L.D. Wang

In situ structure characterization of Pb(Yb1/2Nb1/2)O3-PbTiO3 crystals under high pressure-temperature

The stability field of the piezoelectric/ferroelectric phase of solid solution 0.47Pb(Yb1/2Nb1/2)O3-0.53PbTiO3 (PYN-PT) has been studied using in situ x-ray diffraction (XRD) and Raman spectroscopy techniques under high pressure and high temperature conditions to observe the evolution of features. PYN-PT remains in the piezoelectric tetragonal phase up to approximately 6.5 GPa at room temperature then transforms to a paraelectric cubic phase, which exhibits local disorder. The cubic phase is stable up to 50 GPa. Based on the high pressure and high temperature XRD results, we present a pressure-temperature phase diagram of PYN-PT which constraints the stability region of the ferroelectric phase.

Conventional empirical law reverses in the phase transitions of 122-type iron-based superconductors

Phase transition of solid-state materials is a fundamental research topic in condensed matter physics, materials science and geophysics. It has been well accepted and widely proven that isostructural compounds containing different cations undergo same pressure-induced phase transitions but at progressively lower pressures as the cation radii increases. However, we discovered that this conventional law reverses in the structural transitions in 122-type iron-based superconductors. In this report, a combined low temperature and high pressure X-ray diffraction (XRD) measurement has identified the phase transition curves among the tetragonal (T), orthorhombic (O) and the collapsed-tetragonal (cT) phases in the structural phase diagram of the iron-based superconductor AFe2As2 (A = Ca, Sr, Eu, and Ba). The cation radii dependence of the phase transition pressure (T → cT) shows an opposite trend in which the compounds with larger ambient radii cations have a higher transition pressure.

Local structure of liquid gallium under pressure

In situ high energy X-ray pair distribution function (PDF) measurements, microtomography and reverse Monte Carlo simulations were used to characterize the local structure of liquid gallium up to 1.9 GPa. This pressure range includes the well-known solid-solid phase transition from Ga-I to Ga-II at low temperature. In term of previous research, the local structure of liquid gallium within this domain was suggested a mixture of two local structures, Ga I and Ga II, based on fitting experimental PDF to known crystal structure, with a controversy. However, our result shows a distinctly different result that the local structure of liquid gallium resembles the atomic arrangement of both gallium phase II and III (the high pressure crystalline phase). A melting mechanism is proposed for Ga, in which the atomic structure of phase Ι breaks up at the onset of melting, providing sufficient free volume for atoms to rearrange, to form the melt.

Study of liquid gallium as a function of pressure and temperature using synchrotron x-ray microtomography and x-ray diffraction

The volume change of liquid and solid gallium has been studied as a function of pressure and temperature up to 3.02 GPa at 300 K and up to 3.63 GPa at 330 K using synchrotron x-ray microtomography combined with energy dispersive x-ray diffraction techniques. Two sets of directly measured P-V data at 300 K and 330 K were obtained from 3D tomography reconstruction data, and the corresponding isothermal bulk moduli were determined as 23.6 (0.5) GPa and 24.6 (0.4) GPa, respectively. The existence of a liquid-liquid phase transition region is proposed based on the abnormal compressibility of Ga melt at about 2.44 GPa and 330 K conditions.

Correlated High-Pressure Phase Sequence of VO2 under Strong Compression

Understanding how the structures of a crystal behave under compression is a fundamental issue both for condensed matter physics and for geoscience. Traditional description of a crystal as the stacking of a unit cell with special symmetry has gained much success on the analysis of physical properties. Unfortunately, it is hard to reveal the relationship between the compressed phases. Taking the family of metal dioxides (MO2) as an example, the structural evolution, subject to fixed chemical formula and highly confined space, often appears as a set of random and uncorrelated events. Here we provide an alternative way to treat the crystal as the stacking of the coordination polyhedron and then discover a unified structure transition pattern, in our case VO2. X-ray diffraction (XRD) experiments and first-principles calculations show that the coordination increase happens only at one apex of the V-centered octahedron in an orderly fashion, leaving the base plane and the other apex topologically intact. The polyhedron evolves toward increasing their sharing, indicating a general rule for the chemical bonds of MO2 to give away the ionicity in exchange for covalency under pressure.

Polyamorphism in Yb-based metallic glass induced by pressure

The Yb62.5Zn15Mg17.5Cu5 metallic glass is investigated using synchrotron x-ray total scattering method up to 38.4 GPa. The polyamorphic transformation from low density to high density with a transition region between 14.1 and 25.2 GPa is observed, accompanying with a volume collapse reflected by a discontinuousness of isothermal bulk modulus. This collapse is caused by that distortional icosahedron short range order precedes to perfect icosahedron, which might link to Yb 4f electron delocalization upon compression, and match the result of in situ electrical resistance measurement under high pressure conditions. This discovery in Yb-based metallic glass, combined with the previous reports on other metallic glass systems, demonstrates that pressure induced polyamorphism is the general behavior for typical lanthanide based metallic glasses.

Electrical Properties of AlN and SiO2 Co-Modified Barium Titanate Lead-Free Piezoelectric Ceramics

Ba1-xAlxTi1-ySiyO3-xNx(BAxTS4N) piezoceramics were prepared by solid-state reaction sintering. The effects of AlN and SiO2 co-doping on the phase structure and electrical properties of BaTiO3 ceramics were investigated and analyzed. The ceramics adopt a tetragonal crystal symmetry accompanied by the appearance of secondary phases Ba2TiSi2O8 and Ba4Ti2O27. The results of temperature dependence of dielectric constant indicate that the Curie temperature of BAxTS4N ceramics is independent of doping content. The BAxTS4N ceramic for the composition of x = 3% obtains the optimum ferroelectric properties with the remnant polarization (2Pr) of 14.69 μC/cm2 and the maximum piezoelectric coefficient (d33) of 230 pC/N.

The study of noncrystalline material under high pressure using high energy x-ray pair distribution function combined with synchrotron x-ray microtomography techniques

For non-crystalline samples, such as amorphous materials and melts, due to their lack of periodicity, scattering patterns from amorphous and liquid phases are very broad. Thus, the pair distribution function (PDF) and X-ray microtomography are important methods to investigate structural change for non-crystalline samples under high pressure conditions. Here we present experimental results on liquid gallium using microtomography and Yb-based metallic glass using PDF under high pressure.

A new polymorph phase of LiAlSiO4 in β-LiAlSiO4/Cu composite

β-eucryptite (LiAlSiO4) with a negative thermal expansion coefficient can be used as reinforcement in metal matrix composites to reduce the coefficient of thermal expansion (CTE) of the composites. It is necessary to clarify the phase transformation of β-eucryptite (LiAlSiO4) which influences the CTE of the composite. A copper matrix composite reinforced by β-LiAlSiO4 was fabricated by spark plasma sintering. The composite was characterized using X-ray diffraction and transmission electronic microscope. A new phase of eucryptite has been found, which is a polymorph phase and has primary cubic structure with the lattice constant of 0.435 nm. The reason for the phase transition from β-LiAlSiO4 to the new phase was discussed which may result from the large anisotropic stress in the β-LiAlSiO4 particles of β-LiAlSiO4/Cu composite. The discovery of the new polymorph of LiAlSiO4 in β-LiAlSiO4/Cu composite indicates that the phase transformation caused by thermal mismatch stress may be easier than that caused by hydrostatic pressure.