High-temperature reactions in the Co3Cr4(PO4)6–Cr(PO3)3 system
High-temperature reactions in the Co3Cr4(PO4)6-Cr(PO3)3 system
Monika Bosacka 0 1
Anna Błon´ ska-Tabero 0 1
Elz_ bieta Filipek 0 1
Jana Luxova 0 1
Petra Sˇ ulcova´ 0 1
0 Department of Inorganic Technology, Faculty of Chemical Technology, University of Pardubice , Doubravice 41, 532 10 Pardubice , Czech Republic
1 Department of Inorganic and Analytical Chemistry, Faculty of Chemical Technology and Engineering, West Pomeranian University of Technology , Szczecin, Al. Piasto ́w 42, 71-065 Szczecin , Poland
A new dichromium(III) cobalt(II) diphosphate(V) of the formula CoCr2(P2O7)2 was detected in the Co3Cr4 (PO4)6-Cr(PO3)3 system. The new compound was obtained as a result of high-temperature solid-state reactions between CoCO3, Cr2O3 and (NH4)2HPO4 as well as between Cr(PO3)3 and Co3Cr4(PO4)6. CoCr2(P2O7)2 was characterized using XRD, DTA and IR methods. Results demonstrated that CoCr2(P2O7)2 crystallizes in the triclinic system and its unit cell parameters were calculated. Its infrared spectrum was presented. CoCr2(P2O7)2 melts incongruently at 1270±10 C with a formation of solid a-CrPO4. The compound Co3Cr4 (PO4)6, component of the system under study, was obtained for the first time as a pure phase. Its thermal stability was also investigated. Co3Cr4(PO4)6 is stable in air up to 1410 ± 20 C.
Chromium cobalt phosphate; System CoO-; P2O5-Cr2O3; CoCr2(P2O7)2; XRD; DTA; IR
The phosphates(V) of di- and trivalent metals are the objects
of extensive studies as they have many possibilities of
application. Some of such phosphates exhibit anticorrosion
& Anna Błon´ska-Tabero
properties , and the other can be used as ceramic pigments
[2, 3], as an anode material for lithium-ion batteries  or as
efficient catalysts [5, 6]. Research work aimed at synthesis
and characterization of new phosphates(V) is very important
for designing of new materials which can be applied in
different areas of contemporary technology. Such new
phosphates can be formed inter alia in the ternary oxide system
CoO–P2O5–Cr2O3. Cobalt(II) phosphates(V) and
chromium(III) phosphates(V), forming in the lateral systems of the
mentioned ternary oxide system, are known and widely
investigated [1, 2, 4–9], but only one article  was found
referring to a phase, which is formed with an involvement of
all three oxides, i.e. Co3Cr4(PO4)6. The compound was
synthesized by heating the mixture of Co3O4, Cr2O3 and
NH4H2PO4 at a molar ratio 1:2:6 up to 1000 C for 300 h
. The authors , however, could not obtain Co3Cr4
(PO4)6 as a pure phase. However, they assumed that this
compound is isostructural with the compounds from the
family M32?M43?(PO4)6, and on this basis, the structure of
Co3Cr4(PO4)6 was solved . The compound crystallizes in
the triclinic system, and its unit cell parameters are the
following: a = 7.8561 A˚ , b = 9.2350 A˚ , c = 6.2785 A˚ ,
a = 108.173 , b = 101.808 , c = 105.329 . To the
best of our knowledge, there is no literature information on
thermal stability of Co3Cr4(PO4)6.
Therefore, it follows from literature survey that, till now,
there were no systematic investigations on phase formation
in the CoO–P2O5–Cr2O3 system. The main aim of the
presented work was to check whether in one of the cross sections
of the CoO–P2O5–Cr2O3 system, i.e. in the system
Co3Cr4(PO4)6–Cr(PO3)3, any new phosphate(V) is formed
and if so—the second point of the study was to determine
some of its physicochemical properties. Additionally,
necessary investigations, verifying and supplementing literature
data relating to Co3Cr4(PO4)6, were conducted.
As the initial reagents were used: Cr2O3 (p.a., Aldrich,
Germany), CoCO3 (p.a., Fluka, Switzerland), (NH4)2HPO4
(p.a., POCh, Poland).
Reactions were conducted by the conventional method of
calcining samples [9–14]. Appropriate portions of reacting
substances were homogenized by grinding, pressed into
pellets and heated in air atmosphere in several stages, until an
equilibrium state was attained. The first heating stage (at
350 C) of samples containing in initial mixtures precursors
of CoO and P2O5 was applied mainly to remove NH3, H2O
and CO2, so they were pressed into pellets only from the
second heating stage. Next, the heating temperature was
gradually increased, because it follows from our earlier
studies that heating immediately at higher temperatures
leads to obtaining the samples in glass form. After each
heating stage, samples were gradually cooled down in the
furnace to room temperature and next the pellets were
ground and examined by XRD method or by DTA, too.
