Triclinic Phase Research Articles

High-performance self-biased photodetectors based on Bi-incorporated ReSe2 ternary alloys

Transition Metal Dichalcogenides (TMDCs) have recently been the focus because of their unique physical properties. Bi-incorporated ReSe2 (Bi: ReSe2) is a new member of the TMDC family with a triclinic structure that is optically biaxial and strongly anisotropic. In this work, we reported the effect of Bi doping in layered ReSe2 crystals synthesized by the Direct Vapour Transport (DVT) method and studied the structural and optoelectronic properties in detail. The layered surface of the single crystals, the stoichiometric proportion of elements and the triclinic structure and phase purity of the material has been observed using Scanning electron microscopy (SEM), Energy dispersive analysis of X-ray (EDAX) and X-ray diffraction (XRD) respectively. The shifting in vibrational Raman modes has been observed by Raman spectroscopy at room temperature. The dynamic I-t characteristics of BixRe1-xSe2 (x = 0.1, 0.5) ternary alloy-based photodetectors were studied successively. The photodetector parameters were calculated under different biasing voltages and different wavelengths. The anisotropy in the dynamic I-t characteristics of transition metal chalcogenides was studied due to their layered structure. The device showed higher photoresponse for the perpendicular to c-axis configuration than for the parallel. Our results showed that the ternary alloys based single crystals have great potential for application in photosensitive devices.

Pressure tuning of structure, magnetic frustration, and carrier conduction in the Kitaev spin liquid candidate Cu2IrO3

The layered honeycomb lattice iridate Cu$_2$IrO$_3$ is the closest realization of the Kitaev quantum spin liquid, primarily due to the enhanced interlayer separation and nearly ideal honeycomb lattice. We report pressure-induced structural evolution of Cu$_2$IrO$_3$ by powder x-ray diffraction (PXRD) up to $\sim$17 GPa and Raman scattering measurements up to $\sim$25 GPa. A structural phase transition (monoclinic $C2/c \: \rightarrow$ triclinic $P\bar{1}$) is observed with a broad mixed phase pressure range ($\sim$4 to 15 GPa). The triclinic phase consists of heavily distorted honeycomb lattice with Ir-Ir dimer formation and a collapsed interlayer separation. In the stability range of the low-pressure monoclinic phase, structural evolution maintains the Kitaev configuration up to 4 GPa. This is supported by the observed enhanced magnetic frustration in dc susceptibility without emergence of any magnetic ordering and an enhanced dynamic Raman susceptibility. High-pressure resistance measurements up to 25 GPa in the temperature range 1.4--300 K show resilient non-metallic $R$($T$) behaviour with significantly reduced resistivity in the high-pressure phase. The Mott 3D variable-range-hopping conduction with much reduced characteristic energy scale $T_0$ suggests that the high-pressure phase is at the boundary of localized-itinerant crossover. Using first-principles density functional theoretical (DFT) calculations, we find that at ambient pressure $\rm Cu_2IrO_3$ exists in monoclinic $P2_1/c$ phase which is energetically lower than $C2/c$ phase (both the structures are consistent with experimental XRD pattern). DFT reveals structural transition from $P2_1/c$ to $P\bar{1}$ structure at 7 GPa (involving dimerization of Ir-Ir bonds) in agreement with experimentally observed transition pressure.

Low-temperature structure and thermoelectric properties of ductile Ag2S0.4Te0.6

Ductile Ag2S0.4Te0.6 shows promising thermoelectric properties and interesting phase transitions and structural features above room temperature. In this work, our experiments reveal that Ag2S0.4Te0.6 is composed of a Ag2Te-based glass and a Ag2S-based supercooled liquid at room temperature. The Ag2Te-based glass preserves its amorphous state in the measured temperature range (20–303 K), while the Ag2S-based supercooled liquid crystallizes into a triclinic phase below 183 K, leading to respectively the glass and crystalline-like thermal transport of Ag2S0.4Te0.6 near room temperature and at low temperature. At the crystallization point of the Ag2S-based phase, the electrical conductivity of Ag2S0.4Te0.6 shows an anomalous change. The extremely wide supercooling region (183–420 K) of the Ag2S-based phase leads to the unprecedented ductility of Ag2S0.4Te0.6. The thermoelectric figure of merit zT of Ag2S0.4Te0.6 increases monotonically with increasing temperature and reaches ∼0.17 at 300 K, showing the great potential as a ductile thermoelectric inorganic.

