Infrared properties of $\theta$-type ET charge-transfer salts:   $\theta$-ET$_2$CsZn(SCN)$_4$ vs $\theta$-ET$_2$RbZn(SCN)$_4$
  We performed polarized far-infrared reflectance measurements at different temperatures on $\theta$-ET$_2$RbZn(SCN)$_4$ and $\theta$-ET$_2$CsZn(SCN)$_4$, two typical charge-transfer salts in a series of isostructural $\theta$-ET$_2$MM'(SCN)$_4$. The measurements revealed distinct differences in both electronic and vibronic spectra, which provides a clue towards understanding the different metal-insulator transitions of the two salts. 
N. L. Wang, T. Feng, Z. J. Chen, H. Mori