A7 Phase Research Articles

Metastable square Bismuth allotrope oriented by six-fold symmetric mica

Layered bismuth (Bi) has been focused on two Peierls distortion derivatives, i.e., bulk stable β-phase (A7 phase), black phosphorus-like α-phase (A17 phase), and their mutated structures. Metastable structures beyond Peierls distortion system are yet rarely accessible. Here, Bi in square symmetry (C4v\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${C}_{4v}$$\\end{document}), called s-Bi, was grown via physical vapor deposition. The co-existence of three 120°-associated s-Bi crystal grains was analyzed using transmission electron microscopy. 120°-oriented one-dimensional (1D) nuclei indicate s-Bi heteroepitaxy on six-fold symmetric (C6v\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${C}_{6v}$$\\end{document}) mica (001). A subsequent nanorod-like nuclei coalescing could promote a morphology evolution, resulting in triangular or hexagonal nanosheets with the unique triple s-Bi structure suitable for later β-Bi growth. Lattice misfit and strain calculations suggest a supercell match between 4×7\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$4\ imes 7$$\\end{document}s-Bi and 3×3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$3\ imes 3$$\\end{document} mica (001). This work demonstrates the metastable s-Bi structure via anisotropic heteroepitaxy of C4v\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${C}_{4v}$$\\end{document}s-Bi on C6v\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${C}_{6v}$$\\end{document} mica.

Pressure-tuned structure and electric transport properties in violet phosphorus

Violet phosphorus, as a kind of two-dimensional material, has become an excellent candidate for optoelectronic devices as it makes up for the drawbacks of black phosphorus applications. This study explores the structural and electrical properties of violet phosphorus under high-pressure conditions. Two structure phase transitions are revealed by the spectra and electromagnetic transport measurements. At $\ensuremath{\sim}8$ GPa, a structural transition from monoclinic to A7 phase is observed with the appearance of superconductivity. The transition from the A7 phase to the simple cubic phase starts at $\ensuremath{\sim}13$ GPa and completes after $\ensuremath{\sim}30$ GPa at low temperatures. In addition, the band gap of violet phosphorus decreases linearly with the increase of pressure until it disappears after the transition to the A7 phase. The anisotropic upper magnetic field of superconductivities in A7 and simple cubic phases showed the relative crystal orientations of the high-pressure phases compared with initial violet phosphorus crystal phase.

Electrical and structural properties of binary Ga–Sb phase change memory alloys

Material properties of Ga–Sb binary alloy thin films deposited under ultra-high vacuum conditions were studied for analog phase change memory (PCM) applications. Crystallization of this alloy was shown to occur in the temperature range of 180–264 °C, with activation energy >2.5 eV depending on the composition. X-ray diffraction (XRD) studies showed phase separation upon crystallization into two phases, Ga-doped A7 antimony and cubic zinc-blende GaSb. Synchrotron in situ XRD analysis revealed that crystallization into the A7 phase is accompanied by Ga out-diffusion from the grains. X-ray absorption fine structure studies of the local structure of these alloys demonstrated a bond length decrease with a stable coordination number of 4 upon amorphous-to-crystalline phase transformation. Mushroom cell structures built with Ga–Sb alloys on ø110 nm TiN heater show a phase change material resistance switching behavior with resistance ratio >100 under electrical pulse measurements. TEM and Energy Dispersive Spectroscopy (EDS) studies of the Ga–Sb cells after ∼100 switching cycles revealed that partial SET or intermediate resistance states are attained by the variation of the grain size of the material as well as the Ga content in the A7 phase. A mechanism for a reversible composition control is proposed for analog cell performance. These results indicate that Te-free Ga–Sb binary alloys are potential candidates for analog PCM applications.

