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Evidence of two phase transitions in hexammine cadmium chlorate
Physica 123B (1984) 211-214
North-Holland, A m s t e r d a m
EVIDENCE OF TWO PHASE TRANSITIONS IN HEXAMMINE CADMIUM CHLORATE
L. PIEKARA-SADY, S. IDZIAK, M. KRUPSKI, E. DYNOWSKA*, M. MA(~KOWIAK and J. STANKOWSKI Institute of Molecular Physics, Polish Academy of Sciences, Poznari, Poland Received 20 September 1982 Revised 12 July 1983
T h e results of the N Q R , N M R and dilatometric studies and D S C of h e x a m m i n e c a d m i u m chlorate (HCC) are reported. It has been determined that at 298 K the structure of H C C is cubic with the lattice constant a = 11.31 ,~,. T h e results of studies on H C C prove the existence of two different phase transitions at T = 248 K and T ~-164 K. T h e temperature dependences of proton relaxation times T1 and T m show that in the studied temperature range two kinds of motion occur: NH3 group rotation with E , = 8.8kJ/mol and isotropic tumbling of [Cd(NH3)6] 2÷ complex with Ea = 28.1 kJ/mol. O n e a s s u m e s a nonequivalency of six NH3 groups in [Cd(NH3)6] 2÷ complex in H C C at the low t e m p e r a t u r e phase ( T < 164 K). Volume discontinuity in the phase transition at 248 K as well as the linear expansion coefficients have been determined.
1. Introduction
2. Experimental details
Hexammine cadmium chlorate (HCC) belongs to the complex compounds of general formula [Me(NH3)6]X2 which crystallize in a cubic system [1]. Phase transitions at temperature Tc~ characteristic for each ammonate lead to structures of lower symmetries [2-5]. The temperature Tc~ depends on the lattice constant via a strong dependence on cation and anion radii [6]. Particularly interesting are ammonates with complex anions, e.g. C10~, BF~, NO~, since besides the structural phase transition the existence of another phase transition~ at a temperature Tc2 lower than To has been proved [5,7-9]. Due to general considerations of the nature of phase transitions in ammonates one usually associates the occurrence of two different phase transitions with the changes of dynamics of complex cation and anion [10]. HCC has been chosen for study because of a new ClO~ anion which gives the possibility to study NQR in ammonate for the first time.
Polycrystalline samples of HCC synthesized following the procedure by Ephraim and Jansen [11] were used. In order to determine the structure of H C C X-ray diffraction patterns were obtained with the help of an X-ray diffractometer DRON-1 with CuKa radiation. The DSC curve has been recorded on a PerkinElmer DSC-18 scanning calorimeter. For the thermal expansion studies of H C C a polycrystalline pellet was used, compressed previously under a pressure of about 300 MPa. The length of the sample was measured using a quartz dilatometer. NMR studies were performed at 90MHz on a Bruker SXP-4/100 pulsed spectrometer. 7"1 was measured by the inversion recovery method. In the temperature range 117260 K a nonexponential decay of magnetization is observed; the initial slopes of these decay plots were used to determine the relaxation times [12]. Tm was measured using the Jeener pulse sequence [13]. 35C1 NQR frequency measurements were performed on an ISSZ-I-12 pulsed spectrometer.
* Institute of Physics, Polish A c a d e m y of Sciences, Warsaw, Poland.
0378-4363/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
L. Piekara-Sady et al. / Two phase transitions in hexammine cadmium chlorate
212
3. Results F r o m X-ray patterns it has been established that at 298 K the structure is cubic-like CaF2 with the lattice constant a = (11.31_0.01) A. In the t e m p e r a t u r e range 180-300 K, both on cooling and on heating, a peak on the DSC curve has b e e n observed. Its m a x i m u m corresponds to 248 K on heating and to 247 K on cooling. The results concerning the relative length change of the sample vs. t e m p e r a t u r e are presented in fig. 1. At 248 K a discrete length change is observed which corresponds to the relative volume change Av/vo = 1.5%. Moreover, at 164 K there occurs a change of the thermal expansion coefficient from 7.3 × 10 -5 deg -1 to 5.4 x 10 -5 deg -1. The results concerning proton relaxation times 7"] and Tm vs. 1/T are given in fig. 2. At T =
5
1
0.5
E , , o" ~ " .°
kJ
¢
0.1
~l%J 0
-0~
0.01
/
-0.8 -'1.2 -1.6
.
k3
0.005
-2.0 -2.4
0.00t 1oo
I t20
I t40
I t60
I t80
J 200
I 220
I 240
l 260
I 280
T[K]
2
b
-2.0
.
