Local Structural Units Research Articles

Vibrational Spectroscopic Studies of Germanium-High Bismuthate Glasses and Vitroceramics

Abstract Bi2O3-rich glasses belonging to the system xGd2O3(1−x)[0.857Bi2O3·0.143GeO2] with 0.005≤x≤0.1, have been obtained by rapid cooling of the melts. Polycrystalline samples have been further obtained by applying a heat treatment at 600ºC for 24 h. Both types of samples were structurally characterized by using X-ray diffraction (XRD), and infrared (IR) and Raman spectroscopic techniques. Local structural units for the investigated samples are identified on the basis of vibrational spectroscopic data. The obtained data reveal the presence of GeO4, GeO6, BiO3 and BiO6 groups as basic structural units of the networks. For the vitroceramic samples some changes occur in their network, mainly reflecting the presence of an ordered structure, but much important is that part of the BiO6 structural units from Bi2O3-GeO2 glass network are converting into BiO3 units. The IR and Raman results support the idea that the local structure of the glasses and vitroceramics is less affected by the presence of Gd3+ ions but there is strong long range order influence on developed crystalline phases from vitroceramic samples, as proved by XRD. However, the spectral analyses reveal the formation of a structure with a higher disorder degree for x=0.1 in comparison with the other investigated vitroceramics as a result of the partial replacement of the Bi3+ ions with Gd3+ ions. Starting from the Bi12GeO20 sellenite phase in the samples with low gadolinium content (x≤0.01), the structure is change to Bi24GeO38 phase, via a BiLa type structure, when the Gd2O3 concentration is increasing up to x=0.1.

Structure of a glassy Zr70Pd30 alloy analysed by anomalous X-ray scattering coupled with reverse Monte Carlo simulation (AXS-RMC)

The anomalous X-ray scattering (AXS) technique was applied to the structural analysis of a rapidly quenched Zr70Pd30 glassy alloy. The AXS analysis and subsequent reverse Monte Carlo (RMC) simulation allowed three partial pair distribution functions to be obtained, together with a three-dimensional structural model. A Voronoi polyhedral analysis of the nearest neighbour region confirms the overall preference of an icosahedral-like local structural unit. An ideal icosahedron around Pd, composed of Pd–Zr pairs with covalent nature, is suggested to be one of the important structural features for the easy formation of the nano-icosahedral phase by low temperature annealing.

Combined luminescence and X-ray emission study of self-trapped excitons in oxides

The experimental study of relaxation of anion and cation excitons at the selective VUV-(valence) and soft X-ray (inner-shell) excitations for binary BeO crystal and multicomponent oxide crystals Be2SiO4, Al2Be3Si6O18 and Y2SiO5 is presented. Results show that the relaxation during the time-scale of decay of short-living anion and cation excitations leads to creation of self-trapped excitons at the corresponding low-symmetry (tetrahedral) local structural units of crystalline lattice. This can be caused by increase of dynamical instability of such a local structural units (in comparison with high symmetry ones), which facilitates self-trapping processes in these systems.

Amorphous Calcium Phosphates

Amorphous calcium phosphates (ACPs) represent a unique class of biomedically relevant calcium orthophosphate salts, in which there are neither translational nor orientational long-range orders of the atomic positions. Nevertheless, the constancy in their chemical composition over a relatively wide range of preparation conditions suggests the presence of a well-defined local structural unit, presumably, with the structure of Ca9(PO4)6 – so-called Posner’s cluster. ACPs have variable chemical but rather identical glass-like physicochemical properties. Furthermore, all ACPs are thermodynamically unstable compounds and, unless stored in dry conditions or doped by stabilizers, spontaneously they tend to transform to crystalline calcium orthophosphates. Although some order within general disorder is the most distinguishing feature of ACPs, the solution instability of ACPs and their easy transformation to crystalline phases might be of a great biological relevance. Namely, the initiating role ACPs play in matrix vesicle biomineralization raises the importance of this phase from a mere laboratory curiosity to that of a key intermediate in skeletal calcification. Furthermore, ACPs are very promising candidates to manufacture artificial bone grafts.