Crystallization Properties of Al-Sb Alloys for Phase Change Memory Applications

Material properties of Al-Sb binary alloy thin films deposited under ultra-high vacuum conditions were studied for multi-level phase change memory applications. Crystallization of this alloy was shown to occur in the temperature range of 180 °C–280 °C, with activation energy >2 eV. X-ray diffraction (XRD) from annealed alloy films indicates the formation of two crystalline phases, (i) an Al-doped A7 antimony phase, and (ii) a stable cubic AlSb phase. In-situ XRD analysis of these films show the AlSb phase crystalizes at a much higher temperature as compared to the A7 phase after annealing of the film to 650 °C. Mushroom cell structures formed with Al-Sb alloys on 120 nm TiN heater show a phase change material resistance switching behavior with reset/set resistance ratio >1000 under pulse measurements. TEM and in situ synchrotron XRD studies indicate fine nucleation grain sizes of ∼8–10 nm, and low elemental redistribution that is useful for improving reliability of the devices. These results indicate that Te-free Al-Sb binary alloys are possible candidates for analog PCM applications.

Pnictogens Allotropy and Phase Transformation during van der Waals Growth.

With their ns2 np3 valence electronic configuration, pnictogens are the only system to crystallize in layered van der Waals (vdW) and quasi-vdW structures throughout the group. Light pnictogens crystallize in the A17 phase, and bulk heavier elements prefer the A7 phase. Herein, we demonstrate that the A17 of heavy pnictogens can be stabilized in antimonene grown on weakly interacting surfaces and that it undergoes a spontaneous thickness-driven transformation to the stable A7 phase. At a critical thickness of ∼4 nm, A17 antimony transforms from AB- to AA-stacked α-antimonene by a diffusionless shuffle transition followed by a gradual relaxation to the A7 phase. Furthermore, the competition between A7- and A17-like bonding affects the electronic structure of the intermediate phase. These results highlight the critical role of the atomic structure and substrate-layer interactions in shaping the stability and properties of layered materials, thus enabling a new degree of freedom to engineer their performance.

Ultrafast crystallization in nanoscale phase change film of monobasic antimony

The property of Sb phase change thin film is investigated systematically. Sb thin film has an appropriate crystallization temperature (~165 °C) and data retention (110 °C for 10 year). Irradiated by laser pulse, the Sb thin film can achieve the crystallization operation within 6.05 ns. The monoclinic structure is confirmed by X-Ray Diffraction and the bond vibration mode of A7 phase is demonstrated by Raman scattering spectrum. With the crystallization, the band gap becomes narrow and the polycrystalline structure gradually forms. The Sb-based phase change memory cells are prepared. An ultrafast programing operation (~5 ns) and good endurance of 1.4 × 107 cycles are achieved. The results reveal that Sb thin film has promising potential in phase change memory application.

Pressure-induced phase transitions and superconductivity in a black phosphorus single crystal

We report a thorough study of the transport properties of the normal and superconducting states of black phosphorus (BP) under magnetic field and high pressure with a large-volume apparatus that provides hydrostatic pressure to induce transitions from the layered A17 phase to the layered A7 phase and to the cubic phase of BP. Quantum oscillations can be observed at P ≥ 1 GPa in both resistivity and Hall voltage, and their evolutions with pressure in the A17 phase imply a continuous enlargement of Fermi surface. A significantly large magnetoresistance (MR) at low temperatures is observed in the A7 phase that becomes superconducting below a superconducting transition temperature Tc ∼ 6-13 K. Tc increases continuously with pressure on crossing the A7 to the cubic phase boundary. The strong MR effect can be fit by a modified Kohler's rule. A correlation between Tc and fitting parameters suggests that phonon-mediated interactions play dominant roles in driving the Cooper pairing, which is further supported by our density functional theory (DFT) calculations. The change of effective carrier mobility in the A17 phase under pressure derived from the MR effect is consistent with that obtained from the temperature dependence of the quantum oscillations. In situ single-crystal diffraction under high pressure indicates a total structural reconstruction instead of simple stretching of the A17 phase layers in the A17-to-A7-phase transition. This finding helps us to interpret transport properties on crossing the phase transition under high pressure.

