Showing papers on "Ionic radius published in 2008"

Effect of diamagnetic Ca, Sr, Pb, and Ba substitution on the crystal structure and multiferroic properties of the BiFeO3 perovskite

Abstract: In this work, we studied the effect of heterovalent Ca, Sr, Pb, and Ba substitution on the crystal structure, dielectric, local ferroelectric, and magnetic properties of the BiFeO3 multiferroic perovskite. Ceramic solid solutions with the general formula Bi0.7A0.3FeO3 (A is a doping element) were prepared and characterized by x-ray diffraction, dielectric, piezoresponse force microscopy (PFM), and magnetic measurements. It is shown that the crystal structure of the compounds is described within the space group R3c, permitting the spontaneous polarization, whose existence was confirmed by the PFM data. Magnetic properties of the solid solutions are determined by the ionic radius of the substituting element. Experimental results suggest that the increase in the radius of the A-site ion leads to the effective suppression of the spiral spin structure of BiFeO3, resulting in the appearance of net magnetization.

Optical absorption of divalent metal tungstates: Correlation between the band-gap energy and the cation ionic radius

Abstract: We have carried out optical-absorption and reflectance measurements at room temperature in single crystals of AWO4 tungstates (A=Ba, Ca, Cd, Cu, Pb, Sr, and Zn). From the experimental results their band-gap energy has been determined to be 5.26 eV (BaWO4), 5.08 eV (SrWO4), 4.94 eV (CaWO4), 4.15 eV (CdWO4), 3.9–4.4 eV (ZnWO4), 3.8–4.2 eV (PbWO4), and 2.3 eV (CuWO4). The results are discussed in terms of the electronic structure of the studied tungstates. It has been found that those compounds where only the s electron states of the A2+ cation hybridize with the O 2p and W 5d states (e.g., BaWO4) have larger band-gap energies than those where also p, d, and f states of the A2+ cation contribute to the top of the valence band and the bottom of the conduction band (e.g., PbWO4). The results are of importance in view of the large discrepancies existent in prevoiusly published data.

Charge self-regulation upon changing the oxidation state of transition metals in insulators.

Abstract: Transition-metal atoms embedded in an ionic or semiconducting crystal can exist in various oxidation states that have distinct signatures in X-ray photoemission spectroscopy and 'ionic radii' which vary with the oxidation state of the atom. These oxidation states are often tacitly associated with a physical ionization of the transition-metal atoms--that is, a literal transfer of charge to or from the atoms. Physical models have been founded on this charge-transfer paradigm, but first-principles quantum mechanical calculations show only negligible changes in the local transition-metal charge as the oxidation state is altered. Here we explain this peculiar tendency of transition-metal atoms to maintain a constant local charge under external perturbations in terms of an inherent, homeostasis-like negative feedback. We show that signatures of oxidation states and multivalence--such as X-ray photoemission core-level shifts, ionic radii and variations in local magnetization--that have often been interpreted as literal charge transfer are instead a consequence of the negative-feedback charge regulation.

First-principles study on doping and phase stability of HfO 2

Abstract: Based on density functional methods, relative stabilities between monoclinic, tetragonal, and cubic phases of ${\text{HfO}}_{2}$ with cation dopants or oxygen vacancies are investigated. It is found that dopants such as Si, Ge, Sn, P, Al or Ti with ionic radii smaller than Hf stabilize the tetragonal phase but destabilize the cubic phase. In contrast, larger dopants such as Y, Gd or Sc favor the cubic phase. The ionized oxygen vacancies compensating trivalent dopants greatly stabilize both cubic and tetragonal phases. Microscopic explanations on the results are also given. The metastable phase favored by each dopant is in good agreement with experimental data. Our results can serve as a useful guide in selecting dopants to stabilize a specific phase.