Structural Features of Oxyapatite

One of the most widely known representatives of the apatite family is hydroxyapatite, Ca10(PO4)6(OH)2. This mineral is a part of the human dental and bone tissues, and, therefore, is widely used in medicine. Less known is oxyapatite, Ca10(PO4)6O, which has the same biocompatibility as hydroxyapatite. In this work, it is shown that oxyapatite can be obtained by heating hydroxyapatite powder at 1000 °C in vacuum. IR and NMR spectroscopy proved the absence of the hydroxyl groups in the apatite obtained. In the IR spectrum, the presence of new absorption bands of phosphate groups, indicating a symmetry disorder, was observed. Density functional theory modeling confirmed lowering of symmetry for the oxyapatite structure. Modeling the IR spectrum of oxyapatite made it possible to identify the experimentally observed new absorption bands. According to the modeling, the presence of a vacancy in a hydroxyl channel of the apatite structure lowered the symmetry. Powder X-ray diffraction data confirmed that full dehydroxylation of hydroxyapatite led to a decrease in symmetry to triclinic phase. Comparison of the formation energies showed that formation of the hydroxyapatite phase was more preferable than that of oxyapatite, which explains apatite’s tendency to rehydroxylation. It was shown that the solubility of oxyapatite in water was comparable to that of hydroxyapatite.

Synthesis of Nanocrystalline Yttrium Oxide and Evolution of Morphology and Microstructure during Thermal Decomposition of Y2(C2O4)3·10H2O

A study of the morphology and evolution of the microstructure during thermal decomposition of Y2(C2O4)3·10H2O was conducted, and the stages and factors having the greatest impact on particle size and specific surface area were identified. The effect of the yttrium oxalate hexahydrate phases on the course of decomposition was also investigated. The evolution of the morphology and microstructure of decomposition products was explained from the analysis of volume shrinkage at various stages of the reaction. The formation of oxycarbonate is accompanied by the largest shrinkage during the reaction. At this stage, there is a significant increase in the specific surface area to 60–90 m2/g. Conversely, the morphology and microstructure of the particles during the transformation of oxycarbonate into yttrium oxide change insignificantly. Yttrium oxide powders obtained from the monoclinic and triclinic hexahydrate phases have the same specific surface area, but different morphology and bulk density due to pseudomorph formation. The carbon formed during thermolysis was shown to affect the specific surface area of the decomposition product. Two methods for producing yttrium oxide with high specific surface area have been proposed, and nanocrystalline yttrium oxide with a specific surface area of 65 m2/g was obtained.

Evolution of Structural and Spectral Characteristics of Lu-=SUB=-0.99-x-=/SUB=-Sm-=SUB=-x-=/SUB=-Eu-=SUB=-0.01-=/SUB=-BO-=SUB=-3-=/SUB=- orthoborates

The structure, IR absorption and luminescence spectra, and morphology of Lu0.99-xSmxEu0.01BO3 (0≤ x≤0.99) orthoborates synthesized at 970oC were studied. With an increase in the Sm3+ concentration, a successive change of five structural states is observed: calcite (0≤ x≤ 0.1)-> calcite + vaterite (0.1<x<0.3)-> vaterite (0.3≤ x≤ 0.95)-> vaterite + a triclinic phase (0.95<x≤ 0.98)-> a triclinic phase (0.98<x≤1). A wide range of Sm3+, concentrations at which the vaterite phase (0.3≤ x≤ 0.95) exists and a very narrow region of triclinic phase formation (0.98<x≤1) are important distinctive features of this system. Correspondence between the structure and spectral characteristics of these compounds was established. It is shown that the vaterite phase appears in the bulk of the microcrystals of the samples that have a calcite structure. Keywords: rare earth orthoborates, crystal structure, X-ray diffraction analysis, IR-spectroscopy, luminescence spectra.