Crystal structure of 1/0-2/1-1/0 Cu–Al–Sc approximant

The new crystal structure of an orthorhombic phase formed at the alloy composition Cu48.1Al36.4Sc15.5 was analyzed by means of the Rietveld method using synchrotron radiation diffraction data. The starting model for this analysis was constructed theoretically using the so-called ‘cell constrained melt-quenching technique’. The space group of the final model is Cmmm, and the unit cell includes 49.0 Cu, 39.0 Al and 16.0 Sc atoms. The lattice parameters are a = 8.337(4) Å, b = 22.02(1) Å and c = 8.305(4) Å, which are related to the six-dimensional lattice parameter, a 6D = 6.959 Å, of the corresponding Cu–Al–Sc icosahedral quasicrystal as 1/0, 2/1 and 1/0 approximations, respectively. The characteristics of the structure can be understood as a framework consisting of Sc atoms, which is regarded as a tiling of five local structural units; the largest is an icosahedron similar to that included in the Tsai-type quasicrystal. The second exhibits structural similarity to a Mg2Zn11-type crystal and the third is an octahedron.

Approximant of dodecagonal quasicrystal formed in Mn–Si–V alloy

The crystal structure of a hexagonal phase formed in Mn70Si20V10 has been analyzed by the Rietveld method using synchrotron radiation diffraction data. Analysis was carried out by assuming an average metallic element Mn0.875V0.125 via high resolution electron microscopy. The space group of the refined model is P6/mmm (No. 191) and the unit cell includes 18.9 Si and 78.1 metallic atoms. The lattice parameters are a = 17.058(6) Å, c = 4.640(1) Å. This hexagonal phase is regarded as an approximant of a dodecagonal quasicrystal. The electron diffraction pattern along the c-axis exhibits pseudo 12-fold diffraction symmetry. In the c-projection, the structure is interpreted as the tiling of two types of local structural units, namely an equilateral triangle and square with the same edge length of 4.57 Å, which correspond to A15-type and Zr4Al3-type structures in the three dimensions, respectively. Both bond orientational order and scaling property, characteristic of a dodecagonal quasicrystal, are imitated in this periodic tiling.

Amorphous calcium (ortho)phosphates

Amorphous calcium phosphates (ACPs) represent a unique class of biomedically relevant calcium orthophosphate salts, having variable chemical but essentially identical glass-like physical properties, in which there is neither translational nor orientational long-range ordering of the atomic positions. Normally, ACPs are the first solid phases, precipitated after a rapid mixing of aqueous solutions containing ions of Ca(2+) and PO₄³⁻; however, other production techniques are known. Interestingly, ACPs prepared by wet-chemical techniques were found to have a relatively constant chemical composition over a relatively wide range of preparation conditions, which suggests the presence of a well-defined local structural unit, presumably with the structure of Ca₉(PO₄)₆ - so-called Posner cluster. However, the presence of similar clusters in ACPs produced by other techniques remains uncertain. All ACPs are thermodynamically unstable compounds and, unless stored in dry conditions or doped by stabilizers, spontaneously tend to transform to crystalline calcium orthophosphates, mainly to calcium apatites. This solution instability of ACPs and their easy transformation to crystalline phases are of a great biological relevance. Specifically, the initiating role ACPs play in matrix vesicle biomineralization raises the importance of ACPs from a mere laboratory curiosity to that of a key intermediate in skeletal calcification. In addition, due to significant chemical and structural similarities with calcified mammalian tissues, as well as excellent biocompatibility and bioresorbability, all types of ACPs are very promising candidates for the manufacture of artificial bone grafts. This review summarizes the current knowledge on the occurrence, preparation, composition, structure, major properties and biomedical applications of ACPs. To assist readers in looking for the specific details on ACPs, a great number of references have been collected and systematized.

