A5069959280- X-ray Diffraction in Crystallography
- Crystallization and Solubility Studies
- Magnetic and transport properties of perovskites and related materials
- Advanced Condensed Matter Physics
- Rare-earth and actinide compounds
- Multiferroics and related materials
- Iron-based superconductors research
- High-pressure geophysics and materials
- Physics of Superconductivity and Magnetism
- Nuclear Physics and Applications
- Ferroelectric and Piezoelectric Materials
- Inorganic Chemistry and Materials
- Magnetic Properties of Alloys
- Electronic and Structural Properties of Oxides
- Nuclear Materials and Properties
- Thermal Expansion and Ionic Conductivity
- Thermodynamic and Structural Properties of Metals and Alloys
- Microwave Dielectric Ceramics Synthesis
- Nuclear materials and radiation effects
- Gamma-ray bursts and supernovae
- Advanced Thermoelectric Materials and Devices
- Crystallography and molecular interactions
- Laser-Plasma Interactions and Diagnostics
- Solid-state spectroscopy and crystallography
- Superconducting Materials and Applications
Los Alamos National Laboratory
2015-2025
Dynamic Imaging (United Kingdom)
2022
Czech Academy of Sciences, Institute of Plasma Physics
2016
Czech Academy of Sciences, Institute of Physics
2016
University of California, Santa Barbara
2007-2013
Los Alamos National Security (United States)
2009-2012
Rutgers, The State University of New Jersey
2011
Universidad de Oviedo
2011
University of Virginia
2011
University of Nova Gorica
2011
The oxide garnet Y3Al5O12 (YAG), when substituted with a few percent of the activator ion Ce3+ to replace Y3+, is luminescent material that nearly ideal for phosphor-converted solid-state white lighting. local environments small number ions are known critically influence optical properties phosphor. Using combination powerful experimental methods, nature these determined and correlated macroscopic Ce-substituted YAG. rigidity structure established shown play key role in high quantum yield...
Ultra-high resolution synchrotron diffraction data for $\rm GdBaCo_2O_{5.5}$ throw new light on the metal-insulator transition of Co$^{3+}$ Ba-cobaltites. An anomalous expansion CoO$_6$ octahedra is observed at phase heating, while CoO$_5$ pyramids show normal shrinking closing gap. The insulator-to-metal attributed to a sudden excitation some electrons in ($t_{2g}^6$ state) into Co $e_g$ band (final $t_{2g}^4e_g^2$ state). $t_{2g}^5e_g^1$ state does not change and structural study also...
We report long-range ordered antiferromagnetism concomitant with local iron displacements in the spin-ladder compound BaFe$_2$Se$_3$. Short-range magnetic correlations, present at room temperature, develop into antiferromagnetic order below T$_N$ = 256 K, no superconductivity down to 1.8 K. Built of ferromagnetic Fe$_4$ plaquettes, ground state correlates Fe atoms. These imply significant magnetoelastic coupling FeX$_4$-based materials, an ingredient hypothesized be important emergence...
Geopolymer cement is fast becoming a technologically important alternative to ceramics and traditional cement. However, the amorphous nature of phases which participate in molecular processes occurring during evolution geopolymer gel has made nanoscale research challenging. Here, for first time, local structural correlations metakaolin‐based have been elucidated using situ neutron pair distribution function analysis, following changes due dissolution repolymerization processes. Over initial...
Superconductivity in iron selenides has experienced a rapid growth, but not without major inconsistencies the reported properties. For alkali-intercalated selenides, even structure of superconducting phase is subject debate, part because onset superconductivity affected much more delicately by stoichiometry and preparation than cuprate or pnictide superconductors. If high-quality, pure, intercalated are ever to be made, intertwined physics chemistry must explained systematic studies how...
Structural intricacies of the orange-red nitride phosphor system La(3-x)Ce(x)Si6N11 (0 < x ≤ 3) have been elucidated using a combination state-of-the art tools, in order to understand origins exceptional optical properties this important solid-state lighting material. In addition, end-member (x = compound, Ce3Si6N11, are described for first time. A synchrotron powder X-ray diffraction and neutron scattering is employed establish site preferences rigid nature structure, which characterized by...
