IONIC AND MOLECULAR DIFFUSION AND THE ORDER-DISORDER. PHASE TRANSITION IN THE THALLIUM FORM OF NATROLITE. A. M. Panich, I. A. ...
IONIC AND MOLECULAR DIFFUSION AND THE ORDER-DISORDER PHASE TRANSITION IN THE THALLIUM FORM OF NATROLITE A. M. Panich, I. A. Belitskii, N. K. Moroz, S. P. Gabuda, V. A. Drebushchak, and Yu. V. Seretkin
UDC 549.67:548.0:539.143.43:541.67
The thallium form of the narrow-pore zeolite natrolite has been studied with the aid of NMR spectroscopy, scanning calorimetry, and x-ray powder diffraction analysis. A phase transition accompanied by an abrupt change in the mobility of the TI+ ions and H=O molecules has been discovered at T = 252 K. The transition may be regarded as "melting" of the aqueous and cation sublattices in the channels of the zeolite skeleton. INTRODUCTION The melting of particles inserted into the microvoids of the zeolite skeleton was studied experimentally and theoretically in [1-4]. We recently investigated the behavior of water molecules appearing in the composition of a quasi-unidimensional chain of alternating H=O molecules and Na + ions, which is realized in the channels of the narrow-pore zeolite natrolite Nam[Al=Si3Ol0]'2H20, at high hydrostatic pressures [5, 6]. The compression of natrolite to a pressure of %8 kbar at room temperature in an aqueous medium which penetrates into its channels resulted in a phase transition. In this case, the high-pressure phase was characterized by anomalously high mobility of the H=O molecules in the channels, while in the original modification of natroiite existing at atmospheric pressure appreciable molecular diffusion was previously observed only at T Z 450 K [7, 8]. In the present work we used NMR spectroscopy, scanning calorimetry, and x-ray powder diffraction analysis to study the cation-substituted thallium form of natrolite. A phase transition accompanied by an abrupt change in the mobility of the H20 molecules and the TI+ ions was discovered at T = 252 K and atmospheric pressure. An analysis of the experimental data together with some structural arguments allows us to postulate that the phase transition observed here, like the one discovered at high pressures in natrolite, is associated with redistribution of the molecules and ions among the lattice points and interstitial positions and may be regarded as "melting" of the aqueous and cation sublattices in the channels of the zeolite skeleton. STRUCTURAL FEATURES OF NATROLITE AND THE THALLIUMTFORMOF NATROLITE The structure of natrolite belongs to the orthorhombic system, and the space group is Fdd2 (a = 18.27, b = 18.61, c = 6.59 4, Z = 8) [9, I0]. The aluminosilicate skeleton of the crystal is perforated by narrow channels extending parallel to the c axis that accommodate structurally equivalent Na + cations, which form zigzagged quasi-unidimensional chains (Fig. i). Water molecules are also found in structurally equivalent positions (wl), their positions and orientations being determined by the interactions of two neighboring Na+ ions (by means of their lone pairs) and two skeletal oxygen atoms, viz., Ol and 05 (by means of hydrogen bonds) [i0]. The structure of the thallium form of natrolite was not established. According to the data from the x-ray powder method, this structure, like the structure of natural natrolite, belongs to the Fdd2 space group; the unit-cell parameters at room temperature are: a = 19.668(4), b = 20.01(4), c = 6.535(1) 4. The general character of the distribution of the positions of the cations and water molecules in the channels is apparently~aintained as a whole. The IH NMR spectra of the thallium form of natrolite at room temperature, which Institute of Inorganic Chemistry, Academy of Sciences of the USSR, Siberian Branch. Institute of Geology and Geophysics, Academy of Sciences of the USSR, Siberian Branch. Translated from Zhurnal Strukturnoi Khimii, Vol. 31, No. i, pp. 67-73, January-February, 1990. Original article submitted March 16, 1988.
56
0022-4766/90/3101-0056512.50 9 1990 Plenum Publishing Corporation
9
ry/
1
-
'
Fig. I. Fragment of an ...-Na-H20Na-... chain in a natrolite channel.
