Luminescence of a new class of UV–blue-emitting phosphorsMSi2O22dN2+2/3d:Ce3+ (M 5 Ca, Sr, Ba){

Y. Q. Li,* G. de With and H. T. Hintzen

Received 1st June 2005, Accepted 11th August 2005

First published as an Advance Article on the web 7th September 2005

DOI: 10.1039/b507735d

The luminescence properties of Ce3+,Na+-codoped MSi2O22dN2+2/3d (M 5 Ca, Sr, Ba) are reported.

The undoped and Ce3+,Na+-codoped MSi2O22dN2+2/3d powders were prepared by a solid-state

reaction at temperatures between 1300–1400 uC under N2–H2 (10%) atmosphere in the system

MO–SiO2–Si3N4 (M 5 Ca, Sr, Ba). MSi2O22dN2+2/3d (M 5 Ca, Sr, Ba) crystallizes in the

monoclinic system with different crystal structures. For excitation in the 300–360 nm range,

MSi2O22dN2+2/3d:Ce3+ shows typical broad emission bands peaking at about 392, 473 and 396 nm

for M 5 Ca, Sr and Ba, respectively. In particular, CaSi2O22dN2+2/3d:Ce3+ shows an unusual short-

wavelength emission (y392 nm) with a very small Stokes shift of 2200 cm21; BaSi2O2N2:Ce3+ shows

an interesting white-light emission in the visible range 350–600 nm for excitation at 365 nm.

1. Introduction

The Ce3+ ion has a 4f1 electronic ground state configuration. The

luminescence of the Ce3+ ion originates from a transition from

the lowest 5d level to the ground states which is split by spin–

orbit coupling into two components, 2F5/2 and 2F7/2, separated

by y2000 cm21.1 Since the position of the lowest 5d levels is

strongly influenced by the local coordination, the emission

wavelength of Ce3+ varies with different host lattices from UV to

the visible range corresponding to the emission colors from blue

to red.1 In oxide host lattices, the emission of Ce3+ generally is

located in the UV to blue (300–500 nm) spectral range.1 An

exception is the yellow-emitting YAG:Ce3+ due to its large

crystal field splitting.1,2 A large crystal field splitting can also be

realized by N32 replacement of O22. In addition, nitride-based

host lattices provide more covalent bonding (like in sulfides)

