Thermal Expansion of Martensitic A15 Superconductors: V

The martensite phase morphology of V3Si has been controlled by the application of appropriate stress fields to a single crystal. With this procedure, it is possible to transform the crystal to a single, tetragonal domain, enabling the thermal expansion coefficients for the tetragonal a and c axes to be measured, using high-resolution, capacitance dilatometry. Expansion anomalies were found at low temperatures, well below the superconducting critical temperature, for both the a and c axes. The tetragonality continues increasing on cooling at low temperatures, which, predicted by theory, should have been inhibited by the onset of superconductivity. In addition, anisotropy in thermal expansion is found up to 50 K, which is well above the conventional Ms temperature of 21 K

The aging effect in Au-Cd alloys: A Mössbauer spectroscopy study

The martensite aging effect, observed in a number of alloy systems and through which fundamental properties for particular materials are altered by aging in the martensite state, has attracted considerable interest in recent years. It is important to understand and control the effect, particularly in shape-memory alloys for which stability of properties is fundamental to applications. The most likely model, at least for the Au-Cd system in which it bas been most critically tested, is the “symmetry conforming short-range order” (SC-SRO) model first proposed by Ren and Otsuka. $^{197}$Au Mössbauer spectroscopy offers the possibility to observe the nature of defects associated with the gold sites. The spectra for single crystal and polycrystalline Au-Cd in both the “aged martensite" and “quenched austenite” conditions are presented and discussed. While some uncertainty in interpretation of the spectra remains, particularly in relation to any quadruple splitting to be expected from either Au atoms on Cd-nearest-neighbour sites or the martensite lattice distortion, the best single-line fits provide evidence for two defect types

Phonon anomalies in displacive phase transitions by surface X-ray scattering

Phonon anomalies in displacive phase transitions have long been suspected to aid the nucleation of the new phase. In materials with strong but incomplete phonon softening, the crystal surface might additionally play an important role, since the enhanced softening of associated surface phonons effectively lowers the nucleation barrier. X-ray scattering under grazing angles is discussed with regard to experimental studies of such phase transitions between 2 and 3 dimensions, as in this geometry all x-ray techniques provide information from a controlled depth below the surface adjustable from nanometers to microns. However, the sensitivity to premonitory phonon softening varies greatly with the scattering technique employed. It is argued that soft modes confined to a narrow interval in q need not show up in the Debye-Waller factor while giving a strong signal in thermal diffuse scattering. Synchrotron radiation data from Ni$_2$MnGa provide evidence for this behavior and confirm also the feasibility of depth-resolved studies of the [ $\zeta$$\zeta$0] TA$_2$ phonon softening

Influence of magnetic fields on structural martensitic transitions

We show evidence that a structural martensitic transition is related to significant changes in the electronic structure, as revealed in thermodynamic measurements made in high magnetic fields. The effect of the magnetic field is considered unusual as many influential investigations of martensitic transitions have emphasized that the structural transitions are primarily lattice dynamical and are driven by the entropy due to the phonons. We provide a theoretical framework, which can be used to describe the effect of the magnetic field on the lattice dynamics in which the field dependence originates from the dielectric constant