Far infrared spectra of hexachalcogenohypodiphosphates

The far infrared (fir) reflection spectra of the hexachalcogenohypodiphosphates M₂P₂X₆ (M = Mg, Cd, Mn, Fe, Co, Ni, Pb, Sn; X = S, Se) have been studied. $\text{TO}\text{LO}$ splittings are determined from Kramers-Kronig analyses, the splitting is large for the lattice vibration modes of ferroelectric Sn₂P₂S₆ but very small for Ni₂P₂S₆. Measurements with polarized light permit an assignment of the A_2u and E_u modes of the ethane like P₂S₆ units in Mn₂P₂S₆. Factor group analyses were performed for all hitherto described hexachalcogenohypodiphosphates. The fir absorption spectra of the less symmetric compounds Hg₂P₂S₆, Hg₂P₂Se₆, Ca₂P₂S₆, TiP₂S₆, $\text{In}{}_{\mathrm{<mtext>4</mtext><mtext>3</mtext>}}\text{P}{}_2\text{S}{}_6\text{, In}{}_{\mathrm{<mtext>4</mtext><mtext>3</mtext>}}\text{P}{}_2\text{Se}{}_6$, and CuCrP₂S₆ are given.