Crystal structure transitions, with high

High responsiveness is often found in a stability edge of a physical property, or near a structural phase transition. The commonly referred phase transitions are thermally driven structural instabilities in which ionic displace-ments/rearrangements occur in the crystal structure at a critical temperature Tc- In other words, at the crystal structure of the high temperature phase becomes unstable and the ions will form a new crystal structure with lower crystal symmetry below Tc- As a signature of structural phase transitions, at least one physical quantity vanishes, or appears, or becomes discontinuous. Phase transitions can be induced by temperature or field changes, and are the origin of anomalous responses in many crystalline systems that are considered passive functional materials. 

As is known in the case of binary systems, when their composition is close to equiatomic, d-ina-via metals form, as a rule, stable phases with Bl-type structures. But, as was observed by Vereshchagin and Kabalkina (1979) using a high-pressure treatment, it is possible to get B1 B2 (NaCl-CsCl)-type structural transitions with some oxides. The question of whether this could happen with d-met carbides was discussed by Ivanovsky et al (1988). These authors carried out LMTO band structure calculations for hypothetical TiC, VC and CrC compounds with a B2 structure. The lattice parameters were determined from the condition that the unit cell volumes of the CsCl- and NaCl-type phases were equal. In order to consider the influence of uniform isotropic compression the B2 VC calculations were carried out for crystal lattice volumes of 5 and 10% less than the equilibrium one. 

As is usually the case with a displacive phase transition, the crystal structure of the high-temperature (/3) form (Jeitschko, 1973) comes close to the average of the (two) different orientation states of the low-temperature (jS ) form. In both forms the Gd atoms are seven-coordinated and the Mo atoms tetrahedrally coordinated by oxygen atoms. The oxygen atoms have either two or three near metal neighbors (fig. 30.12). The positional differences between the /3 and /3 ... 

In all cases, broad diffuse reflections are observed in the high interface distance range of X-ray powder diffraction patterns. The presence of such diffuse reflection is related to a high-order distortion in the crystal structure. The intensity of the diffuse reflections drops, the closer the valencies of the cations contained in the compound are. Such compounds characterizing by similar type of crystal structure also have approximately the same type of IR absorption spectra [261]. Compounds with rock-salt-type structures with disordered ion distributions display a practically continuous absorption in the range of 900-400 cm 1 (see Fig. 44, curves 1 - 4). However, the transition into a tetragonal phase or cubic modification, characterized by the entry of the ions into certain positions in the compound, generates discrete bands in the IR absorption spectra (see Fig. 44, curves 5 - 8). 

Class II AF, with a SF field induced transition from the AF ground state dReversed coercivity in the isothermal hysteresis loop Expected structural motif, crystal structure not determined fNo CW behavior, 6 value obtained at high temperatures ECoercive field... 

The urethane-substituted polydiacetylenes exhibit thermo-chromic transition with low and high temperature crystal phases favoring acetylenic and butatriene backbone, respectively (4-6). Our interest in the application of epitaxial polymerization to diacetylenes has been the possibility of substrate control over orientation, structure, and the single crystal nature of thin films. 

---