Twinning modes of Er<Subscript>2</Subscript>Si<Subscript>2</Subscript>O<Subscript>7</Subscript>

Theory of deformation twinning by Bilby and Crocker is applied to calculate the twinning elements for all possible twinning modes in monoclinic Er₂Si₂O₇ TYPE C and TYPE D. The magnitude of shear strain was also calculated for each twinning mode. The criteria of small shear strain and minimum shuffling is applied to predict the operative twinning mode for monoclinic Er₂Si₂O₇ TYPE C and TYPE D. These predications of the theory are compared with the available experimental information and may act as guideline for future experimental work.     

Crystallization kinetics of neodymium disilicate obtained by polymeric xerogels

The kinetics of tetragonal Nd₂Si₂O₇ crystallization from polymeric xerogels was investigated under non-isothermal conditions via differential thermal analysis (DTA). The polymeric xerogels were prepared by the Pechini sol–gel method from tetraethoxysilane and neodymium nitrate. The crystallization fraction and rate of tetragonal Nd₂Si₂O₇ were determined based on the exothermic DTA data. The activation energy of tetragonal Nd₂Si₂O₇ crystallization, which is calculated by the Kissinger equation, is 673 kJ/mol. The crystallization process of tetragonal Nd₂Si₂O₇ could involve the nucleation and three-dimensional growth that follows the diffusion-controlled mechanism when the nucleation rate increased.     

UV Raman spectroscopic study on the surface phase of ZrO2 modified with Nd2O3

The Nd₂O₃ modified ZrO₂ was synthesized using two methods of co-precipitation (Nd-ZrO₂) and wet impregnation (Nd/ZrO₂). The surface and bulk crystalline phases of Nd₂O₃ modified ZrO₂ were investigated by using UV Raman spectroscopy, visible Raman spectroscopy, and X-ray diffraction (XRD). It is observed that the tetragonal phase in the surface region of Nd-ZrO₂ was not effectively stabilized by Nd₂O₃, as Nd₂O₃ is mainly present in the bulk of Nd-ZrO₂. However, in Nd/ZrO₂, it is found that with the impregnation of 0.5 mol% Nd₂O₃ on ZrO₂, the surface tetragonal phase of Nd/ZrO₂ can be stabilized even after calcination at 700 °C. The UV Raman results indicate that a disordered structure, or intermediate structure, which is involved in the transition from the tetragonal to the cubic phase, is formed at the surface region of Nd/ZrO₂. The formation of the aforementioned intermediate structure inhibits the phase transition from tetragonal to monoclinic in the surface region of Nd/ZrO₂. Furthermore, it is observed that the mixed tetragonal and monoclinic phases in the surface region of ZrO₂ which has been impregnated with Nd₂O₃ can also be stabilized after calcination at 700 °C. This work provides a simple method for controlling the surface phase of ZrO₂ at high temperatures.     

Investigations of Bi substitution in NdBa2Cu3Oy

Attempts to substitute Bi for Nd in orthorhombic NdBa₂Cu₃O _y , prepared in air or oxygen at about 950°C led instead to formation of Ba₂NdBiO₆, a new cubic compound witha=0.8703 nm. The possibility was then explored of preparing superconducting (Nd_1?x Bi _x )Ba₂Cu₃O _y , by first forming the tetragonal phase at 880–950°C in nitrogen or argon followed by reheating in oxygen or air at 250–500°C in order to insert the additional oxygen required to yield the orthorhombic form while avoiding oxidation of Bi³⁺ to Bi⁵⁺. X-ray diffraction studies, electrical conductivity measurements, and thermogravimetric analysis of products indicate that Bi does not enter the NdBa₂Cu₃O _y , lattice in either the tetragonal or the orthorhombic phase. Ba₂NdBiO₆ clearly forms on reheating in oxygen or air even at low temperatures, and evidence is presented that a poorly crystallized oxygen-deficient form of this compound is already present prior to the reheating.     

