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NMR study of proton relaxation in Ni(NH3)6(NO3)2
Physica B 154 (1988) 93-96 North-Holland, Amsterdam
N M R STUDY OF P R O T O N R E L A X A T I O N IN Ni(NH3)6(NO3) 2
Jerzy C Z A P L I C K I 1, Norbert W E I D E N 2 and Alarich WEISS 2 ~lnstitute of Molecular Physics, Polish Academy of Sciences, 60-179 Poznah, Poland 2Institut fiir Physikalische Chemie-PC 111, Technische Hochschule Darmstadt, D-6100 Darmstadt, Fed. Rep. Germany
Received 6 May 1988
Proton spin-lattice relaxation times T1 were measured for Ni(NH3)6(NOa) 2 in the temperature range 420-5 K. The paramagnetic nature of the observed relaxation is responsible for constant T1 values in each phase. The changes of T~ occur at 244 K (252 K on heating), at ca 90 K (196 K on heating), and at 37 K. Electronic spin-flippingtimes were found of the order 10-1~ s.
I. Introduction
2. Experimental
Nickel hexammine nitrate ( N H N ) belongs to a large family described by the formula M(NH3)6X2, where M is a divalent metal atom and X is a univalent anion. These complex compounds are cubic in the high-temperature phase. Six ammonia molecules form the octahedral surrounding of M. The anions form a cubic sublattice and the crystal has the fcc structure. In lower temperatures there occur structural phase transitions depending on both M and X. N H N was investigated mostly by ESR [1-4], X-ray studies [5], dilatometric studies [2], incoherent inelastic and quasi-elastic neutron scattering [6,7], calorimetric studies [8] and dielectric studies [9]. A supposition appeared that N H N absorbs gas molecules and hence acquires different properties [3]. We felt a need to verify this statement and performed N M R proton relaxation measurements in the wide range of temperatures. There is little N M R work on this subject [10] which is most probably due to the fact that the dipolar relaxation among the N H 3 groups is masked in N H N by the paramagnetic interaction with the nickel electrons. Hence, the relaxation times T 1 are short and temperature-independent. Here we present our results and interpretation in terms of this interaction.
N H N was obtained [11] by passing ammonia gas through a solution of water-dissolved Ni(NO3) 2 and warmed concentrated liquid ammonia. The precipitate was washed with liquid ammonia and dried over K O H . The powder was sealed in air-free glass tubes. The measurements of the spin-lattice relaxation time T~ were performed with a pulsed M A T E C spectrometer operating at 30.5 MHz. The spectra were taken with a Varian WL-115 CW-NMR spectrometer at 8 M H z . Lowtemperature measurements were done with an Oxford Instruments continuous flow helium cryostat.
3. Results and discussion
The temperature dependence of the spinlattice relaxation time T 1 is shown in fig. 1. The values of T 1 a r e constant in each structural phase and change only on moving from one phase to another. At room temperature (Phase I) T 1 equals 96 ~s and changes on cooling at 244 K, when it acquires a value of 117 ixs (Phase II). This phase transition is characterized by a temperature hysteresis. The transition temperature
0921-4526/88/$03.50 t~) Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
J. Czaplicki et al. / NMR study of proton relaxation in Ni(NH 3)~(N0~)2
94
T1
[ps]
go K
~
100-
~ 196
OOO0
37 K
o
O 0
O0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 O0
0
0
0
0
K
k Phase
I
Phase
II
Phase
III
I t I I I
Phase
IV
10
100
103 T
Fig. 1. T e m p e r a t u r e d e p e n d e n c e of the spin-lattice relaxation time T~ for nickel h e x a m m i n e nitrate at 30.5 MHz.
