L. Macalik et al. / Journal of Alloys and Compounds 509 (2011) 7458–7465
7459
the monoclinic monazite phase. The reversibility of the dehydrata-
tion was studied by Khorvath et al. [18] up to 250 C. It should be
noted that the temperature reported for these transitions depends
on preparing method of hexagonal phases and on its thermal his-
tory [19].
pared by dispersing a small amount of the nanocrystalline powder in pure ethyl
alcohol with ultrasonic agitation. A droplet of the suspension was deposited on
a copper microscope grid covered with carbon thin film. The grains on the sup-
ported films were then examined using conventional TEM (Tesla BS 500, operated
at 90 kV). The microstructure was studied by high-resolution TEM (HRTEM, Philips
CM20 SuperTwin operating at 200 kV).
◦
In the present paper we focus on the characterisation of LaPO4
and CePO4 nanomaterials undoped and doped with Pr3+ ions.
2.4. Spectroscopic measurements
The infrared absorption spectra were measured at room temperature using a
2
. Experimental
−
1
Biorad 575C FT-IR spectrometer in the spectral range of 50–4000 cm . KBr pel-
let and the Nujol mull suspension technique were applied to measurements for
the mid (MIR) and far (FIR) infrared range, respectively. Room temperature Raman
2.1. Material synthesis
−
1
spectra were recorded in the spectral range of 80–4000 cm with a Bruker RFS 100
FT-Raman Spectrometer using the back scattering arrangement. The 1064 nm line
of Nd:YAG laser as an excitation source and a liquid nitrogen-cooled Ge detector
were used. Both IR and Raman spectra were recorded with a spectral resolution of
Both hexagonal and monoclinic types of light lanthanide phosphates could be
obtained by hydrothermal synthesis [15,20–23]. The conditions of synthesis, i.e.
temperature and the pH value of solution influenced the type of obtained prod-
uct. Nanoparticles of cerium and lanthanum orthophosphates, pure and slightly
−
1
3+
2 cm
.
doped −1 at.% – with Pr ions, were prepared by the long- and short-lasting low-
The luminescence and excitation spectra were gathered with a Hamamatsu R-
55 PMT using one channel 750 focal length a Dongwoo Optron spectrophotometer
model DM711). To excite the sample a 180 W ozone-free lamp (Dongwoo Optron)
temperature hydrothermal methods.
9
(
In the former method, high purity LnCl3·7H2O, where Ln = La, Ce and Pr, were
dissolved in 0.1 M HCl and the appropriate volume of crystalline H3PO4 was added.
The mixture with pH 1 was transferred into a Teflon-lined stainless steel vessel and
was used. All measurements were performed at room temperature.
◦
In luminescence decay time measurements, short pulses (4 ns) produced by an
optical parametric oscillator OPO (Continuum, Surelite I) pumped by the third har-
monic of Nd:YAG laser were used to excite directly luminescence levels. The decay
signal was detected, averaged and stored with a Tektronix TDS 3052 digital oscillo-
scope; the all decay data consisted of ten thousand points. The fits of experimental
decay curves were done using the Microcal Origin v5.0 software; the amplitude of
the hydrothermal reaction lasted at 100 C for 24 h. The autoclave was naturally
cooled to room temperature. The obtained powders were separated by centrifuga-
tion and washed several times with double distilled water and methanol. Similar
method was used by Ferhi et al. [20] to obtain lanthanum phosphate microcrys-
talline powder doped with Eu . For samples obtained in the alkaline environment
the stoichiometric amounts of lanthanide chlorides and crystalline H3PO4 were dis-
solved in distilled water and the pH value of the solution was kept about three by
3+
3
1
the curves was calibrated to thousand. The analysed multiplets ( P0 and D2) were
excited directly to avoid the influence of over-lying multiplets onto decay curves
profiles of measured levels. Due to non-exponential character of decay curve of the
◦
adding 1 M HCl. After half an hour, the clear solution was heated to 70 C and the pH
value was adjusted dropwise to 11 with 1 M NaOH. The opalescent colloid was sub-
sequently transferred into a Teflon-lined stainless steel vessel and the hydrothermal
reaction was run in the same manner as for acidic condition of reaction.
1
D2 curves the mean lifetime (ꢂm) of this multiplet can be evaluated following the
Inokuti–Hirayama formula [29]:
The powders of LaPO4 and CePO4 were also obtained under short-lasting
hydrothermal conditions. The chemical reagents, La(III) or Ce(III) nitrate hex-
ahydrate and diammonium hydrogen phosphate, were used in stoichiometric
molar ratios with the concentration of lanthanides of 0.04 mol/dm . The lan-
thanum/cerium nitrate and the phosphate precursors were diluted with water to
obtain 50 ml of milky solution. As obtained sols were products of acidic reaction
ꢀ
∞
tI(t)dt
I(t)dt
t=0
ꢂm =
ꢀ
(1)
∞
3
t=0
where I is the intensity of luminescence and t represents time.