.
•
I
!
!
!
I
I
I
3
4
5
6
7
8
9
10
tO31T r K" ]
-
Fig. 2. The proton spin-lattice relaxation time T] and Tm as a function of temperature for [Cd0NH3)~](CIO3)2 at 90 MHz. The dashed line TI(1/T) is calculated from the O'Reilly and Tsang formula [14] using E= = 8.8 kJ/tool and ra--n= 1.63 ~ .
-2.1
-22 -2.3
5.4. lO'S d q "~ -2.4
I 120
[ t30
I 140
I 150
I 160
I t70
I 180
T [KJ
Fig. 1. (a) Relative change in length of the [Cd(NH3)~](CIOa)2 pellet as a function of temperature; (b) the magnified part of Al/lo(T) curve showing a small change of the linear thermal expansion coefficient.
247 K there is a j u m p of T1 from 4.8 s to 7.2 s. In the lower t e m p e r a t u r e range, after analysis of the experimental points which was p e r f o r m e d according to the O'Reilly and Tsang formula [14], at 1 6 6 K another anomaly in the TI(1/T) dependence is observed (fig. 2). Moreover, at 153--162K a distinct anomaly in the 7"1o ( l / T ) dependence is seen.
213
L. Piekara-Sady et al. / Two phase transitions in hexammine cadmium chlorate
35C1 N Q R frequency in H C C at 77 K is equal to 28.205 MHz. VNOR decreases with increasing temperature and at 186K reaches the value 25.700 MHz. The amplitude of the quadrupole spin-echo signal decreases rapidly with increasing temperature, so the observation of the echo is impossible above 186 K. 4. Discussion
The studies on H C C prove the existence of two different phase transitions: (i) A discrete change of the relaxation time 7"1 occurs at 247 K, a stepwise change of volume takes place at 248K, moreover in the range 242-250 K on the DSC curve t h e r e appears a peak corresponding to the specific heat anomaly. These experimental facts undoubtedly evidence a structural phase transition characteristic for hexammine compounds of the [Me(NH3)6]X2 type. (ii) The following experimental results prove the pertinence of the assumption concerning the existence of low temperature phase transition: anomalies in the temperature dependences of T1 at T = 1 6 6 K and T~D at T = 1 6 2 K , and the change of the thermal expansion coefficient at T = 164K. There is no anomaly evidencing low temperature phase transition in the dependence of the 35C1 N Q R frequency on temperature. Perhaps the lack of anomaly means that the low temperature phase transition is associated with the change of the orientation of C10~ groups, i.e. with the change of the E F G direction at chlorine sites without a change of its value. The minimum of T~(1/T) in H C C at 123 K, as well as in other ammonates [15], should be associated with the NH3 groups rotation around the three-fold axes aligned along the C d - N bonds. The minimum related to this motion should be observed in TID(1/T) dependence at much lower temperature than the one accessible in this experiment. Also, with regard to the vanishing signal above 250 K, a high-temperature minimum of T1D(1/T) associated with the other motion, interpreted in
ammonates as isotropic reorientation of the complex cation [15, 16], was not succeeded. The slope of the accessible part of this high-temperature minimum of TID(1/T) allowed to determine the activation energy of this motion, Ea = 28.1 kJ/mol. The minimum of TI(1/T) allows the calculation of the p r o t o n - p r o t o n distance, rH-H in rotating groups. The O'Reilly-Tsang formula [14] determining T~ml. is 20 Tlmin---- 9
r6
to
(1)
~/4h21 . 4 2 '
where to is the L a r m o r frequency, while the other constants have their usual meanings. The experimental value --TeX~tmi, ----45.2ms yields the o value of the internuclear distance orH_i~= 1.75 A. For the ammonia gas rH-i~= 1.63 A, leading to T l m i n = 29.1 ms. It is worth pointing out that in the high-temperature phase in ammonates the six ammonia molecules form a regular octahedral complex[Me(NH3)6] 2+. The E P R studies in paramagnetic and particularly in diamagnetic ammonates doped with Ni 2÷ ions [17, 18] show, that in the high-temperature phase the crystal field at the Me 2÷ ion site has the cubic symmetry while below the structural phase transition temperature Tc~ the symmetry is lower than cubic. Since the minimum of TI(1/T) in H C C occurs at 123K (low-temperature phase) one may assume that if the p h e n o m e n a observed by means of E S R in other ammonates were associated with a deformation of the octahedron formed by six NH3 molecules, e.g. with its elongation, we would be able to regard the NH3 groups as nonequivalent and the Tlr.i, in H C C might be associated with the mutually independent motion of four NH3 groups while the relaxation of protons of the two remaining ones would be caused by the spin diffusion. Using the Anderson-Slichter formula [19]:
T1
N
NH3rot '
other NH 3
where N is the number of all protons in a molecule, n the number of protons in a rotating
214
L. Piekara-Sady et al. / Two phase transitions in hexammine cadmium chlorate
groups; neglecting the second term of (2) when T~ reaches its minimum value, 1 : Texp"
n (T~mi~)
lmin
NH 3 rot. "
0)
For independent rotation of four NH3 groups n = 12, N = 18 and taking the value Tlmi.= 29.1 ms from (1) one gets Tlmi.= 43.7 ms which is in very good agreement with the experimentally observed value ~-rex~ lniin = 45.2 ms. However, the above considerations do not allow drawing finn conclusion about the dynamical or structural nonequivalence of the six NH3 groups in the [Cd(NH3)6]2÷ complex.