Эволюция структурных и спектральных характеристик ортоборатов Lu-=SUB=-0.99-x-=/SUB=-Sm-=SUB=-x-=/SUB=-Eu-=SUB=-0.01-=/SUB=-BO-=SUB=-3-=/SUB=-

The structure, IR absorption and luminescence spectra, and morphology of Lu0.99xSmxEu0.01BO3 (0 ≤ x ≤ 0.99) orthoborates synthesized at 970°С were studied. With an increase in the Sm3+ concentration, a successive change of five structural states is observed: calcite (0 ≤ x ≤ 0.1) → calcite + vaterite (0.1 < x < 0.3) → vaterite (0.3 ≤ x ≤ 0.95) → vaterite + a triclinic phase (0.95 < x ≤ 0.98) → a triclinic phase (0.98 < x ≤ 1). A wide range of Sm3+ concentrations at which the vaterite phase (0.3 ≤ x ≤ 0.95) exists and a very narrow region of triclinic phase formation (0.98 < x ≤ 1) are important distinctive features of this system. Correspondence between the structure and spectral characteristics of these compounds was established. It is shown that the vaterite phase appears in the bulk of the microcrystals of the samples that have a calcite structure.

Plasma-enhanced atomic layer deposition of WO3-SiO2 films using a heteronuclear precursor

Tungsten oxide–silicon dioxide (WOx–SiOy) composite thin films were deposited for the first time via the remote oxygen plasma-enhanced atomic layer deposition (ALD) process using a novel metal-organic heteronuclear and heteroleptic precursor, bis(tert-butylimido)bis(trimethylsilylmethyl)tungsten. Self-limiting ALD growth was demonstrated over a wide temperature window of 203–328 °C with growth per cycle decreasing with increasing temperature from 0.75 to 0.4 Å/cycle, respectively. Residual gas analysis revealed ligand competition and showed that ligand reaction during ALD nucleation and growth was a function of deposition temperature, thereby affecting the film composition. As the temperature increased from 203 to 328 °C, the film composition [W/(Si + W)] ranged from 0.45 to 0.53. In addition, the carbon impurity content was reduced and the refractive index increased from 1.73 to 1.96, the density increased from 4.63 to 5.6 g/cm3, and the optical bandgap decreased from 3.45 to 3.27 eV. Grazing angle x-ray diffraction indicated that as-deposited films were amorphous. Upon annealing in O2 at 500 °C or higher, depending on deposition temperature, films are crystalized into the triclinic WO3 phase. At the same time, WO3 is sublimed from the surface and films are reduced in thickness.

A scheme to fabricate magnetic graphene-like cobalt nitride CoN4 monolayer proposed by first-principles calculations

Based on the first-principles calculations, we propose a scheme to fabricate the cobalt nitride CoN4 monolayer, a magnetic graphene-like two-dimensional material, in which all Co and N atoms are in a plane. Under the pressure above 40 GPa, the bulk CoN4 is stabilized in a triclinic phase. With the pressure decreasing, the triclinic phase of CoN4 is transformed into an orthorhombic phase, and the latter is a layered compound with large interlayer spacing. At ambient conditions, the weak interlayer couplings are so small that a single CoN4 layer can be exfoliated by the mechanical method.

Ultrafast Sodium Intercalation Pseudocapacitance in MoS2 Facilitated by Phase Transition Suppression

Sodium-ion intercalation pseudocapacitance promises fast energy storage that is cheaper than lithium-ion-based systems. MoS2 is an attractive sodium-ion host due to its large van der Waals gaps, high Na+ mobility, and high electronic conductivity in the 1T phase. In this paper, we have quantified high levels (>90%) of pseudocapacitive charge storage in 30 μm thick MoS2 nanocrystal-based composite electrodes, which can be charged to almost 50% of their 1C capacity in just under 40 s. In addition, very little decay is observed in the delivered capacity (retention of 97%) after 1800 cycles at a rate of 20C. Synchrotron-based operando X-ray diffraction shows that the pseudocapacitive performance is enabled through suppression of the trigonal 1T-MoS2 to triclinic NaxMoS2 phase transition in MoS2 nanocrystals during charge–discharge.

