The synthesis and thermal stability of CaKCe(PO4)2 phosphate. Phase equilibria in the CaKPO4-CePO4 system

Article abstract of DOI:10.1016/j.jallcom.2012.12.027

The compound CaKCe(PO₄)₂ was obtained using respectively the ceramic method, the Pechini method and the hydrothermal method. The low-temperature (Pechini and hydrothermal) methods yielded homogenous powders with a crystallite size up to 24 nm. The compound has a hexagonal structure with parameters of an elementary cell: a = b = 7.0468(8) ? and c = 6.4367(0) ?. The CaKCe(PO₄)₂ phosphate occurs in one polymorphic form. A phase equilibrium diagram of the CaKPO ₄-CePO₄ system has been established on the basis of XRD, DTA/DSC/TGA results. The initial phosphates were found to form one intermediate compound, CaKCe(PO₄)₂, with molar ratio CaKPO ₄:CePO₄ = 1:1, melting incongruently at a temperature of 1530 °C according to the reaction: CaKCe(PO₄)_2(cr) → CePO_4(cr) + L. At a temperature of 1440 °C an eutectic was found to be present in the CaKPO₄-CePO₄ system at x(CePO₄) = 0.25.

Full text of DOI:10.1016/j.jallcom.2012.12.027

Journal of Alloys and Compounds 555 (2013) 138–144
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Journal of Alloys and Compounds
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The synthesis and thermal stability of CaKCe(PO4)2 phosphate. Phase equilibria
in the CaKPO4–CePO4 system
⇑
Aleksandra Matraszek , Irena Szczygieł, Teresa Znamierowska
Department of Inorganic Chemistry, Faculty of Engineering and Economics, Wrocław University of Economics, Komandorska 118/120, 53-345 Wrocław, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 19 October 2012
Received in revised form 6 December 2012
Accepted 10 December 2012
Available online 20 December 2012
The compound CaKCe(PO4)2 was obtained using respectively the ceramic method, the Pechini method
and the hydrothermal method. The low-temperature (Pechini and hydrothermal) methods yielded
homogenous powders with a crystallite size up to 24 nm.
The compound has a hexagonal structure with parameters of an elementary cell: a = b = 7.0468(8) Å
and c = 6.4367(0) Å. The CaKCe(PO4)2 phosphate occurs in one polymorphic form.
A phase equilibrium diagram of the CaKPO4–CePO4 system has been established on the basis of
XRD, DTA/DSC/TGA results. The initial phosphates were found to form one intermediate compound,
CaKCe(PO4)2, with molar ratio CaKPO4:CePO4 = 1:1, melting incongruently at a temperature of 1530 °C
according to the reaction: CaKCe(PO4)2(cr) ? CePO4(cr) + L. At a temperature of 1440 °C an eutectic was
found to be present in the CaKPO4–CePO4 system at x(CePO4) = 0.25.
Keywords:
Rare earth alloys and compounds
Phase diagrams
Thermal analysis
Chemical synthesis
X-ray diffraction
Ó 2012 Published by Elsevier B.V.
1. Introduction
Phosphate CaKPO4 occurs in two structural forms and melts at
1560 °C. The low-temperature form has an orthorhombic structure,
The research data published in recent decades indicate a high
application potential of multicomponent phosphates with the for-
mula CaKLn(PO4)2, where Ln – a rare-earth element [1–10]. The
compounds have a hexagonal structure with unit-cell parameters
a = b ꢀ 7.0 and c ꢀ 6.4 Å [11,12]. The structure is like that of
rhabdophanes, i.e. compounds of the LnPO4ꢁnH2O type. It is
characterized by the presence of longitudinal spaces formed by
(Ln, Ca)–PO4–(Ln, Ca) chains running parallel to axis c. Potassium
atoms are placed in the spaces [12]. This crystal structure suggests
that compounds of this kind may be good ionic conductors. The
CaKLn(PO4)2 compounds, where Ln = La, Eu and Y, crystallizing in
the monoclinic system (the monazite structure), were obtained
through precipitation from aqueous salt solutions [11,13] too.