Optical absorption of divalent metal tungstates: Correlation between the band-gap energy and the cation ionic radius

Abstract: We have carried out optical-absorption and reflectance measurements at room temperature in single crystals of AWO4 tungstates (A = Ba, Ca, Cd, Cu, Pb, Sr, and Zn). From the experimental results their band-gap energy has been determined to be 5.26 eV (BaWO4), 5.08 eV (SrWO4), 4.94 eV (CaWO4), 4.15 eV (CdWO4), 3.9-4.4 eV (ZnWO4), 3.8-4.2 eV (PbWO4), and 2.3 eV (CuWO4). The results are discussed in terms of the electronic structure of the studied tungstates. It has been found that those compounds where only the s electron states of the A2+ cation hybridize with the O 2p and W 5d states (e.g BaWO4) have larger band-gap energies than those where also p, d, and f states of the A2+ cation contribute to the top of the valence band and the bottom of the conduction band (e.g. PbWO4). The results are of importance in view of the large discrepancies existent in prevoiusly published data.

Hydration of Lanthanoid(III) Ions in Aqueous Solution and Crystalline Hydrates Studied by EXAFS Spectroscopy and Crystallography: The Myth of the “Gadolinium Break”

Abstract: The structures of the hydrated lanthanoid(III) ions including lanthanum(III) have been characterized in aqueous solution and in the solid trifluoromethanesulfonate salts by extended X-ray absorption fine structure (EXAFS) spectroscopy. At ambient temperature the water oxygen atoms appear as a tricapped trigonal prism around the lanthanoid(III) ions in the solid nonaaqualanthanoid(III) trifluoromethanesulfonates. Water deficiency in the capping positions for the smallest ions starts at Ho and increases with increasing atomic number in the [Ln(H(2)O)(9-x)](CF(3)SO(3))(3) compounds with x=0.8 at Lu. The crystal structures of [Ho(H(2)O)(8.91)](CF(3)SO(3))(3) and [Lu(H(2)O)(8.2)](CF(3)SO(3))(3) were re-determined by X-ray crystallography at room temperature, and the latter also at 100 K after a phase-transition at about 190 K. The very similar Ln K- and L(3)-edge EXAFS spectra of each solid compound and its aqueous solution indicate indistinguishable structures of the hydrated lanthanoid(III) ions in aqueous solution and in the hydrated trifluoromethanesulfonate salt. The mean Ln--O bond lengths obtained from the EXAFS spectra for the largest ions, La-Nd, agree with estimates from the tabulated ionic radii for ninefold coordination but become shorter than expected starting at samarium. The deviation increases gradually with increasing atomic number, reaches the mean Ln-O bond length expected for eightfold coordination at Ho, and increases further for the smallest lanthanoid(III) ions, Er-Lu, which have an increasing water deficit. The low-temperature crystal structure of [Lu(H(2)O)(8.2)](CF(3)SO(3))(3) shows one strongly bound capping water molecule (Lu-O 2.395(4) A) and two more distant capping sites corresponding to Lu-O at 2.56(1) A, with occupancy factors of 0.58(1) and 0.59(1). There is no indication of a sudden change in hydration number, as proposed in the "gadolinium break" hypothesis.

Experimental and theoretical comparison of actinide and lanthanide bonding in M[N(EPR2)2]3 complexes (M = U, Pu, La, Ce; E = S, Se, Te; R = Ph, iPr, H)

Abstract: Treatment of M[N(SiMe3)2]3 (M = U, Pu (An); La, Ce (Ln)) with NH(EPPh2)2 and NH(EPiPr2)2 (E = S, Se), afforded the neutral complexes M[N(EPR2)2]3 (R = Ph, iPr). Tellurium donor complexes were synthesized by treatment of MI3(sol)4 (M = U, Pu; sol = py and M = La, Ce; sol = thf) with Na(tmeda)[N(TePiPr2)2]. The complexes have been structurally and spectroscopically characterized with concomitant computational modeling through density functional theory (DFT) calculations. The An−E bond lengths are shorter than the Ln−E bond lengths for metal ions of similar ionic radii, consistent with an increase in covalent interactions in the actinide bonding relative to the lanthanide bonding. In addition, the magnitude of the differences in the bonding is slightly greater with increasing softness of the chalcogen donor atom. The DFT calculations for the model systems correlate well with experimentally determined metrical parameters. They indicate that the enhanced covalency in the M−E bond as group 16 is descended arise...

