Abstract
The temperature dependence of the linear birefringence (LB), Δn c , is measured in the range 5 K ≲ T ≲ 500 K on samples of KFeF4, which originate from hydrothermal, flux and Bridgman growth techniques. Pronounced anomalies are found at the orthorhombic-orthorhomic phase transition at T c ∽ 400 K. It is weakly discontinuous with a near-tricritical exponent β ∽ 0.2. Weak anomalies near T i ∽ T c + (25 … 40 K) seem to indicate a transition into an intermediate incommensurate phase. Its XY-model character is reflected by the critical LB exponent β = 0.8 ± 0.1. A smooth LB anomaly below 200 K is due to 2-dimensional ferromagnetic spin-order.