resulting in the 5d band shifting to lower energy.1,3–5 Indeed,

long-wavelength emission in Ce3+-doped rare-earth-(oxy)nitride

and alkaline-earth silicon nitride materials is observed.4,6

In comparison with oxides, nitride and oxynitride-based

materials can give some surprises not only in structure (like an

unusual motif) but also in physical characteristics which are

reflected by their unique mechanical, electrical, thermal and

optical properties.7–13 Definitely, the nitrogen atom is believed

to play a key role due to its high formal charge and large

covalent character in nitride-based materials.7–9

In the system M–Si–O–N (M 5 Ca, Sr, Ba), alkaline-earth

silicon oxynitride compounds with composition MSi2O2N2

(M 5 Ca, Sr, Ba) are known.14–18 This kind of oxynitride is of

interest for luminescent materials because its composition is

situated between the oxide compound M2SiO4 and the pure

nitride compound M2Si5N8 (M 5 Ca, Sr, Ba). Eu2+-doped

M2SiO4 phosphor materials are well-known green (M 5 Ca,

Ba) and yellow (M 5 Sr) emitting phosphors,19–22 while

M2Si5N8:Eu2+ (M 5 Ca, Sr, Ba) is a new family of red-emitting

phosphors showing excellent luminescence properties for

white-light LED applications.23,24 Recently, we have reported

on Eu2+-doped MSi2O22dN2+2/3d phosphor materials with

yellow (M 5 Ca), green to yellow (M 5 Sr) and blue-green

(M 5 Ba) emission colors,17 which are also promising

candidates for use as conversion phosphors for white-light

LED applications.25 In contrast, the luminescence properties

of Ce3+-activated alkaline-earth silicates in the BaO–SrO–SiO2

system have been reported in an earlier work which revealed

that M2SiO4:Ce3+ and MSiO3:Ce3+ (M 5 Sr, Ba) exhibited a

peak emission wavelength at about 390 nm with slight

variations resulting from compositional changes.26 Most

recently, we have reported the luminescence properties of

Ce3+-activated M2Si5N8 (M 5 Ca, Sr, Ba) using Li or Na as a

charge compensator.6 Especially, Sr2Si5N8:Ce3+ turns out to be

a very attractive green-emitting phosphor for use in white-light

LEDs owing to its high conversion efficiency in the UV blue

range (370–450 nm). These peculiar behaviors inspired us to

extend our study to the Ce3+-doped MSi2O22dN2+2/3d system

(M 5 Ca, Sr, Ba). In this study, undoped and Ce3+-doped

MSi2O22dN2+2/3d (M 5 Ca, Sr, Ba) compounds were

synthesized by a solid-state reaction using Na+ as charge-

compensator. Furthermore, new X-ray powder diffraction

data and the lattice parameters of MSi2O22dN2+2/3d (M 5 Ca,

Sr) are presented as we have found that previous studies on

these compounds are imprecise.15,16 Finally, the unconven-

tional luminescence properties of MSi2O22dN2+2/3d:Ce3+

(M 5 Ca, Sr, Ba) are reported.

2. Experimental

2.1. Synthesis

All powder samples of undoped and Ce3+,Na+-codoped

MSi2O22dN2+2/3d (M 5 Ca, Sr, Ba) were prepared by a solid-

state reaction at high temperatures using Na+ as a charge com-

pensator. As found in our previous study,17 the approximate d

Laboratory of Materials and Interface Chemistry, Eindhoven Universityof Technology, P.O. Box 513, 5600 MB, Eindhoven, The Netherlands{ Electronic supplementary information (ESI) available: measured andcalculated XRD powder diffraction data for MSi2O22dN2+2/3d

(M 5 Ca, Sr). See http://dx.doi.org/10.1039/b507735d

PAPER www.rsc.org/materials | Journal of Materials Chemistry

4492 | J. Mater. Chem., 2005, 15, 4492–4496 This journal is � The Royal Society of Chemistry 2005

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value to obtain single-phase compounds is about 1 for M 5 Sr

and close to 0 for M 5 Ca and Ba.

The starting materials were high-purity MCO3 (M 5 Ca, Sr,

Ba) (Merck, .99.0%), SiO2 (Aerosil OX 50, Degussa), Si3N4

(SKW Trostberg, b content: 23.3%, O y0.7%), CeO2 (Rhone-

Poulenc, 99.95%) and NaF (Merck, .99.0%). The Ce concen-

trations in the MSi2O22dN2+2/3d host lattices are 1 mol% for

M 5 Ca and Sr and 1–3 mol% for M 5 Ba with respective to the

M ions (i.e. x 5 0.01 or 0.03 in M122xCexNaxSi2O22dN2+2/3d).

Appropriate amounts of the starting materials were homo-

geneously wet-mixed using a planetary ball mill for 4–5 hours in

isopropanol. After mixing the slurry was dried and ground in an

agate mortar. Subsequently, the dried powder mixtures were

fired in molybdenum or alumina crucibles at 1300–1400 uC for

6–12 h under a reducing atmosphere of N2–H2 (10%) in a

horizontal tube furnace. After firing, the samples were

cooled to room temperature in the furnace and were ground

again with an agate mortar for further measurements.

2.2. X-Ray powder diffraction

All obtained samples were checked by X-ray powder diffrac-

tion (Rigaku, D/MAX-B) using Cu-Ka radiation at 40 kV and

30 mA with a graphite monochromator. The data were collected

on powder samples using a step scan mode with a step size of

0.02u and a counting time of 10 second per step in the range 2h

10 to 90u. In order to avoid the preferential particle orientation

of the obtained samples, the powder samples were mounted

into a flat plate holder by the side filling method.