In [3,14], experimental difficulties in obtaining pure
CaKLn(PO4)2 phases through synthesis in solid state were encoun-
tered. In the same papers it was suggested that the thermal stabil-
ity of the preparations is limited.
whereas the high-temperature form has a hexagonal structure
[15]. The polymorphic change of this phosphate is quite compli-
cated and takes place in a temperature range of 600–660 °C [16].
Cerium orthophosphate has one thermodynamically stable
polymorphic form (monazite) with
a monoclinic structure
[17,18], melting congruently at a temperature of 2072 °C [19].
The aim of the present research was to characterize the thermal
behavior of CaKCe(PO4)2, i.e. to determine its thermal stability and
melting temperature and to verify the previously mentioned re-
ports about the existence of additional polymorphic forms. More-
over, in order to solve the problems with obtaining pure
compounds, alternative synthesis routes of CaKCe(PO4)2 have been
proposed. The compound CaKCe(PO4)2 was for the first time ob-
tained using hydrothermal and modified Pechini methods. The
hydrothermal syntheses consist in heating solutions under ele-
vated pressure, which leads to precipitation of deposits. As a result,
it is possible to obtain morphologically uniform and unagglomerat-
ed crystallites whose size is a function of temperature and/or syn-
thesis time [20]. The Pechini method is a kind of sol–gel techniques
[21]. It ensures good control of the stoichiometry of multicompo-
nent compounds and a considerable reduction in phase sintering
temperature (to below 600 °C). Our team has successfully used this
method to obtain other phosphates [22,23].
Compounds of the formula CaKLn(PO4)2 form in CaKPO4–LnPO4
binary systems. In [11] a phase diagram of the system for Ln = La
was presented. It was found that this phosphate undergoes poly-
morphic transition at 1120 °C. At this temperature the structure
changes from low-temperature-monoclinic (b) to hexagonal (a).
According to the results [11], at a temperature of 1480 °C the ter-
nary phosphate undergoes peritectic decomposition.
Another aim of this research was to determine the phase rela-
tions in the CaKPO4–CePO4 system, with a particular regard for
the possible formation of limited solid solutions between the initial
orthophosphates and the intermediate CaKCe(PO4)2 compound.
⇑
Corresponding author. Tel.: +48 71 368 0321; fax: +48 71 368 0292.
E-mail address: aleksandra.matraszek@ue.wroc.pl (A. Matraszek).
0925-8388/$ - see front matter Ó 2012 Published by Elsevier B.V.
http://dx.doi.org/10.1016/j.jallcom.2012.12.027

A. Matraszek et al. / Journal of Alloys and Compounds 555 (2013) 138–144
139
(PO4)2. The experiments were performed under Ar atmosphere in a horizontal tubu-
lar furnace with molybdenum heating wires. Phase composition of molten and
cooled to room temperature samples was controlled by XRD method.
The existence of phases with a variable composition has been sev-
eral times reported in papers dealing with phosphates systems
with LnPO4 [24–27]. The existence of such solid solutions based
on rhabdophane would enable modification of physicochemical
properties of the compound to a large extent.
3. Results and discussion
3.1. Synthesis and characterization of CaKCe(PO4)2 compound
2. Experimental
The CaKCe(PO4)2 compound was obtained using respectively
the solid state synthesis (ceramic technique), the hydrothermal
and the Pechini methods. The phase purity of the obtained pow-
ders was evaluated by XRD, DTA/DSC/TGA and FTIR techniques.
Fig. 1 shows the X-ray diffraction patterns of the compound
obtained by the methods mentioned above. An analysis of the
diffractogramms revealed presence of a phase with a rhabdophane
structure. The indexed pattern of the CaKCe(PO4)2 obtained by
the ceramic method confirmed the presence of a compound with a
hexagonal structure and unit-cell parameters a = b = 7.0468(8) Å
and c = 6.4367(0) Å. The parameters are slightly higher than the
ones given for the CaKNd(PO4)2 compound [12], due to smaller
ionic radius of Nd3+ than that of Ce3+ [29]. Consequently, the unit
cell volume of CaKCe(PO4)2 should also be larger than that of neo-
dymium phosphate CaKNd(PO4)2.
