A study of thermal behavior of cesium phosphate

Article abstract of DOI:10.1007/s10973-015-5192-x

Cesium phosphates with different Cs/P molar ratios were prepared by a solution evaporation method. X-ray powder diffraction, thermogravimetric, and differential thermal analyses were performed in order to reveal the chemical transformation and phase compositions which take place during the heating of the mixtures Cs₂CO₃/(NH₄)₂HPO₄ and CsNO₃/(NH₄)₂HPO₄ as well as individual compound. The effects of the Cs/P molar ratio, Cs source and treatment temperature on thermal behavior of the cesium phosphates were investigated. The results showed that different reactions take place with molar ratio from 1/2 to 3/1. Meanwhile, the thermostability of Cs₂CO₃ and CsNO₃ intimately affects the phase composition and phase transition during heating process. However, for all the Cs–P oxides samples, the crystallinity was decreased differently after higher temperature treatment.

Full text of DOI:10.1007/s10973-015-5192-x

J Therm Anal Calorim (2016) 124:1063–1070
DOI 10.1007/s10973-015-5192-x
A study of thermal behavior of cesium phosphate
1
,2
1
2
Guoliang Zhang
•
Zhijian Peng
•
Chunshan Li
Received: 18 August 2015 / Accepted: 2 December 2015 / Published online: 28 December 2015
Ó Akad e´ miai Kiad o´ , Budapest, Hungary 2015
Abstract Cesium phosphates with different Cs/P molar
ratios were prepared by a solution evaporation method.
X-ray powder diffraction, thermogravimetric, and differ-
ential thermal analyses were performed in order to reveal
the chemical transformation and phase compositions which
take place during the heating of the mixtures Cs CO /
Introduction
Metal phosphates play an important role in chemical engi-
neering as protonic conductors, ion exchangers, catalysts,
intercalation carriers, catalyst support, and novel functional-
ized materials [1–5]. As such, synthesis and applications of
metal phosphates have received considerable attention [6].
Blanco et al. [7] prepared a series of metal orthophosphates
[Ca (PO ) , Sr (PO ) , and Ba (PO ) ] and metal pyrophos-
2
3
(
NH ) HPO and CsNO /(NH ) HPO as well as individ-
4 2 4 3 4 2 4
ual compound. The effects of the Cs/P molar ratio, Cs
source and treatment temperature on thermal behavior of
the cesium phosphates were investigated. The results
showed that different reactions take place with molar ratio
3
4 2
3
4 2
3
4 2
phates (Ca P O , Sr P O , and Ba P O ) for gas-phase
2 2 7
2
2
7
2
2
7
dehydration of lactic acid. Matsuura et al. [8] also prepared
Ca (PO ) (OH) , Sr (PO ) (OH) , Ba (PO ) (OH) , and
from 1/2 to 3/1. Meanwhile, the thermostability of Cs CO3
2
10
4 6
2
10
4 6
2
10
4 6
2
and CsNO intimately affects the phase composition and
3
other hydroxyapatite catalysts to synthesize acrylic acid from
lactic acid. Tang et al. [9] further studied the catalytic dehy-
dration of lactic acid to acrylic acid over Ba P O .
phase transition during heating process. However, for all
the Cs–P oxides samples, the crystallinity was decreased
differently after higher temperature treatment.
2
2 7
Phosphates exhibit a complex phase transition when
prepared with different molar ratios and at different heating
treatment temperatures [10, 11]. Ai et al. [10] reported that
the structure of iron phosphates prepared with a P/Fe
atomic ratio close to 1 varied from the amorphous phase to
Keywords Phosphate Á Thermal behavior Á Phase
transition Á Thermostability Á Dehydration
the quartz-type phase (FePO ) via an unidentified phase
4
with increasing calcination temperature. The quartz-type
phase was formed, accompanied with the formation of
polycondensed phosphates at P/Fe atomic ratio = 1.3–1.5.
Muneyama et al. [11] further investigated the reduction and
&
Chunshan Li
chunshanli@gmail.com; csli@home.ipe.ac.cn
oxidation of iron phosphates and found that FePO was
4
reduced to Fe P O through Fe (P O ) at Fe/P [ 1.2.
