Pavol Juhas - Academia.edu (original) (raw)

Papers by Pavol Juhas

The dielectric behavior of Pb(Sc_2/3W_1/3)O3 perovskite relaxor shows a completely different resp... more The dielectric behavior of Pb(Sc_2/3W_1/3)O3 perovskite relaxor shows a completely different response to the substitutions of PbTiO3 (PSW-PT) and PbZrO3 (PSW-PZ). For PZ the transition temperature (T_ε,max) grows with x; however it is decreased for additions of Ti up to x <= 0.25, in spite of PT having much higher Curie temperature (763 K) than PZ (503 K). To understand

The pair distribution function (PDF) method provides detailed information about nanostructure and... more The pair distribution function (PDF) method provides detailed information about nanostructure and efficient software is available for refining PDF data to starting structures provided by the user. Starting structures may be developed using intuition, homologous structures from the literature, or by using ab-initio theoretical structures. However, in studies of nanoparticles good starting structures are hard to identify, particularly in systems

The atomic pair distribution function (PDF) is total scattering based technique, which includes b... more The atomic pair distribution function (PDF) is total scattering based technique, which includes both Bragg and diffuse scattering, can provide quantitative information about the local structure of the materials at different length scales. The PDF method is used to quantify intermediate-range order and address the size and structure in series of CdSe nanoparticles (NPs), with diameter sizes ranging from 2

SrRietveld is a highly automated software toolkit for Rietveld refinement. Compared to traditiona... more SrRietveld is a highly automated software toolkit for Rietveld refinement. Compared to traditional refinement programs, it is more efficient to use and easier to learn. It is designed for modern high throughput diffractometers and capable of processing large numbers of data-sets with minimal effort. The new software currently uses conventional Rietveld refinement engines, automating GSAS and FullProf refinements. However, as

Many materials at the nanoscale cannot benefit from crystallographic analysis and are unsuitable ... more Many materials at the nanoscale cannot benefit from crystallographic analysis and are unsuitable for refinement techniques that require an initial guess at the structure. One approach to overcoming these difficulties is the Liga algorithm, which generates structures relying solely on distances extracted from the atomic pair distribution function[1,2]. This method is shown to successfully reconstruct the buckyball from experimental data.

The paper describes an extension of the Liga algorithm for structure solution from atomic pair di... more The paper describes an extension of the Liga algorithm for structure solution from atomic pair distribution function (PDF), to handle periodic crystal structures with multiple elements in the unit cell. The procedure is performed in 2 separate steps - at first the Liga algorithm is used to find unit cell sites consistent with pair distances extracted from the experimental PDF.

The formation and growth mechanisms in the hydrothermal synthesis of SnO(2) nanoparticles from aq... more The formation and growth mechanisms in the hydrothermal synthesis of SnO(2) nanoparticles from aqueous solutions of SnCl(4)·5H(2)O have been elucidated by means of in situ X-ray total scattering (PDF) measurements. The analysis of the data reveals that when the tin(IV) chloride precursor is dissolved, chloride ions and water coordinate octahedrally to tin(IV), forming aquachlorotin(IV) complexes of the form [SnCl(x)(H(2)O)(6-x)]((4-x)+) as well as hexaaquatin(IV) complexes [Sn(H(2)O)(6-y)(OH)(y)]((4-y)+). Upon heating, ellipsoidal SnO(2) nanoparticles are formed uniquely from hexaaquatin(IV). The nanoparticle size and morphology (aspect ratio) are dependent on both the reaction temperature and the precursor concentration, and particles as small as ~2 nm can be synthesized. Analysis of the growth curves shows that Ostwald ripening only takes place above 200 °C, and in general the growth is limited by diffusion of precursor species to the growing particle. The c-parameter in the tetragonal lattice is observed to expand up to 0.5% for particle sizes down to 2-3 nm as compared to the bulk value. SnO(2) nanoparticles below 3-4 nm do not form in the bulk rutile structure, but as an orthorhombic structural modification, which previously has only been reported at pressures above 5 GPa. Thus, adjustment of the synthesis temperature and precursor concentration not only allows control over nanoparticle size and morphology but also the structure.