The CaKPO4 and CaKCe(PO4)2 compounds for the phase equilibria study were
prepared in a solid state by a ceramic method. The following analytical reagents
were used to obtain the compounds: CaCO3, CaHPO4, K2CO3, Ce(NO3)3ꢁ6H2O and
H3PO4 (85%) (analytical grade, POCh Gliwice). The CePO4 was obtained by precipi-
tation from a dilute aqueous solution of H3PO4. The molar ratio of H3PO4:Ce(NO3)3
ꢁ6H2O:H2O was 1:0.023:25. The acidic solution of cerium nitrate was boiled under
reflux for at least 6 h. The obtained precipitate was filtered and washed several
times with hot distilled water. Finally, the powder was calcined at 1400 °C for 2 h
to expel moisture and adsorbed pyrophosphates.
The CaKPO4 compound was synthesized according to the synthesis route pro-
posed in [16] from a stoichiometric amounts of K2CO3 and Ca2P2O7, which were
carefully ground in an vibratory mill (Fritsch, Pulverisette 23) in a presence of ace-
tone. The powder was calcined 10 h at 1000 °C, ground again with acetone in the
vibratory mill, pelletized and sintered 1 h at 1300 °C.
The CaKCe(PO4)2 phosphate was obtained by a ceramic method from stoichiom-
etric mixture of CePO4 and CaKPO4 in a similar route as CaKPO4 with following sin-
tering parameters: 1000 °C/10 h and 1050 °C/10 h.
Since the X-ray pattern of the CaKCe(PO4)2 is similar to the
spectrum of CePO4ꢁnH2O, the obtained powders were additionally
characterized by means of Fourier-transform infra-red spectros-
copy (FTIR). The FTIR spectra of the powders synthesized by the
different methods are shown in Fig. 2. For comparison, spectrum
of CePO4ꢁnH2O (rhabdophane) was included. The presented spectra
of CaKCe(PO4)2 (curves B–D in Fig. 2) and rhabdophane (Fig. 2
curve A) differ in the ranges of 1200–850 and 650–400 cmꢂ1, i.e.
in the ranges in which main vibrations of PO4 groups are observed.
The difference in the symmetry of the PO4 groups in the CaK-
Ce(PO4)2 and CePO4ꢁnH2O phosphates is indicated by:
The samples of the CaKPO4–CePO4 system with the composition
{(1ꢂx)CaKPO4 + (x)CePO4}, where x(CePO4) < 0.50, were synthesized from
CaKCePO4 and CaKPO4. The samples with higher CePO4 concentration were ob-
tained from CaKCe(PO4)2 and CePO4 compounds. The initial mixtures were ground
in acetone, pelletized, and sintered 10 h at 1000 and 10 h at 1080 °C. All samples
were ground in vibratory mill between the consecutive heating steps.
The CaKCe(PO4)2 compound was also obtained by a modified Pechini route and
by a hydrothermal method. In the Pechini process following nitrates were used in
stoichiometric ratios: Ce(NO3)3ꢁ6H2O, Ca(NO3)2ꢁ4H2O and KNO3. The salts were dis-
solved in a small amount of distilled water. Citric acid (CA) and ethylene glycol (EG)
were added to the solution in the CA:EG:Ce molar ratio of 4:8:1. The obtained solu-
tion has been heated at 80 °C under mixing until a viscous liquid obtained. Then
NH4H2PO4 in the appropriate molar ratio was added under stirring. The heating
has been continued until a foam obtained, then the sample was put in an electric
drier for 20 h at 120 °C. Calcination of dried gel was carried out in a furnace pre-
heated at 500 °C for 10 h.
