Crystalline copper phosphates: Synthesis and thermal stability

Article abstract of DOI:10.1111/j.1151-2916.1997.tb03241.x

The syntheses of Cu(PO₃)₂, Cu₂P₂O₇, Cu₄P₂O₉, Cu₅P₂O₁₀, Cu₃(PO₄)₂, and Cu₂PO₄ we

Full text of DOI:10.1111/j.1151-2916.1997.tb03241.x

J. Am. Ceram. Soc., 80 [12] 3133–38 (1997)
ournal
J
Crystalline Copper Phosphates: Synthesis and Thermal Stability
†
Carlos E. Bamberger, E. D. Specht,‡ and Lawrence M. Anovitz†
*
Chemical and Analytical Sciences Division and Metals and Ceramics Division, Oak Ridge National Laboratory,
Oak Ridge, Tennessee 37831–6119
The syntheses of Cu(PO3)2, Cu2P2O7, Cu4P2O9, Cu5P2O10,
Cu3(PO4)2, and Cu2PO4 were accomplished by several new
solid-state routes. Attempts to synthesize CuPO3 were un-
successful, and it is suspected that this compound, which is
reported in the literature, may not exist. The thermal sta-
bilities of Cu(PO3)2, Cu2P2O7 and Cu3(PO4)2 were exam-
ined, and their decomposition was concluded to be strongly
dependent on slow kinetics.
II. Experimental Procedure
The following reagents were used: CuO (99.9% pure,
Johnson–Matthey Co., Ward Hill, MA), Cu2O ( 99% pure,
General Chemical Division, New York), and (NH4)2HPO4
( 99.5% pure, General Chemical Division). BPO4 (Johnson–
Matthey Electronics, Ward Hill, MA) was calcined at 1000°C
overnight and analyzed by X-ray diffractometry (XRD). Cu0
powder of undetermined origin was washed with a 10 vol%
HCl solution, dried, and analyzed by XRD; it contained only a
trace ( 1%) of Cu2O. Containment was via boats and crucibles
of platinum and boats of fused SiO2. In a very limited number
of cases, Al2O3 boats were used. The reagent mixtures were
heated for periods of 1–70 h (typically 16 h) with intermittent
homogenization by grinding. All reactions that contained Cu0
or Cu2O as reagents were performed in a flowing argon-gas
atmosphere, whereas all other reactions were performed in air.
In most cases, the contents were not easily separated from the
SiO2 after reaction. Thus, the attached portions of SiO2 were
ground with the reaction product(s). When the mixture was
analyzed by XRD, the initially amorphous SiO2 was occasion-
ally detected; it was crystallized in the form of cristobalite.
The weight change that was involved in every reaction (e.g.,
representing the evolution of NH3 and H2O when using
(NH4)2HPO4) was recorded. The data were used to confirm or
propose alternate mechanisms of reaction.
XRD data were collected using CuK␣ radiation with a
Bragg–Brentano diffractometer that was equipped with a theta-
compensating incident-beam divergence slit and a graphite
(002) diffracted-beam monochromator. Pattern-processing
software (JADE, Materials Data, Livermore, CA) was used to
strip K␣2 and locate peaks. Lattice parameters were determined
by least-squares fitting (JADE), using silicon powder as an in-
ternal standard (ao ס 0.54306 nm). Thermal analyses (differ-
ential thermal analysis/thermogravimetric analysis, DTA/
TGA) of selected preparations were conducted using a
simultaneous thermal analyzer (Model STA 1500S, Stanton-
Redcroft (now Rheometric Scientific), London, U.K.). Air was
used as the purge gas, at a rate of 50 mL/min, and a heating rate
of 20°C/min was used for all runs. Sample weights were 100
mg, and Al2O3 crucibles were used for containment.
I. Introduction
WIDE variety of copper phosphates have been synthe-
A
sized,1–9 and several of their basic salts occur naturally.
Our interest in crystalline, ternary copper phosphates have led
us to search for new synthesis methods. Very early in this
search, it became evident that, to better understand the results
of some attempts to synthesize such compounds, it was neces-
sary to examine their thermal stability.
