Haruyasu Yamashita

Dielectric Activity and Ferroelectricity in Piezoelectric Semiconductor Li-Doped ZnO

Temperature dependences of dielectric constants, specific heat and D–E hysteresis loops of Li-doped zinc oxide ceramics were investigated. A new dielectric anomaly was found at 330 K in Zn1-xLixO with x=0.17. A cusp-like anomaly was also found in specific heat. A ferroelectric D–E hysteresis loop was successfully observed for the first time. These observations suggest that replacement of host Zn ions by small Li ions induces a ferroelectric phase in the wurtzite-type ZnO piezoelectric semiconductor. This material is a candidate for ferroelectric thin films in integrated ferroelectric devices.

Ferroelectric Properties in Piezoelectric Semiconductor Zn1-xMxO (M=Li, Mg)

Ferroelectric properties and the effect of Li+ and Mg2+ dopants in II–VI semiconductor ZnO were studied. The ferroelectric D–E loops were observed in Zn1-x(Li0.02Mgx-0.02)O, as well as in Zn1-xLixO. The transition temperature (T c) increases in proportion to the concentration of Mg2+ ions, while it is almost constant (330 K) in Zn1-xLixO above x > 0.1. T c was 260 K for Zn0.9(Li0.02Mg0.08)O and 360 K for Zn0.7(Li0.02Mg0.28)O. An additional 20% of Mg increases T c by 100 K. The ferroelectric phase transition is strongly influenced by the introduction of small Li and Mg atoms instead of the host Zn atoms in wurtzite ZnO.

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.

Thermal and Dielectric Properties of a New Ferroelectric LaBGeO5

Thermal and dielectric properties of LaBGeO 5 , a new ferroelectric with a stillwellite-type structure are investigated. The specific heat, C p , shows one clear anomaly (Δ S =0.114 R) at 802.5 K. The basic thermodynamic parameters are determined on the basis of the Landau theory. However, an additional hump in C p was found in a different sample just above T c . This dependence on samples is confirmed by the measurements of dielectric constants. The appearance of the new intermediate phase may be caused by some impurities or defects. The Rhodes-Wohlfarth plot suggests that the nature of the phase transitions is of the displacive-type.

Pressure-Induced Tricritical Nature in Triglycine Selenate

Temperature dependences of dielectric constant and spontaneous polarization in triglycine selenate were measured under hydrostatic pressure. Coefficients of power series expansion of the free energy in respect to electric polarization were calculated front the experimental data. It has been found that the coefficient ξ of the fourth power changes its sign from plus to minus near 5 kbar. This indicates that the tricritical point of triglycine selenate exists at ∼40.6°C and ∼5 kbar.

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.

Third Order Elastic Constants of Ammonium Chloride

Sound velocities along several directions in ammonium chloride crystals have been measured under hydrostatic and uniaxial pressures by means of an ultrasonic method. Second order elastic constants and their pressure derivatives were evaluated in the vicinity of the phase transition point in disordered phase, and third order elastic constants were calculated from the data. It was found that the absolute values of the third order elastic constants C 111 , C 112 and C 123 considerably increased with decreasing temperature near the phase transition point.

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 .

Dielectric and Optical Studies of the Low-Temperature Phase in NaNbO3: Effect of Manganese Dopant

Pure and Mn-doped NaNbO3 crystals have been investigated dielectrically between 380 and 20 K. Below room temperature, no phase transition is observed in pure sodium niobate. With increasing Mn content, a structural phase transition is induced around 140 K. The dielectric constant shows a maximum for the sample with 0.7 wt.%, and shows large thermal hysteresis. In the low-temperature region, ferroelectric activities could not be detected in either pure or lightly doped NaNbO3. The phase diagram of Tc vs the concentration of Mn is obtained. Domain motion associated with this phase transition is observed around the transition temperature. This evidence proved that the low-temperature phase is induced by light doping with Mn ions. The uniaxial stress decreases Tc; dTc/dσ=-80 K/kbar on heating and -600 K/kbar on cooling.