Pressure-induced order–disorder transition in Gd 1.5 Ce 0.5 Ti 2 O 7 pyrochlore

An experimental study on ordered pyrochlore structured Gd 1.5 Ce 0.5 Ti 2 O 7 ( F d 3 ¯ m ) was carried out up to 45 GPa by synchrotron radiation X-ray diffraction and Raman spectroscopy. Experimental results show that Gd 1.5 Ce 0.5 Ti 2 O 7 transfers to a disordered cotunnite-like phase ( Pnma Z = 4) at approximately 42 GPa. Compared with the end member Gd 2 Ti 2 O 7 , the substitution of Ce 3+ for Gd 3+ increases the transition pressure and the high-pressure stability of the pyrochlore phase. This pressure-induced structure transition is mainly controlled by cationic order–disorder modification, and the cationic radius ratio r A / r B may also be effective for predicting the pyrochlore oxides' high-pressure stability. Two isostructural transitions are observed at 6.5 GPa and 13 GPa, and the unit-cell volume of Gd 1.5 Ce 0.5 Ti 2 O 7 as a function of pressure demonstrates its compression behaviour is rather complex.

Formulation of an Unprecedented Potassium Thiadiazolate Framework Via In-Situ Ligand Synthesis

The development of extended structures using s-block metal centres is fairly rare because of the predominance of ionic forces at metal center. The in-situ formation of 1,3,5-thiadiazole-2,5-dithiol ligand and its coordination to potassium metal ions results in the form of air stable green crystals of a novel framework [C2HKN2OS3]. The two-dimensional framework is characterized as P21 space group with the potassium ion, being heptacoordinated. The compound, showed distorted pentagonal bipyramidal coordination geometry due to the larger cationic radius of the potassium. The potassium metal ion is forming a bond with a nitrogen atom of azine nitrogens, two bonds with the oxygen atoms of the two hydroxyl ions and four bonds with the four sulfide ions of thiol moieties of four different thiadiazole rings. The C-S bond distances are in the range of 1.687(4) to 1.760(4) slightly shorter than the ideal value of 1.77 and#197; for a C(sp2)-S single bond. The thermogravimetric analysis indicates that a successive loss of the ligand occurs in the range of 253.06 oC to 357.53 oC that infers the stability of the compound.

Effect of Rare-Earth Oxides Additive on Liquid Phase Sintered SiC

The densification of α-SiC occurred by liquid-phase sintering mechanism with AlN-RE2O3(RE=Nd, Gd, Y, Lu) was studied. The total additive content was fixed at 15 wt%. Cold isostatically pressed samples were sintered at 1800-1950 °C under N2atmosphere for 1 h. The linear shrinkage and weight loss of the samples were about 17-20% and 2-5%, respectively. The mechanical properties and microstructure of sintered samples were investigated. The experimental results showed that the fracture toughness of samples was 6-8 MPa·m1/2, the hardness was in the range of 18-21 GPa and the bending strength was in the range of 400-500 MPa. It was found that a decrease in the cationic radius of the rare-earth oxides was accompanied by an increase in hardness and flexural strength of the SiC ceramics, whereas the fracture toughness was improved by incorporating rare-earth oxides of larger cationic radius. The morphology (SEM) of sintered sample showed a fine grained microstructure with equiaxed grains. Fracture mode was intergranular fracture.

Influence of Additives on Mechanical Properties in Liquid-Phase Sintered Silicon Carbide Ceramics

The effect of sintering additives comprising AlN and RE2O3 (RE=Lu, Yb, Er and Y) in 2:3 and 3:2 molar ratios on mechanical properties of liquid-phase-sintered (LPS) and subsequently annealed SiC ceramics was investigated. The toughness vs cationic radius curve had a maximum in both 2:3 and 3:2 molar ratios; Yb-doped specimens showed maximal toughness of 5.8 and 6.6 MPa..m1/2 in 2:3 and 3:2 molar ratios, respectively. There was no correlation between room temperature (RT) strength and cationic radius. In contrast, the high temperature (HT) strength vs cationic radius curve had a minimum in both 2:3 and 3:2 molar ratios; Yb-doped specimens showed minimal strength of 520 MPa and 550 MPa in 2:3 and 3:2 molar ratios, respectively. The present results suggest that there is a trade-off in improving both RT toughness and HT strength in LPS-SiC ceramics.

Crystal-chemical study of Rc natural oxides along the eskolaite – karelianite – hematite (Cr2O3–V2O3–Fe2O3) join

Six natural crystals from the Sludyanka crystalline complex belonging to the eskolaite (Cr2O3)–karelianite (V2O3)–hematite (Fe2O3) solid solution were studied by means of X-ray diffraction and electron microprobe. The Fe3+-poor samples show a general increase in a and c cell parameters with increasing mean cationic radius (MCR), consistent with that shown by the synthetic crystals along the eskolaite–karelianite join. The Fe3+-richer sample deviates significantly from the behaviour shown by the Fe3+-poor ones, similar to synthetic and natural hematites; with increasing MCR, the a and c cell parameters increase linearly along the eskolaite-karelianite join. However, for the samples rich in Fe3+, from karelianite to hematite, a shows a slightly steeper slope whereas the c parameter decreases strongly. The octahedral distortion increases slightly as a function of MCR along the eskolaite-karelianite join, whereas it increases markedly for Fe3+-rich samples. The evolution of the octahedral edges and of the octahedral distortions as a function of MCR are responsible for the behaviour of the unit-cell parameters along the eskolaite-karelianite-hematite join.