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Sarah L Price - One of the best experts on this subject based on the ideXlab platform.
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modelling organic crystal structures using distributed multipole and polarizability based model intermolecular potentials
Physical Chemistry Chemical Physics, 2010Co-Authors: Sarah L Price, Panagiotis G Karamertzanis, Maurice Leslie, Gareth W A Welch, Matthew Habgood, Louise S Price, Graeme M DayAbstract:Crystal structure prediction for organic molecules requires both the fast assessment of thousands to millions of crystal structures and the greatest possible accuracy in their relative energies. We describe a crystal Lattice simulation program, DMACRYS, emphasizing the features that make it suitable for use in crystal structure prediction for pharmaceutical molecules using accurate anisotropic atom–atom model intermolecular potentials based on the theory of intermolecular forces. DMACRYS can optimize the Lattice Energy of a crystal, calculate the second derivative properties, and reduce the symmetry of the spacegroup to move away from a transition state. The calculated terahertz frequency k = 0 rigid-body Lattice modes and elastic tensor can be used to estimate free energies. The program uses a distributed multipole electrostatic model (Qat, t = 00,…,44s) for the electrostatic fields, and can use anisotropic atom–atom repulsion models, damped isotropic dispersion up to R−10, as well as a range of empirically fitted isotropic exp-6 atom–atom models with different definitions of atomic types. A new feature is that an accurate model for the induction Energy contribution to the Lattice Energy has been implemented that uses atomic anisotropic dipole polarizability models (αat, t = (10,10)…(11c,11s)) to evaluate the changes in the molecular charge density induced by the electrostatic field within the crystal. It is demonstrated, using the four polymorphs of the pharmaceutical carbamazepine C15H12N2O, that whilst reproducing crystal structures is relatively easy, calculating the polymorphic Energy differences to the accuracy of a few kJ mol−1 required for applications is very demanding of assumptions made in the modelling. Thus DMACRYS enables the comparison of both known and hypothetical crystal structures as an aid to the development of pharmaceuticals and other speciality organic materials, and provides a tool to develop the modelling of the intermolecular forces involved in molecular recognition processes.
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a new polymorph of 5 fluorouracil found following computational crystal structure predictions
Journal of the American Chemical Society, 2005Co-Authors: Ashley T Hulme, Sarah L Price, Derek A TocherAbstract:A new polymorph of 5-fluorouracil has been obtained following a manual polymorph screen inspired by a computational crystal structure prediction search. It corresponds to the structure that was predicted to be the global minimum in Lattice Energy. The difficulty of crystallizing this simple structure with a rational hydrogen-bonding motif can be rationalized from the differential solvation of the functional groups.
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characterization of complicated new polymorphs of chlorothalonil by x ray diffraction and computer crystal structure prediction
Journal of the American Chemical Society, 2004Co-Authors: Maryjane Tremayne, Sarah L Price, Leanne Grice, James C Pyatt, Colin C Seaton, Benson M Kariuki, Helen Hoi Yan Tsui, Julian C CherrymanAbstract:A simultaneous experimental and computational search for polymorphs of chlorothalonil (2,4,5,6-tetrachloro-1,3-benzenedicarbonitrile) has been conducted, leading to the first characterization of forms 2 and 3. The crystal structure prediction study, using a specifically developed anisotropic atom−atom potential for chlorothalonil, gave as the global minimum in the Lattice Energy a structure that was readily refined against powder diffraction data to the known form 1 (P21/a). The structure of form 2 was solved and refined from powder diffraction data, giving a disordered structure in the R3m (166) space group (Z = 3). It could also be refined against a P1 ordered model, starting from a low-Energy hypothetical sheet structure found in the computational search. This shows that the disorder could be associated with the stacking of ordered sheets. The disordered structure for form 2 was later confirmed by single-crystal X-ray diffraction. The structure of form 3, determined from single-crystal diffraction, contains three independent molecules in the asymmetric unit in P21 (4) (Z = 6). Powder diffraction showed that this single-herringbone structure was similar to two low-Energy structures found in the search. Further analysis confirmed that form 3 has a similar Lattice Energy and contains elements from both these predicted structures, which can be considered as good approximations to the form 3 structure.
