Rhombohedral Layer Research Articles

Tuning of magnetic and dielectric properties of Gallium doped hematite(α-GaxFe(2−x)O3) nanospheres

In this work, we investigated the impact of Ga doping on the properties of α-Fe2O3 i.e. α-GaxFe(2−x)O3 (x=0.00, 0.20, 0.40, 0.60) nanospheres synthesized via novel hydrothermal technique and explored structural alterations, dielectric response, and magnetic characteristics. The hexagonal framework containing a rhombohedral lattice arrangement of α- GaxFe(2−x)O3 (x=0.00, 0.20, 0.40, and 0.60) nanoparticles has been confirmed by rigorous analysis using X-ray Diffraction (XRD) and Rietveld refinements. Nanospheres with impressive mono dispersity and an average diameter of 63 nm have been determined by scanning electron microscopy. Magnetic investigations revealed diluted antiferromagnetic spin order within Ga doped α- Fe2O3 nanoparticles marking a significant increase in the coercivity from 1595.28–2343.46 Oe (x=0.00–0.60). The observable remnant magnetization patterns illustrate the significance of Ga3+ doping in enhancing the magnetic characteristics of α-Fe2O3 attributed to the presence of uncompensated spin values between neighboring rhombohedral layers where opposite spin orientations coexist. By tracing the interactions and reversible/irreversible components, the FORC technique enabled a deeper comprehension of the magnetic response and made a substantial contribution to the interpretation of the complicated magnetic nature of the α-Ga0.20Fe1.8O3. A detailed dielectric analysis of α-GaxFe(2−x)O3 (x=0.00, 0.20, 0.40, and 0.60) nanoparticles was conducted to determine the dielectric constant (ε), dielectric loss (tanδ), ac-conductivity, complex impedance components, and the hopping probability of electron fluctuation with frequency. In the cole-cole plot for α- GaxFe(2−x)O3 (x=0.20, 0.40, and 0.60), the dominating grain boundary effect in conduction is indicated by a single semi-circle.

Hydrogel-Mediated Mineralization Generates Oriented Crystalline Films Comprising Granular-Rhombohedral Heterogeneous Structures.

Gel-mediated crystallization is a common system to produce self-organized materials, which is fundamental to the development of bottom-up approaches to functional complex materials. Mineralization in hydrogel matrices nevertheless remains empirical in the generation of crystallization products with tailored heterogeneous structures. We demonstrate that the employment of the hydrogels with proper cationic diffusivity can trigger the consecutive growth of oriented, granular-rhombohedral heterogeneous structures. The controllable morphogenesis leads to continuous calcitic CaCO3 films comprising spatial heterogeneity, where epitaxial match assumedly favors the successive deposition of both granular and rhombohedral layers. The scenario of consecutive growth is disclosed, where the thickness of the granular layers can become a valuable indicator to reflect the retardancy degree of crystallization. The evaluation of the physicochemical properties of the hydrogels finally establishes a direct correlation between the cationic diffusivity of the hydrogels and the appearance of the heterogeneous structures. The current work therefore sheds light on the implementation of rational morphogenetic approaches to crystalline materials with tailored complex architectures.

Coexistence of Magnetic Orders in Two-Dimensional Magnet CrI3

The magnetic properties in two-dimensional van der Waals materials depend sensitively on structure. CrI3, as an example, has been recently demonstrated to exhibit distinct magnetic properties depending on the layer thickness and stacking order. Bulk CrI3 is ferromagnetic (FM) with a Curie temperature of 61 K and a rhombohedral layer stacking, whereas few-layer CrI3 has a layered antiferromagnetic (AFM) phase with a lower ordering temperature of 45 K and a monoclinic stacking. In this work, we use cryogenic magnetic force microscopy to investigate CrI3 flakes in the intermediate thickness range (25-200 nm) and find that the two types of magnetic orders, hence the stacking orders, can coexist in the same flake with a layer of ∼13 nm at each surface being in the layered AFM phase similar to few-layer CrI3 and the rest in the bulk FM phase. The switching of the bulk moment proceeds through a remnant state with nearly compensated magnetic moment along the c-axis, indicating formation of c-axis domains allowed by a weak interlayer coupling strength in the rhombohedral phase. Our results provide a comprehensive picture on the magnetism in CrI3 and point to the possibility of engineering magnetic heterostructures within the same material.

