Studies on terbium doped apatite phosphors prepared by precipitation under microwave conditions

Article abstract of DOI:10.1016/j.jallcom.2018.04.274

In this study we present our first attempts to obtain silicate apatite phosphors by precipitation with NaOH under microwave conditions. Y and La precursors fired at different temperatures (800 °C÷1400 °C) led to phosphors with hexagonal apatite structure

Full text of DOI:10.1016/j.jallcom.2018.04.274

Journal of Alloys and Compounds 755 (2018) 135e146
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: http://www.elsevier.com/locate/jalcom
Studies on terbium doped apatite phosphors prepared by precipitation
under microwave conditions
,
L.E. Muresan a *, I. Perhaita a, D. Prodan a, G. Borodi b
a Babes- Bolyai University, Raluca Ripan Institute for Research in Chemistry, Fantanele 30, 400294, Cluj-Napoca, Romania
b National Institute for Research and Development of Isotopic and Molecular Technologies, 65-103 Donath, 400293, Cluj Napoca, Romania
a r t i c l e i n f o
a b s t r a c t
Article history:
In this study we present our first attempts to obtain silicate apatite phosphors by precipitation with
NaOH under microwave conditions. Y and La precursors fired at different temperatures (800 ꢀC÷1400 ꢀC)
led to phosphors with hexagonal apatite structure belonging to the space group P63/m and with crys-
tallites size up to 105 nm. The structure and chemical composition of samples were evaluated based on
FTIR, XRD and ICP-OES while the morphology, surface state and porosity were revealed using SEM and
BET investigations. The impact of the firing temperature on the purity phase, crystalline order degree,
crystallites size and luminescent properties of the apatite phosphors are discussed. It was found that the
nature of rare earth (Y3þ or La3þ) same as the incorporation degree of activator (Tb3þ) is responsible for
the apatites green - turquoise emission. ICP-OES measurements show that the Tb3þ amount incorporated
in the apatite lattice increase with firing temperature and is lower in La-phosphors compared with Y-
phosphors. The 5D4e7F5 transition observed in the green spectral domain was dominant in Y- phosphors
while 5D3-7F6,5,4 transitions from blue spectral region was dominant in La-phosphors. The chromatic
coordinates shifts from green to turquoise with firing temperature and depends on the excitation ra-
diation, indicating the applicative potential of these phosphors in display screens for solid-state lighting
applications.
Received 7 March 2018
Received in revised form
23 April 2018
Accepted 24 April 2018
Available online 25 April 2018
Keywords:
Apatite
Luminescence
Precipitation
Microwave
© 2018 Elsevier B.V. All rights reserved.
1. Introduction
cationic substitution either at A site or X site of which we mention:
NaY9(SiO4)6O2 [8], Ca5La5(SiO4)3(PO4)3O2 [9], Sr3LaNa(PO4)2SiO4
Compounds belonging to the apatite structural family are more
and more researched as effective host lattice due to their chemical
and thermal stability. Apatite compounds crystallizes in hexagonal
system (space group P63/m, Z ¼ 2) with general chemical formula
of A10[XO4]6Z2. In this formulation, A represents divalent cations
(Ca2þ, Sr2þ, Ba2þ, Mg2þ); XO4 is a tetrahedral group (where X ¼ P, V,
As, Si, Ge) and Z can be a variety of anions (Clꢁ, Fꢁ, O2ꢁ, OHꢁ) [1e3].
Rare earth (La3þ, Y3þ, Gd3þ) or alkali metal ions (Naþ, Kþ, Liþ) also
can occupy the A site with isomorphic substitution [4e6]. With
such a wide range of substitutional possibilities, there is a great
potential to design new phosphors and to optimize their lumi-
nescent properties. The substitution of elements in the apatite
structure, usually alter the coordination environments of the acti-
vators, changing the photoluminescent properties [7]. Recently, a
large variety of new apatite compositions have been obtained by
[10] or Sr2La8(SiO4)6O2 [11]. The doping of apatite host lattices with
rare earths (Eu, Tb, Ce, Dy) and/or transitional ions (Bi, Mn)
generate luminescence that can be changed from red [12], blue [13],
green [14] to pinks, turquoises or whites tones [15,16]. As a result,
apatite type phosphors become good candidates in display devices
and white-light emitting diodes (WLEDs) for solid-state lighting
applications [16,17].
Preparation of micro or nanostructured apatite materials to
fulfill the technical requirements imposed by the applicative
domain (i.e. high crystallinity degree, phase purity, spherical
morphology, uniform particle size, high luminescence) is not an
easy task. A rigorous control of the synthesis parameters (i.e. pH,
reagents concentrations, reaction time, temperature etc) is needed
in order to obtain phosphors with desired composition and tar-
geted properties [17e19]. Moreover, choosing an appropriate
method can significantly determine the morphology, structure and
optical properties of material. For example, microrod, microfiber
and microbelt like apatites were prepared by sol-gel or electro-
spinning [9,20].
* Corresponding author.
E-mail address: laura_muresan2003@yahoo.com (L.E. Muresan).
https://doi.org/10.1016/j.jallcom.2018.04.274
0925-8388/© 2018 Elsevier B.V. All rights reserved.