Phase equilibria in the refractory oxide systems of zirconia, hafnia and yttria with rare-earth oxides

Abstract: The systematic study of phase equilibria in the ternary systems HfO2(ZrO2)–Y2O3–Ln2O3 has been first carried out. Phase reactions and crystallization of ceramic alloys in the binary systems ZrO2–Ln2O3, HfO2–Ln2O3, Y2O3–Ln2O3 and phase equilibria in the series of ternary systems HfO2–Y2O3–Ln2O3 and ZrO2–Y2O3–Ln2O3 have been developed at high temperature. The most general regularities of the phase reactions in liquid and solid states inherent in these systems have been considered dependent on lanthanide ion radii. Taking into account literature data and newly developed results in binary and ternary systems, the analysis of the main regularities revealed in the constitution of phase diagrams, particularly its dependence on lanthanide ionic radius, was carried out. It was shown that temperature and composition of eutectic reaction, temperature of the pyrochlore phase decomposition, lattice parameters of solid solutions and other parameters of the binary phases linearly depend on ionic radius of lanthanide. For the first time it has been found that the affiliation of lanthanide oxides to cerium or yttrium subgroups predetermines phase relations in the systems and topology of the ternary phase diagrams. The data obtained are the basis for the novel prospective ceramic materials for both structural and functional applications in energetic, medicine, nuclear industry, thermal barrier coatings, solid oxide fuel cells, etc.

Structural characterization of zinc-substituted hydroxyapatite prepared by hydrothermal method

Abstract: Zinc-substituted hydroxyapatite (Zn-HA) powders were prepared by hydrothermal method using Ca(NO3)2, (NH4)3PO4 and Zn(NO3)2 as reagents. X-ray fluorescence spectroscopy (XRF), X-ray diffraction (XRD), fourier transform infrared spectroscopy (FT-IR) and transmission electron microscopy (TEM) were used to characterize the crystalline phase, microstructure, chemical composition, morphology and thermal stability of Zn-HA. The results show that the substitution content of zinc (Zn) in Zn-HA powders prepared in NaOH solution is higher than that prepared in NH3 solution, and is lower than that of the corresponding amount of starting materials. The substitution of the Zn ion for calcium ion causes a lower crystallinity of Zn-HA and changes the lattice parameters of Zn-HA, since the ionic radius is smaller in Zn2+ (0.074 nm) than in Ca2+ (0.099 nm). Furthermore, the substitution of the Zn ions restrains the growth of Zn-HA crystal and decreases the thermal stability of Zn-HA. Zn-HA powder prepared in NH3 solution starts to decompose at 800 °C when the Zn fraction increases to 15 mol%, while that prepared in NaOH solution start to decompose at 5 mol% Zn. The substitution content of Zn significantly influences the thermal stability, microstructure and morphology of Zn-HA.

Evidence of Trivalent Europium Incorporated in Anatase TiO2 Nanocrystals with Multiple Sites

Abstract: Trivalent europium ions have been successfully incorporated in anatase TiO2 nanocrystals via a sol−gel solvothermal synthesis, in spite of a large mismatch in ionic radius between Eu3+ and Ti4+. The photoluminescence intensity of Eu3+:TiO2 nanocrystals has been significantly improved, which is comparable to that of common red phosphors. Multiple sites of Eu3+ in TiO2 nanocrystals have been identified by the technique of site-selective spectroscopy. Eu3+ ions at two lattice sites exhibit sharp emission and excitation peaks with site symmetries descending from D2d to approximate C2v and D2 as a result of the lattice distortion, whereas Eu3+ ions at disordered site near the surface are analogous to Eu3+ ions located in a glasslike environment. Extended X-ray absorption fine structure is utilized for identifying the local structure and Eu3+ coordination. The luminescence dynamics and crystal-field levels of Eu3+ at different sites have been analyzed. A growth mechanism for the incorporation of Eu3+ in the ana...