A novel magnetoelectric memory cell based on bilayer ferroelectric films of (1 − x)[Ba(Zr0.2Ti0.8)O3]–x(Ba0.7Ca0.3TiO3)

We demonstrate that a thin film magnetoelectric memory cell, comprised of Fe70Ga30 sputtered on top of bilayer ferroelectric films which consists of a tetragonal (T) 0.7Ba(Zr0.2Ti0.8)O3–0.3(Ba0.7Ca0.3TiO3) (70BZT–30BCT) film deposited on a rhombohedral (R) 0.3Ba(Zr0.2Ti0.8)O3–0.7(Ba0.7Ca0.3TiO3) (30BZT–70BCT) film. The insertion of rhombohedral layer improves the quality of the films and enhances the piezoelectric properties, which could be attributed to that the ferroelectric domains are tethered only by a soft R under layer, and not by the hard substrate. The working mechanism is that the 70BZT–30BCT layer acquires different strain states when an electric field is applied to it. Mechanical transduction couples this strain to the mechanically coupled Fe70Ga30 layer, which then changes its magnetic anisotropy and thus the magnetoresistance. The high (low) resistance states were realized when different voltages were applied due to the anisotropy magnetoresistance of Fe70Ga30 films. The demonstrated magnetoelectric memory device using resistance as the media and electric field as the writing field shows great promises towards exploring magnetoelectric devices for low-power and high density magnetic data storage applications.

Atomic structure of hardening precipitates in an Al–Mg–Zn–Cu alloy determined by HAADF-STEM and first-principles calculations: relation to η-MgZn2

The structures of two nanoscale plate precipitates prevalent at maximum strength and over-aged conditions in a 7449 Al–Mg–Zn–Cu alloy were investigated. Models derived from images of high angle annular dark field scanning transmission electron microscopy were supported by first-principles calculations. Both structures are closely linked to the η-MgZn2 Laves phase through similar layers of a rhombohedral atomic subunit. The finest plate contains one such layer together with a layer of an orthorhombic unit. The second plate contains rhombohedral layers only, normally four, but rotated relatively to form different stacking variants, one of which may be likened to η. For both structures, the same atomic planes describe the main interface with Al. Both plates could be described in space group P3. The unit cells comprise interface and arbitrary numbers of {111}Al (habit) planes. Eight Al-planes were included in the first-principles calculations. The enthalpy indicates high layer/unit stability. The plate thickness can be understood by a simple mismatch formulation.

Effect of AlF3Coating on Thermal Behavior of Chemically Delithiated Li0.35[Ni1/3Co1/3Mn1/3]O2

The thermal behavior of chemically delithiated Li0.35[Ni1/3Co1/3Mn1/3]O2 and the AlF3-coated material was studied in the temperature range from room temperature to 600 °C. Thermogravimetric analysis results showed that the uncoated and the AlF3-coated Li0.35[Ni1/3Co1/3Mn1/3]O2 powders experienced distinct weight loss with increasing temperature, of which the weight loss was ascribed to oxygen release from the active materials. The released oxygen amount was less for the AlF3-coated Li0.35[Ni1/3Co1/3Mn1/3]O2 than the uncoated material, probably due to the blocking of the oxygen evolution by the AlF3 coating. The weight loss was associated with the irreversible phase transformation from a rhombohedral layer (R3m̅) structure to a cubic spinel (Fd3m) structure, as confirmed by in situ high-temperature X-ray diffraction. The reduced oxygen release brought about by the AlF3 coating delayed the phase transformation to the cubic spinel structure. This entailed shifts of the main exothermic reactions to higher temperatures for the active material in the presence of an electrolyte. The AlF3 coating remained on the surface of the active material to 300 °C. Thereafter, the layer changed to Li−Al−O with increasing temperature, as observed by the time-of-flight secondary ion mass spectroscopy. The improved thermal properties of the chemically delithiated AlF3-coated Li0.35[Ni1/3Co1/3Mn1/3]O2 were ascribed to the suppression of oxygen release from the active material, and this, in turn, retarded the formation of the cubic spinel phase.

