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J. Phys. Chem. B 2003, 107, 2892-2896
Raman Spectroscopy of Rhombohedral P4O10 Sean J. Gilliam, Scott J. Kirkby, and Clifton N. Merrow* Department of Chemistry, UniVersity of Missouri-Rolla, Rolla, Missouri 65409
Daniel Zeroka Department of Chemistry, Lehigh UniVersity, Bethlehem, PennsylVania 18015
Ajit Banerjee and James O. Jensen US Army Edgewood Chemical and Biological Center, Aberdeen ProVing Ground, Maryland 21010 ReceiVed: December 30, 2002
A complete Raman vibrational analysis has been performed on phase-pure rhombohedral phosphorus pentoxide (P4O10) at room temperature. The “molecular” Td symmetry is reduced to C3V symmetry when bound in the rhombohedral crystalline lattice. Therefore, the 15 fundamental modes of vibration irreducibly represented as Γvib ) 3A1 + 3E + 3T1 + 6T2 reduce to 21 fundamental modes with a factor group analysis resulting in Γvib ) 9A1 + 12E for the irreducible representation, all of which are Raman active. Complete vibrational assignments and relative intensities, including all T2 to E and A1 splittings, are reported. Five new E fundamentals (262, 329, 419, 829, and 846 cm-1) in the C3V symmetry were observed. All peaks show good agreement with theoretical calculations.
Introduction The group V oxides comprise a varied and diverse set of inorganic chemistry. In particular, the adamanantoid cage compounds (X4Y6-10, XdP, As, Sb; YdO, S) have been used extensively as “building blocks” and models for network solids.1-5 One interesting chemical characteristic of these adamanantoids is the extent to which their formally empty 3d orbitals participate in bonding.6 Previous Raman and infrared spectroscopic studies, performed on these and similar materials, have provided useful complementary structural information to that obtained from X-ray diffraction patterns. These studies have also been performed to characterize phase purity7,8 and investigate changes in structure as a function of the degree of modification.9 As such, the electronic structure and vibrational characteristics of these oxides in their vapor and solid forms are of interest. This work will focus entirely on the P4O10 adamanantoid. Recent interest in P4O10 is due, in part, to its use in the development of high-energy laser glasses,10 its application as a host material for the vitrification of spent nuclear wastes,11 and its similarity in structure to an emerging class of chemical warfare agents. As a solid, P4O10 exists as an amorphous glass, two orthorhombic phases, and a rhombohedral phase. The orthorhombic phases are two-dimensional layered structures consisting of joined rings. One, of space group Fdd2,12,13 has flat layers, whereas the other, of space group Pnma,13,14 has the layers puckered with the O atoms out of the plane of the rings. The rhombohedral crystalline phase of P4O10 is “molecular,” and belongs to the R3c (Z)2) space group.15-17 The resulting primitive unit cell contains two P4O10 adamantanoid molecules (P2O5 “dimers”, see Figure 1) situated on the C3V sites of the unit cell. * Corresponding author, E-mail:
[email protected].
Figure 1. Molecular modeling representation of the P4O10 “dimer” found in rhombohedral P2O5. The purple spheres represent the P atoms and the red spheres, the O atoms.
