For decades, numerous attempts have been made to produce polar oxynitride perovskites, where some of the oxygen is replaced by nitrogen, but a polar ordered oxynitride has never been demonstrated. Caracas and Cohen [Appl. Phys. Lett. 91, 092902 (2007)] studied possible ordered polar oxynitrides within density-functional theory (DFT) and found a few candidates that were predicted to be insulating and at least metastable. YSiO2N stood out with huge predicted polarization and nonlinear optic coefficients. In this study, we demonstrate the synthesis of perovskite-structured YSiO2N by using a combination of a diamond-anvil cell and in situ laser-heating techniques. Subsequent in situ x-ray diffraction, second-harmonic generation, and Raman-scattering measurements confirm that it is polar and a strong nonlinear optical material, with structure and properties similar to those predicted by DFT.

Raman spectroscopy was used to measure the frequencies of phonons in Si-24 with an open clathrate structure at temperatures from 80 to 400 K with simultaneous pressures of 0 to 8 GPa. The frequency shifts of the different phonons were substantially different under either temperature or pressure. The quasiharmonic behavior was isolated by varying pressure at low temperatures, and the anharmonic behavior was isolated by varying temperature at low pressures. Phonon modes dominated by bond bending were anomalous, showing stiffening with temperature and softening with pressure. Both the quasiharmonic behavior and the anharmonic behavior changed markedly with simultaneous changes in temperature Delta T and pressure Delta P. With Delta T = 320 K and Delta P = 8 GPa, some frequency shifts that scaled with the product Delta T Delta P were as large as the shifts from Delta T and Delta P alone. The thermodynamic entropy of this material likely has a dependence on Delta T and Delta P that cannot be obtained by adding effects from quasiharmonicity and phonon-phonon anharmonicity.

The role of defects in solids of mixed ionic-covalent bonds such as ferroelectric oxides is complex. Current understanding of defects on ferroelectric properties at the single-defect level remains mostly at the empirical level, and the detailed atomistic mechanisms for many defect-mediated polarization-switching processes have not been convincingly revealed quantum mechanically. We simulate the polarization-electric field (P-E) and strain-electric field (e-E) hysteresis loops for BaTiO3 in the presence of generic defect dipoles with large-scale molecular dynamics and provide a detailed atomistic picture of the defect dipole-enhanced electromechanical coupling. We develop a general first-principles-based atomistic model, enabling a quantitative understanding of the relationship between macroscopic ferroelectric properties and dipolar impurities of different orientations, concentrations, and dipole moments. We find that the collective orientation of dipolar defects relative to the external field is the key microscopic structure feature that strongly affects materials hardening/softening and electromechanical coupling. We show that a small concentration (approximate to 0.1 at. %) of defect dipoles dramatically improves electromechanical responses. This offers the opportunity to improve the performance of inexpensive polycrystalline ferroelectric ceramics through defect dipole engineering for a range of applications including piezoelectric sensors, actuators, and transducers. Published by AIP Publishing.

The pressure-dependent phase behavior of semiconducting chalcopyrite ZnSiP2 was studied up to 30 GPa using in situ X-ray diffraction and Raman spectroscopy in a diamond-anvil cell. A structural phase transition to the rock salt type structure was observed between 27 and 30 GPa, which is accompanied by soft phonon mode behavior and simultaneous loss of Raman signal and optical transmission through the sample. The high-pressure rock salt type phase possesses cationic disorder as evident from broad features in the X-ray diffraction patterns. The behavior of the low-frequency Raman modes during compression establishes a two-stage, order-disorder phase transition mechanism. The phase transition is partially reversible, and the parent chalcopyrite structure coexists with an amorphous phase upon slow decompression to ambient conditions. Published by AIP Publishing./.

Ferroelectric perovskite oxides possess a large electrocaloric (EC) effect, but usually at high temperatures near the ferroelectric/paraelectric phase transition temperature, which limits their potential application as next generation solid-state cooling devices. We use classical molecular dynamics to study the electric-field-induced phase transitions and EC effect in PMN-PT (PbMg1/3Nb2/3O3-PbTiO3). We find that the maximum EC strength of PMN-PT occurs within the morphotropic phase boundary (MPB) region at 300 K. The large adiabatic temperature change is caused by easy rotation of polarization within the MPB region.