For M 5 Ca, Sr, the crystal system of MSi2O22dN2+2/3d was

determined from the X-ray powder patterns using the indexing

program DICVOL0427 using the first 20 lines. The possible

space groups were determined according to the systematic

absences.

2.3. Optical measurements

The diffuse reflection, excitation and emission spectra were

measured at room temperature by a Perkin Elmer LS 50B

spectrophotometer equipped with a Xe flash lamp. The

reflection spectra were calibrated with the reflection of black

felt (reflection 3%) and white barium sulfate (BaSO4, reflection

y100%) in the wavelength region of 230–700 nm. The

excitation and emission slits were set at 2.5 nm. Excitation

spectra were automatically corrected for the variation in the

lamp intensity by a second photomultiplier and a beam-

splitter. The emission spectra were corrected by dividing the

measured emission intensity by the ratio of the observed

spectrum of a calibrated W lamp and its known spectrum from

300 to 900 nm. All the spectra were measured with a scan

speed of 100 nm min21. The optical absorption edge is

estimated by the wavelength value at which the reflection

intensity is halfway between the lowest and highest values of

the overall reflection intensity.

3. Results and discussion

3.1. X-Ray powder diffraction data of MSi2O22dN2+2/3d

MSi2O22dN2+2/3d (M 5 Ca, d # 0; and M 5 Sr, d # 1)

compounds were successfully indexed on the monoclinic

systems with the lattice parameters: CaSi2O2N2, a 5

15.035(4) A, b 5 15.450(1) A, c 5 6.851(2) A, b 5 95.26(2)u;SrSi2ON8/3, a 5 11.320(4) A, b 5 14.107(6) A, c 5 7.736(4) A,

b 5 91.87(3)u. The calculated XRD data based on the crystal

structure reported for CaSi2O2N218 do not fit as well our mea-

sured data in this work. XRD full-pattern simulations support

our proposed cell for polycrystalline CaSi2O22dN2+2/3d.17

This difference is probably ascribed to the exact composition

of the obtained compounds (i.e. different d value in

CaSi2O22dN2+2/3d) arising from the different starting materials

and synthetic approaches,17,18 possibly resulting in two

modifications as found in the case of SrSi2O2N2.15 A newly

found compound BaSi2O2N217 also crystallizes in a monoclinic

cell having different structure with CaSi2O2N2 and SrSi2ON8/3,

the obtained lattice parameters of MSi2O22dN2+2/3d (M 5 Ca,

Sr, Ba) are listed in Table 1. The measured and calculated

XRD powder diffraction data for MSi2O22dN2+2/3d (M 5 Ca,

Sr) are available in as ESI.{

3.2. Optical properties

3.2.1. Diffuse reflection. The observed daylight color is grey–

white for undoped MSi2O22dN2+2/3d (M 5 Ca, Sr, Ba) in

agreement with the measured diffuse reflection spectra which

show that only light in the UV range (i.e. ,300 nm) is

absorbed (Fig. 1). From the diffuse reflection spectra the

optical absorption edge of MSi2O22dN2+2/3d is estimated to be

about 270 nm (4.6 eV), 273 nm (4.55 eV) and 242 nm (5.13 eV)

for M 5 Ca, Sr, Ba, respectively. The drop in the reflection

curve represents the host lattice absorption from the valence to

conduction band. BaSi2O2N2 show a much steeper drop

starting from 275 nm (Fig. 1). In the UV-blue to visible range,

the reflection of CaSi2O22dN2+2/3d and BaSi2O2N2 is higher

(.80%) than that of SrSi2O22dN2+2/3d (,60%).

