitrides are assumed to be decomposed below 
, according to the carrier gas hot extraction studies.11 Thus, at 
, the decomposition of 
to 
is expressed as
pressure has a significant influence on phase formation because the residual dependence of 
free energy on pressure is expressed as Eq. 3, regarding 
as an ideal gas
To estimate the influence of 
pressures of 190 and 
at 
on the reaction expressed by Eq. 3, the residual free-energy value was estimated. The value, 
, had a significant effect on the phase formation expressed by Eq. 2. The chemical potential of Sr can depend on the partial pressure of Sr vapor. However, according to observations, Sr vapor diffuses outside the crucible. Therefore, under the experimental conditions used, it is difficult to estimate the effect of the change in the chemical potential of Sr.
In summary, the observed remarkable effect of 
pressure can be explained by the increase in 
free energy, 
, when 
pressure was increased from 0.92 to 
at 
.
Greater homogeneity and reactivity of the precursor were considered effective in improving the PL intensity, because the calcination temperature and 
pressure were sufficiently high. In addition, residue of 
or other stable nitrides, such as 
, seems to be inevitable when the conventional nitride mixture method is applied, because the estimated free-energy change of formation12 showed that 
or Sr-rich SCASN is relatively thermodynamically unfavorable as compared with 
.
As shown in Fig. 4, using the alloy as a precursor, a single phase of 
was obtained at 
. All of the major XRD peaks were assigned to CASN structure, not to 
, 
or other nitrides, because the observed pattern agreed with the calculated pattern shown in Fig. 4d. The shift in peak position toward lower angles with increasing Sr content indicated the formation of solid solution, 
. The PL intensity of the single-phase 
phosphor was further improved by 17% using the alloy method, up to the level of optimum 
(0.8%). This improvement was attributed to the formation of the single-phase, well-crystallized 
. The composition of the phosphor particles (SEM image shown in Fig. 5) was confirmed by SEM-EDX. As shown in Table I, judging from the average values and standard deviation, the composition of the particles agreed with the nominal composition, 
, except for Eu, the concentration of which was below the limit of detection.
Figure 5. SEM image of 
particles.
Table I. Chemical composition of particles of the 
phosphor synthesized at 
by the alloy method (atomic ratio, SEM-EDX, 10 particles).
|
|
|
| |
|---|---|---|---|---|
| Average | 1.058 | 0.190 | 0.781 | 0.971 |
| Standard deviation | 0.032 | 0.012 | 0.030 | 0.035 |
| Minimum | 1.017 | 0.174 | 0.730 | 0.919 |
| Maximum | 1.114 | 0.220 | 0.822 | 1.041 |
As shown in Table II, the 
atomic ratio determined by ICP was 0.72 for the phosphor synthesized at 
, whereas the ratio was decreased to 0.67 and 0.50 for phosphors synthesized at 0.92 and 
, respectively. The large discrepancy in 
ratio between the source mixture and calcined phosphor was caused by the sublimation of Sr at 0.92 and 
as the reaction expressed by Eq. 2. 
and 
appeared as dominant phases especially below 
, as expected from Eq. 2. In contrast, using HIP, due to the higher pressure, the decomposition of Sr-rich SCASN expressed as Eq. 2 or the sublimation of volatile compound was suppressed,13 and higher Sr substitution in the Ca site of the CASN structure was achieved. In addition, the 
atomic ratio was reduced by 75% in the alloy method as compared to the conventional nitride-mixture method. Thus, the advantage of the alloy-precursor method combined with high-pressure synthesis over the conventional method consists of increased homogeneity and reactivity as well as lower oxygen content, enabling the synthesis of relatively thermodynamically unfavorable nitrides free from contamination of the competing phases.
