Because the refractive index for hard x rays is slightly different from unity, the optical phase of a beam is affected by transmission through an object. Phase images can be obtained with extreme instrumental simplicity by simple propagation provided the beam is coherent. But, unlike absorption, the phase is not simply related to image brightness. A holographic reconstruction procedure combining images taken at different distances from the specimen was developed. It results in quantitative phase mapping and, through association with three-dimensional reconstruction, in holotomography, the complete three-dimensional mapping of the density in a sample. This tool in the characterization of materials at the micrometer scale is uniquely suited to samples with low absorption contrast and radiation-sensitive systems.

Indium selenide undergoes several reversible inter‐ and intrapolytypic phase transitions, above and below room temperature. A number of superstructures of the hexagonal (or rhombohedral) room temperature phase have been discovered by means of electron diffraction. The transition at 200 °C which corresponds to the α → β transformation is accompanied by intense non‐radial diffuse scattering, which is attributed to anisotropic transverse acoustic phonons. On cooling, a one‐dimensional deformation modulated structure is formed, which is interpreted as the frozen‐in configuration of the predominant vibration mode, which becomes soft below the transition temperature in the range 60 to 200 °C. This transition shows a large hysteresis, which is also found in thermal expansion. A new low temperature phase, which is presumably orthorhombic, is discovered on cooling below −125 °C. The structure of this phase can alternatively be described as being the result of pairing of indium ions or as the frozen‐in configuration of a longitudinal mode. Both phases exhibit a domain structure, which in the case of the α‐phase can be revealed by lattice resolution. Several high temperature superstructures are observed by means of electron diffraction. A direct relationship with the β‐phase is established and in a number of cases the superperiods are directly imaged.

The α → β phase transition in quartz and in the isostructural AlPO4 has been studied in situ in an electron microscope. A very high density of defects was observed close to the transition temperature. A diffraction contrast analysis allowed us to identify these defects as dauphiné twins. In the neighbourhood of the transition the twins form columnar domains parallel with the c‐axis; these triangular prisms are arranged following regular (hexagonal) networks, their mesh width becomes smaller close to the transition temperature. Particular “defects” in the networks are analysed. The domain walls (dauphiné twin boundaries) are constantly vibrating, thus transforming continuously the α1 orientation into α2. These observations are suggestive for the interpretation of the β phase as being a time average of α1 and α2 orientations. The particular geometry of the diffuse intensity observed in the electron diffraction patterns is related to the phonon mode that drives the transition from the α to the β phase. In AlPO4 also antiphase boundaries were observed as expected from the structure model.

No abstract

The formation of Ni silicides is studied by transmission electron microscopy during in situ heating experiments of 12 nm Ni layers on blanket silicon, or in patterned structures covered with a thin chemical oxide. It is shown that the first phase formed is the NiSi 2 which grows epitaxially in pyramidal crystals. The formation of NiSi occurs quite abruptly around 400°C when a monosilicide layer covers the disilicide grains and the silicon in between. The NiSi phase remains stable up to 800°C, at which temperature the layer finally fully transforms to NiSi 2. The monosilicide grains show different epitaxial relationships with the Si substrate. Ni 2 Si is never observed.