For all Ce3+-doped MSi2O22dN2+2/3d materials, only one

obvious absorption band centered at about 336, 355 and

308 nm for M 5 Ca, Sr, Ba, respectively, can be seen. In

addition, the absorption intensity of Ce3+ in CaSi2O22dN2+2/3d

and BaSi2O2N2 is stronger than that in nitrogen-richer

SrSi2O22dN2+2/3d, possibly related to the amount of Ce

incorporated. Actually, more often we have found lower

Ce3+ absorption in a nitrogen-richer environment. Typical

examples are M2Si5N8:Ce3+,Li+(Na+) and BaSi7N10:Ce3+,

Li+(Na+), where both the absorption and the luminescence

intensity of the oxygen-poor materials are lower than those of

Table 1 Lattice parameters of MSi2O22dN2+2/3d (M 5 Ca, Sr, Ba)

FormulaCaSi2O2N2

(d # 0)SrSi2ON8/3

(d # 1)BaSi2O2N2

(d 5 0)Crystal system Monoclinic Monoclinic MonoclinicSpace group P21/c P21/m P2/mLattice constants

a/A 15.035(4) 11.320(4) 14.070(4)b/A 15.450(1) 14.107(6) 7.276(2)c/A 6.851(2) 7.736(4) 13.181(3)b/u 95.26(3) 91.87(3) 107.74(6)V/A3 1584.53 1234.67 1285.23

Figures-of-meritM(20) 10.5 10.8 10.3F(20) 15.7(0.0088, 144) 14.9(0.0090, 150) 15.4(0.0095, 137)

Reference This work This work 17

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slightly oxygen-richer materials (i.e. the former using b-Si3N4

instead of a-Si3N4 as a starting material which is well known

to contain more oxygen).28 Similar observations were made

for Eu2+-doped M2Si5N8 (M 5 Ca, Sr, Ba) and BaSi7N10

materials.28

3.2.2. Luminescence of MSi2O22dN2+2/3d:Ce3+. Fig. 2 shows

the excitation and emission spectra of M0.98Ce0.01Na0.01-

Si2O22dN2+2/3d (M 5 Ca, Sr, Ba). For all materials, the

excitation band at short wavelength in the range of 230–250 nm

can be readily assigned to host lattice excitation as indicated

by their reflection spectra (see Fig. 1). Surprisingly, only one

excitation band obvious for Ce3+ can be observed peaking

at about 336, 366 and 308 nm for M 5 Ca, Sr and Ba,

respectively, in fair agreement with the obtained diffuse reflec-

tion spectra (Table 2). In the case of SrSi2O22dN2+2/3d:Ce3+, a

weak shoulder at long wavelength around 410 nm can be

observed, probably originating from a second phase similar to

what we found in SrSi2O22dN2+2/3d:Eu2+.17

In general, the 5d levels of Ce3+ can be split into at most five

different crystal-field components.1 The above observation

suggests that the excitation bands of Ce3+ in MSi2O22dN2+2/3d

are seriously overlapping (although the bandwidth is small)

and/or that some of them may be located in the conduction

band of the host lattice. M0.98Ce0.01Na0.01Si2O22dN2+2/3d

shows a typical broad emission band with maxima at about

392, 473 and 396 nm for M 5 Ca, Sr and Ba, respectively

(Fig. 3), located in the UV–blue spectral range (Table 2). In the

case of CaSi2O22dN2+2/3d:Ce3+ and BaSi2O2N2:Ce3+, the broad

emission band actually contains three subbands (Fig. 2a and c)

at about 368, 378 and 392 nm for the Ca; and 369, 380 and

396 nm for the Ba compound. Because the energy difference

between these subbands significantly deviates from the normal

Fig. 1 Diffuse reflection spectra of undoped (solid line) and 1 mol%

Ce3+,Na+-codoped (dashed line) MSi2O22dN2+2/3d: (a) M 5 Ca, (b)

M 5 Sr, (c) M 5 Ba.