The following Cu2+-ion-containing phosphates have been
described in the literature: Cu2(PO3)4, Cu2P2O7, Cu3(PO4)2,
Cu4P2O9, and Cu5P2O10. Additionally, one phase that contains
the Cu+ ion—CuPO3—and one that contains the Cu+ and Cu2+
ions—Cu2PO4—also are described. In general, these com-
pounds have been prepared by using one of two methods: (i)
reaction in air of a solid copper compound, such as CuCO3,
CuO, or CuCl2, with either (NH4)2HPO4 or H3PO4 or (ii) pre-
cipitation from solution, followed by dehydration and calcina-
tion in air (e.g., excess CuSO4(aq) that is reacted with
Na4P2O7(aq)). The first type of reaction has been used to syn-
thesize Cu2(PO3)4,1–3 Cu2P2O7,1,4 Cu3(PO4)2,1,5 Cu4P2O9,6,7
and Cu5P2O10.8 The second approach has been used to synthe-
9
size Cu2P2O7 and a possible polymorph of Cu3(PO4)2.10 In
addition, using appropriate Cu+:Cu2+ ratios (e.g., using Cu2O
and performing the reaction in an argon atmosphere), Ball1
reported the preparation of CuPO3 and Etheredge and Hwu11
reported the preparation of Cu2PO4.
Our initial attempts to synthesize Cu2P2O7, Cu(PO3)2,
Cu4P2O9, Cu5P2O10, Cu3(PO4)2, and CuPO3 were based on the
reaction of copper oxides with BPO4. This reagent has been
used with success earlier in synthesizing other metallic phos-
phates.12 The use of this reagent was advantageous, because
BPO4 is a solid and the byproduct B2O3 is easy to remove via
dissolution in water. In this study, however, the only satisfac-
tory results via this method were obtained in the synthesis of
Cu2P2O7 and Cu3(PO4)2. Most of the work that has been re-
ported below originated in the exploration of the unsatisfactory
results that were obtained for the other compounds.
III. Results and Discussion
(1) Synthesis of Cu2+-ion-containing Phosphates:
Cu2P2O7, Cu(PO3)2, Cu4P2O9, Cu5P2O10, and Cu3(PO4)2
The synthesis of Cu2P2O7 was performed by means of the
reaction
R. S. Roth—contributing editor
900°C
2CuO + 2BPO4
→ Cu2P2O7 + B2O3
(1)
After removing the B2O3 via extraction with hot water, the
remaining solid was identified as Cu2P2O7 by XRD.4 The fol-
lowing alternative syntheses were also performed successfully:
Manuscript No. 191438. Received October 24, 1996; approved April 21, 1997.
Sponsored by the Division of Materials Sciences, Office of Basic Energy Sciences,
U.S. Department of Energy at Oak Ridge National Laboratory (managed by Lockheed
Martin Energy Research Corp.) under Contract No. DE-AC05-96OR22464.
*
Member, American Ceramic Society.
850°C
†Chemical and Analytical Sciences Division.
‡Metals and Ceramics Division.
2CuO + 2 NH ͒ HPO
→ Cu2P2O7 + 4NH3 + 3H2O (2)
͑
4 2
4
3133

3134
Journal of the American Ceramic Society—Bamberger et al.
Vol. 80, No. 12
→ Cu PO ͒ + 4NH + 3H O
according to Ball,1 and
850°–900°C
3CuO + 2 NH ͒ HPO
͑
͑
3
4 2
4
4 2
3
2
750°C
(11)
CuO + Cu PO ͒
→ Cu2P2O7
(3)
(4)
͑
3 2
This reaction was based on Ball.1 The reactions that are
represented by reactions (1)–(11) were performed in platinum
containers. The following reactions, which were contained in
Al2O3 boats, were also successfully used during this work:
and
900°C
Cu (PO ͒ + Cu PO ͒
3 2
→ 2Cu2P2O7
͑
3
4 2
for the first time here. The synthesis of Cu(PO3)2 and
Cu3(PO4)2 are described below.
900°C
CuO + Cu2P2O7
2CuO + Cu PO ͒
→ Cu PO ͒
(12)
(13)
͑
3
4 2
Not all copper phosphates could be synthesized using BPO4;
the synthesis of Cu(PO3)2 via reaction of CuO + 2BPO4 at 850°
or 1000°C did not proceed as expected; instead, Cu2P2O7,
B2O3, and excess BPO4 were obtained. Attempts to synthesize
Cu4P2O9 via reaction of 4 mol of CuO + 2 mol of BPO4 in the
temperature range of 800°–1000°C also were not successful;
instead, CuB2O4 and Cu3(PO4)2 were obtained.
