W.Kaminsky, U.Bismayer: The Faraday effect near the ferroelastic phase transition of lead phosphate, Pb3(PO4)2. Phase Transitions 46 (1993) 41-46.
The Faraday effect and the absolute variation of the index ellipsoid of Pb3(PO4)2 were studied between 415K and 460K near the ferroelectric transition point (453 K). The experimental findings are in good agreement with the tree-component order-parameter model leading to a nearly tricritical transition. A significant anomaly in the Faraday rotation and the birefringence (n3'-n1) near 430 K is observed indicating a different temperature evolution of the excess volume compared with the symmetry breaking parameters which follow (n2-n1).