07-09-2013, 04:55 PM
A green-yellow emitting oxyfluoride solid solution phosphor Sr2Ba(AlO4F)1Àx(SiO5)x:Ce3+ for thermally stable, high color rendition solid state white lighting
[attachment=57751]
ABSTRACT
A near-UV excited, oxyfluoride phosphor solid solution Sr1.975Ce0.025Ba(AlO4F)1Àx(SiO5)x has been
developed for solid state white lighting applications. An examination of the host lattice, and the local
structure around the Ce3+ activator ions through a combination of density functional theory,
synchrotron X-ray and neutron powder diffraction and total scattering, and electron paramagnetic
resonance, points to how chemical substitutions play a crucial role in tuning the optical properties of
the phosphor. The maximum emission wavelength can be tuned from green (lem 1⁄4 523 nm) to yellow
(lem 1⁄4 552 nm) by tuning the composition, x. Photoluminescent quantum yield is determined to be 70 Æ
5% for some of the examples in the series. Excellent thermal properties were found for the x 1⁄4 0.5
sample, with the photoluminescence intensity at 160 C only decreased to 82% of its room temperature
value. Phosphor-converted LED devices fabricated using an InGaN LED (lmax 1⁄4 400 nm) exhibit high
color rendering white light with Ra 1⁄4 70 and a correlated color temperature near 7000 K. The value of
Ra could be raised to 90 by the addition of a red component, and the correlated color temperature
lowered to near 4000 K.
Introduction
Advances in light emitting diode (LED) technologies,1,2 namely
the development of high efficiency blue and near-UV emitting
InGaN,3 has led to devices with greatly diminished energy
demands. The use of these solid state devices for white lighting in
place of conventional incandescent or fluorescent light sources
has become quite appealing for a wide range of applications.4
Solid state white lighting offers many advantages over traditional
lighting including longer lifetimes, reduced energy consumption,
and an environmentally friendly design without the need for
mercury.5
Experimental and computational methods
Samples of Sr1.975Ce0.025Ba(AlO4F)1Àx(SiO5)x (x 1⁄4 0.1, 0.2,
.0.9) were prepared by mixing stoichiometric amounts of
SrCO3 (99.5% purity, Materion), CeO2 (99.9% purity, Cerac),
BaCO3 (99.9% purity, Cerac), BaF2 (99% purity, Cerac), Al2O3
(high purity, sub-micron SM8, Baikowski) and SiO2 (amorphous
fumed, Degussa Aerosil TT600). Powders were intimately mixed
by grinding in an agate mortar with acetone for approximately 30
min, then pressed into pellets. Pellets were placed on sacrificial
powder in alumina crucibles and heated at 1350 C for 4 h in a
reducing atmosphere of 5% H2/95% N2 with heating and cooling
ramps of 10 h.
Density functional theory (DFT) calculations were performed
using the Vienna ab initio Simulation Package (VASP)23–26 in
order to predict site preferences for the Ce3+ ion in the host
oxyfluoride compound. Projector augmented waves (PAW)27,28
were employed to describe the atomic cores and the valence
electrons were described using plane-waves.
Results and discussion
Sr2BaAlO4F (SBAF) crystallizes in the tetragonal space group
I4/mcm (no. 140) while the nearly isostructural Sr3SiO5 (SSO)
crystallizes in the tetragonal space group P4/ncc (no. 130). In the
SBAF structure, Sr2+ ions reside in the 8h site, Ba2+ ions the 4a
site, Al3+ ions the 4b site, O2À ions the 16l site and FÀ ions the 4c
site.34 In the SSO structure, Sr2+ ions reside in the 8f and 4c sites,
Si4+ ions the 4b site and O2À ions the 16g and 4c sites.35 Fig. 1
portrays the structural differences between the two end members,
highlighted by the tilted SiO4 tetrahedra in the SSO structure
which lends this compound to the lower symmetry space group.
In both structures, there are two sites that Ce3+ can be
substituted into based on ionic size and coordination number,36,37
summarized in Table 1. The first is the 8h site in the SBAF
structure and the 8f site in the SSO structure, which is coordi-
nated with six oxygen and two fluorine atoms, in which the
slightly smaller Ce3+ ion can reside. The second is the 4a site in
the SBAF structure and the 4c site in the SSO structure, which is
coordinated with eight oxygen and two fluorine atoms, in which
the smaller Ce3+ ion can also reside.
Conclusions
In conclusion, a Ce3+-doped oxyfluoride solid solution phosphor
has been prepared from the Sr2BaAlO4F and Sr3SiO5 end
members. Sr1.975Ce0.025Ba(AlO4F)1Àx(SiO5)x phosphors have
easy color tunability, by changing the composition x, with near-
UV excitation and green/yellow emission wavelengths optimal
for solid state white lighting applications. With a highly efficient
photoluminescent quantum yield of 70% at room temperature
and photoluminescence intensity decreasing to only 82% of its
room temperature value at 160 C, this solid solution phosphor
exhibits impressive thermal stability.