The CaKCe(PO4)2 powder was also obtained under short-lasting hydrothermal
conditions. The chemical reagents, Ce(III) nitrate hexahydrate, calcium nitrate tet-
rahydrate and K3PO4ꢁ7H2O were used in the 1:1:22 M ratio with the concentration
of cerium of 0.04 mol/dm3. The nitrates were diluted with water, then potassium
phosphate was added and the solution was diluted to obtain 50 ml of milky sol.
As obtained sol was used for hydrothermal process. The hydrothermal reactions
were performed in a Magnum II autoclave (Ertec, Poland) with heating of the sam-
ples by microwaves of the maximal power of 600 W. The syntheses were controlled
by temperature measurement performed at the bottom part of Teflon reactor. The
synthesis was carried out for 1 h at a maximal temperature of 240 °C. As obtained
fine precipitates were centrifuged, washed several times with distilled water and
dried at 120 °C.
- The presence of two absorption bands at a frequency of
600 cmꢂ1 for CaKCe(PO4)2, instead of one band for CePO4ꢁnH2O.
- The appearance of additional absorption bands for CaKCe(PO4)2
in wave numbers range of 500–400 cmꢂ1
.
- The presence of an intensive absorption band at about
950 cmꢂ1
.
The change in the symmetry of the PO4 from D2 (in rhabdo-
phane) to C2 (in ternary phosphate) was described in detail in
Phase analysis of obtained powders was made using XRD technique and a Sie-
mens D5000 diffractometer equipped with a copper radiation tube. The measure-
ments were performed in 2h angle range of 10–60° with a 0.02° step and at least
2 s per step. Silicon (99.995% ABCR GmbH) was used as an internal standard for
refinement of structure parameters. Lattice constants of the CaKCe(PO4)2 unit cell
were refined using Checkcell software [28].
The surface area value of CaKCe(PO4)2 samples obtained by various methods
were determined by multipoint BET method by nitrogen adsorption using
Autosorb 1, Quantachrome Autosorb Automated Gas Sorption System
(Quantachrome Instruments). The mean grain size of powders was estimated using
6
the relation: dBET
¼
, where SBET is the measured specific surface area of
SBET
ꢁ
qth
powders and qth equal 3.65 g cmꢂ3 is the theoretical density of the CaKCe(PO4)2.
The DTA/TGA experiments were carried out using Derivatograph 3427 (MOM,
Hungary). Samples were heated in a temperature range of 20–1400 °C (heating
rate: 7.5 °C minꢂ1, Pt crucible, sample mass 450–600 mg, air atmosphere). The SET-
SYS™ apparatus (TG-DSC 1500; Setaram) was used for DSC/TGA studies. From 15 to
40 mg of the powdered samples were placed in a Pt crucible and heated to 1400 °C
at a heating rate of 10 °C minꢂ1 using Ar as the purge gas. The temperature and sen-
sitivity calibration factor for the thermal experiments were obtained at melting
points of NaCl (801 °C), Ca2P2O7 (1353 °C) and K2SO4 (1070 °C) and for the phase
transition temperature of the latter compound (583 °C).
The temperatures above 1400 °C were determined by observing the behavior of
pelletized powders by means of an optical pyrometer (Cyclops 100, Ametek Land)
calibrated against melting points of SiO2 (5N, Carl Roth GmbH), Na3PO4 and Ca3
Fig. 1. The XRD patterns of CaKCe(PO4)2 compound obtained by hydrothermal
process dried at 120 °C (A), by Pechini method sintered 10 h at 500 °C (B), and
synthesized in solid state at 1050 °C (C).

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A. Matraszek et al. / Journal of Alloys and Compounds 555 (2013) 138–144
75 nm for the powder synthesized by the Pechini method. The dif-
ference in the determined grain and crystallite sizes of the powders
obtained by the Pechini method may indicate the presence of
aggregates made up of several crystallites. This was confirmed by
TEM imaging of powders. Fig. 3 presents TEM and HRTEM images
of samples obtained by hydrothermal and Pechini techniques.
