Pm3n Structure Research Articles

Effect of Alloying Elements on the Mechanical Properties of Mo3Si

Molybdenum silicides are attractive high-temperature structural materials because of their excellent thermal stability and outstanding oxidation resistance at high temperatures. First-principles calculations were employed to investigate the effect of alloying elements (Cr, Nb, V, W, Al, Ga, and Ge) on the mechanical properties of Mo3Si. The structural stabilities of doped Mo3Si were calculated, showing that the Pm-3n structure was stable at the investigated low-doping concentration. The calculated elastic constants have also evaluated some essential mechanical properties of doped Mo3Si. Cr- and V-doping decreased the elastic modulus, while Al- and Nb-doping slightly increased the shear and Young’s modulus of Mo3Si. Furthermore, V-, Al- and Nb-doping decreased the B/G and Poisson ratio, suggesting that these elements could form strong covalent bonds, and decrease shear deformation and alloy ductility. Based on the three-dimensional contours and two-dimensional projection of the elastic modulus, Cr- and V-doping exhibited a significant influence on the anisotropy of the shear and Young’s modulus. According to charge density and density of states, the electronic structures of alloyed Mo3Si were further analyzed to reveal the doping effects.

Exploring the novel structure, elastic and thermodynamic properties of W3Si silicides from first-principles calculations

Abstract The first-principle calculations were selected to investigate the structure, elastic modulus, electronic properties and thermodynamic properties of three W3Si cubic structures (Pm-3m, Fm-3m and Pm-3n). The influence of pressure on the stability and elastic modulus of W3Si is further studied. Two novel W3Si phases: Pm-3m and Fm-3m are firstly found. The order of thermodynamic stability is Pm-3n > Fm-3m > Pm-3m. The bulk modulus and shear modulus of Pm-3n structure are up to 306.8 GPa and 157.1 GPa, respectively. In particular, the elastic modulus of Pm-3n structure is bigger than the Pm-3m and Fm-3n structures at the range of whole pressure. Compared to Pm-3m and Fm-3m structures, the high elastic modulus of Pm-3n structure is attributed to the role of the W–W and W–Si bonds. The high-temperature thermodynamic properties of W3Si are attributed to the vibration of Si atom. Finally, the order of Debye temperature is Pm-3n > Fm-3m > Pm-3m.

Hierarchical Structures Formed by Flexible Dendrimeric Molecules Based on Gallic Acid

AbstractBehavior of first‐generation dendrons based on gallic acid is found to be strongly dependent on the number of terminal alkyl chains attached to the outer phenyl rings; only molecules having three alkyl chains per phenyl ring are mesogenic. Due to the hydrogen bonding, they form micelles that are arranged into cubic Pm3n structure. The cubic phase is also found for dimeric and trimeric gallic ester analogues; it is shown that the size of the crystallographic unit cell correlates with the isotropization temperature, apparently small micelles with high density of alkyl chains at the micelle surface interact more strongly stabilizing the cubic phase. Also behavior of gallic acid dendrons in solution is defined by the number of terminal chains, formation of either large liposomes or gel is observed.

Magnetic correlations in the intermetallic antiferromagnet Nd3Co4Sn13

Specific heat, magnetic susceptibility, and neutron scattering have been used to investigate the nature of the spin system in the antiferromagnet Nd3Co4Sn13. At room temperature Nd3Co4Sn13 has a cubic, Pm-3n structure similar to Yb3Rh4Sn13. Antiferromagnetic interactions between, Nd3+ ions dominate the magnetic character of this sample and at 2.4 K the Nd spins enter a long range order state with a magnetic propagation vector q = (0 0 0) with an ordered moment of 1.78(2) µB at 1.5 K. The magnetic Bragg intensity grows very slowly below 1 K, reaching ~2.4 µB at 350 mK. The average magnetic Nd3+ configuration corresponds to the 3D irreducible representation Γ7. This magnetic structure can be viewed as three sublattices of antiferromagnetic spin chains coupled with each other in the 120°-configuration. A well-defined magnetic excitation was measured around the 1 1 1 zone centre and the resulting dispersion curve is appropriate for an antiferromagnet with a gap of 0.20(1) meV.

