A new birefringence study of thiourea

The relative linear birefringence of thiourea was measured using the rotating analyzer method in the temperature range from 250 K to 160 K. These measurements show a small repeatable anomaly in the vicinity of 178 K, where an electric field induced structural phase transition from an incommensurate phase to a commensurate l/8c* structure (Pnam setting) has been observed. This would tend to indicate that rather than being a field induced transition, this is an intrinsic lock-in transition to a commensurate state similar to the l/9c* transition at 171 K.