Monoclinic–orthorhombic first-order phase transition in K2ZnSi5O12 leucite analogue; transition mechanism and spontaneous strain analysis.

Hydrothermally synthesised K2ZnSi5O12 has a polymerised framework structure with the same topology as leucite (KAlSi2O6, tetragonal I41/a), which has two tetrahedrally coordinated Al3+ cations replaced by Zn2+ and Si4+. At 293 K it has a cation-ordered framework P21/c monoclinic structure with lattice parameters a = 13.1773(2) Å, b = 13.6106(2) Å, c = 13.0248(2) Å and β = 91.6981(9)°. This structure is isostructural with K2MgSi5O12, the first cation-ordered leucite analogue characterised. With increasing temperature, the P21/c structure transforms reversibly to cation-ordered framework orthorhombic Pbca. This transition takes place over the temperature range 848−863 K where both phases coexist; there is an ~1.2% increase in unit cell volume between 843 K (P21/c) and 868 K (Pbca), characteristic of a first-order, displacive, ferroelastic phase transition. Spontaneous strain analysis defines the symmetry- and non-symmetry related changes and shows that the mechanism is weakly first order; the two-phase region is consistent with the mechanism being a strain-related martensitic transition.

Influence of Sb2O3 doping on the sintering and polarization characteristics of PbTiO3 ceramic materials

In this paper, the effect of Sb2O3 doping on the sintering and polarization stability of PbTiO3 ceramics was investigated. The Sb2O3-doped PbTiO3 ceramic materials were prepared viaa conventional piezoelectric ceramic processing. The results showed that the as-prepared PbTiO3 ceramics exhibit a good sintering stability without cracking during sintering because of reducing spontaneous strain stemmed from decrease the tetragonal degree of PbTiO3 by Sb2O3 doping. Furthermore, the polarization stability of Sb2O3-doped PbTiO3 ceramics was considerably enhanced by the relaxation of strain and stress concentration during poling stemmed from the formation of good microstructure with fine grain and high density, and by decreasing the loss stemmed from reduction of oxygen vacancy concentration due to the substitution of Sb[Formula: see text] ions for Ti[Formula: see text] ions under a polarization condition with temperature of 130–150[Formula: see text]C and applied electric field of 4.5–5.5 kV/mm, and its yield of polarization is achieved ca. 80%.

Composition, Thermal Expansion and Phase Transitions in Framework Silicates: Revisitation and Review of Natural and Synthetic Analogues of Nepheline-, Feldspar- and Leucite-Mineral Groups

Framework silicates form about 70% of the Earth’s crust, mainly feldspars ~50–60% and quartz ~10–15%. Less-abundant feldspathoids include nepheline-, leucite-, and sodalite-group minerals, rich in structurally challenging properties. This review paper deals with anhydrous feldspar-, nepheline-, and leucite/pollucite groups, emphasising the importance of parallel studies on natural and synthetic samples. Four topics are covered. For decades, petrologists have analysed nephelines and recalculated their compositions as endmember molecules but, by not following rules of stuffed-tridymite crystal chemistry, have not estimated reliably the excess SiO2 present in solid solution. Some materials scientists make similar mistakes, and a new approach is described here. Synthesis studies of analogue feldspars, nephelines, and leucite/pollucites led to collaborative studies, mainly using laboratory and synchrotron X-ray powder diffraction methods at room and elevated temperatures, to study thermal expansion and displacive phase transitions. Such work was recently expanded to address the spontaneous strain relations. Topics covered here include work on nepheline/kalsilite analogues in the system SrAl2O4—BaAl2O4; thermal expansion of (K,Na)Al-, RbAl-, RbGa-, and SrAl-feldspars; and thermal expansion and phase transitions in analogue leucites KGaSi2O6 (tetragonal to cubic) and K2MgSi5O12 (monoclinic to orthorhombic). Results are reviewed in the context of research published in mineralogical and more-widely in physical sciences journals.

An unprecedented hexagonal double perovskite organic-inorganic hybrid ferroelastic: (piperidinium)2[KBiCl6]

An unprecedented A2MIMIIIX6-type double perovskite adopting a fully hexagonal BaNiO3-type structure, (piperidinium)2[KBiCl6], undergoes a 2/mF1̅ ferroelastic phase transition at 285 K with a spontaneous strain of 0.0615, arising from order-disorder...

High pressure effect on the crystal structure of the BaTiO3

Crystal structure of [Formula: see text] compound was studied using Atomistix Tool Kit software program at high pressure. It is defined that in this combination, tetragonal–cubic phase transition occurs under high pressure [Formula: see text] GPa. The character of this phase transition is explained in respect of baric dependence of spontaneous strain. Birch–Murnaghan equation is solved, compression ratio is determined. Results gained by theoretical calculations are compared with experimental values.

Symmetry Breakdown in Franckeite: Spontaneous Strain, Rippling and Interlayer Moiré

<p>Franckeite is a naturally occurring layered mineral with a structure composed of alternating stacks of SnS₂-like and PbS-like layers. Although this superlattice is composed of a sequence of isotropic two-dimensional layers, it exhibits a spontaneous rippling that makes the material structurally anisotropic. We demonstrate that this rippling comes hand in hand with an inhomogeneous in-plane strain profile and anisotropic electrical, vibrational and optical properties. We argue that this symmetry breakdown results from a spatial modulation of the van der Waals interaction between layers due to the SnS₂-like and PbS-like lattices incommensurability.</p>