CRYSTAL GROWTH & DESIGN
Influence of Pressure and Temperature on the Crystal Structure of Deuterated Ammonium Copper Tutton Salt, (ND4)2[Cu(D2O)6](SO4)2
2003 VOL. 3, NO. 3 403-407
Arthur J. Schultz,*,† Robert W. Henning,† Michael A. Hitchman,‡ and Horst Stratemeier‡ Intense Pulsed Neutron Source, Argonne National Laboratory, Argonne, Illinois, 60439, USA, and School of Chemistry, University of Tasmania, Box 252-75, Hobart, Tasmania 7001, Australia Received November 26, 2002;
Revised Manuscript Received February 27, 2003
ABSTRACT: The influence of pressure on the unit cell parameters of deuterated ammonium copper(II) sulfate hexahydrate at various temperatures from 50 to 325 K obtained from pulsed neutron powder diffraction is reported. Application of pressure causes the structure to switch to a packing arrangement like that observed for the corresponding hydrogenous compound over the complete temperature range, with a higher pressure being required at lower temperatures. A pressure hysteresis occurs upon the release of pressure in the temperature range 303275 K and below ∼295 K the compound remains in the high pressure (high density) modification at 1 bar. As the temperature is raised from ∼296 to 298 K, the structure reverts progressively to the low-pressure modification. Using the Clapeyron equation, an enthalpy ∆H of 259 cm-1 at 260 K was evaluated for the transition. With zero applied pressure, a negative temperature expansion coefficient is observed above 230 K, apparently due to an increasing thermal population of the high-density packing arrangement at higher temperatures. Introduction Prior to the discovery of X-ray diffraction, Tutton’s interest in preparing salts of A2[M(H2O)6](XO4)2 (A ) alkali metals, ammonium; M ) first row transition metals; X ) S, Se) was to examine systematic variations in an isomorphous series of related crystals primarily by optical techniques.1 Although most of the Tutton salts are isomorphous, today we know that the copper salts of the type A2[Cu(H2O)6](SO4)2 are a unique system in which the interplay of the Jahn-Teller expression and the hydrogen-bonding network determines which of two dimorphs is adopted.2-6 From a crystal engineering perspective, the adoption of one dimorph or the other can be controlled by the type of cation (alkali metal or ammonium) or anion (sulfate or selenate), the isotopic H/D ratio, the application of pressure, and the degree of zinc doping. The conditions for each dimorph are summarized in Table 1. A portion of the ab plane of the two structures in shown in Figure 1. As seen in Figure 1, the Tutton salts can be described as having antiferrodistortive structures with the propagation vector parallel to the a-axis in the monoclinc P21/a crystal lattice. Thus, the direction of the JahnTeller distortion of the [Cu(H2O)6]2+ chromophores alternate in planes with spacings of a/2 between planes. At low temperature, the long Cu-O distances are ∼2.3 Å and the short Cu-O distances are ∼2.0 Å. When the Jahn-Teller elongated axis switches by 90° in the copper chromophore, the antiferrodistortive propagation vector becomes 180° out of phase with the previous * Corresponding author: Arthur J. Schultz, IPNS, Bldg. 360, Argonne National Laboratory, Argonne, IL 60439-4814, USA. Phone: 630-252-3465. Fax: 630-252-4163. E-mail:
[email protected]. Web: http://www.pns.anl.gov/. † Argonne National Laboratory. ‡ University of Tasmania.
Table 1. Structural Forms of Copper Tutton Salts at Room Temperaturea compound M2[Cu(H2O)6](SO4)2, M ) K, Rb, Cs (NH4)2[Cu(H2O)6](SO4)2 (ND4)2[Cu(D2O)6](SO4)2 (ND4)2[(Cu1-xZnx(D2O)6](SO4)2 K2[Cu(H2O)6](XO4)2 a
dimorph L
dimorph H
all d > 50% P < ∼200 bar x < 0.02 X)S
refs 20-23
d < 50% P > ∼ 200 bar x > 0.02 X ) Se
2,3,10,18,24 3,8,9 19 20,25
d ) deuteration.