High pressure x-ray diffraction studies of the nanostructured Ge34Sb66 solid solution produced by mechanical alloying

A nanostructured solid solution of Ge34Sb66 produced by mechanical alloying was investigated by the angle-dispersive X-ray diffraction method from synchrotron radiation by using a diamond anvil cell technique. Pressure induced phase transitions were observed. Starting from A7 symmetry at ambient conditions, Ge34Sb66 was transformed to a high pressure host-guest structure at 12.0 GPa. The compressibility and bond distances of A7 phase were determined by the Rietveld refinement of the x-ray diffraction data. The lattice parameters and cell volume were described by Birch–Murnaghan equation-of-state. The Bragg peaks of the A7 structure show a strong reflection-indices-dependent line-broadening due to the applied pressure. Hence, they were simulated by Stephens's model. The host-guest X-ray pattern was simulated by the Pseudo-two-phase Rietveld method.

Raman spectroscopy of two-dimensional material under high pressure: Black phosphorus ultrathin film, phosphorene

Raman spectroscopy of phosphorene has been carried out at room temperature as a function of applied pressure. At ambient pressure, frequency of the and modes for the monolayer (1L) and bilayer (2L) showed different values from the bulk sample. The difference in frequency was observed both for diamond and hexagonal-BN substrates and was enhanced under high pressure. The 1L and 2L structures are stable up to around 15 and 11 GPa, respectively, while the trilayer (3L) phosphorene is transformed into the As-type (A7) structure at 4.5 GPa so as the bulk material For 2L phosphorene, the Eg and A1g modes of the A7 phase newly appeared in the low-frequency side of the mode while for 1L phosphorene a broad band was observed at 280 cm−1 above 15 GPa corresponding to a structural phase transition. Since the observed pressure effects do not depend on the substrate and the pressure medium, they are intrinsic to phosphorene. Theoretical analyses have been performed for the pressure dependence of the Ag modes by incorporating model geometry to simulate the two-dimensional system in the pressure medium.

Thermodynamic assessment of the Co–Fe–Sb system

The Co–Fe–Sb ternary system is critically assessed by CALPHAD (CALculation of PHAse Diagram) technique in the present work. The NiAs-type structure phase β-(Co,Fe)Sb with a wide non-stoichiometric range is described by three sublattices, (Sb)1/3(Co,Fe,Va)1/3 (Co,Fe,Va)1/3, following the extension of the sublattices of the phases β(CoSb) and β(FeSb) in the related binaries. The intermetallic compounds ζ(Co,Fe)Sb2 and η(Co,Fe)Sb3 are treated as linear stoichiometric compounds. The liquid, γ(A1)-Fcc_A1, α(A2)-Bcc_A2 and ε(A3)-Hcp_A3 phases are assumed to be substitutional solutions. The Rhombo_A7 phase is considered as the pure Sb phase. The order/disorder transformation between α(A2)-Bcc_A2 and α′(B2)-Bcc_B2 phases is described by a two-sublattice model (Co,Fe,Sb)0.5(Co,Fe,Sb)0.5. A set of self-consistent thermodynamic parameters of the Co–Fe–Sb system is obtained to fit satisfactorily a large amount of experimental data from literature. In the mean while, the Fe–Sb binary system is re-optimized critically.