Table II. Overall chemical composition of 
phosphors (atomic ratio, 
, ICP).
| Pressure (MPa) | Precursor | Al | Si | Ca | Sr | Eu | N | O |
|---|---|---|---|---|---|---|---|---|
| 190 | Alloy | 1 | 1.00 | 0.20 | 0.72 | 0.008 | 2.81 | 0.10 |
| 0.92 | Nitrides | 1 | 1.13 | 0.18 | 0.67 | 0.01 | 2.84 | 0.43 |
| 0.01 | Alloy | 1 | 0.89 | 0.30 | 0.51 | 0.01 | 2.93 | 0.10 |
Structure and unit-cell volume of
Figure 4 shows the XRD patterns of the 
-doped 
samples synthesized at 
. The measured patterns indicated that all samples had the same structure as 
with an orthorhombic structure. The changes in XRD peak position indicated increases in the lattice constants due to the increase in the occupancy of Sr, which has a larger ionic radius14 than Ca, in the alkaline-earth element site. As shown in Fig. 10, the unit-cell volume of the 
sample increased up to 
, comparable to the unit-cell volume of previously reported6 
-20%-doped 
, as expected from the comparable ionic radii14 of 
and 
. The unit-cell volume increased linearly with increasing Sr substitution, up to 
. The cell constants, the values of 
and 
, increased linearly with increasing Sr substitution, whereas the value of 
did not increase linearly. The unusual dependency on the degree of substitution can probably be explained by tilting of the chain of 
tetrahedra.15 To elucidate the reasons for the unusual changes in the cell constant, closer examination of the crystal structure16 is currently in progress in our laboratory.
Figure 10. Dependence of emission peaking and unit-cell volume on Sr content 
for 
phosphors (excitation at 
).
The observed XRD pattern of 
phase (Fig. 4c) showed good agreement with the calculated pattern (Fig. 4d) based on the previous report7 (atomic coordinates, isotropic displacement parameters of CASN) and the cell parameters of 
(Table III). The calculated pattern showed that the observed splitting of some diffraction peaks in Sr-rich SCASN was attributable to structural modification caused by Sr substitution for Ca, not to any impurity phases other than CASN structure.
Table III. Lattice constants of 
-doped 
phosphors.
|
 (nm) |
 (nm) |
 (nm) |
|
|---|---|---|---|---|
| 0.2 | 0.98184(3) | 0.56668(2) | 0.50839(1) | 282.860(1) |
| 0.5 | 0.98269(3) | 0.56880(2) | 0.51080(1) | 285.51(1) |
| 0.8 | 0.98152(1) | 0.57365(1) | 0.51491(1) | 289.922(7) |
Thus, in 
samples synthesized at 
with the same structure as CASN, space group 36 
was retained at unit-cell volumes up to 
, exhibiting the maximum solubility of 
in the solid solution, 
.
PL properties of
Figure 6 shows the PL and PL excitation (PLE) spectra of 
[
] and 
for comparison. The PL peak was observed at 627 and 
, respectively. The blueshift of emission was attributable to the decrease in crystal field strength by unit-cell expansion. Excitation spectrum of 
was compared to 
in Fig. 6. 
showed broad excitation bands extending into the visible region like 
.
Figure 6. Photoluminescence-PL and photoluminescence excitation-PLE spectra: (Solid line) 
and (dotted line) 
.
In addition to the shift in the PL spectra, a shift in the excitation edge around 
to a shorter wavelength 
was observed. The overall shape of the excitation bands of 
, which is similar to 
, suggested that the local environment around 
is analogous to that in 
, except for the change in crystal field strength.
Figure 7 shows the dependency of PL intensity on Eu content for 
. The maximum intensity was achieved between 0.6 and 
of Eu. Changes in the peak wavelength in this range of Eu concentration were within 
. Thus, to adjust the peak wavelength with maximum PL intensity, it seems more advantageous to change the Sr content than Eu. As shown in Fig. 8, by changing the 
value from 0.2 to 0.9 in 
, the PL peak wavelength was changed from 650 to 
, while the PL peak intensity was retained. The change in the PL peak causes a marked change in emission color, as indicated by the shift in CIE color coordinates in Fig. 9. The peak wavelength can be tailored by Sr content, retaining the maximum quantum output at the optimum Eu content. As shown in Fig. 10, there was a clear correlation between PL peak wavelength and unit-cell volume of the phosphors, indicating that the blueshift of emission was attributable to the decrease in crystal field strength due to unit-cell expansion.
Figure 7. Dependence of PL intensity on Eu concentration for 
(excitation at 
).