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a study of the known and hypothetical crystal structures of pyridine why are there four molecules in the asymmetric unit cell
CrystEngComm, 2002Co-Authors: A T Anghel, Graeme M Day, Sarah L PriceAbstract:The oldest crystal structure of pyridine is unusually complex, with four molecules in the asymmetric unit cell of Pna21 symmetry. In an attempt to understand why pyridine crystallises with 16 molecules in the unit cell, we have considered its thermodynamic stability relative to hypothetical pyridine structures. These were generated by a search for minima in the Lattice Energy of pyridine amongst the more common space groups, using the crystal structure prediction procedure MOLPAK followed by Lattice Energy minimisation using a distributed multipole-based intermolecular potential. We find over two dozen distinct crystal structures in the Energy gap of less than 6 kJ mol−1 between the corresponding models for the observed and most stable (hypothetical) structure. Adding harmonic phonon estimates of the intermolecular zero point Energy and entropy at the melting point of pyridine slightly improves the relative stability of the observed Z = 16 structure. Several of these hypothetical structures can be eliminated as only just mechanically stable, or because the growth rate of the crystal is estimated to be very slow by the attachment Energy model. Nevertheless, there are still over a dozen structures that appear competitive with the known structure as polymorphs of pyridine. Following these predictions, an intense experimental search has found a new polymorph of perdeutero-pyridine (form II), which was not found in the search. This structure is also predicted to be metastable with a similar Energy to form I. Although there is some evidence for kinetic factors favouring the observed structures, the metastable Z = 16 structure and the new form II remain a challenge for our understanding of crystallisation.
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which organic crystal structures are predictable by Lattice Energy minimisation
CrystEngComm, 2001Co-Authors: Theresa Beyer, Thomas C Lewis, Sarah L PriceAbstract:A survey of the molecules which have been used in crystal structure prediction studies is presented. The results of these studies have been analysed in terms of whether the experimentally observed crystal structures are found at or near the global minimum in the Lattice Energy. The results suggest that whilst some crystal structures can be predicted just on the basis of Lattice Energy searches, there is yet insufficient experience to judge for which molecules this energetic criterion is sufficient, within the limitations of current force-field accuracy. The molecules chosen to test crystal structure prediction methods appear to be biased away from the types that would be expected to be readily predictable and suitable for crystal engineering. The survey highlights the need for more theoretical and experimental collaboration to understand what determines whether a molecule's crystal structure will be so favourable that other polymorphs are unlikely.
Graeme M Day - One of the best experts on this subject based on the ideXlab platform.
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static and Lattice vibrational Energy differences between polymorphs
CrystEngComm, 2015Co-Authors: Jonas Nyman, Graeme M DayAbstract:A computational study of 1061 experimentally determined crystal structures of 508 polymorphic organic molecules has been performed with state-of-the-art Lattice Energy minimisation methods, using a hybrid method that combines density functional theory intramolecular energies with an anisotropic atom–atom intermolecular model. Rigid molecule Lattice dynamical calculations have also been performed to estimate the vibrational contributions to Lattice free energies. Distributions of the differences in Lattice Energy, free Energy, zero point Energy, entropy and heat capacity between polymorphs are presented. Polymorphic Lattice Energy differences are typically very small: over half of polymorph pairs are separated by less than 2 kJ mol−1 and Lattice Energy differences exceed 7.2 kJ mol−1 in only 5% of cases. Unsurprisingly, vibrational contributions to polymorph free Energy differences at ambient conditions are dominated by entropy differences. The distribution of vibrational Energy differences is narrower than Lattice Energy differences, rarely exceeding 2 kJ mol−1. However, these relatively small vibrational free Energy contributions are large enough to cause a re-ranking of polymorph stability below, or at, room temperature in 9% of the polymorph pairs.