The powder synthesized by Pechini method is characterized by
presence of aggregates with nanocrystals with diameter up to
30 nm (Fig. 3A and B). The TEM images of sample obtained under
hydrothermal conditions revealed the sample to be built of round
shaped nanocrystals with the diameter of about 15 nm (Fig. 3C and
D).
The sample obtained by ceramic method was characterized by
much lower BET specific surface. Moreover, grinding (for 5 min at
40 Hz) in a vibrating mill of the sample resulted in increase of spe-
cific surface area value from 0.05 m2gꢂ1 to 0.687 m2gꢂ1. The very
effective grinding is connected with a reduction in grain size from
33 to 2.4 lm.
The DTA/DSC analyses of the samples showed that CaKCe(PO4)2
doesn’t undergo any polymorphic change since no thermal effects
appeared on the curves. However, in a temperature range of 1300–
1400 °C a small mass loss of the compound (up to about 2 wt.%)
was observed. The TGA curves of samples obtained by different
methods are shown in Fig. 4.
The existence of the polymorphic change of compound
CaKLa(PO4)2 at 1120 °C was postulated in [11], where the phos-
phate was obtained from LaPO4 (monazite) and CaKPO4 using the
ceramic method and long-term sintering at 1200 °C. At the same
time the research by Campayo et al. [3,14] showed that the yield
of MeMLn(PO4)2 (Me: Ca, Sr; M: Cs, K) compounds by the synthesis
in solid state does not exceed 78%. The low yield of the reaction is
due to two facts. First of all, monazites are compounds character-
ized by very low reactivity against alkalis and acids and by stability
at high temperatures [32–34]. Moreover, during sintering of sub-
strates at a temperature of 1200 °C the MeMNd(PO4)2 phosphates
are depleted of potassium/caesium and phosphorous, whereby a
secondary phase, i.e. Ca3(PO4)2, forms. This calcium orthophos-
Fig. 2. The FTIR spectra of CePO4ꢁnH2O (rhabdophane) (A) and CaKCe(PO4)2
obtained by hydrothermal process and dried at 120 °C (B), sintered 2 h at 1000 °C
(C) as well as synthesized by Pechini method after sintering at 1000 °C for 10 h (D).
[30] and was correlated to different ionic environment of the PO4
groups in compounds CePO4ꢁnH2O and CaKCe(PO4)2.
In addition, strong bands at about 1630 cmꢂ1 are visible in the
spectrum of the CaKCe(PO4)2 compound obtained by the hydro-
thermal route and in the spectrum of the CePO4ꢁnH2O phosphate
(the curves B and A in the Fig. 2, respectively). The bands are char-
acteristic of the bending vibrations of water molecules. The pres-
ence of the bands in the spectra could be attributed to the
presence of zeolitic water molecules in the rhabdophane structure
and to the humidity in the ternary phosphate.
phate undergoes a polymorphic transition at about 1120 °C
[35,36]. The same transition temperature was reported by Jung-
owska for the CaKLa(PO4)2 compound [11]. This means that the
thermal effects observed on the DTA/DSC curves in [11] may have
been connected with the polymorphic transition b ? a-Ca3(PO4)2.
From experimental results presented in this study and other liter-
ature reports [3,14] it appears that the CaKLn(PO4)2 phase has only
one structural form and the thermal effects in the DTA curves ob-
served in [11] could have been connected with the presence of
Ca3(PO4)2 due to the inhomogeneity of the samples or to the partial
decomposition of the CaKLa(PO4)2 phosphate.
The decrement in the mass of the CaKCe(PO4)2 compound reg-
istered at the TGA curves of the compound was attributed to the
partial decomposition of this phase as a result of an incongruent
sublimation of more volatile components, i.e. probably gaseous
phosphorous and potassium species. The sublimation process
changes the chemical composition of the solid phase, causing the
decomposition of CaKCe(PO4)2. Similar remarks considering the
limited stability of MMeNd(PO4)2 phases as a result of depletion
of Cs/K and phosphorous in the course of high temperature treat-
ment were reported in previously published papers [3,14].
