On the high-temperature phase transitions of NH4HSO4

Article abstract of DOI:10.1002/pssa.200520065

Modulated differential scanning calorimetry (MDSC), thermogravimetric analysis (TGA) and X-ray diffraction (XRD) were used to study the phase behaviour of NH₄HSO₄ (AHS) above room temperature. For powder AHS measured during successive thermal cycles, the MDSC curves show an endothermic peak at 147°C on heating scans and an exothermic peak at about 115°C on subsequent cooling runs. However, after the second thermal cycle, a second endothermic peak starts to appear at about 135°C on heating runs whose intensity increases on subsequent cycles. The thermogravimetric measurements on powder AHS show weight loss above 135°C. X-ray powder diffraction data for AHS heated to 140°C during 24 hours, show the diffraction pattern for (NH₄)₂S₂O₇. We discuss the endothermic effect at 135°C in terms of a partial dehydration setting in at reaction sites distributed on the surface of the crystals.

Full text of DOI:10.1002/pssa.200520065

Original
Paper
phys. stat. sol. (a) 202, No. 10, 1891–1895 (2005) / DOI 10.1002/pssa.200520065
On the high-temperature phase transitions of NH4HSO4
J. E. Diosa*, 1, M. E. Fernández1, R. A. Vargas1, E. Torijano1, and B.-E. Mellander2
1
Departamento de Física, Universidad del Valle, A.A. 25360, Cali, Colombia
Physics and Engineering Physics, Chalmers University of Technology, 412 96 Göteborg, Sweden
2
Received 11 February 2005, revised 23 May 2005, accepted 1 June 2005
Published online 5 July 2005
PACS 61.10.Nz, 64.60.–I, 64.70.Kb, 81.70.Pg
Modulated differential scanning calorimetry (MDSC), thermogravimetric analysis (TGA) and X-ray dif-
fraction (XRD) were used to study the phase behaviour of NH4HSO4 (AHS) above room temperature. For
powder AHS measured during successive thermal cycles, the MDSC curves show an endothermic peak at
147 °C on heating scans and an exothermic peak at about 115 °C on subsequent cooling runs. However,
after the second thermal cycle, a second endothermic peak starts to appear at about 135 °C on heating runs
whose intensity increases on subsequent cycles. The thermogravimetric measurements on powder AHS
show weight loss above 135 °C. X-ray powder diffraction data for AHS heated to 140 °C during 24 hours,
show the diffraction pattern for (NH4)2S2O7. We discuss the endothermic effect at 135 °C in terms of a
partial dehydration setting in at reaction sites distributed on the surface of the crystals.
© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1 Introduction
It is relatively common that salts with hydrogen bonds have a high-temperature phase with high proton
mobility, while the low temperature phases are ferroelectric (FE) or antiferroelectric (AFE). The family
MHSO4 (M = K, Rb, Cs and NH4) forms an interesting group of materials characterized by the presence
of a hydrogen bonding pattern between the HSO4- tetrahedral ions [1]. These bonds are responsible for
the ferroelectric phase transitions found in ammonium and rubidium hydrogen sulphates [2, 3]. From
earlier dielectric measurements [2, 4], it was inferred that NH4HSO4 (AHS) undergoes two phase transi-
tions at normal pressure, at T1 = –3 °C (second order) and T2 = –119 °C (first order), respectively. The
phase behaviour of AHS has been studied by thermal and electrical measurements [5, 6]. The high tem-
perature phase, between –3 °C and 147 °C, has a pseudo-orthorhombic structure and the cell dimensions
are with a = 24.66 0.02 Å, b = 4.60 0.01 Å, c = 14.82 0.02 Å, β = 89.87 0.01° at room tempera-
ture [7] and has been reported to be a protonic conductor [8]. However, Baranow et al. [9] in measure-
ments of superficial conductivity observed an anomaly at 139 °C in addition to that at 147 °C. The tran-
sition at 147 °C was attributed to the melting transition of AHS.
Furthermore, it has been suggested that the high temperature phenomena occurring upon heating
above a characteristic temperature Tp in crystals of the KDP type are due to thermal dehydration [10–14]
and similar behaviour has also been observed in other MHS(Se)O4 (M = alkali metal) salts [15, 16].
Thus, we have decided to reexamine the previous results on phase behaviour of AHS above room tem-
perature. Here, we report the results of modulated differential scanning calorimetry (MDSC), thermogra-
vimetry analysis (TGA) and X-ray diffraction (XRD) studies of AHS in the high-temperature range.
