Average Bond Angles Research Articles

Enhancing structural and magnetic properties of BaFe12O19 nanoparticles through co-substitution (Co-Mn): A promising approach for permanent magnet applications

In-depth study into the creation and improvement of rare-earth free permanent magnets, particularly ferrites, has been sparked by recent economic and environmental concerns. M-type barium hexaferrites (BaFe12O19, pure BHF) are a class of low-cost permanent magnets with an excellent curie temperature and good resistance to oxidation and corrosion that are significant from a technological perspective. In the current study, the experimental conditions are optimized to obtain barium hexaferrite (BaFe12O19) co-substituted with transition elements (Co and Mn) (BaFe12–2xCoxMnxO19) via the chemical co-precipitation method. In this framework, different substitution possibilities will be investigated. The structure, morphology, vibrational spectrum, and thermal stability of the generated nanoparticles were revealed using powder x-ray diffraction (XRD), transmission electron microscope (TEM), Fourier transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA). Rietveld refinement was used to estimate the average bond length and bond angle in order to determine the impact of superexchange interaction. Based on more precise structural characteristics, the atomic occupation and distribution of bonds are depicted from their charge density graph. For all the synthesized materials, the maximum energy product (BH)max and magnetic characteristics (VSM) were also investigated. The potential causes of this phenomenon were examined and discussed at length.

Evolution of Short-Range Order of Amorphous GeTe Upon Structural Relaxation Obtained by TEM Diffractometry and RMC Methods.

Glasses frequently reveal structural relaxation that leads to changes in their physical properties including enthalpy, specific volume, and resistivity. Analyzing the short-range order (SRO) obtained from electron diffraction by transmission electron microscopy (TEM) in combination with Reverse-Monte-Carlo (RMC) simulations is shown to provide information on the atomic arrangement. The technique elaborated here features several benefits including reliability, accessibility, and allows for obtaining detailed structural data quickly. This is demonstrated with a detailed view of the structural changes in the as-deposited amorphous phase change material (PCM) GeTe. The data show a significant increase in the average bond angle upon thermal treatment. At the same time the fraction of tetrahedrally coordinated Ge atoms decreases due to an increase in octahedrally distorted and pyramidal motifs. This finding provides further evidence for the atomic processes that govern structural relaxation in amorphous GeTe and other PCMs. A thorough literature review finally unveils possible origins of the large discrepancies reported on the structure of amorphousGeTe.

Review: Structural, elastic, and thermodynamic properties of cubic and hexagonal ScxAl1−xN crystals

A review of the structural, elastic, and thermodynamic properties of cubic and hexagonal ScxAl1−xN crystals over the range of possible random alloys is provided. Based on measured and simulated lattice and internal cell parameters of NaCl (B1), CsCl (B2), and α-ZnS (B3) type cubic ScxAl1−xN lattices as well as of β-ZnS (B4), lh-MgO (Bk), and NiAs (B81) type hexagonal ScxAl1−xN crystals, their atomic positions, distances to nearest neighbor atoms, geometric dimensions of crystal cells, mass density, as well as their average bond length and bond angles are presented in dependence on the alloy composition. The understanding gained about the crystal lattices is used to provide a model for the transitions from the β-ZnS to the lh-MgO or NaCl lattice induced by the alloying of AlN with ScN. Based on published data sets of stiffness coefficients, the compliance coefficients, Young's modulus, shear modulus, Poisson's ratio, compressibility, and the sound velocities are presented in relation to the orientation of representative crystal planes and axes for rock salt, layered hexagonal, and wurtzite ScxAl1−xN crystals. Particular attention is paid to the directional anisotropies of elastic properties of the different crystal lattices if Sc atoms substitute an increasing number of Al atoms. Based on sound velocities determined, an overview of the fundamental thermodynamic properties of cubic and hexagonal ScxAl1−xN alloys is provided, such as the Debye temperature, heat capacity, minimum heat conduction, and melting temperature.

