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Pyroelectric properties of NH4HSeO4 crystals at low temperatures
0038-1098/85 $3.00 + .00 Pergamon Press Ltd.
Sohd State Commumcatlons, Vol. 55, No. 1, pp. 39-41, 1985. Printed m Great Britam.
PYROELECTRIC PROPERTIES OF NH4HSeO4 CRYSTALS AT LOW TEMPERATURES R. Poprawskl and J. Dzledzic Institute of Physics, Techmcal Umverslty of Wroc~aw, Wybrze~e Wyspmfisklego 27, 50-370 Wroc~aw, Poland and J.V. Shaldm International Laboratory of Strong Magnetic Fields and Low Temperatures, ul. Pr6chnika 95, 53-529 Wroc~aw, Poland
(Received 22 January 1985 by G.S. Zhdanov) Pyroelectnc and dielectric properties of NH4HSeO4 crystals have been investigated m a temperature range of 4.2-200 K along the b-axis. Spontaneous polartzatlon has been found to change ~ts s~gn at a temperature of about 100 K a temperature of 305 K. Salver-paste electrodes have been deposited onto the wafers of 50-mm 2 area cut perpendicularly to the b-axis Investigations of pyroelecmc properties have been performed with the method of pyroelectric-charge integration. Dielectric properties have been studied with an automatic capacity bridge for a frequency of 1 kHz In both cases temperature has been controlled with PID temperature regulator co-operating with germanium and platinum thermometers at hehum and mtrogen temperatures, respectively. Temperature stabilization accuracy has not been worse than 10 -2 K
1 INTRODUCTION AMMONIUM HYDROGEN selenate exhibits both ferroelectric properties along the a-axis m a temperature range of 1 0 0 - 2 5 0 K [1-3] and pyroelectric propemes along the b-axas at temperatures ranging from that of liquid mtrogen up to room temperature [ 3 - 5 ] . The upper temperature lmalt for pyroelectnc properties has not been determined yet since dielectric losses make ~t tmposslble to measure these properties at higher temperatures. It ~s known from the crystaUographlc studies that phase transition at a temperature of 250 K is connected with the ordering of protons m hydrogen bonds [ 6 - 8 ] . At thxs temperature a symmetry change from 12 monochmc to I1 triclmlc system occurs. The spontaneous polarization appears along the a-axis, change of this polarization along the b-axis being not reversable [3, 4]. It results from the specific-heat measurements [9] that phase transitions at temperatures of about 100K and 250 K are the translt~ons of the first kind, the tranm~on at 250 K being the one of near-second order. Up to now there is no reformation concerning a mechanism of the phase transition at about 100K and no results of physical properties investigations of NH4HSeO4 crystals below liqmd-nitrogen temperature. The aLrn of this work has been to complete this gap of knowledge, and thus obtam addltxonal reformation on a character of the phase transition at about 100K and determining a d~rectlon of pyroelectnc-polarisatlon change.
3. RESULTS AND DISCUSSION The obtained experimental dependence of spontaneous polarization change upon temperature is shown m Fig. 1. A change of the "pyroelectric" polarization sign takes place during the phase tranmlon at a temperature of about 100 K, provxded that no sharp changes of the polarization occur m a temperature range of 0 - 4 . 2 K. As can be seen from the figure we deal almost with the "repolarizatlon" of the crystal. Due to the lack of crystallograptuc data it is difficult to indicate precisely groups of atoms the reorientation of which is connected with this polarization change. "Pyroelecmc" polarization can be supposed, based on paper [6], to be connected with dipole moment of SeO4 group and its reonentatlon might cause the observed change in polarization direction. It is seen from the figure that the spontaneous polarxzation changes its sign again at a temperature of about 171 K. The exact position of this point cannot be determined since the measurements of the polarization change have not been performed starting from a temperature of 0 K.