In order to determine the thermal stability of CaKCe(PO4)2 the
powders obtained by the hydrothermal and the ceramic methods
were additionally annealed at a temperature of 1200 °C for 120 h.
The main change in the phase composition of the preparations
was found in the hydrothermally synthesized sample. The X-ray
diffraction patterns of the sample sintered at different tempera-
tures are shown in Fig. 5. An analysis of the diffractograms showed
that CaKCe(PO4)2 had undergo complete decomposition into
Moreover, the X-ray patterns of the samples obtained by the
low-temperature (Pechini and hydrothermal) methods were char-
acterized by a considerable broadening of reflections (curves A and
B in Fig. 1). This is due to the smaller crystallite size of the ternary
phosphate. In order to estimate the average size of the crystallites
Kk
in the prepared samples the Scherrer equation was used: D ¼
,
where D/nm is the average crystallite size in the direction perBpceons h-
dicular to the (hkl) plane of a given reflection. K = 0.9 is the Scher-
rer constant [31], k = 0.154 nm – the wavelength of radiation CuK
a
and B – the corrected full-width at the half-maximum (FWHM) of
the diffraction peak at angle h. The calculated average sizes of the
crystallites in the powders, obtained by the hydrothermal method
and the Pechini method, amounted to respectively about 14 and
24 nm.
The BET specific surface area determined from the nitrogen
adsorption measurements amounted to 102 and 22 m2gꢂ1 for the
CaKCe(PO4)2 powders obtained by respectively the hydrothermal
method and the Pechini method. The average size of the grains in
the obtained samples was calculated on the basis of the deter-
mined specific surface area of the powders. The average grain size
amounts to 16 nm for the hydrothermally obtained phosphate and

A. Matraszek et al. / Journal of Alloys and Compounds 555 (2013) 138–144
141
Fig. 3. The TEM and HRTEM images of samples obtained by Pechini (A and B) and hydrothermal methods (C and D).
Fig. 4. The TGA curves of CaKCe(PO4)2 compound obtained by Pechini method (A),
in solid state (B) and in hydrothermal process (C). Experiments carried out in air
atmosphere.
Fig. 5. The XRD patterns of CaKCe(PO4)2 compound synthesized by the hydrother-
mal method after sintering 1 h at 1000 °C (A) and 120 h at 1200 °C (B), as well as the
sample obtained by ceramic method heated 120 h at 1200 °C (C). [s-CePO4
(monazite), }-CeO2, ꢀ-Ca10K(PO4)7].
phases: Ca10K(PO4)7, CeO2 and CePO4 in the course of long-term
annealing at 1200 °C. In the sample synthesized by the ceramic
method and sintered at elevated temperature no secondary phases
were found to be present. Thus the presence of the secondary
phases in the powders could have been below the detection limit
of the XRD technique.
Unfortunately, the chemical analysis of the sample which had
undergone thermal decomposition was not possible because of
the low reactivity of the formed phases. The solid residues could
not be digested even in aqua regia. However, the analysis of the re-
sults obtained by the XRD method allows to conclude as follows:

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A. Matraszek et al. / Journal of Alloys and Compounds 555 (2013) 138–144
ꢃ at elevated temperature CaKCe(PO4)2 undergoes decomposition
into Ca10K(PO4)7, CeO2 and CePO4.
