Tetrahedral Chains Research Articles

A Conjecture on the Relationship Between Critical Residual Entropy and Finite Temperature Pseudo-transitions of One-dimensional Models

Recently pseudo-critical temperature clues were observed in one-dimensional spin models, such as the Ising-Heisenberg spin models, among others. Here we report a relationship between the zero-temperature phase boundary residual entropy (critical residual entropy) and pseudo-transition. Usually, the residual entropy increases in the phase boundary, which means the system becomes more degenerate at the phase boundary compared with its adjacent states. However, this is not always the case; at zero temperature, there are some phase boundaries where the entropy holds the largest residual entropy of the adjacent states. Therefore, we can propose the following conjecture: If residual entropy at zero temperature is a continuous function at least from the one-sided limit at a critical point, then pseudo-transition evidence will appear at finite temperature near the critical point. We expect that this argument would apply to study more realistic models. Only by analyzing the residual entropy at zero temperature, one could identify a priori whether the system will exhibit the pseudo-transition at finite temperature. To strengthen our conjecture, we use two examples of Ising-Heisenberg models, which exhibit pseudo-transition behavior: one frustrated coupled tetrahedral chain and another unfrustrated diamond chain.

Mixed Mn/In chalcogenides [formula omitted]Q4 (A = Na, K, Rb, Cs; Q = S, Se, Te)

The series of new mixed chalcogenido metallates A3I(MnIn)Q4 containing MnII and InIII in a 1:1 ratio were systematically studied as model systems for the respective structurally diverse mixed-valent ferrate(II,III) analogs. The title compounds were synthesized from stoichiometric melts (Tmax ​= ​700–1000 ​°C) consisting of elemental chalcogen and indium, the manganese monochalcogenides MnQ, and either the alkali metals themselves or the alkali sulfides Na2S, Na2S or Cs2S2 (for the sulfides) as starting materials. The K, Rb and Cs salts of all chalcogens form the orthorhombic Na3Fe2S4-type structure uniformly (Pnma, e.g. a ​= ​725.50(2), b ​= ​1189.63(3), c ​= ​1148.76(3) pm, Z ​= ​4, R1 ​= ​0.0194 for A/Q ​= ​K/S), which contains buckled chains of edge-sharing tetrahedra. They are thus isotypic to the mixed-valent Na, K and Rb sulfido and selenido ferrates. The coordination numbers (CN) of the two crystallographically different A cations are 6 and 6 ​+ ​1. In contrast, the sodium sulfido metallate Na3(MnIn)S4, which is isostructural to Na3(MnGa)S4 and crystallizes in the tetragonal β-Ca3Ga2N4-type structure (I41/acd, a ​= ​1324.55(4), c ​= ​1906.94(7) pm, Z ​= ​4, R1 ​= ​0.0394), has no analog among the ferrates(II/III). The metallate anion consists of supertetrahedra (tetrahedra of corner-sharing tetrahedra) [Mn2In2S6S4/2]6−, which are connected via four vertices to form two interpenetrating diamond-like networks. The coordination numbers of the two Na+ cations are increased to 4 ​+ ​1 and 6. The sodium selenido salt Na3(MnIn)Se4 is isotypic with Na2Mn3Te4 (=Na2Mn[6](Mn2[4])Te4; monoclinic, C2/m, a ​= ​1605.5(2), b ​= ​415.64(4), c ​= ​671.02(7) pm, β ​= ​90.831(7)∘, Z ​= ​2, R1 ​= ​0.0316) and exhibits dimers of two edge-sharing tetrahedra [M2Se6], which are further connected via four vertices to form ribbons running along the short b axis. The Na+ CN is further reduced to 4(+2) Q. In accordance with the very similar ionic radii of Mn2+ and In3+, no superstructures based on Mn/In (which are distinguishable by X-ray diffraction methods) ordering are found. The structure chemistry of the overall eleven new compounds is compared with those of the corresponding mixed-valent ferrates A3Fe2II/IIIQ4 and the alkaline-earth trielates A3II(Al/Ga/In)2(N/P/As)4.