Influence of the static atomic displacement on atomic resolution Z-contrast imaging

Abstract: The influence of static atomic displacements, due to atomic size effects in alloys with atoms having different covalent or ionic radii, on high angle annular dark field image contrast is studied quantitatively by simulations and experiments. We show that the static displacements can have a large influence on the Z contrast, depending on the alloy composition and on the scanning transmission electron microscopy specimen thickness. This influence has to be taken into account for quantitative chemistry measurement based on Z-contrast imaging.

A molecular dynamics study of alkaline earth metal-chloride complexation in aqueous solution.

Abstract: The relative stability of alkaline earth metals (M2+ = Mg2+, Ca2+, Sr2+, and Ba2+) and their chloride complexes in aqueous solution is examined through molecular dynamics simulations using a flexible SPC water model with an internally consistent set of metal ion force field parameters. For each metal−chloride ion pair in aqueous solution, the free energy profile was calculated via potential of mean force simulations. The simulations provide detailed thermodynamic information regarding the relative stability of the different types of metal−chloride pairs. The free energy profiles indicate that the preference for contact ion pair formation increases with ionic radius and is closely related to the metal hydration free energies. The water residence times within the first hydration shells are in agreement with residence times reported in other computational studies. Calculated association constants suggest an increase in metal−chloride complexation with increasing cation radii that is inconsistent with experim...

Crystal structure of NaCl-type transition metal monocarbides MC (M = V, Ti, Nb, Ta, Hf, Zr), a neutron powder diffraction study

Abstract: Crystal structure of NaCl-type transition metal monocarbides MC (M = V, Ti, Nb, Ta, Hf and Zr) has been investigated by Rietveld analysis of neutron powder diffraction data measured at 23 °C. The unit-cell parameter a of MC compounds increased with increasing of ionic radius of the metal species M. Structural change of the tantalum carbide TaC from 23 to 413 °C has been investigated by Rietveld analysis of in situ neutron diffraction data. The unit-cell parameter a, unit-cell volume and atomic displacement parameter of TaC increased with increasing of temperature. The thermal expansion coefficient of TaC was estimated to be (6.4 ± 0.3) × 10−6 °C−1.

Effects of lanthanide doping on electronic structures and optical properties of anatase TiO2 from density functional theory calculations

Abstract: The supercells of pure anatase TiO2 and lanthanide-doped anatase TiO2 were calculated by density functional theory with the plane-wave ultrasoft pesudopotentials method. The effects of lanthanide doping on the crystal structure, electronic structure, optical properties and photocatalytic activity were investigated according to the calculation data. The results indicate that lanthanide doping could remarkably improve the photocatalytic activity of TiO2, and the effect of improvement was sensitive to the atomic electronic configuration and ionic radius. Most lanthanide-doped TiO2 simultaneously realized visible-light response and maintained strong redox potentials. The conclusions would have important significance for understanding and further developing TiO2 photocatalysts that are active under visible-light irradiation.

Intrinsic nature of the magnetization enhancement in heterovalently doped Bi1−xAxFeO3 (A = Ca, Sr, Pb, Ba) multiferroics

Abstract: The mechanism of the formation of heterovalent-substitution-induced defects as well as their influence on the magnetic properties of BiFeO3-based multiferroics has been studied. It has been shown that heterovalent A2+ substitution results in the formation of oxygen vacancies in the host lattices of both antiferromagnetic and weak ferromagnetic Bi1−xAxFeO3 (A = Ca, Sr, Pb, Ba; x = 0.2, 0.3) compounds, thus indicating the intrinsic (i.e. not related to defects themselves) mechanism of doping-induced enhancement of magnetization. A correlation between the ionic radius of the substituting element and the value of the spontaneous magnetization of the corresponding solid solution has been found. The experimental results suggest that A-site substitution with the biggest ionic radius ions effectively suppresses the spiral spin configuration of antiferromagnetic BiFeO3.