136
L.E. Muresan et al. / Journal of Alloys and Compounds 755 (2018) 135e146
The most commonly used method for the synthesis of micro-
sized apatites (~5e20 m) is the solid state reaction even if it re-
heating, higher yields and reduced secondary reactions.
m
The novelty of this study brings together the advantages of both
microwave irradiation and simultaneous addition of reagents with
a controlled flow during the precipitation which is important with
respect to the quality of the precursor and the final product. New
aspects regarding the effect of firing temperature, nature of rare
earth (Y3þ, La3þ) and incorporation degree of activator (Tb3þ) on
morpho - structural and luminescent properties of phosphors were
discussed in detail.
quires high temperature (>1500 ꢀC), long processing time (>10 h),
reducing firing atmosphere, repeated milling and washing stages
[21e23]. In addition, this method does not allow controlling the
uniformity of the particles and the phase purity of the final product.
Several other low-temperature synthesis methods were suggested
as alternatives to high-temperature solid state reaction, such as:
precipitation, hydrothermal, Pechini, or solegel [24e27]. The
literature survey revealed limited information regarding the syn-
thesis of silicate apatite phosphors by precipitation technique. Most
of the studies are focused on silicate apatites as electrolytic material
in solid oxide fuel cell (SOFC) [28e30]. For example, Yang and co-
authors describes the synthesis of un-doped lanthanum silicates
by co-precipitation with ammonia in presence of polyethylene
glycol as dispersant to prevent particle agglomeration [28]. A car-
bonate precipitation combined with an azeotropic distillation
process was reported by Yao and co-workers [29]. Recently, a novel
co-precipitation method, based on chemical reaction between
La(NO3)3 and Na2SiO3 was reported to synthesize apatite type
lanthanum silicates for SOFC [30]. Literature does not report the use
of microwave for the synthesis of silicate apatite phosphors
although the use of microwave radiation for synthesis of inorganic
compounds increased rapidly over the past years. Several inter-
esting reviews on microwave-assisted synthesis of materials have
been published [31e33].
2. Material and methods
2.1. Materials
The synthesis of yttrium silicate apatite phosphors was done by
precipitation under microwave conditions. The starting materials
used for the phosphor synthesis are: nitric acid (65% Alfa Aesar);
yttrium oxide (99.9%, Alfa Aesar), lanthanum oxide (99.9%, Alfa
Aesar), calcium nitrate tetrahydrate (99.0%, Alfa Aesar), tetra ethi-
lortosilicate (97% Alfa Aesar) as components for host lattice,
terbium nitrate penta-hydrate (99.998%, Alfa Aesar) as activator
sources; and sodium hydroxide (98%, Lachema) as precipitating
agent. The amount of reactants was calculated to obtain phosphors
with general formula Ca2RE7.88Tb0.12(SiO4)6O2 (RE ¼ Y or La).
2.2. Synthesis procedure
In this paper, our aim is to synthesis, for the first time, apatite
type phosphors using the microwave irradiation during the entire
process of precipitation (formation of precipitate and maturation)
in order to bring improvements on the structural and luminescent
characteristics of apatite powders. Microwave irradiation can have
a positive impact on the morpho - structural and luminescent
characteristics of the apatite, providing high heating rates, selective
The synthesis of apatite phosphors was accomplished through
three stages namely: (1) precipitation of precursors, (2) thermal
treatment of precursors and (3) conditioning of the fired mixture.
For start, metallic nitrate solution was prepared by dissolving
proper amounts of Y2O3 or La2O3 in nitric acid followed by addition
of calcium and terbium nitrates salts in order to achieve a molar
Fig. 1. FTIR spectra of CYSO (a) and CLSO (b) samples fired at different firing temperatures.

L.E. Muresan et al. / Journal of Alloys and Compounds 755 (2018) 135e146
137
ratio between metallic ions (Ca-Y/La-Tb) of 2: 7.88: 0.12.
to asymmetric stretching in NOꢁ3 group; * ~ 1000 cmꢁ1, 1024 cmꢁ1
related to Si-O-Si asymmetric stretching [34]; *1488 cmꢁ1
The first step of synthesis involves separate channelling and
simultaneously supplying of equal volumes (50 ml) of metallic ni-
trates and NaOH solutions into a bottom diluted solution of metal
salts (150 ml, 0.002 M) mechanical stirred. The addition of reactants
(heated at 80 ꢀC) was done under controlled flow (10 ml/min) using
an HEIDOLPH 5201 peristaltic pump. Precipitation takes place in a
MILESTONE microwave oven programmed at 1000 W at 80 ꢀC for
60 min. The white precipitate was allowed to mature under mi-
crowave for one hour to complete the reaction, separated by
centrifugation, washed three times with double distilled water.
Proper amount of TEOS, according to stoichiometry, was added and
homogenized to the precipitate and let at 80 ꢀC under stirring until
the gel was formed. The final precursor was dried at 110 ꢀC for 4 h.
For this study, two sample series with the same amount of
terbium (1.5% mol Tb/mol REþTb) were prepared. Yttrium was used
for the first series of samples, (coded CYSO) and, lanthanum for the
second series (coded CLSO). The thermal treatment of the pre-
cursors was performed at different temperatures (800 ꢀC, 1100 ꢀC,
1200 ꢀC, 1300 ꢀC and 1400 ꢀC) for 4 h in air. After firing, the ma-
terials were washed, soft milled, sieved and dried.
,
1524 cmꢁ1 due to asymmetric stretching of C-O from CO23ꢁ group;
*~3450 cmꢁ1, ~1647 cmꢁ1 attributed to H-O stretching and bending
from atmosphere absorbed water; *626 cmꢁ1 due to Y-O-H bending
and 653 cmꢁ1 from La-O-H vibration found in Y(OH)3 and La(OH)3
respectively. However, small differences are observed between FTIR
spectra of Y and La precursors that refer to the presence of a sharp
intense vibration line at 3610 cmꢁ1 due to the stretching and
bending OeH vibrations of lanthanum hydroxide [35]. Identifica-
tion of nitrates vibration suggests the presence of un-reacted
metallic nitrates due to incomplete remove after washing stage.
According to Huang [36], Y2(CO3)3 nH2O is formed at a lower pH,
while at higher pH the formation of Y(OH)CO3 occurs. In our case
the precipitation was conducted at a pH of ~11.5, therefore, we can
assume that during the synthesis process, metallic hydroxy car-
bonates are formed. Based on FTIR assignments, the precursors
posses a complex composition that contains metallic nitrates;
metallic hydrated carbonates, and amorphous silicon dioxide in
variable proportions.
The thermal treatment of precursors at different temperatures,
leads to significant changes in the FTIR spectra. Therefore, the
specific bands observed in the range 2500e4000 cmꢁ1 and 1250-
1800 cmꢁ1, decrease at 800 ꢀC and almost disappear with the
2.3. Materials characterisation
FT-IR spectra of the samples were recorded using a JASCO 610
Spectrometer (KBr pellets technique), while X-ray diffraction (XRD)
patterns were evaluated by Bruker D8 Advanced X-ray diffrac-
tometer equipped with a Ge (1 11) monochromator in the incident
beam (in order to obtain only Cu Ka1 radiation) and LynxEye super
speed detector. The XRD patterns were recorded by step scanning
from 2ꢀ to 80ꢀ and then compared with the XRD PDF2 card files of
the Joint Committee on Powder Diffraction Standards using X-
powder diffraction software. The shape and size of the phosphor
powders were put in evidence with INSPECTS IN CO Scanning
Electronic Microscope.
Specific surface area and porosity measurements were made on
decontaminated samples of any traces of moisture or other adsor-
bents, using a Micromeritics, TriStar II 3020 - Surface Area and
Porosity Analyzer. Adsorption-desorption isotherms were
measured close to the boiling point of nitrogen (77 K). Gas pressure
was gradually increased to the saturation value when condensation
of the adsorbate (N2) occurs into the powder pores. In order to
evaluate the incorporation degree of the Tb3þ into the apatite host
lattice, chemical analysis was performed by inductively coupled
plasma-optical emission spectrometry (ICP-OES). Therefore the
phosphors were precisely weighed (0.2 g) and dissolved in 10 ml
mixture of 4:4:1:1 nitric acid (65%); sulfuric acid (95%), hydro-
chloric acid (35%), borofloric acid (50%) using MILESTONE micro-
wave digestor. The solubilised samples were diluted and content of
Ca, Y, La and Tb was measured.
Luminescent characteristics of phosphors were evaluated based
on emission and excitation spectra registered with Jasco - WAVELL
FP-6500 spectrofluorimeter, equipped with PMT R928 photo-
multiplier (glass filter WG 320 Reichmann Feinoptik).
3. Results and discussion
3.1. Structural characterisation of the samples
In order to identify the functional groups present in samples, the
FT-IR spectroscopy was performed. Fig. 1 shows the FTIR spectra of
both CYSO and CLSO samples prepared at different firing
temperatures.
The precursors (dried at 110 ꢀC) show similar FTIR spectra which
consists of several intense bands situated at: * ~1384 cmꢁ1 assigned
Fig. 2. Decomposition of the symmetric stretching mode of SiO4 group into Lorentzian
components for CYSO (a) and CLSO (b) samples fired at 1400 ꢀC.