Using high-resolution neutron powder diffraction we have determined the average and local structure of YCrO3 in order to explain recently reported properties YCrO3.
We report a spectacularly high orbital/charge ordering temperature ${T}_{\mathrm{CO}}=475\mathrm{K},$ in ${\mathrm{Bi}}_{1/2}{\mathrm{Sr}}_{1/2}{\mathrm{MnO}}_{3}.$ The charges order approximately $350\mathrm{K}$ above the predicted by bandwidth tuning mechanism ${\mathrm{Ln}}_{1/2}(\mathrm{C}\mathrm{a},\mathrm{S}\mathrm{r}{)}_{1/2}{\mathrm{MnO}}_{3}$ family. This transition takes place $150\mathrm{K}$ that ${\mathrm{Bi}}_{1/2}{\mathrm{Ca}}_{1/2}{\mathrm{MnO}}_{3}$ which is much more...
Magnetic, transport, and optical measurements performed on ${\mathrm{GdBaCo}}_{2}{\mathrm{O}}_{5.5}$ under pulsed magnetic fields up to 35 T have allowed us determine its complete ${\ensuremath{\mu}}_{0}H\ensuremath{-}T$ phase diagram. A first-order field-induced transition takes place below the N\'eel point ${(T}_{N}\ensuremath{\approx}255 \mathrm{K}),$ which implies appearance of a ferromagnetic component steep changes in conductivity. The...
Inelastic neutron measurements of the high-temperature lattice excitations in NaI show that thermal equilibrium at 555 K an intrinsic mode, localized three dimensions, occurs a single frequency near center spectral phonon gap, polarized along [111]. At higher temperatures mode gains intensity. Higher energy inelastic and x-ray scattering on room-temperature crystal indicate creation ground state is 299 meV.
We present evidence for possibly the highest magnetic ordering temperature in any compound without $3d$ transition elements. Neutron powder diffraction measurements, at both time-of-flight and constant wavelength sources, were performed on two independently prepared ${\mathrm{SrTcO}}_{3}$ powders. adopts a distorted perovskite structure with $G$-type antiferromagnetic has moment of $1.87(4){\ensuremath{\mu}}_{B}$ per Tc cation room an extraordinarily high N\'eel point close to $750\text{...
Here we show that the 2.80(8)${\ensuremath{\mu}}_{\mathrm{B}}$ Fe${}^{\ensuremath{-}1}$ block antiferromagnetic order of BaFe${}_{2}$Se${}_{3}$ transforms into stripe in KFe${}_{2}$Se${}_{3}$ with a decrease moment to 2.1(1)${\ensuremath{\mu}}_{\mathrm{B}}$ Fe${}^{\ensuremath{-}1}$. This reduction is larger than expected from change electron count Ba${}^{2+}$ K${}^{+}$, and occurs loss displacements Fe atoms ideal positions ladders, as found by neutron pair distribution function analysis....
Based on specific heat and magnetoresistance measurements, we report that a "heavy" electronic state exists below $T \approx$ 20 K in KNi$_2$Se$_2$, with an increased carrier mobility enhanced effective band mass, $m$* = 6$m_b$ to 18$m_b$. This evolves into superconductivity at $T_c$ 0.80(1) K. These properties resemble of many-body heavy-fermion state, which derives from the hybridization between localized magnetic states conduction electrons. Yet, no evidence for magnetism or order is...
First principles density functional theory DFT+U calculations and experimental neutron diffraction structure analyses were used to determine the low-temperature crystallographic magnetic of bixbyite Mn2O3. The energies various arrangements, calculated from first principles, fit a cluster-expansion model using Bayesian method that overcomes problem underfitting caused by limited number input configurations. was predict lowest-energy states. Experimental determination benefited optimized...
Calcium scandate (CaSc2O4) substituted with small amounts (<1%) of Ce3+ is a recently discovered bright-green-emitting phosphor favorable light absorption and emission properties robust temperature stability that make it well-suited for solid-state white-lighting applications. Combined analyses scattering, nuclear magnetic resonance (NMR), electron paramagnetic (EPR), photoluminescence measurements establish the compositional structural origins macroscopic optical this material. Simultaneous...
