Title: PHASE DIAGRAMS OF INCOMMENSURATE FERROELECTRIC NH_4HSeO_4: PHENOMENOLOGICAL TREATMENT
Author(s): A.V.Kityk, A.V.Zadorozhna (Institute of Physical Optics, 23 Dragomanov Str., 79005 Lviv, Ukraine)

The phenomenological analysis of the pressure-temperature ($P-T$) phase diagram of NH$_4$HSeO$_4$ crystals is presented. It is shown that the disagreement between the experimental results and the theory may be removed assuming that the coefficient of the Landau free-energy expansion $\kappa$ at the gradient term $(\d q/\d z)(\d q^*/\d z)$ changes the sign in the experimental range of pressures. According to the present model the triple point observed in NH$_4$HSeO$_4$ at $P_{\rm K} \approx$ 455 MPa, $T_{\rm K} \approx$ 236 K may be considered as artificial points which result from the limitation of experimental resolution. Therefore, even above $P_{\rm K}$ there still exist two very close (unresolved) lines of the incommensurate phase transitions.

Comments: Figs. 3, Refs. 9, Tabs. 0.