First-principles study on electronic structure, phase stability, and optical properties of In2X2O7 (X═C, Si, Ge or Sn)

In₂Ge₂O₇ and In₂Si₂O₇ are commonly used as scintillation materials. More studies on In₂X₂O₇ (X═C, Si, Ge, or Sn) are important to explore the possibility of using these materials for optoelectronic devices. This work presents results dealing with structural properties, electronic structure, chemical bonding, carrier effective masses, and optical spectra of polymorphs of In₂X₂O₇ obtained from first-principles calculations. The monoclinic phase of In₂Ge₂O₇, cubic and monoclinic phases of In₂Si₂O₇, as well as cubic phase of In₂Sn₂O₇ are known in scientific literature. From the total energy calculations at high pressure/strain we have found that the monoclinic phase of In₂Si₂O₇, In₂Ge₂O₇, and In₂Sn₂O₇ can be transformed into the cubic phase. The cubic phase of In₂Ge₂O₇ and In₂Sn₂O₇ is found to be more stable than the monoclinic phase. However, the monoclinic phase of In₂C₂O₇ and In₂Si₂O₇ is more stable than the cubic phase. The phase stability study suggests that In₂C₂O₇ is not stable, and that it might dissociate into corresponding binary oxides. Effective masses of electrons and holes have been estimated. Analysis of optical properties shows that in Si solar cells In₂Si₂O₇ and In₂Sn₂O₇ can be used as antireflection coating layer.     

The phase transition in GexV1−xO2 crystals

The first order phase transition in Ge_xV_1?xO₂ (O ? x 0,4) has been studied by means of differential thermal analysis, resistivity, magnetic susceptibility, and X-ray measurements. In agreement with the theory an intermediate, orthorhombic phase has been found between the tetragonal and monoclinic ones. The dependence of the transition temperature on Ge⁴⁺ concentration has been examined and explained on the basis of the Goodenough's theory.     

Two types of ABi<Subscript>2</Subscript>B<Subscript>2</Subscript>O<Subscript>9</Subscript> layered perovskite-like ferroelectrics

Statistical analysis of the literature data on the structural and ferroelectric properties of ABi₂B₂O₉ layered perovskite-like compounds shows that these ferroelectrics can be divided into two distinct groups, one constituted by tetragonal and nearly tetragonal compounds, and the other by compounds with a significant orthorhombic (and possibly monoclinic) distortion. Surprisingly, there are no layered ABi₂B₂O₉ compounds in a wide intermediate range of structural parameters.     

Phase relations and high temperature-XRD studies of Gd2−xNdxTi2O7 solid solutions

A series of compositions with the general formula Gd_2−xNd_xTi₂O₇ (0.0 ≤ x ≤ 2.0) was prepared by ceramic sintering and characterized by powder XRD. Nd³⁺ has been used as a surrogate for Am³⁺, an actinide found in spent nuclear fuel. One end member, Gd₂Ti₂O₇, had the pyrochlore structure and formed solid solutions with Nd₂Ti₂O₇ up to the nominal composition Gd_1.2Nd_0.8Ti₂O₇. An increase in lattice parameter was observed as a function of x in the series Gd_2−xNd_xTi₂O₇ in the composition range 0.0 ≤ x ≤ 0.8. Compositions Gd_2−xNd_xTi₂O₇ in the range 0.8 ≤ x ≤ 1.4 were biphasic. From x ≥ 1.6, the solid solutions are monoclinic, isotypic with Nd₂Ti₂O_7. These results were explained based on the radius (r_A/r_B) ratio of the cations. High temperature-XRD studies on cubic pyrochlores showing thermal expansion are reported.     

Atomic structure of grain boundaries and surfaces in YBa2Cu3O7−δ and LaBaCaCu3O7−δ

High-resolution electron microscopy (HREM) studies of grain boundaries and fractured surfaces in both the tetragonal and orthorhombic phases of dense (>90%) YBa₂Cu₃O_7?δ and tetragonal LaBaCaCu₃O_7?δ have been conducted. Grain boundaries in polycrystalline YBa₂Cu₃O_7?δ are found to be frequently parallel to a (001) plane of one of the adjacent grains, with a structure similar to that of the (001) surface of fractured YBa₂Cu₃O_7-gd. Matching of experimental and calculated images shows that the outermost surface layer in this compound is a deformed BaO layer. Both grain boundaries and fractured surfaces in LaBaCaCu₃O_7?δ show no such tendency for preferential orientation of the interface plane. Results indicate that the low critical currents observed in sintered materials are caused by textured grain growth in combination with the atomic structure of the grain boundary plane, and the intercalation of off-stoichiometric species near the grain boundary.     

The structure identification of epitaxial Ru2Si3 on (111) Si

Epitaxial Ru₂Si₃ has been successfully grown on (111) Si for the first time by using the new method of chemical electroless plating of Ru thin film on silicon with subsequent annealing schemes. The crystal structure of Ru₂Si₃ reported in the literature, which was identified by X-ray diffraction, was ambiguous to be tetragonal or orthorhombic. The study of stereomicroscopy and Kikuchi lines of the Ru₂Si₃ by scanning transmission electron microscopy shows that the structure of Ru₂Si₃ is orthorhombic with four-fold like symmetry rather than tetragonal.     

