J. Phys. Chem. A 2007, 111, 5813-5819
5813
Vibrational Analysis Study of Aluminum Trifluoride Phases
Udo Gross,† Stephan Ru1diger,† Erhard Kemnitz,*,† Klaus-Werner Brzezinka,‡
Sanghamitra Mukhopadhyay,§ Christine Bailey,| Adrian Wander,| and Nicholas Harrison*,§,|
Institute of Chemistry, Humboldt UniVersity, Brook-Taylor-Strasse 2, 12489 Berlin, Germany, Federal Institute
of Materials Research and Testing, 12489 Berlin, Germany, Department of Chemistry, Imperial College
London, Exhibition Road, London, SW7 2AZ, U.K., and Computational Science and Engineering Department,
CCLRC Daresbury Laboratory, Daresbury, Warrington, Cheshire, WA4 4AD, U.K.
ReceiVed: March 27, 2007; In Final Form: May 3, 2007
The vibrational modes of three solid AlF3 phases (R, â, and amorphous high surface area AlF3) are investigated.
Calculations have been performed using hybrid exchange correlation functionals to determine the equilibrium
geometries and Γ-point phonon frequencies for the R-AlF3 and â-AlF3 phases. The calculated optical modes
are in excellent agreement with experiment. The IR absorption of the amorphous, glasslike high surface area
(HS)-AlF3 is also discussed. Deconvolution of the broad envelope of IR stretches and bending vibrations
identifies the components of the observed broad band. From the IR vibrational spectrum it has been shown
that both short-range and medium-range disorder are present within HS-AlF3. Structural phase transitions are
identified by their phase transition temperature Tc, measured by thermal analysis.
1. Introduction
of R-AlF3‚3H2O under a self-generating atmosphere. Heating
to 400 °C gave a powdered material without single crystals.5
In studies of structurally distinct aluminum trifluoride phases
such as crystalline R, â, and the highly disordered high surface
area (HS)-AlF3, the degree of structural disorder is clearly
evident from X-ray diffraction, 27Al MAS NMR, and scanning
electron microscopic (SEM) images as well as from adsorption/
desorption isotherms performed by the BET-N2 method.1,2 From
the combined analysis of the vibrational spectra of both IR and
Raman experiments, we hope to gain an understanding of the
near range order of distorted aluminum fluoride phases,
especially of the amorphous fluoride sample. The hybrid
exchange approximation to density functional theory has been
used to calculate the equilibrium geometries and Γ-point phonon
frequencies of the R- and â-phases of AlF3. Consequently, we
can assign the observed vibrational modes of the crystal to
particular motions of its constituent octahedra. Although the
vibrational spectrum of a matrix-isolated AlF3 molecule with
trigonal planar structure is well-known,3 an assignment of the
vibrational modes of solid AlF3 has not previously been made.
As expected, the IR and Raman data from a crystalline solid
Amorphous HS-AlF3 was obtained by sol-gel fluorination and
post-fluorination with CHClF2 according to ref 2. The material
is mesoporous with a surface area of 309 m2/g. The carbon
content is about 0.5%. Fourier transform infrared (FT-IR) spectra
were recorded of CsI and KBr disks on a Perkin-Elmer 2000
spectrometer in transmission mode. A 1.5 mg sample was
ground with 200 mg of CsI or KBr and pressed. Spectra were
measured in the wavenumber ranges of 200-700 and 400-
4000 cm-1 at room temperature. Deconvolution of IR bands
was achieved by fitting of superimposed envelopes with Origin
software program.
Thermal analysis was carried out with a Netzsch thermoana-
lyzer STA 409 C Skimmer system. A 40 mg sample was heated
in a platinum crucible in a nitrogen gas flow up to 800 °C with
a constant heating rate of 10 K/min. The phase transition thermal
hysteresis was monitored by the reverse heating-cooling mode.
Laser Raman spectra were recorded from powder samples
as well as from a single crystal with an IFS 66v FT-IR
spectrometer along with a FRA 106 Raman device (Bruker,
Ettlingen, Germany) with excitation by a Nd:YAG laser (DPY
421, Adlas, Lu¨beck, Germany) at 1064 nm with power levels
up to 500 mW, and with a Dilor XY Raman spectrometer (Dilor,
Bensheim, Germany) equipped with a nitrogen cooled CCD
camera as a detector coupled to a BH-2 microscope (Olympus,
Hamburg, Germany) in micromode technique (180° backscatter
geometry, objective 50×) with a 514.5 nm excitation line of
an ILA 120 argon ion laser (Carl Zeiss, Jena, Germany) with
power levels of 10-25 mW into the entrance optics, corre-
sponding to an irradiation density at the sample surface of about
are quite different from those of the hexafluoroaluminate ion,
3- 4
AlF6
.
2. Experimental Section
The aluminum fluoride phases were either a commercial
product in the case of R-AlF3 (Acros, 99.9%) or prepared
according to standard procedures and checked by X-ray dif-
fraction (XRD). Additional, R-AlF3 was prepared from
NH4AlF4 by thermal decomposition at 700 °C in flowing
nitrogen. Single crystals of 0.2 × 0.1 mm size were grown from
the vapor phase. â-AlF3 was prepared by thermal decomposition
0.1-0.25 mW µm-2
.
* Corresponding authors. Phone: +493020937555 (E.K.). Fax:
+493020937277 (E.K.); +441925603634 (N.H.). E-mail: erhard.kemnitz@
chemie.hu-berlin.de (E.K.); nicholas.harrison@imperial.ac.uk (N.H.).
† Humboldt University.
3. Theory
Theoretical calculations of the equilibrium structure and
vibrational modes of R-AlF3 and â-AlF3 were carried out using
hybrid exchange density functional theory as implemented
within the CRYSTAL program.12,13 The Bloch orbitals were
‡ Federal Institute of Materials Research and Testing.
§ Imperial College London.
| CCLRC Daresbury Laboratory.
10.1021/jp072388r CCC: $37.00 © 2007 American Chemical Society
Published on Web 06/14/2007