Microwave-induced combustion synthesis and characterization of NixCo1−xFe2O4 nanocrystals (x = 0.0, 0.4, 0.6, 0.8, 1.0)

Abstract: A series of Ni-doped cobalt ferrites NixCo1−xFe2O4 (x = 0.0, 0.4, 0.6, 0.8, and 1.0) were prepared using microwave-induced combustion. Nickel, cobalt, and ferric nitrates were used as starting materials and glycine as fuel. The influence of Ni content on the lattice parameter, stretching vibrations, and magnetization was studied. XRD, FTIR, and SEM were used for structure, composition, and morphology investigation. A porous network structure was observed with average particle size 60–67 nm. All samples had a cubic spinel structure. The unit cell parameter “a” decreases linearly with nickel concentration due to the smaller ionic radius of nickel. Magnetization measurements showed that coercivity decreased as Ni content increased; it increased with decreasing temperature.

Tuning the Transparency of Cu2O with Substitutional Cation Doping

Abstract: Materials derived from copper oxide, Cu2O, are potential p-type transparent conducting oxides. Cu2O displays some unusual features, including p-type semiconductivity and the importance of cation−cation interactions in determining the band gap and transparency. We apply first-principles density functional theory to investigate how dopants with a range of ionic radii, oxidation states, and electronic structure can be used to tune the band gap. Unlike many oxides, in which the band gap is reduced by the appearance of dopant induced states in the host band gap, the band gap of Cu2O can be both increased or decreased by a suitable choice of dopant. Two effects dominate: (i) dopant-induced changes to the Cu−Cu interactions through structural distortions around the dopant site and (ii) the alignment of the dopant electronic states with the valence band or conduction band of Cu2O. Dopants with ionic radii larger than Cu+ (Ba2+, Sn2+, Cd2+, In3+, La3+, and Ce4+) produce strong structural distortions around the dop...

Lattice Parameter and Sintering Temperature Dependence of Bulk and Grain-Boundary Conduction of Garnet-like Solid Li-Electrolytes

Abstract: The dependence of the bulk and grain-boundary lithium-ion conduction on the lattice parameter by substitution of trivalent La by divalent Mg, Ca, Sr, Sr 0.5 Ba 0.5 , or Ba and monovalent Li in garnet-like Li 5 La 3 Ta 2 O 12 , and the effect of sintering temperature were investigated. The ionic bulk conductivity increases with increasing ionic radius of the divalent alkaline earth ion and corresponding increased lattice parameter. An exception is Mg, which is too small for replacing La and forms a second phase. The lattice parameter is also found to increase with increased sintering temperature, except for the mixed Sr 0.5 Ba 0.5 substituted sample. The Ca, Sr, Sr 0.5 Ba 0.5 , and Ba compounds show mainly bulk resistances with minor boundary contribution at room temperature, which decreases with increasing size of the alkaline earth ion. In contrast, the multiphase Mg-substituted sample exhibits an appreciable grain-boundary contribution to the total resistance. Microstructural investigations indicate the dependence of the grain-boundary resistance on the grain size, sinterability, and formation of transient or steady-state phase boundary compositions, which are caused by different chemical diffusion coefficients of the components. This is related to the higher conductivities of Li 6 MgLa 2 Ta 2 O 12 and Li 6 BaLa 2 Ta 2 O 12 annealed at 900°C compared to samples annealed at 950°C.

Synthesis, characterization and photophysical properties of Eu3+ doped in BaMoO4.