900°C
→ Cu PO ͒
4 2
͑
͑
3
3 2
and
900°C
Cu4P2O9 + Cu2P2O7
→ 2Cu PO ͒
(14)
͑
3
4 2
However, alternate routes did provide successful methods
for synthesizing these phases. Cu(PO3)2 was obtained by
means of the following reactions:
The XRD patterns of all the Cu3(PO4)2 that was obtained were
similar to that of the compound that was obtained by means of
reaction (1) and, therefore, also differed from that given in Ref.
13. A list of relative intensities and interplanar spacings, which
have been corrected using silicon as the internal standard, is
given in Table I. These values are in very good agreement with
those that were generated from single-crystal data for
Cu3(PO4)2 that was obtained by Shoemaker et al.5 Careful
examinations of the pattern in Ref. 13 indicates that its extra
lines are due to contamination by Cu2P2O7.
750°–850°C
CuO + 2 NH ͒ HPO
→ Cu PO ͒ + 4NH + 3H O
͑
3 2 3 2
͑
4 2
4
(5)
which is based on Ball,1 and
700°C
Cu P O + 2 NH ͒ HPO
→ 2Cu PO ͒ + 4NH + 3H O
͑
3 2 3 2
͑
2
2
7
4 2
4
(2) Synthesis of Cu+-ion-containing Phosphates
(6)
Very few Cu+-ion-containing phosphates are known. The
binary phase CuPO3 was reportedly synthesized by Ball1 from
Cu2O and (NH4)2HPO4 and was described as being stable up to
its congruent melting point of 1245°C. Our attempts to synthe-
size CuPO3 via reaction of Cu2O and different amounts of
BPO4 in an argon atmosphere at 900°C were unsuccessful. In
all ten attempts, at different experimental conditions, the reac-
tion products consisted of Cu0 that was mixed with other crys-
talline phases such as Cu2P2O7, unreacted BPO4, and a mixed-
valence copper(I)/copper(II) phosphate, which is discussed
below. Because of these results, it was decided to duplicate the
preparation that was reported by Ball1 to synthesize CuPO3.
Mixtures of Cu2O + 2 mol of (NH4)2HPO4 were heated in an
argon atmosphere at temperatures in the range of 643°–900°C
for periods of 3–69 h. These attempts also were unsuccessful.
Containment in SiO2 produced finely divided metallic copper
that was dispersed in a dark brown glass; this is consistent with
the products of the reaction of Cu(PO3)2 and Cu0 in SiO2 at
640°C (which is envisioned as an alternate method to synthe-
size CuPO3), which also produced Cu0 that was dispersed in a
brown glass. When containment was in platinum, however,
other crystalline phases, in addition to Cu0, were observed: in
the temperature range of 643°–790°C, some Cu(PO3)2 was
formed, and at higher temperatures (e.g., 900°C), Cu2P2O7 was
present. The presence of Cu0 strongly suggests that a meta-
stable amorphous phase that contained Cu+ ions formed ini-
tially during the reaction and then disproportionated into me-
tallic copper and crystalline copper(II) compounds, whereas
the bulk remained a glass.
which has been derived during this work.
Successful alternate methods of synthesis for Cu4P2O9 con-
sisted of
850°–950°C
Cu2P2O7 + 2CuO
→ Cu4P2O9
(7)
and
900°C
Cu PO ͒ + 3CuO
→ Cu4P2O9
(8)
͑
3 2
Reaction (8) was expected to produce the desired compound,
because it is a combination of reactions (3) and (7).
Our inability to synthesize Cu4P2O9 in the presence of B2O3
suggested that Cu3(PO4)2 is the stable phase for this bulk com-
position when B2O3 is present. To confirm this theory,
Cu3(PO4)2 and Cu4P2O9 were each mixed with B2O3 and
heated to 1000°C. The products respectively consisted of un-
changed crystalline Cu3(PO4)2 and glassy B2O3 in the first
case, and a mixture of Cu3(PO4)2 with Cu0 and Cu2B4O9 in the
second case, which, thus, indicates the stability of Cu3(PO4)2 at
high temperature in the presence of B2O3. The presence of Cu0
suggests that oxygen was lost at 1000°C from the Cu4P2O9,
which caused the formation of a Cu+-ion-containing species
that disproportionated to produce Cu0.