dimorph. The switch is cooperative and appears to be transmitted by hydrogen bonds.5,7 The ammonium copper Tutton salt, (NH4)2[Cu(H2O)6](SO4)2, is particularly interesting because on deuteration it adopts a structure (dimorph L for low density) in which the long axis of the distorted [Cu(D2O)6]2+ ion occurs to a different pair of water molecules from those in the hydrogenous salt (dimorph H for high density), with the change being accompanied by slight alterations in the orientations and hydrogen bonding interactions of the counterions.2 Both (NH4)2[Cu(H2O)6](SO4)2 and its deuterated analogue exhibit a thermal equilibrium involving the alternative direction of the elongated Jahn-Teller distortion,2 and it has been suggested that cooperative interactions may play a significant role in these.5 Application of pressure to the deuterated compound causes the structure to switch to dimorph H,3 the change exhibiting hysteresis at room temperature when the pressure is subsequently released.8 The structural change may also be induced by optically “pumping” a partially deuterated sample with infrared radiation.7 An EPR study has shown that the pressure required to change the structure of the deuterated salt increases significantly below room temperature.9 Moreover, when the pressure is released at low temperature, the com-
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Schultz et al.
Figure 1. View of the ab plane (a horizontal, b vertical) of dimorph L (left) and dimorph H (right). The red Cu-O bonds are Jahn-Teller elongated with respect to the black bonds (∼2.30 vs ∼2.0 Å, at low temperature). Also shown are selected hydrogen bonds (dashed) between water and sulfate groups. Red hydrogen bonds are elongated in comparison to the same hydrogen bond in the other polymorph. Atom colors: Cu, gray; S, green; O, red; N, blue; H/D, black.
pound remains “frozen” in dimorph H, though it reverts to dimorph L when the temperature is raised to roomtemperature again. The two dimorphs have the same monoclinic space group but with unit cell lattice constants that differ by about 0.1 Å. In the switch from L to H, the a and b axes decrease whereas the c axis increases due to a lengthening of a hydrogen bond approximately normal to the ab plane. In a previous paper, we reported the observation of a hysteresis in the changes of the unit cell constants with pressure at ambient room temperature.8 The purpose of the present experiment is to investigate the pressure hysteresis in detail by studying the influence of pressure upon the structure of (ND4)2[Cu(D2O)6](SO4)2 at a range of temperatures with powder neutron diffraction. Experimental Section The preparation and characterization of the sample of (ND4)2[Cu(D2O)6](SO4)2 have been described previously.5 The extent of deuteration was measured as ∼96%, and it may be noted that the change in polymorph has been found to be relatively insensitive to the degree of deuteration at this level.10 The sample was gently ground with a mortar and pestle under helium to produce a fine powder, which was loaded into an aluminum pressure cell. The cell was then mounted on the end of a Displex closed-cycle helium refrigerator on the HighIntensity Powder diffractometer (HIPD) at the intense pulsed neutron source (IPNS). The pressure cell and the HIPD instrument have been described in a previous publication.11
Results and Discussion The unit cell parameters of a powdered sample of (ND4)2[Cu(D2O)6](SO4)2 were measured at room temperature (298 K) as the pressure was increased in increments of ∼30 bar up to ∼400 bar. Initially,
parameters characteristic of dimorph L were obtained, but above 200 bar these switched abruptly to those of dimorph H. On progressively decreasing the pressure, the compound remained in dimorph H even when the pressure dropped to 1 bar. It remained in this polymorph after standing at this temperature for 24 h, but reverted to dimorph L immediately on being warmed to 308 K. The sample was then cooled to 285 K and the procedure was repeated. This time the abrupt switch to dimorph H occurred when the pressure was above 400 bar, and the compound again remained in this dimorph on progressively reducing the pressure to 1 bar. After conversion to dimorph L by warming, then cooling to 275 K and repeating the procedure, the switch to dimorph H took place just below 700 bar, with the compound staying in this polymorph on releasing the excess pressure. The sample was next warmed to 303 K, when it reverted to dimorph L, and the pressure was progressively increased to 500 bar. A switch to dimorph H took place at 170 bar, but on progressively reducing the pressure a smooth change to dimorph L occurred between 130 and 30 bar. In this case, unit cell parameters intermediate between those of dimorphs L and H were observed at the intermediate pressure of 100 bar. On warming to 307 K and raising the pressure progressively to 350 bar the structural change occurred between ∼130 bar and ∼230 bar, and on lowering the pressure the reverse change occurred over the same pressure range, i.e., no hysteresis was observed. At 315 and 325 K, the unit cell parameters changed smoothly from those characteristic of dimorph L toward those of dimorph H as the pressure was increased, with the change being reversed at the same pressures upon progressively removing the pressure. The dependence of the pressure switch upon temperature is illustrated by the behavior of the unit cell length c at various temperatures in Figure 2.