(
pH 1). The alkaline solutions were derived in a similar way, except for addition
of ammonia to achieve pH 11. The sols were aged for 24 h before the hydrother-
mal treatment. Unfortunately, the doping with Pr was unsuccessful using this
method. The hydrothermal reactions were performed in a Magnum II autoclave
3. Results and discussion
3+
3.1. Structural considerations
(
6
Ertec, Poland) with heating of the samples by microwaves of the maximal power of
00 W. The syntheses were controlled by measuring of total pressure in the Teflon
reactor. The hexagonal type phosphates were obtained by heating of the sols under
pmax = 20 bar for 15 min. As obtained fine precipitates were filtered and washed
several times with distilled water.
Analysis of the X-ray powder diffraction patterns of the
orthophosphate nanopowders shows that the investigated sam-
ples are free of impurity phases and they are independent of the
crystallisation method used. The as-synthesized phosphates have
hexagonal symmetry both for pH 11 and pH 1 for the starting solu-
The final product, powders in the nanocrystalline form with hexagonal sym-
◦
metry, was obtained by drying the as-synthesized powders at 100 C for 2 h and
◦
by subsequent calcination at 500 C for 24 h in an electric furnace in a nitrogen
◦
atmosphere (N2). The monoclinic samples were obtained by calcination of the as-
tion. All diffraction peaks for the samples calcined at 500 C and
◦
◦
synthesized powders at 900 C for 24 h in a N2 atmosphere. The obtained samples
9
00 C could be indexed in the hexagonal and monoclinic unit cells,
were used for further investigation.
respectively. Fig. 1 shows the X-ray powder diffraction patterns of
as-synthesized CePO and hexagonal CePO synthesized under the
4
4
2.2. X-ray powder diffraction
◦
acidic or alkaline conditions and subsequently calcined at 500 C.
X-ray powder diffraction patterns (XRD) were recorded at room temperature
The monoclinic CePO synthesized under the acidic or alkaline con-
ditions and subsequently calcined at 900 C are also presented. For
comparison, the diffraction patterns of corresponding crystals are
added. The praseodymium dopant is not the reason for the shift of
the diffraction peaks. XRD patterns for LaPO4 are shifted towards
4
by using STADI-P powder diffractometer (STOE, Germany) working in the trans-
◦
◦
mission geometry and equipped with a linear 140 -PSD detector. CuK␣1 radiation
ꢀ = 1.54056 A˚ ) in the 2ꢁ range from 10 to 65 with a step of 0.03 a flat rotating
◦ ◦ ◦
(
sample holder was used. DICVOL04 [24] was used for indexing the X-ray powder
diffraction patterns. The structure solution was carried out using FullProf package
25].
[
the lower degrees in comparison to the CePO4 apart from the type
The average size of nanocrystallites was estimated from the broadening of the
3+
of phosphate. In Figure 1 the LaPO :Pr obtained under pH 1 and
4
X-ray diffraction lines using the Scherrer equation D = Kꢀ/( cos ꢁ), where D is the
diameter of the crystallite (in the approximation of a spherical shape), ꢀ is the X-
ray wavelength, ˇ is the full width at half maximum (FWHM) of the diffraction
line (in radians) and ꢁ is the Bragg angle of diffraction peak [26,27]. The Scherrer
constant K is conventionally set to 1.0 [28]. The contribution of the instrumental
broadening was removed by subtracting the FWHM of the respective line observed
for well-annealed bulk crystals. The Halder–Wagner parabolic approximate relation,
pH 11 is shown as an example.
Due to the rather poor quality of the X-ray powder diffraction
patterns (n.b. obvious for nanocrystalline species), solution of the
structures is difficult when using a routine procedure. Fortunately,
the structures of our samples similar to those known from lit-
erature: hexagonal [11] and monoclinic [14]. However, the exact
crystal structure of hydrated orthophosphate is not known. The
key paper by Mooney [11] gives the positions of atoms except
those of zeolite water. The same rigid framework as for our hexag-
onal ortophosphates was found in the structure of KCaNd(PO )
2
ˇ = B − b /B, was used ˇ is the FWHM of the true diffraction profile, B and b are
the measured FWHM of the equivalent diffraction lines in the specimen and the
reference sample, respectively [27,28].
2
.3. Electron microscopy studies
4
2
[
30] where K+ ions are in the channels instead of water molecules.
Unfortunately, the use of such model for the samples calcined
The morphology and particle size of the phosphates were determined using a
transmission electron microscopy (TEM) technique. The TEM specimens were pre-