Ambient Crystal Structure Research Articles

Photoluminescence and Raman Spectra of One-Dimensional Lead-free Perovskite CsCu2I3 Single-Crystal Wires.

Lead-free highly luminescent CsCu2I3 perovskite has attracted much attention recently, but agreements on basic optical properties have remained unsettled. By correlating X-ray diffraction with the photoluminescence (PL) of CsCu2I3 single-crystal wires, we first show that blue PL at 420 nm originates from CuI. We then exclude defect states as a source for the broadband emission centered at 570 nm from the lack of defect absorption, PL under sub-bandgap photoexcitation, observations of a linear dependence of PL intensity on excitation laser power, and a strong spectral blueshift under mild hydrostatic pressure. Finally, using a model of the self-trapped exciton and the associated coordinate configuration diagram, we explain pressure evolutions of PL energy, intensity, and lifetime. Single-crystal wires also enable us to obtain polarization-dependent Raman spectra down to 10 cm-1 and confirm their respective ambient crystal structure of orthorhombic Cmcm and phase transition to Pbnm at ∼5 GPa.

CaCu3Ti4O12: Pressure dependence of electronic and vibrational structures

The effects of pressure in electronic and vibrational properties of the double perovskite CaCu3Ti4O12 have been investigated in the 0-25 GPa range by optical absorption and Raman spectroscopy. Besides a full structural characterization, we aim at unveiling whether the ambient crystal structure is stable under high pressure conditions and how its giant dielectric permitivity and electronic gap varies with pressure. Results show that there is evidence of neither structural phase transition nor metallization in CaCu3Ti4O12 in the explored pressure range. We have observed the eight Raman active modes associated with its crystal phase and obtained their corresponding frequency and pressure shift. Moreover, the direct electronic band gap (2.20 eV), which is mainly associated with the oxygen-to-copper charge transfer states, increases slightly with pressure at a rate of 13 meV GPa−1 from 0 to 10 GPa. Above this pressure is almost constant (Eg = 2.3 eV). The results highlight the high stability of the compound in its phase against compression.

Structural Transformation and Melting in Gold Shock Compressed to 355GPa.

Gold is believed to retain its ambient crystal structure at very high pressures under static and shock compression, enabling its wide use as a pressure marker. Our insitu x-ray diffraction measurements on shock-compressed gold show that it transforms to the body-centered-cubic (bcc) phase, with an onset pressure between 150 and 176GPa. A liquid-bcc coexistence was observed between 220 and 302GPa and complete melting occurs by 355GPa. Our observation of the lower coordination bcc structure in shocked gold is in marked contrast to theoretical predictions and the reported observation of the hexagonal-close-packed structure under static compression.

Role of Lone Pair Cations in Ferroelectric Tungsten Bronzes

The role of lone pair cations in tetragonal tungsten bronze (TTB) ferroelectrics has so far not been addressed in detail despite the importance of lone pairs for the polarization mechanism in the prototype ferroelectric perovskite PbTiO3. We report a combined experimental and computational study of the effect of lone pairs in ferroelectric tungsten bronzes with particular emphasis on the important high-temperature piezoelectric lead metaniobate (PN). The ambient crystal structure of PN is revised based on X-ray and neutron powder diffraction. The most likely cation-vacancy configurations identified by the structural analysis were assessed by electron density functional theory (DFT) calculations. The ferroelectric transition was characterized by high-temperature X-ray diffraction, and the origin of the ferroelectric polarization was studied by DFT, emphasizing the relationship between polarization and cation–vacancy ordering. Covalency between Pb and O is identified as the driving force for the orthorhombic distortion of the unit cell of PN and the polarization in-plane with respect to the chains of corner-sharing octahedra. Finally, to further elucidate the role of lone pairs in ferroelectric TTBs polar lattice instabilities and resulting polarization in the TTB model system K4R2Nb10O30 (R = La, ..., Gd, or Bi) were investigated by DFT.

Structure, thermal expansion and electrical conductivity of Nb-substituted LaCoO3

The effect of Nb-substitution in LaCoO3 has been examined by evaluating the electrical conductivity, thermal expansion and crystal structure of the solid solutions LaCo1−xNbxO3 (x=0 to 1/3). It was demonstrated that Nb-substitution in LaCoO3 was mainly compensated by reduction of Co3+ to Co2+, and that oxidation of Co2+ could destabilise the solid solution. The ambient crystal structure was shown to transform from rhombohedral R3¯c perovskite (x=0) to orthorhombic Pbnm (x=0.15, 0.20) and finally to B-site ordered perovskite P21/n (x=0.25, 1/3) perovskite with increasing Nb-substitution. The thermal expansion of LaCo1−xNbxO3 was shown to be strongly depressed with increasing Nb-content, and significantly lower thermal expansion was observed for LaCo2/3Nb1/3O3 relative to LaCoO3. The electrical conductivity was reduced with increasing Nb-content, and semi-conducting properties was demonstrated for LaCo1−xNbxO3 in contrast to the metallic behaviour of pure LaCoO3 at elevated temperature. The thermal expansion, electrical conductivity and the stability of the materials were discussed with emphasis on Co/Nb ordering, the oxidation state and spin transitions of Co.

The multiscale coarse-graining method: Assessing its accuracy and introducing density dependent coarse-grain potentials

The ability of particle-based coarse-grain potentials, derived using the recently proposed multiscale coarse-graining (MS-CG) methodology [S. Izvekov and G. A. Voth, J. Phys. Chem. B 109, 2469 (2005); J. Chem. Phys. 123, 134105 (2005)] to reconstruct atomistic free-energy surfaces in coarse-grain coordinates is discussed. The MS-CG method is based on force-matching generalized forces associated with the coarse-grain coordinates. In this work, we show that the MS-CG method recovers only part of the atomistic free-energy landscape in the coarse-grain coordinates (termed the potential of mean force contribution). The portion of the atomistic free-energy landscape that is left out in the MS-CG procedure contributes to a pressure difference between atomistic and coarse-grain ensembles. Employing one- and two-site coarse-graining of nitromethane as worked examples, we discuss the virial and compressibility constraints to incorporate a pressure correction interaction into the MS-CG potentials and improve performance at different densities. The nature of the pressure correction interaction is elucidated and compared with those used in structure-based coarse-graining. As pairwise approximations to the atomistic free-energy, the MS-CG potentials naturally depend on the variables describing a thermodynamic state, such as temperature and density. Such dependencies limit state-point transferability. For nitromethane, the one- and two-site MS-CG potentials appear to be transferable across a broad range of temperatures. In particular, the two-site models, which are matched to low and ambient temperature liquid states, perform well in simulations of the ambient crystal structure. In contrast, the transferability of the MS-CG models of nitromethane across different densities is found to be problematic. To achieve better state-point transferability, density dependent MS-CG potentials are introduced and their performance is examined in simulations of nitromethane under various thermodynamic conditions, including shocked states.

X-ray diffraction study of B4C under high pressure

The high pressure X-ray diffraction study of B4C was carried out by using the synchrotron radiation source. The ambient crystal structure remained stable to the highest pressure 126 GPa, and the pressure induced amorphization did not occurs. The c/a ratio decreased with increasing pressure that means the apex angle of the rhombohedron increases with increasing pressure. The bulk modulus was determined by the Birch-Murnaghan's equation to be 305.8 GPa.