Optical properties of Sr₂Nb₂O₇ around the incommensurate phase transition: Refractive index, birefringence and absorption-edge

Abstract

The refractive index, birefringence and optical absorption-edge of Sr₂Nb₂O₇ were examined in the temperature range from room temperature to 350°C. The birefringence Δn_ac (= n_a - n_c ) exhibits two anomalies at the incommensurate phase transition temperature T_i = 216°C and at around 100°C. Similar behavior is seen in n_a vs. T curve, but n_b is almost constant against T in this temperature range. The absorption coefficient α increases exponentially with increasing photon energy in the range of 3·8 ∼ 4·0 eV. The absorption-edge tail broadens with increasing temperature according to the Urbach-rule and shows anomalous behavior at around T_i . Some discussions are made concerning to the incommensurate phase transition of this crystal.

Key Words:

• Sr₂Nb₂O₇
• refractive index
• birefringence
• optical absorption-edge
• incommensurate phase transition