A procedure that was similar to that which was used to make
Cu4P2O9 via reaction (7) was used successfully to make
Cu5P2O10:
850°–950°C
1
Examination of the published diffraction data on CuPO3
Cu2P2O7 + 3CuO
→ Cu5P2O10
(9)
shows that its three most-intense peaks are very similar to the
three most-intense peaks of Cu0, whereas the remaining CuPO3
peaks, of low intensity, do not resemble any known copper(II)
phosphate. This observation suggests that the samples that
were examined by Ball1 may have consisted of Cu0 and some
minor impurity and that CuPO3 may not exist.
The product was contaminated with 5%–10% of Cu4P2O9;
however, efforts were not made to obtain it in a purer form.
The synthesis of Cu3(PO4)2 via reaction of CuO and BPO4,
3CuO + 2BPO4 → Cu3(PO4)2 + B2O3
(10)
Additional attempts were made to synthesize CuPO3 by in-
creasing the ratio of Cu2O to (NH4)2HPO4. The following re-
sults were obtained in an argon atmosphere:
was accomplished by heating the mixture for 24 h in air at
950°C. The products were ground, leached with hot water to
remove the B2O3, and analyzed by XRD. Because the XRD
pattern of the product of this synthesis varied from that of
Cu3(PO4)2 in the Powder Diffraction File,13 several other syn-
thesis routes were also used, e.g.,
550°–750°C
2Cu O + 2 NH ͒ HPO
4
→ 2Cu0 + Cu2P2O7
͑
2
4 2
+ 4NH3 + 3H2O
(15)

December 1997
Crystalline Copper Phosphates: Synthesis and Thermal Stability
3135
Cu0 were present. Our duplication of their synthesis at 550°–
900°C, using (NH4)2HPO4, yielded Cu2PO4 that was mixed
with Cu0, Cu3(PO4)2, and Cu2P2O7, which suggests that
Cu2PO4 forms metastably during synthesis at these tempera-
tures.
Table I. XRD Pattern Data of Cu3(PO4)2 Powder
d-spacing (nm)
h
k
l
Intensity (%)
0.58842
0.46824
0.45154
0.40814
0.37468
0.36984
0.34616
0.32140
0.30702
0.29229
0.27776
0.26592
0.25701
0.25391
0.24062
0.23914
0.23411
0.23201
0.22974
0.22542
0.21821
0.21613
0.21532
0.20841
0.20677
0.20590
0.20422
0.19780
0.19594
0.19466
0.19280
0.18851
0.18674
0.18483
0.18225
0.18138
0.17997
0.17724
0.17679
0.17559
0.17337
0.16789
0.16375
0.16138
0.16076
0.15974
0.15868
0.15809
0.15547
0.15500
0.15434
0.15291
0.15244
0.15025
0.14576
0.14520
0.14376
0.14332
0.14251
0.13890
0.13658
0.13614
0.13574
0
0 1
1 0
0 0
1 0
1 1
7.7
2.7
0
1
1
1
1.9
Examination of the XRD data of previously unidentified
products that were obtained from previously described reac-
tions that involved Cu2O also revealed the presence of Cu2PO4.
For example, the reaction of Cu2O with 1.1 mol of BPO4 at
900°C produced Cu2PO4 and minor amounts of Cu0, Cu2P2O7,
and Cu3(PO4)2. Equilibrium was not attained because the pres-
ence of this number of phases violates the phase rule.
In addition, reaction of Cu2P2O7 with Cu0 in an argon at-
mosphere at 900° and 1200°C produced a melt that, when
cooled, contained the starting materials and a small amount of
Cu2PO4. Reaction of Cu3(PO4)2 with 2Cu0 also produced
Cu2PO4 at 1100°C. A low weight loss (1.36%) was observed in
this latter reaction, which suggests that oxygen rather than
P2O5 was lost, as in the reaction
30.7
2.0
28.2
1.3
−1 0 1
1
0 1
0 −1 1
11.9
73.0
100.0
66.2
20.3
58.9
11.7
29.3
34.1
32.0
10.5
9.3
−1 −1 1
0
1 2
−1 1 0
1
1
1 2
2 1
−1 0 2
2
1
0
1 0
0 2
2 0
1 −1 1
2
2
−1 −1 2
−2 −1 1
−2 0 1
−1 −2 1
1 1
0 0
16.5
11.0
5.8
1100°C
Cu PO ͒ + 2Cu0
→ Cu2PO4 + 0.5Cu2P2O7 + 2Cu0
+ 0.5O2
͑
3
4 2
(17)
5.7
4.5
The reactions that have been depicted by reactions (15)–(17)
were contained in SiO2.
