Abstract
The low-temperature II-III phase transition of TlH 2 PO 4 (ferroelastic in phase II) at 230 K ( T c2 ) was studied by a polarizing microscope between 300 and 80 K. Despite the II-III phase transition at T c2 , the two kinds of ferroelastic domain structures with the (100) and/or (001) and (201) domain boundaries were still observed below T c2 and were nearly temperature-independent. No new domain structures corresponding to antiferroelectric or ferrielectric orientation states were observed in phase III. The possible intermediate structural phase transition near 130 K suggested by vibrational spectroscopic study [B. Pasquier et al .: Chem. Phys. 171 (1993) 203] was not detected. The transition at T c2 without optically detectable new domains indicates that the monoclinic crystal system of the phase II persists in phase III from 300 to 80 K and that the nature of the II-III phase transition of TlH 2 PO 4 is not ferroic, but is nonferroic.
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