2
2
7
3
2 7 2
Zhijian Peng
pengzhijian@cugb.edu.cn
However, the sample did not pass through Fe (P O ) at
3
2 7 2
Fe/P = 1. Masui [12] studied the thermal behavior of
amorphous cerium phosphate and found that crystallization
from the amorphous phase occurred at 500 °C to form
CeP O . CePO was then formed at 650 °C in the presence
1
2
School of Engineering and Technology, China University of
Geosciences, No. 29 Xueyuan Road, Beijing 100083,
People’s Republic of China
2
7
4
Beijing Key Laboratory of Ionic Liquids Clean Process,
Institute of Process Engineering, Chinese Academy of
Sciences, No. 1 Zhongguancun North Second Street,
Beijing 100190, People’s Republic of China
of a small amount of Ce(PO ) . Sun et al. [13] prepared
3
3
zeolites modified with the cesium phosphate catalyst
through wet impregnation and then calcined at 800 °C to
123

1
064
G. Zhang et al.
improve dehydration activity. They discussed the effect of
cesium phosphate modification and cesium content on the
catalytic performance. Previous studies on cesium phos-
phate focused on cesium dihydrogen phosphate (CsH PO )
All precursors were calcined at 450, 650, and 800 °C for
6 h. The obtained samples were sealed in a dry place after
calcination.
2
4
because of its anhydrous proton conductors [14–16] and the
thermodynamics and kinetics of its dehydration reaction at
temperatures lower than 300 °C [17–22]. However, sys-
tematic studies on the thermal behavior of cesium phos-
phate and phase translation at high temperatures have been
rarely reported.
Product characterization
XRD patterns were obtained with an X’Pert PRO MPD
diffractometer at 40 kV and 40 mA with Cu Ka radiation.
-
1
Angle (2h) was measured in steps of 10° min between 5°
and 90°. Mass loss and temperature associated with phase
transformation were determined through TG/DTA on a
Shimadzu DTG-60H analyzer. The samples were heated
As we all know that the thermoanalytical tools such as
thermogravimetry (TG) and differential thermal analysis
-
1
(
DTA) and X-ray powder diffraction (XRD) analysis play
from room temperature to 850 °C at rate of 10 °C min in
air flow (30 mL min ).
-1
an important role in the thermal behavior research and
phase identification of a crystalline material, respectively
[
23–28]. Therefore, in this study, the TG/DTA and XRD
analyses were used to obtain information on the thermal
behavior and phase composition of cesium phosphate at
different Cs/P molar ratios. Different Cs sources were used
to prepare cesium phosphates with Cs/P molar ratios of 1/2,
Results and discussion
Thermal behavior of cesium phosphates
with Cs/P 5 1/2 and 1/1
1
/1, 2/1, and 3/1 through evaporation.
The TG/DTA data of CCP samples with Cs/P molar ratios
of 1/2 and 1/1 were shown in Fig. 1. During heating at a
-
1
Experimental
constant heating rate of 10 °C min up to 850 °C, five
transformations accompanied by mass loss occurred at
Cs/P = 1/1: (1) up to 68 °C, endothermic peak at 35 °C,
mass loss of 1.3 %; (2) between 80 and 174 °C, endothermic
peak at 131 °C, mass loss of 4.0 %; (3) between 174 and
Product preparation
Cesium phosphate was synthesized through reaction of
diammonium hydrogen phosphate [(NH ) HPO , 99.0 %]
4
2
4
with cesium carbonate (Cs CO , 98.0 %) and cesium
3
2
0
nitrate (CsNO , 99.0 %), respectively.