2. EXPERIMENT The NH4HSeO4 crystals have been grown from a water solution by the xsothermal vaporatlon method at 39
NH4HSe04 CRYSTALS AT LOW TEMPERATURES
40
Vol. 55, No. 1 X
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Fig 2. Temperature dependence of pyroelectrlc coefficient for NH4HSeO4 crystals along the b-axzs. It is worth noting that at a temperature of 250K there are no anomahes of pyroelectnc properties along the b-axis [ 3 , 4 ] , therefore coupling coeffic:ent between ferroelectnc and pyroelectrtc polanzatlon depends upon temperature and vamshes at 250 K Ftgure 2 shows temperature dependence of pyroelectnc coefficient obtained
from measurements of polarization changes presented in Fig. 1. It can be seen from the figure that beszde a sharp anomaly corresponding to the phase transition at about 100K and temperature hysterem characteristic for phase transitions of the first order, an extra maximum at a temperature of about 6 K is observed. Stmdar anomalies
Vol. 55, No. 1
NH4HSe04 CRYSTALS AT LOW TEMPERATURES
phase transition connected probably with dtsordenng of protons in hydrogen bonds and, coupled with this, reorientatlon of other groups of atoms. The full understanding of this transitton mechamsm reqmres further investigations (first of all crystallographtc studies).
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Acknowledgements - The authors express their thanks to Mr. Stanlstaw Matyastk for his help during the measurements. zi
REFERENCES r [K]
--
Fig. 3. Temperature dependence of dielectric penmttivity for NH4HSeO4 crystals along the b-axis. of pyroelectnc propemes are observed at low temperatures for other ferroelectrlcs [10, 11] wluch indicates that the anomahes cannot be described w~thm Barn's model [12, 13] m which pyroelectrlc coefficient at low temperatures is proportional to the third power of temperature. Figure 3 shows temperature dependence of dielectric permlttwlty for NH4HSeO4 crystals. A slow increase m dielectric permlttlvlty with the decreasing temperature ~s probabl3, connected w~th a contribution of acoustic vibrations of the crystal to ~ts total permlttwity Dielectric losses m a range of 4 . 2 - 8 0 K are small (tan ~ = 7 x 10 -4) In conclusion ~t should be pointed out that at a temperature of about 100K there occurs a complex
1. 2 3 4 5. 6. 7. 8. 9 10. 11. 12. 13
Z. Czapla, T. Lls & L. Sobczyk, Phys. Status Solidi 51,609 (1979). V.S. Krasikov & A.I. Krughk, Ftzika Tverdogo Tela 21,2534 (1979) (m Russmn) R Poprawskl & J Dziedzlc, J. Phys. Chem. Sohds 45,355 (1984). K Gesl, J Phys. Soc. Japan 48, 1399 (1980) H. Pykacz, Z. Czapla & J. Mr6z, Acta Phys. Polon. A60,325 (1981). K.S Aleksandrov, A.I. Krughk, S.V. Misyul & M.A. Stmonov, Krtstallografiya 25, 1142 (1980) (in Russian). A.I. Krughk, S.V. Mlsyul, K S Aleksandrov, Dokl Akad. Nauk S.S.S.R, Krtstallografiya 295, 344 (1980) (m Russian). I. Brach, D.J. Jones & J Rozlere, J. Solid State Chem 8,401 (1983). R Poprawskl, P.S. Smlrnov & B.A. Strukov, Fiztka Tverdogo Tela 26, 1339 (1984)(in Russian) A. Hadnl & D. Grandjean, J Phys. 30,377 (1969) N.D. Gavrdova, S.N Drozhdm & V.K Novtk, Sohd State Commun. 48, 129 (1983). M. Born, Rev Mod. Phys 13,245 (1945). B. Szlgettl, Phys Rev Lett. 35, 1532 (1975)
Sohd State Commumcatlons, Vol. 55, No. 1, pp. 39-41, 1985. Printed m Great Britam.