* Corresponding author: e-mail: jediosa@calima.univalle.edu.co
© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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J. E. Diosa et al.: On the high-temperature phase transitions of NH4HSO4
2 Experimental
Commercially available high-purity powders of NH4HSO4 (Aldrich, 99.999%) were used. The thermal
characterisation was carried out using a modulated differential scanning calorimetric MDSC 2920 ana-
lyser, TA Instruments. The measurements were performed using a heating rate of 5 °C/min; the samples
were always in the atmosphere of flowing dry N2. The thermogravimetric analysis (TGA) was performed
using a TA Instruments TGA 2050 analyser.
Powder X-ray diffraction studies were performed using a Philips powder diffractometer equipped with
a high-temperature attachment. Ni filtered CuK radiation was used. A platinum sample holder was used
α
for all X-ray diffraction measurements. The temperature measurements were made using a chromel-
alumel thermocouple placed inside the sample holder. The thermal treatment of all AHS samples, before
starting the X-ray measurements, was in situ in a controlled gas atmosphere of helium or air.
3 Results and discussion
The results of MDSC measurements for a powder AHS sample for five heating/cooling cycles above
room temperature are shown in Fig. 1. An endothermic peak at 147 °C is observed on the heating runs
and the corresponding exothermic peak is detected at about 115 °C on the cooling runs. However, after
the second thermal cycle a small endothermic peak begins to appear at about 135 °C on heating runs
whose intensity increases on successive heating runs. Figure 2 shows the MDSC curves for the fourth
thermal cycle.
Figure 3 shows the thermogravimetric curves (% sample weight and its corresponding derivative as a
function of temperature) obtained when a non heat-treated (raw) AHS powder was heated to 170 °C at a
rate of 5 °C/min. Two major variations of weight loss are observed. In the first region there is a rapid
weight loss reaching 1% of weight change at about 90 °C, which is attributed to the removal of super-
ficial water. In the second region (above approximately 135 °C) there is an additional relative slow
weight loss of 0.4% up to 170 °C. It should be noted that the onset temperature (135 °C) of the second
weight loss region is equal to that of the endothermic effect shown by the MDSC heating curves, see
Figs. 2 and 3.
3
2
3
2
NH4HSO4
NH4HSO4
Firts run
Second run
Thirth run
Fourth run
Fifth run
1
1
0
0
-1
-1
Fourth run
100
100
120
140
80
120
140
160
Temperature (oC)
Temperature (oC)
Fig. 1 MDSC curves for five consecutive thermal
cycles of a non heat treated (raw) NH4HSO4 sample in
a nitrogen atmosphere with a flux 80 ml min–1 above
room temperature using a heating/cooling rate of
5 °C/min. The solid line corresponds to the first ther-
mal cycle and the dashed line corresponds to the
subsequent thermal cycles.
Fig. 2 MDSC curve for the fourth thermal cycle of
NH4HSO4.
© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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phys. stat. sol. (a) 202, No. 10 (2005) / www.pss-a.com
1893
0,06
0,04
0,02
0,00
(a) % weight
(b) derivative
100,0
99,6
99,2
98,8
98,4
0
a
5
10
15
20
b
30
60
90
120
150
180
Temperature (oC)
Fig. 3 Thermogravimetric analysis (TGA) of a raw NH4HSO4 sample under a nitrogen atmosphere with
a flux of 10 ml min–1 and a rate of 5 °C/min: (a) first thermal cycle from room temperature to 170 °C; (b)
derivative of the TGA curve.
Baranov et al. [9] reported three conductivity anomalies on heating single crystal samples of AHS.
The first one at 113 °C, is a very pronounced one, which Baranov et al. relate with a phase transition in
the surface layer of the crystal. The second anomaly at 139 °C and the third one at 147 °C (melting point)
show a slow increase in the superficial conductivity. We did not see any thermal effect associated with
the first anomaly at 113 °C. It should be noted that the present measurements were performed on powder
samples while the ones of Baranov et al. were done using single crystals. We will here discuss the endo-
thermic peak we have observed at 135 °C in terms of a partial thermal dehydration that begins at the
surface of the sample and produces the slowly increasing superficial conductivity observed by Baranov
et al. in AHS single crystals at 139 °C.
To test the thermal dehydration of powdered AHS above 135 °C, we performed powder X-ray diffrac-
tion measurements to correlate the previous observations with crystallographic data. Figure 4a shows the
X-ray pattern of a raw sample at room temperature in a dry helium atmosphere and the lines are indexed
on the basis of the pseudo-orthorhombic structure of AHS [7]. The sample was subsequently heat treated
in helium atmosphere: for 24 h at 100 °C before performing the XRD at this temperature whose pattern
is shown in Fig. 4b; then, for 12 h at 140 °C (Fig. 4c) and after cooling the sample to 20 °C (Fig. 4d).
