3R Phases Research Articles

Correlation between X-ray diffraction and Raman spectra of 16 commercial graphene–based materials and their resulting classification

Structural properties of sixteen (16) commercial samples of graphene-based materials (GBM) labelled as graphene, graphene oxide or reduced graphene oxide are investigated at room temperature using X-ray diffraction (XRD) and Raman spectroscopy. Based on the observed correlation between the results obtained with these two techniques, these samples are classified into three groups: Group A of seven samples consisting of graphitic nanosheets with evaluated thickness ≃20 nm and exhibiting both the 2H and 3R phases in XRD; Group B of six samples exhibiting XRD spectra characteristic of either graphene oxides (GO) or carbons with some order; and Group C of three samples with XRD spectra characteristic of disordered carbons. The relative intensities and widths of D, G, D′, 2D and (D + D′) bands in the Raman spectra are equally distinguishable between the samples in groups A, B and C. The width of the D-band is the smallest for Group A samples, intermediate for group B and the largest for group C samples. The intensity ratio I(D)/I(G) of the D and G bands in the Raman spectra of the samples is used to quantify the Raman-active defects whose concentration increases in going from samples in Group A to those in Group C.

Detection and quantification of 2H and 3R phases in commercial graphene-based materials

Graphene-based material (GBM) samples acquired from commercial sources are investigated using X-ray diffraction (XRD). Of the 18 GBM samples investigated here, seven samples show XRD patterns with features characteristic of the graphite structure. The XRD patterns of the seven samples are analyzed showing the presence of both the ABA (2H) structure and the ABCA (3R) structure. After de-convoluting the (101) lines of the 2H and 3R structures, the areas under the peaks are used to determine the relative concentrations of the 2H and 3R phases present, typically yielding the ratio 60/40 for 2H/3R. The presence of the 3R structure is important since the 3R structure is a semiconductor with tunable band gap and it is less stable than the 2H structure. The number of layers determined from the analysis of the XRD data varies between 65 and 109 for different samples yielding thickness of the graphite sheets varying between 22 nm and 37 nm. Scanning electron microscopy and transmission electron microscopy of three representative samples confirms the sheet-like morphology and stacking of the graphene layers in the samples. Relevance of these results in connection with their potential applications and toxicology is briefly discussed.

Sealed-tube synthesis and phase diagram of LixTiS2 (0 ≤ x ≤1)

We reported a new method to prepare LixTiS2 (0≤x≤1) at 600°C in sealed tube using Li2S as lithium source. The LixTiS2 samples were characterized by powder X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy, and differential thermal analysis. The variations of the lattice parameters with lithium content x in LixTiS2 were determined by X-ray powder diffraction analysis for both 1T and 3R phases. The phase transition between low-temperature 1T phase and high-temperature 3R phase was confirmed by the powder X-ray diffraction analysis. Based on the differential thermal analysis and X-ray diffraction results, a schematic phase diagram of the LixTiS2 system has been constructed, providing a guideline to synthesize LixTiS2 in 1T structure or 3R structure.

Superior thermoelectric response in the 3R phases of hydrated NaxRhO2

Density functional theory is used to investigate the thermoelectric properties of the 3R phases of NaxRhO2 for different Na vacancy configurations and concentrations. As compared to the analogous 2H phases, the modified stacking of the atomic layers in the 3R phases reduces the interlayer coupling. As a consequence, the 3R phases are found to be superior in the technologically relevant temperature range. The Rh orbitals still govern the valence band maxima and therefore determine the transport properties. A high figure of merit of 0.35 is achieved in hydrated Na0.83RhO2 at 580 K by water intercalation, which is 34% higher than in the non-hydrated phase.

Effect of Magnesium on the Crystal Transformation and Electrochemical Properties of A2B7-Type Metal Hydride Alloys

The effect of Mg content on crystal transformation and electrochemical characteristics of La2-xMgxNi7 (x = 0.40–0.60) alloys has been studied. It is found that alloys with allotropic structures Ce2Ni7-type (2H) and Gd2Co7-type (3R) phases exist in a range of Mg concentration from 0.48 to 0.50. Beyond that range, further increasing Mg to 0.60 is favorable for the formation of PuNi3-type phase due to the rising ratio of [A2B4] to [AB5] slabs in regard to the demand of La and Ni atoms for Mg to assemble [A2B4] slabs. Conversely, the surplus of La and Ni atoms for Mg to assemble [A2B4] slabs lowers the ratio of [A2B4] to [AB5] slabs, which helps the formation of CaCu5-type phase as Mg decreases to 0.40. Electrochemical studies show that the electrochemical properties of the alloy electrodes with allotropic phases are superior in discharge capacity (388 mAh g−1) and cycling stability (78% capacity retaining rate at 100th cycle). Appearance of PuNi3-type or CaCu5-type phase reduces the cycling stability, while CaCu5-type phase can significantly improve the high rate dischargeablity (HRD) of the alloy electrode.