Molecular dynamics simulation of the cone-jet electrospray: Role of surfactants

Electrospray in the cone-jet regime could produce highly charged and fine monodisperse drops, and further disintegrates into nanoscale drops, even ions due to synergism of Rayleigh instability and evaporation. The jet and its breakup instability into drops are significantly affected by liquid properties including surface tension, conductivity, density and others. Surfactants may play important roles on gas-liquid interface and are usually added in solution to vary liquid property and may further affect electrospray behaviors. In present work, the ethanol containing surfactants is atomized in the cone-jet to discuss the synergistic effect of surfactant types, concentrations and surfactant-salt on the jet evolution behaviors under direct current electric fields using molecular dynamics (MD) method. It is found that the jet length of ethanol solution with Span-80 is the longest (Span-80, SDS, CTAB and TX-100), while it has excellent dispersion. The addition of CTAB could make ethanol solution transform from continuous jet into dripping. SDS interacts most violently with ethanol due to the presence of sodium ions. As surfactant concentration increasing, the number of solution molecules sprayed remarkably increases, while the interaction generated from ethanol decreases. The number of molecular clusters (composed of three or more ethanol molecules) in the cone-jet electrospray increases with increasing Span-80 concentration. It is also found that the addition of Span-80 slightly changes the average bond angle of ethanol and bond angle distribution. The ethanol atomization morphology transforms to spindle becasue of the synergistic effect of surfactant and salt ions. The interaction generated between ethanol molecules plays a dominating role and decreases due to the generation of spindle-shaped drops. The hydrogen bond generated from ethanol decreases with increasing conductivity and surface tension. The conclusions of this paper will be latent value for the optimization of physical parameters in electrospray.

First-principles study of torsional single-walled carbon nanotubes

The controllable band gap of single-walled carbon nanotube (SWCNT) has become a research hotspot. This study introduces a torsional model that involves each rotating carbon atom along the axial direction of SWCNT, and a detailed description of the model creation process. Two guidelines for constructing the model are proposed, and the self-consistency of the torsion model is established through first-principles density functional theory. Initially, the band gap map of SWCNTs under torsion is present. As the twist strength increases, the band gap of SWCNT undergoes several phase transitions, including semiconductor-metal transition and metal-semiconductor transition. Moreover, we investigate the variations in the average bond length, average bond angle, and diameter of SWCNT under torsion. Furthermore, this work turns to the analysis of carbon atomic energy statistics, revealing distinct energy changes for different types of single-walled carbon nanotubes under identical torsion intensity. The findings shed light on the controllable band gap of SWCNTs, offering a theoretical foundation for the development of nanoelectronic devices and microintegrated circuits utilizing single-walled carbon nanotubes. In conclusion, this research presents a novel approach for exploring the controllable band gap of single-walled carbon nanotube through torsional manipulation. Theoretical insights into the behavior of SWCNTs under torsion provide valuable contributions to the field and pave the way for potential applications in nanoelectronics and microintegrated circuits.

Density functional theory studies on graphene/h-boron nitride hybrid nanosheets for supercapacitor electrode applications.

Pristine graphene (C32), hexagonal boron nitride (h-BN), and graphene/h-BN hybrid nanosheets were examined using density functional theory calculations in order to find their suitability as an electrode material for supercapacitor applications. The stability of the structure, charge density, electronic properties, and quantum capacitance of pristine graphene and graphene/h-BN hybrid nanosheets were studied. The structural optimization results reveal that all the nanosheets are stable with zero transverse displacement of atoms along the z-direction. Further, replacing the C-C pair with B-N altered the average bond length and angle, thereby maintaining structural stability. The interaction between graphene and h-BN is higher for C16B8N8 compared to other hybrid nanosheets because of the delocalized distribution of the electron density cloud. The doping of the B-N pair into the graphene nanosheet shifts the Fermi level into either the valence band or the conduction band based on the concentration of the B-N pair. Meanwhile, the effective mass is increased and is relatively high for the hybrid nanosheets with a localized state. The pristine B16N16 nanosheet exhibits a quantum capacitance of 31.539 μF cm-2, while among the hybrid nanosheets, the C4B14N14 nanosheet exhibits a maximum quantum capacitance of 22.518 μF cm-2, and from the outcomes, they are suitable as an electrode for asymmetric supercapacitors.