on heating is 252 K. Proton spectra of N H N in Phase I and Phase II are shown in fig. 2. The resonance line in Phase I is 20% narrower than the line in Phase II. On further cooling one observes a wide transition area that starts at approximately 130 K and ends at 90 K with T~ decreasing to 90 Ixs (Phase III). This phase transition has a very large temperature hysteresis. On heating the relaxation time T 1 reaches the previous value of 117 txs at 196 K. There is one more phase transition in the range of low temperatures that occurs at 37 K with no hysteresis and is characterized by a change of T 1 from 90 to 86 ~s. This change is rather small but distinct. The phase transitions in N H N were studied also by other methods [9], from which the following picture emerges: in the high-temperature region N H N is characterized by a cubic phase with anions undergoing fast reorientations. The transition to Phase II is caused by the motional freezing of anions. At 90 K there occurs a structural phase transition to the rhombic phase (Phase III). However, there is a certain obscurity about the phase transition to Phase IV. Trybula [9] found a phase transition from Phase III to Phase IV by dielectric methods at 63 K
using a frequency of 8 . 8 G H z . Our measurements at 30.5 MHz revealed a phase transition at 37 K. The difference is too significant to be a measuring error. It seems probable that the temperature of this transition is frequency-dependent. Such a feature is characteristic of glassy states. Hence, it is possible [12] that we are observing here a transition to a glassy state in NHN. If we assume that the relation between the observation frequency f and the temperature of the transition to the glassy state Tg has the Vogel-Fulcher form, we may write
In this case,
however, we cannot find the parameters Ea, ]Co, To and 3' from just the two experimental points (63 K at 8.8 G H z and 37 K at 30.5MHz). We may simplify eq. (1) to the Arrhenius relation dropping T 0 and y:
where (as in eq. 1) E a is the activation energy for
J. Czaplicki et al. / NMR study of proton relaxation in Ni(NH3)6(N03) 2
95
Ni (NH3)6 (NO3) 2
4Gs
229 K
Fig. 2. ~H-NMR resonance lines of nickel hexammine nitrate above (265 K) and below (229 K) the 244 K phase I-phase H transition.
the creation of molecular clusters. From the experiment we get f = 2.79 × 1013 exp(-507.87/Tg).
(3)
However, this simplification goes too far and therefore other measurements at different frequencies must be done in order either to accept or to discard the above assumption. The constant values of T 1 impose the interpretation of the results in terms of the paramagnetic relaxation of protons to the unpaired Ni electrons. Solomon [13] gave the following formula for this case: 1 T1
J. 2 _
1
10
n
2
2
YiY~ r6
[-
[ 1 + (091 -- O~e)2~"2
3r 6r ] + 1 + o9~"2 + 1 + (w I + toe)2r 2 J '
(4)
where the subscript I denotes the studied nuclei/
protons in our c a s e / a n d e-electron. Other symbols have their usual meaning. Since toe ~> toI (at %--2~r.30.5 MHz i.e. in the field 0.7T, toe -20 GHz) we may simplify eq. (4) to 1 T1
C
[3"r + 7r] 1 + tour 2 1 +--2 2 , O.)eT
(5)
where C = 9 . 1 5 × 1013 s -2. From eq. (5) and the measured value of T 1 we calculate r, the electron spin-flipping time. The results are given in table I. The flipping times are of the order of tens of picoseconds. However, the detailed analysis of the relation between this parameter and the electron dynamics goes beyond the scope of this work. Laryg et al. [3] reported a 7 K shift upwards in the Phase I-Phase II transition in NHN after heating at 450 K. The~, ascribed this phenomenon to the effect of evacuating gas molecules previously embedded in the lattice. Trying to verify this we found the following.
96
J. Czaplicki et al. / NMR study of proton relaxation in Ni(NH~)6(NO~)2 Table I Electron spin-flipping times as calculated from eq. (5). Phase denotations correspond to those in fig. 1. Phase
Tl(Ixs )
~'(ps)
I 1I III 1V
96 117 90 86
33.9 25.8 36.5 38.6
We measured the phase I-phase II hysteresis loop and found the transition temperatures to be 244 K and 252 K on cooling and heating, respectively. Further, the sample was heated 1.5 h at 400 K and the measurements were repeated. No change in the hysteresis loop was observed. The sample was heated further 3 h at 400 K with the same result. Then the sample was heated 1.5 h at 450K and we noticed a sudden change in the transition temperatures. Now they were 246K and 250 K on cooling and heating, respectively. Subsequent heating for 3h at 450K gave no further temperature shifts. After about 2 weeks the measurement of the transition temperatures was repeated on the same sample and gave the same results as the least ones obtained, i.e. 246/250 K, within the experimental error. In our opinion there is no sign of evacuation of the sample, for the transition temperatures should be dependent on the heating time. The change of the hysteresis loop is due to the fact that at 423 K NHN loses two ammonia groups [14]. Hence, heating at 450 K spoils the sample and from then on the measurments concern nickel tetrammine nitrate.