9 Na 0 H20(W~) ~-, H O(Wo)
were first obtained in [1i], showed that the angle between the proton-proton vector and the axis is altered only slightly in comparison to natural natrolite. EXPERIMENTAL Polycrystalline samples of the thallium form of natrolite were obtained by Na + ~ TI + ion exchange in natural orthorhombic natrolite Na16[AII6Si24080]'I6H20 (from KhiSiny, USSR) in a TINO 3 melt (230~ 120 h). After the treatment of natrolite in the salt melt, it was washed to remove any remaining TINO~ with hot water and then autoclaved in water at 130~ for 24 h. According to the data from chemical analysis, x-ray diffraction analysis, and IR spectroscopy, the samples obtained correspond to the monocationic T1 form of natrolite. The IH, 27AI, 2~ and 2~ spectra were recorded at frequencies equal to 11.651.9 MHz in the 120-350 K temperature range. The measurements of the specific heat of the sample at a constant pressure were carried out on Mettler DSC-30 and CETARAM DSC-III differential scanning calorimeters. EXPERIMENTAL RESULTS A specific-heat anomaly, which attests to a phase transition, was discovered on the temperature dependence of Cp(T) at the temperature of the phase transition (Tpt = 252 K) of the sample of the thallium form of natrolite. The hysteresis of the positions of the maxima of the anomalies of Cp(T) observed during a heating and cooling cycle of the sample amounts to about i0 K. The enthalpy change accompanying the phase transition AH equals 8.3 J/g (Fig. 2). During partial dehydration of the thallium form of natrolite by briefly holdin~ the sample at 340-400 K, AH decreases linearly with the mass loss. However, the form of the anomaly is significantly dependent on the history of the sample: the peak splits in cases in which the measurement of Cp(T) is carried out immediately after partial dehydration, and it returns to the one-component form after prolonged (over the course of 24 h) holding of the sample at room temperature in air. The anomaly of Cp(T) is not observed in the case of a completely dehydrated sample of the thallium form of natrolite. According to the data from the x-ray powder method, the low-temperature phase of the thallium form of natrolite also belongs to the orthorhombic system (space group fdd2) with the unit-cell parameters e = 19.02(3), b = 19.73(4), and c = 6.55(i) ~. The i H NMR spectra of the thallium form of natrolite at T < Tpt consist of Pake doublets, which are characteristics of crystal hydrates with immobile water molecules and correspond to a "rigid" two-spin proton system with a typical dipole-dipole coupling constant for H20 molecules ~H = 3/475R-3 = 5.1 • 0.i G (here R is the interproton distance in a molecule). The 2~ and 2~ NMR spectra of the low-temperature phase consists of relatively broad lines (Fig. 3); plots of the temperature dependence of their second moments are 57
ACp, j.g-i ,deg- I
o,5
J Fig. 2. Temperature dependence of ACp for the thallium form of natrolite.
,
---
I
..,,,.
" - /o-
- '5
,
,
~Y
i ~
i--;
......
203T'I.
y'~''-f6 2O5T1~ T=252K
-2
-1
0
F~-
? .-Ho, G
Fig. 3. =~ and 2~ NMR spectra (first derivative of absorption line) in low-temperature (a, b) and high-temperature (c) phases. presented in Fig. 4. The values of M= depend on the strength of the applied magnetic field H 0, pointing out the significant contribution of the electron nuclear interactions (shielding effects). Then the dependence of M 2 on H 0 may be represented in the form
M~ = (6~ro)~ + M~'.
(1)
The parameters 6 and M 2' obtained from the measurements of M 2 at resonance frequencies equal to 11.8 and 23.1 MHz are presented in Table i. If the structure of the thallium form of natrolite in the low-temperature phase is similar to that of natrolite, all the T1 + ions are structurally equivalent, and the parameter 6 in (i) is determined only by the anisotropy of the magnetic shielding. The contributions of t h e s e c o n d moments which are not dependent on the magnetic field are different for the two isotopes of thallium, and their ratio M2'(=~176 = 2.1 is close to the reciprocal of the ratio between the natural abundances of the isotopes. This indicates that the decisive contribution to M 2' is made by indirect exchange interactions between the nuclear spins [12]. Under the assumption 58
I
c~
~o
co o
+I
o
C 0 0
C
U 0
E-~ c
0
c~
U
%
o
oo
0
C o
o-~ C~ o ~ o o m
4J
~
,-..4
I1) -
9
o
o
. ~
I ~
o
~
o
o
59
--O-----O
C,
0
0%
"f,O"
,5
0,6-
203TI.,23~07MHz 6
~G
G
/,'/Z ,, G 2
o;
ZOSTI,~ 25,07 MHz
2~o 4
2~
3bo T,n
~11,61
2 I ~'0
1
I
!
