Biomedical subjects

Peter J Chupas

Publications and source records attributed to Peter J Chupas.

7 recordsLinked to original sources

PubMed2006

Compositional dependence of negative thermal expansion in the Prussian Blue analogues M(II)Pt(IV)(CN)6 (M = Mn, Fe, Co, Ni, Cu, Zn, Cd).

The effect of M(II) substitution on the magnitude of the negative thermal expansion (NTE) behavior within a series of Prussian Blue analogues, M(II)Pt(IV)(CN)(6) for M(II) = Mn, Fe, Co, Ni, Cu, Zn, Cd, has been investigated using variable-temperature powder X-ray diffraction (100-400 K). The NTE behavior varies widely with M(II) substitution, from near zero thermal expansion in NiPt(CN)(6) (alpha = dl/l dT = -1.02(11) x 10(-)(6) K(-)(1)) up to a maximum in CdPt(CN)(6) (alpha = -10.02(11) x 10(-)(6) K(-)(1)). The trend in the magnitude of the NTE behavior, with increasing atomic number (Z) of the M(II) ion, follows the order Mn(II) > Fe(II) > Co(II) > Ni(II) < Cu(II) < Zn(II) < Cd(II), which correlates with the trends for M(II) cation size, the lattice parameter, and structural flexibility as indicated by the temperature-dependent structural refinements and Raman spectroscopy. Analysis of the temperature dependence of the average structures suggests that the differences in the thermal expansion are due principally to the different strengths of the metal-cyanide binding interaction and, accordingly, the different energies of transverse vibration of the cyanide bridge, with enhanced NTE behavior for more flexible lattices.

Journal Article↗

PubMed2005

Direct observation of a transverse vibrational mechanism for negative thermal expansion in Zn(CN)2: an atomic pair distribution function analysis.

The instantaneous structure of the cyanide-bridged negative thermal expansion (NTE) material Zn(CN)(2) has been probed using atomic pair distribution function (PDF) analysis of high energy X-ray scattering data (100-400 K). The temperature dependence of the atomic separations extracted from the PDFs indicates an increase of the average transverse displacement of the cyanide bridge from the line connecting the Zn(II) centers with increasing temperature. This allows the contraction of non-nearest-neighbor Zn...Zn' and Zn...C/N distances despite the observed expansion of the individual direct Zn-C/N and C-N bonds. Thus, this analysis provides definitive structural confirmation that an increase in the average displacement of bridging atoms is the origin of the NTE behavior. The lattice parameters reveal a slight reduction in the NTE behavior at high temperature from a minimum coefficient of thermal expansion (alpha = dl/ldT) of -19.8 x 10(-6) K(-1) below 180 K, which is attributed to interaction between the doubly interpenetrated frameworks that comprise the structure.

Journal Article↗

PubMed2005

Selective recovery of dynamic guest structure in a nanoporous prussian blue through in situ X-ray diffraction: a differential pair distribution function analysis.

We use in situ X-ray diffraction to obtain data suitable for differential PDF analysis, providing unique insight into the structure of weakly bound, dynamic N2 molecules in the Prussian blue system Mn3[Co(CN)6]2.x{N2}. The differential PDF shows a distributed orientation of N2 molecules constrained to sites centered about the (1/4,1/4,1/4) positions within the pores. The results show a subtle response of the framework to guest loading which corresponds principally to the perturbation of the Mn ion coordination.

Journal Article↗

PubMed2005

Quantitative high-pressure pair distribution function analysis.

The collection of scattering data at high pressure and temperature is now relatively straightforward thanks to developments at high-brightness synchrotron radiation facilities. Reliable data from powders, that are suitable for structure determination and Rietveld refinement, are routinely collected up to about 30 GPa in either a large-volume high-pressure apparatus or diamond anvil cell. In those cases where the total elastic scattering is of interest, as it is in the case of nano-crystalline and glassy materials, technical developments, including the use of focused high-energy X-rays (>80 keV), are advantageous. Recently completed experiments on nano-crystalline materials at the 1-ID beamline at the Advanced Photon Source suggest that quantitative data, suitable for pair distribution function analysis, can be obtained.

