Monte Carlo simulation of crystal-fluid coexistence states in the hard-sphere system under gravity with stepwise control

Monte Carlo (MC) simulations were performed for hard spheres (with diameter sigma and mass m) placed between well-separated upper and lower hard walls. A periodic boundary condition was imposed in the horizontal direction. The system was exposed to the gravitational field with the acceleration due to gravity g. After preparing a melt as the initial state, g was increased stepwise up to mgsigma/k(B)T(identical with g(*))=1.5 or 2.0 with an increment Deltag(*) = 0.1; k(B)T is the temperature multiplied by Boltzmann's constant. We maintained g(*) at each value for 2.0 x 10(5) MC cycles. The transition of the system into a metastable state such as a polycrystalline state due to trapping phenomena was successfully avoided. A monotonic increase and subsequent saturation were observed for the development of the crystalline region formed at the bottom of the system. The development of this region accompanied a shrinkage of the defective (or less ordered) crystalline region that was formed between the bottom region and the fluid phase. As the development of the bottom region almost saturated, the defective region grew upward again.     

Synthesis, structure, optical properties, and electronic structure of NaLiCdS2

The new compound NaLiCdS₂ has been synthesized by the reaction of Cd and a Li₂S/S/Na₂S flux at 773 K. This compound, which has the Ce₂O₂S structure type, crystallizes with one formula unit in space group P3¯m1 of the trigonal system in a cell at T=153 K with a=4.1320(3) Å and c=6.8666(11) Å. The structure consists of two-dimensional layers stacked perpendicular to the [001] direction. The two-dimensional layers are formed by corner-sharing LiS₄ or CdS₄ tetrahedra. The Na atoms are between these layers. Li incorporation in the compound is confirmed by an SIMS chemical composition map and by ICP measurements. The Li and Cd atoms are disordered in the crystal structure. First-principles calculations show that the optical excitations arise primarily from S→Cd charge-transfer transitions at 1.0 eV (very weak) and 2.4 eV (strong). Calculations also indicate that Na contributions around the Fermi level are significant. Polarized single-crystal optical measurements indicate an indirect optical band gap of 2.37 eV for light perpendicular to the (001) crystal face, in good agreement with theory. The compound NaLiZnS₂ has also been synthesized and is found to be isostructural with NaLiCdS₂.     

Investigation of crystal structure and associated electronic structure of Sr6BP5O20

Strontium borophosphate phosphate (Sr₆BP₅O₂₀, SrBP), activated by divalent europium ions is a bluish-green phosphor emitting in a broad band with the emission peak near 480 nm. In this paper, we report the crystal structure of SrBP determined from an analysis of the X-ray diffraction pattern of a prismatic single crystal (size 60 μm×50 μm×40 μm). This crystal was chosen from undoped phosphor powder samples prepared for this purpose by solid-state reaction. SrBP is observed to crystallize in a body-centered tetragonal lattice with the lattice parameters and , the associated space group being (space group 120). Using the structural data from this study, we have also calculated its electronic structure using the augmented spherical wave method and the local density approximation (LDA). We show the ordering of the electronic states by the density of states (DOS) and the partial DOS plots. The LDA gives a direct optical band gap at the Γ point of about 5 eV. The significance of the crystal structure and associated electronic structure is discussed with respect to maintenance of this phosphor in Hg-discharge lamps.     

Chemical deposition of semiconducting cadmium selenide quantum dots in thin film form and investigation of their optical and electrical properties

Cadmium selenide quantum dots with cubic crystal structure are chemically deposited in thin film form using selenosulfate as a precursor for selenide ions and ammonia buffer with double role: as a ligand and as a pH value controller. The optical band gap energies of as-deposited and thermally treated cadmium selenide thin films, calculated within the framework of parabolic approximation for the dispersion relation, on the basis of equations which arise from the Fermi's golden rule for electronic transitions from valence to conduction band, are 2.08 and 1.77 eV, correspondingly. The blue shift of band gap energy of 0.34 eV for as-deposited thin films with respect to the bulk value is due to the quantum size effects (i.e., nanocrystals behave as quantum dots) and this finding is in agreement with the theoretical predictions. During the thermal treatment the nanocrystals are sintered, the increase of crystal size being in correlation with the decrease of band gap energy. The annealed thin films are practically non-quantized. From the resistance-temperature measurements, on the basis of the dependence of ln(R/Ω) vs 1/T in the region of intrinsic conduction, the thermal band gap energy (at 0 K) of 1.85 eV was calculated.     