Pressure-induced phase transition and band-gap decrease in semiconducting Na3Bi(IO3)6

We report a combined experimental/theoretical high-pressure study of Na3Bi(IO3)6 under compression to 11.2 GPa at ambient temperature. Through a combination of single-crystal and powder synchrotron X-ray diffraction, optical absorption measurements and ab initio density functional theory calculations we unambiguously show a first-order pressure-induced phase transition at around 9.5 GPa from the ambient pressure phase, referred to here as α-Na3Bi(IO3)6, to a new crystalline structure referred to here as β-Na3Bi(IO3)6. The triclinic (P-1) to triclinic (P1) phase transition is characterised by a doubling of the primitive cell volume, whereby the crystallographic b-axis doubles in length, and by a decrease in the volume per formula unit of approximately 3 %. The phase transition is also characterised by an indirect → indirect electronic bandgap decrease of approximately 0.1 eV as measured by absorption spectroscopy (3.44(1) → 3.32(1) eV) and calculated via density functional calculations (2.48 → 2.33 eV). We also report the pressure evolution of the crystal lattice parameters and isothermal compressibility tensor of the ambient pressure phase α-Na3Bi(IO3)6, which reveals highly anisotropic compressibility and a bulk modulus of 30.4(7) GPa.

Quantitative spatial mapping of distorted state phases during the metal-insulator phase transition for nanoscale VO2 engineering

ABSTRACT Vanadium dioxide (VO2) material, known for changing physical properties due to metal-insulator transition (MIT) near room temperature, has been reported to undergo a phase change depending on the strain. This fact can be a significant problem for nanoscale devices in VO2, where the strain field covers a large area fraction, spatially non-uniform, and the amount of strain can vary during the MIT process. Direct measurement of the strain field distribution during MIT is expected to establish a methodology for material phase identification. We have demonstrated the effectiveness of geometric phase analysis (GPA), high-resolution transmission electron microscopy techniques, and transmission electron diffraction (TED). The GPA images show that the nanoregions of interest are under tensile strain conditions of less than 0.4% as well as a compressive strain of about 0.7% (Rutile phase VO2[100] direction), indicating that the origin of the newly emerged TED spots in MIT contains a triclinic phase. This study provides a substantial understanding of the strain-temperature phase diagram and strain engineering strategies for effective phase management of nanoscale VO2.

Low-symmetry ferroelectric phases stabilized by anisotropic misfit strain

The complete thermodynamic potential of (111) ferroelectric thin films including anisotropic in-plane misfit strain, in-plane shear strain, and external stress is developed based on the Landau-Ginzburg-Devonshire thermodynamic theory. Anisotropic strain-tuned low-symmetry equilibrium phases in (111) single-domain ferroelectric $\mathrm{PbTi}{\mathrm{O}}_{3}$ films were investigated via the developed nonlinear thermodynamic potential and the misfit strain--misfit strain phase diagram was constructed. It was found that the anisotropic epitaxial strain can effectively control the orientations of polarization vectors, which determine the symmetry of equilibrium phases, and their functionalities. In particular, two types of low-symmetry triclinic phases, disappearing under isotropic strain states, can be stabilized by anisotropic strains. Low-symmetry monoclinic and triclinic ferroelectric phases and field-induced phase transitions were also analyzed in depth. It was indicated that the monoclinic (${M}_{A}^{1}$) symmetry rhombohedral-like--tetragonal-like transition and triclinic (${T}_{\mathrm{r}}^{1}$)-monoclinic (${M}_{B}^{1}$) phase transition would give rise to the superior piezoelectric properties as compared to other structure phase transitions. Electric field-enhanced piezoelectric response in triclinic structure originates from the elastic softening that is related to the shear ${u}_{5}$. These results provide guidance for interpreting and understanding more experimental observations as well as the designing of high-performance piezoelectric films.