Acknowledgements The authors wish to thank Dr. N. Pi~lewski for his helpful comments. We also wish to acknowledge the help of Dr. R. Radomski of the Institute of Organic and Physical Chemistry, Technical University of WrocJ'aw, in recording the DSC curve.
References [1] R.W.G. Wyckoff, Crystal Structure (Interscience, New York, 1965). [2] S.H. Yu, Nature 141 (1938) 158; 150 (1942) 347. [3] E. Dynowska, Phys. Stat. Solidi (a) 31 (1975) K23; E. Dynowska and J. Startkowski, Phys. Stat. Solidi (a) 52 (1979) 381.
[4] L. Asch, G.K. Shenoy, J.M. Friedt and J.P. Adloff, J. Chem. Phys. 62 (1975) 2335. [5] S. Hodorowicz, M. Cienchanowicz-Rutkowska, J.M. Janik and J.A. Janik, Phys. Stat. Solidi (a) 43 (1977) 53; A. Migda$-Mikuli, E. Mikuli, M. Rachwalska and S. Hodorowicz, Phys. Stat. Solidi (a) 47 (1978) 57; S. Hodorowicz and M. Ciechanowicz-Rutkowska, Acta Phys. Polon. A53 (1978) 29. [6] J. Stankowski, Mater. Sci. 11/3 (1976) 57. [7] T. Grzybek, J.A. Janik, J. Mayer, G. Pytasz, M. Rachwalska and T. Waluga, Phys. Stat. Solidi (a) 16 (1973) K165; M. Rachwalska, J.M. Janik, J.A. Janik, G. Pytasz and T. Waluga, Phys. Stat. Solidi (a) 30 (1975) K81. [8] E.A. Long and F.C. Toettcher, J. Am. Chem. Soc. 64 (1942) 629; J. Bousquet, M. Prot and M. Diot, J. Chem. Phys. 6 (1972) 1004. [9] M. Krupski and J. Stankowski, Acta Phys. Polon. A55 (1979) 597. [10] J. Stankowski and A.R. Bates, Proc. Congress.AMPERE Tallinn USSR (1978). [11] F. Ephraim and J. Jansen, Ber. 48 (1915) 41. [12] J.D. Cutnell and W. Venable, J. Chem. Phys. 60 (1974) 3795. [13] J. Jeener, R. DuBois and P. Broekaert, Phys. Rev. 139 (1965) A1959. [14] D.E. O'Reilly and T. Tsang, J. Chem. Phys. 46 (1967) 1291. [15] N. Pi~lewski, Fiz. Diel. Rad. IX (1977) 85 (in Polish); N. Pi~lewski and J. Stankowski and L. Lary~, Phys. Stat. Solidi (a) 31 (1975) 415; N. Pi~lewski and J. Stankowski, Bull. Acad. Pol. Sci. XXVII (1979) 61; N. Pi~lewski and L.T.H. Ferris, Phys. Stat. Solidi (b) 106 (1981) 123. [16] G.R. Murray and J.S. Waugh, J. Chem. Phys. 29 (1958) 207. [17] C. Trapp and Chin-I Shyr, J. Chem. Phys. 54 (1971) 196. [18] P.B. Sczaniecki and J. Stankowski, Acta Phys. Polon. A51 (1977) 117. [19] J.E. Anderson and W.P. Slichter, J. Phys. Chem. 69 (1%5) 9.