138
L.E. Muresan et al. / Journal of Alloys and Compounds 755 (2018) 135e146
increase of firing temperature up to 1400 ꢀC. The small broad bands
at ~3445 cmꢁ1 (CYSO samples, CLSO samples) and the small sharp
lines at 3607 cmꢁ1 (CLSO samples) are due to the tendency of Y2O3
and La2O3 to form hydroxides with atmospheric humidity [37,38].
At the same time, the broad band situated at 1024 cmꢁ1 (La-pre-
cursor) or 1000 cmꢁ1 (Y-precursor) shifts to 954 cmꢁ1 (CYSO) or to
971 cmꢁ1 (CLSO), increase in intensity and splits into two (CYSO) or
three (CLSO) bands, as a result of structural rearrangements due to
crystallization process, suggesting the formation of SiO4 groups
with apatite structure. The FTIR profiles of fired apatites
(1100e1400 ꢀC) are typical for orthosilicates structures with iso-
lated SiO4 tetrahedrons. The bands detected at 983 cmꢁ1, 952 cmꢁ1
(for CYSO 1400 ꢀC) or 967 cmꢁ1, 924 cmꢁ1, 887 cmꢁ1 (for CLSO,
1400 ꢀC) are attributed to Si-O-Si asymmetric stretching vibrations.
No significant shift is observed for stretching bands as the firing
temperature increases from 1100 ꢀC to 1400 ꢀC. Similarly, the O-Si-
O bending vibrations are noticed in 400e550 cmꢁ1 domain.
Moreover, after thermal treatment the asymmetric Y-O-H and La-
O-H bending vibration shifts toward lower wavenumbers because
of water loose during the decomposition of hydroxides. Therefore
Y-O and La-O vibrations are observed at 569 cmꢁ1 and 545 cmꢁ1
specific for Y2O3 and La2O3. Our FTIR results are in concordance
with those reported in the literature on apatite [39].
asymmetric stretching modes (
n
3). The significant differences
observed for asymmetric stretching modes (
n3) may be explained
by the distortions in structure since ionic radius of La3þ (rLa ¼ 1.1 Å
NC ¼ 7; rLa ¼ 1.22 Å NC ¼ 9) is larger than Y3þ (rY ¼ 1.075 Å NC ¼ 7;
rY ¼ 0.96 Å NC ¼ 9) [40,41].
The crystalline structure and structural homogeneity of apatite
samples were investigated by X-ray diffraction. Fig. 3 shows the
XRD patterns of Y and La precursors (110 ꢀC) and the corresponding
phosphors fired at different temperatures (800e1400 ꢀC).
At 110 ꢀC a diffuse XRD pattern, related to an amorphous phase,
is observed for both CYSO and CLSO precursors suggesting a low
degree of crystallinity. The main peaks centered at 28.7ꢀ (Y - pre-
cursor) and at 27.9ꢀ, 31.6ꢀ, 39.6ꢀ, 47.3ꢀ 48.4ꢀ, 49.9ꢀ, 55.3ꢀ (La -
precursor) are attributed to the presence of Y(OH)3 (PDF: 04-0213)
and La(OH)3 (PDF:36-1481) respectively. The crystallite size eval-
uated using Scherrer formula are 7 nm for Y(OH)3 and 6.3 nm for
La(OH)3. Excepting CYSO fired at 800 ꢀC, all the rest apatite phos-
phor samples reveal sharp and well-defined peaks indicating well
crystallised powders. For the CYSO samples, the main peaks situ-
ated at 32.16ꢀ, 32.64ꢀ, 33.2ꢀ corresponding to the crystalline plane
with Miller index of [211], [112], [300] indicate the formation of
hexagonal Ca2Y8(SiO4)6 phase, isostructural with Ca4La6O(SiO4)6O2
(PDF: 270093) and with Ca10-xLax(PO4)6-x(SiO4)x, for x ¼ 8 and
by substituting La with Y and P with Si for having formula Ca2Y8(-
SiO4)6O2 [42]. In these structures Ca, La, and Y are in the same
positions in the unit cell. Based on the atom positions in the unit
cell, XRD diffraction pattern was simulated and compared with
experimental one and are shown in Fig. 4.
The assignments of vibrational broad band in the range
700e1100 cmꢁ1 take in consideration that in SiO4 units exist four
types of vibrations: symmetric stretching
(
n1); asymmetric
stretching ( 3); symmetric bending ( 2); asymmetric bending (n
n
n
4)
[39]. Fig. 2 show the Lorentzian fit spectra of the Si-O-Si symmetric
stretching band in 700e1100 cmꢁ1 range for CYSO and CLSO sam-
ples fired at 1400 ꢀC. The band is decomposed into several main
bands with different relative intensities, depending on the cation
type (Y3þ, La3þ).
Along with hexagonal Ca2Y8(SiO4)6 phase, small amounts of
cubic Y2O3 phase (PDF430661) and trace of monoclinic Y2SiO5
phase (PDF 410004) were identified in fired CYSO samples. It is
difficult to estimate the content of Y2O3 in CYSO samples, since the
diffraction line specific for Y2O3 phase situated at 2
Muller index [222] is superimposed with the diffraction line spe-
The careful examination of Fig. 2, highlights the presence of
some additional bands (shoulders) assigned to symmetric
q
¼ 28.6ꢀ with
stretching
(
n1) which are otherwise hidden by the intense
cific for apatite phase with Muller index [210]. The unit cell
Fig. 3. XRD patterns of CYSO (a) and CLSO (b) samples prepared at different temperatures.