Deuterated Ammonium Copper Tutton Salt
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Figure 2. Pressure dependence of the unit cell c-axis at various temperatures. Pressure was applied to samples initially in dimorph L. Note the hysteresis loop for the 303 K data and that at temperatures of 298 K and lower the samples remain in the H phase even after the applied pressure was decreased to zero. Data at 50, 230, and 260 K are not shown for clarity.
Figure 5. Variation of the unit cell lengths of a sample of (ND4)2[Cu(D2O)6](SO4)2 produced in the metastable dimorph H as this is heated from 290 to 305 K at 1 bar.
parameter exponential decay of the form
P ) P0 + A exp(-BT)
Figure 3. Plot of unit cell volumes vs pressure at various temperatures from 50 to 325 K. Data at 275, 298, 307, and 315 K were omitted for clarity.
Figure 4. Dependence of the pressure which must be applied to switch the unit cell of (ND4)2[Cu(D2O)6](SO4)2 from dimorph L to dimorph H upon temperature. The pressure is the midpoint of the transition shown in Figure 2. The line is a three parameter exponential decay fit.
The change in unit cell volume vs pressure at various temperatures is presented in Figure 3. Included in this figure are data for T ) 260 K, which exhibits a phase transition at slightly above 1100 bar. No transition was observed for temperatures of 230 and 50 K up to 2 kbar, the maximum obtainable with the apparatus. The pressure needed to switch from dimorph L to dimorph H increases smoothly as the temperature is decreased from 307 to 260 K (Figure 4). The behavior agrees well with that deduced from the pressure dependence of the EPR spectrum,9 which showed that further cooling to 150 K causes a progressive increase in the pressure needed to cause the structural change to 4.5 kbar. The P-T data in Figure 4 were fit to a three
where P0 ) -102 bar, A ) 8.86 × 106 bar, and B ) 0.0341 bar/K. However, at lower temperatures from EPR measurements, there appears to be a linear rather than exponential dependence of the switching pressure versus pressure in the range of 150-295 K.9 Below 307 K, the pressure at which dimorph H changes to dimorph L as the pressure is released was lower than that when the pressure is raised, i.e., a pressure hysteresis occurred (Figure 2), the behavior at 303 K being similar to that observed previously at ambient room temperature.8 When the temperature was less than ∼298 K the sample remained “frozen” in dimorph H when the pressure returned to 1 bar. The sample was switched to dimorph H by applying pressure at room temperature. It was then cooled to 290 K, and the pressure was reduced to 1 bar to produce the compound in the metastable dimorph H. The unit cell parameters were monitored as the temperature was raised very slowly, and the sample changed smoothly and progressively from dimorph H to L between 296 and 298.5 K (Figure 5). As reported by others,2,5,12 a further examination of Figure 3 reveals that for temperatures of 230 K and above with the sample in dimorph L (low pressure), at constant pressure the unit cell volume decreases with increasing temperature. This observed negative temperature coefficient of expansion (-0.027 Å3/K) is exhibited in Figure 6 for unit cell volumes at zero applied pressure. This is apparently due to increasing dynamic population of dimorph H at higher temperatures. Qualitatively, one reason the pressure required to change the structure increases as the temperature is lowered probably lies in the thermal equilibrium between the two polymorphs which develops at higher temperatures. At room temperature about one-third of a crystal of the deuterated ammonium Tutton salt is
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Schultz et al. Table 2. Enthalpies of the Phase Transition Based on the Clapeyron Equation T, K
∆V, Å3
dP/dT, bar K-1
∆H, cm-1
260 275 285 298 303
-4.65 -4.24 -2.81 -2.15 -1.92
-42.6 -25.6 -18.2 -11.7 -9.8
259 150 73 38 29
Table 3. Compressibilitiesa
Figure 6. Plot of unit cell volume vs temperature at zero applied pressure. Note the negative thermal expansion (solid line with a slope of -0.027 Å3/K) above 230 K.