The thermodynamic assessment of the ternary Co–Ni–Sb system

The Co–Ni–Sb ternary system is critically assessed using the CALPHAD technique on the basis of three optimized binaries, the Co–Ni, the Co–Sb and the Ni–Sb systems. The NiAs-type structure phase γ-(Co,Ni)Sb with a wide non-stoichiometric range is described by three sublattices, (Sb) 1/3(Co,Ni,V a) 1/3(Co,Ni,V a) 1/3, following the extension of the sublattices of the phases γ(CoSb) and γ(NiSb) in the related binary systems. The sublattice model is also used to describe the terminal ternary solid solution phases: three sublattices (Sb) 1/4(Co,Ni,V a) 1/2(Ni,V a) 1/4 for β(Co,Ni) 3Sb and two sublattices (Co,Ni) 3/4(Ni,Sb) 1/4 and (Co,Ni) 5/7(Ni,Sb) 2/7 for δ(Co,Ni) 3Sb and θ(Co,Ni) 5Sb 2 respectively. The intermetallic compounds ζ(Co,Ni)Sb 2 and η(Co,Ni)Sb 3 are treated as linear stoichiometric compounds. The liquid, the α Fcc_A1 and the ε Hcp_A3 phases are assumed to be substitutional solutions with Redlich–Kister polynomials for the expressions of the excess Gibbs energies. The Sb Rhombo_A7 phase is considered to be the pure Sb due to the very small and negligible solubilities of Co and Ni in it. A set of self-consistent thermodynamic parameters of the Co–Ni–Sb system is obtained according to eleven experimental isothermal sections and one isoplethal section. Most of the experimental data can be satisfactorily reproduced by the present thermodynamic modeling. The liquidus projection and the reaction scheme are also presented.

High-pressure Raman spectroscopy of antimony: As-type, incommensurate host-guest, and bcc phases

The vibrational dynamics of elemental solids that form incommensurate host–guest structures are of fundamental interest. High-pressure Raman scattering has been used to examine the vibrational spectrum of the group-V element Sb up to 33 GPa. A 1 g and E g phonons of the ambient pressure rhombohedral A7 phase display a marked decrease with pressure, i.e., prior to the transition to the tetragonal host–guest Sb-II phase at 8.6 GPa, via the monoclinic host-guest Sb-IV phase. The Raman spectrum of the incommensurate host-guest Sb-II phase, has five bands between 80 cm −1 and 200 cm −1 that increase with pressure. For the bcc structure stable above 28 GPa, we observe one weak disorder-induced band that increases with pressure.

Phase equilibria and thermodynamics of the Bi–Sb–Sn ternary system

Phase equilibria in the Bi–Sb–Sn ternary system have been studied experimentally as well as by using the calculation of phase diagram (CALPHAD) method. New experimental SEM-EDX and DTA results were used together with the literature data in this thermodynamic assessment. Solubility of bismuth in SbSn phase and ternary corrections for Liquid and Rhombo_A7 phase were established. Calculated values of thermodynamic functions were collated with recently published experimental thermodynamic results for liquid phase at 900 K. Calculated isothermal sections at 200 °C and 300 °C were compared with our own experimental results based on the SEM-EDX analysis of composition of phases. Three calculated vertical sections were successfully compared with the DTA results from this work and from literature data. Reasonable agreement between calculation and experimental data was observed in all cases.

Raman study of black phosphorus up to 13 GPa

Raman scattering experiments of a single crystal of black phosphorus have been carried out at room temperature in a wide pressure range from 0.55 to 13 GPa in order to study a band overlapped metallization at 1.5 GPa and the structural phase transitions. Five Raman-active modes in the orthorhombic (A11) phase were detected as a function of pressure. The softening of the B 1 g and B 3 g 1 modes was observed around 2 GPa. Above 4.7 GPa, these modes showed a sudden decrease in the intensity and two new broad bands appeared at 388 and 308 cm −1, corresponding to a transition to the rhombohedral (A7) phase. The broad bands which were assigned to the A 1 g and E g modes of the A7 phase, exhibited remarkable softening with pressure. The A7 phase coexisted with the A11 phase up to 7 GPa. The transition to the simple cubic phase was also observed at 10.5 GPa with vanishment of the A 1 g mode.

First principles investigation of the A7 to simple cubic transformation in As

We investigate by means of first principles variable cell shape molecular dynamics (FP-VCS-MD) the pressure induced A7 to simple cubic ( sc) phase transformation in As. The major experimental facts associated with this phase change and with the individual phases are very well reproduced, including the weakly first order character of the transformation. We also report a visual animation of the atomic motion and charge density evolution during the structural change caused by: (a) a sudden compression of the A7 phase to 30 GPa, and (b) a stepwise compression up to 30 GPa at T = 300 K. The goal of this animation was to shed light into the nature of the trajectories generated by the fictitious VCS-MD.

