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The ferroelectric phase transition in ammonium sulphate
Volume
22, number
THE
CHEMICAL
3
FERROELECTRIC
PHASE
PHYSICS
15 October
LETTERS
TRANSITION
W
AMMONlUM
1973
SULPHATE
Y.S. JAIN, H.D. BIST and C.C. UPRETI Department
of Physics. Indian Institute
Rcccticd
On the basis of the temperature depcndencc inferred that the fcrroelcctric phase transition tions in the SOi- ions.
To
the
best
of our
knowledge
this
is the
of typical near 223°K
first
report
of Technology,
16 July
Kanprtr-16,
India
1973
internal modes associated with SO:- and NH; ions it is in ammonium sulphate is primari!y due to sudden distor-
The
vr mode
of the
SOi-
ion,
which
appears
in the
associating the ferroelectric transition near 223°K in
infrared as a very weak band for the room temperature
(NH,)2S04 with the sudden distortion in the SOiion at that temperature. This conclusion is based on a careful study of the temperature dependence of the infrared bands of the compound between room temperature (” 300°K) and = 110°K. An extremely thin and uniform film of ammonium sulphate (having low general scattering) was obtained by first spreading uniformly a finely powdered ‘dust’ of the compound on a polished AgCl plate, and subsequently wetting the ‘dust’ with few drops of Ccl, and rubbing it gently with a plane glass plate (which was eventually removed) and finally aliowing the CC14to evaporate off completely. The infrared spectra of the thin film of the compound on AgCl plate were recorded in situ using a
phase (e.g., the curve near 300°K in fig. la), exhibits an abnormal enhancement in its intensity near 223”K, the ferroe!.ectric transition temperature, T, [3]. Thereafter a very slow but gradual enhancement in intensity of this band continues right up to the lowest temperature studied in the present study. Likewise, the v2 band of SOi- which has almost imperceptible intensity above T,, suddenly appears at r, and simultaneously exhibits a splitting (fig. lc). The discontinuity in the T-I curl-es of SOi- bands appears to last within 6°C of our monitored tempel ture which of course would represent within our experimental set-up the temperature gradient along the AgCl plate. The above mentioned behaviour of the v1 and v2 bands of SOi- ions indicates that a sudden distortion
conventional
Wagner-Hornig
type
cell
and
a Perkin-
Elmer 521 spectrophotometer [I]. The temperature of the window was monitored with a copper-constantan thermocouple and a millivoltmeter. Typical contours and intensity distributions in the well-established pi and u2 bands of SO:- (which are forbidden in the infrared for isolated ion having Td symmetry) [2] are shown in figs. la and Ic, respectively. The changes of the integrated intensity (I) with temperature (7) for these two bands are depicted in ‘.figs. 1b and Id, respectively. The contours of an NH; band
at 3310
cm-1
are
shown
in fig.
le at various
temperatures noted at the bottom of the figure along with the T-1 curve.(fig; if). 5.72 ;
..’ :
., _.
..
.
:
takes
place
at Tc in the
reguIar
tetrahedral
room-tem-
of SO,Z- ions. The almost T, character of SOi- at room temperature is confirmed on the basis of neutron diffraction studies [4]. There are no such studies regarding the structure of ammoniuin sul?hate just below T,. The bands associated with the NH: ion do exhibit perature
arrangement
a minor
discontinuity
in the integrated
intensities
at
T, as has already been reported by Torrie et al. [5] on the basis of infrared studies. The representative T-I curve (fig. 1fs for the 33 10 cm-r band further illustrates this point.
By monitoring continuously the peak heights of a few temperature-sensitive
infrared bands, Schutte et
Volume 22, number 3
CKEMICAL PHYSICS LETTERS
1.5 October
1973
-5
N-H
3or t -2oc
100
Band e
.-.-,-
-.-_-.1
Q__y?y--T
0
Fig. i. Temperature
Vibrational 33ld ck
200
dependencc
260 240 22 -TEMPERATURE
2
(OK)
100
of typical infrared band CO~~OUISand their integrated
160
intensities
l&O
120
in ynmotium
su:phate. Con-
Lous.61:(a) v, CSO~TI in the range952 to 982 cm-’ (peakat 972 cm-‘), (c) “2(SOL-)in
:he range 41.5 to 475 cm-’ (peaks at 445 and 456 cm-‘); and (e) the N-H vibratiorgl band between 3200 and 3506 cm-’ (peak at 3310 cm-‘). Curyes (b). (d) and (f-j represent the variation of integrated intensities1 of respective bands with temperature T. Temperature scale at the bottom is for the whole tigure.
573
V&me
22, number 3
CHEMICAL
PHYSICS
al. [6,7] have established a correlation between definite discontinuities observed in the peak heights near 163’K and the linewidth changes observed in the proton magnetic resonance near that temperature [S]. : Few other studies in ammonium sulphate need a coherent explanation. On varying the temperature, the deuteron spin-lattice relaxation time T, in (ND&SO4 exhibits three discontinuities near 223, 163 and 113’K indicating that the two different types of (ND:) ions continue to execute large amplitude oscillations even below T, [9]. The 14N quadrupole coupling data demonstrate a high degree of distortion of the NH: tetrahedra in the parae!ectric phase [lo]. The neutron diffraction data depict much less distortions of NH: ions in the ferroelectric phase [3].
The
deuteron
mapetic
resonance
work
on single
crystals of ammonium sulphate between 77 and 296 “K has indicated that the ND;; ion distortions are strongly dependent on temperature, and an orderdisorder
mechanism
of phase
transition
exists
in the
15 October 1973
LETTERS
tude of NH: ion oscillations decreases and one of the ions tends to align its dipoles opposite to the permanent dipoles of SO:- ions. The NH:(I) ion freezes near 163°K [B,V]. The NH$(II) retains moderate oscillations even up to 103”K, below which it also acquires a Fairly stable directed dipole moment. On freezing the net polarization due to one of the NI$ ions opposes the polarization created by SO:- ions at T, [20]. Finally, below 83’K the P, acquires a small negative value [17]. One can roughly calculate the polarization associated with each NH: ions at T, under valid approximations of this model from the data of Unruh [17]. The above model will dispel the controversy regarding the order (first order and/or second order) as well as the nature (displacive and/or order-disorder) of the transition at T, in ammonium sulphate. The transition
would
order-disorder order
phase
naturally type
transition
show
mechanism
characteristics related
if the properties
of an
to a secondof NH: ions
crystal [l 1,121. Hamilton and Ibers [ 131 state that the neutron diffracticn data [3] are incompatible with an order-disorder transfortiation at the Curie point. Ross and Hamilton [14] assert that the orientation of NH;; ion starts at 233°K. The dielectric and
alone are studied. However, it would exhibit the characteristics of a fust-order phase transition if properties related to SOi- or bulk properties are studied, as the maximum change occurs at T, due to sudden dis-
polarization
We hope that a theoretical model will soon veloped for this class of improper ferroelectrics
show unusual and attempts menological
studies
in ammonium
sulphate
behaviour with temperature are being made to develop theory
[15--181
variation a new pheno-
for this class of improper
ferro-
[! 91. Our infrared results could be explained and other anomalous results could perhaps be correlated if the following model is adopted for this ciass of ferroelectrics in general and ammonium sulphate in pa,rticular. The main spontaneous polarization (P,) at the ferroelectric transition temperature near 223°K (T,) is associated with sudden distortions in the SOf- ion. This is a first-order ferroelectric mechanism and results in a maximum P,value of 0.6 .&/cm2_ The NH:(I) and NH:@) tetrahedra do have enougll distortions at room temperature and could have a large dipole moment associated with each of them. However, they execute large amplitude motions at :oom temperaelectrics
ture and even below T,, and thus contribute very pcorly to the spontaneous polarization at T,. The fact that the transition temperature near.223”K is insensitive to deuteiation of Nl$ group, further corroborates this Suggestion..With decreasing temperature the ampli-
tortions
in SOi-
ions. be deex-
their anomalous properties in the light of the above-mentioned model based on experimental observations. plaining
Thanks are due to Professor C.J.H. Schutte (Pretoria) for a helpful communication regarding the procedure for depositing the thin fdms of the sample on the window.
References
[ lj E.W. Wagner and D.F. Hornig, J. Chem. Phys 18 (1950) 296.
