J. Phys. Chem. B 1997, 101, 2811-2814
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NMR Study of Phase Transitions in Guest-Free Silica Clathrate Melanophlogite Shuangxi Liu,† Mark D. Welch,‡ and Jacek Klinowski*,† Department of Chemistry, UniVersity of Cambridge, Lensfield Road, Cambridge CB2 1EW, U.K., and Department of Mineralogy, The Natural History Museum, Cromwell Road, London SW7 5BD, U.K. ReceiVed: NoVember 19, 1996; In Final Form: February 3, 1997X
Reversible phase transitions in the silica clathrate melanophlogite have been studied by powder X-ray diffraction and 29Si magic-angle-spinning (MAS) NMR on a sample previously heated to 950 °C in order to remove volatile guest molecules. At 200 °C the 29Si MAS NMR spectrum contains three peaks in the 12:8:3 intensity ratio corresponding the cubic Pm3n polymorph. On gradual cooling, systematic peak splitting is observed, indicating the presence of cubic-cubic and cubic-orthorhombic phase transitions at ca. 140 °C and ca. 60 °C, respectively. A powder X-ray diffraction study of the thermal evolution of the cubic unit cell parameter suggests that the phase transitions are second-order. The possible symmetry breaking involved and the nature of the room temperature polymorph are examined by considering supergroup-subgroup relations for space group Pm3n in relation to 29Si MAS NMR spectra. The analysis suggests Pm3n T Pm3 T Pmmm symmetry breaking for the 140 °C and 60 °C transitions, respectively. The spectra suggest the splitting of the 512 cages in the Pm3 melanophlogite structure into two crystallographically distinct types located at 000 and 1/21/21/2.
Introduction The silica clathrate melanophlogite, 46SiO2‚6M14‚2M12 (M12 ) CH4, N2; M14 ) CO2, N2), is a rare naturally-occurring silica polymorph which contains up to 8% C, H, O, N, and S.1 Melanophlogite is isostructural with the cubic gas hydrates of type I.2 Corner-sharing SiO4 tetrahedra form a three-dimensional framework containing two types of cages: two pentagondodecahedra (V ≈ 97 Å3) and six tetrakaidecahedra (V ≈ 136 Å3) per unit cell. The guest molecules can be completely removed by heating to above 600 °C. For brevity, guest-bearing and guest-free melanophlogite will be referred to as GBmelanophlogite and GF-melanophlogite, respectively. Melanophlogite has a very high framework density of 18.9 TO4 tetrahedra per 1000 Å3 and is of considerable interest in connection with the problem of the storage of radioactive gases, especially since it can be synthesized hydrothermally.3 On cooling, melanophlogite is known to undergo a reversible phase transition from cubic to tetragonal at temperatures of 3080 °C. The transition temperature Tc depends upon the type and relative populations of guest molecules. However, it is not clear whether or not guest molecules actually cause the phase transition or merely influence the value of Tc. Displacive phase transitions occur in other silica polymorphs (e.g. high-low transitions in quartz, tridymite, and cristobalite) and involve framework distortions primarily effected by rigid rotations of SiO4 tetrahedra. Studies of displacive phase transitions in GFmelanophlogite provide a valuable reference point for exploring the energetics of interactions between guest molecules and the tetrahedral framework. The cubic high-temperature polymorph of melanophlogite has space group Pm3n.4,5 This structure has three distinct Si sites with symmetries 4m2, 3, and m, and multiplicities of 6, 16, and 24, respectively (Z ) 46). However, the nature of the * Corresponding author. Address: Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, U.K. Telephone: +(44)-1223-33 65 14. FAX: +(44)-1223-33 63 62. E-mail: jk18@ cam.ac.uk. † University of Cambridge. ‡ The Natural History Museum. X Abstract published in AdVance ACS Abstracts, March 15, 1997.