L.E. Muresan et al. / Journal of Alloys and Compounds 755 (2018) 135e146
139
Ca2La8(SiO4)6 apatite structure (PDF 29-0337) belonging to the
space group P63/m with lattice parameters a ¼ b ¼ 9.65, c ¼ 7.15,
a
¼ 90,
b
¼ 90,
g
¼ 120. Beside the hexagonal apatite phase, La2O3
phase (PDF 83-1344) was identified only in CLSO sample fired at
800C0 in proportion of 24%.
The structure of apatite offers two types of sites for cations,
which are the 4f (C3) site with nine-coordination (Ca1) and the 6 h
(Cs) site with seven-coordination (Ca2). The cell parameters of
CLSO samples (a ¼ b ¼ 9.65) are higher than CYSO sample
(a ¼ b ¼ 9.355) due to the differences between La3þ and Y3þ ionic
radius [40,41].
Firing time and temperature plays a decisive role in the for-
mation of the apatite structure. During the thermal treatment, the
material is subjected to several changes that refer to: formation of
the inert amorphous structure; formation of inert crystalline lattice
due to sintering, rearrangements of the host crystal components in
order to obtain luminescent phase and formation of different
luminescent centers. These processes will inevitably affect both the
size of the crystals and the deformation of the host lattice. To
highlight the importance of firing temperature, the crystalline
mean size (Deff) and root mean square (rms) of the microstrains < ε2
1/2
>
were calculated on the basis of XRD data. Fig. 5 reveals the
dependence of Deff and < ε2
>
on firing temperature for both
1/2
CYSO and CLSO samples.
The crystallite size was estimated using the Scherrer equation,
Deff ¼ k /( cos ), where Deff is the average grain size, k (0.9) is a
shape factor, is X-ray wavelength (1.5406 Å), is the full width at
half maximum (FWHM) of the highest reflection peak and is the
l
b
q
l
b
q
diffraction angle of the observed peak. The strongest diffraction
peak at 2
q
¼ 32.14ꢀ (CYSO) and 2
q
¼ 31.00ꢀ (CLSO) was used to
calculate the crystallite sizes of samples fired at different temper-
atures. The microstrains were evaluated using Williams-Hall plot.
The larger amount of amorphous component in CYSO sample fired
at 800 ꢀC compared with CLSO sample (Fig. 5a,b) may explain why
the crystallites size are 2.3 times smaller even if the microstrains
values are close (15.44$10ꢁ4 for CYSO and 14.09$10ꢁ4 for CLSO).
Following Fig. 5, we observe that the apatite nanocrystals grow up
to 103.5 nm (CYSO) and 105 nm (CLSO) with the firing temperature
from 800 ꢀC to 1400 ꢀC. Decrease of the microstrains with the
crystallite growing, suggests that some lattice disorder are removed
Fig. 4. Experimental and simulated XRD patterns for CYSO sample fired at 1400 ꢀC.
parameters of CYSO fired at 800 ꢀC (a ¼ b ¼ 9.35, c ¼ 6.79,
a
¼ 90,
b
¼ 90,
g
¼ 120) present no significant variation with firing tem-
perature and are in good agreement with PDF 270093 file.
XRD patterns of CLSO samples fired over 800 ꢀC indicate the
formation of single phase materials with pure hexagonal
1/2
Fig. 5. Dependence of Deff and < ε2
>
with firing temperature for CYSO (a) and CLSO (b) samples.

140
L.E. Muresan et al. / Journal of Alloys and Compounds 755 (2018) 135e146
and that the apatite structure is approaching the ideal shape. Due to
decrease of defects in the lattice with increase of temperature, the
microstrains reach minimum values at 1400 ꢀC (3.68$10ꢁ4 for CYSO
and 4.24$10ꢁ4 for CLSO).
particles with fluffy aspect due to decomposition processes sug-
gesting the beginning of the apatite formation. Changes in shape
and dimensions of particles for the samples fired at elevated tem-
perature (1400 ꢀC) draws attention in Fig. 6 c, e. As can be seen, the
powders consist of rounded particles with spongy aspect with size
that vary between 0.6 mm and 2 mm.
3.2. Morphology and surface state characterisation of samples
Surface area and porosity measurements were performed in
order to illustrate the surface states changes of the apatite powders
during the thermal treatment process. Fig. 7 shows adsorption/
desorption isotherms of CYSO and CLSO samples. Isotherms shape
of the precursors (dried at 110 ꢀC) are similar to type IV, which are
usually meet in the mezoporous materials [43,44].
The hysteresis loops are identified to be H1 type, which indicate
the presence of uniform pores [44]. After the thermal treatment,
the shape of isotherms are significantly changed from type IV to
The shape and the size of precursors and corresponding CYSO
and CLSO phosphors fired at different temperatures are illustrated
in Fig. 6.
Precursors consist from small agglomerated particles of about
1
m
m on the surface of irregular larger aggregates with size up to
m. After the thermal treatment, at 800 ꢀC, it is observed that
SiO2 aggregates break into smaller pieces with size between 15 and
20 m. At this temperature, it is observed the increase of submicron
40
m
m
Fig. 6. SEM images of CYSO (a - 110 ꢀC, b - 800 ꢀC, c - 1400 ꢀC) and CLSO samples (d - 110 ꢀC, e - 800 ꢀC, f - 1400 ꢀC).