The featured local structural units in liquid Al80Si15P5 alloy and their relationship with Si modification

The structure of liquid Al80Si15P5 alloy was investigated by ab initio molecular dynamics simulation in order to understand the mechanism of modification of Si primary phases by P in Al–Si alloys. It is found that the chemical environment around P atoms is crucial to provide insight into the structure of liquid Al80Si15P5 alloy. The P-centered Al6P structural units exist at 1373 K but are slightly distorted at 2873 K. One P atom usually has other P atoms at the next neighborhood bridged by Al atoms, and also tends to exclude Si atoms from its first coordination shell. Based on the structure information, some aspects related with the modification mechanism are discussed.

Structural, optical and thermal investigations on Dy3+ doped NaF–Li2O–B2O3 glasses

Abstract Structural, optical and thermal properties of Dy 3+ doped lithium fluoroborate glasses have been studied for various concentrations of Dy 3+ from 0.5 to 5 wt%. The XRD studies confirm the amorphous nature of the glasses while the FTIR spectra reveal the presence of BO 3 and BO 4 local structural units. The UV–VIS–NIR absorption studies were carried out to calculate the bonding parameters ( β ¯ and δ ), to identify the ionic/covalent nature of the glasses. The JO parameters, experimental and theoretical oscillator strengths were also determined and reported. The luminescence spectra have been studied to determine the radiative transition probability ( A ), stimulated emission cross section ( σ P E ) and the experimental and calculated branching ratios ( β R ) for the excited levels that include 4 F 9/2 → 6 H 11/2 , 6 H 13/2 , and 6 H 15/2 transitions. The variation of optical properties with varying concentrations of dysprosium oxide content in the glasses are reported and discussed. The thermal behavior of Dy 3+ doped lithium fluoroborate glasses have been reported by recording DSC thermograms.

3d Transition metal decorated B-C-N composite nanostructures for efficient hydrogen storage: A first-principles study

Ti decorated BC_4N nanotube has been studied using first-principles density functional approach, to explore the storage of molecular hydrogen. It combines the advantages of carbon nanotube, together with the thermal stability of BN nanotube. The local structural unit of BN3 and NB3 linked with B-N bonds are responsible for the extra stability of BC_4N nanotube as compared with CNT. While the host carbon nanotube is metallic, the substitutional doping of B and N with a large enough concentration (33%) turns it to semiconducting. Endohedral decoration, although energetically favourable, encounters a rather high barrier height of ∼4 eV, as obtained from our nudge elastic band calculation of the minimum energy path. Exohedral Ti@BC4N can bind up to four H2 molecules. For full Ti coverage, the system can absorb up to 5·6 wt% of hydrogen. Ab initio molecular dynamics simulation reveals that at 500 K hydrogen gets released in molecular form. We believe that this novel composite nanotube, functionalized by Ti atoms from outside, serves as a promising system for hydrogen storage.

Experimental Evidence for the Existence of a Stable Half-Barrel Subdomain in the (β/α) 8-Barrel Fold

The (β/α) 8-barrel is one of the most common folds functioning as enzymes. The emergence of two (β/α) 8-barrel enzymes involved in histidine biosynthesis, each of which has a twofold symmetric structure, has been proposed to be a consequence of tandem duplication and fusion of a (β/α) 4-half-barrel. However, little evidence has been found for the existence of an ancestral half-barrel in the evolution of other (β/α) 8-barrel proteins. In order to detect remnants of an ancestral half-barrel in the (β/α) 8-barrel structure of Escherichia coli N-(5′-phosphoribosyl)anthranilate isomerase, we engineered three potential half-barrel units, (β/α) 1–4, (β/α) 3–6, and (β/α) 5–8. Among these three arrangements, only (β/α) 3–6 is stable; it exists in equilibrium between monomeric and dimeric forms. Thus, the central segment of N-(5′-phosphoribosyl)anthranilate isomerase from E. coli can serve as a half-barrel precursor. A tandem duplication of (β/α) 3–6 yielded predominantly monomeric structures that were quite stable. This result exemplified that the structural characteristics of noncovalently assembled half-barrels could be improved by duplication and fusion. Moreover, our results may provide information regarding the local structural units that encompass interactions important for the early folding events of this ubiquitous protein conformation.