Synthesis, crystal structure, and properties of the rhombohedral modification of the thiospinel CuZr1.86(1)S4

The rhombohedral modification of the thiospinel, CuZr_1.86(1)S₄, has been synthesized by the reaction of the constituent elements in an alkali metal halide flux and structurally characterized by single crystal X-ray diffraction techniques. The title compound crystallizes in the rhombohedral space group (#166, a=7.3552(2) Å, c=35.832(2) Å, V=1678.76(13) Å³, Z=12, and R/wR=0.0239/0.0624). The structure is composed of close packed S layers, with a stacking order of ?ABCBCABABCACAB?·along the c axis. The Zr and Cu atoms occupy the octahedral and tetrahedral holes between S layers, respectively. Three different kinds of S-M-S layers exist in the structure: layer I has fully occupied Zr and Cu sites, layer II has fully occupied Zr sites but no Cu, and layer III has partially occupied Zr and fully occupied Cu sites. Transport and optical properties indicate that the title compound is a small band gap (1.26 eV) n-type semiconductor.     

Vibrational spectra and reinvestigation of the crystal structure of a polymeric copper(II)-orotate complex, [Cu(μ-HOr)(H2O)2]n: The performance of new DFT methods, M06 and M05-2X, in theoretical studies

The crystal and molecular structure of a polymeric Cu(II)-orotate complex, [Cu(μ-HOr)(H₂O)₂]_n, has been reinvestigated by single crystal X-ray diffraction. It is shown that several synergistic interactions: two axial Cu-O interactions; intramolecular and intermolecular hydrogen bonds; and π-π stacking between the uracil rings contribute to the stability of the crystal structure. The Raman and FT-IR spectra of the title complex are reported for the first time. Comprehensive theoretical studies have been performed by using three unrestricted DFT methods: B3LYP; and the recently developed M06, and M05-2X density functionals. Clear-cut assignments of all the bands in the vibrational spectra have been made on the basis of the calculated potential energy distribution, PED. The very strong Raman band at 1219 cm⁻¹ is diagnostic for the N1-deprotonation of the uracil ring and formation of the copper-nitrogen bond, in this complex. The Cu-O (carboxylate) stretching vibration is observed at 287 cm⁻¹ in the IR spectrum, while the Cu-N (U ring) stretching vibration is assigned to the strong Raman band at 263 cm⁻¹. The molecular structure and vibrational spectra (frequencies and intensities) calculated by the M06 functional method are very similar to the results obtained by the B3LYP method, but M06 performs better than B3LYP in calculations of the geometrical parameters and vibrational frequencies of the interligand O-H?O hydrogen bonding. Unfortunately, the M05-2X method seriously overestimates the strength of interligand hydrogen bond.     

Synthesis, structure, and electronic structure of K2CuSbS3

The new compound K₂CuSbS₃ has been synthesized by the reaction of K₂S, Cu, Sb, and S at 823 K. The compound crystallizes in the Na₂CuSbS₃ structure type with four formula units in space group P2₁/c of the monoclinic system in a cell at 153 K of a=6.2712 (6) Å, b=17.947 (2) Å, c=7.4901 (8) Å, β=120.573 (1)°, and V=725.81 (12) Å³. The structure contains two-dimensional layers separated by K atoms. Each layer is built from CuS₃ and SbS₃ units. Each Cu atom is pyramidally coordinated to three S atoms with the Cu atom about 0.4 Å above the plane of the S atoms. Each Sb atom is similarly coordinated to three S atoms but is about 1.1 Å above its S₃ plane. First-principles calculations indicate an indirect band gap of 1.9 eV. These calculations also indicate that there is a bonding interaction between the Cu and Sb atoms. An optical absorption measurement performed with light perpendicular to the (0 1 0) crystal face of a red block-shaped crystal of K₂CuSbS₃ indicates an experimental indirect band gap of 2.2 eV.     

Synthesis, crystal structure and optical properties of BiMgVO5

The new vanadate BiMgVO₅ has been prepared and its structure has been determined by single crystal X-ray diffraction: space group P2₁/n, , , , β=107.38(5)°, wR₂=0.0447, R=0.0255. The structure consists of [Mg₂O₁₀] and [Bi₂O₁₀] dimers sharing their corners with [VO₄] tetrahedra. The ranges of bond lengths are 2.129-2.814 Å for Bi-O; 2.035-2.167 Å for Mg-O and 1.684-1.745 Å for V-O. V-O bond lengths determined from Raman band wavenumbers are between 1.679 and 1.747 Å. An emission band overlapping the entire visible region with a maximum around 650 nm is observed.     