3
131 °C
04 °C
5 °C
281 °C
1
00
Cs/P = 1/2
2
(a)
Cesium phosphate was prepared using Cs CO (signed as
2
3
3
–15
–30
–45
CCP). Briefly, 0.025 mol Cs CO (8.3117 g)wasdissolved in
3
95
2
169 °C
DTA
TG
5
0 mL deionized water and added with 0.05 mol (NH4)2
335 °C
9
0
HPO (6.6695 g) solution under constant stirring for 1 h. The
4
630 °C
mixture was vaporized at 150 °C, and the precipitated pre-
cursor with Cs/P molar ratios of 1/1 was obtained. Samples
85
–60
8
0
with Cs/P molar ratios of 1/2 [0.025 mol Cs CO and 0.1 mol
2
3
–75
(
(
(
NH ) HPO ], 2/1 [0.025 mol Cs CO and 0.025 mol
4
75
74 %
2
4
2
3
72.4 %
–90
0
NH ) HPO ], and 3/1 [0.03 mol Cs CO and 0.02 mol
4
2
4
2
3
131 °C
209 °C
Cs/P = 1/1
(b)
NH ) HPO ] were prepared using the same procedure.
4
280 °C
2
4
35 °C
Another group of cesium phosphate was prepared using
100
–15
9
8.7 %
473 °C
cesium nitrate (signed as CNP) at different Cs/P molar
ratios of 1/2, 1/1, 2/1, and 3/1. Generally, 0.05 mol CsNO3
235 °C
3
35 °C
–
30
45
94.7 %
95
DTA
94 %
(
9.8439 g) was dissolved in 50 mL deionized water and
–
added with 0.05 mol (NH ) HPO (6.6695 g) solution
4
2
4
8
9.6 %
under constant stirring for 1 h. The mixture was vaporized
at 150 °C, and the precipitated precursor with Cs/P molar
ratios of 1/1 was obtained. Samples with Cs/P molar ratios
of 1/2 [0.05 mol CsNO and 0.1 mol (NH ) HPO ], 2/1
90
–60
75
735 °C
TG
8
6.2 %
–
85
100
200
300
400
500
600
700 800
3
4 2
4
Temperature/°C
[
0.05 mol CsNO and 0.025 mol (NH ) HPO ], and 3/1
3 4 2 4
[
0.06 mol CsNO and 0.02 mol (NH ) HPO ] were pre-
3
4 2
4
Fig. 1 TG and DTA curves of samples with Cs/P molar ratios of 1/2
(a) and 1/1 (b) at a heating rate of 10 °C min
-1
pared using the same procedure.
1
23

A study of thermal behavior of cesium phosphate
1065
2
0
–
–
–
–
0
10
0
1
00
1
00
1
20 °C
8
7.1 %
TG
90
–10
9
8
7
6
5
4
3
0
0
0
0
0
0
0
1
74 °C
8
7
0
0
20
40
60
80
–20
–30
–40
1
24 °C
67.4 %
DTA
4
96 °C
60
50
3
82 °C
–
50
60
285 °C
–
4
3
2
1
0
0
0
0
DTA
TG
–70
80
–
–100
–120
200 °C
–90
–100
1
00 200 300 400 500 600 700 800 900
Temperature/°C
100
200 300
400 500
600
700
800 900
Temperature/°C
Fig. 3 TG and DTA curves of (NH HPO at a heating rate of 10 °C
4
)
2
4
Fig. 2 TG and DTA curves of Cs
1
min
2
CO
3
at a heating rate of 10 °C
-
min
1
-
2
24 °C, endothermic peak at 209 °C, mass loss of 0.7 %; (4)
temperatures, a second endothermic peak appeared at
285 °C; this peak is related to the elimination of the second
ammonium molecule (Eq. 3) and further dehydration of the
corresponding phosphates (Eqs. 4, 5). This process started
at temperatures higher than 200 °C [27, 33, 34] and
accompanied by gradual mass loss on the TG curve (Fig. 3)
immediately after the formation of NH H PO . The theo-
between 224 and 300 °C, endothermic peak between 235
and 280 °C, mass loss of 4.4 %; and (5) between 300 and
4
42 °C, endothermic peak at 335 °C, mass loss of 3.4 %.
Two endothermic peaks were further observed at 473 and
35 °C, but without mass loss as the temperature increased
7
to 850 °C. For Cs/P = 1/2, four evident endothermic peaks
were observed at 35, 131, 169, and 204 °C, with a mass loss
of 0.7 % up to 51 °C and continuous drop of mass to 74 %.
