The electrical conductivity of the cold sintered salt KH2As04 has been investigated within temperature range from room temperature to about 500°C during ...
CHANGES OF ELECTRICAL AND STRUCTURAL CHARACTERISTICS OF COLD SINTERED POTASSIUM DIHYDROGEN ARSENATE WITH TEMPERATURE D. Minic, R. Dimitrijevic* and M. susie Institute of Physical Chemistry, Faculty of Chemistry Belgrade University, Belgrade, Yugoslavia *Faculty of Mining and Geology, Department of Mineralogy and Crystallography, Belgrade University, Yugoslavia ABSTRACT The electrical conductivity of the cold sintered salt KH2As04 has been investigated within temperature range from room temperature to about 500°C during multiple heating and cooling cycles. The reversibility of the dehydration process KH2As04! S-KAs03 has been established. The electrical characteristics were correlated with thermal and structural transformations. The unit cell dimensions of the S-KAs03 phase were determined: a 0 = 14.092(5) R, b 0 = 13.099(4) R, c 0 = 8.891(2) R, S0 = 96.37(2) 0 and V0 = 1631(1) R3.
INTRODUCTION Many solid electrolytes, acid salts and crystallohydrates are known for their conductivity, which is characteristically based on the migration of protons. 1- 4 The conductivity of these substances at room temperature ran1es within wide limi~s, from about 1o-2s/m for LiH2PO~ up to below 10- S/m for MgHP04·3Hz0 and is closely connected with the crystal structure of the respective electrolyte. In these structures hydrogen forms a series of hydrogen bonds along which the protons are moving by a complex mechanism which also involved a tunnelling step. Besides the latter, the complex mechanism of transfer includes also the step in which the charge carrier is prepared as well as the 5reparation step of the acceptor group to capture an approaching proton. The latter two steps require a definite amount of energy and therefore they determine the total activation energy of the conductivity. In some crystal structures from the class of crystallohydrates, the aforementioned steps do not require a significant energy consumption, and in that case, these systems acquire superconductive characteristics.1-3 However, upon heating even the most promising of the hitherto known systems lose the superconductive characteristic irreversibly. Heating involves the processes of dehydration and structural transformations, where the dehydration step most often takes place between 50° and zoooc. Therefore, there is still a need to discover a system which would retain good conductive properties within a wider temperature
Sc1ence of Sintering Edited by D. P. Uskokovic et a/. Plenum Press, New York
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range. From this standpoint, of particular interest are those systems in which the phase transformations are reversible within the aforementioned temperature range. To attain this aim, we studied electrical, thermal and X-ray crystallographic characteristics of the solid acid salt KH2As04, within the range from room temperature to about 500°C, during multiple heating and cooling cycles. The conductivity of this salt, measured in the range from room temperature to about 100°C, having an activation energy of about 92 kJ/mol, classifies this salt within the group of good solid conductors.6 EXPERIMENTAL The polycrystalline powder of the acid salt KH2As04 (p.a. Carlo Erba) was cold sintered under the pressure of 5.1 x 1o8 Pa into pellets of 8 mm diameter and 1-3 mm thickness, the density of which was 2.85 g/cm3. The electrical conductivity of the sintered sample was measured during multiple heating within the temperature range from room temperature to about 500°C by an alternating current of 1 kHz frequency, as described previously.2 The thermochemical investigations were carried out with a Du Pont Thermal Analyzer at a heating rate of 20°/min, if not otherwise stated, using DSC and high-temperature DTA cell (1200°C). The peak temperature and the peak area were determined by using INTERACTIVE DSC V2.0 program. The TG curves were recorded on a TG-951 Analyzer in nitrogen atmosphere. The X-ray powder diffractograms were obtained using a Philips PW-1051 diffractometer with Cuka radiation and §raphite monochromator, at room temperature. Fully automatic programs], for finding the crystal symmetry, and a program for refinement9 of unit cell dimensions from powder data, were used. RESULTS AND DISCUSSION The electrical conductivity measured within the temperature range from room temperature to 500°C during two heating cycles, is shown in Fig. 1. During the first heating, the logarithmic dependence of the conductivity as a function of reciprocal temperature in the investigated temperature range, showed two linear regions, namely from 20° to 220°C and from 360° to 500°C, with a sudden increase and decrease of the conductivity in the region from 220° to 360°C and with a maximum at 305°C. The activation energies of the conductivity in the regions of these two linear dependences, log x= f(l/T), determined from the slopes, were found to be 91.9 kJ/mol and 160.8 kJ/mol, respectively, which is in good agreement with data reported in the literature for the region of the first linear dependenceft The discontinuity in the dependence shown, line ~ in Fig. 1, points to structural changes of the sample within the investigated temperature range. These changes are also seen on the DTA diagram, Fig. 2, and are confirmed and followed by recording X-ray powder diffractograms. The same temperature dependence of the logarithm of the conductivity on the reciprocal temperature was obtained during the
556
-2.00
a -3.00
•e
1/)
P
I I
'-----------·-t-
Temperature ( •c
Cll
00
)
Fig. 4. TGA diagram for solid KHzAs04; heating rate 200C/min; in nitrogen atmosphere.
