Materials for high temperature acoustic and vibration sensors: A review

Crystallographic texturing of polycrystalline ferroelectric ceramics offers a means of achieving significant enhancements in the piezoelectric response. Templated grain growth (TGG) enables the fabrication of textured ceramics with single crystal-like properties, as well as single crystals. In TGG, nucleation and growth of the desired crystal on aligned single crystal template particles results in an increased fraction of oriented material with heating. To facilitate alignment during forming, template particles must be anisometric in shape. To serve as the preferred sites for epitaxy and subsequent oriented growth of the matrix, the template particles need to be single crystal and chemically stable up to the growth temperature. Besides templating the growth process, the template particles may also serve as seed sites for phase formation of a reactive matrix. This process, referred to as Reactive TGG (RTGG), has been used to obtain highly oriented Pb(Mg1/3Nb2/3)O3-PbTiO3, Sr0.53Ba0.47Nb2O6, and (N...

Templated Grain Growth of Textured Piezoelectric Ceramics

Applications of metal–organic coordination polymers as precursors for preparation of nano-materials

The description of displacive distorted structures in terms of symmetry-adapted modes is reviewed. A specific parameterization of the symmetry-mode decomposition of these pseudosymmetric structures defined on the setting of the experimental space group is proposed. This approach closely follows crystallographic conventions and permits a straightforward transformation between symmetry-mode and conventional descriptions of the structures. Multiple examples are presented showing the insight provided by the symmetry-mode approach. The methodology is shown at work, illustrating its various possibilities for improving the characterization of distorted structures, for example: detection of hidden structural correlations, identification of fundamental and marginal degrees of freedom, reduction of the effective number of atomic positional parameters, quantitative comparison of structures with the same or different space group, detection of false refinement minima, systematic characterization of thermal behavior, rationalization of phase diagrams and various symmetries in families of compounds etc. The close relation of the symmetry-mode description with the superspace formalism applied to commensurate superstructures is also discussed. Finally, the application of this methodology in the field of ab initio or first-principles calculations is outlined. At present, there are several freely available user-friendly computer tools for performing automatic symmetry-mode analyses. The use of these programs does not require a deep knowledge of group theory and can be applied either a posteriori to analyze a given distorted structure or a priori to parameterize the structure to be determined. It is hoped that this article will encourage the use of these tools. All the examples presented here have been worked out using the program AMPLIMODES [Orobengoa et al. (2009). J. Appl. Cryst. 42, 820–833].

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Mode crystallography of distorted structures

Experimental studies of structurally incommensurate crystal phases

Ferroelectricity was observed in a lanthanum titanate single crystal (La2Ti2O7), a monoclinic crystal of the space group C22-P21. The Curie temperature was found to be about 1500°C. It was determined by a D-E hysteresis loop at room temperature that the spontaneous polarization Ps = 5 μC/cm2 and that the coercive field Ec = 45 kV/cm. Ferroelectric domains were observed using a crystal etched by an aqueous solution of nitric acid. The La2Ti2O7crystal showed strong piezoelectric and linear electro-optic effects: electromechanical coupling factor k22 = 0.29; half-wave voltage [E l]δ/2 = 2000 V at a wavelength of 6328 A.

A new ferroelectric: La2Ti2o7

Structures of cubic and orthorhombic phases of langbeinite-type K 2 Mn 2 (SO 4 ) 3 have been determined at 20°C and -90°C by X-ray diffraction. Thermal parameters of oxygen atoms in the cubic phase are remarkably anisotropic and show unusually high values, whereas those in the orthorhombic phase are rather normal. The change in bond lengths and angles at the transition clarifies a rigid body motion of SO 4 group. It is concluded that the principal structural change at the cubic-orthorhombic transition is characterized by ordering of sulfate groups.

Structures of Langbeinite-Type K2Mn2(SO4)3 in Cubic and Orthorhombic Phases

The modulated structure in the incommensurate phase of K 2 SeO 4 has been analyzed at 113 K by X-ray diffractometry based on the concept of four-dimensional space. The final R -factor was 0.046 for all 905 reflections and 0.087 for 326 satellite reflections. The modulation is characterized by the rotation of SeO 4 groups around b -direction and the associated translation of K atoms along c -axis. The eigenvector of the modulation mode was determined from the observed atomic displacement pattern and it is compared with that of Σ 2 soft mode determined by neutron experiment. 1)

Incommensurately Modulated Structure of K2SeO4

The crystal structure of K 2 SeO 4 in the ferroelectric phase was analyzed and the structural change at the transition from incommensurate to ferroelectric phase was elucidated. The phase transition is characterized by a commensuration of structural modulation as well as an additional homogeneous structural change both in rotation of SeO 4 radicals around the y -axis and in translation of K ions along the z -axis. The spontaneous polarization in the ferroelectric phase is estimated on the basis of these structural changes and the contribution of the latter additional part is presumed to attain 60% or so. The symmetry coordinate components of Σ 2 soft mode were determined and the feature of atomic displacement responsible to this transition is discussed.

A Structural Study of the Incommensurate-to-Ferroelectric Phase Transition in K2SeO4

X-ray and dielectric measurements were made on Cs 2 CdBr 4 (CCB) crystals, and optical observation was also made. CCB undergoes successive phase transitions at T 1 =-21°C, T 2 =-36°C and T 3 =-65°C. The space groups in all phases have been determined: I: Pnam, II: P Pnam 1ss , III: P2 1 /n and IV: P\bar1. The space group for the incommensurate II-phase is determined on the basis of four-dimensional space group concept. Temperature dependence of the position and the intensity of satellite reflections in the incommensurate phase was measured. Rotation of optical indicatrix was found in III- and IV-phases and its temperature dependence was measured. Ferroelastic activities in III- and IV-phases were found.

Noboru Yamada

Papers

A new ferroelectric: La2Ti2o7

Structures of Langbeinite-Type K2Mn2(SO4)3 in Cubic and Orthorhombic Phases

Incommensurately Modulated Structure of K2SeO4

A Structural Study of the Incommensurate-to-Ferroelectric Phase Transition in K2SeO4

Successive Phase Transitions in Cs 2 CdBr 4