Infrared and Raman spectra of oriented CdAl2Se4 single crystals and of as-g
rown CdAl2S4 single crystals have been measured in the wavenumber range fro
m 600 to 25 cm(-1). Both ordered vacancy compounds grown by the chemical va
pour transport method crystallize in the thiogallate structure with space g
roup I (4) over bar. All 13 optical phonon modes are Raman active; ten of t
hese are in addition infrared active. We report for the first time the comb
ined infrared and Raman study of the optical phonon modes of both compounds
. With the complementary information from infrared and Raman spectroscopy a
ll optical phonon modes have been identified and assigned to their correspo
nding symmetry classes. The fact that all five modes of E symmetry exhibit
a counterpart of B symmetry in the close spectral environment is unambiguou
s evidence of space group I (4) over bar. The measured frequencies of the n
onpolar modes are in a very good agreement with data calculated in the fram
ework of a simplified Keating model. For an oriented CdAl2Se4 single crysta
l the directional dispersion of the polar phonons has been studied.