Panchromatic photomultiplication-type organic photodetectors with planar/bulk heterojunction structure

Broadband organic photodetectors (OPDs) can be integrated into various wearable devices and show great application potential in health monitoring and other fields. Here, we demonstrated high-performance broadband photomultiplication-type OPDs (PM-OPDs) based on MoO3 trapping electron-assisted hole tunneling injection mechanism with small-molecule lead (II) phthalocyanine (PbPc) and C70 fullerene as the donor and acceptor, respectively. In order to control the crystal phase structure formed by PbPc molecules, we prepared PbPc:C70 bulk heterojunction (BHJ) and PbPc/C70 planar heterojunction (PHJ) devices. It can be seen that the PHJ device exhibits stronger near-infrared (NIR) absorption characteristics, which is more conducive to the formation of the triclinic phase structure of the PbPc molecule. We further prepared flat panchromatic PM-OPDs in the range of 300–1000 nm by using materials (PbPc and SubPc) with absorption complementary characteristic as the active layers and PHJ/BHJ hybrid heterojunction structure. The external quantum efficiency of the resulting panchromatic PM-OPDs exceeds 1000% in the whole spectral response range under −8 V reverse bias. Finally, we prepared flexible panchromatic PM-OPDs on polyethylene terephthalate substrate and successfully realized the detection of the human pulse signal. This work provides a new strategy for obtaining high-performance panchromatic PM-OPDs by designing device structures and selecting appropriate materials.

Effect of External Pressure on the Metal–Insulator Transition of the Organic Quasi-Two-Dimensional Metal κ-(BEDT-TTF)2Hg(SCN)2Br

The metal–insulator transition in the organic quasi-two-dimensional metal κ-(BEDT-TTF)2Hg(SCN)2Br at TMI ≈ 90 K has been investigated. The crystal structure changes during this transition from monoclinic above TMI to triclinic below TMI. A theoretical study suggested that this phase transition should be of the metal-to-metal type and brings about a substantial change of the Fermi surface. Apparently, the electronic system in the triclinic phase is unstable toward a Mott insulating state, leading to the growth of the resistance when the temperature drops below TMI ≈ 90 K. The application of external pressure suppresses the Mott transition and restores the metallic electronic structure of the triclinic phase. The observed quantum oscillations of the magnetoresistance are in good agreement with the calculated Fermi surface for the triclinic phase, providing a plausible explanation for the puzzling behavior of κ-(BEDT-TTF)2Hg(SCN)2Br as a function of temperature and pressure around 100 K. The present study points out interesting differences in the structural and physical behaviors of the two room temperature isostructural salts of κ-(BEDT-TTF)2Hg(SCN)2X with X = Br, Cl.

Structural stabilities, mechanical and thermodynamic properties of chalcogenide perovskite ABS3 (A = Li, Na, K, Rb, Cs; B = Si, Ge, Sn) from first-principles study

In this study, first-principles calculations have been used to study the mechanical and thermodynamic properties of chalcogenide perovskite ABS3 (A = Li, Na, K, Rb, Cs; B = Si, Ge, Sn) in the triclinic phase. The structural stabilities of perovskite were investigated through Goldschmidt’s tolerance factor (t) and phonon dispersion. It was indicated that all of the investigated materials construct stable perovskite structures. The mechanical properties of chalcogenide perovskites ABS3 were systematically investigated by density functional theory (DFT). The DFT method was considered within the meta-generalized gradient approximation revTPSS. The elastic properties of materials give the data necessary in understanding the bonding property between adjacent atomic planes, stiffness, bonding anisotropic, and structural stability of the material. The independent elastic constants Cij have been used for the prediction of mechanical properties like bulk modulus (B), Shear modulus (G), Young’s modulus (E) Poisson’s ratio (ν), and the universal anisotropic index (AU). The mechanical stability, brittleness, and ductility behaviors of materials were discussed. The covalent, ionic, and metallic nature of the materials were also discussed. The thermodynamic parameters including heat capacity, entropy, enthalpy, and free energy were also computed and discussed with a wide range of temperatures (0–1000 K).