[attachment=57751]
ABSTRACT
A near-UV excited, oxyfluoride phosphor solid solution Sr1.975Ce0.025Ba(AlO4F)1Àx(SiO5)x has been
developed for solid state white lighting applications. An examination of the host lattice, and the local
structure around the Ce3+ activator ions through a combination of density functional theory,
synchrotron X-ray and neutron powder diffraction and total scattering, and electron paramagnetic
resonance, points to how chemical substitutions play a crucial role in tuning the optical properties of
the phosphor. The maximum emission wavelength can be tuned from green (lem 1⁄4 523 nm) to yellow
(lem 1⁄4 552 nm) by tuning the composition, x. Photoluminescent quantum yield is determined to be 70 Æ
5% for some of the examples in the series. Excellent thermal properties were found for the x 1⁄4 0.5
sample, with the photoluminescence intensity at 160 C only decreased to 82% of its room temperature
value. Phosphor-converted LED devices fabricated using an InGaN LED (lmax 1⁄4 400 nm) exhibit high
color rendering white light with Ra 1⁄4 70 and a correlated color temperature near 7000 K. The value of
Ra could be raised to 90 by the addition of a red component, and the correlated color temperature
lowered to near 4000 K.
Introduction
Advances in light emitting diode (LED) technologies,1,2 namely
the development of high efficiency blue and near-UV emitting
InGaN,3 has led to devices with greatly diminished energy
demands. The use of these solid state devices for white lighting in
place of conventional incandescent or fluorescent light sources
has become quite appealing for a wide range of applications.4
Solid state white lighting offers many advantages over traditional
lighting including longer lifetimes, reduced energy consumption,
and an environmentally friendly design without the need for
mercury.5
Experimental and computational methods
Samples of Sr1.975Ce0.025Ba(AlO4F)1Àx(SiO5)x (x 1⁄4 0.1, 0.2,
.0.9) were prepared by mixing stoichiometric amounts of
SrCO3 (99.5% purity, Materion), CeO2 (99.9% purity, Cerac),
BaCO3 (99.9% purity, Cerac), BaF2 (99% purity, Cerac), Al2O3
(high purity, sub-micron SM8, Baikowski) and SiO2 (amorphous
fumed, Degussa Aerosil TT600). Powders were intimately mixed
by grinding in an agate mortar with acetone for approximately 30
min, then pressed into pellets. Pellets were placed on sacrificial
powder in alumina crucibles and heated at 1350 C for 4 h in a
reducing atmosphere of 5% H2/95% N2 with heating and cooling
ramps of 10 h.
Density functional theory (DFT) calculations were performed
using the Vienna ab initio Simulation Package (VASP)23–26 in
order to predict site preferences for the Ce3+ ion in the host
oxyfluoride compound. Projector augmented waves (PAW)27,28
were employed to describe the atomic cores and the valence
electrons were described using plane-waves.
Results and discussion
Sr2BaAlO4F (SBAF) crystallizes in the tetragonal space group
I4/mcm (no. 140) while the nearly isostructural Sr3SiO5 (SSO)
crystallizes in the tetragonal space group P4/ncc (no. 130). In the
SBAF structure, Sr2+ ions reside in the 8h site, Ba2+ ions the 4a
site, Al3+ ions the 4b site, O2À ions the 16l site and FÀ ions the 4c
site.34 In the SSO structure, Sr2+ ions reside in the 8f and 4c sites,
Si4+ ions the 4b site and O2À ions the 16g and 4c sites.35 Fig. 1
portrays the structural differences between the two end members,
highlighted by the tilted SiO4 tetrahedra in the SSO structure
which lends this compound to the lower symmetry space group.
In both structures, there are two sites that Ce3+ can be
substituted into based on ionic size and coordination number,36,37
summarized in Table 1. The first is the 8h site in the SBAF
structure and the 8f site in the SSO structure, which is coordi-
nated with six oxygen and two fluorine atoms, in which the
slightly smaller Ce3+ ion can reside. The second is the 4a site in
the SBAF structure and the 4c site in the SSO structure, which is
coordinated with eight oxygen and two fluorine atoms, in which
the smaller Ce3+ ion can also reside.
Conclusions
In conclusion, a Ce3+-doped oxyfluoride solid solution phosphor
has been prepared from the Sr2BaAlO4F and Sr3SiO5 end
members. Sr1.975Ce0.025Ba(AlO4F)1Àx(SiO5)x phosphors have
easy color tunability, by changing the composition x, with near-
UV excitation and green/yellow emission wavelengths optimal
for solid state white lighting applications. With a highly efficient
photoluminescent quantum yield of 70% at room temperature
and photoluminescence intensity decreasing to only 82% of its
room temperature value at 160 C, this solid solution phosphor
exhibits impressive thermal stability.