Ferroelastic interactions in bilayered ferroelectric thin films

We present a theoretical investigation of the elastic interactions in a heteroepitaxial bilayer consisting of a (001) tetragonal PbZrxTi1−xO3 and (001) rhombohedral PbZr1−xTixO3 on a thick (001) passive substrate. Analytical expressions for the elastic interaction energies between the layers and the resultant ferroelastic twin formation have been derived as a function of the lattice misfit strain (between layers and the substrate), composition of the ferroelectric and thickness. It is found that the elastic coupling between the tetragonal and rhombohedral layers leads to the equilibrium domain fraction in the tetragonal layer several time larger than that in single-layer films of similar thickness. Most critically, the model finds a significant change in the ferroelastic domain volume fraction in the presence of an applied electric field and hence enhanced piezoelectric properties compared to single-layered epitaxial PZT thin films.

Ferroelastic domains in bilayered ferroelectric thin films

We investigate theoretically ferroelastic domain fractions in a heteroepitaxial bilayer consisting of (001) tetragonal PbZrxTi1−xO3 and (001) rhombohedral PbZr1−xTixO3 on a thick (001) passive substrate as a function of the lattice misfit strain between layers and the substrate. By considering the self-strain in each layer and the indirect elastic interaction between the layers, we provide a numerical analysis of the relative domain fractions in the tetragonal layer of a (001)PbZr0.2Ti0.8O3/(001)PbZr0.8Ti0.2O3 and (001)PbZr0.4Ti0.6O3/(001)PbZr0.6Ti0.4O3 bilayer structure as a function of the tetragonal layer thickness on (001)LaAlO3, (001)SrTiO3, and (001) MgO. It is found that the elastic coupling between the tetragonal and rhombohedral layers leads to an excess elastic energy in the tetragonal layer, resulting in a two to three times increase in the ferroelastic domain volume fraction of the tetragonal layer compared to single-layer films of similar thickness. These results show alternate ways of engineering ferroelastic domain structures in ferroelectric thin films.

Relative stability of polymerized phases of C 60: Depolymerization of a tetragonal phase

Differential scanning calorimetry and IR-spectroscopy have been used to study the depolymerization of the 2D tetragonal (T) polymerized phase of C 60 at p = 1 atm. The depolymerization enthalpy obtained was 17.7 ± 1.7 kJ per mole of C 60. Experimental enthalpies of depolymerization along with the lattice energies calculated by the atom–atom potential method were combined into the thermochemical cycles to account for the trends in the relative stability of the polymerized phases of C 60. Surprisingly low depolymerization enthalpy of 2D rhombohedral (R) phase compared to 1D orthorhombic (O) and 2D T-phases was explained by the unfavorable packing energy of the isolated rhombohedral layers into R-crystal lattice, though the averaged C 60 = C 60 bond energy was also lower for R- than for O- and T- phases, being 31, 42 and 43 kJ/mol, respectively. Results of DFT quantum chemical calculations of the energetics of C 60 polymers were in qualitative agreement with this trend. The mechanism of depolymerization appeared to be significantly different for R-phase compared to other polymers. While decomposition of O- and T-phases occurred in one step without any IR-detectable intermediate species, depolymerization of R-phase was found to be at least a two-step process. Comparison of experimental and DFT simulated IR-spectra suggested that intermediate species were cyclic trimers or similar oligomers.

7Li NMR Studies of Chemically-Delithiated Li1-xCoO2

7Li magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy and Rietveld analysis of X-ray diffraction data have been used to study chemically delithiated Li1-xCoO2. Samples of LiCoO2 were prepared at 400 (LT), 600 (MT), and 800 °C (HT), chemically delithitated to Li0.5CoO2 composition, and heated at 200 °C. It was found that all HT materials and the as-prepared MT-Li0.5CoO2 displayed layered structure, whereas all LT materials and the 200 °C heated MT- Li0.5CoO2 displayed spinel structure. The NMR results suggest that the local atomic and electronic structures of the as-prepared MT-Li0.5CoO2 approach that of spinel phase although X-ray refinement results show the rhombohedral layer structure. The 7Li MAS NMR results provide evidence for electronic phase segregation in layered and spinel Li1-xCoO2 materials. Several samples showed coexisting NMR peaks arising from lithium in a diamagnetic Co3+ environment and in a paramagnetic mixed-valence Co3+/4+ environment. Li0.5CoO2 samples derived from LiCoO2 fired at 600 °C and 800 °C gave rise to only one NMR peak, associated with a mixed-valence cobalt environment, and produced a low NMR signal intensity, arising from an environment containing localized t2g holes (Co4+ ions). A large NMR shift was observed for lithium in the mixed-valence environment, attributed to a Knight shift for the as-prepared HT-Li0.5CoO2 and to a hyperfine shift in the other samples. Variable-field studies showed homonuclear dipolar coupling to be the dominant source of residual line broadening in Li1-xCoO2, and chemical shift dispersion to be a probable secondary source.

