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S. J. L. Billinge

Publications and source records attributed to S. J. L. Billinge.

At least 37 records · Page 2Linked to original sources
arXiv2007

Quantitative size-dependent structure and strain determination of CdSe nanoparticles using atomic pair distribution function analysis

The size-dependent structure of CdSe nanoparticles, with diameters ranging from 2 to 4 nm, has been studied using the atomic pair distribution function (PDF) method. The core structure of the measured CdSe nanoparticles can be described in terms of the wurtzite atomic structure with extensive stacking faults. The density of faults in the nanoparticles ~50% . The diameter of the core region was extracted directly from the PDF data and is in good agreement with the diameter obtained from standard characterization methods suggesting that there is little surface amorphous region. A compressive strain was measured in the Cd-Se bond length that increases with decreasing particle size being 0.5% with respect to bulk CdSe for the 2 nm diameter particles. This study demonstrates the size-dependent quantitative structural information that can be obtained even from very small nanoparticles using the PDF approach.

cond-mat.soft↗
arXiv2006

Phase separation and nanostructuring in the thermoelectric material PbTe1-xSx

The average and local structures of the (PbTe)1-x(PbS)x system of thermoelectric materials has been studied using the Rietveld and atomic pair distribution function (PDF) methods. Samples with 0:25 < x are macroscopically phase separated. Phase separation was suppressed in a quenched x = 0:5 sample which, nonetheless, exhibited a partial spinodal decomposition. The promising thermoelectric material with x = 0:16 showed intermediate behavior. Combining TEM and bulk scattering data suggests that the sample is a mixture of PbTe rich material and a partially spinodally decomposed phase similar to the quenched 50% sample. This results in a nano-meter scale inhomogeneous material that accounts for its very low thermal conductivity.

cond-mat.mtrl-sci↗
arXiv2006

Understanding the insulating phase in CMR manganites: Shortening of the Jahn-Teller long-bond across the phase diagram of La_{1-x} Ca_x Mn O_3

The detailed evolution of the magnitude of the local Jahn-Teller (JT) distortion in La_{1-x} Ca_x Mn O_3 is obtained across the phase diagram for 0 < x < 0.5 from high quality neutron diffraction data using the atomic pair distribution function (PDF) method. A local JT distortion is observed in the insulating phase for all Ca concentrations studied. However, in contrast with earlier local structure studies its magnitude is not constant, but decreases continuously with increasing Ca content. This observation is at odds with a simple small-polaron picture for the insulating state.

cond-mat.str-el↗
arXiv2006

Local atomic structure and discommensurations in the charge density wave of CeTe3

The local structure of CeTe3 in the incommensurate charge density wave (IC-CDW) state has been obtained using atomic pair distribution function (PDF) analysis of x-ray diffraction data. Local atomic distortions in the Te-nets due to the CDW are larger than observed crystallographically, resulting in distinct short and long Te-Te bonds. Observation of different distortion amplitudes in the local and average structures are explained by the discommensurated nature of the CDW since the PDF is sensitive to the local displacements within the commensurate regions whereas the crystallographic result averages over many discommensurated domains. The result is supported by STM data. This is the first quantitative local structural study within the commensurate domains in an IC-CDW system.

cond-mat.mtrl-sci↗
arXiv2006

Study of temperature dependent atomic correlations in MgB$_{2}$

We have studied the evolution with temperature of the local as well as the average crystal structure of MgB$_2$ using the real-space atomic pair distribution function (PDF) measured by high resolution neutron powder diffraction. We have investigated the correlations of the B-B and B-Mg nearest neighbor pair motion by comparing, in the wide temperature range from T=10 K up to T=600 K, the mean-square displacements (MSD) of single atoms with the mean-square relative displacements (MSRD) obtained from the PDF peak linewidths. The results show that the single atom B and Mg vibrations are mostly decoupled from each other, with a small predominance of positive (in phase) correlation factor for both the B-B and B-Mg pairs. The small positive correlation is almost temperature independent, in contrast with our theoretical calculations; this can be a direct consequence of the strong decay processes of the $E_{2g}$ anharmonic phonons.

cond-mat.supr-con↗
arXiv2005

Nominal doping and partition of doped holes between planar and apical orbitals in La2-xSrxCuO4

By considering both the average structural parameters obtained from Rietveld refinement of neutron powder diffraction data, and the local structural parameters obtained from the atomic pair distribution function, we have tested the recent hypothesis of Perry et al [Perry et al., Phys. Rev. B v.65, 144501 (2002)] that doping in La2-xSrxCuO4 system occurs as localized defects of predominantly Cud3z^2-r^2--Opz character associated with the Sr dopants accompanied by a local destruction of the Jahn-Teller distortion. While the structural parameters behave qualitatively according to the prediction of this model, a quantitative analysis indicates that doped holes predominantly appear in the planar Cudx^2-y^2--Opx,y band as is normally assumed. However, a small amount of the doped charge does enter the Cud3z^2-r^2--Opz orbitals and this should be taken into account when theoretical phase diagrams are compared to experiment. We present a calibration curve, p = x (1.00(1) - 0.45(7) x), for the planarcharge doping, p, vs strontium content, x, for the La2-xSrxCuO4 system.