328.0"C
a
b
'
409_g•c
Fig. 5. Determination of the peak temperature and the enthalpies from peak areas of DSC curves of KH2As04·
559
279.0 "C
Fig. 6. Determination of the peak temperature and the enthalpies from peak areas in successive heating of KHzAs04.
atmosphere for several hours, Fig. 7; namely, on the DSC diagram the appearance of poorly separated peaks I and II is repeated. The total area of these peaks corresponds to the dehydration enthalpy of 376 J/g, which is very close to the enthalpy of 389 J/g determined during the first heating of the sample. This interesting phenomenon indicates a definite reversibility of the process: dehydration t hydration, which can explain to some extent similar electrical behaviour of the investigated sample during the repeated heating. The dehydrated phase KAs03 heated at 450°C whose X-ray diffractogram is shown in Fig. 8~, as well as an indexed powder pattern presented in Table I, corres~onds to the beta-KAs03 polyarsenate phase described in the literature. 0 However, X-ray data for the beta KAs03 phase found in the JSPDS file and which had been taken from the former paper, differ considerably from our results. That was the reason to carry out the calculation of unit cell dimensions of the beta-KAs03 phase. By using programs VISSER, 7 TREOR-4, 8 and LSUCRIPC~ 9 a monoclinic unit cell with a =14.092(5)A, b =13.099(4)!, c =8.891(2)A, Bo=96.37(2) and V0 =163l(l)A~ was found and refined. From Fig. 8b, it can be seen that the transformation from KHzAs04 phase to a dehydrated beta KAs0 3 phase at 35QOC is a solid-solid transformation, the same being valid
0
~ -; -40 0
i:L
~-aof
288.0 °C
I
80
160 240 320 Temperaturt- ( • C)
Fig. 7. Determination of the peak temperature and the enthalpies from peak areas of DSC curves after longer standing heated sample in open atmosphere •
560
b
I J
a
I
_,t 60
j
50
40
.l
llJ. 30
20
Fig. 8. X-ray powder diffractograms of KHzAs04 ; at room temperature (a), calcined at 330°C(b), calcined at 450°C (c).
561
Table I. Powder diffraction data for S-KAsOJ phase HKL -1 -1 -2 2 -1 2 -2 -3 -3 3 -1 0 -1 2 -2 -3 -1 -3 3 1 -4 -1 2 -5 2 4 -5 -1 2 5 -2 2 3
1 1 0 0 2 1 2 1 1 0 3 2 2 1 2 3 3 3 0 3 0 1 4 1 0 0 0 5 4 2 5 5 1
I/Io Dobs(A)
0 30 1 6 1 60 1 20 1 5 1 3 0 15 0 10 2 1 1 5 1 53 2 6 2 12 2 11 2 32 0 100 2 11 1 2 11 2 45 2 11 3 32 1 10 1 15 3 3 2 3 2 4 1 3 2 4 1 1 4 1 3 3
9.5534 6.7315 5.8034 5.2179 5.0271 4.8484 4. 7774 4.3987 4.1109 3.9563 3.8140 3.6661 3.6322 3.4356 3.3724 3.1858 3.0792 3.0448 2.9822 2.9101 2.8800 2.7755 2.6966 2.6094 2.6062 2.4915 2.4835 2.4093 2.3898 2.3399
0
Dca1c(A) 9. 56 71 6. 7511 5.8109 5.2135 5.0366 4.8440 4.7835 4.3974 4.1120 3.9504 3.8192 3.6626 3.6261 3.4374 3.3756 3.1890 3.0832 3.0750 3.0445 2.9832 2.9054 2.8769 2. 7733 2.7004 2.6132 2.6067 2.4041 2.4861 2.4109 2.4074 2.3885 2.3410 2.3367
HKL -6 -1 -6 6 -2 0 -3 5 7 -7 7 -6 -3 4 -5 -4 4 -8 -7 0 -3 3 -8 -7 -6 -7 2 1 3 -4 -8 3
0 5 2 0 1 2 0 4 0 0 0 4 3 5 0 6 4 1 4 2 1 7 2 2 1 3 2 3 6 2 0 1
1 2 0 1 4 4 4 1 0 1 1 0 4 2 4 1 3 1 1 5 5 1 0 3 4 3 5 5 3 5
3 5
0
0
I/I 0
Dobs (A)
Dca1c(A)
3 2 7 6 4 2
2.3198 2.2455 2.1999 2.1950 2.1489 2.0908
7 30 28 5 3 2 4 4
2.0304 2.0011 1.9984 1.9063 1. 9025 1.8833 1.8481 1.8360
3 5 8
1.7916 1.7401 1.7063
7 6
1.7028 1.6916
6 5 10 8 7
1.6900 1. 6867 1.6217 1. 6177 1. 6075
6 2
1.6034 1.5833
2.3212 2.2449 2.1987 2.1973 2.1476 2.0931 2.0883 2.0308 2.0007 1. 9997 1. 9062 1.9008 1. 8839 1.8479 1.8364 1.8350 1.7914 1. 7392 1. 706 7 1. 7061 1.7024 1.6913 1.6912 1.6885 1.6873 1.6225 1. 6182 1.6080 1.6073 1.6029 1.5840 1.5834
for the reverse process. The X-ray diffractogram of the beta-KAs03 phase, after standing of the latter for several hours in an open atmosphere, is identical with that obtained for the starting KH2 As03 shown in Fig. 8a. However, it seems that these are not the only phase transformations~ The poorly defined endothermic maximum at about 300°C and the "shoulder" at 280°C on the DTA curve in Fig. 2, probably indicate additional interphases in the transformation KH2As04 + beta-KAs03. For checking of this assumption, and for its confirmation, high-temperature X-ray investigations in the range from room temperature to 450°C will need to be performed. From all our results, it may be concluded that the transformation process: hydrated-dehydrated K-arsenate in the temperature range from 20 to 450°C, is reversible, which, as a phenomenon, by itself improves and prolongs the electrical conductivity. The results obtained also show that the mechanism of phase transformations in this system deserve further investigation.
562
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