Windmountainite, □Fe3+2Mg2□2Si8O20(OH)2(H2O)4·4H2O, a new modulated, layered Fe3+-Mg-silicate-hydrate from Wind Mountain, New Mexico: Characterization and origin, with comments on the classification of palygorskite-group minerals

ABSTRACTWindmountainite, ideally □Fe3+2Mg2□2Si8O20(OH)2(H2O)4·4H2O, is a new mineral species and member of the palygorskite group discovered as orange-brown, radiating aggregates that commonly fill vesicles (average 1.5 × 2.5 mm) within a phonolite dike at Wind Mountain, Otero County, New Mexico, USA. The mineral develops as tightly bound bundles (up to 0.02 × 6 mm) of acicular to bladed crystals that are elongate on [001] and flattened on the pinacoid {010}. Associated minerals include albite, aegirine, fluorapophyllite-(K), natrolite, neotocite, and montmorillonite, the last of these being observed to replace primary windmountainite. It has a dull luster, silky in aggregates, is translucent and has an orange-brown streak. It does not fluoresce under short-, medium-, or long-wave ultraviolet radiation. Windmountainite is brittle with a splintery fracture and has two good cleavages (predicted) on {110}, an estimated hardness of 2, a calculated density of 2.51 g/cm3, and a calculated navg of 1.593. A total of n = 30 EMPA (WDS) analyses from six grains yielded an average of (wt.%): Na2O 0.08, MgO 3.47, Al2O3 1.15, SiO2 49.76, Cl 0.07, K2O 0.40, CaO 0.68, TiO2 0.30, MnO 5.64, Fe2O3 20.17, H2O (calc.) 16.59, O=Cl –0.02, total 98.29. The empirical formula [based on Σ(T1, T2, M2, M3) = 12 cations pfu, excluding Ca, K, and Na] is: (□0.78Ca0.12K0.08Na0.02)Σ1.00(Fe3+1.93Al0.04Ti0.02)Σ1.99 (Mg0.81Mn2+0.75Fe3+0.44)Σ2.00□2(Si7.81Al0.17Ti0.01Fe3+0.01)Σ8.00O20[(OH)1.98Cl0.02]Σ2.00[(H2O)3.38(OH)0.62]Σ4.00·4H2O, yielding the simplified formula, □Fe3+2Mg2□2Si8O20(OH)2(H2O)4·4H2O. The predominance of Fe3+ is based on color, results from the crystal-structure refinement, the crystal-chemistry of palygorskite-group minerals, the association with Fe3+-dominant minerals, and considerations regarding the late-stage geochemical evolution of agpaitic rocks. The presence of H2O and OH was determined based on results from the refined crystal structure and Fourier-transform infrared spectroscopy. Windmountainite crystallizes in the space group C2/m with a 13.759(3), b 17.911(4), c 5.274(1) Å, β 106.44(3)°, V 1246.6(1) Å3, and Z = 2. The seven strongest powder X-ray diffraction lines are [d in Å (I), (hkl)]: 10.592 (100) (110), 5.453 (16) (130), 4.484 (19) (040), 4.173 (28) , 3.319 (53) (221, 400), 2.652 (30) , 2.530 (27) . The crystal structure was determined from single-crystal X-ray diffraction data and refined to R = 4.01% and wR2 = 10.70% using data from 902 reflections (Fo > 4σFo). It is based on sheets of inverted double chains of SiO4 tetrahedra that sandwich ribbons of Mφ6 octahedra (φ = O, OH, H2O, Cl), giving rise to large channels (∼6.5 × 9 Å) that are occupied by loosely held H2O groups. A modified classification of the palygorskite group [general crystal-chemical formula M1M22M32M42T14T24O20(OH)2(H2O,OH)4·W] is proposed based on the occupants of the four M sites. Within this scheme, windmountainite is the □-Fe3+-Mg-□ member. The palygorskite group includes six members: palygorskite (monoclinic and orthorhombic polytypes), yofortierite, tuperssuatsiaite, raite, windhoekite, and windmountainite. Windmountainite is considered to have formed from late-stage fluids that were alkaline, oxidized, and rich in both Fe3+ and H2O; high aH2O conditions are reflective of abundant, hydrated feldspathoids (natrolite and analcime) forming as primary rock-forming minerals in the phonolite at Wind Mountain.

