The fundamental lattice vibrations of the trigonal CdCl2 structure have been analyzed group theoretically. All k=0 optical modes are either infrared or Raman active, so a complete vibrational analysis is possible. Results of far-infrared measurements on crystal and powder samples using a Fourier-transform spectrometer together with single-crystal Raman spectra are presented for CdCl2, CdBr2, MnCl2, and CoCl2. Symmetry assignments of the observed frequencies are made on the basis of polarization behavior. Second-order infrared and Raman spectra are also presented; peaks in the two-phonon axial infrared absorption spectra are assigned to allowed fundamental combinations at k=0. The infrared absorption spectra exhibit additional features in the range 200–4000 cm−1 that are ascribed to impurities.
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Symmetry coordinates for the k=0 modes in CdCl2. The cartesian-coordinate subscripts 1, 2, and 3 refer to the two chlorine atoms and the cadmium atom, respectively. The coordinate system was chosen so that the z axis corresponds to the crystal c axis, and the x axis is one of the three twofold axes corresponding to the rotations C2′.
Representation of D3d
Symmetry coordinate
A1g
Q1 = z1−z2
A2u (acoustic)
Q2 = z1+z2+z3
A2u (optic)
Q3 = z1+z2−2z3
Eg
Q4 = x1−x2
Q5 = −(y1−y2)
Eu (acoustic)
Q6 = x1+x2+x3
Q7 = −(y1+y2+y3)
Eu (optic)
Q8 = x1+x2−2x3
Q9 = −(y1+y2−2y3)
Table III
Frequencies (cm−1), linewidths (cm−1), and assignments of first-order Raman and infrared lines observed in CdBr2 CdCl2, MnCl2, and Cocl2 at room temperature.
Symmetry coordinates for the k=0 modes in CdCl2. The cartesian-coordinate subscripts 1, 2, and 3 refer to the two chlorine atoms and the cadmium atom, respectively. The coordinate system was chosen so that the z axis corresponds to the crystal c axis, and the x axis is one of the three twofold axes corresponding to the rotations C2′.
Representation of D3d
Symmetry coordinate
A1g
Q1 = z1−z2
A2u (acoustic)
Q2 = z1+z2+z3
A2u (optic)
Q3 = z1+z2−2z3
Eg
Q4 = x1−x2
Q5 = −(y1−y2)
Eu (acoustic)
Q6 = x1+x2+x3
Q7 = −(y1+y2+y3)
Eu (optic)
Q8 = x1+x2−2x3
Q9 = −(y1+y2−2y3)
Table III
Frequencies (cm−1), linewidths (cm−1), and assignments of first-order Raman and infrared lines observed in CdBr2 CdCl2, MnCl2, and Cocl2 at room temperature.
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