Volume 1, Issue 3-4
December 1979, pages 129-240
pp 129-170 December 1979
This paper presents a survey of soft modes and their relationship to structural phase transitions. After introducing the concept of a soft mode, the origin of softening is considered from a lattice-dynamical point. The Landau theory approach to structural transitions is then discussed, followed by a generalisation of the soft-mode concept through the use of the dynamic order-parameter susceptibility. The relationship of soft modes to broken symmetry is also examined. Experimental results for several classes of crystals are next presented, bringing out various features such as the co-operative Jahn-Tellev effect. The survey concludes with a discussion of the central peak, touching upon both the experimental results and the theoretical speculations.
pp 171-179 December 1979
The distinction between first-order and second-order phase transformations is brought out. The general characteristics of first-order transformations are described. The salient features of transformations controlled by long-range diffusion, short-range diffusion and by (diffusionless) shear mechanisms are discussed. Martensitic trasnformations, which belong to the shear category, occur in many metallic and non-metallic systems and deserve an intensive study.
pp 181-193 December 1979
Principal features of glass transition phenomenon are summarised. Various theoretical approaches such as free volume, entropy, energy surface, etc., have been briefly discussed.
pp 195-199 December 1979
The remarkable remanence effects observed in spin glasses are discussed. Some theoretical approaches and results are reviewed.
pp 201-206 December 1979
A review of the recent applications of renormalisation group techniques to the calculations of the critical properties of random magnetic systems is presented.
pp 207-212 December 1979
A brief review of one-dimensional conductors with emphasis on the synthetic criterion, the model systems of radical ion salts, charge transfer complexes, metal chain systems, polymeric conductors and A3B compounds is given.
pp 213-222 December 1979
In general a solid-solid phase boundary does not terminate in a critical point due to symmetry requirements. However, there exists systems like pure cerium metal and rare earth compounds like samarium sulphide where an isostructural solid-solid phase boundary terminates at a critical point. The critical point for the pressure-induced semiconductor-metal phase transition in SmS has been determined for the first time using the thermoelectric power data collected upto 12 kbar pressure and 850° C. These results are discussed on the basis of some theoretical models developed for the mixed valent compounds.
pp 223-225 December 1979
The nature of the NH4 network in the order-disorder transition of ammonium copper chloride dehydrate is examined.
pp 227-240 December 1979
A brief review is provided of the subject of nonequilibrium phase transitions occurring in various systems in different disciplines. All these systems exhibit a similar kind of ordering phenomena at a macroscopic level and can therefore be classified in terms of appropriate order parameters. Typical order parameter equations are discussed in the light of Landau’s phenomenological theory of phase transitions. The instabilities occurring in a few chemical reactions are discussed in some detail and the role of fluctuations pointed out. Mention is also made of the possibility of viewing some metallurgical phenomena as examples of nonequilibrium phase transitions.
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