Below its ordering temperature at 105 K, perovskite-type SrTiO(3) exhibits a tetragonal phase with three different structural domains that are strongly influenced by the application of uniaxial mechanical stresses and electric fields. A careful neutron diffraction study of superlattice reflections provides full quantitative information about the varying domain distributions under external loads as a function of temperature. It is shown that electric field and uniaxial stress exhibit competitive effects and the simultaneous application leads to a complex redistribution behaviour of the tetragonal domains. The results are discussed in the context of the formation of a field induced ferroelectric phase at low temperatures. The experimental findings demonstrate that its polarization is always perpendicular to the tetragonal axis and the polar phase has orthorhombic symmetry.
SrTiO 3 is a well-studied perovskite system which undergoes an antiferrodistortive phase transition at 105 K. The low-temperature tetragonal phase exhibits different domains corresponding to the orientation of the symmetry axis. The domain structure can be changed drastically by applying electric fields. Using time-resolved neutron diffraction, the kinetics of the domain redistribution has been investigated on a time scale between microseconds and seconds. It is shown that there are at least two different types of relaxation processes on different time scales which govern the transformation. These can be related to the motion of hard and easy domain walls. Moreover, it is demonstrated that at low temperatures the field induced transition into the ferroelectric phase reveals a significant hysteresis which is reflected by the distribution of tetragonal domains. This finding supports the view of the zero field socalled quantum-paraelectric or coherent-paraelectric phase consisting of polar clusters which are removed only by heating above 40 K.
Polyoxometalates form a class of compounds that is unique in its versatile properties and applications. Despite of enormous work focused on the functionnalisation of POMs done so far, synthesis of derivatized polyoxoxanions is still a challenging and demanding task. We are particularely interested in the in the derivatisation of Lindqvist type polyoxoanions with organosilyl and organotin moieties. Such reactions require two steps; the first one consists of the synthesis of dimeric (nBu 4 N) 4 [(MW 5 O 18) 2 O] (M = Nb, Ta) as starting material; (nBu 4 N) 4 [(TaW 5 O 18) 2 O] was characterized by IR spectroscopy and single crystal X-ray diffraction. It crystallizes in the orthorhombic system, space group Pbnb with lattice parameters a = 15.7981(14), b = 17.939(3), c = 35.216(6)Å, V= 9980 Å 3 and Z = 4. The grafting of organic moieties onto the dimeric forms leads to the formation of monomeric plenary Lindqvist hybrids (nBu 4 N) 2 [MW 5 O 18 EOR 3 ] (E= Si, Sn). These derivatives were characterized in the solid state by IR and in solution by multinuclear NMR (1 H, 13 C, 29 Si, 119 Sn, 183 W). The crystallographic study of (nBu 4 N) 2 [(W 5 O 18 Ta(O)SiPh 3)] indicates that the structure of the POM is conserved and {SiPh3} + is grafted selectively on the surface of the polyanion through the terminal O t-Ta oxygen atom.
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