Disorder-Driven Pretransitional Tweed in Martensitic Transformations

Defying the conventional wisdom regarding first--order transitions, {\it solid--solid displacive transformations} are often accompanied by pronounced pretransitional phenomena. Generally, these phenomena are indicative of some mesoscopic lattice deformation that ``anticipates'' the upcoming phase transition. Among these precursive effects is the observation of the so-called ``tweed'' pattern in transmission electron microscopy in a wide variety of materials. We have investigated the tweed deformation in a two dimensional model system, and found that it arises because the compositional disorder intrinsic to any alloy conspires with the natural geometric constraints of the lattice to produce a frustrated, glassy phase. The predicted phase diagram and glassy behavior have been verified by numerical simulations, and diffraction patterns of simulated systems are found to compare well with experimental data. Analytically comparing to alternative models of strain-disorder coupling, we show that the present model best accounts for experimental observations.Comment: 43 pages in TeX, plus figures. Most figures supplied separately in uuencoded format. Three other figures available via anonymous ftp

Zigzag Filamentary Theory of Broken Symmetry of Neutron and Infrared Vibronic Spectra of YBa2Cu3O(6+x)

Filamentary high-temperature superconductivity (HTSC) theory differs fundamentally from continuous HTSC theories because it emphasizes self-organized, discrete dopant networks and does not make the effective medium approximation (EMA). Analysis of neutron and infrared (especially with c-axis polarization) vibrational spectra, primarily for YBa2Cu3O(6+x), within the filamentary framework, shows that the observed vibronic anomalies near 400 cm-1 (50 meV) are associated with curvilinear filamentary paths. these paths pass through cuprate chains and planes, as well as resonant tunneling centers in the BaO layers. The analysis and the data confirm earlier filamentary structural models containing ferroelastic domains of 3-4 nm in the CuO2 planes; it is these nanodomains that are responsible for the discrete glassy nature of both electronic and vibronic properties. Chemical trends in vibronic energies and oscillator strengths, both for neutron and photon scattering, that were anomalous in continuum models, are readily explained by the filamentary model.Comment: 45 pages, 17 figures, PD

EPR studies of phase transitions in cadmium calcium acetate hexahydrate as a function of different paramagnetic impurity-ion concentrations

The phase tt':lnsition in cadmium calcium acetate hexahydrate (CCDAH) has been studied in detail with electron paramagnetic resonance (J;PR) as a function of two different paramagnetic ion concentrations. namely. Cu:• and Mn:• ions. The change in transition temperature (1:!2-143 Kl with Cuz• ion concentrations is explained in terms of mean-field theory and a soft vibrational mode of the -Ca-Cd1 _ ,Cu,-Ca- chain along the c axis of the crystal. While the same theory can also explain our observed transition temperature ( 118-128 K) as a function of the Mn2• ion concentration in this crystal. it does not explain why the limiting value of the transition temperature (i.e .• 145 K) of CaCd1 -.,CuzCCH3C00)4 ·6H~O as x tends to zero, is strikingly different from the limiting value of ( -128..+ K) of CaCd1_.,Mn,(CH3C00)4·6H:O as x tends to zero. The same theory also successfully c:xplains the absence of any phase transition in isomorphous CaCu(CH 3C00)~·6H 20. The value of -dT~Id.t is significantly higher with Mn:• than with Cu!• in CCDAH. [50163-1829(97)01329-5

A review of mechanoluminescence in inorganic solids : compounds, mechanisms, models and applications

Mechanoluminescence (ML) is the non-thermal emission of light as a response to mechanical stimuli on a solid material. While this phenomenon has been observed for a long time when breaking certain materials, it is now being extensively explored, especially since the discovery of non-destructive ML upon elastic deformation. A great number of materials have already been identified as mechanoluminescent, but novel ones with colour tunability and improved sensitivity are still urgently needed. The physical origin of the phenomenon, which mainly involves the release of trapped carriers at defects with the help of stress, still remains unclear. This in turn hinders a deeper research, either theoretically or application oriented. In this review paper, we have tabulated the known ML compounds according to their structure prototypes based on the connectivity of anion polyhedra, highlighting structural features, such as framework distortion, layered structure, elastic anisotropy and microstructures, which are very relevant to the ML process. We then review the various proposed mechanisms and corresponding mathematical models. We comment on their contribution to a clearer understanding of the ML phenomenon and on the derived guidelines for improving properties of ML phosphors. Proven and potential applications of ML in various fields, such as stress field sensing, light sources, and sensing electric (magnetic) fields, are summarized. Finally, we point out the challenges and future directions in this active and emerging field of luminescence research