Figure 4e shows the diffraction pattern at 20 °C after keeping the sample at this temperature for 40 h in
air. The diffraction patterns shown in Figs. 4a and b are essentially the same, except for small changes in
the intensity at the peaks maybe as a consequence of absorbed water molecules weakly bound to the
surface that are released at 100 °C. However, the diffraction pattern at 140 °C (Fig. 4c) is quite different
to those observed previously indicating a new lattice symmetry of the sample which remains when the
sample is cooled to room temperature in a dry helium atmosphere (Fig. 4d). However, when the sample
was kept for 40 h in air at 20 °C it recovers the room temperature AHS structure (Fig. 4a).
Comparing of the results of XRD with those of MDSC and TGA gives crucial information on the
dynamics of AHS. The XRD results indeed show that the diffraction pattern that is obtained when AHS
is heated above 135 °C for a specific time (12 h) corresponds to that of ammonium pyrosulphate
(NH4)2S2O7 [17] which is formed due to the thermal dehydration of AHS.
Heating AHS (by using the thermal cycles of the MDSC equipment (5 °C/min)) leads to the onset of a
thermal dehydration of AHS and the formation of the salt (NH4)2S2O7 which take place over the surface
of the crystals of AHS, so a double salt is formed as previously proposed for the thermal decomposition
of MHS(Se)O4 salts [15, 16] after their melting points, according to the reaction:
2 MHS(Se)O4 → M2S(Se)2O7 + H2O (M = alkali metals) .
(1)
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J. E. Diosa et al.: On the high-temperature phase transitions of NH4HSO4
2400
1200
(a)
NH4HSO4
0
2400
NH4HSO4
(b)
(c)
(d)
1200
0
2400
(NH4)2S2O7
1200
0
2400
(NH4)2S2O7
1200
0
2400
NH4HSO
45
4 (e)
1200
0
20
25
30
35
40
50
2
θ
(degrees)
Fig. 4 X-ray diffraction patterns of a powdered AHS as a function of temperature and always in a dry
helium atmosphere, except for the last one: (a) the raw AHS sample at 20 °C; (b) the same sample at
100 °C after 24 h annealing at this temperature; (c) the same sample at 140 °C after 12 h annealing at this
temperature; (d) the same sample at 20 °C after cooled rapidly from 140 °C; (e) the same sample at 20 °C
after keeping the sample at this temperature for 40 h in air.
For AHS, our results indeed show that the thermal dehydration begins to take place before its previously
reported melting temperature at 147 °C. Thus, the phase transition at 139 °C reported in Ref. [9] corre-
sponds to the onset of the thermal dehydration according to the reaction (2) for M = NH4, which occurs
at particular localities on the surface of the sample. (NH4)2S2O7 cover the surface of the AHS crystals and
some of the constitutional water that is formed in the reaction is evaporated which is in agreement with
the thermogravimetric measurements. It should be stressed that, for the specific time taken by the TGA
to run the experiment, only 0.4% of the sample weight was lost when an AHS raw sample was heated to
170 °C, see Fig. 3. Thus, the conversion AHS to (NH4)2S2O7 have only occurred partially, since accord-
ing to the reaction (2), when all the constitutional water is released 9.2% of the sample weight is lost
(A vertical scale for the percentage of water weight loss is added in Fig. 3 such that 100% means
the total dehydration of the sample according to Eq. (1)). In fact, heating AHS to 140 °C for 12 h pro-
duces the pyrosulphate and the thermal dehydration is completed as indicated by the XRD results
(Fig. 4c and 4d).
The reverse reaction
(NH4)2S2O7 + H2O → 2 NH4HSO4 ,
(2)
is very slow, because there is not enough available free water neither at the surface, nor in the surround-
ings. Thus, we did not observe the reversibility of the anomaly at 135 °C in the MDSC measurements.
However, if (NH4)2S2O7 is kept at room temperature in air for a specific time (24 h) a total conversion to
AHS takes place, see Fig. 4e.
4 Conclusions
On the basis of our measurements using TG, MDSC and X-ray diffraction, we suggest that the anomaly
observed by Baranov at 139 °C measuring the superficial conductivity is not related with a structural
change of AHS. Instead, this anomaly is a consequence of a thermal dehydration process starting on the
surface of the sample, which is described by the reaction: 2 NH4HSO4 → (NH4)2S2O7 + H2O. The total
conversion of AHS to (NH4)2S2O7 is obtained after heating the sample just above this characteristic tem-
perature for a specific time (12 h).
© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Original
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phys. stat. sol. (a) 202, No. 10 (2005) / www.pss-a.com
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Acknowledgements The authors would like to acknowledge the support of the Banco de la República de Colom-
bia, and the International Program in the Physical Sciences, IPPS, of Uppsala, Sweden.
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© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim