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STUDY OF SLOW MOTION OF WATER MOLECULES IN ROCHELLE SALT AND IN
AMMONIUM ROCHELLE SALT BY SPIN-LATTICE RELAXATION OF PROTONS IN ROTATING FRAME
Z. Trontelj
To cite this version:
Z. Trontelj. STUDY OF SLOW MOTION OF WATER MOLECULES IN ROCHELLE SALT AND IN AMMONIUM ROCHELLE SALT BY SPIN-LATTICE RELAXATION OF PROTONS IN ROTATING FRAME. Journal de Physique Colloques, 1972, 33 (C2), pp.C2-189-C2-191.
�10.1051/jphyscol:1972264�. �jpa-00215000�
JOURNAL DE PHYSIQUE Colloque C2, suppl&ment au no 4 , Tome 33, Avril 1972, page C2-189
STUDY OF SLOW MOTION OF WATER MOLECULES IN ROCHELLE SALT AND IN AMMONIUM ROCHELLE SALT BY SPIN-LATTICE
RELAXATION OF PROTONS IN ROTATING FRAME
Z. TRONTELJ
Physics Department, University of Ljubljana and Institute of Mathematics Physics and Mechanics, Ljubljana, Yugoslavia
Resume. - On mesure la dkpendance en temperature et en frequence du temps de relaxation spin-reseau des protons dans le sel de Rochelle et dans le sel de Rochelle ammoni6. On Btudie deux groupes de molkcules d'eau, appelks par Bjorkstam [I] A, B, C , D et a, /?, y, S; on determine le temps de corrklation z, pour les 2 groupes. On ne trouve pas de contribution des mol&ules d'eau i la ferro- Blectricite. Nos conclusions sont confirmees par nos mesures de Tlr des protons dans le sel de Rochelle ammonik a 7,8 % molaire. On compare aux mesures dielectriques du sel de Rochelle A 298 OK.
Abstract. - Temperature and frequency dependence of spin-lattice relaxation time in rotating frame Tlr of protons in Rochelle salt and in ammonium Rochelle salt was measured. Two groups of water molecules, called by Bjorkstam [I] A, B, C, D and a, D , y , S water molecules were studied and the correlation time zc was determined for both groups. No direct contribution of water molecules to ferroelectric dipoles was found. Measurements of Tlr of protons in 7.8 mol% ammonium Rochelle salt were performed to check our conclusions, obtained for Rochelle salt. Comparison is made with the dielectric measurements on Rochelle salt at 298 OK.
Introduction. - The first known ferroelectric Rochelle salt [2] (R. S.) NaKC,H,O,. 4 H,O is still interesting since there are several open questions in the dynamics of crystal lattice, in particular at both ferro- electric transitions. R. S. is one of the few ferro- electrics with two transition temperatures and with the ferroelectric phase between the transition tempera- tures : 24 OC and - 18 OC for the normal R. S. and 35 OC and - 22OC for the deuterated R. S. Consi- derable change in the temperature interval of ferro- electric region and in the magnitude of spontaneus polarization after the replacement of hydrogen atoms with the deuterium atoms shows the influence of hydrogen atoms in the phenomenon of ferroelectricity of R. S.
The number of experimental methods being used to study R. S. is quite big [3] and nuclear magnetic resonance (N. M. R.) is one which gave us some interesting results. The following nuclei of R. S.
molecule were studied up till now by N. M. R. : 'H, 2D and 23Na. Mainly the shape of the absorption signal, quadrupole splitting, electric field gradient tensor and quadrupole coupling constant were investi- gated. There are not so many experiments considering the spin-lattice relaxation of 'H, 2D and 23Na, giving data for studies of lattice dynamics of R. S. [4].
We expect to get some additional informations on the dynamics of slow motion of protons in R. S. and in ammonium R. S. (A. R. S.) by performing the TI, measurements of protons as a function of temperature
and frequency.
Experimental. - In order t o avoid the spurious signals, all measurements were done on the powder sample of R. S. Bruker pulsed N. M. R. spectrometer BKR-304 s was used and a standard pulse sequence (900-t,,,) was applied to obtain T i , results. TI, measurements were performed at HI equal to 14.7 G and 9.2 G for R. S. and at 6.35 G for A. R. S., respecti- vely. Temperature was measured with an accuracy of 0.5 degree. Maximal error in TI, measurements is 10 %.