250
i
Fig. 4. Temperature dependence of the second moments M= of the 2~ and 2~ spectra and dipole-dipole coupling constants of the IH spectrum ~ (the plots at T < 292 K were obtained when the sample was cooled). Plots of (T) for two samples of the thailium form of natrolite with different H20 contents (xm/x 1 % 1.05) are presented in the figure, that the arrangement of the TI + ions in a chain is uniform, the exchange coupling constant J determined from the values of M=' is equal to 8.0 • 0.7 kHz. Alternative mechanisms of indirect exchange are possible in the thallium form of natrolite. If the distance between the TI+ ions does not differ significantly from the interaction distance in the original sodium form (3.65 ~), the interaction between the nuclear spins can appear as a result of the direct overlap of the wave functions of the TI+ ions. On the other hand, it was shown in [13] that this interaction can be realized through the electronic orbitals of the intermediate atoms (skeletal oxygen atoms or H20 molecules in the present case). However, in the case of the 2~ and =~ spectra of a completely dehydrated sample of the thallium form of natrolite, the contributions to M~ caused by indirect exchange and the coupling constant J are close to those found for the low-temperature phase of the hydrated form and have the.following values: M='(=~ = 4.3 • 0.4 G 2, M='(2~ = 1.8 • 0.2 G 2, J = 8.5 • 1 kHz. This allows us to assume that the participation of the electronic orbitals of the water molecules in the shaping of the mechanisms of indirect nuclear exchange is not decisive in the thallium form of natrolite. The 2VAI NMR spectra, whose form is determined by the second-order quadrupole effects, were recorded for the first time for the thallium form of natrolite in [14]; a difference between the values of the quadrupole coupling constant e2qzzQ/h at T = 293 and 233 K was discovered. ~he measurements performed in the present work showed that the value of e2qzzQ/h jumps from 2.4 • 0.4 to 1.8 • 0.i MHz at the point of the phase transition. The transition to the high-temperature phase is accompanied by drastic narrowing of the NMR spectra of protons and thallium nuclei (Figs. 3-5). The observed transformation of the IH spectrum points out the appearance of the translational diffusion of H20 molecules with a correlation frequency of the jumps v c ~ l0 s sec ~l at T > Tpt. The fine structure of the spectra (Fig. 5a) corresponds to a dipole--dipole interaction averaged by the motion of the molecules in a two-spin system with an axial tensor and a C0upling constant = Kd= equal to 0.9 G immediately above T t (here K d is the averaging factor). In the 252-320 K region the values of and thus t~e values of K d are greatly dependent on the temperature (see Fig. 4). When the temperature is raised further (T ~ 330 K), a narrow singlet peak, into which the intensity of the wings of the fine structure is "pumped" as the temperature is increased, forms in the center of the IH spectrum (Fig. 5b). An analysis of the changes in the form of the line occurring here reveals that intermolecular proton transfer is responsible for them. However, this process cannot be associated with the observed dependence of (T) at lower temperatures (252-320 K). One interesting special feature of this dependence is its high sensitivity to small changes in the hydration of the sample. A decrease in the water content, which does not exceed 10% according to the 60
?
29! t~
jJ
580~
-1
Fig. 5. IH NMR spectra (first derivative of the absorption line) in the high-temperature phase of the thallium form of natrolite.
.dH, G
- 6 ,ppm
1 O,
,
,
~0
0,5
0,5
0,7 ~ G
Fig. 6. Changes in the splitting and shifts of the 2~ NMR lines as a function of upon variation of the temperature (T % Tft). The shifts of the lines were measured relative to the signal of 2~ in a saturated solution of TINO~. thermogravimetric data, causes a significant decrease in the slope of the plots of (T) (see Fig. 4). In addition, the value of remains practically unchanged in the immediate vicinity of the point of the phase transition. The effect is reversible, and the course of the plots of (T) is restored when the sample is held in water vapor. Above the phase transition the values of M 2 of the spectra of both thallium isotopes (see Table i) coincide within the range of the experimental error [Ma'(2~ = Ma(2~ 0], and at T = 253 K they are determined entirely by the shielding effects with a parameter 6 times smaller than in the low-temperature phase. The significant averaging of the electron-nuclear interactions upon the transition to the high-temperature phase and the accompanying disappearance of the indirect exchange interactions in the system of nuclear spin indicate the appearance of mobility for the thallium ions (most probably translational diffusion). Immediately above the point of the phase transition, the 2~ 2~ spectra have the form of asymmetric lines with a well expressed fine structure (see Fig. 3), which vanishes when the temperature is increased further; in addition, some displacement of the lines toward weaker fields is observed. When the water content is increased, the transformation of the spectra just described occurs at lower temperatures, but the form and positions of the lines near the point of the phase transition are practically identical for all the samples investigated. X~he transitional character of the spectra ~n the 252-350 K range hampers their interpretation. The observed fine structure can be attributed with equal success to shielding effects which are partially averaged by diffusion and to the presence of large-scale heterogeneity in the crystal and the associated distribution of the shielding constants of the