already in dimorph H because of this thermal equilibrium.5 This means that the change in structure induced by pressure is much smaller than at low temperature, when virtually none of the crystal is in this polymorph. Similar arguments apply to the pressure hysteresis. This hysteresis represents the energy barrier to be overcome when the change in structure occurs. This will decrease with the development of the thermal equilibrium between the two polymorphs, explaining why the hysteresis increases as the temperature is lowered. However, there appears to be no simple relationship between the pressures at which the switch occurs and the extent of the thermal equilibrium at any temperature. The thermal equilibrium develops rapidly above ∼250 K,5 while the pressure needed to cause the structural switch falls exponentially as the temperature rises from 150 to 300 K. It should be noted that the structural change is quite complicated, involving not only the Jahn-Teller distortion axis of the complexes, but also the hydrogen bonding network of the surrounding lattice.3,5 A sharp discontinuity and a hysteresis in the unit cell volume with the application of pressure are characteristics of a first-order phase transition. For this type of structural phase transition, the enthalpy or latent heat ∆H of the transformation can be obtained from the Clapeyron equation:13-15
dP ∆H ) dT T∆V In our case, the transitions show appreciable deviation from an ideal first order transition, especially at the higher temperatures where the transition becomes smoother rather than sharp. This is more appropriately characteristic of an order-disorder transition,15,16 which in the case of (ND4)2[Cu(D2O)6](SO4)2 involves thermal occupation of the metastable Jahn-Teller distortion. Herbstein discusses application of the Clapeyron equation to adamantine and C60, which are described as deviating from first-order and exhibiting some characteristics of second-order transitions.15 Thus, it appears that application of the Clapeyron equation to the data presented in this paper is justified. Values for dP/dT were obtained from the exponential decay fit to the data points in Figure 4 and values for ∆V where estimated from data shown in Figure 3. These are tabulated in Table 2 with their associated temperatures and the calculated ∆H values. The enthalpy values range from 29 cm-1 at slightly above room temperature to 259 cm-1
a
temp, K
dimorph L
dimorph H
50 230 260 275 285 298 303
-0.0025 -0.0036 -0.0042 -0.0049 -0.0060 -0.0074 -0.0061
-0.0027 b -0.0043 -0.0022 -0.0044
Å3/bar. b Transforms, but sufficient data not available.
at 260 K. For comparison, values ranging from 184 to 250 cm-1 for the energy difference, ∆E, between the two Jahn-Teller minima, have been reported in the literature.3,5,17 Moreover, the dramatic fall in the energy difference as the temperature increases mirrors that deduced from the analysis of the average bond lengths and g-values.5,9 It has been suggested that this may be caused by cooperative interactions between the copper(II) complexes.5 Compressibility data at different temperatures are tabulated in Table 3. In all cases, the data were fit to a linear equation of unit cell volume versus pressure. It is seen that after switching from dimorph L to H, the compressibilities are lower for dimorph H compared to dimorph L. Previous investigations of the structural switch found no evidence of any intermediate between the two polymorphs.8-10,18,19 In the EPR study, spectra characteristic of both polymorphs were observed as the sample changed phase at 138 K.9 However, in the present study, unit cell parameters intermediate between those of the two polymorphs occurred for the pressure-induced changes in structure at higher temperatures (Figure 2). As the temperature was increased from 303 to 315 K the change altered from a rather sharp “switch” to a gradual shift in the unit cell parameters from those characteristic of one polymorph to those of the other. The unit cell parameters also changed smoothly from those of dimorph H to dimorph L as the temperature increased from 296 and 298.5 K at 1 bar (Figure 5). It is interesting to speculate on the nature of the crystal structure when the unit cell parameters lie midway between those of the