Ferroelastic domain structures of the low-temperature phase transition in TlH₂PO₄

The low-temperature II-III phase transition of TlH₂PO₄ (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 TlH₂PO₄ is not ferroic, but is nonferroic.