Acknowledgements The research fellowship from the Alexander von Humboldt Foundation for JC is gratefully acknowledged. We thank Prof. J. Stankowski for encouragement to undertake the studies on the complex hexammine compounds.
References [1] J. Stankowski, Proc. RAMIS Conf. (1979) p.l13, and references cited therein. [2] J. Lesiak, L. Piekara-Sady, P.B. Sczaniecki and M. Krupski, Proc. RAMIS Conf. (1981) p. 165. [3] L. Lary~, J. Stankowski and M. Krupski, Acta Phys. Polon. A 50 (1976) 351. [4] J. Stankowski, Mater. Sci. II (3) (1976) 57. [5] S. Hodorowicz, J. Czerwonka, J.M. Janik and J.A. Janik, Physica B 111 (1981) 155. [6] A.V. Belushkin, J.A. Janik, J.M. Janik, I. Natkaniec and K. Otnes, Physica B 128 (1985) 289. [7] J.A. Janik, J.M. Janik, A. Migdal-Mikuli, E. Mikuli and K. Otnes, Physica B 111 (1981) 62. [8] A. MigdaI-Mikuli, E. Mikuli, M. Rachwalska, T. Stanek, J.M. Janik and J.A. Janik, Report No. l l I 5 / P S (Institute of Nuclear Physics, Kark6w, 1980), [9] Z. Trybula, Ph.D. Thesis (Institute of Molecular Physics, Polish Academy of Sciences, Poznafi, 1985). [10] N. Pi~lewski, Fizyka Dielektryk6w i Radiospektroskopia, IX(I) (1977) 85 [in Polish]. [11] Gmelins Handbuch der Anorganischen Chemie 8. Auflage, "Nickel", TI.C, Lfg. 1, No 57 (1968) p. 54. [12} J. Stankowski, private communication. [13] I. Solomon, Phys. Rev. 99 (1955) 559. [14] T.D. George and W.W. Wendlandt, J. Inorg. Nucl. Chem. 25 (1963) 395.
N M R STUDY OF P R O T O N R E L A X A T I O N IN Ni(NH3)6(NO3) 2
Jerzy C Z A P L I C K I 1, Norbert W E I D E N 2 and Alarich WEISS 2 ~lnstitute of Molecular Physics, Polish Academy of Sciences, 60-179 Poznah, Poland 2Institut fiir Physikalische Chemie-PC 111, Technische Hochschule Darmstadt, D-6100 Darmstadt, Fed. Rep. Germany
Received 6 May 1988
Proton spin-lattice relaxation times T1 were measured for Ni(NH3)6(NOa) 2 in the temperature range 420-5 K. The paramagnetic nature of the observed relaxation is responsible for constant T1 values in each phase. The changes of T~ occur at 244 K (252 K on heating), at ca 90 K (196 K on heating), and at 37 K. Electronic spin-flippingtimes were found of the order 10-1~ s.
I. Introduction
2. Experimental
Nickel hexammine nitrate ( N H N ) belongs to a large family described by the formula M(NH3)6X2, where M is a divalent metal atom and X is a univalent anion. These complex compounds are cubic in the high-temperature phase. Six ammonia molecules form the octahedral surrounding of M. The anions form a cubic sublattice and the crystal has the fcc structure. In lower temperatures there occur structural phase transitions depending on both M and X. N H N was investigated mostly by ESR [1-4], X-ray studies [5], dilatometric studies [2], incoherent inelastic and quasi-elastic neutron scattering [6,7], calorimetric studies [8] and dielectric studies [9]. A supposition appeared that N H N absorbs gas molecules and hence acquires different properties [3]. We felt a need to verify this statement and performed N M R proton relaxation measurements in the wide range of temperatures. There is little N M R work on this subject [10] which is most probably due to the fact that the dipolar relaxation among the N H 3 groups is masked in N H N by the paramagnetic interaction with the nickel electrons. Hence, the relaxation times T 1 are short and temperature-independent. Here we present our results and interpretation in terms of this interaction.