Dielectric properties and physical features of phosphate glasses containing iron oxide

Microwave techniques were applied to the study of dielectric properties of phosphate glasses on the basis of contributions from permanent and induced dipolar polarization of local structural units interacting with the electrical component of the electromagnetic radiation. The dielectric constant of the selected glass system (100 − x)(50P 2O 5·25Li 2O·25Na 2O)· xFe 2O 3, where 0 ≤ x ≤ 21 is in mol%, was measured using a microwave setup assembled to measure the phase shift of the standing wave pattern produced by the insertion of the sample. It is shown that the Fe 2+ ions contribute effectively to the dielectric constant, as expected from the interactions of the dipoles of the local charge compensation pairs with the microwave radiation. However, there is the possibility of occurrence of some ions Fe 3+, in general, at low iron content, which reinforces the glass structure and, therefore, decreases the dielectric constant. There is a gradual conversion from Fe 3+ to Fe 2+ as the iron ions increases. This is possibly the reason of the anomaly in the dielectric constant values observed in the results. These assumptions can be checked by results of electronic paramagnetic resonance (EPR) and optical absorption (OA). The dielectric constant of the glasses studied in this work was found to increase with the temperature in the range of 25–330 °C.

Oriented solids as a colloid suspension in nematic liquid crystal – New tool for IR-spectroscopic and structural elucidation of inorganic compounds and glasses

Possibilities of the linear-polarized infrared (IR-LD) spectroscopy of oriented colloid suspensions in nematic liquid crystals, for structural and local structural elucidation for first time are demonstrated of inorganic compounds and glasses. The advantages of the method for tellurite and borate glasses are shown. The IR-band assignment of the typical local structural units in the glasses are proposed by a comparison with the IR-characteristics of appropriate crystalline analogues as α-TeO 2, V 2O 5, MoO 3 · H 2O and its high temperature form. The IR-spectroscopic characteristics of BO 3, BO 4 and boroxol ring are elucidated, using crystalline β-BaB 2O 4, SrB 4O 7, H 3BO 3 and B 2O 3 as model systems, where the structural moieties have been refined by single crystal X-ray diffraction.

Quadrupolar Ordering inLaMnO3Revealed from Scattering Data and Geometric Modeling

Many strongly correlated materials display quadrupolar (Jahn-Teller) distortion of the local octahedral structural units. It is common for these distortions to be observed by probes of local structure but absent in the crystallographic average structure. The ordering of these quadrupoles is important in determining the properties of manganites and cuprates, and the nature of the disorder in these structures has been an unsolved problem. We combine high resolution scattering data and novel geometrical modeling techniques to obtain a detailed picture of the local atomic structure, and also to extract the quadrupolar order parameter associated with the distorted octahedra. We show that in LaMnO3, quadrupoles undergo a strong first-order phase transition at 730 K, but with nonzero order parameter remaining in the high-temperature phase.

準結晶研究の最近の展開 ２．準結晶をつくる（準結晶の作製）新合金の準結晶：その構造と形成条件

Recent progress on a new group of icosahedral quasicrystals is reviewed. This group is characterized by Tsai-type atomic cluster as a local structural unit, which is different from Mackay and Bergman-types. This group can be regarded as largest including two Cd-based binary, seven Zn-based and ten Cd-based ternary stable phases as well as Cu- and Ag-based phases. They are all P-type, and some of them exhibit higher degree of structural perfection than the “perfect” Al-Pd-Mn quasicrystal. In the Tsai-type quasicrystals, there are substitution rules in the constituent elements. Hume-Rothery rules, namely the equality in e/a and in the ratio of atomic radii, play important role in the substitution rules.