The expected vibrational modes, using factor group analysis, of isolated P4O10 molecules gives
Γvib ) 3A1 + 3E + 3T1 + 6T2
(1)
for the irreducible representation. However, in the crystal, the P4O10 molecules reside in sites with C3V rather than the isolated molecule’s inherent Td symmetry. The result is to reduce the symmetry of the molecule from Td to C3V. This is observable in the single-crystal X-ray diffraction structure which has three
10.1021/jp027854l CCC: $25.00 © 2003 American Chemical Society Published on Web 03/07/2003
Raman Spectroscopy of Rhombohedral P4O10
J. Phys. Chem. B, Vol. 107, No. 13, 2003 2893
identical terminal O-P bonds of 338 pm and a fourth at roughly 7% greater length.17 Thus, using the site symmetry of C3V results in a symmetry analysis for the vibrations of
Γvib ) 9A1 + 3A2 + 12E
(2)
for the internal representation. The A1 and E modes in this C3V symmetry group are both Raman and IR active, whereas the A2 modes are neither Raman nor IR active. Hence, there should be 21 (9A1 + 12E) fundamental modes in both the Raman and IR spectra. As would be expected, several studies of the Raman and infrared spectra do exist for P4O10. These may be divided into four distinct groups. The first is of gas-phase P4O10, a sphericaltop dimer with Td symmetry.18,19 The experimental IR20,21 and Raman22 studies are, however, incomplete with several ambiguities in the assignments involving the labeling of the A1 and T2 modes. Also, there are differences in the observed peaks of up to 27 cm-1 for some of the vibrations. Most importantly, two of the expected three E modes have not been observed. Data from recent theoretical studies21,23,24 have essentially resolved the ambiguities of the A1 and T2 assignments, and suggested possible wavelength positions for the two missing E bands. The second group focused on investigating P4O10 molecules that were isolated in Ar matrixes. Three of the studies in this group measured trapped P4O10 following the reaction of P2 + O2,25 P2 + O3,26 and P4O6 + O3.27 A fourth matrix isolation study, also in an Ar matrix, reported the laser ablation and photoinduced decomposition of an unspecified orthorhombic phase and the pure rhombohedral crystal.28 However, for all of these Ar matrix studies, none of the trapped (P4O10)n products exhibited any crystallinity and only the infrared spectra were measured. The third group encompasses the amorphous glass phases. Raman and infrared spectra have been reported for several forms of vitreous “P2O5,” and can usually be found reported in glass modification studies.29,30 In the last group, there are four reported studies, which may possibly contain measurements of the three crystalline phases of P4O10. The results of these reports were compiled and critically reviewed by Chapman.31 The compilation includes Chapman’s own experimental results and those of the earliest measurements by Gerding and de Decker in 1942,32 Sidorov and Sobolev in 1957,33 and Zipj in 1962.34 The results from the latter three are incomplete, and in some instances are either lacking or differ in their vibrational assignments. Nevertheless, Chapman attempted to reconcile these differences, change or added assignments where appropriate, and produced a “nearly” complete set of vibrational modes. In the Td symmetry used for the analysis of the reported Raman spectra, there were still two missing E vibrations (as in the gas phase), and a possible incorrect assigned T2. With respect to the particular crystalline phase used in Chapman’s work, no confirmation of a certain phase or phase purity analysis was reported in his study invoking P4O10 molecules. This lack of vibrational information for any of the pure crystalline forms of P4O10, and our general interest in using Raman and IR measurements to better understand the properties of the solid phases of the Group V oxides, formed the basis for undertaking this study. Thus, given that the rhombohedral phase is a molecular crystal, and would be expected to have a vibrational spectrum more closely related to the gas-phase spectrum, as compared
Figure 2. Powder X-ray diffraction pattern for the P4O10 sample used in this work. The sample has excellent crystallinity with only the rhombohedral phase detectable. Nor is there any evidence of a significant amount of amorphous phase present.
with the other phases, it was selected as a starting point for our studies. The objective then was to measure a complete set of Raman and infrared vibrations, including the missing E modes, and determine their relative intensities. Experimental Section The P4O10 was purchased from Aldrich (purity 99.998% metals basis), and checked for phase purity by obtaining an X-ray powder diffraction pattern. This diffraction pattern was recorded using a Scintag (now Thermo Arl) XDS-2000 diffractometer with a liquid nitrogen cooled solid-state germanium detector (EG&G Ortec, model number GLP-L0195107-S) and Cu KR radiation. The samples for Raman analysis were sealed in glass capillaries under a dry nitrogen (BOC Gas, 99.990%) atmosphere. The spectrometer was a SPEX model 1403 double monochromator with 0.85-m path lengths and 1800 gr/mm gratings. The monochromator’s exit slit was removed and a Princeton Instruments model NTE CCD array detector installed. This array is approximately 27 mm wide with 1340 × 100 pixels. Given the linear dispersion of the monochromator, each spectral segment or “window” was approximately 200 cm-1 wide. The excitation source consisted of a Coherent Inova 90-5 Argon Ion CW laser emitting at 514.5 nm with an output power of 1.6W. This output was transmitted through a SPEX plasma filter to remove the unwanted emission lines. Additionally, a Kaiser Optical Systems Holographic Super Notch-Plus Filter (part number HSPF-514.5-2.0) was used to reduce the intensity of the resonant laser scatter from the sample. The entrance slit to the monochromator was set to 50 µm. The integration time for the CCD array was 1.0 s with 20 scans coadded to form the final spectrum. The Raman shifts were calibrated using the ASTM E1840-96 standard values for naphthalene.35 The resolution of the collected spectra was less than 0.5 cm-1 with a reproducibility of
