Study of the Ferroelastic Phase Transition of TlH_2PO_4 Acoustic Phonon Softening

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Elastic, thermal and dielectric measurements of T]H.P0. were carried out in a fernperature range covering the upper phase transition point at 357 K. The specific heatshows a 1-type anomaly associated with the upper phase transition of second orderand yields the transition entropy J5= 1.46 J / mo]?K. The elastic stiffness c, amongc, (i= 1, 2, - I I , 6) determined from the ultrasonic velocity data at a frequency of 10MHz shows a large anomaly with a tendency to approach zero in value at the upperphase transition point. It was shown that the upper, ferroelastic phase transition iscaused by an instability of the B.. transverse acoustic mode, and the elastic, thermaland dielectric behaviours near the ferroelastic phase transition point can bereasonably explained by a phenomenological theory.[thaIIiumther- vj mat 'Vspon- jl taneousldihydrogen phosphate, T]H.P0., ferroelastic, dielectric constant,expansuon, specufic heat, ultrasonic velocity, soft acoustuc mode,stramn, phase transition
社団法人日本物理学会の論文
1991-01-15