A mixture of Cu2O with Cu2P2O7, with the stoichiometry of
Cu2PO4 and contained in SiO2 at 650°C and in platinum at
870°C, reacted as follows instead:
2
2
2
2 1
0 1
2 0
4.8
6.3
41.0
15.1
38.8
6.1
0 −2 1
0
2
1
0 3
1 2
1 3
650°–870°C
8.5
Cu2O + Cu2P2O7
→ Cu0 + Cu PO ͒
(18)
͑
−1 2 1
1.3
3
4 2
2
2 2
3.1
−2 0 2
−1 2 0
13.3
13.5
5.8
Because the products of reaction (18) are the same as the re-
agents in reaction (17), it was expected that reaction (18), when
conducted at higher temperatures (e.g., 1200°C), would pro-
duce the same products as reaction (17). This hypothesis was
confirmed experimentally in a SiO2 container; thus,
1
2 3
−1 1 3
−1 2 2
30.7
7.0
2
1
2
1
2 0
0 3
0 2
3 0
2.5
5.4
1200°C
→ Cu2PO4 + 0.5Cu2P2O7 + Cu0
+ 0.5O2
6.3
Cu2O + Cu2P2O7
8.9
(19)
0 −1 3
1 0
0 −2 2
44.2
5.9
3
(3) Thermal Stability in Air
12.8
1.6
0
2
2
3 2
2 3
3 2
The thermal stability of several copper(II) phosphates was
tested to establish their interrelationships and explain the oc-
casional presence of minor amounts of additional phases during
their preparation. The behavior of Cu(PO3)2 at increasing tem-
peratures was examined using DTA/TGA by heating a speci-
men in air at a rate of 20°C/min over a temperature range of
800°–1200°C. Melting and solidification or crystallization oc-
curred at 932°C; the total weight loss on heating to 1200°C was
1%, with an additional 1% loss by holding at 1200°C for 10
min. When cooled to 800°C, no weight gain was observed. The
weight loss very likely resulted from the vaporization of P2O5,
as indicated in reaction (20), which is given below.
10.5
2.4
−1 2 3
3.5
3
0
2 0
1 4
2.9
8.2
−2 0 3
2.5
1
3
0
3 3
0 0
3 3
7.4
7.9
13.8
21.4
13.5
13.9
21.6
13.8
6.7
−3 −2 1
−2 −3 1
3
0 1
−1 0 4
−2 2 0
−1 3 2
Heating larger amounts (0.300–1.5 g) of Cu(PO3)2 at 900°,
950°, 1050°, and 1200°C in SiO2 and platinum for periods of
16–116 h produced Cu2P2O7 and glass as the main products,
which suggests that the main decomposition path is
3
3 0
4.0
5.8
−1 −2 3
2Cu(PO3)2 → Cu2P2O7 + P2O5
(20)
The measured weight loss varied from 1.2% for 30 h at 900°C,
to 4.7% for 8 h at 950°C, to 12.0% for 17 h at 1050°C, and
finally to 32.2% for 16 h at 1200°C. If all the P2O5 is volatil-
ized, the weight loss for reaction (20) should be 32.04%, which
agrees well with our final value. The lower weight-loss values
at temperatures <1200°C and the presence of a glass that co-
exists with crystalline Cu2P2O7 suggests that a P2O5-rich melt
occurred as an intermediate reaction product.
650°–900°C
3Cu O + 2 NH ͒ HPO
→ 2Cu0 + Cu2PO4
͑
2
4 2
4
+ 4NH3 + 3H2O
(16)
The compound Cu2PO4 (Cu+Cu2PO4) was reported11 while this
work was in progress; this allowed the identification of the
products in reaction (16), as well as the products of other
reactions that are described below. The reaction of a mixture of
Cu2O + 2 mol of CuO + P2O5 that was reported by Etheredge
and Hwu11 did not yield 100%-pure Cu2PO4; Cu3(PO4)2 and
The above-mentioned results also suggest that Cu2P2O7
should be stable at temperatures as high as 1200°C. A sample

3136
Journal of the American Ceramic Society—Bamberger et al.
Vol. 80, No. 12
Table II. Thermal Decomposition of Cu2P2O7
Temperature
(°C)
Time
(h)
Weight loss
(%)
Atmosphere/container
Products†
1100
1100
1200
1200
1200
1200
1200
1400
20 Air/platinum
2.7
2.3
Cu2P2, Cu3P2(s)
16 Argon/SiO2
16 Air/platinum
21 Air/SiO2
Cu2P2, Cu0
9.4
Cu2P2, Cu0, Cu3P2(t), Cu2P(t)
Cu2P2, Cu3P2, Cu0, Cu2P(?)