Anion substitution effects on structure and magnetism in the chromium chalcogenide Cr5Te8—Part I: Cluster glass behavior in trigonal Cr(1+x)Q2 with basic cell (Q=Te, Se; Te:Se=7:1)

The effect of substitution of the anion Te by Se in non-stoichiometric Cr₅Te₈ has been investigated with respect to its crystal structure, magnetic properties, and electronic structure. The compounds Cr_(1+x)Q₂ (Q=Te, Se; Te:Se=7:1; (1+x)=1.234(6), 1.264(6), 1.300(7)) were synthesized at elevated temperatures followed by quenching the samples to room temperature. The crystal structures have been refined with X-ray powder diffraction data with the Rietveld method in the trigonal space group with lattice parameters a=3.8651(1)-3.8831(1) Å and c=5.9917(2)-6.0528(2) Å. The structure is related to the NiAs structure with full and deficient metal layers stacking alternatively along the c-axis. The irreversibility in the field-cooled/zero-field-cooled magnetization suggests that the substitution effects of one Te by one Se is strong enough to cause cluster-glass behavior, from ferromagnetic Cr₅Te₈ to cluster-glass Cr_(1+x)Q₂. Non-saturation magnetizations at 5.5 T and the magnetic relaxation results further support the existence of cluster-glass behavior. Accompanying SPR-KKR (spin-polarized relativistic Korringa-Kohn-Rostoker) band structure calculations strongly support the observation that the Cr(1) sites are preferentially occupied by Cr atoms and predict that these compounds are metallic. Results for the spin-resolved DOS and magnetic moments on each crystallographic sites are presented.     

Synthesis and structure of Ba6Co6ClO16, a new cobalt oxychloride with a layered perovskite-related structure

Well-developed single crystals of the title compound were prepared using a BaCl₂ flux and investigated by X-ray diffraction methods using Mo(Kα) radiation and a Charge Coupled Device (CCD) detector. The crystal structure was solved and refined in the hexagonal symmetry with space group, a=5.6698(2) Å and c=14.4654(5) Å to a final R₁=0.022 for 44 parameters with 1418 individual reflections. The structure of Ba₆Co₆ClO₁₆, which is related to the 6H-perovkite-type structure of BaMnO_2.88, is formed by the periodic stacking along [001] of five [BaO₃] layers separated by a [BaOCl] with a (hhhchc) stacking sequence. The [BaO₃] stacking creates tetranuclear face sharing octahedra units Co₄O₁₅ containing Co(III) connected by dimers of corner-sharing CoO₄ tetrahedra. This new oxychloride belongs to the family of compounds formulated as [BaOCl]M′₂[Ba_n+1M_nO_3n+3] where n represents the thickness of the octahedral string in hexagonal perovskite slabs.     

Interpenetrated structure and compressibility studies in pressure frozen pentafluoropyridine crystals at 0.3 and 1.1 GPa

Pentafluoropyridine has been pressure frozen in situ in a diamond-anvil cell (DAC) and its structure determined at 0.30(5) and 1.10(5) GPa and at room temperature by single-crystal X-ray diffraction. The freezing pressure of pentafluoropyridine has been determined to be 0.10(5) GPa. The crystals are monoclinic, space group P2₁/c. The crystal packing is governed by F?F and C/N?F van der Waals contacts, but no ring stacking is observed. The intermolecular interactions are non-directional, and the crystal compresses nearly isotropically between 0.3 and 1.1 GPa.     

Synthesis of the novel perovskite-type oxyfluoride PbScO2F under high pressure and high temperature

We synthesized a novel perovskite-type oxyfluoride, PbScO₂F, and investigated its crystal structure, thermal stability and dielectric properties. PbScO₂F has a cubic perovskite-type structure with Pb ions displaced from the ideal A-site positions along the 〈110〉 direction. By thermal gravity and differential thermal analyses, we found that this compound is stable up to 963 K (690 °C) under ambient atmosphere. The dielectric permittivity of PbScO₂F is approximately 80, at room temperature, which is almost the same as KTiO₂F and smaller than that of PbFeO₂F. Furthermore, an anomaly in the dielectric permittivity was observed in the vicinity of 100 K that seems to be related to the displacement of the Pb ions.     