Rigidity and intermediate phases in glasses driven by speciation

The rigid to floppy transitions and the associated intermediate phase in glasses are studied in the case where the local structure is not fully determined from the macroscopic concentration. The approach uses size increasing cluster approximations and constraint counting algorithms. It is shown that the location and the width of the intermediate phase and the corresponding structural, mechanical and energetical properties of the network depend crucially on the way local structures are selected at a given concentration. The broadening of the intermediate phase is obtained for networks combining a large amount of flexible local structural units and a high rate of medium range order.

STRUCTURAL STUDY OF x(Fe2O3·V2O5)·(100-x)[P2O5·Li2O] GLASS SYSTEM BY FTIR SPECTROSCOPY

Glasses of the x( Fe 2 O 3· V 2 O 5)·(100-x)[ P 2 O 5· Li 2 O ] system, with 0≤x≤50 mol %, were prepared and investigated by FTIR spectroscopy in an attempt to determine the local structure of glasses. The interpretation of the obtained IR spectra revealed the presence and the dependence of the local structural units in the studied glasses on the iron and vanadium ions content. The results showed that phosphate units are the main structural units of the glass system and the iron and vanadium ions are located in the network. The increasing of iron and vanadium ions content indicate a gradual decreasing in the number of bridging oxygen ions and an increasing in the number of nonbridging oxygen ions.

Variations of Titanium Interactions in Solid State NMRCorrelations to Local Structure

Direct solid state NMR observation of titanium opens up new possibilities for characterization of many chemical problems. Here, solid state 47,49Ti NMR spectra are reported for a large variety of crystalline ternary and quaternary titanates. A wide range of values for the NMR interaction parameters is observed, with chemical shifts covering ∼300 ppm and quadrupole interactions (CQ) up to 24 MHz. The isotropic chemical shift (δiso) ranges for TiO6, TiO5, and TiO4 deduced from these samples suggest that NMR can help to determine coordinations in materials where it is unknown, although the strong overlap of these ranges may not allow unequivocal identification in all cases. For TiO6 units, there is a good correlation of the shear strain of the local TiO6 unit to CQ. Similar correlations are found for the much rarer TiO4 coordination. The correlation of CQ is generalized through the concept of a distortion index of the local structural unit, which applies to all TiOx units, including TiO5. Static NMR data from several samples (e.g., PbTiO3, Sr2TiO4) show unequivocally that for some compounds titanium has a significant chemical shift anisotropy, which needs to be included for accurate simulations of static titanium NMR spectra. The electric field gradient determined by NMR does not agree with values previously obtained by perturbed angular correlation. Calculations of the quadrupole parameters using the WIEN97 code, based on the linearized augmented plane wave method, have been carried out for some perovskite- and ilmenite-structured titanates and agree well with the values determined from NMR (i.e., within a few percent), except for the ilmenite form of CdTiO3 where the current structure appears unreliable. The 43Ca NMR spectrum is reported for CaTiO3 showing that CQ = 2.15 ± 0.03 MHz, η = 0.70 ± 0.07, and δiso = 17.2 ± 1.0 ppm.

STRUCTURAL STUDY OF Fe2O3–B2O3–CaO GLASSES BY IR SPECTROSCOPY

lasses belonging to the x Fe 2 O 3·(100–x)[ 3B 2 O 3· CaO ] system (with 0 ≤ x ≤ 50 mol %) have been investigated by IR spectroscopy. The influence of the gradual increase of Fe 2 O 3 content on the glass structure is revealed. Using FT-IR spectroscopy, the local structural units in glasses studied were found to have been modified by the presence of and the dependence on the Fe 2 O 3 content.

IR STRUCTURAL INVESTIGATION OF THE xFe2O3 · (100 - x) [3B2O3 · KCl] GLASS SYSTEM

Samples belonging to the x Fe 2 O 3 · (100 - x) [3 B 2 O 3 · KCl ] glass system with 0 ≤ x ≤ 20 mol% were investigated by IR spectroscopy in order to obtain information about the influence of Fe 2 O 3 on the local order in the 3B 2 O 3 · KCl glass matrix. The interpretation of the obtained vibrational spectra revealed the presence and the dependence of the local structural units in glasses studied by the Fe 2 O 3 content.