PHASE-TRANSITION-INDUCED CHANGES IN OPTICAL-PROPERTIES OF FERROELECTRIC (LA,ND)P5O14

LaP5O14 and other pentaphosphates undergo a ferroelastic phase transition mmm-2/m. Optical index ellipsoid rotation, characterised by the angle phi , and birefringence Delta n31 have been measured as a function of temperature in the ferroelastic phases for LaP5O14 and La0.25Nd0.75P 5O14 single crystals. It is found that tan 2 phi is proportional to (Tc-T)beta, beta =0.5, within several degrees below Tc. At lower temperatures, significant departure from the simple Landau behaviour is observed. A theoretical expression for tan 2 phi has been obtained on the assumption based on published data that in LaP 5O14 a bare optical mode B2g softens and causes the crystal to become unstable against a mixed, predominantly acoustic mode. It is shown that the elastooptic coefficient p55 may be temperature dependent in the parent phase. Below Tc in the linear approximation, tan 2 phi approximately p55(Ic)u5s. The value of mod p55(Tc) mod is estimated to lie between 0.145 and 0.16.