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Specific heat of some phosphate compounds
Solid State Communications, Vol. 45, No. 2, pp. 133-135, 1983. Printed in Great Britain.
0038-1098/83/020133-03503.00/0 Pergamon Press Ltd.
SPECIFIC HEAT OF SOME PHOSPHATE COMPOUNDS N. Tjapkin 'Boris Kidri~' Institute of Nuclear Sciences, Vin~a, Belgrade, Yugoslavia
(Received
10 January 1982
by A. Blandin)
Specific heats Cue of some phosphates have been measured on powder samples, in the temperature range 100-373 K. Anomalous behaviour of Cue(T) has been observed for Co(H2PO4)2 • 2HaPO4 and Mn(H2PO4)2 • 2HaPO4 around 190 K. In triclinic Ba(H2PO4): a phase transition was found at about 147 K, i.e. at a somewhat higher temperature than according to previous dielectric measurements.
FERROELECTRIC and antiferroelectric nondeuterated phosphate compounds studied up till now have transitions from the paraelectric phase at temperatures between 122 K (KH2PO4) and 310 K (PbHPO4) [1 ]. These phase transitions are accompanied by a welldefmed step in the specific heat. New phosphate crystals suitable for dielectric measurements are very difficult to obtain [2]. That is why, in order to find new KDP type ferroelectrics or antiferroelectrics, we decided to perform specific heat measurements on polycrystalline samples, in the temperature range 100-373 K. Data were taken on the following compounds, stable in air: monoclinic Mn(H2PO4)2 • 2H20 and Sr(H2PO4)2, triclinic Sr(H2PO4)2, Sr(H2PO4)2 • H20 and Ba(H2PO4)2, and orthorhombic Ba(H2PO4)2; as well as on hygroscopic monoclinic Fe(H2POa)a and triclinic Mn(H2PO4)2" 2H3PO4, Co(H2PO4)2 • 2H3PO4 and Pb(H2PO4)2. Also, in order to check the validity of the obtained results, measurements were performed on NH4H2PO4 and PbHPO4 for comparison with literature values. The powder samples studied were obtained from small single crystals grown by slow evaporation from water solutions.* Samples of 2 0 - 3 0 mg were held in aluminum containers. The Perkin-Elmer differential scanning calorimeter was used, which is suitable for measurements on powder in such a wide temperature range. Its advantage, so far as the samples with crystal water are concerned, is that it does not require vacuum. However, the method followed has several disadvantages: (1) the sample was not a single crystal and was not in vacuum; (2) the measurements were not isothermal (the rates were 10, 20 and 40 K min-1); (3) there was a sensitivity to the change of the liquid nitrogen level in the reservoir and the flow of gaseous nitrogen through the dry box. For these reasons • The author is indebted to M. ~uri6 for the great help in preparing the samples and for the discussion about their crystal systems.
we wanted to test our results, no matter how good their reproducibility, by comparing them with those obtained earlier on some KDP type crystals. We found a heat of transition of 642.8 J mole -1 in NH4H2PO4, which is in excellent agreement with the value 644.8 +- 2.1 J mole -1 , obtained formerly on a single crystal sample by another method [3]. As another example, Fig. 1 shows the temperature dependence of the PbHPO4 specific heat in the region near the phase transition. Our measurements are in good agreement with recently published results obtained by the same method on a sample consisting of several single crystals [4]. The step in the specific heat at the transition, 1.8 x 10 -2 J g - l K - t , is somewhat smaller than in [4] (2 x 10-2 J g - l K - 1 ) , but is still considerably larger than the values which we calculated by applying the Devonshire thermodynamic theory of second order phase transitions to the temperature depen. dence of the dielectric permittivity and the spontaneous polarization [5, 6] (7.5 x 10 -a j g - l K - 1 and 9.25 x 10- 3 j g- 1K- 1, respectively). In case of the other investigated materials, we found an anomalous behaviour of Cue(T) in triclinic Ba(HzPO4)2, Co(H2PO4)2 • 2HaPO4 and Mn(H2PO4)2 • 2H3PO4 only. Triclinic Ba(H2PO4)2 has already been investigated, but perhaps because of the difficulties in crystallization, a small number of papers have been published so far [2, 7, 8]. The temperature dependence of the dielectric permittivity was measured and the temperature of the antiferroelectric phase transition was determined as 133 K [2]. From our results (Fig. 2) it can be seen that the step in specific heat is not very pronounced. Although there is an indication for a second order phase transition similar as in PbHPO4, this cannot be claimed with certainty. The transition temperature would be about 147 K, which is in agreement with dielectric measurements which were performed on powder samples formed in pills (Fig. 2), but which differs considerably from the earlier published value [2].
