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The energies of IR active A 2u modes in both HfSe 2 and HfS 2 are not known, since it is experimentally difficult to perform IR spectroscopy measurements for E || c . The experimental results of our first and second order resonance Raman measurements show that A 2u (TO) = 180cm −1 and A 2u (LO) = 295cm −1 in HfS 2 ; in HfSe 2 we obtain A 2u (TO) = 120cm −1 and A 2u (LO) = 175cm −1 . Our results are consistent with those theoretically calculated by means of the polarizable ion model.
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Cingolani et al. (1988) studied this question.
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