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determination of the crystal structure of a new polymorph of theophylline
Chemistry: A European Journal, 2013Co-Authors: Mark D Eddleston, Graeme M Day, Katarzyna E Hejczyk, Erica G Bithell, William JonesAbstract:A new approach to crystal structure determination, combining crystal structure prediction and transmission electron microscopy, was used to identify a potential new crystal phase of the pharmaceutical compound theophylline. The crystal structure was determined despite the new polymorph occurring as a minor component in a mixture with Form II of theophylline, at a concentration below the limits of detection of analytical methods routinely used for pharmaceutical characterisation. Detection and characterisation of crystallites of this new form were achieved with transmission electron microscopy, exploiting the combination of high magnification imaging and electron diffraction measurements. A plausible crystal structure was identified by indexing experimental electron-diffraction patterns from a single crystallite of the new polymorph against a reference set of putative crystal structures of theophylline generated by global Lattice Energy minimisation calculations.
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modelling organic crystal structures using distributed multipole and polarizability based model intermolecular potentials
Physical Chemistry Chemical Physics, 2010Co-Authors: Sarah L Price, Panagiotis G Karamertzanis, Maurice Leslie, Gareth W A Welch, Matthew Habgood, Louise S Price, Graeme M DayAbstract:Crystal structure prediction for organic molecules requires both the fast assessment of thousands to millions of crystal structures and the greatest possible accuracy in their relative energies. We describe a crystal Lattice simulation program, DMACRYS, emphasizing the features that make it suitable for use in crystal structure prediction for pharmaceutical molecules using accurate anisotropic atom–atom model intermolecular potentials based on the theory of intermolecular forces. DMACRYS can optimize the Lattice Energy of a crystal, calculate the second derivative properties, and reduce the symmetry of the spacegroup to move away from a transition state. The calculated terahertz frequency k = 0 rigid-body Lattice modes and elastic tensor can be used to estimate free energies. The program uses a distributed multipole electrostatic model (Qat, t = 00,…,44s) for the electrostatic fields, and can use anisotropic atom–atom repulsion models, damped isotropic dispersion up to R−10, as well as a range of empirically fitted isotropic exp-6 atom–atom models with different definitions of atomic types. A new feature is that an accurate model for the induction Energy contribution to the Lattice Energy has been implemented that uses atomic anisotropic dipole polarizability models (αat, t = (10,10)…(11c,11s)) to evaluate the changes in the molecular charge density induced by the electrostatic field within the crystal. It is demonstrated, using the four polymorphs of the pharmaceutical carbamazepine C15H12N2O, that whilst reproducing crystal structures is relatively easy, calculating the polymorphic Energy differences to the accuracy of a few kJ mol−1 required for applications is very demanding of assumptions made in the modelling. Thus DMACRYS enables the comparison of both known and hypothetical crystal structures as an aid to the development of pharmaceuticals and other speciality organic materials, and provides a tool to develop the modelling of the intermolecular forces involved in molecular recognition processes.
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beyond the isotropic atom model in crystal structure prediction of rigid molecules atomic multipoles versus point charges
Crystal Growth & Design, 2005Co-Authors: Graeme M Day, W Sam D Motherwell, William JonesAbstract:The Lattice energies of predicted and known crystal structures for 50 small organic molecules with constrained (rigid) geometries have been calculated with a model potential whose electrostatic component is described by atom-centered multipoles. In comparison to previous predictions using atomic point charge electrostatics, there are important improvements in the reliability of Lattice Energy minimization for the prediction of crystal structures. Half of the experimentally observed crystal structures are found either to be the global minimum Energy structure or to have calculated Lattice energies within 0.5 kJ/mol (0.1 kcal/mol) of the global minimum. Furthermore, in 69% of cases, there are five or fewer unobserved structures with Lattice energies calculated to be lower than that of the observed structure. The results are promising for the advancement of global Lattice Energy minimization for the ab initio prediction of crystal structures and confirm the utility of representing electrostatic contributions to the Energy with atom-centered multipoles.