Effects of Aluminum Incorporation in Tobermorite Structure on Chloride Diffusion: A Molecular Dynamics Simulation Study

In this paper, the effects of different aluminum to silicon ratios in silicate chains of calcium silicate hydrates (C-S-H) are evaluated on the diffusion coefficient of chloride ions by molecular dynamics method. Tobermorite is a crystalline phase that is used for studying C-S-H properties in nano scale, because of its analogous chemical composition to C-S-H. Aluminum incorporation in C-S-H and the formation of calcium aluminosilicate hydrates (C-A-S-H) is due to both of hydration of tricalcium aluminate (C3A) in portland cement and aluminum oxides in pozzolans. There exist different Al/Si ratios in the tetrahedral chains of C-A-S-H depending on available aluminum oxides in cementitious raw materials. In order to compare the simulation results with previously-published experimental researches on cement pastes, a novel method is introduced here to calculate Al/Si ratio in tetrahedral chains of C-A-S-H using pozzolan replacement ratio in cementitious paste. MK (metakaolin) and FC3R (Fluid Catalytic Cracking Catalyst Residue) are the pozzolans that are used to validate the obtained results in this paper. Results showed that diffusion coefficients of chloride ions in C-A-S-H decrease by Al/Si ratio increasing in the tetrahedral chains as it was observed experimentally in previous researches.

Synthesis, structure and magnetic properties of a novel double layered brownmillerite Ca2LaFe1.75Cr0.25GaO8

We report the structural and magnetic properties of a novel double layered Brownmillerite Ca2LaFe1.75Cr0.25GaO8. Synchrotron X-ray diffraction and neutron powder diffraction data confirms orthorhombic structure having Pbma space group with tetrahedral chain ordering and ∼ 25 % of cation (Fe/Ga) disorder between octahedral and tetrahedral sites. Ca2LaFe1.75Cr0.25GaO8 exhibits antiferromagnetic transition at TN∼630 K. The magnetic structure is determined as canted G-type antiferromagnetic with weak ferromagnetic component along a direction. The ordered magnetic moments obtained at 4 K are 2.24 (4) μB (Fe3+/Cr3+) for octahedral sites and 2.34 (4) μB (Fe3+) for tetrahedral sites.

Mullite-2<i>c</i> – a natural polytype of mullite

Abstract. A single crystal (∼ 20 µm × 20 µm × 330 µm) of mullite-2c, a natural polytype of mullite, was separated from a radially grown cluster of acicular crystals from Ettringer Bellerberg (Quarternary Eifel volcanic fields, Germany). The chemical composition determined from electron microprobe analysis (EMPA) is Na0.01Mg0.05Al8.52Fe0.293+Si3.13Ti0.02O19.55, corresponding to x=0.22(8) in the generalised mineral formula My+Mgz2+M8+4x+y-2z3+M4-4x-y+z4+O20-2x. Only Fe3+ as foreign cation was considered in the refined structure model, partially replacing Al3+ in the octahedral chains. A crystal of a similar type, though exhibiting a significantly different composition with x=0.02, was first described in 2015, tentatively named “sillimullite” by Fischer et al. (2015). This crystal and our new sample have similar structural properties, now classified as a polytype of mullite, designated mullite-2c. Single-crystal X-ray diffraction showed that the mullite-2c crystal investigated here exhibits partial Si ∕ Al ordering in the double chains of (Si,Al)O4 tetrahedra in contrast to the sample described in 2015 as being completely ordered. The ordering in mullite-2c results in a doubled c lattice parameter with respect to mullite. It crystallises in space group Pnam, with cell parameters for the new sample of a=7.5432(5) Å, b=7.7048(5) Å, c=5.7965(3) Å, V=336.89(6) Å3 and Z=1. X-ray powder diffraction data are presented with a detailed discussion of the differences between the diffraction patterns of sillimanite, mullite and mullite-2c. Crystals of mullite-2c are translucent to lightly violet, they possess a vitreous lustre and the calculated density is 3.199 g cm−3. The optical character is biaxial (+), with refractive indices determined by spindle-stage microscopy of nx=1.6673, ny=1.6687 and nz=1.680(4) (adjusted to conform to 2VZ=39(4)∘). Applying the Gladstone–Dale approach, the compatibility index is 0.007, representing superior compatibility. In terms of chemical composition and structural features mullite-2c is an outstanding example of mullite-type compounds falling into the postulated miscibility gap between sillimanite and mullite. Its crystal structure combines characteristics from both mullite (oxygen vacancies, triclusters of tetrahedral building units) and sillimanite (high degree of Si ∕ Al ordering in the tetrahedral building units, causing the doubled c parameter). The lattice parameters (normalised to 1c) of the new sample lie between those of sillimanite and 3 / 2 mullite; the chemical composition is close to 3 / 2 mullite and thus differs significantly from the silica-rich composition of the species previously determined by Fischer et al. (2015), indicating a relatively large compositional variation.