North-Holland, A m s t e r d a m
EVIDENCE OF TWO PHASE TRANSITIONS IN HEXAMMINE CADMIUM CHLORATE
L. PIEKARA-SADY, S. IDZIAK, M. KRUPSKI, E. DYNOWSKA*, M. MA(~KOWIAK and J. STANKOWSKI Institute of Molecular Physics, Polish Academy of Sciences, Poznari, Poland Received 20 September 1982 Revised 12 July 1983
T h e results of the N Q R , N M R and dilatometric studies and D S C of h e x a m m i n e c a d m i u m chlorate (HCC) are reported. It has been determined that at 298 K the structure of H C C is cubic with the lattice constant a = 11.31 ,~,. T h e results of studies on H C C prove the existence of two different phase transitions at T = 248 K and T ~-164 K. T h e temperature dependences of proton relaxation times T1 and T m show that in the studied temperature range two kinds of motion occur: NH3 group rotation with E , = 8.8kJ/mol and isotropic tumbling of [Cd(NH3)6] 2÷ complex with Ea = 28.1 kJ/mol. O n e a s s u m e s a nonequivalency of six NH3 groups in [Cd(NH3)6] 2÷ complex in H C C at the low t e m p e r a t u r e phase ( T < 164 K). Volume discontinuity in the phase transition at 248 K as well as the linear expansion coefficients have been determined.
1. Introduction
2. Experimental details
Hexammine cadmium chlorate (HCC) belongs to the complex compounds of general formula [Me(NH3)6]X2 which crystallize in a cubic system [1]. Phase transitions at temperature Tc~ characteristic for each ammonate lead to structures of lower symmetries [2-5]. The temperature Tc~ depends on the lattice constant via a strong dependence on cation and anion radii [6]. Particularly interesting are ammonates with complex anions, e.g. C10~, BF~, NO~, since besides the structural phase transition the existence of another phase transition~ at a temperature Tc2 lower than To has been proved [5,7-9]. Due to general considerations of the nature of phase transitions in ammonates one usually associates the occurrence of two different phase transitions with the changes of dynamics of complex cation and anion [10]. HCC has been chosen for study because of a new ClO~ anion which gives the possibility to study NQR in ammonate for the first time.
Polycrystalline samples of HCC synthesized following the procedure by Ephraim and Jansen [11] were used. In order to determine the structure of H C C X-ray diffraction patterns were obtained with the help of an X-ray diffractometer DRON-1 with CuKa radiation. The DSC curve has been recorded on a PerkinElmer DSC-18 scanning calorimeter. For the thermal expansion studies of H C C a polycrystalline pellet was used, compressed previously under a pressure of about 300 MPa. The length of the sample was measured using a quartz dilatometer. NMR studies were performed at 90MHz on a Bruker SXP-4/100 pulsed spectrometer. 7"1 was measured by the inversion recovery method. In the temperature range 117260 K a nonexponential decay of magnetization is observed; the initial slopes of these decay plots were used to determine the relaxation times [12]. Tm was measured using the Jeener pulse sequence [13]. 35C1 NQR frequency measurements were performed on an ISSZ-I-12 pulsed spectrometer.
* Institute of Physics, Polish A c a d e m y of Sciences, Warsaw, Poland.
0378-4363/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
L. Piekara-Sady et al. / Two phase transitions in hexammine cadmium chlorate
212
3. Results F r o m X-ray patterns it has been established that at 298 K the structure is cubic-like CaF2 with the lattice constant a = (11.31_0.01) A. In the t e m p e r a t u r e range 180-300 K, both on cooling and on heating, a peak on the DSC curve has b e e n observed. Its m a x i m u m corresponds to 248 K on heating and to 247 K on cooling. The results concerning the relative length change of the sample vs. t e m p e r a t u r e are presented in fig. 1. At 248 K a discrete length change is observed which corresponds to the relative volume change Av/vo = 1.5%. Moreover, at 164 K there occurs a change of the thermal expansion coefficient from 7.3 × 10 -5 deg -1 to 5.4 x 10 -5 deg -1. The results concerning proton relaxation times 7"] and Tm vs. 1/T are given in fig. 2. At T =
5
1
0.5
E , , o" ~ " .°
kJ
¢
0.1
~l%J 0
-0~
0.01
/
-0.8 -'1.2 -1.6
.
k3
0.005
-2.0 -2.4
0.00t 1oo
I t20
I t40
I t60
I t80
J 200
I 220
I 240
l 260
I 280
T[K]
2
b
-2.0
.