Journal of Applied Crystallography, 2015

ABSTRACT The methods currently used to calculate atomic pair distribution functions (PDFs) from o... more ABSTRACT The methods currently used to calculate atomic pair distribution functions (PDFs) from organic structural models do not distinguish between the intramolecular and intermolecular distances. Owing to the stiff bonding between atoms within a molecule, the PDF peaks arising from intramolecular atom–atom distances are much sharper than those of the intermolecular atom–atom distances. This work introduces a simple approach to calculate PDFs of molecular systems without building a supercell model by using two different isotropic displacement parameters to describe atomic motion: one parameter is used for the intramolecular, the other one for intermolecular atom–atom distances. Naphthalene, quinacridone and paracetamol were used as examples. Calculations were done with the DiffPy-CMI complex modelling infrastructure. The new modelling approach produced remarkably better fits to the experimental PDFs, confirming the higher accuracy of this method for organic materials.

Physical review letters, Jan 31, 2014

We present an x-ray pair distribution function (XPDF) analysis and extended x-ray absorption fine... more We present an x-ray pair distribution function (XPDF) analysis and extended x-ray absorption fine structure (EXAFS) data for ZrW2O8 (10-500 K) with a focus on the stiffness of the Zr-O-W linkage. The XPDF is highly sensitive to W-Zr and W-W correlations, but much less so to O-O or W-O correlations. The Zr-W peak in the XPDF data has a weak temperature dependence and, hence, this linkage is relatively stiff and does not permit bending of the Zr-O-W link. We propose that the low energy vibrational modes that lead to negative thermal expansion involve correlated rotations of ZrO6 octahedra that produce large <111> translations of the WO4 tetrahedra, rather than a transverse motion of O atoms that imply a flexible Zr-O-W linkage.

Journal of Applied Crystallography, 2004

An atomic pair distribution function (PDF) neutron powder diffraction roundrobin experiment was p... more An atomic pair distribution function (PDF) neutron powder diffraction roundrobin experiment was performed on six diffractometers at three spallation sources. Instrument-speci®c effects on the real-space PDF were investigated, such as ®nite measurement range, the instrument resolution and the asymmetric shape of diffraction peaks. Two illustrative samples, a perfectly long-rangeordered element, Pb, and a locally strained alloy ZnSe 0.5 Te 0.5 , were measured at low temperatures. Various aspects of the PDF were explored, either qualitatively by direct comparison or quantitatively via structural modelling. Future implementation of modelling codes incorporating some of these instrumental effects are also discussed.

Extending the technique of the perfectly matched layer ͑PML͒ to discrete lattice systems, a multi... more Extending the technique of the perfectly matched layer ͑PML͒ to discrete lattice systems, a multiscale method was proposed by To and Li ͓Phys. Rev. B 72, 035414 ͑2005͔͒, which was termed the perfectly matched multiscale simulation ͑PMMS͒. In this paper, we shall revise the proposed PMMS formulation, and extend it to multiple dimensions. It is shown in numerical simulations that the perfectly matched layer between the fine scale region and the coarse scale region can provide an efficient remedy to reduce spurious phonon reflections. We have found ͑i͒ the bridging projection operator stems from minimization of the temperature of an equilibrium system; ͑ii͒ for discrete lattice systems, the perfectly matched layer ͑PML͒ can be constructed by stretching the lattice constant, or the equilibrium atomic spacing, in the Fourier domain; ͑iii͒ the dispersive relation in the PML zone is significantly different from the one in the original lattice system, and the PML usually behaves like a low-frequency pass filter. This may be one of the mechanisms to eliminate the reflective waves at the multiscale interface. Moreover, we apply the multidimensional PMMS algorithm to simulate a screw dislocation passing through different scales.