Rochelle salt. - Frazer [ 5 ] , [6] concluded on the base of neutron diffraction measurements on R. S.
that O(5)-H groups (*) contribute the major part t o the total dip01 moment in ferroelectric phase. Since the N. M. R. methods are especially suitable for studies of static and dynamic behaviour of hydrogen atoms, it is obvious that the first R. S. nuclei, studied by N. M. R., were protons. The second moment measurements of H,O protons [8] in R. S., as well as measurements of temperature and frequency depen- dence of Zeeman spin-lattice relaxation time TI [9]
for the same protons, show us that the contribution of water molecules to the ferroelectric dipoles is unobser- vable within the experimental error.
Deuterium resonance investigations of deuterated R. S. (D. R. S.) carried out by Bjorkstam and Will- morth [I], [lo] tell us that heavy water molecules (and consequently H 2 0 molecules in R. S.) contribute
(*) The notation described in the reference [7] is used.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1972264
C2-190 2. TRONTELJ considerably to the spontaneous polarization P,. They
found that there are two groups of water molecules in D. R. S. called a, j , y, 6 D 2 0 molecules which are flipping rapidly and A, B, C, D water molecules flipping slowly compared to the reciprocal difference in quadrupole splitting frequency at the two deuteron sites in a water molecule.
To obtain some informations on the dynamics of slow motion of water molecules in R. S., the spin- lattice relaxation time of protons as a function of temperature and frequency in the rotating frame was measured. Results are shown on figure 1. We notice the contribution of two groups of water molecules, what was also observed by Blinc and coworkers [ll]. The minimum between two Curie temperatures comes from a, p, y and 6 H 2 0 molecules, and the decreasing of TI, in the upper paraelectric phase is due to A, B, C and D H 2 0 molecules, starting to move at this temperature.
It is not reasonable to continue with the measurement of TI, as a function of temperature and find the expect- ed second minimum, since in the vicinity of melting point (326 OK) the R. S . loses its bound crystal water.
FIG. 1. - Temperature and frequency dependence of log Tlr
for H z 0 protons in polycrystalline Rochelle salt.
On the base of TI, measurements the correlation time for both groups of water molecules can be found.
It is seen from figure 1 that the TI, consists of two contributions :
where (l/Tlr)A indicates the contribution of A, B, C, D H 2 0 molecules and (l/T13, the contribution of a, j?, y, 6 H 2 0 molecules, respectively.
The well known expression for l/Tl,
where K contains all temperature independent quan- tities, gives us, when taking into account that (l/T,J, is frequency dependent (w2 2; 9 1) and (l/T,3, is frequency independent ( a 2 2: 4 1)
When evaluating eq. (3) and (4) it was assumed that the motion of water molecules is thermally activated and we can write therefore
Here E A , is the activation energy for both groups of water molecules and k is the Boltzmann constant.
In order to calculate 2, for both groups of water molecules, we have to know z0,,, and EA,, Activation energies EA and E, are obtained from the plot log TI, versus 1/T. z,, is calculated from the condition or,, = 1 at the minimum, and z,,, follows from the equality of (l/TIJA and (l/Xlr), at the position where the function log Tl,(l/T) has a maximum. It is also assumed that KA and K, are equal. The following values were obtained
EA = (0.40 f 0.05) eV E, = (0.20 4 0.02) eV z,, = (1.40 f 0.15) x 10-I2 s
zoa = (4.68 + 0.45) x 10-1°s.
On figure 2 is shown the log of correlation times zcA and z,, as a function of reciprocal temperature. Motion of A, B, C and D H 2 0 molecules started at 303 OK is more strongly dependent on temperature than the motion of a, p, y and 6 H 2 0 molecules.
1 -. 1 = K '=c (2) FIG. 2. -Temperature dependence of correlation times rc*
2 2 '
Ti, ~ + O Z , and r,,.
STUDY OF SLOW MOTION OF WATER MOLECULES IN ROCHELLE SALT C2-191
It is interesting to compare our results of spin- lattice relaxation with the results of dielectric relaxation, obtained by Miiser (*). At 298 OK he found for the correlation time of water molecules z = s. At the same temperature the T I , measurements gave us z,, = (1.17 + 0.12) x l o w 6 s and
The agreement is quite good. It was shown by the T I , measurements that there are two kinds of motion of water molecules with the correlation times differing approximately for a factor of 10 a t 298 OK.