N H N was obtained [11] by passing ammonia gas through a solution of water-dissolved Ni(NO3) 2 and warmed concentrated liquid ammonia. The precipitate was washed with liquid ammonia and dried over K O H . The powder was sealed in air-free glass tubes. The measurements of the spin-lattice relaxation time T~ were performed with a pulsed M A T E C spectrometer operating at 30.5 MHz. The spectra were taken with a Varian WL-115 CW-NMR spectrometer at 8 M H z . Lowtemperature measurements were done with an Oxford Instruments continuous flow helium cryostat.
3. Results and discussion
The temperature dependence of the spinlattice relaxation time T 1 is shown in fig. 1. The values of T 1 a r e constant in each structural phase and change only on moving from one phase to another. At room temperature (Phase I) T 1 equals 96 ~s and changes on cooling at 244 K, when it acquires a value of 117 ixs (Phase II). This phase transition is characterized by a temperature hysteresis. The transition temperature
0921-4526/88/$03.50 t~) Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
J. Czaplicki et al. / NMR study of proton relaxation in Ni(NH 3)~(N0~)2
94
T1
[ps]
go K
~
100-
~ 196
OOO0
37 K
o
O 0
O0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 O0
0
0
0
0
K
k Phase
I
Phase
II
Phase
III
I t I I I
Phase
IV
10
100
103 T
Fig. 1. T e m p e r a t u r e d e p e n d e n c e of the spin-lattice relaxation time T~ for nickel h e x a m m i n e nitrate at 30.5 MHz.
on heating is 252 K. Proton spectra of N H N in Phase I and Phase II are shown in fig. 2. The resonance line in Phase I is 20% narrower than the line in Phase II. On further cooling one observes a wide transition area that starts at approximately 130 K and ends at 90 K with T~ decreasing to 90 Ixs (Phase III). This phase transition has a very large temperature hysteresis. On heating the relaxation time T 1 reaches the previous value of 117 txs at 196 K. There is one more phase transition in the range of low temperatures that occurs at 37 K with no hysteresis and is characterized by a change of T 1 from 90 to 86 ~s. This change is rather small but distinct. The phase transitions in N H N were studied also by other methods [9], from which the following picture emerges: in the high-temperature region N H N is characterized by a cubic phase with anions undergoing fast reorientations. The transition to Phase II is caused by the motional freezing of anions. At 90 K there occurs a structural phase transition to the rhombic phase (Phase III). However, there is a certain obscurity about the phase transition to Phase IV. Trybula [9] found a phase transition from Phase III to Phase IV by dielectric methods at 63 K
using a frequency of 8 . 8 G H z . Our measurements at 30.5 MHz revealed a phase transition at 37 K. The difference is too significant to be a measuring error. It seems probable that the temperature of this transition is frequency-dependent. Such a feature is characteristic of glassy states. Hence, it is possible [12] that we are observing here a transition to a glassy state in NHN. If we assume that the relation between the observation frequency f and the temperature of the transition to the glassy state Tg has the Vogel-Fulcher form, we may write
In this case,
however, we cannot find the parameters Ea, ]Co, To and 3' from just the two experimental points (63 K at 8.8 G H z and 37 K at 30.5MHz). We may simplify eq. (1) to the Arrhenius relation dropping T 0 and y:
where (as in eq. 1) E a is the activation energy for
J. Czaplicki et al. / NMR study of proton relaxation in Ni(NH3)6(N03) 2
95
Ni (NH3)6 (NO3) 2
4Gs
229 K
Fig. 2. ~H-NMR resonance lines of nickel hexammine nitrate above (265 K) and below (229 K) the 244 K phase I-phase H transition.
the creation of molecular clusters. From the experiment we get f = 2.79 × 1013 exp(-507.87/Tg).