Generally, to avoid the melting of the samples the maximum
temperature of their heating was several dozen of C lower
than their melting temperatures, which were read from their
The XRD method was used to determine the type of the
phases occurring in particular samples [13, 14]. The powder
diffraction patterns of samples were recorded with the aid of
the Empyrean II diffractometer (PANalytical, Netherlands)
using CuKa radiation with graphite monochromator. The
identification of phases, present in the samples, was
conducted based on their XRD characteristics contained in the
PDF cards. The powder diffraction pattern of the new
compound was indexed by means of the POWDER programme
. The internal standard was a-Al2O3. The parameters of
the unit cell were refined by the refinement programme of
Thermal stability of some obtained samples was
investigated using the F.Paulik–J.Paulik–L.Erdey-type
derivatograph Q – 1500 D (MOM, Hungary), in the temperature
range 20–1400 C, at a heating rate of 7.5 C min-1 and
sample mass 500 mg. The investigations were conducted
also in temperature range 20–1500 C using an optical
pyrometer (Raytek, model RAYMM1MHSF2V, Germany).
The density of the new compound was determined with
the help of an Ultrapyc 1200e ultrapycnometer
(Quantachrome Instruments, USA) using argon (5 N purity) as a
pycnometric gas. The measurements were taken in five
repetitions using *2 g of each sample for the test.
The IR spectroscopic measurement (the Specord M 80
spectrometer, Carl Zeiss, Germany) was conducted
applying the technique of pressing pastilles of the sample with
KBr [13, 16, 17] at the ratio 1:300 by weight.
Results and discussion
Synthesis and thermal stability of Co3Cr4(PO4)6
Preliminary stage of the study was devoted to verification
and supplementation of the information needed for
realization of the main aim of the work and concerning one of
the components of Co3Cr4(PO4)6–Cr(PO3)3 system, i.e.
The authors  have synthesized a compound of the
composition corresponding to the formula Co3Cr4(PO4)6.
Despite the prolonged heating time of the reactants mixture
(300 h) in the temperature range of 800–1000 C, they have
not obtained a monophase sample. Therefore, the aim of the
first stage of the study was to obtain pure Co3Cr4(PO4)6
compound and to determine its thermal stability in air
atmosphere. The stoichiometric mixture of CoCO3, Cr2O3
and (NH4)2HPO4 was heated in the following stages:
350 C(12 h) ? 500 C(12 h) ? 650 C(12 h) ? 700 C
(12 h) ? 950 C(12 h) ? 1050 C(12 h) 9 2 ? 1100 C
(12 h) 9 2 ? 1200 C(12 h). XRD phase analysis of the
sample obtained after its last heating stage has proved that it
is monophase, because its diffractogram contains only a set
of lines characteristic for Co3Cr4(PO4)6 (PDF 49-0499).
These results testify that shorter heating stages, but at
higher temperatures (in comparison with those given in the
literature ) allowed to obtain pure Co3Cr4(PO4)6
compound, according to the complete stoichiometric reaction:
3 CoCO3ðsÞ þ 2 Cr2O3ðsÞ þ 6 ðNH4Þ2HPO4ðsÞ
¼ Co3Cr4ðPO4Þ6ðsÞþ3 CO2ðgÞ þ 12 NH3ðgÞ þ 9 H2OðgÞ:
As thermal stability of Co3Cr4(PO4)6 has not been
known, it was subjected to DTA measurements in air in the
temperature range 20–1400 C. In this range on DTA
curve, no thermal effects were recorded. This result means
that Co3Cr4(PO4)6 undergoes decomposition or melting at
temperature higher than 1400 C. In order to assess its
thermal stability, the sample containing Co3Cr4(PO4)6 was
heated in air in a horizontal tube furnace equipped with an
optical pyrometer. Based on the result of the study, it was
found that the resulting compound is stable in air up to
1410 ± 20 C.
It is known from the literature that the second
compound, constituting the system under study, i.e. Cr(PO3)3,
decomposes to Cr2P4O13 and P2O5 at 1325 C .
