High-pressure structural anddielectric studies of the phase transitions in lithium thallium tartratemonohydrate

Abstract

We have investigated the structure of lithium thallium tartratemonohydrate (LiTlC4H4O6·H2O) single crystalby means of neutron diffraction at a hydrostatic pressure of120 MPa. Atomic positions including those of hydrogen atoms havebeen refined for the orthorhombic paraelectric phase(P21212) at 36 K and for the monoclinic ferroelectric(FE) phase (P2111) at 6 K. Moreover, our dielectricresults, confirmed by the temperature variation of themonoclinic splitting, provide evidence for the existence of anintermediate FE phase in the temperature range betweenTc' = 24 K and Tc = 34 K. The two FE phases are probablyisomorphous but differ in their degree of monoclinic distortionand in their value of the piezoelectric coefficients. Both phasetransition temperatures drop with decreasing hydrostaticpressure and only a single FE phase is observed below 11 K atambient pressure. The spontaneous polarization appears along thecrystallographic axis a and, according to our structure data,it can be mainly attributed to displacements of the Li atomsaccompanied by a realignment of crystal-watermolecules.