Transmission electron microscopy study of a new silicon nitride phase

An abnormally large phase, which was found in the precursor-derived Si 3.0 B 1.1 C 5.3 N 3.0 ceramics after crystallization under a nitrogen pressure of 100bar at 1800C for 3h, has been characterized by means of transmission electron microscopy and electron-energy-loss spectroscopy (EELS). EELS analysis shows that this phase consists of only silicon and nitrogen, no other elements being detected. The analysis of selected-area diffraction and convergent-beam electron diffraction in conjunction with EELS reveals that the unknown phase is a variant of silicon nitride. It has a hexagonal structure with lattice parameters a =0.737nm and c =0.536nm, and the space group P62c. .     

Use of electron channelling contrast imaging to assess near-surface mechanical damage

The crystal structure of a cubic phase in the system Zn-Mg-Er has been solved by a combination of high-resolution electron microscopy and X-ray powder diffraction. This phase is considered to be related to that of the quasicrystalline phase. The structure consists of 448 atoms in the unit cell with lattice constant of a ₀ = 20.20Å and the space group is F43m. Important structural elements in the cubic structure are interpenetrating icosahedral units around Zn and Mg atoms and Frank-Kasper polyhedra around the Mg atoms. No giant icosahedral atomic cluster, such as the 136-atom Bergman cluster, was found in the stucture.     

Frank's 'cubic' hexagonal phase: An intermetallic cluster compound as an example

Frank introduced in 1965 the novel idea of projection from a four-dimensional cube to recover the points of a hexagonal lattice with a special c / a ratio of ( 3 - 2 ½. This was called a cubic hexagonal crystal, as there was a similarity to the conventional cubic crystals in that directions were perpendicular to planes with the same Miller-Bravais indices. While a number of crystals in the NiAs-Ni 2 In family have been reported with this special axial ratio, the number of atoms in the unit cell is small. As the first example of an intermetallic cluster compound, we identify µ-Al 4 Mn, µ-Al 4 Cr, Zn-Mg-Sm and a host of related intermetallics featuring a special aggregate of icosahedra as Frank's 'cubic' hexagonal phase or its variant. The metric is generated by the Friauf polyhedra and the icosahedral linkages and leads to a multimetric crystal and several interesting connections with hexagonal quasicrystals, hexagonal phases and derivative orthorhombic and lower-symmetry structures.     

Nanoprecipitates of icosahedral phase in quasicrystal-strengthened Mg-Zn-Y alloys

Extensive microstructural studies have been performed with respect to the formation of the icosahedral quasicrystalline phase and its relationship to other phases in Mg 95 Zn 4.2 Y 0.8 alloy. The icosahedral phase forms as an intergranular eutectic phase as well as precipitates in the matrix. The precipitates are nanosized (typically 50 nm) with a definite orientation relationship with the matrix, sharply faceted on twofold planes which are on the basal and prismatic planes of the matrix. The detailed crystallographic relationship with the matrix is described. The icosahedral phase is occasionally found to coexist with the cubic W-Zn 3 Mg 2 Y 3 phase with a definite crystallographic relationship.     

Local chemistry of a nanocrystalline high-strength Mg 97 Y 2 Zn 1 alloy

The local chemistry of a high-strength nanocrystalline Mg 97 Y 2 Zn 1 alloy produced by a rapidly solidified powder metallurgy process has been characterized by a three-dimensional atom probe (3DAP). The consolidated and extruded rod consists of hcp Mg grains of approximately 150nm size with their basal planes normal to the radial direction of the rod. 3DAP results revealed that the Y and Zn contents vary depending on the grains. Y- and Zn-depleted grains have a hcp structure, while Y- and Zn-enriched grains have a unique long-period stacking of the basal plane designated as 6H-Mg. In the 6H-Mg phase, the enrichment of Y and Zn occurs in one or two atomic layers in the unit cell. The chemical composition of the solute-enriched layer was estimated to be approximately 10 at.% Y and 3 at.% Zn using the 3DAP.     