[ 21 G. Herzberg, hlolecular spectra and molecular structure, .Yol. 2 (Van Nostrand, Princeton, 1945). [3] B.T. Mntthies and J-P_ Remeika, Phys. Rev. 103 (1956) 262. [4] E.O. Schlemper and W.C. Hamilton, J. Chem. Phys. 44 (1966) 4498. [5] B.H.Torrie, C.C. Lin, O.S. Binbrek and A. Anderson, J. Phys Chem. Solids 33 (1?72) 697.
Volume 22, number 3 [6] C.J.H. Schutte and A.M. Heyns, (1968) 511. [7] C.J.H. Schutte (1970) 864.
and A.hl.
1.5 October
CHEMICAL PHYSICS LETTERS.
Heyns,
Chcm.
Phys.
J. Chem.
Phys
Letters
J
52
[ 81 R. Blinc and I. Levstek, J. Phys. Chem. Solids 12 (1960) 295. [9] D.W. Kydon, M. Pinter and H.E. Petch, J. Chem. Phys. 47 (1967) 118.5. [lo] R. Blinc, M. Mali, R. Osredkar, A. Prclcsnik, J. Selinaer and 1. Zupancic, Chcm. Phys. Letters 14 (1972) 49. [ 1 I] D.E. O’Reilly and T. Twang, J. Chem. Phys. 46 (1967) 1291. [12] DE. O’Reilly and T. Tsang, J. Chem. Phys. 50 (1969) 2274.
[13]
[ 141 [ 1.51 [ 161 [ 171 [ 181
[ 191 [ZO]
WC. Hamilton
and J.A. Ibers,
Hydrogen
bonding
1973 iu
solids (Benjamin, New Yo:k, 1968) pp. 248, 249. F.K. Ross and WC. Hamilton. Proc. Am. Cryst. Assoc. Xleet. Columbia (1971). S. Hoshino, K. Vedam, Y. Okaya and R. Pepinsky, Phys. Rev. It2 (1958) 405. H. Ohshima and E. Nakarnura, J. Phys. Chcm. Solids 27 (1966) 481. H.G. Unruh, Solid State Commun. 6 (1970) 1951. T. lkeda and K. Fuzibayashi, J. Phys. Sac. Japan 33 (1972) 1487. J. Kabayashi, Phys. Stat. Sol. 5Ob (1952) 335. Y.S. Jnin and H.D. Bist, Phys. Rev. Letters, submitted for publication.
575
22, number
THE
CHEMICAL
3
FERROELECTRIC
PHASE
PHYSICS
15 October
LETTERS
TRANSITION
W
AMMONlUM
1973
SULPHATE
Y.S. JAIN, H.D. BIST and C.C. UPRETI Department
of Physics. Indian Institute
Rcccticd
On the basis of the temperature depcndencc inferred that the fcrroelcctric phase transition tions in the SOi- ions.
To
the
best
of our
knowledge
this
is the
of typical near 223°K
first
report
of Technology,
16 July
Kanprtr-16,
India
1973
internal modes associated with SO:- and NH; ions it is in ammonium sulphate is primari!y due to sudden distor-
The
vr mode
of the
SOi-
ion,
which
appears
in the
associating the ferroelectric transition near 223°K in
infrared as a very weak band for the room temperature
(NH,)2S04 with the sudden distortion in the SOiion at that temperature. This conclusion is based on a careful study of the temperature dependence of the infrared bands of the compound between room temperature (” 300°K) and = 110°K. An extremely thin and uniform film of ammonium sulphate (having low general scattering) was obtained by first spreading uniformly a finely powdered ‘dust’ of the compound on a polished AgCl plate, and subsequently wetting the ‘dust’ with few drops of Ccl, and rubbing it gently with a plane glass plate (which was eventually removed) and finally aliowing the CC14to evaporate off completely. The infrared spectra of the thin film of the compound on AgCl plate were recorded in situ using a
phase (e.g., the curve near 300°K in fig. la), exhibits an abnormal enhancement in its intensity near 223”K, the ferroe!.ectric transition temperature, T, [3]. Thereafter a very slow but gradual enhancement in intensity of this band continues right up to the lowest temperature studied in the present study. Likewise, the v2 band of SOi- which has almost imperceptible intensity above T,, suddenly appears at r, and simultaneously exhibits a splitting (fig. lc). The discontinuity in the T-I curl-es of SOi- bands appears to last within 6°C of our monitored tempel ture which of course would represent within our experimental set-up the temperature gradient along the AgCl plate. The above mentioned behaviour of the v1 and v2 bands of SOi- ions indicates that a sudden distortion
conventional
Wagner-Hornig
type
cell
and
a Perkin-
Elmer 521 spectrophotometer [I]. The temperature of the window was monitored with a copper-constantan thermocouple and a millivoltmeter. Typical contours and intensity distributions in the well-established pi and u2 bands of SO:- (which are forbidden in the infrared for isolated ion having Td symmetry) [2] are shown in figs. la and Ic, respectively. The changes of the integrated intensity (I) with temperature (7) for these two bands are depicted in ‘.figs. 1b and Id, respectively. The contours of an NH; band
at 3310
cm-1
are
shown
in fig.