cond-mat.supr-con↗
arXiv2005

Nanoscale clusters in the high performance thermoelectric AgPbmSbTem+2

The local structure of the AgPbmSbTem+2 series of thermoelectric materials has been studied using the atomic pair distribution function (PDF) method. Three candidate-models were attempted for the structure of this class of materials using either a one-phase or a two-phase modeling procedure. Combining modeling the PDF with HRTEM data we show that AgPbmSbTem+2 contains nanoscale inclusions with composition close to AgPb3SbTe5 randomly embedded in a PbTe matrix.

cond-mat.mtrl-sci↗
arXiv2004

The local atomic quasicrystal structure of the icosahedral Mg25Y11Zn64 alloy

A local and medium range atomic structure model for the face centred icosahedral (fci) Mg25Y11Zn64 alloy has been established in a sphere of r = 27 A. The model was refined by least squares techniques using the atomic pair distribution (PDF) function obtained from synchrotron powder diffraction. Three hierarchies of the atomic arrangement can be found: (i) five types of local coordination polyhedra for the single atoms, four of which are of Frank-Kasper type. In turn, they (ii) form a three-shell (Bergman) cluster containing 104 atoms, which is condensed sharing its outer shell with its neighbouring clusters and (iii) a cluster connecting scheme corresponding to a three-dimensional tiling leaving space for few glue atoms. Inside adjacent clusters, Y8-cubes are tilted with respect to each other and thus allow for overall icosahedral symmetry. It is shown that the title compound is essentially isomorphic to its holmium analogue. Therefore fci-Mg-Y-Zn can be seen as the representative structure type for the other rare earth analogues fci-Mg-Zn-RE (RE = Dy, Er, Ho, Tb) reported in the literature.

cond-mat.mtrl-sci↗
arXiv2004

Orbital correlations in the pseudo-cubic \emph{O} and rhombohedral ${R}$-phases of LaMnO$_3$

The local and intermediate structure of stoichiometric LaMnO$_3$ has been studied in the pseudocubic and rhombohedral phases at high temperatures (300 to 1150 K). Neutron powder diffraction data were collected and a combined Rietveld and high real space resolution atomic pair distribution function analysis carried out. The nature of the Jahn-Teller (JT) transition around 750 K is confirmed to be orbital order to disorder. In the high temperature orthorhombic ($O$) and rhombohedral ($R$) phases the MnO$_6$ octahedra are still fully distorted locally. The data suggest the presence of local orbitally ordered clusters of diameter $\sim 16$ Å($\sim$four MnO$_6$ octahedra) implying strong nearest neighbor JT anti-ferrodistortive coupling.

cond-mat.str-el↗
arXiv2003

Temperature-induced barium de-trapping from a double-well potential in Ba6Ge25

The crystal structure of barium-germanium clathrate Ba6Ge25 was studied using neutron powder diffraction in the temperature range 20-300K. The compound was found to be cubic (S.G. P4_1 23) in the entire temperature range. However, the fully-ordered model of the crystal structure (no split sites) is marginal at room temperature, and clearly fails at low temperature. A much better description of the crystal structure below 250K is given in terms of two split Ba sites, with random occupancies, for two out of three types of cages present in the Ba6Ge25 structure. The Ba atoms were found to interact strongly with the Ge host. The separation of the split Ba sites grows with decreasing temperature, with a sudden increase on cooling through the 200-250K temperature range, accompanied by an expansion of the entire crystal structure. We propose a simple model for this transition, based on temperature-induced de- trapping of Ba from a deep double-well potential. This transition is associated with sizeable anomalies in the transport and magnetic properties. The most significant of these effects, that is, the drop in electrical conductivity on cooling, can be easily explained within our model through the enhanced structural disorder, which would affect the relaxation time for all portions of the Fermi surface. We suggest that the other anomalies (increase in the absolute value of the negative Seebeck coefficient, decrease in the magnetic susceptibility) can be explained within the framework of the one-electron semi- classical model, without any need to invoke exotic electron-electron interaction mechanisms.