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a study of the known and hypothetical crystal structures of pyridine why are there four molecules in the asymmetric unit cell
CrystEngComm, 2002Co-Authors: A T Anghel, Graeme M Day, Sarah L PriceAbstract:The oldest crystal structure of pyridine is unusually complex, with four molecules in the asymmetric unit cell of Pna21 symmetry. In an attempt to understand why pyridine crystallises with 16 molecules in the unit cell, we have considered its thermodynamic stability relative to hypothetical pyridine structures. These were generated by a search for minima in the Lattice Energy of pyridine amongst the more common space groups, using the crystal structure prediction procedure MOLPAK followed by Lattice Energy minimisation using a distributed multipole-based intermolecular potential. We find over two dozen distinct crystal structures in the Energy gap of less than 6 kJ mol−1 between the corresponding models for the observed and most stable (hypothetical) structure. Adding harmonic phonon estimates of the intermolecular zero point Energy and entropy at the melting point of pyridine slightly improves the relative stability of the observed Z = 16 structure. Several of these hypothetical structures can be eliminated as only just mechanically stable, or because the growth rate of the crystal is estimated to be very slow by the attachment Energy model. Nevertheless, there are still over a dozen structures that appear competitive with the known structure as polymorphs of pyridine. Following these predictions, an intense experimental search has found a new polymorph of perdeutero-pyridine (form II), which was not found in the search. This structure is also predicted to be metastable with a similar Energy to form I. Although there is some evidence for kinetic factors favouring the observed structures, the metastable Z = 16 structure and the new form II remain a challenge for our understanding of crystallisation.
Leslie Glasser - One of the best experts on this subject based on the ideXlab platform.
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ionic hydrates mpxq nh2o Lattice Energy and standard enthalpy of formation estimation
Inorganic Chemistry, 2002Co-Authors: Donald Brooke H Jenkins, Leslie GlasserAbstract:This paper is one of a series (see: Inorg. Chem. 1999, 38, 3609; J. Am. Chem. Soc. 2000, 122, 632; Inorg. Chem. 2002, 41, 2364) exploring simple approaches for the estimation of Lattice energies of ionic materials, avoiding elaborate computation. Knowledge of Lattice Energy can lead, via thermochemical cycles, to the evaluation of the underlying thermodynamics involving the preparation and subsequent reactions of inorganic materials. A simple and easy to use equation for the estimation of the Lattice Energy of hydrate salts, UPOT(MpXq·nH2O) (and therefore for solvated salts, MpXq·nS, in general), using either the density or volume of the hydrate, or of another hydrate, or of the parent anhydrous salt or the volumes of the individual ions, is derived from first principles. The equation effectively determines the hydrate Lattice Energy, UPOT(MpXq·nH2O), from a knowledge of the (estimated) Lattice Energy, UPOT(MpXq), of the parent salt by the addition of nθU where θU(H2O)/kJ mol-1 = 54.3 and n is the number...
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Lattice potential Energy estimation for complex ionic salts from density measurements
Inorganic Chemistry, 2002Co-Authors: Donald Brooke H Jenkins, David Tudela, Leslie GlasserAbstract:This paper is one of a series exploring simple approaches for the estimation of Lattice Energy of ionic materials, avoiding elaborate computation. The readily accessible, frequently reported, and easily measurable (requiring only small quantities of inorganic material) property of density, ρm, is related, as a rectilinear function of the form (ρm/Mm)1/3, to the Lattice Energy UPOT of ionic materials, where Mm is the chemical formula mass. Dependence on the cube root is particularly advantageous because this considerably lowers the effects of any experimental errors in the density measurement used. The relationship that is developed arises from the dependence (previously reported in Jenkins, H. D. B.; Roobottom, H. K.; Passmore, J.; Glasser, L. Inorg. Chem. 1999, 38, 3609) of Lattice Energy on the inverse cube root of the molar volume. These latest equations have the form UPOT/kJ mol-1 = γ(ρm/Mm)1/3 + δ, where for the simpler salts (i.e., UPOT/kJ mol-1 < 5000 kJ mol-1), γ and δ are coefficients dependent u...