L.E. Muresan et al. / Journal of Alloys and Compounds 755 (2018) 135e146
141
Fig. 7. Adsorptionedesorption isotherms of CYSO (a) and CLSO (b) samples. SBET are shown for each sample.
type II, while the hysteresis loop becomes similar to H3 with slit
shape pores features. The quantity of adsorbed nitrogen continu-
ously decrease with firing temperature from 350 cm3/g STP to
3 cm3/g STP for CYSO samples and from 250 cm3/g STP to 13 cm3/g
STP for CLSO samples. Moreover, the specific surface areas of
samples (SBET) decrease dramatically as the firing temperature in-
crease as can be seen in Fig. 7. According to this observation we can
say that the powders become almost nonporous under thermal
treatment. BJH analysis (Baret, Joyner, and Halenda method) was
used to determine area and volume of the mezoporous using
adsorption and desorption techniques. Fig. 8 shows the pore vol-
ume distributions in 2÷150 nm range together with the cumulative
pores volume values for each sample.
The pore volume distribution curve of precursors possess
broaden and single peaked shape in the mezo-porous domain with
pores diameter situated at 15.8 nm (CYSO) and 15 nm (CLSO),
indicating homogeneous powders. When CYSO precursor is heated
at 800 ꢀC the pore diameter is shifted to higher values (51.7 nm)
while the pore volumes significantly decrease from 0.650 cm3/g to
0.336 cm3/g. The changes of the pore volume distribution curve are
more evident in the case of CLSO precursor (Fig. 8 b). After the
thermal treatment at 800 ꢀC, CLSO sample becomes already
nonporous with pores volumes of seven times smaller than corre-
sponding precursor.
samples) and oxide phosphors (Y2O3:Tb, La2O3:Tb) prepared in the
same conditions. As can be seen, the excitation spectra show sig-
nificant differences depending on the firing temperature, nature of
the cations (Y3þ, La3þ) and type of the host lattice. As a general
trend, the intensity of apatites excitation bands steadily increases
with the firing temperature due to crystallinity improvements.
CYSO samples exhibit more complex excitation spectra than CLSO
samples, due to the secondary phases (Y2O3, Y2SiO5) present even
at higher temperature (see Fig. 3). Normally the excitation spec-
trum of Tb3þ consists of sharp lines due to fef transitions in 4f8 and
broad bands due to fed transitions [19]. The fed transitions of Tb3þ
contain two groups: one at higher energies corresponding to spin-
allowed (SA) transitions and the other at lower energies assigned to
spin-forbidden (SF) transitions [45]. In our samples, Tb3þ may
accommodate in different crystallographic sites with specific
symmetry for hexagonal apatite (C3, Cs) or cubic oxide (C2, C3i)
[13,46]. Analysing the excitation spectra of oxide phosphors
(Y2O3:Tb and La2O3:Tb), it is clear the presence of an intense broad
excitation band at 246 nm followed by other smaller bands at
274 nm, 284 nm, 305 nm and 319 nm assigned to spin allowed and
spin forbidden 4f-5d transition in Tb3þ [46,47]. The bands observed
between 320 nm and 400 nm are due to f-f transitions from the
ground level (7F6) to excited levels (5D2, 5D3) in Tb3þ [19,46,48].
Interestingly, the PLE spectra of samples containing amorphous
components (samples fired at 800 ꢀC) exhibit different excitation
shape in the range 225e300 nm in comparison to corresponding
crystalline samples. At lower firing temperature (800 ꢀC), CYSO
sample present one broad excitation band with maximum situated
3.3. Luminescent characterisation of the samples
Fig. 9 shows the excitation spectra of apatite (CYSO and CLSO

142
L.E. Muresan et al. / Journal of Alloys and Compounds 755 (2018) 135e146
Fig. 8. Pore volume distribution curves of CYSO (a) and CLSO (b) samples.
Fig. 9. Excitation spectra of CYSO (a) and CLSO (b) samples prepared at different temperatures (
comparison.
l
¼ 545 nm). Excitation spectra of Y2O3:Tb and La2O3:Tb are shown for
em.

L.E. Muresan et al. / Journal of Alloys and Compounds 755 (2018) 135e146
143
Fig. 10. Emission spectra of CYSO (a) and CLSO (b) samples prepared at different temperatures (
comparison.
l
¼ 235 nm). Emission spectra of Y2O3:Tb and La2O3:Tb are shown for
exc.
Fig. 11. Variation of emission intensity with firing temperature of CYSO (a) and CLSO (b) samples. Variation of real amounts of Y3þ, Ca3þ, La3þ, Tb3þ with firing temperature for CYSO
(c) and CLSO (d) samples.

