Fourier Transform Infrared (FTIR) Measurement On Lewis Base Intercalates Of Iron Oxychloride

AbstractFourier transform infrared spectroscopy is shown to be a useful technique for structural studies on intercalates of iron oxychloride. Mid-infrared measurement yields information regarding the orientation of the guest molecule in the host lattice. Far- infrared studies reveal the effect of the guest molecule on the iron oxychloride lattice.IntroductionIntercalation compounds, consisting of organic molecules inserted into the layers of inorganic matrices, are becoming increasingly important due to their unique electrical, magnetic and chemical properties. (-1-) The orientation of the "guest" molecule as well as its effect on the host lattice has been the subject of spectroscopic investigation since the properties of these compounds are directly related to their molecular structure. Moreover, the physical properties of intercalate compounds are often temperature dependent. Of particular interest in this study are the Lewis base intercalates of iron oxychloride (FeOCl). We use Fourier transform infrared spectro­ scopy (FTIR) in the spectral range 100 to 4000 cm"-*- to study the structure of these compounds.ExperimentalFTIR measurements were obtained using the IBM Instruments, Inc. IR/98 vacuum spectrometer. Mid-infrared spectra were obtained using a globar source, Ge coated KBr beamsplitter, and DTGS detector. Samples were prepared as KBr pellets. Far-infrared spectra were obtained using a mercury arc or globar source, 8 line/mm metal mesh, or 6 x/ mylar beamsplitter, and DTGS detector. Samples were prepared as nujol mulls. Variable temperature experiments were carried out using an Oxford Industries cryostat system, adapted to the vacuum spectrometer. The 2 or 4 cm"1 resolution spectra are the result of 256 co-added interferograms.Iron oxychloride and the intercalate compounds were prepared by methods previously discussed in the literature. ^ 'Results and di'scussionIron (III) oxychloride is a layered compound belonging to the orthorhombic space group(Die) with two formula units per unit cell. (3,4) single crystal X-ray measure­ ments reveal a stack-like structure consisting of double layer sheets of cis FeCl204 octahedra joined together with shared edges. Chlorine atoms lie in a plane at the edge of each layer and the interaction between planes, assumed to be van der Waals type, is quite weak. It is between these planes that molecules can intercalate with a subsequent increase in the plane-to-plane (b-axis) distance.One of the consequences of this structure is that for both virgin FeOCl and the intercalates, there exist inter-layer vibrations across the van der Waals gap which should be infrared active in the low frequency spectral region.Far-infrared measurement of FeOCl at 300°K reveals three bands at 113, 191, and 260 cm"1. The Lewis base intercalates of FeOCl exhibit a far infrared profile similar to that of the virgin compound, however, spectral shifts are observed. For example, the spectrum of FeOCl (pyridine) -^3 at a measurement temperature of 300°K exhibits bands at 115, 164, and 261 cm"1. Crystallographic measurements of FeOCl reveal tjiat the b oarystallographic axis (axis perpendicular to the FeOCl planes) expands from 7.9A to 11A upon intercalation. As the layers separate the energy of the interlayer vibration decreases which should be observed as red shifted bands in the far-infrared. The 191 cm"1 band in FeOCl is red shifted to 164 cm"1 in the pyridine intercalate and this