The atomic pair distribution function (PDF) is total scattering based technique, which includes b... more The atomic pair distribution function (PDF) is total scattering based technique, which includes both Bragg and diffuse scattering, can provide quantitative information about the local structure of the materials at different length scales. The PDF method is used to quantify intermediate-range order and address the size and structure in series of CdSe nanoparticles (NPs), with diameter sizes ranging from 2

ABSTRACT A thiospinel CuIr2S4 exhibits a metal-insulator (MI) transition at T=230 K, with simulta... more ABSTRACT A thiospinel CuIr2S4 exhibits a metal-insulator (MI) transition at T=230 K, with simultaneous spin-dimerization and charge-ordering [1]. The transition can also be driven by extended exposure to the x-rays at low T [2, 3]. Total x-ray scattering study of CuIr2S4 was carried out using 100 KeV synchrotron beam and rapid acquisition pair distribution function (RAPDF) approach. The RAPDF results indicate consistency of the local and average structure at high T. At 100 K long x-ray exposure melts the long-range order (LRO) of the dimerization pattern, without affecting the local structure, in agreement with diffuse scattering result [3]. The dependence of the LRO related superlattice peak intensity on the exposure time reveals that the melting occurs within approximately 15 seconds of exposure under experimental conditions used. At 100 K the LRO is recovered without temperature increase quickly after the cessation of the beamtime exposure. Results on Cr doped samples will be addressed as well. [1] P.G. Radaelli et al., Nature 416, 155 (2002). [2] V. Kiryukhin et al., Phys. Rev. Lett. 97, 225503 (2006). [3] H. Ishibashi et al., Phys. Rev. B 66, 144424 (2002). This work is supported by the NSF under grant DMR-0304391. ANL is supported under DOE contract No. DE-AC02-06CH11357.

Neutron powder diffraction based atomic pair distribution functions (PDFs) are reported from the ... more Neutron powder diffraction based atomic pair distribution functions (PDFs) are reported from the new wide angular-range chopper spectrometer ARCS at the Spallation Neutron Source (SNS) at Oak Ridge National Laboratory (ORNL). The spectrometer was run in white-beam mode with no Fermi chopper. The PDF patterns of Ni and Al 2 O 3

The dielectric behavior of Pb(Sc_2/3W_1/3)O3 perovskite relaxor shows a completely different resp... more The dielectric behavior of Pb(Sc_2/3W_1/3)O3 perovskite relaxor shows a completely different response to the substitutions of PbTiO3 (PSW-PT) and PbZrO3 (PSW-PZ). For PZ the transition temperature (T_ε,max) grows with x; however it is decreased for additions of Ti up to x <= 0.25, in spite of PT having much higher Curie temperature (763 K) than PZ (503 K). To understand

The pair distribution function (PDF) method provides detailed information about nanostructure and... more The pair distribution function (PDF) method provides detailed information about nanostructure and efficient software is available for refining PDF data to starting structures provided by the user. Starting structures may be developed using intuition, homologous structures from the literature, or by using ab-initio theoretical structures. However, in studies of nanoparticles good starting structures are hard to identify, particularly in systems

The atomic pair distribution function (PDF) is total scattering based technique, which includes b... more The atomic pair distribution function (PDF) is total scattering based technique, which includes both Bragg and diffuse scattering, can provide quantitative information about the local structure of the materials at different length scales. The PDF method is used to quantify intermediate-range order and address the size and structure in series of CdSe nanoparticles (NPs), with diameter sizes ranging from 2

SrRietveld is a highly automated software toolkit for Rietveld refinement. Compared to traditiona... more SrRietveld is a highly automated software toolkit for Rietveld refinement. Compared to traditional refinement programs, it is more efficient to use and easier to learn. It is designed for modern high throughput diffractometers and capable of processing large numbers of data-sets with minimal effort. The new software currently uses conventional Rietveld refinement engines, automating GSAS and FullProf refinements. However, as

Many materials at the nanoscale cannot benefit from crystallographic analysis and are unsuitable ... more Many materials at the nanoscale cannot benefit from crystallographic analysis and are unsuitable for refinement techniques that require an initial guess at the structure. One approach to overcoming these difficulties is the Liga algorithm, which generates structures relying solely on distances extracted from the atomic pair distribution function[1,2]. This method is shown to successfully reconstruct the buckyball from experimental data.