Abstract: In this work Ba0.99Eu0.01MoO4 (BEMO) powders were prepared by the first time by the Complex Polymerization Method. The structural and optical properties of the BEMO powders were characterized by Fourier Transform Infra-Red (FTIR), X-ray Diffraction (XRD), Raman Spectra, High-Resolution Scanning Electron Microscopy (HR-SEM) and Photoluminescent Measurements. XRD show a crystalline scheelite-type phase after the heat treatment at temperatures greater than 400 °C. The ionic radius of Eu3+ (0.109 nm) is lower than the Ba2+ (0.149 nm) one. This difference is responsible for the decrease in the lattice parameters of the BEMO compared to the pure BaMoO4 matrix. This little difference in the lattice parameters show that Eu3+ is expected to occupy the Ba2+ site at different temperatures, stayed the tetragonal (S4) symmetry characteristic of scheelite-type crystalline structures of BaMoO4. The emission spectra of the samples, when excited at 394 nm, presented the 5D1→7F0, 1 and 2 and 5D0→7F0, 1, 2, 3 and 4 Eu3+ transitions at 523, 533, 554, 578, 589, 614, 652 and 699 nm, respectively. The emission spectra of the powders heat-treated at 800 and 900 °C showed a marked increase in its intensities compared to the materials heat-treated from 400 to 700 °C. The decay times for the sample were evaluated and all of them presented the average value of 0.61 ms. Eu3+ luminescence decay time follows one exponential curve indicating the presence of only one type of Eu3+ symmetry site.

Nanocrystals (x = 0.0, 0.4, 0.6, 0.8, 1.0)

Abstract: A series of Ni-doped cobalt ferrites Ni x Co 1-x Fe 2 O 4 (x = 0.0, 0.4, 0.6, 0.8, and 1.0) were prepared using microwave-induced combus- tion. Nickel, cobalt, and ferric nitrates were used as starting materials and glycine as fuel. The influence of Ni content on the lattice parameter, stretching vibrations, and magnetization was studied. XRD, FTIR, and SEM were used for structure, composition, and mor- phology investigation. A porous network structure was observed with average particle size 60-67 nm. All samples had a cubic spinel structure. The unit cell parameter "a" decreases linearly with nickel concentration due to the smaller ionic radius of nickel. Magnetiza- tion measurements showed that coercivity decreased as Ni content increased; it increased with decreasing temperature.

Evolution of the crystal structure of RVO3 (R = La, Ce, Pr, Nd, Tb, Ho, Er, Tm, Yb, Lu, Y) perovskites from neutron powder diffraction data.

Abstract: RVO 3 perovskites have been prepared in the widest range of R 3+ ionic size, from LaVO 3 to LuVO 3 . Pure polycrystalline samples have been obtained by a citrate technique leading to reactive RVO 4 precursors, followed by thermal treatments in a reducing H 2 /N 2 (15/85%) flow to stabilize V 3+ cations. These oxides have been studied at room temperature by high-resolution neutron powder diffraction to follow the evolution of the crystal structures along the series. The distortion of the orthorhombic perovskite (space group Pbnm), characterized by the tilting angle of the VO 6 octahedra, progressively increases from La to Lu due to simple steric factors. Additionally, all of the perovskites show a subtle distortion of the VO 6 octahedra which significantly increases from La to Tb, and then slightly decreases for the last terms of the series. The stability of the crystal structure is also discussed in light of bond-valence arguments.

Correlation between ionic charge and the lattice constant of cubic perovskite solids

Abstract: In this paper we present lattice constant values for cubic perovskite solids with the product of ionic charges and average ionic radii rav (A). The lattice constant of these compounds exhibit a linear relationship when plotted on a log–log scale against the average ionic radii rav (A), but fall on different straight lines according to the ionic charge product of the compounds. We have applied the proposed relation to ABX3 (A: large cation with different valence, B: transition metal and X: oxides and halides) and found a better agreement with the experimental data as compared to the values evaluated by earlier researchers. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)