We report Hugoniot measurements on a mixture of cubic boron nitride (cBN) and hexagonal boron nitride (hBN, similar to 10% in weight) to investigate the shock compression behavior of BN at Hugoniot stresses up to 110 GPa. We observed a discontinuity at similar to 77 GPa along the Hugoniot and interpreted it as the manifestation of the shock-induced phase transition of hBN to cBN. The experimental stress at 77-110 GPa shows significant deviation from the hydrodynamic Hugoniot of cBN calculated using the Mie-Gruneisen model coupled with the reported 300 K-isotherms of cBN. Our investigation reveals that material strength in cBN increases with the experimental stress at least up to 110 GPa. The material strength might be preserved at higher stress if we consider the previously reported high stress data. Published by AIP Publishing.

The optical and electronic properties of semiconducting materials are of great importance to a vast range of contemporary technologies. Diamond-cubic germanium is a well-known semiconductor, although other 'exotic' forms may possess distinct properties. In particular, there is currently no consensus for the band gap and electronic structure of ST12-Ge (tP12, P4(3)2(1)2) due to experimental limitations in sample preparation and varying theoretical predictions. Here we report clear experimental and theoretical evidence for the intrinsic properties of ST12-Ge, including the first optical measurements on bulk samples. Phase-pure bulk samples of ST12-Ge were synthesized, and the structure and purity were verified using powder X-ray diffraction, transmission electron microscopy, Raman and wavelength/energy dispersive X-ray spectroscopy. Optical measurements indicate that ST12-Ge is a semiconductor with an indirect band gap of 0.59 eV and a direct optical transition at 0.74 eV, which is in good agreement with electrical transport measurements and our first-principles calculations.

Performing well-controlled metal-silicate partitioning experiments at conditions directly simulating those of a deep magma ocean is difficult. It is therefore common to perform experiments at lower pressures and temperatures, which are used to determine the effects of salient variables. Often, these effects are determined by multiple linear regression of a data set covering a large range of P-T-composition space. In particular, these data sets often contain the results of experiments performed both with and without sulfur in the system. Data are often regYressed, however, using a relationship based only upon the formation of oxide species in the silicate melt. Several studies have suggested that when sulfur is present in the system, siderophile trace metals may also dissolve into silicate melt as S-bearing species. We have derived a relationship for regressing experimental metal-silicate partitioning data that considers the formation of both oxide and sulfide species in the silicate melt. Using model data sets, we have assessed the ability of this relationship, and the more typical single-species relationship, to accurately parameterize data in which the formation of S-bearing species is important. We have also applied this new relationship to experimental results on the metal-silicate partitioning of gold and find it is able to reconcile the conflicting pressure dependencies of lnD(Au)(met/sil) found in previous studies.

Black phosphorus (BP) has recently attracted significant attention due to its exceptional physical properties. Currently, high-quality few-layer and thin-film BP are produced primarily by mechanical exfoliation, limiting their potential in future applications. Here, the synthesis of highly crystalline thin-film BP on 5 mm sapphire substrates by conversion from red to black phosphorus at 700 degrees C and 1.5 GPa is demonstrated. The synthesized approximate to 50 nm thick BP thin films are polycrystalline with a crystal domain size ranging from 40 to 70 mu m long, as indicated by Raman mapping and infrared extinction spectroscopy. At room temperature, field-effect mobility of the synthesized BP thin film is found to be around 160 cm(2) V-1 s(-1) along armchair direction and reaches up to about 200 cm(2) V-1 s(-1) at around 90 K. Moreover, red phosphorus (RP) covered by exfoliated hexagonal boron nitride (hBN) before conversion shows atomically sharp hBN/BP interface and perfectly layered BP after the conversion. This demonstration represents a critical step toward the future realization of large scale, high-quality BP devices and circuits.

We study the electrocaloric effect in the classic ferroelectric BaTiO3 through a series of phase transitions driven by applied electric field and temperature. We find both negative and positive electrocaloric effects, with the negative electrocaloric effect, where temperature decreases with applied field, in monoclinic phases. Macroscopic polarization rotation is evident through the monoclinic and orthorhombic phases under applied field, and is responsible for the negative electrocaloric effect.