cond-mat.mtrl-sci↗
arXiv2003

Temperature dependent total scattering structural study of CaCu3Ti4O12

X-ray and neutron powder diffraction data as a function of temperature are analyzed for the colossal dielectric constant material CaCu3Ti4O12. The local structure is studied using atomic pair distribution function analysis. No evidence is found for enhanced oxygen displacement parameters suggesting that short-range octahedral tilt disorder is minimal. However, an unusual temperature dependence for the atomic displacement parameters of calcium and copper is observed. Temperature dependent modeling of the structure, using bond valence concepts, suggests that the calcium atoms become underbonded below approximately 260 K, which provides a rationale for the unusually high Ca displacement parameters at low temperature.

cond-mat.mtrl-sci↗
arXiv2002

Lattice dynamics and correlated atomic motion from the atomic pair distribution function

The mean-square relative displacements (MSRD) of atomic pair motions in crystals are studied as a function of pair distance and temperature using the atomic pair distribution function (PDF). The effects of the lattice vibrations on the PDF peak widths are modelled using both a multi-parameter Born von-Karman (BvK) force model and a single-parameter Debye model. These results are compared to experimentally determined PDFs. We find that the near-neighbor atomic motions are strongly correlated, and that the extent of this correlation depends both on the interatomic interactions and crystal structure. These results suggest that proper account of the lattice vibrational effects on the PDF peak width is important in extracting information on static disorder in a disordered system such as an alloy. Good agreement is obtained between the BvK model calculations of PDF peak widths and the experimentally determined peak widths. The Debye model successfully explains the average, though not detailed, natures of the MSRD of atomic pair motion with just one parameter. Also the temperature dependence of the Debye model largely agrees with the BvK model predictions. Therefore, the Debye model provides a simple description of the effects of lattice vibrations on the PDF peak widths.

cond-mat.mtrl-sci↗
arXiv2002

Structural compliance, misfit strain and stripe nanostructures in cuprate superconductors

Structural compliance is the ability of a crystal structure to accommodate variations in local atomic bond-lengths without incurring large strain energies. We show that the structural compliance of cuprates is relatively small, so that short, highly doped, Cu-O-Cu bonds in stripes are subject to a tensile misfit strain. We develop a model to describe the effect of misfit strain on charge ordering in the copper oxygen planes of oxide materials and illustrate some of the low energy stripe nanostructures that can result.

cond-mat.supr-con↗
arXiv2002

Chemical short range order obtained from the atomic pair distribution function

Many crystalline materials show chemical short range order and relaxation of neighboring atoms. Local structural information can be obtained by analyzing the atomic pair distribution function (PDF) obtained from powder diffraction data. In this paper, we present the successful extraction of chemical short range order parameters from the x-ray PDF of a quenched Cu_3Au sample.

cond-mat.mtrl-sci↗
arXiv2001

Structure of nanocrystalline materials using atomic pair distribution function analysis: study of LiMoS2

The structure of LiMoS2 has been experimentally determined for the first time. The approach of atomic pair distribution function (PDF) analysis was used because of the lack of well defined Bragg peaks due to the short structural coherence (~ 50 A) in this intercalation compound. This represents one of the first instances where the structure of such poorly diffracting material was determined from the total scattering including diffuse as well as Bragg scattering. The reduction of Mo by Li results in Mo-Mo bonding with the formation of chains of distorted Mo-S6 octahedra. Using the refined structural parameters the electronic band-structure for this material has been calculated and is in good agreement with observed material properties.

cond-mat.mtrl-sci↗
arXiv2001

Local Atomic Strain in ZnSe(1-x)Te(x) from High Real Space Resolution Neutron Pair Distribution Function Measurements

High real-space resolution atomic pair distribution functions (PDFs) have been obtained from ZnSe(1-x)Te(x) using neutron powder diffraction. Distinct Zn-Se and Zn-Te nearest neighbor (nn) bonds, differing in length by delta_r= 0.14Angstroms, are resolved in the measured PDF allowing the evolution with composition of the individual bond-lengths to be studied. The local bond-lengths change much more slowly with doping than the average bond-length obtained crystallographically. The nn bond-length distributions are constant with doping but higher-neighbor pair distributions broaden significantly indicating that most of the strain from the alloying is accommodated by bond-bending forces in the alloy. The PDFs of alloys across the whole doping range are well fit using a model based on the Kirkwood potential. The resulting PDFs give excellent agreement with the measured PDFs over the entire alloy range with no adjustable parameters.

cond-mat.mtrl-sci↗
arXiv2000

Structural response to local charge order in underdoped but superconducting La2-x(Sr,Ba)xCuO4

We report an anomalous local structural response in the CuO2 planes associated with the appearance of charge inhomogeneities at low temperature in underdoped but superconducting La2-x(Sr,Ba)xCuO4. We used pair distribution function analysis of neutron powder diffraction data. The increase in the Cu-O bond length distribution at low temperature has an onset temperature which correlates with observations of charge and spin freezing seen by other probes.

cond-mat.supr-con↗