High-precision refractive index measurements revealing order parameter fluctuations in KMnF3 and NiO

The temperature dependence of the refractive index n of cubic crystals undergoing phase transitions (PT) is measured with an accuracy of the order δn=10−6 at temperatures between 3.5 and 1000 K with a computer-controlled Jamin–Lebedeff interferometer. Microscopic monitoring for in situ sample orientation and single domain work is provided. Results are presented for KMnF3 (antiferrodistortive PT) and NiO (antiferromagnetic PT) and discussed within the framework of a generalized phenomenological indicatrix perturbation theory. It predicts the proportionality between the order parameter autocorrelation function 〈δη2〉 and the refractive index anomaly preceding the PT. Comparison with recent theoretical predictions is performed.