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Lattice potential Energy estimation for complex ionic salts from density measurements
Inorganic Chemistry, 2002Co-Authors: Donald Brooke H Jenkins, David Tudela, Leslie GlasserAbstract:This paper is one of a series exploring simple approaches for the estimation of Lattice Energy of ionic materials, avoiding elaborate computation. The readily accessible, frequently reported, and easily measurable (requiring only small quantities of inorganic material) property of density, rho(m), is related, as a rectilinear function of the form (rho(m)/M(m))(1/3), to the Lattice Energy U(POT) of ionic materials, where M(m) is the chemical formula mass. Dependence on the cube root is particularly advantageous because this considerably lowers the effects of any experimental errors in the density measurement used. The relationship that is developed arises from the dependence (previously reported in Jenkins, H. D. B.; Roobottom, H. K.; Passmore, J.; Glasser, L. Inorg. Chem. 1999, 38, 3609) of Lattice Energy on the inverse cube root of the molar volume. These latest equations have the form U(POT)/kJ mol(-1) = gamma(rho(m)/M(m))(1/3) + delta, where for the simpler salts (i.e., U(POT)/kJ mol(-1) 5000, gamma/kJ mol(-1) cm = 10(-7) AI(2IN(A))(1/3) and delta/kJ mol(-1) = 0 where A is the general electrostatic conversion factor (A = 121.4 kJ mol(-1)), I is the ionic strength = 1/2 the sum of n(i)z(i)(2), and N(A) is Avogadro's constant.
Ping Zhang - One of the best experts on this subject based on the ideXlab platform.
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phase composition crystal structure complex chemical bond theory and microwave dielectric properties of high q materials in a nd1 xyx nbo4 system
RSC Advances, 2015Co-Authors: Yonggui Zhao, Ping ZhangAbstract:In this paper, (Nd1−xYx)NbO4 ceramics are prepared via a conventional solid-state reaction method and their microwave dielectric properties have been reported for the first time. The Rietveld refinement was used to investigate the crystal structure of (Nd1−xYx)NbO4 ceramics. Based on the refined results, the NdNbO4 ceramics have a monoclinic fergusonite structure (I2/a (15) space group, Z = 4). The XRD patterns present a single monoclinic phase of NdNbO4 in the range of x = 0.02 to 0.1, with a further increase in the substitution content of Y3+ ions, few impurity phases are formed. In order to evaluate the correlations between complex chemical bond theory and microwave dielectric properties, the ionic polarization, Lattice Energy and bond Energy were calculated using the refined Lattice parameters and bond length. The effects of substituting Y3+ ions for Nd3+ ions on the microwave dielectric properties of the (Nd1−xYx)NbO4 ceramics were also discussed. The increase in the dielectric constant er is due to increasing the corrected theoretical dielectric constant erc. For high relative density samples, the Q × f values and τf values are really dependent upon the calculated Lattice Energy and bond Energy. High-quality factor microwave dielectric materials can be obtained with x = 0.08 in the (Nd1−xYx)NbO4 system, and show excellent dielectric properties of er = 19.87, Q × f = 81 100 GHz and τf = −18.84 ppm °C−1.
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bond ionicity Lattice Energy bond Energy and microwave dielectric properties of znzr nb1 xax 2o8 a ta sb ceramics
Dalton Transactions, 2015Co-Authors: Ping Zhang, Yonggui Zhao, Wu HaitaoAbstract:The dependence of microwave dielectric properties on the structural characteristics of ZnZr(Nb1−xAx)2O8 (A = Ta, Sb) (0 ≤ x ≤ 0.10) ceramics is investigated. All the compounds were prepared by a conventional solid-state reaction method and analyzed via multiphase structure refinement. The diffraction patterns of ZnZr(Nb1−xAx)2O8 (A = Ta, Sb) show the monoclinic wolframite structure of ZrZrNb2O8 which consists of an oxygen octahedron, with the Nb ion in the center of the oxygen octahedron. For the ZnZr(Nb1−xAx)2O8 (A = Ta, Sb) ceramics, the dielectric constant (er) decreased with the decrease in Nb-site bond ionicity. The quality factor (Q × f) of ZnZr(Nb1−xSbx)2O8 ceramics was found to be the highest (89 400 GHz), which is explained in terms of the average of the Nb-site Lattice Energy. With the decrease in the bond Energy of the Nb-site, the temperature coefficient of resonant frequency (|τf|) value increased. The substitution of A5+ (A = Ta, Sb) for Nb5+ effectively influences the microstructure and microwave dielectric properties of ZrZrNb2O8 ceramics.
