An intermediate orthorhombic ferroelectric phase has been found in oriented crystals of (0.92)Pb(Zn1/3Nb2/3)O3–0.08PbTiO3 (PZN-PT 92/8). Investigations have been performed by electrically induced polarization and strain methods, and reciprocal phase space mapping. The lattice parameters of this intermediate ferroelectric state have been shown to be equal to those predicted by the adaptive martensite theory. A hierarchy of symmetries is believed to exist on the mesoscale which are due to symmetry reductions by domain averaging.

Symmetry-adaptive ferroelectric mesostates in oriented Pb(BI1/3BII2/3)O3–PbTiO3 crystals

The thermosalient effect is an extremely rare propensity of certain crystalline solids for self-actuation by elastic deformation or by a ballistic event. Here we present direct evidence for the driving force behind this impressive crystal motility. Crystals of a prototypical thermosalient material, (phenylazophenyl)palladium hexafluoroacetylacetonate, can switch between five crystal structures (α-e) that are related by four phase transitions including one thermosalient transition (α↔γ). The mechanical effect is driven by a uniaxial negative expansion that is compensated by unusually large positive axial expansion (260 × 10(-6)  K(-1)) with volumetric expansion coefficients (≈250 × 10(-6)  K(-1)) that are among the highest values reported in molecular solids thus far. The habit plane advances at ~10(4) times the rate observed with non-thermosalient transitions. This rapid expansion of the crystal following the phase switching is the driving force for occurrence of the thermosalient effect.

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Colossal positive and negative thermal expansion and thermosalient effect in a pentamorphic organometallic martensite.

Publisher Summary This chapter discusses vibrational properties of shape-memory alloys. The crystal structure of a given material depends on its energy and also its entropy, which plays an increasingly important role as temperature is raised. In the case of nonmagnetic metallic systems, the entropy has two major contributions: the electronic contribution arising from electronic states near the Fermi level and the vibrational contribution, related to the vibrations of the atoms around their equilibrium positions in the crystal lattice. The present chapter is concerned with a class of bcc-based metallic alloys, commonly called shape-memory alloys. These materials have the unique property of being able to recover from large deformations by slightly increasing their temperature. The physical mechanism behind this effect is a diffusionless, first-order structural transition, usually referred to as the martensitic transition. Martensites form when a high-temperature crystalline phase undergoes a first-order phase transition to a lower symmetry crystalline phase. This structural phase transition usually proceeds by means of the nucleation of the new phase within the initial one, and may be induced either by changing the temperature or by applying a mechanical stress.

Vibrational properties of shape-memory alloys

An elastic organic crystal, 2,6-dichlorobenzylidine-4-fluoro-3-nitroaniline (DFNA), which also shows thermosalient behavior, is studied. The presence of these two distinct properties in the same crystal is unusual and unprecedented because they follow respectively from isotropy and anisotropy in the crystal packing. Therefore, while both properties lead from the crystal structure, the mechanisms for bending and thermosalience are quite independent of one another. Crystals of the low-temperature (a) form of the title compound are bent easily without any signs of fracture with the application of deforming stress, and this bending is within the elastic limit. The crystal structure of the a-form was determined (P2(1)/c, Z = 4, a = 3.927(7) angstrom, b = 21.98(4) angstrom, c = 15.32(3) angstrom). There is an irreversible phase transition at 138 degrees C of this form to the high-temperature beta-form followed by melting at 140 degrees C. Variable-temperature X-ray powder diffraction was used to investigate the structural changes across the phase transition and, along with an FTIR study, establishes the structure of the beta-form. A possible rationale for strain build-up is given. Thermosalient behavior arises from anisotropic changes in the three unit cell parameters across the phase transition, notably an increase in the b axis parameter from 21.98 to 22.30 angstrom. A rationale is provided for the existence of both elasticity and thermosalience in the same crystal. FTIR studies across the phase transition reveal important mechanistic insights: (i) increased pi...pi repulsions along 100] lead to expansion along the a axis; (ii) change in alignment of C-Cl and NO2 groups result from density changes; and (iii) competition between short-range repulsive (pi...pi) interactions and long-range attractive dipolar interactions (C-Cl and NO2) could lie at the origin of the existence of two distinctive properties.

Dual Stress and Thermally Driven Mechanical Properties of the Same Organic Crystal: 2,6-Dichlorobenzylidene-4-fluoro-3-nitroaniline.

When heated, single crystals of enantiomerically pure d- and l-pyroglutamic acid (PGA) are capable of recurring self-actuation due to rapid release of latent strain during a structural phase transition, while the racemate is mechanically inactive. Contrary to other thermosalient materials, where the effect is accompanied by crystal explosion due to ejection of debris or splintering, the chiral PGA crystals respond to internal strain with unprecedented robustness and can be actuated repeatedly without deterioration. It is demonstrated that this superelasticity is attained due to the low-dimensional hydrogen-bonding network which effectively accrues internal strain to elicit propulsion solely by elastic deformation without disintegration. One of the two polymorphs (β) associated with the thermosalient phase transition undergoes biaxial negative thermal expansion (αa = −54.8(8) × 10–6 K–1, αc = −3.62(8) × 10–6 K–1) and exceptionally large uniaxial thermal expansion (αb = 303(1) × 10–6 K–1). This second examp...

Perpetually self-propelling chiral single crystals.

Over recent years new experimental information from X-ray and neutron scattering, electron diffraction and imaging, computer simulation, and first-principles electronic structure calculations have greatly increased the amount of information we have on the microscopic mechanisms of structural transformations. In this paper, a particular class of martensitic transformation which exhibits both “soft modes” (out in the Brillouin zone, q ≠ 0) and elastic instabilities are discussed. The intent is to provide a heuristic review of some background in phonon theory, symmetry constrained elastic distortions and a pedagogical statistical mechanical model which is appropriate to NiAl and FePd and similar alloy transformations.

Y. Yamada

Papers

Some new aspects of first-order displacive phase transformations: Martensites