Quasiperiodic interfaces: HREM observations of grain and phase boundaries

The atomic structures of internal interfaces have recently received considerable attention, not only because of their importance in determining many materials properties, but also because the atomic structure of many interfaces has become accessible to direct atomic-scale observation by modem HREM instruments. In this communication, several interface structures are examined by HREM in terms of their structural periodicities along the interface.It is well known that heterophase boundaries are generally formed by two low-index planes. Often, as is the case in many fcc metal/metal and metal/metal-oxide systems, low energy boundaries form in the cube-on-cube orientation on (111). Since the lattice parameter ratio between the two materials generally is not a rational number, such boundaries are incommensurate. Therefore, even though periodic arrays of misfit dislocations have been observed by TEM techniques for numerous heterophase systems, such interfaces are quasiperiodic on an atomic scale. Interfaces with misfit dislocations are semicoherent, where atomically well-matched regions alternate with regions of misfit. When the misfit is large, misfit localization is often difficult to detect, and direct determination of the atomic structure of the interface from HREM alone, may not be possible.

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Structural Relaxations At Metal / Metal Oxide Interfaces

MRS Proceedings ◽

10.1557/proc-238-763 ◽

1991 ◽

Vol 238 ◽

Cited By ~ 1

Author(s):

W. Mader

Keyword(s):

Metal Oxide ◽

Relative Strength ◽

Model Systems ◽

Misfit Dislocations ◽

Orientation Relationships ◽

Oxide Systems ◽

Oxide Interfaces ◽

Metal Metal ◽

High Resolution Tem ◽

Interacting Systems

ABSTRACTRecent work is reviewed on the structure of metal/metal oxide interfaces in model systems with well defined orientation relationships and boundary inclination. Structural relaxations established upon interface formation may be described as misfit dislocations which can be investigated using conventional and high resolution TEM. The conditions for obtaining informations at an atomistic scale using HRTEM are critically discussed. Specifically, geometrical restrictions are found to be critical in HRTEM study of {111} interfaces in fee metal -fee oxide systems. Different misfit dislocation networks at {100} interfaces in fee metal - fee oxide systems were observed which may be correlated to the relative strength of metal-anion and metal-cation bonds at the interface. In strongly interacting systems misfit dislocations can possess an equilibrium stand-off distance from the interface. In the system Nb-Al2O3 the interface is shown to be coherent by the registry of atomic columns adjacent to the interface. In this configuration energy is minimized by unbroken strong interfacial bonds and misfit localization in the elastically softer metal.

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High-resolution electron microscopy of metal/metal and metal/metal-oxide interfaces in the Ag/Ni and Au/Ni systems

Journal of Materials Research ◽

10.1557/jmr.1990.1995 ◽

1990 ◽

Vol 5 (9) ◽

pp. 1995-2003 ◽

Cited By ~ 42

Author(s):

Y. Gao ◽

Karl L. Merkle

Keyword(s):

Electron Microscopy ◽

High Resolution ◽

Metal Oxide ◽

High Resolution Electron Microscopy ◽

Misfit Dislocations ◽

Oxide Systems ◽

Lattice Statics ◽

Embedded Atom ◽

Resolution Electron ◽

Metal Metal

The atomic structures of heterophase interfaces with large misfits (>14% in Ag/Ni and Au/Ni) and with small misfits (∼2% in Ag/NiO and Au/NiO) have been studied by high-resolution electron microscopy (HREM). It is found that all interfaces are strongly faceted on (111) planes. This indicates that (111) interfaces have the lowest interfacial energy in both metal/metal and metal/metal-oxide systems. For the metal interfaces, this also agrees with determinations of interfacial energies by lattice statics calculations. The large misfit of Ag/Ni and Au/Ni interfaces is accommodated by misfit dislocations. Observations of misfit localization by HREM are in good agreement with images derived from computer simulation, based on relaxed structures, obtained in embedded atom calculations. All misfit dislocations at the Ag/Ni and Au/Ni interfaces lie exactly in the plane of the interfaces, while the dislocations at Ag/NiO and Au/NiO interfaces reside at a stand-off distance, 3 to 4 (111)Ag or (111)Au interplanar spacings from the interfaces.