Machine Learning Diffusion Monte Carlo Forces

Diffusion Monte Carlo (DMC) is one of the most accurate techniques available for calculating the electronic properties of molecules and materials, yet it often remains a challenge to economically compute forces using this technique. As a result, ab initio molecular dynamics simulations and geometry optimizations that employ Diffusion Monte Carlo forces are often out of reach. One potential approach for accelerating the computation of "DMC forces" is to machine learn these forces from DMC energy calculations. In this work, we employ Behler-Parrinello Neural Networks to learn DMC forces from DMC energy calculations for geometry optimization and molecular dynamics simulations of small molecules. We illustrate the unique challenges that stem from learning forces without explicit force data and from noisy energy data by making rigorous comparisons of potential energy surface, dynamics, and optimization predictions among ab initio density functional theory (DFT) simulations and machine-learning models trained on DFT energies with forces, DFT energies without forces, and DMC energies without forces. We show for three small molecules─C2, H2O, and CH3Cl─that machine-learned DMC dynamics can reproduce average bond lengths and angles within a few percent of known experimental results at one hundredth of the typical cost. Our work describes a much-needed means of performing dynamics simulations on high-accuracy, DMC PESs and for generating DMC-quality molecular geometries given current algorithmic constraints.

Influence of alloying and structural transition on the directional elastic and isotropic thermodynamic properties of wurtzite and layered hexagonal ScxAl1−xN crystals

The structural, elastic, and basic thermodynamic properties of hexagonal ScxAl1−xN crystals are calculated and discussed over the whole range of possible random alloys, including the transition from wurtzite to the layered hexagonal structure. Based on a review of lattice and internal parameters in combination with complete datasets of stiffness coefficients published in the literature, differing in the considered alloying intervals and the predicted structural transitions, changes in the crystal lattices caused by the substitution of aluminum by scandium atoms are discussed and illustrated. Crystal properties like the mass densities, average bond angles, and bond lengths are calculated, and the compliance coefficients, Young's modulus, shear modulus, Poisson's ratio, compressibility, and sound velocities are determined depending on the alloy composition and in relation to the orientation of crystal planes and axes. Particular attention is paid to the occurring directional anisotropies and the changes in structural and elastic properties in the alloy region of the structural transition between wurtzite and layered hexagonal ScxAl1−xN crystals. The acoustic velocities determined are used to calculate basic thermodynamic properties such as the Debye temperature, heat capacity, and minimum heat conduction, as well as to evaluate both the influence of the alloying and the structural transition on these properties.

Molecular Dynamics Simulation to Investigate the Effect of Al2O3 Doping and Compression on the Structural Properties of Aluminium Silicate Glass

In this paper, we have performed molecular dynamics simulation to investigate the effect of Al2O3 doping and compression on the structural properties of aluminium silicate (AS2) glass. The structural properties are examined via TOx units, OTy linkages, the average bond angle distributions, order parameters, and cluster function. The result shows that the network structure of AS2 is mainly built by TOx units and OTy linkages (T = Si or Al, x = 3-7, y = 2-4). We found that in AS2 glass, the subnet structure performed by the perfect SiOx units is not transfigured. Meanwhile, the AlOx units are significantly distorted. The structural organization of the SiO4-network in AS2 glass is not dependent on Al2O3doping. Moreover, the degree of structural homogenous significantly depends on compression. In the range of 15–25 GPa, the structural homogenous of AS2 is caused by the mobility of all atoms, tightly related to the glass-glass transition. Our work is expected to support finding the AS2 compositions that can produce hard and damage-tolerant glasses in the higher pressure region. Doi: 10.28991/HEF-2022-03-02-03 Full Text: PDF