High-pressure atomic phases of solid nitrogen

Several candidate atomic phases for solid nitrogen at high pressures have been studied theoretically. The semimetallic A7 (\ensuremath{\alpha}-arsenic) phase is found to be the most energetically favored of the atomic phases studied which agrees with previous calculations. At extremely high pressures, this phase is found to deform continuously into the metallic simple cubic structure which is unstable at lower pressures. The possibility that a metallic simple tetragonal phase is metastable at lower pressures is discussed.

Strucrural Stability and Band Structure of the Metallic Phase of Phosphorus

The total energies of the rhombohedral A7 and the simple cubic (SC) phases of phosphorus are calculated within the local-density-functional formalism by using the norm-conserving pseudopotential. The calculated results successfully explain the phase transition from the A7 phase to the SC one under pressure. The calculated electronic structure of the metallic SC phase reveals that the bonding in this phase has an appreciable amount of covalency. It is also found that the stabilization of the SC structure under high pressure is due to be fact that the contribution of the electrostatic energy to the total energy increases on the compression. Moreover, it is shown that the absence of the core d -orbitals of a phosphorus atom is very important in explaining the difference between phosphorus and other VB-elements in the phase transition under pressure.

Revised results for the structural properties of arsenic.

We present new results for the total energy of arsenic in the simple-cubic and A7 structures, using a more accurate sampling of the Brillouin zone than in our previous work. In cases where direct comparison can be made, the calculated energy differences between the structures agrees with those of Mattheiss, Hamann, and Weber and of Chang and Cohen. The present results support our previous conclusions about the stability and the structure of the A7 phase as a function of volume.

Pressure-induced superconductivity of arsenic: Evidence for a structural phase transition

The group VB elements P, As, Sb and Bi are normally nonsuperconductors since they are either semiconductors or semimetals at ambient pressure. Under high pressure all of them are turned into superconductors. We have investigated the pressure dependence of T c for As up to a pressure of 28 GPa quantitatively for the first time. Employing a 3He/ 4He dilution refrigetor we have discovered that T c increases monotonously and steeply with pressure in the A7-rhombohedral phase from below 0.05 K at 10 GPa to a pronounced maximum at approximately 24 GPa with T c, max2.7 K. Our results are in strong disagreement with a previous publication. In addition, a faint resistance anomaly is observed in the same pressure range. It is concluded that both phenomena point to the occurence of a structural phase transition in agreement with an unpublished X-ray investigation by another group of authors. The continuous rise of T c with pressure in the A7 phase is so far a rather rare phenomenon among the B-group elements. Interestingly enough, quite similar behavior has also recently been reported for the A7 phases of P and Sb. We suggest that this unusual feature is related to a pressure-induced phonon softening in connection with a gradual weakening of the covalent bonds. The effect is believed to stand in close connection with the continuous reduction of the rhombohedral distortion of the crystal lattice towards simple cubic with increasing pressure. An interesting prediction for the shape of the melting curve of As at very high pressure can be made.

The pressure-induced black phosphorus to A7 (arsenic) phase transformation: An analysis using the concept of orbital symmetry conservation

Orbital correlation diagrams for the black phosphorus to A7 (arsenic) phase transformation for various points in the Brillouin zone are presented and analyzed in symmetry terms. The level doublings observed at the black phosphorus geometry for several points are directly attributable to the nonsymmorphic nature of the space group (Herring's theorem). Doublings observed at the A7 structure derive from the choice of a four-atom, rather than two-atom, unit cell to view the geometrical transformation. The space group requirements at the beginning and end points of the transformation process simply control the form of orbital correlation diagrams. In general, HOMOLUMO crossings are found at k = (12,α,β) where bonds are made and broken along the x direction, a result with simple analogies to more easily visualized one-dimensional problems.