K. Yoshio

Crystal Structure of High-Temperature Paraelectric Phase in Bi-Layered Perovskite Sr0.85Bi2.1Ta2O9

The crystal structure of Bi-layered perovskite Sr0.85Bi2.1Ta2O9 was determined at 750 K by X-ray Rietveld analysis. Sr0.85Bi2.1Ta2O9 with space group A21am at room temperature transforms to the paraelectric phase above 670 K, passing through a probable intermediate state at 510 K. Assuming two possible orthorhombic Fmmm and tetragonal I4/mmm space groups, analysis of the crystal structure of the paraelectric phase gave the final discrepancy factors Rwp of 11.3% and 9.9%, respectively. Examination of possible reflections and peak positions at high-angles proved that the true space group is I4/mmm for the high temperature paraelectric phase. The lattice constants are a=3.927 (1), c=25.142 (1) A at 750 K. The TaO6 octahedron is not perfect and is slightly distorted along the c-axis, having the dipole moment of 0.478 debye.

Structural Study of Intermediate Phase in Layered Perovskite SrBi2Ta2O9 Single Crystal

The crystal structure of an intermediate phase of Bi-layered ferroelectric SrBi2Ta2O9 single crystals was studied by means of X-ray diffraction. An analysis of the extinction rules and X-ray intensities demonstrated that the crystal structure is orthorhombic with space group A21am in the ferroelectric phase and Amam in the intermediate phase; this conclusion is in good agreement with the findings of previous powder neutron diffraction studies.

Crystal structure of ZnO:Li at 293 K and 19 K by x-ray diffraction

Abstract Crystal structures of single crystals of ferroelectric Zn1−xLixO and paraelectric ZnO were studied by X-ray diffraction at 293K and 19K. At 293 K, the Zn-O bond length decreases by an amount of 0.007 Å along c-axis associated with Li-substitution. It is about two orders smaller than the changes in typical ferroelectrics. At 19 K, the clear change of Zn-3d z2 electron is observed in the difference Fourier maps. This change in electronic distribution is important in the ferroelectricity in Zn1−xLixO.

Ferroelectric Phase Transition in Layered Perovskite SrBi 2 Ta 2 O 9

Crystal structure of Bi-layered perovskite SrBi 2 Ta 2 O 9 was studied at 1000 K by X-ray diffraction. Rietveld analysis confirmed antiparallel arrangements of TaO 6 octahedra in the high-temperature paraelectric phase (tetragonal 14/mmm, R wp =7.37%). The TaO 6 octahedra are distorted along the c -axis. This crystal favors canted octahedral arrangements below T c (608 K). These result in net spontaneous polarization along the a -axis. We report the first observation of canted ferroelectricity or weak ferroelectricity in SrBi 2 Ta 2 O 9 .

Electronic ferroelectricity in ZnO

Abstract Novel ferroelectricity in II–VI semiconducting ZnO was studied. The mechanism of ferroelectricity was examined based on size-mismatch model and electronic model. If the ionic size-mismatch between substituted and the host ions is important, the introduction of Be2+ (ionic radius 0.3 Å) may be more effective rather than Li+ (0.60 Å). On the other hand, if the changes in electronic configuration are important, the Mg2+ may play a different role from the isoelectronic Li+ and Be2+. Dielectric measurements indicate that the introduction of Mg2+ instead of Li+ suppresses T c, while isoelectronic Be2+ shows almost the same Tc. This suggests that ZnO is the first ferroelectric with electronic origin.

Ferroelectric Phase Transition and New Intermediate Phase in Bi-Layered Perovskite SrBi 2 Ta 2 O 9

Thermal and structural studies showed that SrBi 2 Ta 2 O 9 undergoes ferroelectric-intermediate and intermediate-paraelectric phase transitions at 608 K ( T c ) and 850 K ( T *), respectively, while Bi-rich Sr 0.85 Bi 2.1 Ta 2 O 9 at 510 K ( T o ) and 670 K ( T c ). On heating, SrBi 2 Ta 2 O 9 still remains to be orthorhombic above T c, and then changes to tetragonal above T *. The TaO 6 is a distorted octaheron along the c -axis even in the paraelectric phase, and locates in antiparallel manner above T *. Below T c, this crystal favors canted arrangements of TaO 6, resulting in the net polarization along the a -axis. It was clarified that SrBi 2 Ta 2 O 9 shows a canted weak ferroelectricity, analogous to weak ferromagnetism with Dzialoshinski-Moriya interaction.

Thermal behavior in ferroelectric SrBi2Ta2O9 thin films

Abstract Specific heat and X-ray diffraction of Bi-layered perovskite SrBi2Ta2O9 and Bi-rich Sr0.8Bi2.2Ta2O9 thin films prepared by the sol-gel method were measured from room temperature to 800 K. The specific heat of stoichiometric SrBi2Ta2O9 film shows one slight anomaly at 520 K (Tc). In Sr0.8Bi2.2Ta2O9 films, the ferroelectric Tc shifted to 620 K and an additional shoulder in Cp was observed at 410 K (T*). The X-ray diffraction of Sr0.8Bi2.2Ta2O9 films showed structural changes at T* and Tc . This evidence suggests the existence of a new intermediate phase in Bi-rich Sr0.8Bi2.2Ta2O9.

Structural Changes and Ferroelectricity in Bi-Layered SrBi2Ta2O9

Structural changes of bismuth-layered perovskite SrBi2Ta2O9 associated with ferroelectric phase transition were studied by X-ray powder Rietveld analysis. The crystal structure of SrBi2Ta2O9 is tetragonal in the paraelectric phase at 1000 K (I4/mmm, Rp=5.64%, Rwp=7.37%). The TaO6 octahedra are distorted along the a-axis and aligned in an antiparallel manner. In the ferroelectric phase, this crystal favors canted arrangements of octahedra below Tc (608 K). These result in net spontaneous polarization along the a-axis. SrBi2Ta2O9 undergoes a canted phase transition, analogous to canted ferromagnets with an antisymmetrical Dzialoshinski-Moriya-type interaction.

Li-Induced ferroelectricity and its structural phase transition in ZnO

Abstract Ferroelectricity in semiconducting ZnO by Li-substitution was studied. A ferroelectric phase transition occurs at 470 K in Zn0.91Li0.09O. The revised Tc – x phase diagram was given. The Rietveld analysis of X-ray powder pattern showed a small displacement of Zn of 0.002 A along the polar c-axis in Zn0.91Li0.09O.

Novel ferroelectricity in polar semiconductor ZnO by Li-substitution

Ferroelectricity in Li-substituted II-VI semiconductor ZnO ceramics was studied. A ferroelectric phase transition temperature T/sub c/ is 370 K in hot-pressed ceramics synthesized by the spark plasma sintering method, although the previous cold-pressed ceramics showed a relative low T/sub c/ (330 K). The Rietveld analysis of X-ray powder diffraction pattern showed R/sub WP/=7.4%, u=0.3821 (3) for Zn (Li) and x=0.09 at 293 K in Zn/sub 1-x/Li/sub x/O hot-pressed ceramics. The ferroelectricity in Zn/sub 1-x/Li/sub x/O results from small structural distortion (of order 10/sup -3/ /spl Aring/) induced along the polar c-axis by substitutional Li atoms, instead of the host Zn atoms in the wurtzite structure.