Sn4As3 revisited: Solvothermal synthesis and crystal and electronic structure

A facile one pot method of synthesis of tin arsenide Sn₄As₃ starting from metallic tin and elemental arsenic under mild solvothermal conditions in ethylenediamine in the presence of ammonium chloride is offered. The dissolving of the tin metal in ethylenediamine and the role of NH₄Cl are discussed. The crystal structure of Sn₄As₃ has been re-determined. It is shown to crystallize in the trigonal non-centrosymmetric space group R3m, (a=4.089(1) Å, c=36.059(6) Å, Z=3), which differs from the previously reported centrosymmetric structure . The crystal structure of Sn₄As₃ consists of alternating layers of arsenic and tin atoms that are combined into seven-layer blocks and build up along the c-axis. The major structural feature is the short tin-tin distances (3.24 Å) between the adjacent blocks. The analysis of the density of states and band structure reveals that Sn₄As₃ should have metallic properties, which is in line with the previously reported experimental observations. Analysis of chemical bonding employing the electron localization function shows that only for the shortest Sn-As contacts the bonding is pairwise, while four-center bonds are formed between arsenic and tin atoms at relatively long distances (>2.85 Å). Moreover, each tin atom holds an electron lone-pair.     

Synthesis of zirconium titanate with an ordered M-fergusonite (beta) structure

We report a new zirconium titanate compound (Zr,Ti)O₂ with 27.5-35 mol% titania (TiO₂) formed from the oxides at 35-38 kbar, 1400-1500 °C. Crystal structure investigations at atmospheric conditions with powder X-ray diffraction (XRD) and transmission electron microscopy (TEM) revealed a monoclinic structure related to that of M-fergusonite (beta). Unit-cell dimensions (from 27.5 to 35 mol% TiO₂): a=7.267(20)-7.340(2) Å, b=10.435(3)-10.429(1) Å, c=5.023(11)-5.040(1) Å, β=136.45(12)-137.55(1)°, V=262.44(92)-260.40(12) Å³, Z=4. Rietveld refinement (R_F=1.55) of a sample with 32.8 mol% TiO₂ indicates that site A is 8-fold coordinated, mostly occupied by Zr, while site B has 6-fold average coordination, occupied by Ti and Zr. Site B is at least partly ordered, as indicated by superstructure reflections 0 0 1 and −2 0 1 detected with TEM, reducing the space group from C2/c to C2. Pronounced streaking of selected diffraction spots is linked to the boundaries of lamellar domains in twin orientation, with twin planes either (200) or (20−2). Adjacent lamellae differ slightly in composition, causing subtle asymmetry of the twin diffraction patterns.     

Dispersion of electron–hole excitations in pentacene along (1 0 0)

The dispersion of the lowest lying singlet electron–hole excitation of pentacene along the reciprocal lattice vector (1 0 0) has been studied using electron energy-loss spectroscopy. This exciton shows a clear dispersion along the direction with a band width of about 100 meV. Moreover, the translational symmetry indicated by the exciton band structure does not agree with that reported from diffraction studies. This might be related to the interaction responsible for the dispersion which is of next nearest neighbor type only.     

Crystal and molecular structure of 5-trifluorothymine, a metabolite from human urine: Role of fluorine in stacking and hydrogen bonded interactions

The crystal structure of the metabolite from urine, 5-trifluorothymine [5F₃T] has been determined by single crystal X-ray diffractometric methods. Crystals of 5F₃T are monoclinic, space group P2₁/c with cell dimensions a = 6.7468(2), b = 15.0740(6), c = 13.4405(6), β = 90.412(2), V = 1366.88(8), Z = 8 (two molecules per asymmetric unit). Crystal structure of 5F₃T was determined with 3039 independent data and refined by full-matrix least squares methods to a final reliability factor of 0.047. Molecules of 5F₃T are connected by dimeric type of NH?O hydrogen bonding linking molecules related by a center of inversion into an extensive layer of dimeric molecules. These layers are stacked on top of each other at a stacking distance of 3.280 Å with a head-to-head stacking of the fluorine atoms on top of each other with no hydrogen bonding involving the fluorine atoms.     