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Direct Determination of Grain Boundary Atomic Structure in SrTiO3

MRS Proceedings ◽

10.1557/proc-357-115 ◽

1994 ◽

Vol 357 ◽

Cited By ~ 2

Author(s):

M.M. McGibbon ◽

N.D. Browning ◽

A.J. McGibbon ◽

M.F. Chisholm ◽

S.J. Pennycook

Keyword(s):

Grain Boundary ◽

Atomic Structure ◽

Light Element ◽

Direct Determination ◽

Atomic Scale ◽

Boundary Structure ◽

Structure Property ◽

Contrast Imaging ◽

Grain Boundary Structure ◽

Tilt Boundaries

AbstractIn the electroceramic SrTiO3 the grain boundary atomic structure governs a variety of electrical properties such as non-linear I-V characteristics. An understanding of this atomic structure-property relationship for individual grain boundaries requires a technique which probes both composition and chemical bonding on an atomic scale. Atomic structure models for [001] tilt boundaries in SrTiO3 bicrystals have been determined directly from experimental data, by combining high-resolution Z-contrast imaging to locate the cation columns at the boundary, with simultaneous electron energy loss spectroscopy to examine light element coordination at atomic resolution. In this paper we compare and contrast the grain boundary structure models of symmetric and asymmetric boundaries in SrTiO3.

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Atomic Scale Analysis of Cubic Zirconia Grain Boundaries

MRS Proceedings ◽

10.1557/proc-589-323 ◽

1999 ◽

Vol 589 ◽

Author(s):

E.C. Dickey ◽

X. Fan ◽

M. Yong ◽

S.B. Sinnott ◽

S.J. Pennycook

Keyword(s):

Grain Boundary ◽

Grain Boundaries ◽

Mirror Symmetry ◽

Fluorite Structure ◽

Atomic Scale ◽

Contrast Imaging ◽

Cubic Zirconia ◽

Atomic Structures ◽

Periodic Arrays ◽

Z Contrast

AbstractThe core structures of two symmetric tilt [001” grain boundaries in yttria- stabilized cubic zirconia are determined by Z-contrast imaging microscopy. In particular, near-σ=13 (510) and σ=5 (310) boundaries are studied. Both grain boundaries are found to be composed of periodic arrays of basic grain-boundary structural units, whose atomic structures are determined from the Z-contrast images. While both grain boundaries maintain mirror symmetry across the boundary plane, the 36° boundary is found to have a more compact structural unit than the 24° boundary. Partially filled cation columns in the 24° boundary are believed to prevent cation crowding in the boundary core. The derived grain boundary structural models are the first developed for ionic crystals having the fluorite structure

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Atomic Structure of Ag/Ni Interfaces

MRS Proceedings ◽

10.1557/proc-183-39 ◽

1990 ◽

Vol 183 ◽

Cited By ~ 7

Author(s):

Y. Gao ◽

K. L. Merkle

Keyword(s):

Atomic Structure ◽

High Resolution Electron Microscopy ◽

Lattice Parameter ◽

Special Technique ◽

Misfit Dislocations ◽

Lattice Statics ◽

Embedded Atom ◽

Resolution Electron ◽

Small Lattice ◽

Model Structures

AbstractWhile in heterophase systems of small lattice parameter differences, misfit dislocations are often formed at the interface, it is not known, whether and in which form, misfit localization occurs when the misfit is very large. The atomic structure of Ag/Ni interfaces (misfit 14%) was studied by high-resolution electron microscopy (HREM). A special technique was developed to prepare interface specimens suitable for HREM observations.Lattice statics calculations, using embedded-atom potentials, were performed to determine the structure and energies of Ag/Ni interfaces. The lowest interfacial energy was found for the cube-on-cube orientation and (111) interfaces. This is in agreement with the experimental observation, that all interfaces are strongly faceted with (111)Ag/(111)Ni facets.Misfit localization was found by HREM and computer simulation. The HREM observations will be compared to images derived from image simulations, based on model structures obtained from embedded atom calculations.