133
134
SPECIFIC HEAT OF SOME PHOSPHATE COMPOUNDS
o3~I~<-)~ 0 ~I
~bHP04
280
320
340
Fig. 1. Temperature dependence of the specific heat of PbHPO4. II
045
041
Er
037
w
o
o
0
E
o
I0 -
9
£3 (H2 P 0 4 ) 2 0 33
I 130
I0
I 150 T,
8 170
K
Fig. 2. Temperature dependence of the specific heat and of the relative permittivity of triclinic Ba(H2PO4)2. 0.75
v 070 Co(H~PO~ -)
065
uJ~ 060
~~1
055 160
Mn(H2PO4)2"2H3P04 I 200
180 T,
220
K
Fig. 3. Temperature dependence of the specific heat of Co(H2PO4)2 • 2H3PO4 and Mn(H2PO4)2 • 2H3PO4.
Co( H 2 P 0 4 ) 2 2 H 3 P 0 4
P b ( H2 P 0 4 ) z
Base Line I
160
I
I
180
200 T,
It should be mentioned that the dependence of our results on varying the rate of temperature change in the range 10-40 Kmin -1 is insignificant. A slight shifting of the curves toward higher temperatures with the rise of this rate was only noticed. The shifting is in good agreement with that predicted in the Perkin-Elmer instructions, 0.85 K every 10 K rain -t ; the required correction of the temperature scale was done. The actual rate of temperature change for the data in Figs. 1 - 4 was the same, 20 Kmin -1 . Another thing we would like to point out is that the anomalous behaviour of the dielectric permittivity of the Ba(H2PO4)2 powder, although weak compared with a single crystal measurement of the same compound [2], is still clear enough for determining the phase transition temperature. The dielectric permittivity was measured at 1 kHz with a field smaller than 3 V cm-t. The temperature dependence of the specific heat found for Co(H2PO4)2 • 2H3PO4 and Mn(H2PO4)2 • 2H3PO4 (Fig. 3) is very similar, which could be expected since the compounds are isostructural [9]. The anomaly which appears in the range 180-190 K for Mn(H2PO4)2 • 2H3PO4 and 185-195 K for Co(H2PO4)2 • 2H3PO4, is even less clear than for Ba(H2PO4)2 and has a rather unusual shape for this kind of material. Dielectric measurements on pills did not confirm the existence of a phase transition. In order to exclude any doubt about the existence of an anomalous behaviour of the specific heat, and to show its shape better, Fig. 4 is given. The curves, copied directly from the DSC recorder chart, are drawn one above the other. Measurement conditions were the same for all the curves, save that for each the slope was separately adjusted, making them run parallel to the direction of the recorder chart movement (temperature axis). In that way, the deviation of from linearity is best seen. The curve obtained for the container with the Pb(H2PO4)2 powder has approximately the same shape as the base line (empty container). It is evident, therefore, that Pb(H2PO4)2 has no anomalous behaviour of the specific heat in the observed temperature interval 160-220 K. The curve obtained for the container with the Co(H2PO4)2 • 2H3PO4 powder, besides this weakly pronounced nonlinearity caused by the experimental conditions (base line nonlinearity), has also a welldefined step in the range 185-195 K. It is obvious that this step is the consequence of the anomalous behaviour of the specific heat of C o ( H 2 P O 4 ) 2 • 2H3PO4, since measurements were repeated several times on several samples. The same is true for Mn(H2PO4)2 • 2H3PO4. These results, including those for Ba(H2PO4)2 as well, should be considered as preliminary. More precise isothermic measurements need to be done, if possible on single crystals, to clarify the nature of these transitions.
Cff(T)
I
T, K
300
Vol. 45, No. 2
I
220
K
Fig. 4. The evidence for the specific heat anomaly of Co(H2PO4)2 " 2H3PO4.
Cff(T)
Vol. 45, No. 2
SPECIFIC HEAT OF SOME PHOSPHATE COMPOUNDS REFERENCES
1.
2. 3. 4.
See for example, appendix E in M.E. Lines & A.M. Glass, Principles and Application of Ferroelectrics and Related Materials, Clarendon Press, Oxford (1977). M. Huter & M. ~ivanovi6, Fizika 8, Suppl. 272 (1976). C.C. Stephenson & A.C. Zettlemoyer, J. Am. Chem. Soc. 66, 1405 (1944). A. Fouskova & B. Brezina, Ferroelectrics 19, 119 (1978).
5. 6. 7. 8. 9.
135
T.J. Negran, A.M. Glass, C.S. Brickenkamp, R.D. Rosenstein, R.K. Osterheld & R. Susott, Ferroelectrics 6, 179 (1974). F. Smutny & J. Fousek, Ferroelectrics 21, 1-4, 385 (1978). R. Herak, lOth Conf. of the Yugoslav Centre for Crystallography, Kumrovec, 5-7 June (1975). N. Tjapkin & M. ~ivanovi6, Fizika 8, Suppl. 235 (1976). R. Herak & M. ~uri6, (private communication).
