December 2, 2016 01:43 PM

Thermoelectric materials have a wide range of applications, such as commercial refrigeration, energy-efficient engines, etc.  Their effectiveness has certain limitations, however, expressed by a dimensionless figure of merit, ZT = (α2σ/κ)T, where α is the Seebeck coefficient, σ the electrical conductivity, κ the thermal conductivity, and T the temperature. To obtain insight into the effects of structure and phase variation on the figure of merit, a technique for in-situ high-pressure thermoelectricity measurement has been developed at the HPCAT by utilizing a Paris-Edinburgh press. The dedicated sample cell design is composed of an asymmetric heat source, directional heat flow guides, thermally transparent but electrically insulating windows, and thermocouple electrode wires of which the Seebeck coefficients are already known.

December 2, 2016 01:59 PM

Among thousands of known chemical interactions between common elements in the periodic table, there remain bonds that are curiously absent. Iron-bismuth is such a system, exhibiting complete immiscibility even in the liquid state. Using laser-heated diamond anvil cells, recent experiments at HPCAT have revealed a new form of intermetallic FeBi2. The new material, formed at 1500 K and 30 GPa, possesses the Al2Cu-type structure featuring Fe coordinated by eight Bi atoms in face-sharing square antiprisms along the c-axis. By combining the paramagnetism of iron with the spin-orbit coupling inherent to bismuth, the new material may display transformative magnetic properties. This research, conducted at beamline 16-ID-B, is a collaboration between Danna Freedman (Chemistry) and Steve Jacobsen (Earth and Planetary Sciences) from Northwestern and Yue Meng (Carnegie Institution of Washington).

September 22, 2016 12:44 PM

A phase transition with an abrupt volume collapse of >20% could be associated with dramatic changes of electronic orbitals and/or spin-states of involved transition metals. Using HPCAT capabilities of high-pressure X-ray diffraction and X-ray emission spectroscopy, the mechanism of the pressure-driven lattice collapse in MnS and MnSe is studied. Both MnS and MnSe exhibit a rocksalt-to-MnP phase transition under compression with ~22-23% unit-cell volume changes at a similar pressure range around 25GPa. The phase transition was found to be coupled with the Mn2+(d5) spin-state transition from S=5/2 to S=1/2 and the formation of Mn-Mn intermetallic bonding. The mutual relationship between pressure-driven lattice collapse and the orbital/spin-state of Mn2+ in manganese chalcogenides provides an insight toward the exploration of new metastable phases with exceptional functionalities. (Published in Wang, et al., Angew. Chem. Int.

September 9, 2016 03:03 PM

Electronic topological transition (ETT), where new features occur in the topology of the Fermi surface, has strong influences on structural, mechanical, and electronic properties of materials. Recent nuclear resonant inelastic x-ray scattering experiments at HPCAT strongly support a thermally driven ETT that drives anomalous changes in phonon dynamics. The thermally induced ETT in FeTi causes an increased electronic screening for the atom displacements in the M5 phonon mode and an adiabatic electron phonon interaction with an unusual temperature dependence. This behavior is consistent with the atomic displacement pattern, in which (110) planes slide in opposite [110] directions. From the phonon polarization vectors, it is found that the magnitude of the Fe displacement is at least twice as that of Ti. [F. C. yang et al, Phys. Rev. Lett., 117, 076402, (2016)]

September 9, 2016 02:53 PM

A hallmark of the iron-based superconductors is the strong coupling between magnetic, structural and electronic degrees of freedom. Using HPCAT capabilities of high-pressure x-ray diffraction and time-domain Mössbauer spectroscopy, the nematicity and magnetism in FeSe under applied pressure are found to be strongly coupled. Distinct structural and magnetic transitions are observed for pressures between 1.0 and 1.7 GPa and merge into a single first-order transition for pressures ≳1.7 GPa, reminiscent of what has been found for the evolution of these transitions in the prototypical system Ba(Fe1−xCox)2As2. Our results are consistent with a spin-driven mechanism for nematic order in FeSe and provide an important step towards a universal description of the normal state properties of the iron-based superconductors. [K. Kothapalli et al, Nature Comm. 7, 12728 (2016)]


August 17, 2016 12:13 PM

Perovskite solar cells (PSCs) have attracted much attention owing to their high power conversion efficiency and low manufacturing cost.  Power conversion efficiencies of over 20% have recently been achieved with lead-based organic-inorganic halide PSCs, but these have relatively low structural stability, and the use of lead in the manufacturing process introduces problems of toxicity.  Recently, organotin perovskites have been synthesized as lead-free alternatives, and a number of chemical and processing modifications have been applied to these compounds in an effort to tune their crystal structure and electronic properties, with the hope of yielding more stable compounds which exhibit superior absorption and conductivity.

August 4, 2016 11:53 AM

Molybdenum disulfide (MoS2) attracts extensive interests because of its peculiar bandgap properties and the two-dimensional structure. The layer-dependent band structure has been studied by many theoretical calculations, including its evolution with pressure, e.g., the increase of the direct band gap by applying pressures. It is believed that the band-gap behavior is closely related to lattice strains. However, no general description has been established. A recent study conducted at HPCAT reveals a structural characteristic that is strongly correlated to bandgap behavior, regardless of its form as monolayer or bulk.  There are two unique structural parameters describing the strain of MoS2: in-plane S-Mo-S bond-angle φ and its out-of-plane pair θ. The trends in φ and θ have been determined as a function of external pressure using high-pressure X-ray powder diffraction for bulk MoS2.

August 4, 2016 01:07 PM

The lanthanide elements show many interesting phenomena such as heavy fermion metal properties and unconventional superconductivity due to their ƒ electrons. Among all lanthanide metals, only Ce and Eu are known to become superconducting under pressure. In order to understand the interplay between magnetism and superconductivity in europium at extreme pressures, experiments using X-ray emission spectroscopy (XES) at HPCAT and nuclear forwarding scattering (NFS) at sector 3 of APS have been done recently. With pressure above 80GPa, Eu has a critical superconducting temperature around 2K. Magnetic order in Eu is found to collapse just above 80GPa in association with the superconducting state according NFS spectra; however no change in the Lγ1 line of Eu in XES is observed up to 119GPa. These results indicate that the highly localized character of the 4ƒ7 magnetic state in Eu remains intact and Eu remains nearly divalent to at least 119GPa.

August 1, 2016 06:24 PM

Materials combining the hardness and strength of diamond with the higher thermal stability of cubic boron nitride (cBN) have broad potential value in science and engineering. Reacting nanodiamond with cBN at high pressures and high temperatures provides a pathway to such materials. Postdoctoral associate Dr. Xiaobing Liu, with CDAC partner Steve Jacobsen at Northwestern University, report the fabrication of C2-BN nanocomposite (2:1 ratio of diamond to cBN), measuring up to 10 mm in longest dimension. The nanocomposite consists of randomly-oriented, diamond and cBN domains of 50-250 nm, stitched by sp3 hybridized C-B and C-N bonds. B-C-N solid solution within the sutures leads to bulk p-type semiconductivity with an activation energy of 6.2 meV. The new study, published in the 27 July issue of Scientific Reports, shows that dislocations near the sutures accommodate lattice mismatch between diamond and cBN.