Commensurate-incommensurate phase transition in muthmannite,AuAgTe2: first evidence of a modulated structure at low temperature

To study the temperature-dependent structural changes and to analyze the crystal chemical behavior of silver as a function of temperature, a crystal of muthmannite, AuAgTe₂, has been investigated by X-ray single-crystal diffraction methods at 300 K and 110 K. At room temperature, muthmannite was confirmed as belonging to the space group P2/m, while at low temperature (110 K) it undergoes a reversible commensurate–incommensurate phase transition with a modulation wave vector q = 0.215(1)a* + 0.379(2)c*. Muthmannite reconverts to the commensurate type upon returning to room temperature, thus indicating that the phase transition is completely reversible in character. The average structure of the low-temperature muthmannite remains monoclinic, space group P2/m, and shows only normal thermal compression over the entire temperature range investigated. Crystal-chemical characteristics are compared with published data on the other members of the system Au–Ag–Te. Speculations on the possible origin of the modulated structure at low temperature are also given.     

Analysis of a displacive superlattice in nickel arsenide

Abstract

The structural analysis by convergent-beam electron diffraction is described for a weak orthorhombic superlattice observed in both mineral and synthetic samples of NiAs. The three equivalent directions for the commensurate wavevector, q = ?(a? + b?), defined a domain structure within the hexagonal subcell. Within a single domain, the symmetry around the c axis was reduced from 6mm to 2mm. The internal structure of higher-order Laue-zone reflections in diffraction patterns recorded at 100 K was used to associate a specific phonon eigenvector or condensed soft mode with the atomic displacement pattern.     

Crystal structure of the ξ phase in the Al-Si-Mg-Fe system studied by electron channelling

We have re-examined the structure of the ~ phase in a quaternary alloy previously found to have the composition Al 8 Si 6 Mg 3 Fe. Electron probe microanalysis shows that the composition of the phase is actually Al 9 Si 5 Mg 3 Fe and, based on this new chemical formula, we propose a revision of the atomic positions which does not require any substitutional disorder within the hexagonal crystal unit cell with space group P ¥ 62 m (no. 189). These Wyckoff positions are as follows: for Al, 6i and 3f; for Si, 4h and 1b; for Mg, 3g; and for Fe, 1a. Results from electron channelling experiments are consistent with these new positions while ruling out those previously reported in the literature.     

Structure images of unaged martensite containing a high carbon content

Abstract

A highly tetragonal (large c/a ratio) martensite in a Fe-4 wt% Al-1·6 wt% Mn-2 wt% Calloy, which is not aged and considered to be as ‘fresh’ as possible, has been observed by high-resolution electron microscope. From the arrangement of atom rows in the [010] structure image and the corresponding electron diffraction pattern, it is concluded that carbon atoms are clustered on the (305) plane. The unit structure of these carbon-clustered regions is similar to that of the modulated structure observed in Fe-C alloys, although a periodic layer distribution of carbon atoms is not observed.     

Hexagonal surface structure during the first stages of niobium growth on sapphire (1120)

A detailed reflection high-energy electron diffraction study of the first stages of the niobium growth on (1120)s sapphire is presented for several substrate temperatures. It is shown that the niobium film exhibits an hexagonal surface structure when the deposited thickness is smaller than a critical value, which, depending on the substrate temperature, varies between 5 and 15 A. For thicknesses larger than this critical thickness, the surface hexagonal structure relaxes to the (110) bcc niobium structure. The hexagonal surface structure is observed for high substrate temperatures (820-410oC) but does not appear when the substrate temperature is 270oC. The epitaxial relationships between the substrate, the surface hexagonal structure of niobium and the cubic niobium phase are presented.     