0038-1098/83/020133-03503.00/0 Pergamon Press Ltd.
SPECIFIC HEAT OF SOME PHOSPHATE COMPOUNDS N. Tjapkin 'Boris Kidri~' Institute of Nuclear Sciences, Vin~a, Belgrade, Yugoslavia
(Received
10 January 1982
by A. Blandin)
Specific heats Cue of some phosphates have been measured on powder samples, in the temperature range 100-373 K. Anomalous behaviour of Cue(T) has been observed for Co(H2PO4)2 • 2HaPO4 and Mn(H2PO4)2 • 2HaPO4 around 190 K. In triclinic Ba(H2PO4): a phase transition was found at about 147 K, i.e. at a somewhat higher temperature than according to previous dielectric measurements.
FERROELECTRIC and antiferroelectric nondeuterated phosphate compounds studied up till now have transitions from the paraelectric phase at temperatures between 122 K (KH2PO4) and 310 K (PbHPO4) [1 ]. These phase transitions are accompanied by a welldefmed step in the specific heat. New phosphate crystals suitable for dielectric measurements are very difficult to obtain [2]. That is why, in order to find new KDP type ferroelectrics or antiferroelectrics, we decided to perform specific heat measurements on polycrystalline samples, in the temperature range 100-373 K. Data were taken on the following compounds, stable in air: monoclinic Mn(H2PO4)2 • 2H20 and Sr(H2PO4)2, triclinic Sr(H2PO4)2, Sr(H2PO4)2 • H20 and Ba(H2PO4)2, and orthorhombic Ba(H2PO4)2; as well as on hygroscopic monoclinic Fe(H2POa)a and triclinic Mn(H2PO4)2" 2H3PO4, Co(H2PO4)2 • 2H3PO4 and Pb(H2PO4)2. Also, in order to check the validity of the obtained results, measurements were performed on NH4H2PO4 and PbHPO4 for comparison with literature values. The powder samples studied were obtained from small single crystals grown by slow evaporation from water solutions.* Samples of 2 0 - 3 0 mg were held in aluminum containers. The Perkin-Elmer differential scanning calorimeter was used, which is suitable for measurements on powder in such a wide temperature range. Its advantage, so far as the samples with crystal water are concerned, is that it does not require vacuum. However, the method followed has several disadvantages: (1) the sample was not a single crystal and was not in vacuum; (2) the measurements were not isothermal (the rates were 10, 20 and 40 K min-1); (3) there was a sensitivity to the change of the liquid nitrogen level in the reservoir and the flow of gaseous nitrogen through the dry box. For these reasons • The author is indebted to M. ~uri6 for the great help in preparing the samples and for the discussion about their crystal systems.
we wanted to test our results, no matter how good their reproducibility, by comparing them with those obtained earlier on some KDP type crystals. We found a heat of transition of 642.8 J mole -1 in NH4H2PO4, which is in excellent agreement with the value 644.8 +- 2.1 J mole -1 , obtained formerly on a single crystal sample by another method [3]. As another example, Fig. 1 shows the temperature dependence of the PbHPO4 specific heat in the region near the phase transition. Our measurements are in good agreement with recently published results obtained by the same method on a sample consisting of several single crystals [4]. The step in the specific heat at the transition, 1.8 x 10 -2 J g - l K - t , is somewhat smaller than in [4] (2 x 10-2 J g - l K - 1 ) , but is still considerably larger than the values which we calculated by applying the Devonshire thermodynamic theory of second order phase transitions to the temperature depen. dence of the dielectric permittivity and the spontaneous polarization [5, 6] (7.5 x 10 -a j g - l K - 1 and 9.25 x 10- 3 j g- 1K- 1, respectively). In case of the other investigated materials, we found an anomalous behaviour of Cue(T) in triclinic Ba(HzPO4)2, Co(H2PO4)2 • 2HaPO4 and Mn(H2PO4)2 • 2H3PO4 only. Triclinic Ba(H2PO4)2 has already been investigated, but perhaps because of the difficulties in crystallization, a small number of papers have been published so far [2, 7, 8]. The temperature dependence of the dielectric permittivity was measured and the temperature of the antiferroelectric phase transition was determined as 133 K [2]. From our results (Fig. 2) it can be seen that the step in specific heat is not very pronounced. Although there is an indication for a second order phase transition similar as in PbHPO4, this cannot be claimed with certainty. The transition temperature would be about 147 K, which is in agreement with dielectric measurements which were performed on powder samples formed in pills (Fig. 2), but which differs considerably from the earlier published value [2].