Tremolite-actinolite fiber coatings of sub-nanometer silica-rich particles in lungs from deceased Quebec miners.

Tremolite-actinolite (TA) fibers from the lungs of deceased former Quebec mine workers were found to be coated with sub-nanometer particles. Qualitative chemical analyses were performed on the particles indicating that they were composed of silicon and oxygen. The crystal lattice structure of all amphibole minerals, including the TA series, is arranged as pairs of linear chains of SiO4 tetrahedra that are linked together to form double chains. Our observations of the TA fibers from miner's lungs, made using a high-resolution transmission electron microscope, indicated that the tetrahedral silica chains were progressively split, forming dispersed sub-nanometer particles. The non-tetrahedral sites were removed at the surface of the TA fibers, presumably by the oxidation process involved in attempted phagocytosis, which also resulted in fragmentation of the tetrahedral chains. It was found that the silicon-rich particles (SRPs) were variable in diameter, consistent with fragments formed from the splitting of the tetrahedral chains. The TA fibers from lungs displayed coatings and linear interior zones of SRP parallel to the planes of longitudinal fiber splitting. The literature on very small nanoparticles is consistent with deep penetration of SRP into cell DNA interiors, oxidative stress, and carcinogenesis.

Anisotropic structure of alkali metaphosphate glasses

AbstractOrientation anisotropy, which is well known in organic polymers with appropriate network structures, is less common in oxide glasses. We present the intermediate‐range order in anisotropic alkali metaphosphate glass which consists of oriented PO4 tetrahedral chains and intervening alkali cations along the elongation direction. The X‐ray total structure factor S(Q) indicates that the inter‐chain spacing depends on the size of alkali cations and varies from 5.03 to 6.28 Å. The mixed alkali effect is primarily related to an increase of the separation. The total correlation function T(r) provides the first definite evidence that the anisotropic structure is composed of phosphorus‐bridging oxygen bonds (P–OB) lying along the elongation direction and phosphorus‐non‐bridging oxygen bonds (P–OT) oriented perpendicular to the elongation direction. The present result unveils fundamental aspects of the anisotropic structure of an oxide glass and provides essential information for the development of oxide glasses to control structural anisotropy.

Observation of polymer-like flow mechanism in a short-chain phosphate glass-forming liquid.

The dynamics of the phosphate chains and the attendant shear relaxation in a short-chain silver phosphate glass-forming liquid with the composition 51.5%Ag2O-48.5% P2O5 are studied using a combination of high-temperature 31P NMR spectroscopy and parallel plate rheometry. The temperature-dependent evolution of the 31P NMR spectral line shapes indicates that the constituent PO4 tetrahedral chains in this liquid undergo rapid rotational reorientation. The time scale of this dynamics is in complete agreement with that of shear relaxation and, thus, must be responsible for the viscous flow of this liquid. These results demonstrate for the first time that, although the shear relaxation of the network oxide glass-forming liquids is typically controlled by the scission and renewal of bonds between the network-forming cations and oxygen atoms, such a scenario may not always be tenable for liquids with low-dimensional structures consisting of chains.

Assessment of the influence of additives in concrete by the Raman spectroscopy method

The effect of nano SiC additives with a particle size less than 50 nm, carbon black with a specific surface of about 80 m2/g, SiO2 and Al2O3 zol, AlCl3, FeCl3 solutions on the composition of silicon-oxygen anions in the pure C-S-H phase and hydrated cement was studied. The mixture was pressed at a specific pressure of 100 MPa and tested for compressive strength after 1, 7 and 28 days of normal aging. The Raman spectroscopy method was used to determine the composition of silicon-oxygen anions (SOA) in the C-S-H phase of cement and compared it with the strength of the stone, as well as with the SOA composition in the pure C-S-H phase. It was found that the initial powder of the C-S-H phase mainly contains isolated Q0 and terminal Q1 tetrahedra [SiO4]. Pressing the powder at 100 MPa leads to the advanced formation of chains of cross-linking Q2 and Q3 tetrahedra. The addition of silica to C-S-H promotes maximum chain formation from Q2. The Raman spectrum of C-S-H with silica zol is identical to the spectrum of hydrated cement. Iron chloride prevents the formation of extended chains of silicon-oxygen tetrahedrons. Carbon-containing nano-additives significantly alter the spectrum of C-S-H calcium substructures.