Fig. 2 Excitation (solid line) and emission spectra (dashed line) of

M0.98Ce0.01Na0.01Si2O22dN2+2/3d: (a) M 5 Ca, (b) M 5 Sr, (c) M 5 Ba.

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value between the ground states of 2F5/2 and 2F7/2

(y2000 cm21),1 these subbands suggest the presence of

multi-emission centers of the Ce3+ ions. This hypothesis is

confirmed by the fact that the shape (for M 5 Ca) and position

(for M 5 Ba) of the emission subbands can be changed

by varying the excitation wavelength (Fig. 3). In agreement

with this observation, for CaSi2O2N2 the large number of six

crystallographic Ca sites in a unit cell was reported,18 which

could also apply to BaSi2O2N2.

As mentioned before, from the compositional point of

view, MSi2O22dN2+2/3d apparently lies between alkaline-earth

silicates and alkaline-earth silicon nitrides. Normally, a lower

energy 5d Ce3+ excitation band together with a longer

wavelength emission band is expected due to a highly covalent

bonding and a large crystal field splitting in the nitride or

oxynitride compounds.4 Therefore, the local coordination

around M ions in MSi2O22dN2+2/3d can be probed by

luminescent ions, such as Ce3+ and Eu2+. With respect to the

luminescence properties, both the excitation and emission

spectra of MSi2O22dN2+2/3d:Ce3+ are more like those of Ce3+-

doped alkaline-earth silicates26 rather than those of

M2Si5N8:Ce3+.6 For example, for MSiO3:Ce3+ (M 5 Sr, Ba)

and M2SiO4:Ce3+ (M 5 Sr, Ba), the main excitation band is

around 300–335 nm and the emission band is around 390 nm,26

while for M2Si5N8:Ce3+ the principle excitation band is

around 400 nm and the emission band is found at about

470–560 nm depending on the type of M.6 For Eu2+-doped

MSi2O22dN2+2/3d (M 5 Ca, Sr, Ba),17 the luminescence

properties are also close to those of Eu2+-doped alkaline-earth

silicates19–22 while significantly different from M2Si5N8:Eu2+

(M 5 Ca, Sr, Ba).23,24 Therefore it can be concluded that the

M ions in MSi2O22dN2+2/3d are dominantly coordinated by

O atoms, in agreement with the structure elucidation of

CaSi2O2N2, which is a layer silicon oxynitride in which Ca2+

ions are connected by six O atoms and one N atom in

the range of 2.28–2.79 A.18 In addition, based on the fact

that these alkaline-earth silicates consist of layers (i.e. SrSiO3),

chains (i.e. BaSiO3) and isolated [SiO4] tetrahedra,29

MSi2O22dN2+2/3d (M 5 Sr, Ba) is possibly also composed of

layers of [Si(O,N)4] tetrahedral groups similar to the reported

CaSi2O2N2 structure.18

The estimated Stokes shifts are about 2200, 6500 and

5000 cm21 for M 5 Ca, Sr and Ba, respectively (Table 2).

These results are completely contrary to what we have found

for Eu-doped MSi2O22dN2+2/3d (M 5 Ca, Sr, Ba), where

BaSi2O2N2:Eu2+ has the smallest Stokes shift y1700 cm21

while CaSi2O22dN2+2/3d:Eu2+ has a significantly larger Stokes

shift (y5100 cm21).17 Exactly similar opposing trends for Ce3+

and Eu2+ were also found for M2SiO4, i.e., 2900 (M 5 Sr) vs.

4800 cm21 (M 5 Ba) for Ce3+;26 and 5500–6000 (M 5 Sr) vs.