The paper describes an extension of the Liga algorithm for structure solution from atomic pair di... more The paper describes an extension of the Liga algorithm for structure solution from atomic pair distribution function (PDF), to handle periodic crystal structures with multiple elements in the unit cell. The procedure is performed in 2 separate steps - at first the Liga algorithm is used to find unit cell sites consistent with pair distances extracted from the experimental PDF.

The formation and growth mechanisms in the hydrothermal synthesis of SnO(2) nanoparticles from aq... more The formation and growth mechanisms in the hydrothermal synthesis of SnO(2) nanoparticles from aqueous solutions of SnCl(4)·5H(2)O have been elucidated by means of in situ X-ray total scattering (PDF) measurements. The analysis of the data reveals that when the tin(IV) chloride precursor is dissolved, chloride ions and water coordinate octahedrally to tin(IV), forming aquachlorotin(IV) complexes of the form [SnCl(x)(H(2)O)(6-x)]((4-x)+) as well as hexaaquatin(IV) complexes [Sn(H(2)O)(6-y)(OH)(y)]((4-y)+). Upon heating, ellipsoidal SnO(2) nanoparticles are formed uniquely from hexaaquatin(IV). The nanoparticle size and morphology (aspect ratio) are dependent on both the reaction temperature and the precursor concentration, and particles as small as ~2 nm can be synthesized. Analysis of the growth curves shows that Ostwald ripening only takes place above 200 °C, and in general the growth is limited by diffusion of precursor species to the growing particle. The c-parameter in the tetragonal lattice is observed to expand up to 0.5% for particle sizes down to 2-3 nm as compared to the bulk value. SnO(2) nanoparticles below 3-4 nm do not form in the bulk rutile structure, but as an orthorhombic structural modification, which previously has only been reported at pressures above 5 GPa. Thus, adjustment of the synthesis temperature and precursor concentration not only allows control over nanoparticle size and morphology but also the structure.

Journal of Applied Crystallography, 2015

ABSTRACT The methods currently used to calculate atomic pair distribution functions (PDFs) from o... more ABSTRACT The methods currently used to calculate atomic pair distribution functions (PDFs) from organic structural models do not distinguish between the intramolecular and intermolecular distances. Owing to the stiff bonding between atoms within a molecule, the PDF peaks arising from intramolecular atom–atom distances are much sharper than those of the intermolecular atom–atom distances. This work introduces a simple approach to calculate PDFs of molecular systems without building a supercell model by using two different isotropic displacement parameters to describe atomic motion: one parameter is used for the intramolecular, the other one for intermolecular atom–atom distances. Naphthalene, quinacridone and paracetamol were used as examples. Calculations were done with the DiffPy-CMI complex modelling infrastructure. The new modelling approach produced remarkably better fits to the experimental PDFs, confirming the higher accuracy of this method for organic materials.

Physical review letters, Jan 31, 2014

We present an x-ray pair distribution function (XPDF) analysis and extended x-ray absorption fine... more We present an x-ray pair distribution function (XPDF) analysis and extended x-ray absorption fine structure (EXAFS) data for ZrW2O8 (10-500 K) with a focus on the stiffness of the Zr-O-W linkage. The XPDF is highly sensitive to W-Zr and W-W correlations, but much less so to O-O or W-O correlations. The Zr-W peak in the XPDF data has a weak temperature dependence and, hence, this linkage is relatively stiff and does not permit bending of the Zr-O-W link. We propose that the low energy vibrational modes that lead to negative thermal expansion involve correlated rotations of ZrO6 octahedra that produce large <111> translations of the WO4 tetrahedra, rather than a transverse motion of O atoms that imply a flexible Zr-O-W linkage.