Preparation and catalytic properties of mesoporous titanosilicate of cubic Pm3n structure

Mesoporous titanosilicate of cubic Pm3n structure was prepared for the first time using cetyltriethylammonium (CTEA+) bromide as pore-directing agent in alkaline condition (pH around 9) with the aid of NaCl salt. The alkaline synthesis condition facilitated the incorporation of Ti(IV) into the silica framework as well as the assembly of mesoporous silica with zeolite seeds. Two kinds of silica sources were used. The material prepared with sodium silicate was named Ti-SBA-1-alk, and the other named Ti-ZSBA-1 was prepared with the seeds of titanosilicalite-1 (TS-1). The studies by UV–Vis and Ti K-edge XANES spectra indicated that the Ti(IV) in Ti-SBA-1-alk was favorably in tetrahedral (Td) coordination and should be incorporated into the framework of mesoporous silica, whereas a larger amount of Ti(IV) in Ti-ZSBA-1 was in octahedral coordination. In oxidation of 2,3,6-trimethylphenol (TMP) to 2,3,5-trimethylbenzoquinone (TMBQ) using hydrogen peroxide as the oxidant, the mesoporous titanosilicate materials were found to be efficient catalysts, while only trace TMP conversion was observed over TS-1 due to its small zeolitic pores. In comparison of mesoporous titanosilicate materials in similar pore diameters, Ti-SBA-1-alk and Ti-ZSBA-1 both of cubic Pm3n structure gave higher TMBQ selectivities than Ti-MCM-41 of 2D channeling pores, attributing to the better diffusion of the phenoxyl radicals in the three-dimensional pores. Moreover, Ti-SBA-1-alk which contained more Td-coordinated Ti(IV) showed higher catalytic activities than Ti-ZSBA-1.

Synthesis of cubic Pm3n supermicroporous silicas and aluminosilicates under alkaline condition

Ordered cubic Pm3n supermicroporous silica and aluminosilicate materials were synthesized using dodecyltriethylammonium bromide as template and sodium silicate as silicon source under alkaline condition. The cubic Pm3n structures were confirmed by XRD patterns and TEM images. Ordered supermicroporous silica could be obtained facilely at room temperature. As the incorporation of Al resulted in a decrease in the cubic ordering, ordered supermicroporous aluminosilicates should be synthesized at lower temperature. A series of aluminosilicates was prepared and the SiO2/Al2O3 molar ratio in products could be achieved as low as 32 with the cubic Pm3n structure well-preserved. Nitrogen sorption data further proved the supermicropore size of the materials.

Highly ordered supermicroporous aluminosilicates with cubic Pm3n symmetry fabricated in weakly acidic solution

Highly ordered supermicroporous aluminosilicates with cubic Pm3n symmetry were prepared for the first time in weakly acidic solutions of succinic acid and malonic acid using dodecyltriethylammonium bromide as a template. The polycarboxylic acids acted as auxiliaries for fabricating the cubic Pm3n structure as well as weak acids. Thermally stable aluminosilicates with a high utilization ratio of Al could be obtained in the presence of succinic acid by either facilely adding aluminum source into the initial synthesis gel or by grinding the as-made silica-surfactant composite with aluminum nitrate. In contrast, thermally stable aluminosilicates could only be prepared via a post-synthesis approach in the presence of malonic acid, probably because of the powerful coordination tendency between malonic acid and Al cations. The thermal stability of the aluminosilicates was enhanced because of the incorporation of Al into the framework or grafting of Al onto the pore walls. Aluminosilicates prepared via the post-synthesis approach preserved the cubic Pm3n structure better than the directly-synthesized ones. In spite of the kind of polycarboxylic acids or the preparation methods, Al was predominantly tetrahedrally coordinated. The aluminosilicates showed a high specific area and pore volume, especially for the post-synthesized ones. The pore size was in the supermicroporous range evidenced by N2 physisorption, though that of the aluminosilicates prepared via the post-synthesis approach was larger. TEM images verified the Pm3n symmetry of the materials herein. The cubic Pm3n aluminosilicates prepared in our research exhibited equal reactivity but a much pronounced deactivation resistance property in the acetalization of cyclohexanone with pentaerythritol.