.
•
I
!
!
!
I
I
I
3
4
5
6
7
8
9
10
tO31T r K" ]
-
Fig. 2. The proton spin-lattice relaxation time T] and Tm as a function of temperature for [Cd0NH3)~](CIO3)2 at 90 MHz. The dashed line TI(1/T) is calculated from the O'Reilly and Tsang formula [14] using E= = 8.8 kJ/tool and ra--n= 1.63 ~ .
-2.1
-22 -2.3
5.4. lO'S d q "~ -2.4
I 120
[ t30
I 140
I 150
I 160
I t70
I 180
T [KJ
Fig. 1. (a) Relative change in length of the [Cd(NH3)~](CIOa)2 pellet as a function of temperature; (b) the magnified part of Al/lo(T) curve showing a small change of the linear thermal expansion coefficient.
247 K there is a j u m p of T1 from 4.8 s to 7.2 s. In the lower t e m p e r a t u r e range, after analysis of the experimental points which was p e r f o r m e d according to the O'Reilly and Tsang formula [14], at 1 6 6 K another anomaly in the TI(1/T) dependence is observed (fig. 2). Moreover, at 153--162K a distinct anomaly in the 7"1o ( l / T ) dependence is seen.
213
L. Piekara-Sady et al. / Two phase transitions in hexammine cadmium chlorate
35C1 N Q R frequency in H C C at 77 K is equal to 28.205 MHz. VNOR decreases with increasing temperature and at 186K reaches the value 25.700 MHz. The amplitude of the quadrupole spin-echo signal decreases rapidly with increasing temperature, so the observation of the echo is impossible above 186 K. 4. Discussion
The studies on H C C prove the existence of two different phase transitions: (i) A discrete change of the relaxation time 7"1 occurs at 247 K, a stepwise change of volume takes place at 248K, moreover in the range 242-250 K on the DSC curve t h e r e appears a peak corresponding to the specific heat anomaly. These experimental facts undoubtedly evidence a structural phase transition characteristic for hexammine compounds of the [Me(NH3)6]X2 type. (ii) The following experimental results prove the pertinence of the assumption concerning the existence of low temperature phase transition: anomalies in the temperature dependences of T1 at T = 1 6 6 K and T~D at T = 1 6 2 K , and the change of the thermal expansion coefficient at T = 164K. There is no anomaly evidencing low temperature phase transition in the dependence of the 35C1 N Q R frequency on temperature. Perhaps the lack of anomaly means that the low temperature phase transition is associated with the change of the orientation of C10~ groups, i.e. with the change of the E F G direction at chlorine sites without a change of its value. The minimum of T~(1/T) in H C C at 123 K, as well as in other ammonates [15], should be associated with the NH3 groups rotation around the three-fold axes aligned along the C d - N bonds. The minimum related to this motion should be observed in TID(1/T) dependence at much lower temperature than the one accessible in this experiment. Also, with regard to the vanishing signal above 250 K, a high-temperature minimum of T1D(1/T) associated with the other motion, interpreted in
ammonates as isotropic reorientation of the complex cation [15, 16], was not succeeded. The slope of the accessible part of this high-temperature minimum of TID(1/T) allowed to determine the activation energy of this motion, Ea = 28.1 kJ/mol. The minimum of TI(1/T) allows the calculation of the p r o t o n - p r o t o n distance, rH-H in rotating groups. The O'Reilly-Tsang formula [14] determining T~ml. is 20 Tlmin---- 9
r6
to
(1)
~/4h21 . 4 2 '
where to is the L a r m o r frequency, while the other constants have their usual meanings. The experimental value --TeX~tmi, ----45.2ms yields the o value of the internuclear distance orH_i~= 1.75 A. For the ammonia gas rH-i~= 1.63 A, leading to T l m i n = 29.1 ms. It is worth pointing out that in the high-temperature phase in ammonates the six ammonia molecules form a regular octahedral complex[Me(NH3)6] 2+. The E P R studies in paramagnetic and particularly in diamagnetic ammonates doped with Ni 2÷ ions [17, 18] show, that in the high-temperature phase the crystal field at the Me 2÷ ion site has the cubic symmetry while below the structural phase transition temperature Tc~ the symmetry is lower than cubic. Since the minimum of TI(1/T) in H C C occurs at 123K (low-temperature phase) one may assume that if the p h e n o m e n a observed by means of E S R in other ammonates were associated with a deformation of the octahedron formed by six NH3 molecules, e.g. with its elongation, we would be able to regard the NH3 groups as nonequivalent and the Tlr.i, in H C C might be associated with the mutually independent motion of four NH3 groups while the relaxation of protons of the two remaining ones would be caused by the spin diffusion. Using the Anderson-Slichter formula [19]:
T1
N
NH3rot '
other NH 3
where N is the number of all protons in a molecule, n the number of protons in a rotating
214
L. Piekara-Sady et al. / Two phase transitions in hexammine cadmium chlorate
groups; neglecting the second term of (2) when T~ reaches its minimum value, 1 : Texp"
n (T~mi~)
lmin
NH 3 rot. "
0)
For independent rotation of four NH3 groups n = 12, N = 18 and taking the value Tlmi.= 29.1 ms from (1) one gets Tlmi.= 43.7 ms which is in very good agreement with the experimentally observed value ~-rex~ lniin = 45.2 ms. However, the above considerations do not allow drawing finn conclusion about the dynamical or structural nonequivalence of the six NH3 groups in the [Cd(NH3)6]2÷ complex.