ꢃ from the elemental balance and the data on the sublimation of
orthophosphates [37] the following decomposition route of ter-
nary phosphate could be proposed:
10CaKCeðPO4Þ2ðcrÞ ! Ca10KðPO4Þ7ðcrÞ þ ð10 ꢂ xÞCePO4ðcrÞ
þ ðxÞCeO2ðcrÞ þ 9KðgÞ þ ð3 þ xÞPO2ðgÞ þ 3O2ðgÞ
:
The scheme indicates considerable change in the phase compo-
sition of ternary phosphate in the course of sintering. This is a re-
sult of sublimation of potassium and phosphorous species from
solid phase. At the same time it should be noted that the chemical
composition of compound Ca10K(PO4)7 corresponds to a concentra-
tion range in which a solid solution of CaKPO4 in b-Ca3(PO4)2
exists [16]. In addition Ca10K(PO4)7 is isostructural with the low-
temperature Ca3(PO4)2 form [38]. This means that it is difficult to
distinguish compounds Ca10K(PO4)7 and b-Ca3(PO4)2 on the basis
of XRD spectra, and b-Ca3(PO4)2 instead of Ca10K(PO4)7 may be
actually observed in the sample after sintering. Thus the decompo-
sition of CaKCe(PO4)2 phase may proceed gradually until potas-
sium completely evaporates from the sample and b-Ca3(PO4)2
forms. The proposed decomposition scheme is also in accordance
with previously mentioned studies of thermal stability of ternary
phosphates [3,14].
The observed differences in the intensity of the decomposition
of the ternary phosphate obtained by the ceramic method and
the hydrothermal method can be explained by the different spe-
cific surface areas of the samples (102 and 0.05 m2gꢂ1 respectively
for powders obtained by hydrothermal and ceramic method), since
the sublimation processes largely depend on the size and quality of
the material surface.
Fig. 6. The DSC heating curves of samples with x(CePO4) = 0 (A), 0.25 (B) and 0.45
(C).
CePO4 system are shown in Fig. 7. The same figure summarizes also
samples melting points, determined by the experiments in hori-
zontal furnaces. Due to very high temperatures of experiments
and limited equipment stability it was not possible to determine
melting temperatures of samples with x(CePO4) > 0.75. The phase
composition of samples ground after melting experiments was
examined by XRD and microscopic observations of polished cross
sections of samples cooled from melt. The XRD results did not
show presence of phases other then CaKCe(PO4)2, CaKPO4 or CePO4.
Nevertheless, because of the limited thermal stability of the phases
present in the system, the presented in the Fig. 7 liquidus shape in
the area rich in CePO4 is as proposed.
The chemical composition of an eutectic point and its tempera-
ture were determined from the distribution of samples melting
temperatures. Fig. 8 shows microphotographs of cross sections of
samples with x(CePO4) = 0.25 and 0.50. The microstructure of the
sample with x(CePO4) = 0.25 (Fig. 8A) is characteristic of an eutec-
tic point. Whereas in the cross section of the sample with x = 0.50,
i.e. compound CaKCe(PO4)2, large polyhedral grains of about
3.2. Phase equilibria in the CaKPO4–CePO4 system
Because of the limited thermal stability of compounds present
in the system, it was necessary to reduce the temperature and sin-
tering time of samples used in the phase equilibrium study. Conse-
quently, the samples were several times pelletized and ground
between heating steps. Owing to this the phase equilibrium in
the samples could be reached easier.
An analysis of the X-ray spectra of the samples sintered from
the CaKPO4–CePO4 system showed all the samples, except for
CaKPO4, CaKCe(PO4)2 and CePO4 compounds, to be diphase. In
the samples containing less then 50 mol.% of CePO4, CaKPO4 and
CaKCe(PO4)2 phases were found to coexist.
The DSC results confirmed the presence of phase CaKPO4 in the
samples. Fig. 6 shows the DSC curves for the heating of the samples
selected from the CaKPO4–CaKCe(PO4)2 phase area. In the temper-
ature range of 600–700 °C, two endothermic effects were observed
on the heating curves. Both effects are connected with the poly-
morphic transition of CaKPO4 [16]. As the molar fraction of CePO4
increased, the intensity of the effects decreased, but the change of
chemical composition of samples had no influence on the onset
temperatures of effects. Since the temperatures were constant
and independent of the CaKCe(PO4)2 content in the samples, the
formation of any solid solution within the composition range of
CaKPO4-CaKCe(PO4)2 could be ruled out.
The samples with more then 50 mol.% of CePO4 were also di-
phase. The diffraction spectra of the synthesized powders showed
compounds CaKCe(PO4)2 and CePO4 to be present in them. The
DTA-heating curves for the samples did not show any thermal ef-
fects in the temperature range of 20–1400 °C.