Periodic Mesoporous Organosilicas with Controlled Pore Symmetries for Peptides Enrichment

Periodic mesoporous organosilicas (PMOs) with controlled structures have been synthesized by using cetyltrimethylammonium bromide (CTAB) and sodium perfluorooctanoate (PFONa) as co-templates, 1,2-bis (triethoxysilyl)ethane (BTEE) as an organosilica precursor. By increasing the weight ratio of PFONa/CTAB, a structure transformation from a cubic (Pm-3n) to a two-dimensional hexagonal (p6m) mesostructure and then to multilamellar vesicles can be observed. The cubic and hexagonal samples have similar particle size (200-750 nm), pore size (2.6 and 2.8 nm, respectively), total pore volume (approximately 0.7 cm3/g), and surface area (approximately 900 m2/g), providing ideal candidates to study the peptide enrichment performance influenced simply by pore symmetries. Matrix-assisted laser desorption ionization time-of-flight mass spectroscopy (MALDI-TOF MS) analysis indicates that PMO with a cubic (Pm-3n) structure is more effective in small molecular weight peptides enrichment compared with PMO with a hexagonal structure, showing the importance of mesostructural control for targeted applications. The phenomena can be attributed to the cage-type structure of the Pm-3n symmetry, which possesses cages with a relatively larger pore size and connectivity with a relatively smaller size. It is suggested that the pore entrances with small size are responsible for entrapping small molecular weight peptides. Our study may shed light on the designed synthesis of functional porous materials with controlled structures and enhanced performance in peptides enrichment.

AlH3 between 65 and 110GPa: Implications of electronic band and phonon structures

A first-principles density-functional-theory method has been used to reinvestigate the mechanical and dynamical stability of the metallic phase of AlH3 between 65-110 GPa. The electronic properties and phonon dynamics as a function of pressure are also explored. We find electron-phonon superconductivity in the cubic Pm-3n structure with critical temperature Tc = 37 K at 70 GPa which decreases rapidly with the increase of pressure. Further unlike a previously calculated Tc value of 24 K at 110 GPa, we do not find any superconductivity of significance at this pressure which is consistent with experimental observation.

Thermal Stability of Periodic Mesoporous SiCO Glasses

Periodic Mesoporous Organosilicas (PMOs) with 2D-hexagonal and cubic Pm3n structures have been prepared from bis(trialkoxysilyl)ethane and cetyltrimethylammonium chloride. The two samples have been pyrolyzed under argon up to 1000 °C. Study by X-ray diffraction (synchrotron radiation) allows the thermal stability of both structures to be followed as a function of the pyrolysis temperature. While the 2D-hexagonal structure collapses after pyrolysis at 800 °C, the cubic Pm3n structure is retained up to 1000 °C. Further characterizations were performed by 29Si MAS-NMR, N2 adsorption-desorption experiments and elemental analysis. At 1000 °C, the first sample can be described as a mixture of silica and a free C phase, while the cubic one is a true SiCO glass with mixed SiC x O4 − x units (x = 0,1,2) and a very large surface area of 730 m2/g.

Periodic Mesoporous SiCO Glasses with Cubic Symmetry Stable at 1000°C

AbstractA periodic mesoporous organosilica (PMO) with cubic Pm3n structure has been prepared from bis(trimethoxysilyl)ethane and cetytriethylammonium chloride. The sample has been heat treated under argon up to 1000°C, and the pyrolysis intermediates were analysed by X-ray diffraction using synchrotron radiation, multinuclear solid state NMR (29Si, 13C and 1H) and N2 adsorption-desorption experiments. The cubic Pm3n structure is retained after pyrolysis at 1000°C. The sample is a SiCO glass with mixed SiCxO4-x units (x = 0, 1, 2) and a very large surface area of 730 m2/g.

Raman Study of Nitrogen at High Pressure

Raman spectra of the nitrogen loaded in a diamond anvil cell with the compressed gas have been investigated at the pressure up to 40 GPa. Pressure was measured in terms of the ruby fluorescence. The curve of frequency vs pressure was obtained by micro-Raman spectrometer. According to the experimental results, the frequency of two stretching mode peaks increases steadily from 2340 and 2347 cm-1 at 5.2 GPa to 2396 and 2434 cm-1 at 40 GPa, respectively. The interval between two peaks also increases gradually with pressure increasing. A new peak (2402 cm-1) of the stretching mode was observed near 40 GPa, which may be associated with the distortion of the Pm3n structure or the formation of low-symmetry structure.