PYROELECTRIC PROPERTIES OF NH4HSeO4 CRYSTALS AT LOW TEMPERATURES R. Poprawskl and J. Dzledzic Institute of Physics, Techmcal Umverslty of Wroc~aw, Wybrze~e Wyspmfisklego 27, 50-370 Wroc~aw, Poland and J.V. Shaldm International Laboratory of Strong Magnetic Fields and Low Temperatures, ul. Pr6chnika 95, 53-529 Wroc~aw, Poland
(Received 22 January 1985 by G.S. Zhdanov) Pyroelectnc and dielectric properties of NH4HSeO4 crystals have been investigated m a temperature range of 4.2-200 K along the b-axis. Spontaneous polartzatlon has been found to change ~ts s~gn at a temperature of about 100 K a temperature of 305 K. Salver-paste electrodes have been deposited onto the wafers of 50-mm 2 area cut perpendicularly to the b-axis Investigations of pyroelecmc properties have been performed with the method of pyroelectric-charge integration. Dielectric properties have been studied with an automatic capacity bridge for a frequency of 1 kHz In both cases temperature has been controlled with PID temperature regulator co-operating with germanium and platinum thermometers at hehum and mtrogen temperatures, respectively. Temperature stabilization accuracy has not been worse than 10 -2 K
1 INTRODUCTION AMMONIUM HYDROGEN selenate exhibits both ferroelectric properties along the a-axis m a temperature range of 1 0 0 - 2 5 0 K [1-3] and pyroelectric propemes along the b-axas at temperatures ranging from that of liquid mtrogen up to room temperature [ 3 - 5 ] . The upper temperature lmalt for pyroelectnc properties has not been determined yet since dielectric losses make ~t tmposslble to measure these properties at higher temperatures. It ~s known from the crystaUographlc studies that phase transition at a temperature of 250 K is connected with the ordering of protons m hydrogen bonds [ 6 - 8 ] . At thxs temperature a symmetry change from 12 monochmc to I1 triclmlc system occurs. The spontaneous polarization appears along the a-axis, change of this polarization along the b-axis being not reversable [3, 4]. It results from the specific-heat measurements [9] that phase transitions at temperatures of about 100K and 250 K are the translt~ons of the first kind, the tranm~on at 250 K being the one of near-second order. Up to now there is no reformation concerning a mechanism of the phase transition at about 100K and no results of physical properties investigations of NH4HSeO4 crystals below liqmd-nitrogen temperature. The aLrn of this work has been to complete this gap of knowledge, and thus obtam addltxonal reformation on a character of the phase transition at about 100K and determining a d~rectlon of pyroelectnc-polarisatlon change.
3. RESULTS AND DISCUSSION The obtained experimental dependence of spontaneous polarization change upon temperature is shown m Fig. 1. A change of the "pyroelectric" polarization sign takes place during the phase tranmlon at a temperature of about 100 K, provxded that no sharp changes of the polarization occur m a temperature range of 0 - 4 . 2 K. As can be seen from the figure we deal almost with the "repolarizatlon" of the crystal. Due to the lack of crystallograptuc data it is difficult to indicate precisely groups of atoms the reorientation of which is connected with this polarization change. "Pyroelecmc" polarization can be supposed, based on paper [6], to be connected with dipole moment of SeO4 group and its reonentatlon might cause the observed change in polarization direction. It is seen from the figure that the spontaneous polarxzation changes its sign again at a temperature of about 171 K. The exact position of this point cannot be determined since the measurements of the polarization change have not been performed starting from a temperature of 0 K.
2. EXPERIMENT The NH4HSeO4 crystals have been grown from a water solution by the xsothermal vaporatlon method at 39
NH4HSe04 CRYSTALS AT LOW TEMPERATURES
40
Vol. 55, No. 1 X
1c,
XO~X xxx X
X o 0
E k_)
X X
0 X
o0
on
O
X
o
X
0
o_"
Q
<::3
X
0 0
OrS
X 0
X
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0 X
X
0
X X
-OS
X X X o
x x
-10 x
x
x*
x* o
xo x° o
-1S i
i
i
~10
.
.
.
.
.
. 100.
.