Synthesis and Anion Exchange Chemistry of Rhombohedral Li/Al Layered Double Hydroxides

A new Li/Al layered double hydroxide, [LiAl2(OH)6]Cl·2H2O, having a rhombohedral layer stacking sequence has been synthesized by the reaction of the Al(OH)3 polymorphs, bayerite and nordstrandite, with LiCl. As is the case with the hexagonal analogue, rhombohedral [LiAl2(OH)6]Cl·2H2O has been found to readily undergo anion exchange reactions with a range of dicarboxylate anions although these reactions differ significantly in that no crystalline staging intermediates could be isolated. The rhombohedral stacking is maintained in the dicarboxylate derivatives.

Gold-gold interactions as crystal engineering design elements in heterobimetallic coordination polymers.

A series of coordination polymers containing Cu(II) and [Au(CN)(2)](-) units has been prepared. Most of their structures incorporate attractive gold-gold interactions, thus illustrating that such "aurophilic" interactions can be powerful tools for increasing structural dimensionality in supramolecular systems. [Cu(tren)Au(CN)(2)][Au(CN)(2)] (1, tren = tris(2-ethylamino)amine) forms a cation/anion pair, which is weakly linked by hydrogen bonds but not by aurophilic interactions. [Cu(en)(2)Au(CN)(2)][Au(CN)(2)] (2-Au, en = ethylenediamine) is a 2-D system composed of a chain of [Au(CN)(2)](-) anions and another chain of [(en)(2)Cu-NCAuCN](+) cations; short Au-Au bonds of 3.1405(2) A connect the anions. This bond is shorter than that observed in the analogous silver(I) structure, 2-Ag. The average M-C bond lengths of 1.984(8) A in 2-Au are significantly shorter than those found in 2-Ag, suggesting that Au(I) is smaller than Ag(I). Cu(dien)[Au(CN)(2)](2) (3, dien = diethylenetriamine) forms a 1-D chain of tetranuclear [Au(CN)(2)](-) units that are bound to [Cu(dien)] centers. Aurophilic interactions of ca. 3.35 A hold the tetramer together. Cu(tmeda)[Au(CN)(2)](2) (4, tmeda = N,N,N',N'-tetramethylethylenediamine) forms a 3-D network by virtue of aurophilic interactions of 3.3450(10) and 3.5378(8) A. Altering the Cu:Au stoichiometry yields Cu(tmeda)[Au(CN)(2)](1.5)(ClO(4))(0.5) (5), which has an unusual 2-D rhombohedral layer structure (space group R32). Complex 5 is composed of three mutually interpenetrating Cu[Au(CN)(2)](1.5) networks which are interconnected by aurophilic interactions of 3.4018(7) and 3.5949(8) A. Weak antiferromagnetic coupling is observed in 2 and 5.

Niobium and tantalum diselenides

AbstractNiobium and tantalum diselenides have been found to exist in the following polymorphous forms: 2s‐NbSe2, 3s‐NbSe2, 4s‐NbSe2, 2s‐TaSe2, 3s‐TaSe2, 4s‐TaSe2, and 6s‐TaSe2. Unit‐cell dimensions are collected in Table I and the crystal structures of the various forms are schematically shown in Fig. 1. All the polymorphs have hexagonal or rhombohedral layer structures. In the 2s‐, 3s‐ and 4s‐ phases the metal atoms have a trigonal‐prismatic environment, but in 6s‐TaSe2 half of the metal atoms have octahedral and half have trigonal‐prismatic coordination. The conditions of preparation of the various forms are discussed. Excess of selenium over the composition MSe2 leads to mixtures of the MSe3 and MSe2 phases. Excess of metal results in the formation of phases of the type M1+xSe2, where the additional metal atoms are accommodated in the octahedral interstices between the MSe2 slabs.

The crystal structure of potassium thioferrite KFeS2 and sodium thiochromite NaCrS2

AbstractKFeS2 is monoclinic, pseudohexagonal; it consists of chains (FeS2)n with K in the interstices. NaCrS2 has a rhombohedral layer lattice. The relation of these compounds to the mineral “sulpho‐salts” is discussed.