Barium titanate, order-disorder

Barium titanate BaTi03 is usually considered as the prototype of compounds having a purely displacive ferroelectric phase transition, which exhibits a soft mode describable by an anharmonic phonon. The nonferro-electric compound NH4C1 is another example of compound with pure order-disorder phase transition, with two phases that differ from the ordering of the ammonium (NH/) cation in the unit cell. 

B. Zalar, V.V. Laguta, R. Blinc, NMR evidence for the coexistence of order-disorder and displacive components in barium titanate, Phys. Rev. Lett. 90 (2003) 037601 (1)-037601(4). 

BaTi409 shows a characteristic strong Raman band at 860 cm . This is absent for other barium titanates." Low-temperature Raman spectra have been reported for bismuth titanate (Bi2Tt40u) ceramics. There was evidence for an order-disorder transition at about 250 K." ... 

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