1~0
. . . .
T[K] F:g. 1. Temperature dependence of spontaneous-polarization changes along the b-axis.
l
0.3
v
0~
01
8 10 ~o
~0 '
~0
'
~
' ",~
'° I~)"
li,0 '
~0
'T[I(]...~._.
Fig 2. Temperature dependence of pyroelectrlc coefficient for NH4HSeO4 crystals along the b-axzs. It is worth noting that at a temperature of 250K there are no anomahes of pyroelectnc properties along the b-axis [ 3 , 4 ] , therefore coupling coeffic:ent between ferroelectnc and pyroelectrtc polanzatlon depends upon temperature and vamshes at 250 K Ftgure 2 shows temperature dependence of pyroelectnc coefficient obtained
from measurements of polarization changes presented in Fig. 1. It can be seen from the figure that beszde a sharp anomaly corresponding to the phase transition at about 100K and temperature hysterem characteristic for phase transitions of the first order, an extra maximum at a temperature of about 6 K is observed. Stmdar anomalies
Vol. 55, No. 1
NH4HSe04 CRYSTALS AT LOW TEMPERATURES
phase transition connected probably with dtsordenng of protons in hydrogen bonds and, coupled with this, reorientatlon of other groups of atoms. The full understanding of this transitton mechamsm reqmres further investigations (first of all crystallographtc studies).
xXXXx Xxxx x x x x
x x
41
x x x x x x x x x x~ii(xxxxxxxxxxxxxxxx
Acknowledgements - The authors express their thanks to Mr. Stanlstaw Matyastk for his help during the measurements. zi
REFERENCES r [K]
--
Fig. 3. Temperature dependence of dielectric penmttivity for NH4HSeO4 crystals along the b-axis. of pyroelectnc propemes are observed at low temperatures for other ferroelectrlcs [10, 11] wluch indicates that the anomahes cannot be described w~thm Barn's model [12, 13] m which pyroelectrlc coefficient at low temperatures is proportional to the third power of temperature. Figure 3 shows temperature dependence of dielectric permlttwlty for NH4HSeO4 crystals. A slow increase m dielectric permlttlvlty with the decreasing temperature ~s probabl3, connected w~th a contribution of acoustic vibrations of the crystal to ~ts total permlttwity Dielectric losses m a range of 4 . 2 - 8 0 K are small (tan ~ = 7 x 10 -4) In conclusion ~t should be pointed out that at a temperature of about 100K there occurs a complex
1. 2 3 4 5. 6. 7. 8. 9 10. 11. 12. 13
Z. Czapla, T. Lls & L. Sobczyk, Phys. Status Solidi 51,609 (1979). V.S. Krasikov & A.I. Krughk, Ftzika Tverdogo Tela 21,2534 (1979) (m Russmn) R Poprawskl & J Dziedzlc, J. Phys. Chem. Sohds 45,355 (1984). K Gesl, J Phys. Soc. Japan 48, 1399 (1980) H. Pykacz, Z. Czapla & J. Mr6z, Acta Phys. Polon. A60,325 (1981). K.S Aleksandrov, A.I. Krughk, S.V. Misyul & M.A. Stmonov, Krtstallografiya 25, 1142 (1980) (in Russian). A.I. Krughk, S.V. Mlsyul, K S Aleksandrov, Dokl Akad. Nauk S.S.S.R, Krtstallografiya 295, 344 (1980) (m Russian). I. Brach, D.J. Jones & J Rozlere, J. Solid State Chem 8,401 (1983). R Poprawskl, P.S. Smlrnov & B.A. Strukov, Fiztka Tverdogo Tela 26, 1339 (1984)(in Russian) A. Hadnl & D. Grandjean, J Phys. 30,377 (1969) N.D. Gavrdova, S.N Drozhdm & V.K Novtk, Sohd State Commun. 48, 129 (1983). M. Born, Rev Mod. Phys 13,245 (1945). B. Szlgettl, Phys Rev Lett. 35, 1532 (1975)