Modeling of a point set which belongs to a spatial monotone curve

The formation of one-dimensional contours on the basis on the given conditions is one of the most popular geometric modeling problems. The problem is solved by variative discrete geometric modeling, which assumes the formation of intermediate points for the initial set of condensation points. The discrete model of the curve consists of a point sets, given geometric characteristics and a condensation algorithm.A discretely presented curve (DPC) is formed by a condensation of the initial point set of an arbitrary configuration in areas on which it is possible to ensure a monotonic change in the values of its characteristics. Monotone plots are joined at special points. Every three consecutive points of the duodenum define an adjacent plane. Four adjacent planes passing through two consecutive points limit the tetrahedron. The chain of consecutive tetrahedrons defined on all segments is the region of the location of the smooth curve of the constant stroke line interpolating the initial point sets. The torsion on the sections of the DPC is estimated by the value of the ratio of the angle between adjacent adjacent planes to the length of the corresponding chord of the accompanying broken line. The point of thickening is assigned inside the tetrahedron of the DPC. As a result of successive condensations, we obtain a continuous bypass of a constant stroke, at each point of which there is a single position of the main trihedron. The point of condensation is assigned in such a way that the values of the torsion in the duodenum points change monotonically. This ensures that the torsion values are regular at the points of the bypass.The imposition of additional conditions on the DPC formed requires the definition of the corresponding region of a possible solution within the DPC arrangement tetrahedron.

The Long-Periodic Loop-Branched Chain Structure of the Oxonitridophosphate La21 P40 O46 N57 , Elucidated by a Combination of TEM and Microfocused Synchrotron Radiation.

The lanthanum oxonitridophosphate La21P40O46N57 was synthesized by high‐pressure metathesis from partially hydrolysed LiPN2 and LaCl3 at 750–950 °C and 7–9 GPa. The combination of transmission electron microscopy (TEM) and diffraction using microfocused synchrotron radiation revealed a monoclinic crystal structure (space group P21/n, a=14.042(4), b=7.084(3), c=41.404(10) Å, β=97.73(3)° and Z=2), which is characterized by loop‐branched 21 member single chains of P(O,N)4 tetrahedra that extend along [2 0 1]. These chains are related to the loop‐branched dreier single chains with dreier‐ring loops in stillwellite (CeBSiO5). In La21P40O46N57, these chains are characterized by a complex long‐periodic conformation and exhibit disorder that involves La/N and P split positions. This is an extraordinarily long periodicity with respect to branched single chains of tetrahedra. La21P40O46N57 constitutes the first rare‐earth oxonitridophosphate exhibiting a chain structure. Single‐crystal data are consistent with electron and powder X‐ray diffraction.

Order, Disorder, and Dynamics in Brownmillerite Sr2Fe2O5.

The room-temperature structure of brownmillerite-type Sr2Fe2O5 remains controversial, despite numerous published crystallographic studies utilizing X-ray, neutron, and electron diffraction data collected on single-crystalline and powder samples. The main disagreements concern the ordering of twisted FeO4 tetrahedral chains within and between the layers stacked along the b axis, and the impact of this ordering on oxide-ionic conductivity. Here, we present new data along with a reinterpretation of previously published diffraction images, including the reassignment of satellite reflections, which harmonize the results of past studies in a unified description of tetrahedral chain ordering in Sr2Fe2O5 at length scales relevant to X-ray and neutron diffraction. Implications for the prevailing model of oxide ion transport in this material are also discussed.

Synthesis and Characterization of Sodium-Iron Antimonate Na2FeSbO5: One-Dimensional Antiferromagnetic Chain Compound with a Spin-Glass Ground State.

