Precise linear birefringence measurements were performed for two isomorphic
crystals: (CH3)(2)NH2H2AsO4 and (CH3)(2)NH2H2PO4. Anomalous cusp-like maxi
ms of linear birefringence increments, i.e. changes of birefringence caused
at the phase transition, were revealed in the ferroelectric phase. Dilatom
etric studies confirmed that these increments can be connected with similar
anomalies of spontaneous strain.