144
L.E. Muresan et al. / Journal of Alloys and Compounds 755 (2018) 135e146
Fig. 12. The chromatic coordinates of CYSO and CLSO samples under excitation with 235 nm (a) and 303 nm (b).
at 240 nm attributed to 4f8 /4f75d1 transition in Tb3þ. The low
crystallinity of sample and presence of Y2O3 phase may explain the
broadened band that gathers several overlapping bands. Through
Gaussian curve-fitting, these bands are observed at 233 nm,
244 nm and 260 nm (see Fig. 9 a). Comparing the excitation spectra
of CYSO fired at 800 ꢀC with that of Y2O3:Tb, it is obvious that band
at 240 nm brings together components arising from the accom-
modation of Tb3þ in C2, C3i sites of Y2O3 (bands at 244 nm, 260 nm)
and C3, Cs sites in hexagonal Ca2Y8(SiO4)6O2 lattice (band at
233 nm) [13,46]. In case of CLSO sample fired at 800 ꢀC, the small
broad band at 273 nm is due to Tb3þ spin forbidden f-d transition in
La2O3. Further increase of temperature up to 1100 ꢀC significantly
changes the shape of the excitation spectrum by shifting the
240 nm band toward lower wavelength (233 nm) indicating the
formation of Tb3þ luminescent centers in apatite lattice (spin
allowed f-d transitions). Moreover, the two bands with maxima at
274 nm and 303 nm, which are extended into a large spectral
domain may hidden other bands observed in Y2O3:Tb while the
shoulder at 246 nm (seen as an intense band in Y2O3:Tb) become
more evident with the increase of firing temperature. The very
small forbidden fef transitions of Tb3þ are observed in the range
350e450 nm for CYSO samples, but not for CLSO samples (Fig. 9).
The spin allowed f-d transition of Tb3þ in CLSO sample presents a
small shift from 230 nm to 235 nm due to some lattice distortions
caused by the heating process.
The highest excitation peak centered at 235 nm was fixed as
excitation wavelength for recording the emission spectra. Fig. 10
illustrates the emission spectra of apatite (CYSO and CLSO sam-
ples) and oxide phosphors (Y2O3:Tb, La2O3:Tb). The emission
spectra of samples consist of several peaks from 350 nm to 650 nm
with a prominent green emission at 545 nm and 551 nm due to
5D4-7F5 transition. The emission lines at 380 nm, 416 nm, 437 nm,
458 nm are from 5D3-7F6,5,4 transitions, those from 487 nm to
495 nm are attributed to 5D4-7F6, while lines at 585 nm, 592 nm and
625 nm are the results of 5D4-7F4,3 transitions.
A quick observation of PL spectra reveals that the blue emission
dominates in CLSO samples, while green emission is dominant in
case of CYSO samples. This suggests that terbium ions occupy sites
with different symmetry in CYSO and CLSO samples. Furthermore,
correlation with XRD data leads to an interesting observation. Even
though CLSO samples exhibit higher phase purity than CYSO
samples, the PL emission is up to 4 times smaller. This behaviour
can be explained by the incorporation degree of Tb3þ in the ma-
terial during both precipitation process and thermal treatment
stage. In order to make some correlations regarding the emission
intensity and incorporation of Tb3þ in the apatite lattice, the ICP-
OES measurements were performed. Fig. 11 depicts the variation
of emission intensity and the real amount of cations (Ca2þ, Y3þ
,
La3þ, Tb3þ) in phosphor samples with firing temperature.
As a general trend, emission intensity increase with firing
temperature due to improvements of powders crystallinity and
Tb3þ incorporation degree from 0.101 mol to 0.121 mol for CYSO
samples and from 0.108 mol to 0.115 mol for CLSO samples. During
the thermal treatment more and more Tb ions accommodate in the
apatite lattice enhancing the probability of radiative transitions. It
is also observed that the Tb amount in CLSO samples is smaller than
in CYSO samples, this being a possible explanation for the lower PL
intensity of CLSO samples. According with ICP-OES results, we can
say that CYSO samples are closed to theoretical formula
-
Ca2Y7.88Tb0.12(SiO4)6O2 with values for Y3þ and Ca2þ slowly higher
than the theoretical ones, which can be explained by the presence
of Y2O3 as secondary phase in concordance with XRD results. As for
CLSO samples the real amount of Ca2þ is half from the theoretical
values, which indicate that the stoichiometry of the final product is
modified due to un-reacted or redissolved cations. The deviation
from the theoretical composition is expected since the precipita-
tion is an extremely complex process involving a large variety of
ions (Ca2þ, Y3þ, La3þ, Tb3þ) having different precipitation rates and
different solubility products (KCa(OH)2 ¼ 5 ꢂ 10ꢁ6
;
KLa(OH)
¼ 2 ꢂ 10ꢁ21; KY(OH)3 ¼ 1 ꢂ 10ꢁ22) [49]. On the basis of the emission
3
spectra, the chromatic coordinates of CYSO and CLSO phosphors
were calculated and presented in Fig. 12.
It can be observed that CYSO samples are grouped in the green
region of the map, while CLSO samples are situated in the turquoise
region either the excitation is 235 nm (Fig.12a) or 303 nm (Fig.12b).
On the other hand, the firing temperature seems to generate
important color shifts. From Fig. 12a, it is clear that the emitting
color is changed from greenish to turquoise as the firing tempera-
ture increases and the CIE coordinates varies from (0.2137, 0.329) to
(0.260, 0.565). Under 303 nm excitation the color shift is not so