Reactions in the Co3Cr4(PO4)6–Cr(PO3)3 system
In order to determine the kind of phases forming in the
system Co3Cr4(PO4)6–Cr(PO3)3 11 mixtures of CoCO3,
Cr2O3 and (NH4)2HPO4 were prepared. The composition of
initial mixtures, in terms of the components of the system
Co3Cr4(PO4)6–Cr(PO3)3 as well as of the system CoO–
P2O5–Cr2O3, is given in Table 1. All the samples were
heated in the following stages: 350 C(12 h) ? 500 C(12 h)
? 650 C(12 h) ? 700 C(12 h) ? 950 C(12 h) ? 1050 C
(12 h) 9 2, while samples 5–11 were additionally heated at
1100 C (12 h) twice. Table 1 presents XRD analysis
results for the samples after their last heating stage. In the
diffractograms of all the investigated samples, a set of
unidentified lines denoted by X was detected. However,
only as a result of heating a mixture initially containing
33.33 mol% Co3Cr4(PO4)6 and 66.67 mol% Cr(PO3)3 [in
terms of the components of the system Co3Cr4(PO4)6
–Cr(PO3)3] a monophase sample was obtained whose
diffractogram consisted only of a set of unidentified lines X.
These lines were not assigned to any initial reactants as well
as to any previously known phases that belong to the lateral
binary systems constituting the ternary oxide system CoO–
P2O5–Cr2O3. It has been concluded that the recorded set of
lines X is an XRD characteristic of a new compound of the
formula CoCr2(P2O7)2 that is formed according to the
complete stoichiometric reaction:
CoCO3ðsÞ þ Cr2O3ðsÞ þ 4 ðNH4Þ2HPO4ðsÞ
¼ CoCr2ðP2O7Þ2ðsÞþCO2ðgÞ þ 8 NH3ðgÞ þ 6 H2OðgÞ:
study, Cr(PO3)3 also occurs. In the remaining component
concentration range, i.e. above 33.33 mol% of Co3Cr4
(PO4)6, apart from CoCr2(P2O7)2, Co3Cr4(PO4)6 is formed.
The new compound has been also obtained by reaction
occurring by heating the stoichiometric mixture of Cr(PO3)3
and Co3Cr4(PO4)6 in the following stages: 700 C(12 h) ?
950 C(12 h) ? 1050 C(12 h) 92 ? 1100 C(12 h) 92:
2CrðPO3Þ3ðsÞþCo3Cr4ðPO4Þ6ðsÞ¼ 3 CoCr2ðP2O7Þ2ðsÞ:
In the next stage of the study, two mixtures (Table 1,
samples 12 and 13) were prepared with their compositions
being very close to that which corresponds to the formula
CoCr2(P2O7)2 but not representing the system under study
(Fig. 1). Heating conditions for sample 13 were the same
as for samples 1–4, while sample 12 was additionally
heated at 1100 C (12 h) twice (as samples 5–11). After the
last heating stage, besides of CoCr2(P2O7)2, the other
compounds were also identified in sample 12 and 13
(Table 1). The obtained results additionally prove that the
composition of the new phase corresponds to the formula
Figure 1 shows the positions of: the Co3Cr4(PO4)6–
Cr(PO3)3 cross section studied, samples 12 and 13 as well
as the new obtained compound in the component
concentration triangle of the ternary oxide system.
Some properties of the new compound
In the component concentration range up to 33.33 mol%
of Co3Cr4(PO4)6 (in terms of the components of the Co3
Cr4(PO4)6–Cr(PO3)3 system), in the cross section under
In a further part of the work, some physicochemical
properties of the new compound were investigated.
CoCr2(P2O7)2 has a patina colour; its density amounts to
Table 1 Composition of initial mixtures and their composition after the last heating stage
No. Composition of initial mixtures in terms of the components Composition of initial mixtures in terms
of the system Co3Cr4(PO4)6–Cr(PO3)3/mol% of oxides percentage/mol%
Phase composition of samples
after their last heating stage
Fig. 1 Positions of the Co3Cr4(PO4)6–
Cr(PO3)3 cross section studied, samples 12
and 13 as well as the new obtained
compound in the component concentration
triangle of the CoO–P2O5–Cr2O3 system
dobs = 3.63(5) g cm-3. Figure 2 shows a powder
diffraction pattern of the new compound, whereas Table 2
presents the results of its indexing. CoCr2(P2O7)2 crystallizes
in the triclinic system. The parameters of its primitive unit
cell are as follows: a = 6.684(3) A˚ , b = 10.295(5) A˚ ,
c = 20.136(9) A˚ , a = 121.41(8) , b = 94.25(4) , c =
86.04(4) . The unit cell volume V = 1178.5 A˚ 3; the
number of stoichiometric formula units in the unit cell Z = 5;
the XRD calculated density dcalc = 3.60 g cm-3.