Journal of Applied Crystallography, 2004

An atomic pair distribution function (PDF) neutron powder diffraction roundrobin experiment was p... more An atomic pair distribution function (PDF) neutron powder diffraction roundrobin experiment was performed on six diffractometers at three spallation sources. Instrument-speci®c effects on the real-space PDF were investigated, such as ®nite measurement range, the instrument resolution and the asymmetric shape of diffraction peaks. Two illustrative samples, a perfectly long-rangeordered element, Pb, and a locally strained alloy ZnSe 0.5 Te 0.5 , were measured at low temperatures. Various aspects of the PDF were explored, either qualitatively by direct comparison or quantitatively via structural modelling. Future implementation of modelling codes incorporating some of these instrumental effects are also discussed.

Extending the technique of the perfectly matched layer ͑PML͒ to discrete lattice systems, a multi... more Extending the technique of the perfectly matched layer ͑PML͒ to discrete lattice systems, a multiscale method was proposed by To and Li ͓Phys. Rev. B 72, 035414 ͑2005͔͒, which was termed the perfectly matched multiscale simulation ͑PMMS͒. In this paper, we shall revise the proposed PMMS formulation, and extend it to multiple dimensions. It is shown in numerical simulations that the perfectly matched layer between the fine scale region and the coarse scale region can provide an efficient remedy to reduce spurious phonon reflections. We have found ͑i͒ the bridging projection operator stems from minimization of the temperature of an equilibrium system; ͑ii͒ for discrete lattice systems, the perfectly matched layer ͑PML͒ can be constructed by stretching the lattice constant, or the equilibrium atomic spacing, in the Fourier domain; ͑iii͒ the dispersive relation in the PML zone is significantly different from the one in the original lattice system, and the PML usually behaves like a low-frequency pass filter. This may be one of the mechanisms to eliminate the reflective waves at the multiscale interface. Moreover, we apply the multidimensional PMMS algorithm to simulate a screw dislocation passing through different scales.

The atomic pair distribution function (PDF) is total scattering based technique, which includes b... more The atomic pair distribution function (PDF) is total scattering based technique, which includes both Bragg and diffuse scattering, can provide quantitative information about the local structure of the materials at different length scales. The PDF method is used to quantify intermediate-range order and address the size and structure in series of CdSe nanoparticles (NPs), with diameter sizes ranging from 2

ABSTRACT A thiospinel CuIr2S4 exhibits a metal-insulator (MI) transition at T=230 K, with simulta... more ABSTRACT A thiospinel CuIr2S4 exhibits a metal-insulator (MI) transition at T=230 K, with simultaneous spin-dimerization and charge-ordering [1]. The transition can also be driven by extended exposure to the x-rays at low T [2, 3]. Total x-ray scattering study of CuIr2S4 was carried out using 100 KeV synchrotron beam and rapid acquisition pair distribution function (RAPDF) approach. The RAPDF results indicate consistency of the local and average structure at high T. At 100 K long x-ray exposure melts the long-range order (LRO) of the dimerization pattern, without affecting the local structure, in agreement with diffuse scattering result [3]. The dependence of the LRO related superlattice peak intensity on the exposure time reveals that the melting occurs within approximately 15 seconds of exposure under experimental conditions used. At 100 K the LRO is recovered without temperature increase quickly after the cessation of the beamtime exposure. Results on Cr doped samples will be addressed as well. [1] P.G. Radaelli et al., Nature 416, 155 (2002). [2] V. Kiryukhin et al., Phys. Rev. Lett. 97, 225503 (2006). [3] H. Ishibashi et al., Phys. Rev. B 66, 144424 (2002). This work is supported by the NSF under grant DMR-0304391. ANL is supported under DOE contract No. DE-AC02-06CH11357.

Neutron powder diffraction based atomic pair distribution functions (PDFs) are reported from the ... more Neutron powder diffraction based atomic pair distribution functions (PDFs) are reported from the new wide angular-range chopper spectrometer ARCS at the Spallation Neutron Source (SNS) at Oak Ridge National Laboratory (ORNL). The spectrometer was run in white-beam mode with no Fermi chopper. The PDF patterns of Ni and Al 2 O 3