An unobvious endothermic peak was further observed at
4
2
4
retical mass loss of this process is 26.5 %, which was
reached on the recorded TG curve at 376 °C. A third
endothermic peak appeared at 382 °C is mainly associated
with the condensation of H PO (Eq. 5) and dehydration of
6
30 °C with a mass loss of 1.6 %.
In order to identify process that causes mass loss and
3
4
H P O –HPO (Eq. 6) [30]. NH PO (Eq. 7) was simul-
4 2 7 3 4 3
verify reaction during heating, the thermoanalytical curves
of the pure Cs CO and (NH ) HPO were invested and
taneously formed at temperature higher than 400 °C [31].
As these processes are complex, accurate interpretation of
the TG/DTA results is difficult. Moreover, the sample
started to decompose (Eqs. 8, 9) and evaporate at 550 °C.
2
3
4 2
4
shown in Figs. 2 and 3, respectively. TG and DTA curves
of Cs CO showed that Cs CO was stable with only two
2
3
2
3
small endothermic peaks at 124 and 174 °C, which could
be due to desorption of water and decomposition of
impurities [29]. After that, it started to melt and meanwhile
decompose (Eq. 1).
ðNH4Þ HPO4 ! NH4H2PO4 þ NH3
NH4H2PO4 ! H3PO4 þ NH3
ð2Þ
ð3Þ
ð4Þ
ð5Þ
ð6Þ
ð7Þ
ð8Þ
ð9Þ
2
2
NH H PO ! ðNH Þ H P O þ H O
4
2
4
4 2
2
2
7
2
Cs2CO3 ! Cs2O þ CO2
Based on TG/DTA curves of (NH ) HPO (Fig. 3), the
ð1Þ
2
H PO ! H P O þ H O
3
4
4
2
7
2
4
2
4
H P O ! 2HPO þ H O
4
2
7
3
2
thermoanalytical analysis revealed three main endothermic
peaks at 208, 285, and 382 °C. Intensive mass loss started
at temperatures higher than 120 °C and continued
throughout the whole heating range. The first endothermic
peak at 200 °C is mainly correlated with the melting of
ðNH4Þ H2P2O7 ! 2NH4PO3 þ H2O
2
2HPO ! P O þ H O
3
2
5
2
2NH4PO3 ! P2O5 þ 2NH3 þ H2O
(
NH ) HPO and partial elimination of ammonia (Eq. 2)
The XRD patterns of CCP sample with Cs/P = 1/2 and
4
2
4
[
30]. The theoretical mass loss of the elimination of one
1/1 (Fig. 4) showed that the crystal phase was mainly
composed of CsH PO phase (PDF NO. 35-0746). As such,
ammonia molecule is 12.9 %, which was reached on the
recorded TG curve at 210 °C. However, according to the
literature, a small amounts of H PO (Eq. 3), (NH4)2
2
4
the reaction (Eq. 10) occurred when Cs CO and (NH )
4 2
2
3
HPO were mixed. According to previous studies, phase
4
3
4
H P O (Eq. 4), and H P O (Eq. 5) were also formed at
7
transition and dehydration occur upon heating of CsH PO
4
2
2
7
4
2
2
temperature lower than 210 °C [31, 32]. When the
NH ) HPO sample was continuously heated at higher
[17, 18, 20, 21]. The phase transition from the low tem-
perature phase (paraelectric phase) to the high temperature
(
4
2
4
123

