The defect structures of impurity-doped congruent lithium niobates (c-LN) were investigated by combining lattice constant measurements with thermodynamic analyses. On the basis of the “Li site vacancy model”, six kinds of defect structures in impurity-doped c-LN are possible. Using thermodynamic constraints, these can be narrowed down to two kinds. The thermodynamic analyses showed impurities are located at only the Li or Nb site in the range where antisite niobium exists. Based on the thermodynamic analyses, lattice constant measurement revealed that divalent and trivalent impurities were located at Li sites in LN, whereas tetravalent impurities were located at Nb sites. This result provides good clues for controlling intrinsic point defects.
Shape-controlled langasite-type piezoelectric single crystals have been developed by the micro-pulling-down (μ-PD) method using specially designed Pt-Rh-Au crucibles with suitable wetting to the melt of langasite-type materials. The shape-controlled langasite-type piezoelectric single crystals indicated the comparable piezoelectric properties to the crystals grown by the conventional methods.
Ca3Nb(Ga1-xAlx)3Si2O14 (CNGAS) and Ca3Ta(Ga1-xAlx)3Si2O14 (CTGAS) single crystals have been developed and the effects of Al substitution on their crystal growth, phase formation and piezoelectric properties were revealed. CNGAS and CTGAS single crystals could be grown by the μ-PD method in the range of 0 ≤ x ≤ 0.6 and 0 ≤ x ≤ 1, respectively. Piezoelectric properties of the CNGAS and CTGAS bulk single crystals grown by the Czochralski (Cz) method were investigated, and the piezoelectric constant d11 and electromechanical coupling factor k12 were increased by the Al substitution. In addition, the precise material constants of the CNGAS and CTGAS single crystals have been clarified by the ultrasonic material characterization system, and the 2 inch bulk single crystals have been developed by the Cz method.
In this paper, we report with regard to the growth of potassium tantalate niobate (KTaxNb1-xO3:KTN) single crystals with compositional uniformity by the vertical Bridgman (VB) technique. First, relationship between the primary phases at the initial solidification region in KTN boules was investigated by directional solidification from melts with various compositions. Second, KTN crystals were grown by the normal VB technique using a -oriented KTaO3 seed from K-rich melts. KTN single crystals were obtained above the KTaO3 seed. But, the proportions of Ta and Nb in the grown crystal gradually changed with the progress of growth, the tendency of the composition to change qualitatively obeys the binary phase diagram of KTaO3－KNbO3. Finally, based on results of the directional solidification and the normal VB crystal growth, KTN crystals with compositional uniformity were grown by the modified VB technique with the special continuous supplying system.
A solid solution of KTaO3 and KNbO3 is well known as KTa1-xNbxO3 (KTN). KTN single crystals have attractive optical properties including the photo-refractive and electro-optic effects. We have been studying and developing optical devices using KTN, which exhibits a large relative permittivity around the cubic-tetragonal phase-transition. A large Kerr effect, where the refractive index changes in proportion to the square of an applied electric field, is also available based on this large permittivity. A waveguide optical modulator, a spectrometer, a beam deflector and a varifocal lens using the Kerr effect of KTN single crystals were investigated by using a fast kHz or MHz response without any mechanical systems. In this paper, first we introduce the operating principle of the beam deflector and its applications, and then we describe the operating principle of a varifocal lens and its performance.
The photovoltaic (PV) effect in polar materials offers great potential for light-energy conversion that generates a voltage beyond the bandgap limit of present semiconductor-based solar cells. Ferroelectrics have received renewed attention because of the ability to deliver a high voltage in the presence of ferroelastic domain walls (DWs). We report an unusually large PV response induced by ferroelastic DWs — termed ‘DW’-PV effect. The precise estimation of the bulk PV tensor in single crystals of barium titanate enables us to quantify the giant PV effect driven by 90°DWs. We show that the DW-PV effect arises from an effective electric field consisting of a potential step and a local PV component in the 90° DW region. This work offers a starting point for further investigation into the DW-PV effect of alternative systems and opens a reliable route for enhancing the PV properties in ferroelectrics based on the engineering of domain structures in either bulk or thin-film form.
Ferroelectric materials are widely used for electronic materials and optical materials from ancient days. Useful shapes are ceramics in past research. Materials are usually used lead-based oxides such as Pb(Zr,Ti)O3. However, the trend keyword is lead-free. In recent years, research trend is moved from ceramics to single crystals and thin films. Furthermore, some ferroelectric materials show ferromagnetic characteristics such as multiferroic materials. In this article, we will show our current research topics on lead-free ferroelectric and multiferroic materials.
Perovskite-type oxynitride, having oxide and nitride ions together in a compound, is a new class of dielectric material. Its dielectric property is related to the anionic configuration of short-range cis-type O/N ordering in crystal structure. Dielectric property including piezoelectric response was found out on its dense sintered ceramics. In this report, our recent studies on perovskite-type oxynitride dielectrics are reviewed with a characterization of crystal structure, development of sintering process and observation of a ferroelectric response in an oxynitride bulk ceramics.
In order to suppress the pyroelectric properties of LiTaO3(LT) substrates, a technique of controlling the volume resistivity of LT via a reduction process was investigated using metallic Al powder. After embedding the LT substrate in mixed powder of Al and Al2O3, the reduction process was carried out for 20 hours at 580℃, Before the reduction, the volume resistivity of the substrates was about 1×1015Ω･cm or more, but after the reduction, it became 1-10×109Ω･cm. In addition, after the reduction, the surface charges generated by the pyroelectric effect were neutralized within a few seconds. Oxygen vacancy concentration formed in the LT substrates by the reduction process was estimated to be tens of ppm order at most.