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the correlations among bond ionicity Lattice Energy and microwave dielectric properties of nd1 xlax nbo4 ceramics
Physical Chemistry Chemical Physics, 2015Co-Authors: Ping Zhang, Yonggui ZhaoAbstract:(Nd1−xLax)NbO4 ceramics were prepared via a conventional solid-state reaction route and the correlations among bond ionicity, Lattice Energy, phase stability and microwave dielectric properties were investigated. The diffraction patterns showed that the (Nd1−xLax)NbO4 ceramics possessed a monoclinic fergusonite structure. The chemical bond ionicity, bond covalency and Lattice Energy were calculated using the empirical method. The phase structure stability varied with the Lattice Energy which resulted due to the substitution content of La3+ ions. With the increase of La3+ ion contents, the decrease of Nd/La–O bond ionicity was observed, which could be attributed to the electric polarization. er has a close relationship with the Nd/La–O bond covalency. The increase of the Q × f values and τf values could be attributed to the change in the Lattice Energy. The microwave dielectric properties of (Nd1−xLax)NbO4 ceramics with a monoclinic fergusonite structure were strongly dependent on the chemical bond ionicity, bond covalency and Lattice Energy.
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the relationship between bond ionicity Lattice Energy coefficient of thermal expansion and microwave dielectric properties of nd nb1 xsbx o4 ceramics
Dalton Transactions, 2015Co-Authors: Ping Zhang, Yonggui Zhao, Xiuyu WangAbstract:The crystalline structure refinement, chemical bond ionicity, Lattice Energy and coefficient of thermal expansion were carried out for Nd(Nb1−xSbx)O4 ceramics with a monoclinic fergusonite structure to investigate the correlations between the crystalline structure, phase stability, bond ionicity, Lattice Energy, coefficient of thermal expansion, and microwave dielectric properties. The bond ionicity, Lattice Energy, and coefficient of thermal expansion of Nd(Nb1−xSbx)O4 ceramics were calculated using a semiempirical method based on the complex bond theory. The phase structure stability varied with the Lattice Energy which was resulted by the substitution constant of Sb5+. With the increasing of the Sb5+ contents, the decrease of Nb/Sb–O bond ionicity was observed, which could be contributed to the electric polarization. The er had a close relationship with the Nb/Sb–O bond ionicity. The increase of the Q × f and |τf| values could be attributed to the Lattice Energy and the coefficient of thermal expansion. The microwave dielectric properties of Nd(Nb1−xSbx)O4 ceramics with the monoclinic fergusonite structure were strongly dependent on the chemical bond ionicity, Lattice Energy and coefficient of thermal expansion.
Yonggui Zhao - One of the best experts on this subject based on the ideXlab platform.
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phase composition crystal structure complex chemical bond theory and microwave dielectric properties of high q materials in a nd1 xyx nbo4 system
RSC Advances, 2015Co-Authors: Yonggui Zhao, Ping ZhangAbstract:In this paper, (Nd1−xYx)NbO4 ceramics are prepared via a conventional solid-state reaction method and their microwave dielectric properties have been reported for the first time. The Rietveld refinement was used to investigate the crystal structure of (Nd1−xYx)NbO4 ceramics. Based on the refined results, the NdNbO4 ceramics have a monoclinic fergusonite structure (I2/a (15) space group, Z = 4). The XRD patterns present a single monoclinic phase of NdNbO4 in the range of x = 0.02 to 0.1, with a further increase in the substitution content of Y3+ ions, few impurity phases are formed. In order to evaluate the correlations between complex chemical bond theory and microwave dielectric properties, the ionic polarization, Lattice Energy and bond Energy were calculated using the refined Lattice parameters and bond length. The effects of substituting Y3+ ions for Nd3+ ions on the microwave dielectric properties of the (Nd1−xYx)NbO4 ceramics were also discussed. The increase in the dielectric constant er is due to increasing the corrected theoretical dielectric constant erc. For high relative density samples, the Q × f values and τf values are really dependent upon the calculated Lattice Energy and bond Energy. High-quality factor microwave dielectric materials can be obtained with x = 0.08 in the (Nd1−xYx)NbO4 system, and show excellent dielectric properties of er = 19.87, Q × f = 81 100 GHz and τf = −18.84 ppm °C−1.