L.E. Muresan et al. / Journal of Alloys and Compounds 755 (2018) 135e146
145
photoluminescence properties of red-emitting phosphors Ba2Gd8(SiO4)6O2:
Eu3þ, Mater. Lett. 129 (2014) 130e133.
[13] Z. Sun, M. Wang, Z. Yang, K. Liu, F. Zhu, Crystal structure and luminescence
properties of Bi3þ activated Ca2Y8(SiO4)6O2 phosphors under near UV exci-
tation, J. Solid State Chem. 239 (2016) 165e169.
[14] H. Liu, Y. Zhang, L. Liao, Z. Xia, Synthesis, structure and green luminescence
evolution of apatite-type Sr3.5Y6.5O2(PO4)1.5(SiO4)4.5:Eu2þ,Tb3þ phosphors,
J. Lumin. 156 (2014) 49e54.
dramatically, samples being grouped in more restricted areas.
These results suggest that the emitting color of the synthesized
phosphors can be tuned by properly adjusting the firing tempera-
ture (which also determines the phase structure) making from
silicate apatite phosphors promising candidates for color displays
and solid-state lighting.
[15] S.A. Khan, W. Ji, L. Hao, X. Xu, S. Agathopoulos, N.Z. Khan, Synthesis and
characterization of Ce3þ/Tb3þ coedoped CaLa4Si3O13 phosphors for applica-
tion in white LED, Opt. Mater. 72 (2017) 637e643.
4. Conclusion
[16] G. Li, D. Geng, M. Shang, Y. Zhang, C. Peng, Z. Cheng, Jun Lin, Color tuning
luminescence of Ce3þ/Mn2þ/Tb3þ-Tri-activated Mg2Y8(SiO4)6O2 via energy
transfer: potential single-phase white-light-emitting phosphors, J. Phys.
Chem. C 115 (2011) 21882e21892.
[17] Z. Xia, Q. Liu, Progress in discovery and structural design of color conversion
phosphors for LEDs, Prog. Mater. Sci. 84 (2016) 59e117.
[18] A. Kitai, Luminescent Materials and Applications, John Wiley&Somns, Ltd,
2008.
[19] S. Shionoya, W.M. Yen, Handbook of Phosphors, CRC Press, Boca Raton, 1999.
[20] C. Peng, G. Li, Z. Hou, M. Shang, J. Lin, Electrospinning synthesis and lumi-
nescent properties of one-dimensional Ca2Gd8(SiO4)6O2:Eu3þ microfibers
and microbelts, Mater. Chem. Phys. 136 (2012) 1008e1014.
Green and turquoise emitting phosphors with hexagonal apatite
structure are successfully synthesized by precipitation under mi-
crowave conditions. Morphology, surface state porosity and phase
purity are influenced by the nature of rare earth (Y3þ or La3þ) in the
apatite lattice. La-phosphors consist of pure apatite with crystallites
size that varies between 60.8 nm and 105 nm. Y-phosphors contain
cubic -Y2O3 as secondary phase along apatite with crystallites size
that increase from 26.1 nm to 103.2 nm with firing temperature.
The microstrains decrease for both La-, Y- phosphors, meaning that
some lattice disorder is removed through temperature rising. Ac-
cording to the excitation spectra, multiple emission Tb3þ centers
are formed in Y-phosphors and single emission centers in La-
phosphors. All samples present emission lines situated in the
entire visible spectrum specific for 5D3-7F6,5,4 and 5D4-7F6,5,4,3
transitions in Tb3þ. The chromatic coordinates of the samples shift
from greenish to turquoise and depends both on the nature of rare
earth (Y3þ, La3þ) and firing temperature. In order to achieve higher
purity phase for this promising material, further researches are to
be performed to optimize the precipitation synthesis conditions.
[21] A. Vincent, S. Beaudet Savignat, F. Gervais, Elaboration and ionic conduction of
apatite-type lanthanum silicates doped with Ba, La10ꢁxBax(SiO4)6O3ꢁx/2 with
x ¼ 0.25e2, J. Eur. Ceram. Soc. 27 (2007) 1187e1192.
[22] H. Liu, Y. Zhang, L. Liao, Z. Xia, Synthesis, structure and green luminescence
evolution of apatite-type Sr3.5Y6.5O2(PO4)1.5(SiO4)4.5:Eu2þ,Tb3þ phosphors,
J. Lumin. 156 (2014) 49e54.
[23] Y. Guo, S.H. Park, B.C. Choi, J.H. Jeong, J.H. Kim, Effect of La3þ ion doping on the
performance of Eu2þ ions in novel Sr3CeNa(PO4)2SiO4 phosphors, J. Alloys
Compd. 724 (2017) 763e773.
[24] T. Kharlamova, O. Vodyankina, A. Matveev, V. Stathopoulos, A. Ishchenko,
D. Khabibulin, V. Sadykov, The structure and texture genesis of apatite-type
lanthanum silicates during their synthesis by co-precipitation, Ceram. Int.
41 (2015) 13393e13408.
[25] A.R. Noviyanti, D.R. Eddy, A. Anshari, Synthesis of the Bi-doped apatite-type
phases La10-xBixSi6O27 (x¼ 0.5, and 1) by hydrothermal method, Proc. Chem.
17 (2015) 16e20.
[26] D. Kioupis, G. Kakali, Structural and electrical characterization of Sr-and Al-
doped apatite type lanthanum silicates prepared by the Pechini method,
Ceram. Int. 42 (2016) 9640e9647.
[27] C. Tian, J. Liu, J. Cai, Y. Zeng, Direct synthesis of La9.33Si6O26 ultrafine powder
via solegel self-combustion method, J. Alloys Compd. 458 (2008) 378e382.
[28] T.Y.H. Zhao, J. Han, N. Xu, Y.S. Zhihong Du, J. Wang, Synthesis and densification
of lanthanum silicate apatite electrolyte for intermediate temperature solid
oxide fuel cell via co-precipitation method, J. Eur. Ceram. Soc. 34 (2014)
1563e1569.
[29] H.-C. Yao, J.-S. Wang, D.-G. Hu, J.-F. Li, X.-R. Lu, Z.-J. Li, New approach to
develop dense lanthanum silicate oxyapatite sintered ceramics with high
conductivity, Solid State Ionics 181 (2010) 41e47.
[30] Y. Ma, M. Moliere, Z. Yu, N. Fenineche, O. Elkedim, Novel chemical reaction co-
precipitation method for the synthesis of apatite-type lanthanum silicate as
an electrolyte in SOFC, J. Alloys Compd. 723 (2017) 418e424.
[31] G.A. Tompsett, W.C. Conner, K.S. Yngvesson, Microwave synthesis of nano-
porous materials, ChemPhysChem 7 (2006) 296e319.
[32] M. Baghbanzadeh, L. Carbone, P.D. Cozzoli, C.O. Kappe, Microwave-assisted
synthesis of colloidal inorganic nanocrystals, Angew Chem. Int. Ed. Engl. 50
(2011) 11312e11359.
[33] I. Bilecka, M. Niederberger, Microwave chemistry for inorganic nanomaterials
synthesis, Nanoscale 2 (2010) 1358e1374.
Formatting of funding sources
This research did not receive any specific grant from funding
agencies in the public, commercial, or not-for-profit sectors. This
study is part of the research that will lead to the elaboration of a Ph.
D thesis.
References
ꢁ
ꢀꢁ
ꢀ
ꢁ
ꢀ
[1] P. Ptacek, T. Opravil, F. Soukal, J. Tkacz, J. Masilko, E. Bartoníckova, The field of
solid solutions in ternary system of synthetic apatite-type alkaline earth
element yttrium silicate oxybritholite phases of the composition: AEEdY10-
d[SiO4]6O3-0.5d, where AEE ¼Ca, Sr and Ba, Ceram. Int. 42 (2016) 6154e6167.
[2] H. Njema, K. Boughzala, A. Chaabe` ne, K. Bouzouita, Study of the ionic con-
ductivity of Ca6La4(PO4)2(SiO4)4F2 and Ca4La6(SiO4)6F2, C. R. Chimie 17 (2014)
1237e1241.
[3] Feng Zhang, Bitao Liu, An intense red-emitting phosphor Ca3Gd7(SiO4)5(PO4)
O2:Eu3þ for NUV-LEDs application, J. Alloys Compd. 542 (2012) 276e279.
[4] J. Felsche, Rare earth silicates with the apatite structure, J. Solid State Chem. 5
(1972) 266e275.
[34] M.-A. Muroya, Correlation between the formation of silica skeleton structure
and Fourier transform reflection infrared absorption spectroscopy spectra,
Colloids Surf., A 157 (1999) 147e155.
[5] Xue Lu, Haitao Liu, Xinyu Yang, Yiguang Tian, Xiaoli Gao, Liyuan Han, Qin Xu,
A single-phase white-emitting La10(SiO4)6O3:Eu2þ/Eu3þ phosphor for near-UV
LED-based application, Ceram. Int. 43 (2017) 11686e11691.
[35] M. Salavati-Niasari, G. Hosseinzadeh, F. Davar, Synthesis of lanthanum car-
bonate nanoparticles via sonochemical method for preparation of lanthanum
hydroxide and lanthanum oxide nanoparticles, J. Alloys Compd. 509 (2011)
134e140.
[6] Lahrich, M.A. Elmhammedi, B. Manoun, Y. Tamraoui, F. Mirinioui, M. Azrour,
P. Lazor, Elaboration, Rietveld refinements and vibrational spectroscopic study
of Na1-xKxCaPb3(PO4)3 lacunar apatites (0ꢃxꢃ1), Spectrochim. Acta A 145
(2015) 493e499.
[36] Z.G. Huang, X.D. Sun, Z.M. Xiu, S.W. Chen, C.T. Tsai, Precipitation synthesis and
sintering of yttria nanopowders, Mater. Lett. 58 (2004) 2137e2142.
[37] K. Chen, J. Peng, C. Srinivasakannan, S. Yin, S. Guo, L. Zhang, Effect of tem-
perature on the preparation of yttrium oxide in microwave field, J. Alloys
Compd. 742 (2018) 13e19.
[38] B. Klingenberg, M. Albert Vannice, Influence of Pretreatment on lanthanum
nitrate, carbonate, and oxide powders, Chem. Mater. 8 (1996) 2755e2768.
[39] R. El Ouenzerfi, C. Goutaudier, G. Panczer, B. Moine, M.T. Cohen-Adad,
M. Trabelsi-Ayedi, N. Kbir-Ariguib, Investigation of the CaO-La2O3-SiO2-P2O5
quaternary diagram. Synthesis, existence domain, and characterization of
apatitic phosphosilicates, Solid State Ionics 156 (2003) 209e222.
[40] J. Zhou, T. Ya, J. Lian, Y. Shen, Z. Dong, F. Lu, Radiation-induced amorphization
of Ce-doped Mg2Y8(SiO4)6O2 silicate apatite, Nucl. Instrum. Methods Phys.
Res., Sect. B 379 (2016) 102e106.
[7] S. Unithrattil, H.J. Kim, K.H. Gil, N.H. Vu, V.H. Hoang, Y.H. Kim, P. Arunkumar,
W.B. Im, Engineering the lattice site occupancy of apatite-structure phosphors
for effective broad-band emission through cation pairing, Inorg. Chem. 56
(2017) 5696e5703.
[8] X.H. Chuai, H.J. Zhang, F.Sh Li, Sh. Z. Lu, J. Lin, Sh.B. Wang, Kou Chi-Chou,
Synthesis and luminescence properties of oxyapatite NaY9Si6O26 doped with
Eu3þ, Tb3þ, Dy3þ and Pb2þ, J. Alloys Compd. 334 (2002) 211e218.
[9] Bing Yan, Honghua Huang, Solegel synthesis and luminescence of unexpected
microrod crystalline Ca5La5(SiO4)3(PO4)3O2:Dy3þ phosphors employing
different silicate sources, Opt. Mater. 29 (2007) 1706e1709.
[10] Y. Guo, B.K. Moon, B.C. Choi, J.H. Jeong, J.H. Kim, H. Choi, Fluorescence prop-
erties with red-shift of Eu2þ emission in novel phosphor-silicate apatite
Sr3LaNa(PO4)2SiO4 phosphors, Ceram. Int. 42 (2016) 18324e18332.
[11] H. Liu, W. Gu, Y. Hai, L. Zhang, L. Liao, L. Mei, Facile combustion synthesis and
photoluminescence properties of Ce3þ doped Sr2La8(SiO4)6O2 phosphors, Opt.
Mater. 42 (2015) 553e555.
[41] Q. Guo, L. Liao, Z. Xia, Luminescence properties and energy transfer in La6B-
a4(SiO4)6F2:Ce3þ,Tb3þ phosphors, J. Lumin. 145 (2014) 65e70.
[42] H. Njema, K. Boughzala, H. Boughzala, K. Bouzouita, Structural analysis by
[12] Y. Liu, Z. Wang, J. Zhong, F. Pan, H. Liang, Z. Xiao, Synthesis and