(3)
However, this simplification goes too far and therefore other measurements at different frequencies must be done in order either to accept or to discard the above assumption. The constant values of T 1 impose the interpretation of the results in terms of the paramagnetic relaxation of protons to the unpaired Ni electrons. Solomon [13] gave the following formula for this case: 1 T1
J. 2 _
1
10
n
2
2
YiY~ r6
[-
[ 1 + (091 -- O~e)2~"2
3r 6r ] + 1 + o9~"2 + 1 + (w I + toe)2r 2 J '
(4)
where the subscript I denotes the studied nuclei/
protons in our c a s e / a n d e-electron. Other symbols have their usual meaning. Since toe ~> toI (at %--2~r.30.5 MHz i.e. in the field 0.7T, toe -20 GHz) we may simplify eq. (4) to 1 T1
C
[3"r + 7r] 1 + tour 2 1 +--2 2 , O.)eT
(5)
where C = 9 . 1 5 × 1013 s -2. From eq. (5) and the measured value of T 1 we calculate r, the electron spin-flipping time. The results are given in table I. The flipping times are of the order of tens of picoseconds. However, the detailed analysis of the relation between this parameter and the electron dynamics goes beyond the scope of this work. Laryg et al. [3] reported a 7 K shift upwards in the Phase I-Phase II transition in NHN after heating at 450 K. The~, ascribed this phenomenon to the effect of evacuating gas molecules previously embedded in the lattice. Trying to verify this we found the following.
96
J. Czaplicki et al. / NMR study of proton relaxation in Ni(NH~)6(NO~)2 Table I Electron spin-flipping times as calculated from eq. (5). Phase denotations correspond to those in fig. 1. Phase
Tl(Ixs )
~'(ps)
I 1I III 1V
96 117 90 86
33.9 25.8 36.5 38.6
We measured the phase I-phase II hysteresis loop and found the transition temperatures to be 244 K and 252 K on cooling and heating, respectively. Further, the sample was heated 1.5 h at 400 K and the measurements were repeated. No change in the hysteresis loop was observed. The sample was heated further 3 h at 400 K with the same result. Then the sample was heated 1.5 h at 450K and we noticed a sudden change in the transition temperatures. Now they were 246K and 250 K on cooling and heating, respectively. Subsequent heating for 3h at 450K gave no further temperature shifts. After about 2 weeks the measurement of the transition temperatures was repeated on the same sample and gave the same results as the least ones obtained, i.e. 246/250 K, within the experimental error. In our opinion there is no sign of evacuation of the sample, for the transition temperatures should be dependent on the heating time. The change of the hysteresis loop is due to the fact that at 423 K NHN loses two ammonia groups [14]. Hence, heating at 450 K spoils the sample and from then on the measurments concern nickel tetrammine nitrate.
Acknowledgements The research fellowship from the Alexander von Humboldt Foundation for JC is gratefully acknowledged. We thank Prof. J. Stankowski for encouragement to undertake the studies on the complex hexammine compounds.
References [1] J. Stankowski, Proc. RAMIS Conf. (1979) p.l13, and references cited therein. [2] J. Lesiak, L. Piekara-Sady, P.B. Sczaniecki and M. Krupski, Proc. RAMIS Conf. (1981) p. 165. [3] L. Lary~, J. Stankowski and M. Krupski, Acta Phys. Polon. A 50 (1976) 351. [4] J. Stankowski, Mater. Sci. II (3) (1976) 57. [5] S. Hodorowicz, J. Czerwonka, J.M. Janik and J.A. Janik, Physica B 111 (1981) 155. [6] A.V. Belushkin, J.A. Janik, J.M. Janik, I. Natkaniec and K. Otnes, Physica B 128 (1985) 289. [7] J.A. Janik, J.M. Janik, A. Migdal-Mikuli, E. Mikuli and K. Otnes, Physica B 111 (1981) 62. [8] A. MigdaI-Mikuli, E. Mikuli, M. Rachwalska, T. Stanek, J.M. Janik and J.A. Janik, Report No. l l I 5 / P S (Institute of Nuclear Physics, Kark6w, 1980), [9] Z. Trybula, Ph.D. Thesis (Institute of Molecular Physics, Polish Academy of Sciences, Poznafi, 1985). [10] N. Pi~lewski, Fizyka Dielektryk6w i Radiospektroskopia, IX(I) (1977) 85 [in Polish]. [11] Gmelins Handbuch der Anorganischen Chemie 8. Auflage, "Nickel", TI.C, Lfg. 1, No 57 (1968) p. 54. [12} J. Stankowski, private communication. [13] I. Solomon, Phys. Rev. 99 (1955) 559. [14] T.D. George and W.W. Wendlandt, J. Inorg. Nucl. Chem. 25 (1963) 395.