1
066
G. Zhang et al.
2
00000
60000
20000
8000
7000
(
a)
CsH PO
Cs/P =1/2
CsPO3
2
4
Cs P O
7
1
1
(NH ) HPO
4
4
2
4
2
6
5
4
3
2
1
000
000
000
000
000
000
0
Cs/P =1/2
800 °C
8
4
0000
0000
0
6
50 °C
Cs/P =1/1
4
50 °C
1
0
20
30
40
50
60
1
0
20
30
40
50
60
2
θ/°
2
θ/°
Fig. 4 XRD patterns of CCP precursor samples with Cs/P = 1/2 and
/1
2
2
5000
0000
1
(b)
Cs/P =1/1
CsPO3
Cs P O
4
2
7
800 °C
phase (super-protonic phase) was observed at 230 °C
[
22, 27, 28]. Pyrophosphate (Cs H P O ) was formed
15000
2
2 2 7
(
Eq. 11) at 235 °C [28] and then converted to cesium
polyphosphate (Eq. 12) temperatures higher than 260 °C
650 °C
1
0000
[
(
(
17, 18, 20, 28]. At higher temperatures, phosphate such as
CsPO3)n (n » 1) polymerized at about 330 °C [28]
Eq. 13). The (CsPO ) phase underwent crystallographic
5
000
0
3
n
transition at 480 °C and melting at 735 °C with increasing
temperature [22, 28].
450 °C
1
0
20
30
40
50
60
Cs2CO3 þ 2ðNH4Þ HPO4 ! 2CsH2PO4 þ 4NH3 þ CO2
2
2θ/°
þ H2O
ð10Þ
ð11Þ
ð12Þ
ð13Þ
ð14Þ
Fig. 5 XRD patterns of CCP samples with Cs/P molar ratios of 1/2
a) and 1/1 (b) and calcination temperatures of 450, 650, and 800 °C,
respectively
(
2
CsH2PO4 ! Cs2H2P2O7 þ H2O
nCs2H2P2O7 ! 2CsnH2PnO3nþ1 þ ðn À 2ÞH2O
nCsH2PO4 ! ðCsPO3Þ þnH2O
Cs3PO4 þ CsH2PO4 ! Cs4P2O7 þ H2O
n
Cs/P = 1/2. The crystallinity slightly increased when the
calcination temperature was increased to 650 °C. The dif-
ferences between the two CCP samples may be attributed to
polycondensation and decomposition of phosphoric acid
derived from the decomposition of excess (NH ) HPO
Aside from the CsH PO phase, (NH ) HPO (PDF NO.
2
4
4 2
4
2
9-0111) was also observed at in Cs/P = 1/2. The decom-
position and dehydration of (NH ) HPO led to continuous
2
4
4
4 2
4
mass loss without separation step, similar to that of pure
NH ) HPO . In this regard, the endothermic peak of DTA
(Eq. 3) during heating. When the calcination temperature
was increased to 800 °C, the crystal phase of all the samples
disappeared and only the amorphous phase was observed
because of the melting of (CsPO ) leading to formation of
(
4
2
4
at 630 °C is due to the formation and evaporation of P O .
2
5
Samples were calcined at 450, 650, and 800 °C to analyze
the crystal phase structure through XRD analysis (Fig. 5a, b)
and information on processes that occur during heating at
high temperatures. The CsH PO phase disappeared because
3
n
the glassy state [20].