30 ꢄ 100
lm in size are visible (Fig. 8B). The grains were visible
in all samples with 0.25 < x(CePO4) < 0.60. Small acicular and light
in color occlusions can be distinguished within the polyhedral
grains. The amount of the segregations increased with CePO4 con-
tent. Analysis of XRD patterns of the CaKCe(PO4)2 compound after
crystallization from melt showed that the sample was diphase. Be-
sides ternary phosphate a small amount of CePO4 was found to be
present. The appearance of the latter phase in the sample can be
indicative of the partial decomposition of CaKCe(PO4)2 at temper-
atures above 1200 °C, as already discussed above. However, con-
sidering that no Ca3(PO4)2, in any of its polymorphic forms, was
found to be present in the sample, another probable explanation
of CePO4 presence is possible. The distribution of the melting tem-
peratures of samples with 45–75 mol.% CePO4 content does not
show any maximum. This kind of temperatures distribution is
All the registered onset temperatures of thermal effects re-
flected on the heating DTA/DSC curves of samples in the CaKPO4–

A. Matraszek et al. / Journal of Alloys and Compounds 555 (2013) 138–144
143
Fig. 7. Phase diagram of CaKPO4–CePO4 system. [
determined by optical observation of pellets].
r
– onset temperatures of endothermic effects registered on DTA/DSC heating curves, s – melting temperatures of samples
Fig. 8. The microstructure of samples cooled from melt with x(CePO4) = 0.25 (A) and 0.50 (B).
indicative of peritectic reaction of compound CaKCe(PO4)2, pro-
4. Conclusions
ceeding according to the scheme:
The results of the study provided the basis for proposing a dia-
gram representing the phase equilibria in the CaKPO4–CePO4 sys-
tem (Fig. 7). Compound CaKCe(PO4)2 forms in the CaKPO4–CePO4
system and an eutectic point was found to be present at
T = 1444 °C for x(CePO4) = 0.25. The results of XRD and thermal
analysis experiments did not show any solid solution to be present
in the system examined.
Compound CaKCe(PO4)2 has a hexagonal structure of the rhab-
dophane type, with unit-cell parameters a = b = 7.0468(8) Å and
c = 6.4367(0) Å. The compound does not undergo any polymorphic
transitions. It is characterized by limited thermal stability at tem-
peratures above 1200 °C, however rate of the decomposition of the
phosphate depends on the specific surface area value of powder.
Compound CaKCe(PO4)2 melts incongruently at a temperature of
1530 °C.
CaKCeðPO4Þ2ðcrÞ ! CePO4ðcrÞ þ L:
During crystallization of sample with x(CePO4) = 0.50 from tem-
peratures higher than the temperature of the peritectic reaction,
monazite (CePO4) crystallizes first and only when T 6 Tper the latter
compound starts reacting with liquid being in equilibrium, forming
CaKCe(PO4)2. The crystallization of ternary phosphate takes place
at liquid/monazite interface. Thus if the sample is cooled down
from the high temperatures too quickly (in non- equilibrium con-
ditions) the crystallization of CaKCe(PO4)2 may be incomplete. As a
result, unreacted CePO4 may be present inside the grains of the ter-
nary phosphate. This should be accompanied by the appearance of
vitrified liquid phase in the cooled samples. This microstructure of
compound CaKCe(PO4)2 is visible in Fig. 8B. Thus the presence of
the secondary crystalline phase (CePO4) may be due to the non-
equilibrium state of the sample, caused by the too rapid cooling
from melt.
According to the previously published studies on the subject,
the ceramic method’s yield of phase-pure MMeLn(PO4)2 samples
is limited. In this study, in order to improve the yield of
CaKCe(PO4)2, the solid-phase syntheses were conducted in stages,

144
A. Matraszek et al. / Journal of Alloys and Compounds 555 (2013) 138–144
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Acknowledgements
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The study was financially supported by Polish Ministry of Sci-
ence and Higher Education under grant JP 2010 024870 and by Na-
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