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bond ionicity Lattice Energy bond Energy and microwave dielectric properties of znzr nb1 xax 2o8 a ta sb ceramics
Dalton Transactions, 2015Co-Authors: Ping Zhang, Yonggui Zhao, Wu HaitaoAbstract:The dependence of microwave dielectric properties on the structural characteristics of ZnZr(Nb1−xAx)2O8 (A = Ta, Sb) (0 ≤ x ≤ 0.10) ceramics is investigated. All the compounds were prepared by a conventional solid-state reaction method and analyzed via multiphase structure refinement. The diffraction patterns of ZnZr(Nb1−xAx)2O8 (A = Ta, Sb) show the monoclinic wolframite structure of ZrZrNb2O8 which consists of an oxygen octahedron, with the Nb ion in the center of the oxygen octahedron. For the ZnZr(Nb1−xAx)2O8 (A = Ta, Sb) ceramics, the dielectric constant (er) decreased with the decrease in Nb-site bond ionicity. The quality factor (Q × f) of ZnZr(Nb1−xSbx)2O8 ceramics was found to be the highest (89 400 GHz), which is explained in terms of the average of the Nb-site Lattice Energy. With the decrease in the bond Energy of the Nb-site, the temperature coefficient of resonant frequency (|τf|) value increased. The substitution of A5+ (A = Ta, Sb) for Nb5+ effectively influences the microstructure and microwave dielectric properties of ZrZrNb2O8 ceramics.
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the correlations among bond ionicity Lattice Energy and microwave dielectric properties of nd1 xlax nbo4 ceramics
Physical Chemistry Chemical Physics, 2015Co-Authors: Ping Zhang, Yonggui ZhaoAbstract:(Nd1−xLax)NbO4 ceramics were prepared via a conventional solid-state reaction route and the correlations among bond ionicity, Lattice Energy, phase stability and microwave dielectric properties were investigated. The diffraction patterns showed that the (Nd1−xLax)NbO4 ceramics possessed a monoclinic fergusonite structure. The chemical bond ionicity, bond covalency and Lattice Energy were calculated using the empirical method. The phase structure stability varied with the Lattice Energy which resulted due to the substitution content of La3+ ions. With the increase of La3+ ion contents, the decrease of Nd/La–O bond ionicity was observed, which could be attributed to the electric polarization. er has a close relationship with the Nd/La–O bond covalency. The increase of the Q × f values and τf values could be attributed to the change in the Lattice Energy. The microwave dielectric properties of (Nd1−xLax)NbO4 ceramics with a monoclinic fergusonite structure were strongly dependent on the chemical bond ionicity, bond covalency and Lattice Energy.
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the relationship between bond ionicity Lattice Energy coefficient of thermal expansion and microwave dielectric properties of nd nb1 xsbx o4 ceramics
Dalton Transactions, 2015Co-Authors: Ping Zhang, Yonggui Zhao, Xiuyu WangAbstract:The crystalline structure refinement, chemical bond ionicity, Lattice Energy and coefficient of thermal expansion were carried out for Nd(Nb1−xSbx)O4 ceramics with a monoclinic fergusonite structure to investigate the correlations between the crystalline structure, phase stability, bond ionicity, Lattice Energy, coefficient of thermal expansion, and microwave dielectric properties. The bond ionicity, Lattice Energy, and coefficient of thermal expansion of Nd(Nb1−xSbx)O4 ceramics were calculated using a semiempirical method based on the complex bond theory. The phase structure stability varied with the Lattice Energy which was resulted by the substitution constant of Sb5+. With the increasing of the Sb5+ contents, the decrease of Nb/Sb–O bond ionicity was observed, which could be contributed to the electric polarization. The er had a close relationship with the Nb/Sb–O bond ionicity. The increase of the Q × f and |τf| values could be attributed to the Lattice Energy and the coefficient of thermal expansion. The microwave dielectric properties of Nd(Nb1−xSbx)O4 ceramics with the monoclinic fergusonite structure were strongly dependent on the chemical bond ionicity, Lattice Energy and coefficient of thermal expansion.