Cu2P2, Cu0, Cu3P2(s), Cu2P(s)
Cu3P2, Cu2P2, Cu0, Cu2P(t)
Cu0, Cu2P2
15.7
22.7
42.3
10.1
67.2
41 Air/platinum
112 Air/platinum
62 Argon/SiO2
20 Air/platinum
Cu2P2, Cu3P2, Cu0, Cu2O, Cu2P(?),
unidentified phase
†Cu2P2 is Cu2P2O7, Cu3P2 is Cu3(PO4)2, and Cu2P is Cu2PO4; ‘‘(s)’’ denotes small (5%–10%) and ‘‘(t)’’ denotes
trace (2%–3%). The symbol ‘‘(?)’’ means that the presence of the product is questionable. The order of the products
indicates their abundance.
of 15 mg, analyzed by TGA/DTA, confirmed this observation;
it showed practically no weight loss when it was heated at a
rate of 20°C/min from 800°C to 1170°C, where it melted and
started to decompose. When cooled, there was a gradual weight
gain, probably due to oxygen, and a sharp exothermic effect at
930°C, which was due to solidification or crystallization. Dif-
ferent preparations of Cu2P2O7, in sample sizes of 0.35–0.60 g,
were heated in air at temperatures in the range of 950°–1400°C
for periods of 16–112 h in SiO2 and platinum (as indicated in
Table II). The remaining material was analyzed by XRD; no
changes in composition were detected at temperatures up to
1050°C. The results that were obtained for the higher-
temperature experiments (where changes were observed) are
summarized in Table II. The major product of the decomposi-
tion of Cu2P2O7 in air at 1100°C was Cu3(PO4)2; at higher
temperatures, Cu3(PO4)2 and Cu0 were obtained, whereas in an
argon atmosphere, only unreacted Cu2P2O7 and Cu0 were ob-
tained at 1100° and 1200°C.
Because Cu3(PO4)2 was present in the residues that remained
after heating Cu2P2O7, its thermal stability also was investi-
gated. TGA/DTA analysis indicated no weight loss at tempera-
tures up to 1020°C. Above this temperature, decomposition and
melting began and a 0.3% weight loss was observed. When
cooled, two-thirds of the weight loss was recovered and solidi-
fication or crystallization occurred at 900°C. Thermal treat-
ments at temperatures in the range of 950°–1400°C of
Cu3(PO4)2 (sample sizes of 0.3–1 g) and XRD analyses of the
remnants confirmed that this compound was stable, even after
90 h at 950°C. Results that were obtained at temperatures
>950°C and the container materials that were used are sum-
marized in Table III. In all the reactions that were performed in
air, even at 1400°C, remnant Cu3(PO4)2 was always present.
The decomposition products Cu2P2O7, Cu2PO4, Cu0, and Cu2O
and their relative amounts varied with the temperature. In an
argon atmosphere, however, the only products were Cu2P2O7
and Cu0; these also are the products that were observed when
Cu2P2O7 decomposed in an argon atmosphere (Table II). The
weight losses that were measured for the decompositions of
Cu2P2O7 and Cu3(PO4)2 in an argon atmosphere at 1200°C
were 10.1% and 7.8% respectively; this indicates that oxygen
was evolved and P2O5 must also have been lost, because a
reaction such as
Cu3(PO4)2 → Cu2P2O7 + Cu0 + 0.5O2
(21)
involves a weight loss of only 4.2%. Here again, it is reason-
able to assume that the loss of oxygen leads to the formation of
a metastable Cu+-ion-containing compound that disproportion-
ates and produces Cu0. However, this compound does not ap-
pear to be Cu2PO4, because it has been observed among reac-
tion products that were obtained at 1200°C.
Our best interpretation of the phase equilibria of the copper–
PO2.5–O2 system, based on the experiments that have been
reported here, is shown in Fig. 1. Although this system is
actually a subset of the copper–phosphorus–O2 system, we
have chosen to address the smaller system because none of our
data have any bearing on the more-reduced phases on the oxy-
gen-poor side of the copper–PO2.5 tie line nor suggest that this
tie line breaks at 1 bar (105 Pa) within the range of tempera-
tures that we have investigated.