It is known from the literature that there exist
compounds of the formulae CuFe2(P2O7)2  and CuIn2
(P2O7)2 , i.e. with a composition analogous to the
composition of the obtained new compound. However,
some considerable differences between the powder
diffraction patterns of CoCr2(P2O7)2 and CuM2(P2O7)2
(M = Fe, In) indicate that these compounds are not
In the DTA curve of CoCr2(P2O7)2, two endothermic
effects were recorded (Fig. 3), with onset temperatures
1270 and 1350 C, respectively. In order to determine the
type of the transformation that the first effect is due to, a
sample of CoCr2(P2O7)2 was heated for 2 h at 1285 C, i.e.
at the temperature of the maximum of this effect, and next
Fig. 2 Powder diffraction pattern of CoCr2(P2O7)2
it was rapidly cooled to room temperature and subjected to
XRD analysis. The obtained results indicate that the first
endothermic effect is due to incongruent melting of
CoCr2(P2O7)2 with a formation of solid a-CrPO4.
Table 2 Indexing results for the CoCr2(P2O7)2 powder diffraction
The IR spectrum of CoCr2(P2O7)2 is shown in Fig. 4.
Univocal attribution of the recorded absorption bands to
the specific vibrations is not possible, because the full
structure of the new compound is unknown. However,
probable attribution can be made in the light of literature
data [20–25]. IR spectrum of CoCr2(P2O7)2 is complex,
but in general, four characteristic groups of bands are
observed in the range 1500–300 cm-1. The broadening
character observed in the region 1270–1190 cm-1
corresponds to asymmetric stretching vibration of O–P–O
groups, while the next broad band in the region
1120–1000 cm-1 is related to the symmetric stretching
vibration of those linkages, e.g. O–P–O [20–22]. The
absorption bands at 970, 945, 925 are assigned to the
asymmetric stretching vibration of P–O–P linkages, while
Fig. 3 Fragment of DTA curve of CoCr2(P2O7)2
Fig. 4 IR spectrum of CoCr2(P2O7)2
the relatively weak band around 725 cm-1 is due to the
symmetric stretching vibration of those linkages: P–O–P
[20–23]. A weak band registered at 745 cm-1 occurring in
the IR spectrum of CoCr2(P2O7)2 is probably due to
symmetric stretching vibration of the P–O bonds with an
internal oxygen atom [23, 24]. The shoulder at about
625 cm-1 can be assigned to asymmetric bending
vibrations of the O–P–O groups [20, 21] and to stretching
vibrations of the Cr–O bonds [20, 23, 25]. The absorption
bands at 570, 540, 510 and 460 cm-1 may be assigned
either to the harmonics of P–O–P bending vibration or to
the characteristic frequency of the P2O72- group
[21, 22, 25]. Stretching vibration of Co–O bonds appears
in the spectra below 400 cm-1 [22, 25].
Studies on the other properties of the new obtained
compound CoCr2(P2O7)2, especially from the point of view
of its application as ceramic pigment, are in progress.
The results obtained in this study showed that in the ternary
oxide system CoO–P2O5–Cr2O3, besides Co3Cr4(PO4)6, a
new compound of the stoichiometric formula CoCr2(P2O7)2 is
also formed in air atmosphere. The new compound, forming
in the cross section Co3Cr4(PO4)6–Cr(PO3)3, was obtained as
a result of reaction between CoCO3, Cr2O3 and (NH4)2HPO4,
mixed at a molar ratio 1:1:4 as well as in the reaction of
mixture of Cr(PO3)3 and Co3Cr4(PO4)6 (2:1). CoCr2(P2O7)2
has a patina colour and crystallizes in the triclinic system
with the primitive unit cell parameters: a = 6.684(3) A˚ ,
b = 10.295(5) A˚ , c = 20.136(9) A˚ , a = 121.41(8) , b =
94.25(4) , c = 86.04(4) . The volume of such selected unit
cell is V = 1178.5 A˚ 3, and the number of stoichiometric
formula units in the unit cell Z = 5 and the density, calculated
from the unit cell parameters, is 3.60 g cm-3. CoCr2(P2O7)2
melts incongruently at 1270±10 C with a formation of solid
a-CrPO4. Co3Cr4(PO4)6 compound, one of the components of
the system studied, was obtained for the first time as a pure
phase. It is stable in air up to 1410 ± 20 C.
Acknowledgements This work was partly supported by Ministry of
Science and Higher Education (Poland) and funded from science
resources: UPB-DZS 518-10-020-3101-01/18 (Faculty of Chemical
Technology and Engineering, West Pomeranian University of
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