An infrared spectroscopic study of the incommensurate transitions and related phase changes in $K_2Pb[Cu(NO_2)_2]$ and $[N(CH_3)_4]_2MX_4$ (M = Mn, Co, Cu or Zn and X = Cl or Br)

$K_2Pb[Cu(NO_2)_6$] and $N(CH_3)_4]_2MX_4$ (M = Mn, Co, Cu or Zn and X = Cl or Br) undergo phase transitions which involve incommensurate phases. The transitions have been investigated by examining the changes in the $NO_2$ and $CH_3$ vibration bands in the i.r. spectra. Splitting and broadening of some of the bands across the incommensurate transitions are discussed in the context of geometrical restrictions in the incommensurate phases. The phase transitions have also been characterized using differential scanning calorimetry