Thermochromic phase transitions in two aromatic tetrachlorocuprates

Bis(para-toluidinium) 2 tetrachlorocuprate and bis(para-chloroanilinium) 2 tetrachlorocuprate crystallize in a perovskite-related layer structure. The former crystallizes in an orthorhombic unit cell with a = 6.911 Å, b = 7.052 Å and c = 33.182 Å. It undergoes a thermochromic first order phase transition from a yellow low temperature phase to a dark orange high temperature phase at T = 300 ± 3K with a 10° thermal hysteresis. The latter compound undergoes two thermochromic transitions expressed by the relation. Orange Phase (I) ⇄ 294 K Yellow Phase (II) ⇄ 214K Green Phase (III). Both compounds are ferromagnetic at low temperture with exchange interactions J / k = 17.5° and 20° for the two compounds respectively.