Figure 8. Emission spectra of 
phosphors (excitation at 
).
Figure 9. CIE color coordinates varied with Sr content for 
phosphors (excitation at 
). The figures indicate Sr occupancy, 
.
The temperature dependence of the PL intensity above room temperature is shown in Fig. 11 for 
and 
compared to 
(Kasei Optonix, P46-Y3). The thermal quenching was small for 
, comparable to 
. Stokes shifts, estimated as described in a previous report6 based on the PL and PLE spectra, were 
for 
and 
for 
. Both were smaller than the values reported previously for 
, 
, 
, and 
, respec tively.5 The magnitude of the Stokes shifts of 
, which was larger than that of 
, seemed contradictory with the observation that the thermal properties of 
are comparable to those of 
. Furthermore, the larger host-lattice metal site usually leads to smaller Stokes shifts by preventing shrinking of a luminescent center caused by excitation.17 Therefore, further investigations are required to determine the local environment of 
in 
; both the bond length and symmetry of 
coordination should be studied intensively. The observations presented here may be explained by the improved symmetry of the 
coordination due to substitution of 
by 
, which has a larger ionic radius than 
; the substitution probably reduced the difference in the 
bond lengths in the CASN structure and improved the symmetry of the 
coordination. In 
structure, the coordination is distorted; the longest, the shortest, and the differences in the 
bond lengths are 0.2405(3), 0.2586(5), and 
, respectively, regarding the 
site as five-coordinated.6 Further studies of the relationships between detailed structure and excitation spectra, as well as temperature properties, based on the crystal structure refinements17 will be helpful in further improving our understanding and lead to improved phosphors.
Figure 11. Temperature dependence of PL intensity (excitation at 
).
Figure 12 shows the spectrum of the white LED composed of 
blue LED 
, green 
,18 and red 
phosphors. In comparison with the white LED containing only yellow 
phosphor, the white LED containing both 
and 
exhibited improved color-rendering features, with improvement of the average CRI 
from 80 to 90. In comparison with the white LED containing 
and 
, the power efficiency was improved by 14%, without loss of 
, due to the high quantum output and optimized red-emission spectrum for lighting applications.
Figure 12. Spectrum of the white LED composed of 
blue LED 
and green 
and red 
phosphors.
In addition, 
is highly stable; the PL intensity and crystal structure did not change after the aging tests at 298, 358, and 
for 
in ambient conditions. 
is also insoluble in water or dilute acids; the PL intensity and crystal structure were fully retained after soaking in boiling water or dilute 
hydrochloric acid for at least 
.
Conclusions
A single-phase 
-doped 
red phosphor was obtained for the first time by nitridation of 
alloy powder under high 
pressure 
and at 
. Unlike nitrides of alkaline-earth or rare-earth metals, the intermetallic alloy powder was stable under ambient conditions and suitable for industrial production. Nitridation of these starting materials at lower pressures of 0.92 and 
resulted in sublimation of Sr, formation of 
impurity phase, and decreases in PL intensity. Under optimized synthesis conditions, at 
and 
, increasing the 
value resulted in unit-cell expansion as well as a blueshift of emission from 650 to 
due to the decrease in crystal field strength. At the optimum Eu concentration of 
, high PL intensity was retained throughout the 
value range. Due to the outstanding temperature characteristics, high external quantum efficiency, tuned emission characteristics, and the broad excitation band, ranging from UV to blue matching the emission of 
-based LEDs, the red phosphor is one of the most promising candidates for red-emitting conversion phosphors for white and warm white LED lighting applications.
In the future, we will report on the investigation of detailed changes in the cell parameters, the local environment of 
in 
, and synthesis of 
phase, 
.
Acknowledgments
The authors are grateful to Hideaki Kaneda for fabrication of the white LED and to Chihiro Yoshida and Masahiro Yamamoto for help with the postsynthesis treatments and PL measurements of the phosphor. The authors also thank Dr. Youji Arita for his helpful advice, Dr. Masayoshi Mikami for discussion on thermodynamics, and Dr. Kyota Uheda for his careful reading of this manuscript.
Mitsubishi Chemical Group Science and Technology Research Center Incorporated assisted in meeting the publication costs of this article.