5000 cm21 (M 5 Ba) for Eu2+.19–21 In general, with the ionic

radius of M increasing going from Ca, Sr to Ba the Stokes shift

is expected to decrease for isostructural compounds as we

indeed have found in MYSi4N7:Ce3+ (M 5 Sr, Ba)30,31 with

homovalent Ce3+/Y3+substitution and M2Si5N8:Ce3+,Li+(Na+)

(M 5 Sr, Ba)6 with heterovalent Ce3+/M2+substitution. The

reason for the deviation of the Ca . Sr . Ba sequence

evidently is the fact that the MSi2O22dN2+2/3d compounds have

different crystal structures. In the case of different behaviors of

the Ce3+ and Eu2+ ions, this is possibly related to their different

site preferences as influenced by charge compensation neces-

sary for Ce3+ in contrast to Eu2+ in MSi2O22dN2+2/3d,

eventually resulting in significantly different trends in lumines-

cence properties.

Finally, it is worth noting that CaSi2O22dN2+2/3d:Ce3+ is a

high potential UV–blue-emitting phosphor material with high

efficiency and low thermal quenching (i.e. a small Stokes shift).

With respective to Ce3+-doped BaSi2O2N2, first, when the Ce

concentration increases from 1 to 3 mol% both the excitation

and emission bands show significant shifts to long-wavelength

(Fig. 4). As there is no significant change in the Stokes

shift (,200 cm21), this red-shift is mainly attributed to the

BaSi2O2N2 lattice shrinkage caused by the replacement of the

large Ba2+ ion (1.35 A, CN 5 6) by the smaller Ce3+ (1.01 A,

CN 5 6) and Na+ (1.02 A, CN 5 6) ions.32 Correspondingly,

the BaCe–O(N) distances become shorter which leads to the

increase in the crystal-field splitting. As a consequence, the

lowest 5d level shifts to lower energy. Second, an attractive

feature of BaSi2O2N2:Ce3+ is that it shows white light for

excitation under 365 nm, especially for high Ce concentrations.

As far as we know, no such studies have been reported. Just

Table 2 Optical properties of MSi2O22dN2+2/3d:Ce3+,Na+ (1 mol%) (M 5 Ca, Sr, Ba)

M d Absorption band/nm 5d excitation band/nm Emission band/nm CIE coordinates (x, y) Stokes shift/cm21

Ca 0 336 336 392 (0.165, 0.061) y2200Sr 1 355 366 473 (0.197, 0.263) y6500Ba 0 308 308 396 (0.161, 0.096) y5000

Fig. 3 Color coordinates deduced from the emission band of

M0.98Ce0.01Na0.01Si2O22dN2+2/3d, (m) M 5 Ca, (&) M 5 Sr, ($)

M 5 Ba (lexc 5 337, 364, 308 nm for M 5 Ca, Sr and Ba, respectively).

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using a single activator within a host lattice to generate

white light is rather unique, but it is indeed realized in

BaSi2O2N2:Ce3+.

4. Conclusions

A new class of UV-blue-emitting phosphor materials

MSi2O22dN2+2/3d:Ce3+ (M 5 Ca, Sr, Ba) has been

found. X-Ray powder diffraction analysis showed that

MSi2O22dN2+2/3d crystallized in the monoclinic system with

different crystal structures. Ce3+-doped MSi2O22dN2+2/3d

shows UV–blue emission with maxima at about 392, 473

and 396 nm for M 5 Ca, Sr and Ba, respectively, under

excitation in the UV range (300–360 nm). Unexpectedly,

CaSi2O22dN2+2/3d:Ce3+ emits light at very high energies for

nitride-based materials, ascribed to predominantly coordina-

tion with oxygen atoms combined with a small Stokes shift due

to a rigid lattice. For BaSi2O2N2:Ce3+, with increasing the Ce

concentration both excitation and emission bands show a red

shift and it can emit white light when excited with 365 nm light.

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Fig. 4 Diffuse reflection (dotted line), excitation (solid line) and

emission (dashed line) spectra of Ba0.94Ce0.03Na0.03Si2O2N2.

4496 | J. Mater. Chem., 2005, 15, 4492–4496 This journal is � The Royal Society of Chemistry 2005

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