le at various
temperatures noted at the bottom of the figure along with the T-1 curve.(fig; if). 5.72 ;
..’ :
., _.
..
.
:
takes
place
at Tc in the
reguIar
tetrahedral
room-tem-
of SO,Z- ions. The almost T, character of SOi- at room temperature is confirmed on the basis of neutron diffraction studies [4]. There are no such studies regarding the structure of ammoniuin sul?hate just below T,. The bands associated with the NH: ion do exhibit perature
arrangement
a minor
discontinuity
in the integrated
intensities
at
T, as has already been reported by Torrie et al. [5] on the basis of infrared studies. The representative T-I curve (fig. 1fs for the 33 10 cm-r band further illustrates this point.
By monitoring continuously the peak heights of a few temperature-sensitive
infrared bands, Schutte et
Volume 22, number 3
CKEMICAL PHYSICS LETTERS
1.5 October
1973
-5
N-H
3or t -2oc
100
Band e
.-.-,-
-.-_-.1
Q__y?y--T
0
Fig. i. Temperature
Vibrational 33ld ck
200
dependencc
260 240 22 -TEMPERATURE
2
(OK)
100
of typical infrared band CO~~OUISand their integrated
160
intensities
l&O
120
in ynmotium
su:phate. Con-
Lous.61:(a) v, CSO~TI in the range952 to 982 cm-’ (peakat 972 cm-‘), (c) “2(SOL-)in
:he range 41.5 to 475 cm-’ (peaks at 445 and 456 cm-‘); and (e) the N-H vibratiorgl band between 3200 and 3506 cm-’ (peak at 3310 cm-‘). Curyes (b). (d) and (f-j represent the variation of integrated intensities1 of respective bands with temperature T. Temperature scale at the bottom is for the whole tigure.
573
V&me
22, number 3
CHEMICAL
PHYSICS
al. [6,7] have established a correlation between definite discontinuities observed in the peak heights near 163’K and the linewidth changes observed in the proton magnetic resonance near that temperature [S]. : Few other studies in ammonium sulphate need a coherent explanation. On varying the temperature, the deuteron spin-lattice relaxation time T, in (ND&SO4 exhibits three discontinuities near 223, 163 and 113’K indicating that the two different types of (ND:) ions continue to execute large amplitude oscillations even below T, [9]. The 14N quadrupole coupling data demonstrate a high degree of distortion of the NH: tetrahedra in the parae!ectric phase [lo]. The neutron diffraction data depict much less distortions of NH: ions in the ferroelectric phase [3].