The TG/DTA data indicated that the endothermic peak at
131 °C is related to water desorption. The two endothermic
peaks at 169 and 209 °C are related to the melting of
(NH ) HPO companied with the elimination of ammonia
2
4
of dehydration and polymerization. At Cs/P = 1/1, the
sample was mainly composed of the metaphosphate phase
4
2
4
(
CsPO , PDF NO. 38-0135 and Eqs. 12, 13) and pyrophos-
3
(Eqs. 2, 3) and dehydration reaction, respectively. The
endothermic peak at 235 °C, with a corresponding mass
loss, is related to the superionic phase transition and dehy-
dration of CsH PO –Cs H P O (Eq. 11); Moreover, the
phate (Cs P O , PDF NO. 38-0001, and possible reaction as
4 2 7
Eq. 14) after calcination at 450 and 650 °C. However, the
sample was mainly composed of non-crystalline compound,
in addition to metaphosphate and pyrophosphate phases at
2
4
2 2 2 7
endothermic peaks at 280 and 335 °C are attributed to the
1
23

A study of thermal behavior of cesium phosphate
1067
7
1 °C
12000
Cs/P = 2/1
DTA
(a)
(b)
800
140 °C
0
99
96
93
Cs CO
3
2
04 °C
2
CsH PO
4
341 °C
2
–20
9
6
3
000
000
Cs HPO ·1.5H2O
2 4
–
40
60
80
90
–
TG
Cs/P =2/1
87
–
0
000
0
100
Cs/P =3/1
Cs/P =3/1
95
–
20
DTA
TG
10
20
30
40
50
60
90
–40
–60
–80
2
θ/°
8
5
0
Fig. 7 XRD patterns of CCP precursors with Cs/P molar ratios of 2/1
and 3/1
8
1
00 200
300
400 500
600 700
3
3
2
5000
0000
5000
Temperature/°C
Cs/P =2/1
Cs PO4
(a)
3
CsPO3
Fig. 6 TG and DTA curves of samples with Cs/P molar ratios of 2/1
a) and 3/1(b)
Cs P O
4
2
7
(
8
00 °C
dehydration reaction forming a liquid phase (Eq. 12) and
conversion of Cs H P O into cesium polyphosphate with
20000
5000
10000
2
2 2 7
1
the melting of CsH PO , respectively. The endothermic
2
4
6
50 °C
peaks at 473 and 735 °C without mass loss are attributed to
the crystallographic transition from polyphosphate to
5000
metaphosphate and melting of CsPO , respectively. In
3
4
50 °C
60
sample with a Cs/P molar ratio of 1/2, an endothermic peak
at 630 °C is present, which may be attributed to the evap-
oration of P O , as calculated Eq. 8 and 9.
0
10
20
30
40
50
2
θ
/°
2
5
1
1
1
1
1
8000
6000
4000
2000
0000
Thermal behavior of CCP samples with Cs/P molar
ratios of 2/1 and 3/1
Cs/P =3/1
Cs PO4
CsPO3
(b)
3
Cs P O
4
2
7
Cesium phosphates with Cs/P molar ratios of 2/1 and 3/1
were also prepared, and the thermal behavior of these
samples was investigated using TG/DTA (Fig. 6). The TG
and DTA curves show that the two samples exhibited similar
behavior during continuous heating. Four endothermic peaks
accompanied by mass loss were observed at 71, 140, 204,
and 341 °C, accompanied with certain mass loss. The XRD
patterns of the CCP precursor (Fig. 7) showed the presence
of the cesium hydrogen phosphate hydrate phase (Cs2
HPO Á1.5H O, PDF NO. 37-0106), in addition to the
8
00 °C
8
6
000
000
650 °C
4000
2000
4
50 °C
0
10
20
30
40
50
60
2
θ
/°
4
2
CsH PO phase and reactant Cs CO (PDF NO. 35-0962).
2
4
2
3
Fig. 8 XRD patterns of CCP samples with Cs/P molar ratios of 2/1
a) and 3/1 (b) at different calcination temperature
The two samples were treated at 450, 650, and 800 °C. The
XRD patterns of the corresponding samples (Figs. 8a, b)
showed that all the samples exhibited poor crystallinity, and
the Cs PO phase (PDF NO. 26-1097) was observed because
(
increased from 650 to 800 °C. This finding suggested that
the structure of CCP samples with Cs/P molar ratios of 2/1
and 3/1 is stable after calcination at 450 °C.
3
4
of sufficient Cs source in the reaction. However, Cs PO
3
4
phase can still be observed as the heating temperature was
123

1
068
G. Zhang et al.
The first endothermic peak at 71 °C during heating is
because of the decomposition of partial (NH ) HPO . The
4
2
4
correlated with the desorption of the adsorbed water,
whereas the peak at 140 °C is related to the decomposition
of Cs HPO Á1.5H O [11]. The third endothermic peak at
CsH PO phase cannot be observed in all CNP precursors.
2 4
This finding suggested that the reaction of CsNO with
3
NH H PO did not occur at low temperatures.