Thermodynamics of the tetragonal to monoclinic phase transformation in constrained zirconia microcrystals

End-point thermodynamic analyses were made of the tetragonal to monoclinic transformation (tm) occurring in ZrO₂ precipitates in a Ca-PSZ alloy and particles in Al₂O₃-ZrO₂ composites. Calculated plots of the reciprocal critical size for transformation temperature were in excellent agreement with experimental data for both systems. Contributions to the total free energy change included bulk chemical, dilatational and residual shear strain energies and also interfacial energies. The latter term consisted of contributions from the change in the chemical surface free energy, the presence of twin boundaries in the precipitate (particle)-matrix interfacial energy. The major impediment to the transformation was the shear strain energy which could not be reduced sufficiently by twinning alone. The t m reaction proceeded spontaneously when the energy barrier was reduced by the response of the particle-matrix interface. The response comprised loss of coherency and grain boundary microcracking for the Ca-PSZ and Al₂O₃-ZrO₂ alloys, respectively. These results are in accord with recent suggestions that either a stress-free strain or a free surface is a necessary conditions for the initiation of a martensitic transformation.     

Obtention par trempe a partir de l'etat liquide d'une nouvelle phase dans le systeme Nd2O3.Al2O3. etude comparative avec Nd4Ga2O9

By using splat cooling devices associated with laboratory solar furnace a new phase is obtained in the neodymium rich part of the Al₂O₃Nd₂O₃ system. X ray and optical absorption studies and comparisons with Nd₄Ga₂O₉ show that the two phases are isostructural and monoclinic (space group P_21/C) with Al³⁺ in tetrahedral coordination. Optical examinations show that substitution of Al³⁺ by Si⁴⁺ induced a disorder on the neodymium site.     

The orthorhombic/monoclinic transition in single crystals of zeolite ZSM-5

X-ray photographs of single crystals of zeolite H-ZSM-5 (Si_11.96Al_0.04H_0.04O₂₄) at different temperatures are presented. The reversible orthorhombic/monoclinic transition, previously observed with XRD and ²⁹Si m.a.s. n.m.r. on powder samples of H-ZSM-5, is confirmed. Upon cooling, the orthorhombic H-ZSM-5 single crystal changes into an aggregate of monoclinic twin domains. The directions of a and c in the orthorhombic single crystal and in the monoclinic twinned crystal (a ≈ 90.5°) are identical. The deviation of α from 90° can be ascribed to a mutual shift of successive (010) pentasil layers along c, induced by a distortion or rotation of the T₄ and T₆ rings interconnecting the layers. The space group of the monoclinic phase most probably is P₁²n11.     

Studies on the Gyromagnetic Factors of the Tetragonal NdBa2Cu3O6.1

The electron paramagnetic resonance (EPR) g factors g _?? and g _?? of the tetragonal NdBa₂Cu₃O_6.1 are theoretically studied using the perturbation formulas of the g factors for a 4f ³ ion in tetragonal symmetry. In the calculations, the influences of the admixtures of various energy levels are considered, while the tetragonal field parameters are determined from the local structural data of Nd³⁺ site using the superposition model. The theoretical g factors are in reasonable agreement with the observed values. The positive anisotropy g _?? ? g _?? is attributable to the elongation of the oxygen cube around Nd³⁺ along the C ₄ axis in NdBa₂Cu₃O_6.1. The magnitude (average) of the g factors for NdBa₂Cu₃O_6.1 is lower than that for the previously observed NdBa₂Cu₃O_7??? (0<??<1). This can be ascribed to the different oxygen amount and the dissimilar local environments around Nd³⁺ under different preparation conditions and parameters. Further, the lowest Kramers doublet of the present NdBa₂Cu₃O_6.1 is the unresolved ?? _7,8, while that of NdBa₂Cu₃O₇ is ?? ₆.     

Thermal Transformations of the Nd2P6O18 · 12H2O and Li3Nd3(P6O18)2 · 26H2O Cyclohexaphosphates into Polyphosphates

The thermal dehydration and transformations of the cyclohexaphosphates Nd₂P₆O₁₈ · 12H₂O and Li₃Nd₃(P₆O₁₈)₂ · 26H₂O are investigated in the range 20–1000°C using differential thermal analysis and high-temperature x-ray diffraction, in conjunction with chromatographic determination of the anion composition of reaction products. The results demonstrate that, during heating, the cyclohexaphosphates convert into the polyphosphates Nd(PO₃)₃ (monoclinic and orthorhombic forms) and LiNd(PO₃)₄ (monoclinic form). These processes are not accompanied by P₂O₅ removal. The temperature stability limits of the polyphosphates are determined, and the possible mechanisms of the cyclohexaphosphate–polyphosphate transformations are discussed.     

