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[Masayoshi Mikami <mmikami@rc.m-kagaku.co.jp>]

Dear Whoming,

> > The paper says "the optical modes at Gamma are classified into
> > 2A1g + 2 Eg + 2A2u + 2Eu". It never says, "8 IR modes were
> > reported including 4 Eu and 4 Au modes" (This is your mental  
> > picture !)
> > Please cite correctly... :-(
>
> ... In your paper you state "The present calculation thus predicts the
> existence of eight IR modes for Y2O2S (Table I)." Page 2, Right  
> Column,
> Line 16 from the bottom. I mean that 4 frequencies labled Eu and 4  
> labled
> A2u in Table. I.

You should have cited the context. The IR spectrum (performed in 1970s)
indicates only six peaks, not eight. The "eight" intended to mean the  
number
of the IR peaks to be observed.

> But when I writting here, I know what the meaning of 8 IR-active  
> modes is:
> When E//a,b the IR-active modes are:
> A2u symmetry TO modes, 315.3 cm-1, 475.2 cm-1;
> Eu symmetry  TO modes, 235.7 cm-1, 485.2 cm-1;
>              LO modes, 266.3 cm-1, 599.6 cm-1;
> here totally 6 modes;
>
> When E//c, the IR-active modes are
> A2u symmetry LO modes, 360.9 cm-1, 587.4 cm-1;
> Eu  symmetry TO modes, 235.7 cm-1, 485.2 cm-1.
>
> Noticed that the Eu's TO modes having the same frequecies no matter  
> E//c
> or E//a,b, the total number of IR-active modes are 8?
>
> Am I right?

Yes, one mode of the Eu may remain the same frequency (TO)
even when q vector is perpendicular to c (// ab plane).
Please consult with textbooks to sketch LO-TO modes for this case ...
(Visualization of the phonon modes (with vectors) would help,
 from the phonon eigenmodes. Then, imagine "q" vector ... )

The rest issue (the numerical difference) should be explained
from PPs, geometry, etc. (as I pointed out before)
You may have chances to have better/worse agreement with
the experimental data.

That is all.
Masayoshi