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tetragonal low-temperature phase is not well-understood, mainly because transformation twinning is always present and makes single-crystal structure determination very difficult. To our knowledge, no such study of the tetragonal phase has been performed. Zˇ a´k6 recognized systematic absences and superlattice reflections in Weissenberg photographs which suggested a supercell with space group P42/nbc and aT ) 2aC (Z ) 184). He also proposed that at room temperature, the melanophlogite previously heated at 1050 °C for 12 h to remove guest molecules has space group P4232 and no superstructure (a ) 13.4 Å). Fyfe and Gies7 measured the 29Si magic-angle-spinning (MAS) NMR spectra with cross-polarization (CP) of Sicilian melanophlogite at 90 °C and -10 °C. The 90 °C spectrum comprises three peaks at -118.2, -119.8, and -121.4 ppm in the intensity ratio 12:8:3, corresponding exactly to the site multiplicities of the Pm3n phase (24:16:6). The -10 °C spectrum consists of at least four peaks at approximately -117.5, -118, -120, and -121 ppm, although these are not wellresolved. Fyfe and Gies also found that the -15 °C 29Si CP/ MAS spectrum of a synthetic melanophlogite with methylamine guests has at least four peaks, again not well-resolved. No detailed NMR studies of the phase transition have been made. We report the results of NMR, optical, and diffraction studies of the phase transition in GF-melanophlogite from room temperature to 200 °C and demonstrate the existence of two transitions: a previously unrecognized cubic-cubic transition at ca. 140 °C and a cubic-tetragonal transition at ca. 60 °C. Experimental Section Sample Characterization and Preparation. A sample of melanophlogite from Livorno, Italy, was obtained from the Natural History Museum (London), specimen number BM1973.116. In this specimen melanophlogite occurs as colorless spherules coating sulfur crystals. The spherules were easily detached, carefully cleaned, and examined optically for inclusions, but none was observed. Light-element analysis indicated the presence of C, N and H. Electron microprobe analysis for elements heavier than O confirmed that the only element present was Si. Infrared spectroscopy indicated the © 1997 American Chemical Society
2812 J. Phys. Chem. B, Vol. 101, No. 15, 1997
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Figure 1. Thermal evolution of the 29Si MAS NMR spectra of GF-melanophlogite and their simulation using Gaussian lineshapes on cooling from 200 to 20 °C. The spectrum labeled “second cycle 20 °C” refers to an experiment in which the sample was heated again to 200 °C for 6 h and then cooled to 20 °C. The second 200 °C spectrum (not shown) is identical to the first and so are the two 20 °C spectra. The experiment demonstrates that the spectra are completely reproducible, and thus the phase transition is reversible.
presence of CdO, C-H, and O-H (water) bonds, indicating CO2, a hydrocarbon, and water guests. The sample is therefore a typical melanophlogite. The present study concerns GFmelanophlogite prepared by heating the natural melanophlogite for 6 h at 950 °C in a muffle furnace. Infrared spectroscopy indicated that all guest molecules had been removed. The absence of H was confirmed by 1H MAS NMR. 29Si MAS NMR. 29Si MAS NMR spectra were measured at 79.45 MHz using a Chemagnetics CMX-400 spectrometer operating at 9.4 T and a probehead with a zirconia rotor 7.5 mm in diameter driven by nitrogen gas and spinning at 4 kHz. A sample of solid sodium bromide was used to set the magic angle accurately using the 79Br NMR resonance. Single-pulse 29Si MAS NMR spectra were obtained with a long pulse delay (240 s) after a 30° pulse (2 µs) was used, and about 240-900 transitions were acquired. The chemical shift was measured using tetramethylsilane (TMS) as an external standard. To ensure thermal re-equilibration, a 30-min delay was used before the NMR data were collected in all experiments. Sample temperature was calibrated using the 207Pb resonance of solid lead(II) nitrate,8 and the accuracy of sample temperature control was (4 °C. The spectra were deconvoluted using Gaussian line shapes, a well-established procedure.9 We estimate the error with which the spectral intensities of the individual component lines (corresponding to populations of the various sites) were determined was less than 8% of each value. Powder X-ray Diffraction. Variations in cell parameters with temperature were studied by powder X-ray diffraction (XRD) at 20-230 °C using a Huber high-temperature Guinier
camera. Monochromatic Cu KR1 radiation was selected by a bent quartz monochromator. Diffraction patterns were recorded on single-sided emulsion film from a thin powdered sample held in a platinum wire loop in transmission geometry. Si powder was mixed with the sample to act as an internal 2θ standard, allowing film shrinkage corrections and zero-point corrections to be made for each exposure. The furnace stability was better than (1°, with a control thermocouple positioned