Development of Twins in YBa2Cu3Oy Superconductor

It is significant to study the problems how the tetragonal and orthorhombic structures of YBa₂Cu₃O _y generate, since the structures closely relate to superconductivity. To deal with these problems, we approach the generation of tetragonal and orthorhombicity and their morphologies by SEM, TEM, and X-ray diffraction analysis, respectively. For interpreting the experimental phenomena, we put forward a crystal growth modal: the tetragonal grows at sintering temperatures, and the tetragonal transits to orthorhombicity and forms twins along [110] faces at annealing temperatures. This model agrees with experimental results well.     

Phase relationships in the system Si3N4-SiO2-La2O3

Phase relationships in the system Si₃N₄-SiO₂-La₂O₃ have been investigated after cooling from 1700° C. Two phases, 2Si₃N₄·La₂O₃ (monoclinic) and La₅ (SiO₄)₃N (hexagonal), were identified; the other two phases in the system, LaSiO₂N (monoclinic) and La₄Si₂O₇N₂ (monoclinic), were found to dissociate to La₅ (SiO₄)₃N and a glass after cooling from temperatures above 1650° C. The unit cells of 2Si₃N₄·La₂O₃, LaSiO₂N and La₄Si₂O₇N₂ have been determined and compared with those of preceding works. The results are discussed in relation to the intergranular phases observed when Si₃N₄ is sintered with La₂O₃ additions.     

Oxides formed between ZrO2 and Nb2O5

High-purity monoclinic ZrO₂ and monoclinic Nb₂O₅ powder samples were mixed in varying ratios, and heated at 1300°C for 24 h before furnace cooling. X-ray diffraction analysis showed that two compounds, Nb₂O₅-6ZrO₂ and Nb₂O₅·8ZrO₂ (or (Zr, Nb)₈O₁₇ and (Zr, Nb)₁₀O₂₁), were formed. Some of the reflections from the two compounds were overlapped and interfered with those from the zirconium oxide, especially the tetragonal ZrO₂. A thermodynamic analysis was used to demonstrate that it is possible to have ZrO₂ as well as Nb₂O₅ formed on Zr-Nb alloys in an oxidizing environment. The relevance of these results (experimental and calculated) to the corrosion of a Zr-Nb alloy in high temperature aqueous environments is discussed. It is suggested that the incorporation of niobium into the oxide during the corrosion of Zr-Nb alloys could be by the formation of compounds such as (Zr, Nb)₈O₁₇ and (Zr, Nb)₁₀O₂₁. Also, the tetragonal ZrO₂, which has been found to be a barrier layer oxide, could, in fact, be either of the ternary compounds Nb₂O₅·6ZrO₂ and Nb₂O₅·8ZrO₂ both of which are orthorhombic with lattice parameters very close to those of the tetragonal ZrO₂.     

Influence of the heat treatment on the structure of SmBa2Cu3O7−x

In this paper we present the results of our study of different heat treatment regimes on the structure of SmBa₂Cu₃O_7−x samples. We have shown that using a light microscope it is possible, in polarized light, to distinguish between a tetragonal and an orthorhombic phase and to study their morphology. The independence of the thickness of orthorhombic phase twins on the transition temperature was quantitatively determined, this may be connected with a constant oxygen content in the SmBa₂Cu₃O_7−x phase during tetragonal to orthorhombic transition.     

Les disilicates de terres rares Ln2Si2O7: Precisions sur leur nouvelle forme cristalline X et leur polymorphisme sous pression

The new X form of rare earth disilicates Ln₂Si₂O₇ (Ln=Lu, Yb, Tm) is described by means of X-ray powder data. The X-ray indexation is made using single microcristal electronic diffraction. The investigation of structural relation between Ln₂Si₂O₇ and M₂A₂O₇ compounds as also the X-ray powder data of X-Ln₂Si₂O₇ show that this X form is isostructural with tetragonal Er₂Ge₂O₇. The rare earth coordination polyhedra and the (Si₂O₇)^6? configuration are compared with those actually known in neighbouring phases and discussed in terms of pressure effect.