146
L.E. Muresan et al. / Journal of Alloys and Compounds 755 (2018) 135e146
Rietveld refinement of calcium and lanthanum phosphosilicate apatites,
J. Rare Earths 31 (2013) 897e904.
[43] K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, J. Rouquerol,
T. Siemieniewska, Reporting physisorption data for gas/solid systems with
special reference to the determination of surface area and porosity (Recom-
mendations 1984), Pure Appl. Chem. 57 (1985) 603e619.
B. Kozankiewicz, E. Mijowska, M. Godlewski, Reduction of Tb4þ ions in
luminescent Y2O3:Tb nanorods prepared by microwave hydrothermal
method, J. Rare Earths 34 (2016) 774e781.
[47] L. van Pieterson, M.F. Reid, G.W. Burdick, A. Meijerink, 4fn -4Fn-15d transitions
of the heavy lanthanides: experiment and theory, Phys Rew.B 65 (2002),
045114-1-04511-13.
[44] P.A. Webb, C. Orr, Analytical Methods in fine Particle Technology, Micro-
meritics Instrument Corp., Norcross, USA, 1997.
[48] M.A. Flores-Gonzalez, G. Ledoux, S. Roux, K. Lebbou, P. Perriat, O. Tillement,
Preparing nanometer scaled Tb-doped Y2O3 luminescent powders by the
polyol method, J. Solid State Chem. 178 (2005) 989e997.
[49] T. Moeller, H.E. Kremers, The basicity characteristics of scandium, yttrium, and
the rare earth elements, Chem. Rev. 37 (1945) 97e159.
[45] F. Zhang, Y. Wang, Y. Huang, Y. Tao, Vacuum ultraviolet excited luminescence
properties of Ca3Gd7(SiO4)5(PO4)O2:Re3þ (Re3þ ¼Tb3þ
,
Dy3þ
)
phosphors,
J. Phys. Chem. Solid. 74 (2013) 1499e1503.
ꢀ
[46] Jaroslaw Kaszewski, B.S. Witkowski, Lukasz Wachnicki, H. Przybylinska,