61
thalliumnuclei. In our opinion, the significant finding here is the symbatic character of the transformation of the T1 spectra and the change in upon variation of the temperature. We note that the quadrupole coupling constant of the 27AI nuclei remains unchanged in the range of the experimental error in the 252-350 K range. This points out the absence of any deformation of the aluminosilicate skeleton in the temperature range under consideration. RESULTS AND DISCUSSION One important property of the thallium form of natrolite in the high-temperature phase is the existence in it of developed molecular (H20) and ionic mobility, which includes both the diffusion of T1 + ions and intermolecular proton transfer. In the case of rapid transla~tional diffusion, the parameters of motionally averaged interactions (dipole-dipole and electron-nuclear interactions) are known to be determined by the entire set of structural positons occupied by a molecule or an ion and generally vary to a weak extent with the temperature [15, 16]. For this reason, the temperature dependences of the parameters of the IH, 2~ and =~ spectra observed for the high-temperature phase of the thallium form of natrolite are unusual and apparently reflect the further development (as the temperature is increased) of definite orientation-translation disorder in the system of mobile particles, under which the set of positions occupied by water molecules and T1 + ions continuously changes w i t h t h e temperature. In [5] attention was focused for the first time on the presence in the channels not only of occupied lattice positions (wl), but also of a certain number of interstitial positions (w 2) that are capable of acconmlodating water molecules, but remain vacant in the structure of natural natrolite (see Fig. i). It was postulated in [5] that the'partial populating of these positions by H20 molecules when a natrolite crystal is compressed in water provides of the total translational diffusion in the aqueous sublattice. A further increase in the "forced" populating of the interstitial positions b y H 2 0 molecules results in the disordering of the aqueous subsystem and the natrolite + natrolite II phase transition. As in the case of natural natrolite, the phase transition in the thallium formDf natrolite may be associated with disordering of the water molecules among two types of posit~o~s, viz., w I and w 2. However, the aforementioned feature of the behavior of the parameters of the NMR spectra of the compound above Tpt allow us to assume that the "order-disorder" transition takes place according to a more complicated scheme in the thallium form o~ natrolite and may be classified as a transition involving "melting" of the sublattice of the'particles inserted into the channels. It may be assumed that the dimensions of the sublattice of H20 mol~cules and ions does not match the aluminosilicate skeleton of the compound. One of the consequences of this effect may be the axial character of the tensors of themotionally averaged dipole-dipole and electron-nuclear interactions in the orthorhombic skeleton of natrolite. LITERATURE CITED i. 2. 3. 4. 5. 6. 7. 8. 9. i0. Ii. 12.
62
V. N. Bogomolov, T. I. Volkonskaya, A. I. Zodorozhnyi, et al., Fiz. Tverd. Tela, 1__7, No. 6, 1707-1710 (1975). u N. Bogomolov, Usp. Fiz. Nauk, 124, No. i, 171-183 ~1978). Yu. A. Alekseev, V. N. Bogomolov, u A. Egorov, et al., Pis'ma Zh. Eksp. Teor. Fiz., 36, No. ii, 384-386 (1982). E. V. Kholopov, Fiz. Tverd. Tela, 28, No. 4, 1265-1268 (1986). I. A. Belitskii, S. P. Gabuda, and N. K. Moroz, Dokl. Akad. Nauk SSSR, 292, No. 5, 1232-1234 (1987). O . V . Kholdeev, I. A. Belitskii, B. A. Fursenko, and S. ~. Goryainov, Dokl. Akad. Nauk SSSR, 297, No. 4, 946-950 (1987). S. P. Gabuda, Dokl. Akad. Nauk SSSR, 146, No. 3, 840-843 (1962). R. T. Thompson, R. R. Rnispel, and N. E. Petch, Can. J. Phys. 52, No. 21, 2164-2173 (1974). W. M. Meier, Z. Kristallogr., 113, No. 8, 430-444 41960). G. Artioli, T. V. Smith, and A. Kvick, Acta Crystallogr., C40, No. ii, 1658-1662 (1984). I. A. Belitsky and S. P. Cabuda, Chem. Erde, 27, No. i, 79-90 (1968). T. H. Van u Phys. Rev. 474, No. 9, 1168-1183 (1948).
13. 14.
E. V. Kholopov, A. M. Panich, N. K. Moroz, and Yu. G. Kriger, Zh. Eksp. Teor. Fiz., 8__44,No. 3, 1091-1096 (1983). C. P. Gabuda, I. A. Belitskii, and V. N. Shcherbakov, Geokhimiya, No. 10, 1556-1559
(1973). 15. 16.
S. P. Gabuda and A. G. Lundin, Zh. Eksp. Teor. Fiz., 5_55, No. 3, 1066-1073 (1968). S. P. Gabuda and A. F. Rzhavin, NMR in Crystal Hydrates and Hydrated Proteins [in Russian], Nauka, Sib. Otd., Novosibirsk (1978).
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