Ultra-Low Reflectivity in Visible Band of Vanadium Alumina Nanocomposites

The high-temperature annealing effect of a V2O5/Al2O3 bilayer on the reflectivity of visible band was studied in the present work. It was found that the V2O5 (200 nm)/Al2O3 (30 nm) sample (A-0) has a nano-granular surface morphology without a definite crystalline phase. The reflectance of A-0 overreaches 60% as the wavelength is larger than 650 nm, and its minimum reflectance (20%) occurs at the wavelength of ~500 nm. After in situ annealing treatment at 750 °C for 30 min, a triclinic AlVO4 phase appears while the nano-granular surface morphology remains (sample A-30). The reflectivity of A-30 is well suppressed in comparison with that of A-0 especially in the long wavelength range. Its minimum reflectivity approaches 5% at ~500 nm wavelength. It is speculated that the appearance of a new AlVO4 nanophase is the main reason for the decline of reflectivity. Extending the annealing time to 60 min, the reflectivity spectrum, as well as the surface morphology, are almost the same. These results can be helpful for applications of vanadium alumina nanocomposites in high-temperature environments.

Disappearance of spiral spin order and field induced spin reorientation transition in Fe[formula omitted]Cr[formula omitted]VO[formula omitted] (0.1[formula omitted][formula omitted][formula omitted]0.3) multiferroic: 57Fe Mössbauer spectroscopic studies

Here we report a progressive suppression of spiral spin order and evidence for disappearance of spin reorientation transition (SRT) in Fe1−xCrxVO4 (0.1 ≤x≤ 0.3) studied by low temperature and high magnetic field (LTHM) Mössbauer spectroscopy (MS). Variation in the area ratio of lines 2 and 3 (A23) with external magnetic field confirms the presence of antiferromagnetic (AFM) ordering for all the compositions at the lowest temperatures (5 K). Field induced spin reorientation transition is being observed in both triclinic (x = 0.10) and mixed phase (x = 0.175) systems, evidenced by the change in the sign of quadrupole shift (QS) value at low temperatures, where as the SRT is absent in the monoclinic phase (x = 0.20 and 0.30). MS line width (Γ) values indicate the presence of spiral spin order in T-phase, partial suppression in mixed phase (T + M) and finally complete suppression for the M-phases.

Absence of precursor incommensurate charge order in electronic nematic Ba0.35Sr0.65Ni2As2

Recent discoveries of charge order and electronic nematic order in the iron-based superconductors and cuprates have pointed towards the possibility of nematic and charge fluctuations playing a role in the enhancement of superconductivity. The Ba1-xSrxNi2As2 system, closely related in structure to the BaFe2As2 system, has recently been shown to exhibit both types of ordering without the presence of any magnetic order. We report single crystal X-ray diffraction, resistance transport measurements, and magnetization of \BaSrLate, providing evidence that the previously reported incommensurate charge order with wavevector $(0,0.28,0)_{tet}$ in the tetragonal state of \BaNi~vanishes by this concentration of Sr substitution together with nematic order. Our measurements suggest that the nematic and incommensurate charge orders are closely tied in the tetragonal state, and show that the $(0,0.33,0)_{tri}$ charge ordering in the triclinic phase of BaNi2As2 evolves to become $(0,0.5,0)_{tri}$ charge ordering at $x$=0.65 before vanishing at $x$=0.71.

Single-crystal X-ray diffraction of fluorapatite to 61 GPa

Abstract Apatite is a mineral of widespread importance in Earth and planetary science. Here we examine the behavior of a natural fluorapatite (FAp) crystal from Durango (Mexico) under compression to 61 GPa. Single-crystal X-ray diffraction experiments were carried out in a diamond-anvil cell using a synchrotron source. The apatite structure persists up to 32.4 GPa. Birch-Murnaghan equation of state parameters were fit to the pressure-volume data for fluorapatite for two cases: fixing V0 at its measured ambient value resulted in a bulk modulus, K0T, of 97.0(8) GPa and a pressure derivative of the bulk modulus, K0T′, of 3.3(1), while fixing V0 and K0T at its ambient value 90.5 GPa (derived from ultrasonically measured elastic constants) resulted in a K0T′ value of 4.1(1). At 35.6 GPa, fluorapatite transforms to a triclinic phase (P1, Z = 4), designated here as fluorapatite II (FAp-II). This phase persists up to at least 61 GPa. The major structural differences between FAp and FAp-II involve the buckling of the Ca polyhedra along the c-axis and changes in the number and coordination of the Ca sites. Our study extends the pressure range over which fluorapatite has been examined by more than a factor of three, providing new insights into its structural response to high-pressure conditions.