Sulfur-catalyzed phase transition in MoS2 under high pressure and temperature

We report phase transition and stability of MoS2 with and without the presence of sulfur melt under high-pressure and high-temperature conditions. Rhombohedral (3R) phase is found to be a high-temperature phase of MoS2 at high pressures. Excess sulfur melt catalyzes the hexagonal (2H) to rhombohedral (3R) phase transformation and lowers the conversion temperature by more than 280K. Boundary between 2H and 3R phases has been delineated with a negative slope. Based on experimental observations, sulfur-catalyzed 2H→3R transformation mechanisms are proposed involving atomic exchange between MoS2 and sulfur, which is different from the case of without excess sulfur that proceeds through rotation and translation of the S–Mo–S sandwich layers.

Phase Stability, Structural Transition, and Hydrogen Absorption−Desorption Features of the Polymorphic La4MgNi19 Compound

The phase stability, structural transition, and hydrogen storage characteristics of the polymorphic La4MgNi19 compound are systematically investigated in the present work. The La4MgNi19 compound has two variants in the temperature range of 840−960 °C with a preference for a hexagonal structure (2H) at a higher temperature and a rhombohedral structure (3R) at a lower temperature. Because the structure and composition are almost identical, the polymorphic transition between the 2H and 3R phases may proceed through short-range displacements of their subunit layers. Detailed analyses on XRD patterns for the hydrided or dehydrided samples indicate that the equilibrium pressures of hydrogen absorption and desorption for the 2H phase are lower than the pressures for the 3R phase, but the pressure gap between the two phases is too small to be identified from the measured pressure−composition (P−C) isotherms. The average hydriding and dehydriding enthalpies are determined to be −32.1 and 31.5 kJ/mol H2, respectively. As hydrogen atoms enter into the lattices, the strains in both the 2H and the 3R phases increase first in the region of the dilute H-dissolved solid solutions and then dramatically decrease once the dilute solid solutions transform into the corresponding hydrides.

Graphite structure and lithium intercalation

The various commercial graphite structures and their lithium intercalation properties were investigated using X-ray diffraction (XRD) and electrochemical voltage spectroscopy (EVS) techniques. It has been found that most commercially available graphites are a mixture of hexagonal (2H) and rhombohedral (3R) phases. For some graphites, the 3R phase content may exceed 30%. The correlation between graphite lithium intercalation capacity, 2H, 3R phase contents and structural disorder in each phase is discussed. A model is proposed to facilitate the understanding of the relationship between the graphite structures and the lithium intercalation properties. It was found that the total reversible lithium capacity is related to both the 2H and the 3R phase contents, and that the lithium intercalation mechanism in the 2H and 3R structures are similar.

Defect structures of Nb1+αS2 sulfidation scales

One of the major purposes in the present work is to study the high temperature sulfidation properties of Nb in severe sulfidizing environments. Kinetically, the sulfidation rate of Nb is satisfactorily slow, but the microstructures and non-stoichiometry of Nb1+αS2 challenge conventional oxidation/sulfidation theory and defect models of non-stoichiometric compounds. This challenge reflects our limited knowledge of the dependence of kinetics and atomic migration processes in solid state materials on their defect structures.Figure 1 shows a high resolution image of a platelet from the middle portion of the Nb1+αS2 scale. A thin lamellar heterogeneity (about 5nm) is observed. From X-ray diffraction results, we have shown that Nb1+αS2 scale is principally rhombohedral structure, but 2H-NbS2 can result locally due to stacking faults, because the only difference between these 2H and 3R phases is variation in the stacking sequence along the c axis. Following an ABC notation, we use capital letters A, B and C to represent the sulfur layer, and lower case letters a, b and c to refer to Nb layers. For example, the stacking sequence of 2H phase is AbACbCA, which is a ∼12Å period along the c axis; the stacking sequence of 3R phase is AbABcBCaCA to form an ∼18Å period along the c axis. Intergrowth of these two phases can take place at stacking faults or by a shear in the basal plane normal to the c axis.

Lithium intercalation in Li xZrS 2

We report electrochemical and x-ray diffraction studies of Li xZrS 2. lT-ZrS 2 undergoes a reversible first order transition to the 3R polytype when intercalated with lithium. Co-existing 1T and 3R phases are observed for 0.01 ± 0.01 < x < 0.23 ± 0.01.