The series of phase diagrams in Fig. 1 begin at temperatures
below 870°C. Because most of the phases that were synthe-
sized in this study were formed at temperatures <900°C, either
from (NH4)2HPO4 or from other phases in the copper–
phosphorus–oxygen system, all phases except Cu2PO4 have
been shown as being stable in this temperature range. The fact
that reaction (18), which only involves phases on crossing tie
lines at the Cu2PO4 composition, did not yield Cu2PO4 at tem-
peratures in the range of 650°–870°C suggests that this com-
pound was not stable at these conditions. Although Cu2PO4
was not synthesized from phases that were adjacent to it on any
tie line below 1100°C (reaction (17)), its synthesis as the major
phase from Cu2O and BPO4 at 900°C suggests that it probably
becomes stable at 870°C.
Because the data for reaction (20) clearly demonstrate that
CuP2O6 becomes unstable before attaining a temperature of
900°C, its decomposition to Cu2P2O7 and P2O5 is shown in the
diagram in the upper right-hand corner of Fig. 1. Beyond this
temperature, however, the sequence of reactions becomes less
clear. This ambiguity occurs because (i) there are no data that
constrain the decomposition temperatures of Cu4P2O9 and
Cu5P2O10 and (ii) some of the data suggest that Cu2P2O7 de-
composes to a Cu3(PO4)2-bearing assemblage, whereas other
results suggest the opposite. The fact that, in an argon atmo-
sphere, Cu2P2O7 has remained stable at temperatures >1050°C
Table III. Thermal Decomposition of Cu3(PO4)2
Temperature
(°C)
Time
(h)
Weight loss
(%)
Atmosphere/container
Products†
1050
1050
1100
1200
1200
1200
1200
1400
23
63
Air/platinum
Air/platinum
Argon/SiO2
Air/platinum
Air/platinum
Air/platinum
Argon/SiO2
Air/platinum
1.6
2.4
Cu3P2, Cu0, Cu2P2, Cu2P(s)
Cu3P2, Cu0, Cu2P2, Cu2P(s)
Cu2P2, Cu0
24
21
41
9.0
15.3
34.5
7.8
Cu2P2, Cu3P2, Cu0
Cu3P2, Cu2P2, Cu0, Cu2P(t)
Cu3P2, Cu0, Cu2O(s), Cu2P2(t)
Cu 0, Cu2P2
112
62
25
42.8
Cu3P2, Cu0, Cu2P2(t)
†Cu3P2 is Cu3(PO4)2, Cu2P2 is Cu2P2O7, and Cu2P is Cu2PO4; ‘‘(s)’’ denotes a small amount (5%–10%) and
‘‘(t)’’ denotes a trace amount (2%–3%). The order of the products indicates their abundance.

December 1997
Crystalline Copper Phosphates: Synthesis and Thermal Stability
3137
Fig. 1. Ternary phase diagram PO2.5–copper–O2, showing some of the joins established from present results.
while Cu3(PO4)2 has reacted to form Cu2P2O7, Cu2PO4, and
Cu0 suggests that the observed decomposition of Cu2P2O7 may
be related to changes in the bulk chemistry of the material.
Thus, Cu3(PO4)2 is assumed to decompose before Cu2P2O7.
The decomposition of Cu3(PO4)2 probably begins at 1020°C,
where it is first observed in the TGA/DTA data. The final
reaction that is shown in Fig. 1 is the decomposition of Cu2O,
which occurs at 1150°C.
Several caveats need to be noted in regard to Fig. 1. The first
is that, although many half-reversals have been performed,
none of the reactions that are shown have been fully reversed;
therefore, their temperatures are somewhat uncertain. In addi-
tion, many of the reactions do not appear to have remained
isochemical. Individual starting materials commonly yielded
reaction products whose bulk chemistry was not possible un-
less changes in bulk composition, probably the loss of P2O5 or
O2, occurred during the run. Therefore, the phase diagrams that
are presented have been derived with the assumption that this
discussion explains some of the discrepancies that are ob-
served.
A major limitation in Fig. 1 is that, despite its common
presence in our experimental results, no attempt was made to
define the field of melt stability, because the compositional
data that are necessary for such a determination were not ob-
tained. In addition, the first diagram in Fig. 1 shows stable tie
lines between Cu2O and Cu3(PO4)2, Cu4P2O9, and Cu5P2O10,
rather than between Cu0 and the same copper phosphates. This
condition has simply been assumed, because there are no data

3138
Journal of the American Ceramic Society—Bamberger et al.
