Juan C. Nino

Dielectric relaxation in Bi2O3–ZnO–Nb2O5 cubic pyrochlore

Frequency dispersion associated with dielectric relaxation phenomena in polycrystalline cubic pyrochlore with normal composition (Bi1.5Zn0.5)(Zn0.5Nb1.5)O7 is analyzed. Measurements at cryogenic temperatures and high frequencies reveal a broad distribution of relaxation times. The associated dielectric loss data can be modeled with a function convoluting the Vogel–Fulcher law and a Gaussian distribution. This function, dependent only on measuring frequency and temperature, describes exceptionally well the phenomena observed over a frequency range covering seven decades and over 300 K.

Crystal Structure of the Compound Bi2Zn2/3Nb4/3O7

The crystal structure of Bi 2 Zn 2/3 Nb 4/3 O 7 was determined using a combination of electron, x-ray, and neutron powder diffraction. The compound crystallizes with a monoclinic zirconolite-like structure [C2/c (No.15) space group, a = 13.1037(9) A, b = 7.6735(3) A, c = 12.1584(6) A, β = 101.318(5)°]. According to structural refinement using neutron diffraction data, Nb preferentially occupies six-fold coordinated sites in octahedral sheets parallel to the (001) planes, while Zn is statistically distributed between two half-occupied (5 + 1)-fold coordinated sites near the centers of six-membered rings of [Nb(Zn)O 6 ] octahedra. The Nb/Zn cation layers alternate along the c -axis with Bi-layers, in which Bi cations occupy both eight- and seven-fold coordinated sites. The eight-fold coordinated Bi atoms exhibited strongly anisotropic thermal displacements with an abnormally large component directed approximately along the c -axis (normal to the octahedral layers).

Higher ionic conductive ceria-based electrolytes for solid oxide fuel cells

Codoping is used to enhance the ionic conductivity of ceria-based electrolytes. Sm3+ and Nd3+ were selected as codopants to promote low migration energy paths for oxygen vacancy diffusion, thereby increasing the ionic conductivity. Moreover, the use of codopants also increases the pre-exponential factor in the Arrhenius relationship, thus further improving the ionic conductivity. The ionic conductivity of SmxNdxCe1−2xO2−δ solid solutions is measured using electrochemical impedance spectroscopy. It was observed that for Sm0.075Nd0.075Ce0.85O2−δ, the grain ionic conductivity was 14.0×10−3Scm−1 at 550°C, which makes it one of the most promising ceria-based electrolytes for intermediate temperature solid oxide fuel cells.

The role of polar, lamdba (Λ)-shaped building units in noncentrosymmetric inorganic structures.

A methodology for the design of polar, inorganic structures is demonstrated here with the packing of lambda (Λ)-shaped basic building units (BBUs). Noncentrosymmetric (NCS) solids with interesting physical properties can be created with BBUs that lack an inversion center and are likely to pack into a polar configuration; previous methods to construct these solids have used NCS octahedra as BBUs. Using this methodology to synthesize NCS solids, one must increase the coordination of the NCS octahedra with maintenance of the noncentrosymmetry of the bulk. The first step in this progression from an NCS octahedron to an inorganic NCS solid is the formation of a bimetallic BBU. This step is exemplified with the compound CuVOF(4)(H(2)O)(7): this compound, presented here, crystallizes in an NCS structure with ordered, isolated [Cu(H(2)O)(5)](2+) cations and [VOF(4)(H(2)O)](2-) anions into Λ-shaped, bimetallic BBUs to form CuVOF(4)(H(2)O)(6)·H(2)O, owing to the Jahn-Teller distortion of Cu(2+). Conversely, the centrosymmetric heterotypes with the same formula MVOF(4)(H(2)O)(7) (M(II) = Co, Ni, and Zn) exhibit ordered, isolated [VOF(4)(H(2)O)](2-) and [M(H(2)O)(6)](2+) ionic species in a hydrogen bond network. CuVOF(4)(H(2)O)(7) exhibits a net polar moment while the heterotypes do not; this demonstrates that Λ-shaped BBUs give a greater probability for and, in this case, lead to NCS structures.

Correlation between infrared phonon modes and dielectric relaxation in Bi2O3–ZnO–Nb2O5 cubic pyrochlore

Analysis of the dielectric relaxation phenomena in Bi2O3–ZnO–Nb2O5 cubic pyrochlore utilizing the Arrhenius equation is revisited to include recent high-frequency measurements. The fitting parameters obtained are further analyzed by comparison with theoretical and experimental activation energies of better understood relaxation systems such as dipolar glasses and relaxor ferroelectrics. The attempt jump frequency obtained from the Arrhenius fit (ν0∼4.385 THz) is found in correspondence with the resonant frequencies of the polar phonon modes previously identified by infrared spectroscopy. This correlation, and in particular the role of the O–A–O and O′–A–O′ bending phonon modes in driving the relaxation, is discussed.