New N-heterocyclic carbene silver(I) and mercury(II) 2-D supramolecular layers by the π-π stacking interactions

The precursor 1-(9-anthracenylmethyl)-3-alkylbenzimidazolium chlorides (1a, alkyl = C₄H₉, 1b, alkyl = C₆H₁₃) and their three new NHC silver(I) and mercury(II) complexes {[1-(9-anthracylmethyl)-3-alkylbimy]MCl}₂ (2a, alkyl = C₄H₉, M = Ag; 2b, alkyl = C₆H₁₃, M = Ag; 3a, alkyl = C₄H₉, M = Hg; bimy = benzimidazol-2-ylidene) have been prepared and characterized. The crystal structures of 2a, 2b and 3a showed that 2-D supramolecular layers are formed by both benzimidazole ring head to tail π-π stacking interactions and anthracene ring face-to-face π-π stacking interactions.     

Local symmetry breaking in PrFeO3 thin films and other similar systems after Ni doping: A brief Raman study

Raman study of PrFe_1−xNi_xO₃ (0 ≤ x ≤ 0.5) thin films deposited on LaAlO₃ substrate by pulsed laser deposition is presented. With doping, a new band is developed at around 574 cm⁻¹. After a certain doping range, there is a decrease in the observed intensity of this band and it becomes hard to resolve it from other observed bands. The generation of this new band in other systems with Ni is also established. The present study is a direct indication of symmetry breaking. The established orthorhombic structure for the present system is doubtful.     

Crystal structure and phase transformations of calcium yttrium orthophosphate, Ca3Y(PO4)3

Crystal structure and phase transformations of calcium yttrium orthophosphate Ca₃Y(PO₄)₃ were investigated by X-ray powder diffraction, selected-area electron diffraction, transmission electron microscopy and optical microscopy. The high-temperature phase is isostructural with eulytite, cubic (space group ) with a=0.983320(5) nm, V=0.950790(8) nm³, Z=4 and D_x=3.45 Mg m⁻³. The crystal structure was refined with a split-atom model, in which the oxygen atoms are distributed over two partially occupied sites. Below the stable temperature range of eulytite, the crystal underwent a martensitic transformation, which is accompanied by the formation of platelike surface reliefs. The inverted crystal is triclinic (space group P1) with a=1.5726(1) nm, b=0.84267(9) nm, c=0.81244(8) nm, α=109.739(4)°, β=90.119(5)°, γ=89.908(7)°, V=1.0134(1) nm³, Z=4 and D_x=3.24 Mg m⁻³. The crystal grains were composed of pseudo-merohedral twins. The adjacent twin domains were related by the pseudo-symmetry mirror planes parallel to with the composition surface . When the eulytite was cooled relatively slowly from the stable temperature range, the decomposition reaction of Ca₃Y(PO₄)₃→β-Ca₃(PO₄)₂+YPO₄ occurred.     

Three new nickel(III) compounds based on 2-thioxo-1,3-dithiole-4,5-dithiolate: Syntheses,structures, magnetic properties and theoretical analyses

Three new molecular solids, (ADP)₂[Ni(dmit)₂]₂ (1), (BDP)[Ni(dmit)₂]·CH₃COCH₃ (2) and (CP)₂[Ni(dmit)₂]₃ (3) (dmit = 2-thioxo-1,3-dithiole-4,5-dithiolate, ADP = 1-(2,4-dichlorobenzyl)-3,5-dimethylpyridinium, BDP = 1-(3,4-dichlorobenzyl)-3,5-dimethylpyridinium and CP = 1-(2,4-dichlorobenzyl)-3-methylpyridinium) have been prepared and characterized by elemental analysis, IR and single-crystal X-ray diffraction. All the compounds formed stacking columns containing cations and anions. In the crystal structure of 1, both ADP cations and [Ni(dmit)₂] anions were found to form stacked dimers, and these dimers piled up alternately in columns. In 2, the anions of [Ni(dmit)₂] anions stack into dimers, which further construct into a 1D zig-zag chain through lateral S?S interactions. Compound 3 shows stacks built from trimers of Ni(dmit)₂ units, which further construct into 3D network structure through short S···S interactions, S···Cl interactions and C–H?S hydrogen bonds. Magnetic susceptibility measurements for 1–3 in the temperature range 2–300 K show that the overall magnetic behavior indicates the presence of antiferromagnetic interaction, while 3 exhibits an activated magnetic behavior in the high-temperature region (HT) together with a Curie tail in the low-temperature region (LT).