Neutron powder diffraction study of deuterium ordering in β phase Pd[sbnd]D at low temperatures

We recently briefly reported the first observation by powder diffraction of a superlattice in β-PdD annealed at low temperatures. In this paper we give more details of our investigation by neutron powder diffraction of the low temperature phase diagram at several concentrations and temperatures. Two different superlattice were observed, at (1 (2n + 1)/2 0) positions in PdD0.62 and PdD0.65 annealed at 54 K, and at (h ±2/5h ±4/50) and equivalent positions in PdD0.77 annealed at 70 K. At the lower concentrations, contrary to the previous reports of a tetragonal I41/amd structure, a cubic superstructure in Pm3n with a doubled cell constant relative to the disordered structure was determined. The diffusion of D in 6d sites into neighbouring 6c sites generates the order, driven by a repulsive nearest-neighbour interaction. The ordering at [D]/[M] = 0.77 developed very slowly at 70 K and is described in an MoNi4 structure as in previous observations. No sign of ordering was found in PdD0.82 and PdD0.86 at temperatures down to 20 K. Enhanced anisotropic line broadening during ordering was observed in the Pm3n structure. It was successfully accounted for in the refinements of the cubic superlattice by assuming that it originates from strain along [001] which propagates anisotropically in other directions.

Heavy fermion behaviour in CeRuSn 3 compound

The electrical resistivity under pressure (up to 23 kbar), magnetic susceptibility and specific heat of CeRuSn 3 with cubic Pm3n structure were measured between 30 mK and 300 K. The magnetic scattering of the resistivity behaves like a dilute Kondo type. Below 1 K, the ratio C/T remarkably increases and reaches a broad maximum around 0.5 K. The magnetic susceptibility also shows a maximum at the same temperature. These results indicate a antiferromagnetic-tpye ordering induced by the RKKY-interaction between 4f-electrons below 20.5 K. CeRuSn 3 is a new heavy fermion compound with specific heat coefficient of 1.67 J/K .mol at 0.6 K.

Heavy fermion behaviour in CeRuSn3 compound

The electrical resistivity under pressure (up to 23 kbar), magnetic susceptibility and specific heat of CeRuSn3 with cubic Pm3n structure were measured between 30 mK and 300 K. The magnetic scattering of the resistivity behaves like a dilute Kondo type. Below 1 K, the ratio C/T remarkably increases and reaches a broad maximum around 0.5 K. The magnetic susceptibility also shows a maximum at the same temperature. These results indicate a antiferromagnetic-tpye ordering induced by the RKKY-interaction between 4f-electrons below 20.5 K. CeRuSn3 is a new heavy fermion compound with specific heat coefficient of 1.67 J/K .mol at 0.6 K.

ChemInform Abstract: New Phase of an Oxygen‐Nitrogen Alloy at High Pressure and Room Temperature Detected by Raman Spectroscopy.

AbstractThe new solid N‐O phase that forms at pressures > 11 GPa appears to be a variant of the Pm3n structure of δ‐N2 in which O2 molecules replace some of the N2 molecules.

High-pressure static and dynamic properties of the R3-barc phase of solid nitrogen.

The zero-temperature equilibrium structure and orientations of solid ${\mathrm{N}}_{2}$ are calculated at pressures 20\ensuremath{\le}P\ensuremath{\le}415 kbar by optimizing the lattice energy of an eight-molecule unit cell with respect to a rhombohedral distortion of the crystal from the cubic Pm3n structure. The resulting equilibrium structures have R3\ifmmode\bar\else\textasciimacron\fi{}c(${C}_{3v}^{6}$) symmetry, upon which the pressure-volume relation and the libron, vibron, and lattice phonon frequencies at zone center are calculated using lattice dynamics. These quantities are compared with experiment and the symmetry assignments of the calculated and observed modes are given. Also calculated are the second virial coefficients, the \ensuremath{\alpha}-${\mathrm{N}}_{2}$ molar volume and sublimation energy, and the \ensuremath{\alpha}- and \ensuremath{\gamma}-${\mathrm{N}}_{2}$ vibron frequencies, versus pressure.

The compression of solid N2 at 296 K from 5 to 10 GPa

The volume of solid N2 having Pm3n space group was measured at 296 K from 5 to 10 GPa using a high pressure, x-ray diffraction technique. The compression of NaF was used as the pressure gauge. From a fit of the compression of the Pm3n structure data converted to the velocity plane, the volumes and bulk moduli are calculated as a function of pressure. From this fit and simultaneous volume measurements of the β-N2 phase and the Pm3n structure, the pressure of transformation from the β-N2 phase to the Pm3n structure is found to be 4.8 GPa. Recently published a priori calculations are shown to agree well with the present data.