Wurtzite ScAlN, InAlN, and GaAlN crystals, a comparison of structural, elastic, dielectric, and piezoelectric properties

We present detailed investigations of the structural, elastic, dielectric, and piezoelectric properties of scandium aluminum nitride (ScxAl1−xN) with the wurtzite crystal structure by means of first-principles calculations based on density functional theory in order to enable a detailed comparison to the corresponding physical properties of GaAlN and InAlN. The goal of our approach is to use atomistic simulations to extract the novel solid state characteristics of ScxAl1−xN crystals by the determination of complete sets of coefficients for the elastic, compliance, and piezoelectric tensor and to confirm the theoretical predictions by experimental measurements of selected tensor coefficients. The calculation of the tensor components is accompanied by a detailed analysis of the crystal structures, e.g., average bond length, bond angles, lattice parameters, and mass density in dependence on alloy composition of ScxAl1−xN. If an increasing number of Al atoms of up to x = 0.5 are replaced by Sc atoms, we observe a nonlinear change of the ratio of lattice parameter c(x)a(x) and average bond angles of about 10% and 5%, respectively, which give an indication of an increasing deviation of the crystal structure of ScxAl1−xN from an ideal hexagonal lattice. As a consequence of the deformed crystal structure and the iconicity of the Sc–N bond, we predict a change in value of the elastic coefficient C33ScAlN(x), the piezoelectric coefficient e33ScAlN(x), and the value of spontaneous polarization PSPScAlN(x) of up to 65%, 150%, and 230%, respectively. Based, on these simulation results, physical features of practical use are derived, like the average bulk, shear, and the Young modulus as well as the reciprocal Young's modulus and Poisson ratio. Furthermore, the spontaneous polarization of ScxAl1−xN is approximated, taking nonlinear effects into account as well as the piezoelectric polarization caused by uniaxial, biaxial, and hydrostatic stresses in dependency on alloy composition and strain. A detailed comparison of the structural and polarization related properties of GaAlN and InAlN allows us to point out the peculiarity of wurtzite ScAlN crystals within the group III-nitrides.

In situ and tunable structuring of semiconductor-in-glass transparent composite.

SummarySemiconductor-in-glass composites are an exciting class of photonic materials for various fundamental applications. The significant challenge is the scalable elaboration of composite with the desirable combination of tunable structure, high semiconductor loading ratio, and excellent transparency. Here we report that the topological engineering strategy via hybridization of the glass network former enables to surmount the aforementioned challenge. It not only facilitates the in situ precipitation of (Ga2-xAlx)O3 domains with continuously tunable composition but also allows to simultaneously refine the grain size and enhance the crystallinity. In addition, the composites exhibit excellent transparency and can host various active dopants. We demonstrate the attractive broadband optical response of the composite and achieve the pulse laser operation in mid-infrared waveband. The findings are expected to provide a fundamental principle of in situ modification in hybrid system for generation of high-performance semiconductor-in-glass composites.

Role of Potassium Substitution in the Magnetic Properties and Magnetocaloric Effect in La0.8−xKxBa0.05Sr0.15MnO3 (0 ≤ x ≤ 0.20)

The magnetic and magnetocaloric effects of potassium-substituted La0.8−xKxBa0.05Sr0.15MnO3 (0 ≤ x ≤ 0.20) manganite were explored. The samples in polycrystalline form were synthesized by the sol–gel method, with a final sintering temperature of 1100 °C. Powder X-ray diffraction (XRD) patterns refined by Rietveld refinement show that all samples crystallized in rhombohedral structure with R-3c space group. The unit cell volume of the samples decreases with increasing potassium concentration. In addition, small changes in average bond length and bond angle are also observed in the samples. Scanning electron microscope (SEM) images reveal that the largest average grain size was observed for x = 0.10. Field-cooled (FC) magnetization measurements show that the Curie temperature ( T C ) of the samples increases from 320 K for x = 0 to 360 K for x = 0.2. The largest magnetocaloric (MCE) effect, which is represented by maximum magnetic entropy change (− Δ S M , M A X ), reaches its greatest value for the x = 0.10 sample. The monotonous increase in T C suggests that TC is mainly governed by the ferromagnetic coupling between Mn ions induced by the changes on average bond length and bond angle. The obtained − Δ S M , M A X value suggests that MCE property is more sensitive to Zener theory of double exchange, which is strongly related to the Mn3+/Mn4+ ratio of the samples.