Vol. 80, No. 12
to constrain the choice. Therefore, these tie lines are shown as
dashed lines until their presence is required by other reactions.
Finally, it has also been assumed that the decomposition tem-
peratures of Cu4P2O9 and Cu5P2O10 lie between those of
CuP2O6 and Cu3(PO4)3, and that Cu4P2O9 decomposes at
higher temperatures than Cu5P2O10, although there are no data
to support either contention.
solidus will occur at the CuPO3 composition along the PO2.5
Cu2O pseudobinary that Ball treated.
–
References
1M. C. Ball, ‘‘Phase-Equilibrium Relationships in the Systems CuO–P2O5,’’
J. Chem. Soc. (A), 1113–18 (1968).
2M. Beuch and J.-C. Grenier, ‘‘Donnees Cristallographiques Sur les Tetra-
metaphosphates du Type M2IIP4O12 (MII ס Ni, Mg, Zn, Cu, Co, Mn),’’ Mater.
Res. Bull., 3, 643–48 (1968).
IV. Conclusions
3Powder Diffraction File, Card 29-0572. International Centre for Diffraction
Data, Newtowne Square, PA, 1995.
Several new routes for the syntheses of Cu2P2O7, Cu(PO3)2,
Cu4P2O9, Cu5P2O10, and Cu3(PO4)2 have been found, and the
powder XRD pattern of pure Cu3(PO4)2 is reported. Several
reactions that produce Cu2PO4, although not as a single phase,
have also been found.
The thermal decompositions of Cu(PO3)2, Cu2P2O7, and
Cu3(PO4)2 have been studied, and the results suggest that, al-
though Cu2P2O7 and Cu3(PO4)2 are quite stable at <1200°C,
their decomposition paths may be complex and controlled by
slow kinetics. Hence, the results that have been obtained very
likely reflect the fact that the decomposition reactions have not
reached completion.
As evidence of the existence of the putative CuPO3 phase,
Ball1 has presented thermodynamic data that are interpreted as
a transition at the CuPO3 composition from the Cu2O + CuPO3
solidus at 805°C to the CuPO3 + X solidus at 935°C (X is an
unknown). Here, we propose a reinterpretation of the Ball re-
sults; this interpretation is motivated by our inability to syn-
thesize CuPO3. As shown in Fig. 1, a transition from the copper
+ CuP2O6 + PO2.5 solidus to the copper + CuP2O6 + Cu2P2O7
4Powder Diffraction File, Card 44-0182, International Centre for Diffraction
Data, Newtowne Square, PA, 1995.
5G. L. Shoemaker, J. B. Anderson, and E. Kostiner, ‘‘Copper(II) Phosphate,’’
Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem., B33, 2969–72
(1977).
6M. Brunel-Lau¨gt, A. Durif, and J.-C. Guitel, ‘‘Structure Cristalline de
Cu4(PO4)2O,’’ J. Solid State Chem., 25, 39–47 (1978).
7T. B. Anderson, G. L. Shoemaker, and E. Kostiner, ‘‘The Crystal Structure
of Cu4(PO4)2O,’’ J. Solid State Chem., 25, 49–57 (1978).
8M. Brunel-Lau¨gt and J.-C. Guitel, ‘‘Structure Cristalline de Cu5O2(PO4)2,’’
Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem., B33, 3465–68
(1977).
9B. E. Robertson and C. Calvo, ‘‘The Crystal Structure and Phase Transfor-
mation of ␣-Cu2P2O7,’’ Acta Crystallogr., 22, 665–72 (1967).
10M. R. Hussain, A. Hussain, A. Zafar Jamil, and B. Rama Rao, ‘‘Phase
Transformations in Tribasic Copper Phosphate Trihydrate,’’ Indian J. Chem.,
Sect. A., 21A, 160–61 (1982).
11K. M. S. Etheredge and S.-J. Hwu, ‘‘Synthesis, Structure and Magnetic
Properties of a Novel Mixed-Valence Copper(I/II) Phosphate, Cu2PO4,’’ Inorg.
Chem., 34, 5013–16 (1995).
12C. E. Bamberger, ‘‘Preparation of Metal Phosphates by Metathesis Reac-
tion with BPO4,’’ J. Am. Ceram. Soc., 65, C-107–C-108 (1982).
13Powder Diffraction File, Card 21-298. International Centre for Diffraction
Data, Newtowne Square, PA, 1995.
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