Journal Article↗

PubMed2005

Short- and long-range order in the positive electrode material, Li(NiMn)0.5O2: a joint X-ray and neutron diffraction, pair distribution function analysis and NMR study.

The local environments and short-range ordering of LiNi(0.5)Mn(0.5)O(2), a potential Li-ion battery positive electrode material, were investigated by using a combination of X-ray and neutron diffraction and isotopic substitution (NDIS) techniques, (6)Li Magic Angle Spinning (MAS) NMR spectroscopy, and for the first time, X-ray and neutron Pair Distribution Function (PDF) analysis, associated with Reverse Monte Carlo (RMC) calculations. Three samples were studied: (6)Li(NiMn)(0.5)O(2), (7)Li(NiMn)(0.5)O(2), and (7)Li(NiMn)(0.5)O(2) enriched with (62)Ni (denoted as (7)Li(ZERO)Ni(0.5)Mn(0.5)O(2)), so that the resulting scattering length of Ni atoms is null. LiNi(0.5)Mn(0.5)O(2) adopts the LiCoO(2) structure (space group Rm) and comprises separate lithium layers, transition metal layers (Ni, Mn), and oxygen layers. NMR experiments and Rietveld refinements show that there is approximately 10% of Ni/Li site exchange between the Li and transition metal layers. PDF analysis of the neutron data revealed considerable local distortions in the layers that were not captured in the Rietveld refinements performed using the Bragg diffraction data and the LiCoO(2) structure, resulting in different M-O bond lengths of 1.93 and 2.07 Angstroms for Mn-O and Ni/Li-O, respectively. Large clusters of 2400-3456 atoms were built to investigate cation ordering. The RMC method was then used to improve the fit between the calculated model and experimental PDF data. Both NMR and RMC results were consistent with a nonrandom distribution of Ni, Mn, and Li cations in the transition metal layers; both the Ni and Li atoms are, on average, close to more Mn ions than predicted based on a random distribution of these ions in the transition metal layers. Constraints from both experimental methods showed the presence of short-range order in the transition metal layers comprising LiMn(6) and LiMn(5)Ni clusters combined with Ni and Mn contacts resembling those found in the so-called "flower structure" or structures derived from ordered honeycomb arrays.

Journal Article↗

PubMed2004

Measuring Brønsted acid densites in zeolite HY with diphosphine molecules and solid state NMR spectroscopy.

The use of 31P MAS NMR spectroscopy, combined with diphenyldiphosphine Ph2P(CH2)nPPh2 probe molecules with two basic groups (n = 1, 3, and 6, corresponding to maximum P-P separations of approximately 3.0, 5.6, and 9.4 A, respectively), to investigate both acidities and distances between Brønsted acid sites in zeolite HY (Si/Al = 2.6) is demonstrated in this communication. More than 90% of the Ph2P(CH2)6PPh2 molecules are doubly protonated on zeolite HY at a loading level of 12 molecules per unit cell, indicating that there are at least 12 pairs of Brønsted acid sites about 9 A apart. Similarly, experiments involving Ph2P(CH2)3PPh2 show that there are only six pairs of Brønsted acid sites separated by a distance of 6 A. Only approximately 60% of the Ph2PCH2PPh2 molecules were doubly protonated for a loading level of 4 molecules/unit cell, as not all of the Brønsted acid sites were sufficiently acidic to protonate both ends of this molecule. 31P 2D double quantum NMR spectroscopy was used to confirm the spectral assignments.

Journal Article↗

PubMed2004

Probing local and long-range structure simultaneously: an in situ study of the high-temperature phase transition of alpha-AlF3.

We show in this Communication that the combination of Rietveld and pair distribution function (PDF) analyses allows unique insight into the nature and driving force of the phase transition of alpha-AlF3, which was not available from conventional structural analysis methods alone. The use of image plate technology allows structural changes to be followed in "real time" and reduces the time required to collect high-resolution PDF data from hours (with a conventional solid state detector) to seconds. This methodology produces raw data that can simultaneously be analyzed by both Rietveld and PDF analysis.

Journal Article↗