Phase formation and reactions in the Bi 2 O 3 –ZnO–Nb 2 O 5 –Ag pyrochlore system

Mixed-phase compositions in the Bi 2 O 3 –ZnO–Nb 2 O 5 pyrochlore system have been previously identified as potential low-fire dielectric composites for high-frequency multilayer devices. However, there has been a problem with the ability to co-process this material with silver. It was found that an intermediate BiNbO 4 phase that occurs during calcination can interact with silver during co-sintering, forming a deleterious phase. Processing routes to control the presence of remanent BiNbO 4 were investigated, such as ZnO stoichiometric adjustment, low partial pressure of oxygen sintering atmosphere, and specific temperature-time dwells.

Band Gap and Structure of Single Crystal Bii3: Resolving Discrepancies In Literature

Bismuth tri-iodide (BiI3) is an intermediate band gap semiconductor with potential for room temperature gamma-ray detection applications. Remarkably, very different band gap characteristics and values of BiI3 have been reported in literature, which may be attributed to its complicated layered structure with strongly bound BiI6 octahedra held together by weak van der Waals interactions. Here, to resolve this discrepancy, the band gap of BiI3 was characterized through optical and computational methods and differences among previously reported values are discussed. Unpolarized transmittance and reflectance spectra in the visible to near ultraviolet (UV-Vis) range at room temperature yielded an indirect band gap of 1.67 ± 0.09 eV, while spectroscopic ellipsometry detected a direct band gap at 1.96 ± 0.05 eV and higher energy critical point features. The discrepancy between the UV-Vis and ellipsometry results originates from the low optical absorption coefficients (α ∼ 102 cm−1) of BiI3 that renders reflection...

Origin of colossal permittivity in BaTiO3 via broadband dielectric spectroscopy

Barium titanate (BT) ceramics with Ba/Ti ratios of 0.95 and 1.00 were synthesized using spark plasma sintering (SPS) technique. Dielectric spectroscopy (frequency range from 40 Hz to 1 MHz and temperature range from 300 K to 30 K) was performed on those ceramics (SPS BT). SPS BT showed extremely high permittivity up to ∼105, which can be referred to as colossal permittivity, with relatively low dielectric loss of ∼0.05. Data analyses following Debye relaxation and universal dielectric response models indicate that the origin of colossal permittivity in BT ceramics is the result of a hopping polaron within semiconducting grains in combination with interfacial polarization at the insulating grain boundary. Furthermore, the contributions of each polarization mechanism to the colossal permittivity in SPS BT, such as a hopping polarization, internal barrier layer capacitance effect, and electrode effect, were estimated.

Time-resolved and orientation-dependent electric-field-induced strains in lead zirconate titanate ceramics

Electric-field-induced lattice strains in a tetragonal ferroelectric lead zirconate titanate bulk ceramic are characterized under application of subcoercive cyclic electric fields using neutron diffraction and a stroboscopic data collection technique. At a driving electric field equal to half of the coercive field, the field-induced lattice strains are found to be a function of orientation with the greatest electric-field-induced strain coefficient of 680pm∕V in crystal orientations such that the 211 pole is parallel to the electric field. A time dependence of the 111 strain was also observed. Suggestions as to the nature of these dependences are discussed.

Complex ceramic structures. I. Weberites

The weberite structure (A2B2X7) is an anion-deficient fluorite-related superstructure. Compared with fluorites, the reduction in the number of anions leads to a decrease in the coordination number of the B cations (VI coordination) with respect to the A cations (VIII coordination), thus allowing the accommodation of diverse cations. As a result, weberite compounds have a broad range of chemical and physical properties and great technological potential. This article summarizes the structural features of weberite and describes the structure in several different ways. This is the first time that the stacking vector and stacking angle are used to represent the weberite structure. This paper also discusses the crystallographic relationship between weberite, fluorite and pyrochlore (another fluorite-related structure). The cation sublattices of weberite and pyrochlore are correlated by an axial transformation. It has been shown that the different coordination environment of anions is due to the alternating layering of the AB3 and A3B close-packed cation layers. A stability field of weberite oxides is proposed in terms of the ratio of ionic radius of cations and relative bond ionicity. In addition, a selection of weberite compounds with interesting properties is discussed.