Molecular dynamics simulation of a lignite structure simplified model absorbing water

ABSTRACTTo study the formation mechanism of hydration films, a lignite molecular model based on FTIR analysis was established. Both the positive and negative extreme values (±184.76 kJ/mol) of the electrostatic potential appear around oxygen-containing functional groups. Water molecules are mainly adsorbed near oxygen-containing functional groups. This paper studies the absorption of lignite for water molecules using MD simulation. With the addition of water molecules, the number of polar groups increases, and the electrostatic potential level of the system increases, causing an increase in the number of active sites. When the amount of water is inadequate for covering the surface of lignite, water molecules adsorb at active sites and places with lower z values. As the number of water molecules increase, the average length of hydrogen bonds between water molecules and that between lignite and water are close to the set values (1.97 and 1.8 Å, respectively). The short-range order structure of the interface has little effect on the average bond length of water molecules, but it reduces the average bond angle by 1°~2°. After a hydration film forms, the increase to the thickness of the hydration film is primarily attributed to hydrogen-bond interaction between water molecules.

Structural, magnetic and dielectric properties of lanthanum substituted Mn0.5Zn0.5Fe2O4

Mn0.5Zn0.5LaxFe2-xO4 (x = 0.0, 0.025, 0.050, 0.075 and 0.10) ferrites have been synthesized using coprecipitation method. Effect of substitution of La3+ on structural, magnetic and dielectric properties of Mn0.5Zn0.5Fe2O4 was studied. Structural properties have been investigated using X-ray diffraction (XRD) and morphological by transmission electron microscopy (TEM). XRD analysis revealed the formation of cubic spinel structure. Different structural parameters like lattice parameter, crystallite size, interionic distances, average cationic radii, bond angles and bond lengths have been calculated and compared with various theoretically predicted parameters. TEM images confirmed that particle size decreases with La content. Magnetic behavior of synthesized samples was studied employing vibrating sample magnetometer (VSM). It was noted that as x increased from 0 to 0.1, maximum magnetization showed decreasing trend from 34.57 emu/gm to 15.34 emu/gm (noted at 15000 G) and Curie temperature from 193 °C to 147 °C at constant field of 500 G. Real (є ̍) and imaginary part (є ̎) of dielectric constant, dielectric loss (tan δ), ac conductivity (σac), real (Z ̍) and imaginary part (Z ̎) of impedance were studied with frequency (0.1 Hz–1 MHz) and composition. є̍, є ̎, tan δ and σac were observed to decrease with substitution of La3+ ions. This variation can be attributed to the hopping of electrons between Fe2+ ions and Fe3+ ions at octahedral sites. Cole-cole plot showed a single semicircular arc for Mn0.5Zn0.5LaxFe2-xO4 clearly indicating the dominant effect of grain boundary in conduction.

Density Functional Theory Calculations of Geometrical and Electronic Properties of Neutral and Charged Silicon Oxide Clusters

We investigate neutral and charged silicon oxide clusters (Si nO n, n = 1–7) using density functional theory (DFT) calculations. For searching a local minimum structure of neutral, cationic, and anionic silicon oxide clusters, ab initio Monte Carlo (MC) simulations are used. Local minimum structures founded from simulations are again calculated using DFT calculations to find a global minimum structure. Relative energies based on total energy differences between the most stable structure and the possible isomers obtained from the calculation are reported. For studying structural properties of silicon oxide clusters, the PBE functional is used with DGDZVP and CEP‐121G* basis sets. These basis sets are compared with each other and with the DGDZVP and 6–311 + G* basis sets, which are also compared with each other in terms of their electronic properties. Thus, the PBE/DGDZVP model is the best choice for large silicon oxide clusters calculations. Examinations are performed on the SiO distances, Si‐O‐Si average bond angles, atomization energies per atom, second difference in energies, HOMO–LUMO gaps, adiabatic ionization potentials, and adiabatic electron affinities. All calculations use Gaussian09 program.