We observed no anomalies in T I , at both Tc and in their vicinity what could indicate to the ferroelectric contribution of H 2 0 protons. That is what one expects, since there is a difference for a factor of lo3 between the frequency of ferroelectric branch of dielectric relaxation [12] and the correlation frequency for the a, p, y and 6 H 2 0 molecules which are the most mobile ones. I t can be said that the thermally activated reorien- tations of water molecules do not contribute directly to the formation of ferroelectric dipoles in the ferroelectric phase.
Ammonium Rochelle salt. - A. R. S. is one of the several isomorphous forms of R. S. where the K ion is replaced by the NH, ion. Measurements of T I , were done on a polycrystalline sample containing 7.8 rnol % of A. R. S. and 92.2 rnol % of R. S. Crystal with this combination of R. S, and A. R. S. does not show the ferroelectric properties. We expect from T I , measure- ments on this sample some additional informations about the water molecules and ferroelectricity in R. S.
Figure 3 shows us the log T I , versus 1/T for the 7.8 mol % A. R. S. Comparingthe T I , measurements on R. S. and the T I , measurements on 7.8 rnol % A. R. S.
we see that there is a qualitative agreement between both measurements in a temperature interval from 265 OK to 315 OK. However, there is a difference between 240 OK and 265 OK where T I , for R. S.
increases with decreasing temperature, while T I , for 7.8 rnol % A. R. S. decreases with decreasing tempe-
(*) Private communication of Prof. H. E. Miiser.
Refer [I] BJORKSTAM (J. L.), J. Phys. Soc. Japan, !970,28, 101.
Proceedings of the Second International Meeting on Ferroelectricity 1969.
[2] VALASEK (J.), Phys. Rev., 1921, 17,475.
[3] JONA (F.) and SHIRANE (G.), Ferroelectric Crystals.
Pergamon Press, New York, 1962.
[4] BLINC (R.) and UEHLING (E. A.), Phys. Letters, 1966, 20.337.
FIG. 3. - Temperature dependence of log Tlr for Hz0 protons
in polycrystalline 7.8 rnol % ammonium Rochelle salt.
logT1, IT,, PSI
3.6
rature. Different temperature dependence of T I , for R. S. and 7.8 rnol % A. R. S. at the temperature below 265 OK might be due to structural changes in both nonpolar phases. The qualitative agreement of T I , for H 2 0 protons of R. S. and 7.8 rnol % A. R. S. in the most important temperature interval supports our previous statement about the water molecules in R. S . In conclusion we can say that the study of proton T I , in R. S. and in 7.8 rnol % A. R. S. as a function of temperature and frequency tells us some new facts about the slow motion of water molecules in R. S. At the same time we can see that this slow motion of water molecules cannot explain the ferroelectric pro- perties of R. S .
Hl = 6.35 gauss
-
Acknowledgment. - The author is greatly obliged to Prof. R. Blinc for suggesting this problem and for many helpful discussions.
[6] FRAZER (B. C.), J. Phys. Soc. Japan, 1961, 17, B-11.
[7] BEEVERS (L. A.) and HUGHES (W.), Proc. Roy. Soc.
(London), 1941, A 177,251.
[8] BLINC (R.) and PRELESNIK (A.), J. Chem. Phys., 1960, 32, 387.
[9] BLINC (R.) and UEHLING (E. A.), Phys. Letters, 1966, 20, 337.
BLINC (R.), JAMSEK (M.), STEPISNIK (J.), TRONTELJ ( Z . ) [lo] BJORKSTAM (J. L.) and WILLMORTH (J. H.). Magnetic and BJORKSTAM (J. L.), Sol. State Comm., 1968, Resonance and Relaxation, Proc. 14th Colloque
6, 821. Ampere, Ljubljana 1966, p. 728, North Holiand,
BLINC (R.), PETERSON (E. M.) and OREILLY (D. E.),
Phys. Letters, 1969, 28A, 624. Amsterdam, 1966.
BONERA (G.), BORSA (F.) and RIGAMONTI (A*), phYS. [111 BLINC (R.). PETERSON (E. M.) and O'REILLY (D. E.),
Letters, 1969, 29A, 88. Phys. Letters, 1969, 28A, 624.
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