A new oxide, sodium-iron antimonate, Na2FeSbO5, was synthesized and structurally characterized, and its static and dynamic magnetic properties were comprehensively studied both experimentally by dc and ac magnetic susceptibility, magnetization, specific heat, electron spin resonance (ESR) and Mössbauer measurements, and theoretically by density functional calculations. The resulting single-crystal structure (a = 15.6991(9) Å; b = 5.3323 (4) Å; c = 10.8875(6) Å; S.G. Pbna) consists of edge-shared SbO6 octahedral chains, which alternate with vertex-linked, magnetically active FeO4 tetrahedral chains. The 57Fe Mössbauer spectra confirmed the presence of high-spin Fe3+ (3d5) ions in a distorted tetrahedral oxygen coordination. The magnetic susceptibility and specific heat data show the absence of a long-range magnetic ordering in Na2FeSbO5 down to 2 K, but ac magnetic susceptibility unambigously demonstrates spin-glass-type behavior with a unique two-step freezing at Tf1 ≈ 80 K and Tf2 ≈ 35 K. Magnetic hyperfine splitting of 57Fe Mössbauer spectra was observed below T* ≈ 104 K (Tf1 < T*). The spectra just below T* (Tf1 < T < T*) exhibit a relaxation behavior caused by critical spin fluctuations, indicating the existence of short-range correlations. The stochastic model of ionic spin relaxation was used to account for the shape of the Mössbauer spectra below the freezing temperature. A complex slow dynamics is further supported by ESR data revealing two different absorption modes presumably related to ordered and disordered segments of spin chains. The data imply a spin-cluster ground state for Na2FeSbO5.

Atomic occupancy mechanism in brownmillerite Ca2FeAlO5 from a thermodynamic perspective

AbstractBrownmillerite‐type oxides are a large family of structural and functional materials that are of great concern. In this study, the occupancy mechanism of Fe and Al ions in brownmillerite Ca2FeAlO5 is revealed and the specific configuration of Ca2FeAlO5 is proposed for the first time. Through the energy analysis of a series of conjectured models based on well‐defined ab initio calculations, we find that the distribution of Al and Fe in Ca2FeAlO5 follows the “homogeneous layer principle”, namely, Al and Fe tend to form homogeneous octahedral layers or tetrahedral chains rather than mixed layers or chains of two elements along the b axis. The theoretical Ca2FeAlO5 structure with specific atomic occupancy is proposed based on energy analysis and previous experiments, which is of fundamental significance for further study of its physical and chemical properties. Moreover, the influence of entropy increment on the stability of Ca2FeAlO5 crystal is clarified, which paves the way to investigate the structures and properties of Ca2FeAlO5 and other brownmillerite members.

Brownmillerite-Type Sr2ScGaO5 Oxide Ion Conductor: Local Structure, Phase Transition, and Dynamics.

Brownmillerite-type Sr2ScGaO5 has been investigated by a range of experimental X-ray and neutron scattering techniques (diffraction, total scattering, and spectroscopy) and density functional theory calculations in order to characterize its structure and dynamics. The material undergoes a second-order phase transition on heating during which a rearrangement of the (GaO4/2)∞ tetrahedral chains occurs, such that they change from being essentially fully ordered in a polar structure at room temperature to being orientationally disordered above 400 °C. Pair distribution function analysis carried out using neutron total scattering data suggests that GaO4 tetrahedra remain as fairly rigid units above and below this transition, whereas coordination polyhedra in the (ScO6/2)∞ layers distort more. Inelastic neutron scattering and phonon calculations reveal the particular modes that are associated with this structural change, which may assist ionic conductivity in the material at higher temperatures. On the basis of the correlations between these findings and the measured conductivity, we have synthesized a derivative compound with increased conductivity and suggest a possible conduction mechanism in these brownmillerite-type solid electrolytes.

Charge transport properties of Ca2FeGaO6-δ and CaSrFeGaO6-δ: The effect of defect-order

The ability to achieve different charge-transport properties in oxygen-deficient perovskites, an important class of materials in the fuel cell technology, has been demonstrated through manipulation of the defect-order. This correlation has been investigated through the synthesis of a new non-centrosymmetric oxygen-deficient perovskite, CaSrFeGaO6-δ, as well as its centrosymmetric analogue, Ca2FeGaO6-δ. X-ray photoelectron spectroscopy and iodometric titration studies indicate the same degree of oxygen-deficiency in both compounds. Neutron diffraction shows the transformation of the defect-arrangement, leading to variation in the orientation of tetrahedral chains in the material structure. The variation in the defect-order results in remarkable changes in charge-transport properties, where CaSrFeGaO6-δ shows significantly greater conductivity than Ca2FeGaO6-δ at room temperature. Variable-temperature electrical conductivity in a wide temperature-range, 25 °C–800 °C, points to semiconducting behavior for both materials, where the superior conductivity of CaSrFeGaO6-δ persists in the entire temperature-range.