4
2
4
2
4
2
2
04 °C is possibly associated with the dehydration of
The TG and DTA curves of CsNO are also shown in
3
Cs HPO ; moreover, the peak at 341 °C is attributed to the
Fig. 11 to identify transformation that occurred during
heating. Two apparent endothermic peaks at 160 and
406 °C without mass loss were observed due to melting
2
4
dehydration reaction and conversion of CsH PO to cesium
2
4
polyphosphate accompanied by melting of CsH PO
2
4
[
22–26, 28, 30]. The TG curves are stable without mass
impurities in CsNO [26, 31] and the melting point of
3
loss from 450 to 850 °C because of the formation of the
Cs PO phase. Mass losses were observed at 800 °C
CsNO , which was reported at 414 °C in the literatures.
3
Most of the mass loss, accompanied with two broad
endothermic DTA peaks from 500 to 800 °C, is attributed
to decomposition and volatilization of CsNO3.
3
4
because of the decomposition of excess Cs CO .
2
3
Thermal behavior of the CNP samples with different
Cs/P molar ratios
Therefore, the thermal behavior of CNP samples (Fig. 9)
with endothermic peak at 152 °C without mass loss is cor-
related with melting impurities from CsNO . The second
3
Cesium phosphates with Cs/P molar ratios of 1/2, 1/1, 2/1,
peak at 188 °C with starting mass loss attributed to the
and 3/1 were prepared using CsNO to compare and ana-
3
decomposition of (NH ) HPO and dehydration of phos-
4 2
4
lyze the influence of using different Cs sources. Figure 9
shows the TG/DTA cures of the four CNP samples. The
thermal behavior is similar below 400 °C. At high tem-
peratures, CNP samples with Cs/P molar ratios of 1/2 and
phate (Eq. 11). The third peak at 266 °C is possibly
7
6
0000
0000
CsNO3
1
/1 are stable and exhibit no mass loss. However, one
(NH ) HPO
4
4
2
broad endothermic peak at 600 °C accompanied with mass
loss and another one at 400 °C without mass loss can be
observed at Cs/P = 2/1 and 3/1, respectively. The XRD
patterns of the CNP precursors are shown in Fig. 10. Aside
NH H PO
4 2 4
50000
0000
30000
Cs/P = 1/2
Cs/P = 1/1
4
from the characteristic peaks of the reactants CsNO (PDF
3
NO. 09-0403) and (NH ) HPO (PDF NO. 29-0111), only
4
2
4
Cs/P = 2/1
Cs/P = 3/1
2
1
0000
0000
0
the NH H PO phase (PDF NO. 37-1479) is observed
4
2
4
1
52 °C
0
–
1
00
2
66 °C
4
00 °C
20
9
8
7
6
0
0
0
0
10
30
60
20
40
50
DTA
DTA
DTA
188 °C
–40
–60
–80
0
6
00 °C
2
θ
/°
Cs/P = 1/2
Cs/P = 1/1
TG
Fig. 10 XRD patterns of the CNP precursor with different Cs/P
molar ratios
1
1
1
00
9
8
7
6
0
0
0
0
–20
–
–
40
60
80
TG
0
1
00
–
0
00
–
–
–
–
–
15
30
45
60
75
TG
–
–
–
20
40
60
80
80
9
8
7
6
0
0
0
0
160 °C
Cs/P = 2/1
TG
TG
60
40
20
–
0
DTA
00
–
–
20
40
9
8
7
6
0
0
0
0
DTA
406 °C
Cs/P = 3/1
700 800
–60
–80
0
1
00 200 300
400
500 600
Temperature/°C
100 200 300 400 500 600 700 800 900
Temperature/°C
Fig. 9 TG and DTA curves of CNP samples with different Cs/P
molar ratios
Fig. 11 TG and DTA curves of CsNO
3
1
23

A study of thermal behavior of cesium phosphate
1069
6
5
4
3
2
1
0000
0000
0000
0000
0000
0000
0
30000
25000
CsPO3
Cs P O
CsNO3
4
2
7
Cs P O
Cs PO4
Cs O
x
4
2
7
3
Cs/P = 1/2
Cs/P = 1/2
Cs/P = 1/1
20000
15000
10000
5000
0
Cs/P = 1/1
Cs/P = 2/1
Cs/P = 2/1
Cs/P = 3/1
Cs/P = 3/1
10
20
30
40
50
60
10
20
30
40
50
60
2
θ/°
2θ/°
Fig. 12 XRD patterns of CNP samples calcined at 450 °C with
different Cs/P molar ratios
Fig. 14 XRD patterns of CNP samples calcined at 800 °C with
different Cs/P molar ratios
also formed during heating process from 450 to 650 °C.