Pressure-Induced transitions in amorphous TlxSe100−x alloys

Abstract

The electrical resistivity of bulk semiconducting amorphous Tl_xSe_100?x alloys with 0 ≤ x ≤ 25 has been investigated up to a pressure of 14 GPa and down to liquidnitrogen temperature by use of a Bridgman anvil device. All the glasses undergo a discontinuous pressure-induced semiconducting-to-metal transition. X-ray diffraction studies on the pressure-recovered samples show that the high-pressure phase is the crystalline phase. The pressure-induced crystalline products are identified to be a mixture of Se having a hexagonal structure with a = 4·37 Å and c = 4·95 Å and TlSe having a tetragonal structure with a = 8·0 Å and c = 7·0 Å     

Identification of a new tetragonal phase in the Nb-Ti-Al system

During the course of investigation of the phase equilibria in the Nb-Ti-Al system, a new phase, designated theta, was observed in three Nb-Ti-40at.%Al alloys following ageing at low temperatures (less than 1000 C). Using X-ray diffraction and transmission electron microscopy techniques, this phase was determined to have a bct structure, with lattice parameters a₀ = 5.106 A and c₀ = 28.168A, a space group of I 4₁/amd (number 141) and composition near 45at.%Nb25at.%Ti-30at.%Al. An orientation relationship between the new tetragonal phase and the gamma-TiAl phase of [101]_gamma//[120]_theta and (111)_gamma//(001)_theta was observed within the microstructure.     

Structure of a phase induced with Xe-ion irradiation-implantation in γ-TiAl

A phase transformation in γ-TiAl intermetallic alloy was found to be induced with 50keV Xe-ion irradiation-implantation at doses larger than 2.2 x 10¹⁸ ions m^-2 at room temperature. The structure and the chemical composition of the induced phase were investigated with high-resolution transmission electron microscopy and energy-dispersive X-ray spectroscopy. The zones of the induced phase have sizes up to about several tens of nanometres. The phase has a hexagonal structure with a = 0.286 nm and c = 0.462nm. The crystallographic orientation relationship between the phase (P) and the gamma-TiAl matrix is (001)_P//(111)_γ and \[100]_P//[011]_γ. The \[Al]/[Ti] atomic composition ratio in the phase is analysed to be 56/44, slightly different from that of the matrix, 51/49. These results suggest that the induced phase is an Al solid solution of α-Ti alloy phase, which has different structural parameters and chemical composition from those of the reported phase. It is suggested that the size of the ions is important in the phase transformation.     

A new modulated structure in GaN nanoparticles

A new modulated structure with a superlattice having parameters a = 2.209nm, b = 3.826nm, c = 1.037nm and f = g = n = 90 has been found in GaN nanoparticles synthesized by a dc arc plasma method. The nanoparticles transformed further into particles with holes at their centres under electron-beam irradiation during high-resolution electron microscopy observations. At the same time, Ga atoms were extruded on to the surface of the nanoparticles and formed an amorphous layer. A series of simulations of high-resolution images and electron diffraction patterns revealed that the modulation could be attributed to aggregations of N vacancies founded during the electron bombardment. Molecular mechanics calculations show that the aggregation of N vacancies is far more energetically favourable than that of Ga vacancies. The stability of the GaN particles is discussed.     

Sign determination of the 57 Fe electric-field gradient in Al-based crystals and icosahedral quasicrystals

Abstract

The icosahedral quasicrystals i-AIMn, isomorphically substituted by 28 at.% Fe or by a mixture of (CrFe) atoms, have been studied for the first time by in-field Mössbauer spectroscopy in order to determine the sign and asymmetry parameter of the dominant electric-field gradient (EFG) term. In addition, the orthorhombic o-Al(MnFe) and cubic α- and hexagonal β-Al(MnFe)Si crystalline phases have been studied. We show that the previous Mössbauer results are inadequate for determining whether there are two sites in the quasicrystalline structure in the ratio of the golden number. Our results for i-Al(MnFe) show that the dominant EFG is negative, with an asymmetry parameter of about 0·6. For i-Al(CrFe), essentially no deviations are found from the model of Czjzek or the Gaussian isotropic model. One crystalline phase, the hexagonal β phase, is found to have a very similar quadrupole effect to that found in i-Al(MnFe). In addition it is found that this phase undergoes a change which is at least partly of magnetic origin.     