Acknowledgements The authors wish to thank Dr. N. Pi~lewski for his helpful comments. We also wish to acknowledge the help of Dr. R. Radomski of the Institute of Organic and Physical Chemistry, Technical University of WrocJ'aw, in recording the DSC curve.
References [1] R.W.G. Wyckoff, Crystal Structure (Interscience, New York, 1965). [2] S.H. Yu, Nature 141 (1938) 158; 150 (1942) 347. [3] E. Dynowska, Phys. Stat. Solidi (a) 31 (1975) K23; E. Dynowska and J. Startkowski, Phys. Stat. Solidi (a) 52 (1979) 381.
[4] L. Asch, G.K. Shenoy, J.M. Friedt and J.P. Adloff, J. Chem. Phys. 62 (1975) 2335. [5] S. Hodorowicz, M. Cienchanowicz-Rutkowska, J.M. Janik and J.A. Janik, Phys. Stat. Solidi (a) 43 (1977) 53; A. Migda$-Mikuli, E. Mikuli, M. Rachwalska and S. Hodorowicz, Phys. Stat. Solidi (a) 47 (1978) 57; S. Hodorowicz and M. Ciechanowicz-Rutkowska, Acta Phys. Polon. A53 (1978) 29. [6] J. Stankowski, Mater. Sci. 11/3 (1976) 57. [7] T. Grzybek, J.A. Janik, J. Mayer, G. Pytasz, M. Rachwalska and T. Waluga, Phys. Stat. Solidi (a) 16 (1973) K165; M. Rachwalska, J.M. Janik, J.A. Janik, G. Pytasz and T. Waluga, Phys. Stat. Solidi (a) 30 (1975) K81. [8] E.A. Long and F.C. Toettcher, J. Am. Chem. Soc. 64 (1942) 629; J. Bousquet, M. Prot and M. Diot, J. Chem. Phys. 6 (1972) 1004. [9] M. Krupski and J. Stankowski, Acta Phys. Polon. A55 (1979) 597. [10] J. Stankowski and A.R. Bates, Proc. Congress.AMPERE Tallinn USSR (1978). [11] F. Ephraim and J. Jansen, Ber. 48 (1915) 41. [12] J.D. Cutnell and W. Venable, J. Chem. Phys. 60 (1974) 3795. [13] J. Jeener, R. DuBois and P. Broekaert, Phys. Rev. 139 (1965) A1959. [14] D.E. O'Reilly and T. Tsang, J. Chem. Phys. 46 (1967) 1291. [15] N. Pi~lewski, Fiz. Diel. Rad. IX (1977) 85 (in Polish); N. Pi~lewski and J. Stankowski and L. Lary~, Phys. Stat. Solidi (a) 31 (1975) 415; N. Pi~lewski and J. Stankowski, Bull. Acad. Pol. Sci. XXVII (1979) 61; N. Pi~lewski and L.T.H. Ferris, Phys. Stat. Solidi (b) 106 (1981) 123. [16] G.R. Murray and J.S. Waugh, J. Chem. Phys. 29 (1958) 207. [17] C. Trapp and Chin-I Shyr, J. Chem. Phys. 54 (1971) 196. [18] P.B. Sczaniecki and J. Stankowski, Acta Phys. Polon. A51 (1977) 117. [19] J.E. Anderson and W.P. Slichter, J. Phys. Chem. 69 (1%5) 9.