two limiting polymorphs. While this presumably represents a situation when the JahnTeller distortion occurs with equal probability to two different pairs of water molecules, this may not occur randomly. On the basis of synchrotron X-ray peak analyses, Figgis et al. present evidence for domain separation at 12 K.12 It has been suggested that cooperative interactions between the complexes may cause the formation of ordered domains, though the position of these fluctuates rapidly at higher temperatures.5 Midway through the phase change, half of these domains would have a packing arrangement corresponding to dimorph L, the other half to dimorph H, and the present results show that they must be too
Deuterated Ammonium Copper Tutton Salt
small to produce a diffraction pattern. However, this aspect remains speculative at present, and is hoped to clarify it by studying a crystal as it changes from the metastable dimorph H to L. We can speculate that the equilibrium involves a Boltzmann distribution of the two Jahn-Teller distortions which are correlated cooperatively with other minor changes in the lattice. In dimorph L, there exist domains of dimorph H which fluctuate rapidly in position. At some critical pressure, corresponding to a critical concentration of domains of dimorph H, the crystal switches from one of mostly dimorph L to one of mostly dimorph H. The pressure required at lower temperatures (260 K) is greater than at higher temperatures (303 K) because of the larger ∆V and the small compressibilities (Table 3) at the lower temperatures. Acknowledgment. The work at Argonne National Laboratory was supported by the U.S. Department of Energy, Office of Science, under Contract W-31-109ENG-38. MAH and HS acknowledge the financial support of the Australian Research Commission. References (1) Tutton, A. E. H. Crystalline Structure and Chemical Constitution; Macmillan: London, 1910. (2) Hathaway, B. J.; Hewat, A. W. J. Solid State Chem. 1984, 51, 364-375. (3) Simmons, C. J.; Hitchman, M. A.; Stratemeier, H.; Schultz, A. J. J. Am. Chem. Soc. 1993, 115, 11304-11311. (4) Cotton, F. A.; Daniels, L. M.; Murillo, C. M.; Quesada, J. F. Inorg. Chem. 1993, 32, 4861-4867. (5) Hitchman, M. A.; Maaskant, W.; van der Plas, J.; Simmons, C. J.; Stratemeier, H. J. Am. Chem. Soc. 1999, 121, 14881501. (6) Falvello, L. R. J. Chem. Soc., Dalton Trans. 1997, 44634475.
Crystal Growth & Design, Vol. 3, No. 3, 2003 407 (7) Chen, Z.; Fei, S.; Strauss, H. L. J. Am. Chem. Soc. 1998, 120, 8789-8796. (8) Schultz, A. J.; Hitchman, M. A.; Jorgensen, J. D.; Lukin, S.; Radaelli, P. G.; Simmons, C. J.; Stratemeier, H. Inorg. Chem. 1997, 36, 3382-3385. (9) Augustyniak, M. A.; Krupski, M. Chem. Phys. Lett. 1999, 311, 126-130. (10) Henning, R. W.; Schultz, A. J.; Hitchman, M. A.; Kelly, G.; Astley, T. Inorg. Chem. 2000, 39, 765-769. (11) Henning, R. W.; Schultz, A. J.; Thieu, V.; Halpern, Y. J. Phys. Chem. A 2000, 104, 5066-5071. (12) Figgis, B. N.; Reynolds, P. A.; Hanson, J. C.; Mutikainen, I. Phys. Rev. B 1993, 48, 13372-13377. (13) Dolino, G. In Structural and Magnetic Phase Transitions in Minerals; Ghose, S., Coey, J. M. D., Salje, E., Eds.; Springer-Verlag: New York, 1988; pp 17-38. (14) McKie, D.; McKie, C. Crystalline Solids; John Wiley & Sons: New York, 1974; pp 474-483. (15) Herbstein, F. H. J. Mol. Struct. 1996, 374, 111-128. (16) Thorn, R. J. J. Chem. Thermodyn. 2002, 34, 973-985. (17) Iversen, B. B.; Larsen, F. K.; Reynolds, P. A.; Figgis, B. N. Acta Chem. Scand. 1994, 48, 800-809. (18) Rauw, W.; Ahsbahs, H.; Hitchman, M. A.; Lukin, S.; Reinen, D.; Schultz, A. J.; Simmons, C. J.; Stratemeier, H. Inorg. Chem. 1996, 35, 1902-1911. (19) Simmons, C.; Hitchman, M. A.; Stratemeier, H.; Astley, T. Inorg. Chem. 2000, 39, 4651-4653. (20) Robinson, D. J.; Kennard, C. H. L. Cryst. Struct. Commun. 1972, 1, 185-188. (21) Van der Zee, J. J.; Shields, K. G.; Graham, A. J.; Kennard, C. H. L. Cryst. Struct. Commun. 1972, 1, 367-369. (22) Smith, G.; Moore, F. H.; Kennard, C. H. L. Cryst. Struct. Comm. 1975, 4, 407-412. (23) Hathaway, B. J. Struct. Bonding 1984, 57, 55-118. (24) Brown, G. M.; Chidambaram, R. Acta Crystallogr. B 1969, 25, 676-687. (25) Whitnall, J.; Kennard, C. H. L.; Nimmo, J. K.; Moore, F. H. Cryst. Struct. Comm. 1975, 4, 709-712.
CG025611C