EPDF tools, a processing and analysis package of the atomic pair distribution function for electron diffraction

We present a new processing & analysis package of the atomic pair distribution function (PDF) for the electron diffraction (ED) pattern based on the total scattering theory which has been broadly used in the X-ray or neutron diffraction to reveal the local atomic structures, but PDF analysis based on the electron diffraction is still rarely used for the lack of an efficient PDF tool. The program was written as a package of DigitalMicrograph, a very popular software to record and analyze the data of transmission electron microscopes (TEM) in the most of TEM laboratories. The azimuthal rotation-average projection algorithm was implemented to enhance the signal-noise ratio of the intensity profile. In order to obtain the reduced structure function, the cubic spline fitting method was utilized to subtract the background scattering. The real-time displayed PDF makes data normalization easier and more accurate, and results of coordination numbers and averaged bond angles can be extracted from calibrated PDFs in real time. Program summaryProgram Title: ePDF toolsProgram Files doi:http://dx.doi.org/10.17632/ff94wp42wd.1Licensing provisions: GPLv3Programming language: DigitalMicrograph scripting languageNature of problem: The total scattering technique has been widely used in X-ray or neutron diffraction, but for the electron diffraction it still lacks an efficient analysis tool for the atomic pair distribution function.Solution method: Intensity profile is obtained by azimuthal rotation-average projection of 2D SAED pattern, the background scattering is subtracted by the cubic spline method, data normalization is real-time performed based on the total scattering theory, the atomic pair distribution functions obtained by Fourier transform will be used to quantitative analysis.

Density functional theory calculation of structure and electronic properties in N-methane hydrate

As a clean and efficient unconventional energy source, natural gas hydrate has been highly valued and vigorously developed by many countries in recent years. In order to solve the problem that the existing hydrate structure symmetry is not high, which leads the theoretical research to be restricted, it is imperative to explore a new type of methane hydrate structure with high symmetry. Using the first-principles method which is based on the density functional theory (DFT), the structure and electronic properties of N-methane hydrate are calculated in the generalized gradient approximation (GGA) for Grimme dispersion correction. The obtained results are shown below. 1) The water cage structure of N-methane hydrate is a truncated octahedron (4<sup>6</sup>6<sup>8</sup>), which is composed of 8 regular hexagons and 6 squares, and the average length of the hexagons and the average length of the squares are both 2.723 Å. The average bond length of water molecules is optimized to be 1.056 Å, and the average bond angle of water molecules is 107.738°. The average bond length of methane molecules is 1.0973 Å. The average distance from methane molecules to water molecules is 4.2831 Å that is longer than the distance in the I- methane hydrate. So N-methane hydrate can accommodate larger volumes of gas molecules. The symmetric group is <inline-formula><tex-math id="M2">\begin{document}${\rm{IM}}\bar 3{\rm{M}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20182230_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20182230_M2.png"/></alternatives></inline-formula> for N-methane hydrate, which has a simple and strict periodic stable structure. 2) The lattice parameter of N-methane hydrate is 7.70 Å, and the density is 0.903 g/cm<sup>3</sup>, which is greater than I-, II- and H-type hydrate density. 3) The x-ray diffraction(XRD) pattern of N-methane hydrateis calculated and is close to that of of I-methane hydrate, while the water cage of N-methane hydrate is larger. 4) The interaction between methane molecules and the water cage is van der Waals force, and the formation energy of N- methane hydrate is –0.247 eV, which indicates that the N-methane hydrate is easy to form. Both the density of states and partial density of states indicate that the interaction between methane and water cage is weak, and it relies on molecular force. 5) In addition, N-methane hydrate is an insulator material with the energy gap greater than 5 eV.