The
deuteron
mapetic
resonance
work
on single
crystals of ammonium sulphate between 77 and 296 “K has indicated that the ND;; ion distortions are strongly dependent on temperature, and an orderdisorder
mechanism
of phase
transition
exists
in the
15 October 1973
LETTERS
tude of NH: ion oscillations decreases and one of the ions tends to align its dipoles opposite to the permanent dipoles of SO:- ions. The NH:(I) ion freezes near 163°K [B,V]. The NH$(II) retains moderate oscillations even up to 103”K, below which it also acquires a Fairly stable directed dipole moment. On freezing the net polarization due to one of the NI$ ions opposes the polarization created by SO:- ions at T, [20]. Finally, below 83’K the P, acquires a small negative value [17]. One can roughly calculate the polarization associated with each NH: ions at T, under valid approximations of this model from the data of Unruh [17]. The above model will dispel the controversy regarding the order (first order and/or second order) as well as the nature (displacive and/or order-disorder) of the transition at T, in ammonium sulphate. The transition
would
order-disorder order
phase
naturally type
transition
show
mechanism
characteristics related
if the properties
of an
to a secondof NH: ions
crystal [l 1,121. Hamilton and Ibers [ 131 state that the neutron diffracticn data [3] are incompatible with an order-disorder transfortiation at the Curie point. Ross and Hamilton [14] assert that the orientation of NH;; ion starts at 233°K. The dielectric and
alone are studied. However, it would exhibit the characteristics of a fust-order phase transition if properties related to SOi- or bulk properties are studied, as the maximum change occurs at T, due to sudden dis-
polarization
We hope that a theoretical model will soon veloped for this class of improper ferroelectrics
show unusual and attempts menological
studies
in ammonium
sulphate
behaviour with temperature are being made to develop theory
[15--181
variation a new pheno-
for this class of improper
ferro-
[! 91. Our infrared results could be explained and other anomalous results could perhaps be correlated if the following model is adopted for this ciass of ferroelectrics in general and ammonium sulphate in pa,rticular. The main spontaneous polarization (P,) at the ferroelectric transition temperature near 223°K (T,) is associated with sudden distortions in the SOf- ion. This is a first-order ferroelectric mechanism and results in a maximum P,value of 0.6 .&/cm2_ The NH:(I) and NH:@) tetrahedra do have enougll distortions at room temperature and could have a large dipole moment associated with each of them. However, they execute large amplitude motions at :oom temperaelectrics
ture and even below T,, and thus contribute very pcorly to the spontaneous polarization at T,. The fact that the transition temperature near.223”K is insensitive to deuteiation of Nl$ group, further corroborates this Suggestion..With decreasing temperature the ampli-
tortions
in SOi-
ions. be deex-
their anomalous properties in the light of the above-mentioned model based on experimental observations. plaining
Thanks are due to Professor C.J.H. Schutte (Pretoria) for a helpful communication regarding the procedure for depositing the thin fdms of the sample on the window.
References
[ lj E.W. Wagner and D.F. Hornig, J. Chem. Phys 18 (1950) 296.
[ 21 G. Herzberg, hlolecular spectra and molecular structure, .Yol. 2 (Van Nostrand, Princeton, 1945). [3] B.T. Mntthies and J-P_ Remeika, Phys. Rev. 103 (1956) 262. [4] E.O. Schlemper and W.C. Hamilton, J. Chem. Phys. 44 (1966) 4498. [5] B.H.Torrie, C.C. Lin, O.S. Binbrek and A. Anderson, J. Phys Chem. Solids 33 (1?72) 697.
Volume 22, number 3 [6] C.J.H. Schutte and A.M. Heyns, (1968) 511. [7] C.J.H. Schutte (1970) 864.
and A.hl.
1.5 October
CHEMICAL PHYSICS LETTERS.
Heyns,
Chcm.
Phys.
J. Chem.
Phys
Letters
J
52
[ 81 R. Blinc and I. Levstek, J. Phys. Chem. Solids 12 (1960) 295. [9] D.W. Kydon, M. Pinter and H.E. Petch, J. Chem. Phys. 47 (1967) 118.5. [lo] R. Blinc, M. Mali, R. Osredkar, A. Prclcsnik, J. Selinaer and 1. Zupancic, Chcm. Phys. Letters 14 (1972) 49. [ 1 I] D.E. O’Reilly and T. Twang, J. Chem. Phys. 46 (1967) 1291. [12] DE. O’Reilly and T. Tsang, J. Chem. Phys. 50 (1969) 2274.
[13]
[ 141 [ 1.51 [ 161 [ 171 [ 181
[ 191 [ZO]
WC. Hamilton
and J.A. Ibers,
Hydrogen
bonding
1973 iu
solids (Benjamin, New Yo:k, 1968) pp. 248, 249. F.K. Ross and WC. Hamilton. Proc. Am. Cryst. Assoc. Xleet. Columbia (1971). S. Hoshino, K. Vedam, Y. Okaya and R. Pepinsky, Phys. Rev. It2 (1958) 405. H. Ohshima and E. Nakarnura, J. Phys. Chcm. Solids 27 (1966) 481. H.G. Unruh, Solid State Commun. 6 (1970) 1951. T. lkeda and K. Fuzibayashi, J. Phys. Sac. Japan 33 (1972) 1487. J. Kabayashi, Phys. Stat. Sol. 5Ob (1952) 335. Y.S. Jnin and H.D. Bist, Phys. Rev. Letters, submitted for publication.
575