Birefringence Imaging Measurements on Various Structural Phase Transitions in (CnH2n+1NH3)2MnCl4 with n = 1, 2, and 3 Using Multiple Wavelengths

Birefringence imaging microscopy is well suited for comprehensively investigating various structural phase transitions but its weak point is that it imposes an inherent limit on the maximum retardance that can be detected. To detect retardance above this limit, we performed simultaneous birefringence measurements at multiple wavelengths on (CnH2n+1NH3)2MnCl4 with \(n = 1,2,3\). For n = 1, we successfully detected large jumps in retardance (over six times greater than the limit) in the first-order phase transitions at Tc1 \( \simeq \) 88 K and Tc2 \( \simeq \) 251 K. For n = 2, a gradual change in retardance appears in the second-order phase transition at Tc = 214 K. High-definition images of optical fast-axis azimuths provided information on the misorientation of the alkylammonium chains. For n = 3, a retardance kink appears at Tc4 \( \simeq \) 152–155 K, and a retardance jump appears in the commensurate-incommensurate phase transition at Tc5 \( \simeq \) 96–99 K.