Quasicrystalline Al-Fe phase formed by ion mixing

Abstract

A quasicrystalline Al-Fe phase has been formed by room-temperature ion mixing of AI-Fe multiple layers with no additional postannealing. The quasicrystalline phase was dispersed as grain in an amorphous matrix. The size of the grains was about 10-30nm, and the composition was determined to be Al₈₀Fe₂₀. The structure of the quasicrystalline phase was identified by calculating some 20 sharp diffraction rings in the selected-area diffraction pattern and found to be similar to that of the rapidly quenched AI-Mn quasicrystalline phase.     

Icosahedral quasicrystal in annealed ZnMgSc alloys

A new icosahedral quasicrystal has been found in Zn 85-x Mg x Sc 15 ( x = 3,4,5) alloys annealed at 922K for 5-100h. Electron diffraction and powder X-ray diffraction experiments indicate that this quasicrystal has a primitive icosahedral quasilattice (P type) and a six-dimensional lattice parameter a 6D =0.7115nm. The stoichiometric composition of the icosahedral quasicrystal was estimated to be close to Zn 80 Mg 5 Sc 15, since the icosahedral quasicrystal coexists in the x = 5 samples with small amounts of a MgZn 2 -type Laves phase and a Zn 17 Sc 3 -type cubic phase with lattice parameter a =1.3854nm. The latter is interpreted to be a 1/1-type approximant crystal of the icosahedral quasicrystal. The atomic cluster included in the Zn 17 Sc 3 -type cubic phase indicates a structural similarity between the Zn-Mg-Sc quasicrystal and the Cd-Yb (and Cd-Ca) icosahedral quasicrystals recently reported.     

Atomic and electronic structure of a carbon nitride compound prepared by magnetron sputtering

The atomic structure, chemical composition and chemical bonding state of a carbon nitride compound, synthesized by rf magnetron sputtering, have been studied by high-resolution electron microscopy, nanobeam diffraction and electron energy loss spectroscopy. It is revealed that the grain sizes of a crystalline compound in an amorphous matrix lie in the range 5-10 nm, and that the compound has a hexagonal structure with a = 0.324 nm and c = 0.404 nm. The C and N K edges are observed in the high-energy region of the electron-energy-loss spectrum. Only features corresponding to C sigma* bonds are found in the C K edge spectrum, which provides evidence for sp³ hybridization of C orbitals in the compound. Quantifying the electron-energy-loss spectra suggests that the compound is C₇N₁₀.     

Electron-microscope study of a non-stoichiometric phase in InSb heated in a vacuum

Abstract

When thin-foil specimens of InSb are heated in the vacuum of an electron microscope, a depletion of Sb takes place because of the preferential evaporation of Sb. This results in the formation of a novel non-stoichiometric (In-rich) solid phase having a cubic structure with a lattice parameter of 5·9 Å, which is about 10% smaller than that of InSb. It has been shown that the twin-orientation relationship exists between the matrix and the non-stoichiometric phase.     

Stacking of hexagonal layers in the structure of β-Mg2Al3

An analysis of cross-sections perpendicular to the main diagonal of a cubic elementary cell of a Samson structure–β-Mg₂Al₃ has been conducted. It has been proved that all skeleton atoms, i.e. which occupy their positions with probability of 100%, form a framework of hexagonal planes. These planes are a part of three domains shifted with respect to each other by one-third of the length of cube's diagonal. Space between domains is filled up by clusters with partially occupied atomic positions. For hexagonal domains, the elementary cell and positions of decorating atoms have been determined.