133
134
SPECIFIC HEAT OF SOME PHOSPHATE COMPOUNDS
o3~I~<-)~ 0 ~I
~bHP04
280
320
340
Fig. 1. Temperature dependence of the specific heat of PbHPO4. II
045
041
Er
037
w
o
o
0
E
o
I0 -
9
£3 (H2 P 0 4 ) 2 0 33
I 130
I0
I 150 T,
8 170
K
Fig. 2. Temperature dependence of the specific heat and of the relative permittivity of triclinic Ba(H2PO4)2. 0.75
v 070 Co(H~PO~ -)
065
uJ~ 060
~~1
055 160
Mn(H2PO4)2"2H3P04 I 200
180 T,
220
K
Fig. 3. Temperature dependence of the specific heat of Co(H2PO4)2 • 2H3PO4 and Mn(H2PO4)2 • 2H3PO4.
Co( H 2 P 0 4 ) 2 2 H 3 P 0 4
P b ( H2 P 0 4 ) z
Base Line I
160
I
I
180
200 T,
It should be mentioned that the dependence of our results on varying the rate of temperature change in the range 10-40 Kmin -1 is insignificant. A slight shifting of the curves toward higher temperatures with the rise of this rate was only noticed. The shifting is in good agreement with that predicted in the Perkin-Elmer instructions, 0.85 K every 10 K rain -t ; the required correction of the temperature scale was done. The actual rate of temperature change for the data in Figs. 1 - 4 was the same, 20 Kmin -1 . Another thing we would like to point out is that the anomalous behaviour of the dielectric permittivity of the Ba(H2PO4)2 powder, although weak compared with a single crystal measurement of the same compound [2], is still clear enough for determining the phase transition temperature. The dielectric permittivity was measured at 1 kHz with a field smaller than 3 V cm-t. The temperature dependence of the specific heat found for Co(H2PO4)2 • 2H3PO4 and Mn(H2PO4)2 • 2H3PO4 (Fig. 3) is very similar, which could be expected since the compounds are isostructural [9]. The anomaly which appears in the range 180-190 K for Mn(H2PO4)2 • 2H3PO4 and 185-195 K for Co(H2PO4)2 • 2H3PO4, is even less clear than for Ba(H2PO4)2 and has a rather unusual shape for this kind of material. Dielectric measurements on pills did not confirm the existence of a phase transition. In order to exclude any doubt about the existence of an anomalous behaviour of the specific heat, and to show its shape better, Fig. 4 is given. The curves, copied directly from the DSC recorder chart, are drawn one above the other. Measurement conditions were the same for all the curves, save that for each the slope was separately adjusted, making them run parallel to the direction of the recorder chart movement (temperature axis). In that way, the deviation of from linearity is best seen. The curve obtained for the container with the Pb(H2PO4)2 powder has approximately the same shape as the base line (empty container). It is evident, therefore, that Pb(H2PO4)2 has no anomalous behaviour of the specific heat in the observed temperature interval 160-220 K. The curve obtained for the container with the Co(H2PO4)2 • 2H3PO4 powder, besides this weakly pronounced nonlinearity caused by the experimental conditions (base line nonlinearity), has also a welldefined step in the range 185-195 K. It is obvious that this step is the consequence of the anomalous behaviour of the specific heat of C o ( H 2 P O 4 ) 2 • 2H3PO4, since measurements were repeated several times on several samples. The same is true for Mn(H2PO4)2 • 2H3PO4. These results, including those for Ba(H2PO4)2 as well, should be considered as preliminary. More precise isothermic measurements need to be done, if possible on single crystals, to clarify the nature of these transitions.
Cff(T)
I
T, K
300
Vol. 45, No. 2
I
220
K
Fig. 4. The evidence for the specific heat anomaly of Co(H2PO4)2 " 2H3PO4.
Cff(T)
Vol. 45, No. 2
SPECIFIC HEAT OF SOME PHOSPHATE COMPOUNDS REFERENCES
1.
2. 3. 4.
See for example, appendix E in M.E. Lines & A.M. Glass, Principles and Application of Ferroelectrics and Related Materials, Clarendon Press, Oxford (1977). M. Huter & M. ~ivanovi6, Fizika 8, Suppl. 272 (1976). C.C. Stephenson & A.C. Zettlemoyer, J. Am. Chem. Soc. 66, 1405 (1944). A. Fouskova & B. Brezina, Ferroelectrics 19, 119 (1978).
5. 6. 7. 8. 9.
135
T.J. Negran, A.M. Glass, C.S. Brickenkamp, R.D. Rosenstein, R.K. Osterheld & R. Susott, Ferroelectrics 6, 179 (1974). F. Smutny & J. Fousek, Ferroelectrics 21, 1-4, 385 (1978). R. Herak, lOth Conf. of the Yugoslav Centre for Crystallography, Kumrovec, 5-7 June (1975). N. Tjapkin & M. ~ivanovi6, Fizika 8, Suppl. 235 (1976). R. Herak & M. ~uri6, (private communication).