Several oxy-apatite materials La10−xSrx(TO4)6O3−0.5x (T = Ge, Si; 10−x 9.00, 8.80, 8.65 and 8.00) La9.33(Si1−xGexO4)6O2 (x 0, 0.5, 0.67) have been prepared as highly crystalline phases. The impedance study showed that all samples are oxide ion conductors. However, bulk conductivities changed by more than 2 orders of magnitude at a given temperature for some compositions. A thorough on the oxygen sublattices oxygen-stoichiometric oxy-apatites has carried out. Constant-wavelength neutron...
We report magnetic neutron-diffraction and electrical resistivity studies on single crystals of the heavy-fermion antiferromagnet ${\mathrm{CeRhIn}}_{5}$ at pressures up to 2.3 GPa. These experiments show that staggered moment Ce incommensurate structure change weakly with applied pressure 1.63 GPa, where resistivity, specific heat nuclear quadrupole resonance measurements confirm presence bulk superconductivity. This work places important constraints an interpretation relationship between...
The isovalent substitution of Te for Se in the superconducting $\ensuremath{\alpha}\text{-FeSe}$ raises ${T}_{C}$ where average chalcogen--Fe bond angle decreases and chalcogen-Fe distance increases. Locally, however, ions do not share same site have two distinct $z$ coordinates, contrast to what is presumed $P4/nmm$ symmetry. local between chalcogens Fe increases with substitution, consistent rise ${T}_{C}$, Fe--Te bonds become shorter than binary FeTe, while Fe--Se stay as binary. Ab...
Displacive phase transitions in perovskite-like solid solutions AgNb1/2Ta1/2O3 were studied using several diffraction and spectroscopic techniques sensitive to average local structures. The room-temperature of (M2) is analogous that the end-member AgNbO3 exhibits Pbcm orthorhombic symmetry with lattice parameters √2ac × 4ac (ac ≈ 4 Å refers an ideal cubic perovskite cell). This structure combines complex octahedral tilting antipolar B-cation (Nb, Ta) displacements. Similar AgNbO3, at higher...
We have succeeded in increasing up to 150 K the Curie temperature ${\text{Fe}}_{64}{\text{Ni}}_{36}$ invar alloy by means of a severe mechanical treatment followed heating 1073 K. The behavior is still present as revealed combination magnetic measurements with neutron and x-ray techniques under extreme conditions, such high pressure. proposed explanation based selective induced microstrain around Fe atoms, which causes slight increase Fe-Fe interatomic distances, thus reinforcing...
Oxygen-deficient BaTiO${}_{3\ensuremath{-}\ensuremath{\delta}}$ exhibits an insulator-metal transition with increasing $\ensuremath{\delta}$. We performed neutron total scattering measurements to study structural evolution across in BaTiO${}_{3\ensuremath{-}\ensuremath{\delta}}$. Despite its significant impact on resistivity, slight oxygen reduction ($\ensuremath{\delta}=0.09$) caused only a small disturbance the local doublet splitting of Ti-O bond. This finding implies that polarization is...
We report the results of a systematic investigation phase diagram iron-based superconductor system, Ba1-xNaxFe2As2, from x = 0.1 to 1.0 using high resolution neutron and x-ray diffraction magnetization measurements. find that coincident structural magnetic transition an orthorhombic (O) structure with space group Fmmm striped antiferromagnet (AF) F(C)mm'm' in Ba1-xNaxFe2As2 is first order. A complete suppression observed by 0.30, bulk superconductivity occurs at critical concentration near...
The local distortion indicated by the split of Cu/Sn-Mn bonds for negative thermal expansion (NTE) materials Cu1−xSnxNMn3 (x = 0.1 and 0.5) was observed using neutron pair distribution function. upon Sn doping is suggested to be attributable fluctuation in hybridization Mn d with p orbitals. Accordingly, antiferromagnetic (AFM) coupling mediated p-d fluctuates strength. Consequently, AFM transition closely coupled volume change broadened, leading NTE.