Synthesis of photoluminescent β-Ga2O3 nanostructures using electrospinning method, and control of length-diameter ratio by calcination heating rates

Nanofiber precursors of PVP-Ga(NO3)3 were synthesized through the electrospinning technique, and monoclinic β-Ga2O3 patterns were later obtained through the calcination method. During the annealing process, the nanofibers’ pores decreased, and their lengths were uniform up to several micrometers due to the surface-to-core extension that comprises the crystallization through Ostwald ripening process. Synthesis on the structure and morphology of materials were investigated using scanning and transmission electron microscopy equipped with an energy dispersive spectrometer, X-ray diffraction, Raman and Fourier-transform infrared (FTIR) spectroscopies. The β-Ga2O3 optical properties disclosed very broad and intense photoluminescence emission spectrum in the blue region of the wavelength, whose driving force was the presence of oxygen vacancies in the structures. Two types of Ga3+ ions (GaO6 octahedral and GaO4 tetrahedral chains) were demonstrated to come from different vibrations of Ga–O bonds in the Raman and FTIR spectra. And Ga3+–CO adducts formed on coordinatively Ga3+ ion located at edges and corners of β-Ga2O3 crystallites. Thus, successful results of this work included the control of length-diameter ratio by calcination heating rates, as well as the broad blue emission band, representing a strong potential of β-Ga2O3 materials in optoelectronic applications.

Preparation and performances of novel Na2FeSiO4/C composite with more stable polymorph as cathode material of sodium-ion batteries

The sodium orthosilicates as the potential candidates of cathode materials in sodium-ion batteries capture tremendous interests because of the low costs, robustly crystal structure and high specific capacity. However, like lithium orthosilicates, the crystal structure, morphology and electrochemical properties of the polycrystalline sodium orthosilicates need to be properly designed so that sodium ion can be smoothly intercalated/deintercalated. Herein, a novel orthorhombic Pb21a–Na2FeSiO4/C, which is obtained from the conversion of C2221–Na2FeSiO4/C, is prepared and used as cathode material for the first time. Meanwhile, the pure phase C2221–Na2FeSiO4/C is firstly prepared by sol-gel method. Field emission scanning and transmission electron microscopy demonstrate that C2221–Na2FeSiO4/C has quasi-spherical morphology, and the active nanoparticles completely disperse within the cross-linked carbon nanolayer. The results from X-ray diffraction and Rietveld analysis reveal that the crystal structure of Pb21a–Na2FeSiO4/C, similarly to C2221–Na2FeSiO4/C, can be formed by three-dimensional channel networks which generate from the alternate tetrahedral geometry chains of [FeO4] and [SiO4], but Pb21a–Na2FeSiO4/C composite is more stable than C2221–Na2FeSiO4/C. After the first sodiation/desodiation process, Pb21a–Na2FeSiO4/C shows the specific capacity of 118 mAh g−1 at 0.1 C and a capacity retention of 57.3% after 100 cycles.

Symmetry of Structures That Can Be Approximated by Chains of Regular Tetrahedra

The noncrystallographic symmetries of chains of regular tetrahedra are determined by mapping the system of algebraic geometry and topology designs to the structural level. It has been shown that the basic structural unit of such a chain is a tetrablock: a seven-vertex linear aggregation over faces of four regular tetrahedra, which is implemented in linear (right- and left-handed) and planar versions. The symmetry groups of linear and planar tetrablocks are isomorphic, respectively, to the projective special linear group PSL(2, 7) of order 168 and the projective general linear group PGL(2, 7) of order 336. A class of structures formed by an assembly of tetrablocks having no common tetrahedra is introduced. Examples of tetrablock assembly over common face, leading to a Boerdijk–Coxeter helix, an α helix, and a helix used as one of collagen models are presented.