Crystal structure of 9,9′-(4,4′-(silinane-1,1-diyl)bis(4,1-phenylene))bis(9H-carbazole) and its photophysical and electrochemical properties

The title compound 9,9′-(4,4′-(silinane-1,1-diyl)bis(4,1-phenylene))bis(9H-carbazole) (6-bisCz) has been designed and prepared to investigate its crystal structure and photophysical and electrochemical properties. 6-bisCz is crystallized in the centrosymmetric space group P−1. Interestingly, the asymmetric unit contains two molecules (Z′ = 2), and hence, the entire cell contains four units (Z = 4). The central silicon atom was arranged in pseudo-tetrahedral geometry defined by the two ipso-carbon atoms of the phenyl groups and two carbon atoms in the endocyclic hexyl ring, with average bond angles of 104.5(1)° and 108.5(1)°, respectively. UV/Vis absorption and photoluminescence spectra showed that the central silicon atom successfully disconnected electronic communication between the two peripheral phenylcarbazole units, yielding similar absorption and emission spectra to an unsubstituted carbazole monomer. The influence of the central silicon atom between the two phenylcarbazole units is also simulated by density functional theory calculations. Finally, electrochemical properties of 6-bisCz were investigated, and the analysis shows that an electropolymerized film was successfully formed during the electrochemical process.

A Standard Structure for Bile Acids and Derivatives

The crystal structures of two ester compounds (a monomer in its methyl ester form, with an amino isophthalic group, and a dimer in which the two steroid units are linked by a urea bridge recrystallized from ethyl acetate/methanol) derived from cholic acid are described. Average bond lengths and bond angles from the crystal structures of 26 monomers and four dimers (some of them in several solvents) of bile acids and esters (and derivatives) are used for proposing a standard steroid nucleus. The hydrogen bond network and conformation of the lateral chain are also discussed. This standard structure was used to compare with the structures of both progesterone and cholesterol.

A2MnXO4 Family (A = Li, Na, Ag; X = Si, Ge): Structural and Magnetic Properties.

Four new manganese germanates and silicates, A2MnGeO4 (A = Li, Na) and A2MnSiO4 (A = Na, Ag), were prepared, and their crystal structures were determined using the X-ray Rietveld method. All of them contain all components in tetrahedral coordination. Li2MnGeO4 is orthorhombic (Pmn21) layered, isostructural with Li2CdGeO4, and the three other compounds are monoclinic (Pn) cristobalite-related frameworks. As in other stuffed cristobalites of various symmetry (Pn A2MXO4, Pna21 and Pbca AMO2), average bond angles on bridging oxygens (here, Mn-O-X) increase with increasing A/X and/or A/M radius ratios, indicating the trend to the ideal cubic (Fd3̅m) structure typified by CsAlO2. The sublattices of the magnetic Mn2+ ions in both structure types under study (Pmn21 and Pn) are essentially the same; namely, they are pseudocubic eutaxy with 12 nearest neighbors. The magnetic properties of the four new phases plus Li2MnSiO4 were characterized by carrying out magnetic susceptibility, specific heat, magnetization, and electron spin resonance measurements and also by performing energy-mapping analysis to evaluate their spin exchange constants. Ag2MnSiO4 remains paramagnetic down to 2 K, but A2MnXO4 (A = Li, Na; X = Si, Ge) undergo a three-dimensional antiferromagnetic ordering. All five phases exhibit short-range AFM ordering correlations, hence showing them to be low-dimensional magnets and a magnetic field induced spin-reorientation transition at T < TN for all AFM phases. We constructed the magnetic phase diagrams for A2MnXO4 (A = Li, Na; X = Si, Ge) on the basis of the thermodynamic data in magnetic fields up to 9 T. The magnetic properties of all five phases experimentally determined are well explained by their spin exchange constants evaluated by performing energy-mapping analysis.