The XRD patterns of CNP samples calcined at 800 °C
showed that the Cs P O crystal phase was observed at Cs/P
3
0000
Cs PO4
Cs P O
Cs O
x
3
4
2
7
CsNO3
CsPO3
P O
2 5
4
2 7
2
5000
molar ratios of 1/2 and 1/1. These changes are different
from those of the CCP samples, which could be due to the
melting of (CsPO ) that destroys the polycondensation
Cs/P = 1/2
Cs/P = 1/1
2
0000
3
n
structure, resulting in formation of the Cs P O crystal
4 2 7
1
1
5000
0000
phase.
Cs/P = 2/1
Cs/P = 3/1
5
000
Conclusions
0
The thermal behavior of cesium phosphate is significantly
1
0
20
30
40
50
60
influenced by Cs/P molar ratio and Cs source. The CsH2
2
θ/°
PO phase was formed at low temperatures, with Cs CO
3
4
2
Fig. 13 XRD patterns of the CNP samples calcined at 650 °C with
different Cs/P molar ratios
as the Cs source at Cs/P molar ratios of 1/2 and 1/1.
Moreover, CsPO and Cs P O were formed during heating
3
4 2 7
process due to the decomposition and dehydration reac-
tions. Crystal phases disappeared, and only the amorphous
phase was observed when the temperature was increased to
800 °C. At Cs/P molar ratios of 2/1 and 3/1, only the
Cs PO phase was formed, in addition to non-crystalline
attributed to conversion into polyphosphate [17, 18, 20, 28].
Finally, the peaks at 400 and 600 °C are associated with the
melting and decomposition of CsNO respectively.
3
,
Base on the XRD patterns of CNP samples calcined at
3
4
4
50 °C (Fig. 12), the (NH ) HPO and NH H PO phases
compounds, during continuous heating from 450 to 800 °C.
4
2
4
4
2
4
disappeared, whereas the metaphosphate (CsPO ) and
The CsH PO phase was not formed, and only the
3
2
4
pyrophosphate (Cs P O ) phases appeared. Moreover, the
4
decomposition of partial (NH ) HPO was observed when
4 2 4
2
7
corresponding characteristic peaks decreased, and the
CaNO was used as the Cs source. Similarly, the CsPO
3 3
CsNO phase still can be observed as the Cs/P molar ratio
3
and Cs P O phases were presented at 450 °C, but disap-
4 2 7
was increased from 1/2 to 3/1. However, based on the XRD
patterns of CNP samples calcined at 650 °C (Fig. 13), the
crystallinity of the CNP samples obviously decreased and
the CsPO and Cs P O phases disappeared at Cs/P molar
peared with further increase in temperature. Only non-
crystalline compounds were observed at temperatures up to
650 °C, in addition to cesium oxides, because of decom-
position of CsNO . The Cs P O phase was observed again
3
4
2
7
3
4 2 7
ratio of 1/2 and 1/1 (Fig. 14). The CsNO phase disap-
3
when the temperature was increased to 800 °C at Cs/P
peared, and some cesium oxides were further observed at
Cs/P molar ratio of 2/1 and 3/1 because of the decompo-
sition of CsNO . Besides, weak Cs PO crystal phase was
molar ratios 1/2 and 1/1. The melting of (CsPO ) and the
3
n
decomposition and volatilization of CsNO possibly led to
3
the reconstruction of these compounds.
3
3
4
123

1
070
G. Zhang et al.
Acknowledgements This work was supported by the National
Basic Research Program of China (2015CB251401), The National
Natural Science Funds for Distinguished Young Scholar (No.
2 4
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1425625), and National Science Fund for Excellent Young Scholars
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