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Comparisons Of Observed And Simulated Atomic Structures Of Pd/NiO Heterophase Interfaces

MRS Proceedings ◽

10.1557/proc-295-109 ◽

1992 ◽

Vol 295 ◽

Cited By ~ 2

Author(s):

M. I. Buckett ◽

J. P. Shaffer ◽

Karl L. Merkle

Keyword(s):

Electron Microscopy ◽

High Resolution ◽

Rigid Body ◽

Atomic Structure ◽

High Resolution Electron Microscopy ◽

Boundary Plane ◽

Misfit Dislocations ◽

Atomic Structures ◽

Resolution Electron ◽

Image Simulations

AbstractHigh-resolution electron microscopy (HREM) and image simulations using the multislice algorithm have been used to study the atomic structure of a Pd/NiO (111) interface in an intemally oxidized sample. Samples prepared in this way result in cube-on-cube oriented or twin-related precipitates whose (111) interfaces exhibit a contrast modulation along the boundary plane in HREM images. Previous studies have reported that the observed structural period of this modulation corresponds qualitatively to the expected spacing if the boundary was composed of a network of misfit dislocations. In this study, rigid models of the (111) interface as viewed from the [110] direction were simulated using the EMS suite of programs. The questions we address are: (1) whether the terminating plane on the oxide side is made up of a Ni or an 0 layer, and (2) whether a rigid body translation normal to the interface exists. Finally, the results of the simulations are compared and contrasted to through-focal experimental images to investigate the origin of the contrast modulations and their possible relation to the extent of the misfit localization in these systems.

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Atomic structure of stair rod misfit dislocations at the GaAs on Si(100) interface

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100174849 ◽

1990 ◽

Vol 48 (4) ◽

pp. 342-343

Author(s):

D. Gerthsen

Keyword(s):

Lattice Constant ◽

Atomic Structure ◽

Spherical Aberration ◽

Stacking Faults ◽

Misfit Dislocations ◽

Gaas On Si ◽

Thermal Expansion Coefficients ◽

Transmission Electron ◽

Intersection Line ◽

Oriented Parallel

The prospect of technical applications has induced a lot of interest in the atomic structure of the GaAs on Si(100) interface and the defects in its vicinity which are often studied by high resolution transmission electron microscopy. The interface structure is determined by the 4.1% lattice constant mismatch between GaAs and Si, the large difference between the thermal expansion coefficients and the polar/nonpolar nature of the GaAs on Si interface. The lattice constant mismatch is compensated by misfit dislocations which are characterized by a/2<110> Burgers vectors b which are oriented parallel or inclined on {111} planes with respect to the interface. Stacking faults are also frequently observed. They are terminated by partial dislocations with b = a/6<112> on {111} planes. In this report, the atomic structure of stair rod misfit dislocations is analysed which are located at the intersection line of two stacking faults at the interface.A very thin, discontinous film of GaAs has been grown by MBE on a Si(100) substrate. Fig.1.a. shows an interface section of a 27 nm wide GaAs island along [110] containing a stair rod dislocation. The image has been taken with a JEOL 2000EX with a spherical aberration constant Cs = 1 mm, a spread of focus Δz = 10 nm and an angle of beam convergence ϑ of 2 mrad.

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Ab initio molecular dynamics and materials design for embedded phase-change memory

npj Computational Materials ◽

10.1038/s41524-021-00496-7 ◽

2021 ◽

Vol 7 (1) ◽

Cited By ~ 1

Author(s):

Liang Sun ◽

Yu-Xing Zhou ◽

Xu-Dong Wang ◽

Yu-Han Chen ◽

Volker L. Deringer ◽

...

Keyword(s):

Phase Change ◽

High Performance ◽

Materials Design ◽

Atomic Scale ◽

Ab Initio Molecular Dynamics ◽

Core Material ◽

Structure Property ◽

Switching Speed ◽

Atomic Structures ◽

Memory Applications

AbstractThe Ge2Sb2Te5 alloy has served as the core material in phase-change memories with high switching speed and persistent storage capability at room temperature. However widely used, this composition is not suitable for embedded memories—for example, for automotive applications, which require very high working temperatures above 300 °C. Ge–Sb–Te alloys with higher Ge content, most prominently Ge2Sb1Te2 (‘212’), have been studied as suitable alternatives, but their atomic structures and structure–property relationships have remained widely unexplored. Here, we report comprehensive first-principles simulations that give insight into those emerging materials, located on the compositional tie-line between Ge2Sb1Te2 and elemental Ge, allowing for a direct comparison with the established Ge2Sb2Te5 material. Electronic-structure computations and smooth overlap of atomic positions (SOAP) similarity analyses explain the role of excess Ge content in the amorphous phases. Together with energetic analyses, a compositional threshold is identified for the viability of a homogeneous amorphous phase (‘zero bit’), which is required for memory applications. Based on the acquired knowledge at the atomic scale, we provide a materials design strategy for high-performance embedded phase-change memories with balanced speed and stability, as well as potentially good cycling capability.

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