Observation of a birefringence anomaly at the 93-K phase transition in K2SeO4

The relative birefringence of K2SeO4 was measured by the rotating-analyzer method, over a temperature range from 140 to 80 K. We report here clear observation of anomalous birefringence at the incommensurate-ferroelectric phase transition at 93 K. The observation of anomalous birefringence at both the paraelectric-incommensurate and the incommensurate-ferroelectric phase transitions is consistent with low-frequency dielectric measurements and illustrates the complementary nature of these two measurements.