Physical metallurgy of Ti–Ni-based shape memory alloys

Potassium-sodium niobate lead-free piezoelectric materials: past, present, and future of phase boundaries.

Irreversible random sequential adsorption (RSA) on lattices, and continuum "car parking" analogues, have long received attention as models for reactions on polymer chains, chemisorption on single-crystal surfaces, adsorption in colloidal systems, and solid state transformations. Cooperative generalizations of these models (CSA) are sometimes more appropriate, and can exhibit richer kinetics and spatial structure, e.g., autocatalysis and clustering. The distribution of filled or transformed sites in RSA and CSA is not described by an equilibrium Gibbs measure. This is the case even for the saturation "jammed" state of models where the lattice or space cannot fill completely. However exact analysis is often possible in one dimension, and a variety of powerful analytic methods have been developed for higher dimensional models. Here we review the detailed understanding of asymptotic kinetics, spatial correlations, percolative structure, etc., which is emerging for these far-from-equilibrium processes.

https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=1405&context=physastro_pubs

Random and cooperative sequential adsorption

Halide perovskites have attracted much attention over the past 5 years as a promising class of materials for optoelectronic applications. However, compared to hybrid organic–inorganic perovskites, the study of their pure inorganic counterparts, like cesium lead halides (CsPbX3), lags far behind. Here, a catalyst-free, solution-phase synthesis of CsPbX3 nanowires (NWs) is reported. These NWs are single-crystalline, with uniform growth direction, and crystallize in the orthorhombic phase. Both CsPbBr3 and CsPbI3 are photoluminescence active, with composition-dependent temperature and self-trapping behavior. These NWs with a well-defined morphology could serve as an ideal platform for the investigation of fundamental properties and the development of future applications in nanoscale optoelectronic devices based on all-inorganic perovskites.

Solution-Phase Synthesis of Cesium Lead Halide Perovskite Nanowires

This Perspective outlines basic structural and optical properties of lead halide perovskite colloidal nanocrystals, highlighting differences and similarities between them and conventional II–VI and III–V semiconductor quantum dots. A detailed insight into two important issues inherent to lead halide perovskite nanocrystals then follows, namely, the advantages of defect tolerance and the necessity to improve their stability in environmental conditions. The defect tolerance of lead halide perovskites offers an impetus to search for similar attributes in other related heavy metal-free compounds. We discuss the origins of the significantly blue-shifted emission from CsPbBr3 nanocrystals and the synthetic strategies toward fabrication of stable perovskite nanocrystal materials with emission in the red and infrared parts of the optical spectrum, which are related to fabrication of mixed cation compounds guided by Goldschmidt tolerance factor considerations. We conclude with the view on perspectives of use of th...

Lead Halide Perovskite Nanocrystals in the Research Spotlight: Stability and Defect Tolerance

Neutron-scattering experiments have elucidated the mechanism of the successive phase transitions of CsPb${\mathrm{Cl}}_{3}$ at 47, 42, and 37\ifmmode^\circ\else\textdegree\fi{}C. These phase transitions are basically associated with the condensation of rotational modes of Pb${\mathrm{Cl}}_{6}$ octahedra around the three principal axes. The phase transition at 47\ifmmode^\circ\else\textdegree\fi{}C is caused by the condensation of the nondegenerate ${M}_{3}$ phonon mode at the zone boundary along the [110] direction of the cubic lattice. The axis of rotation of the Pb${\mathrm{Cl}}_{6}$ octahedra is along the [001] axis. The second phase transition at 42\ifmmode^\circ\else\textdegree\fi{}C is associated with the condensation of one of doubly degenerate modes ${Z}_{5}^{x}$ and ${Z}_{5}^{y}$ at the zone boundary along the [001] direction of the tetragonal lattice, which are derived from the triply degenerate ${R}_{25}$ mode in the cubic phase. The direction of the rotation axis is along the [100] axis when we assign ${Z}_{5}^{x}$ as the condensing mode. The third phase transition at 37\ifmmode^\circ\else\textdegree\fi{}C is caused by the condensation of the remaining ${Z}_{5}^{y}$ mode. The crystal systems and the space groups of three low-temperature phases were determined from the eigenvectors of the condensing modes as tetragonal ${D}_{4h}^{5}$, orthorhombic ${D}_{2h}^{17}$, and monoclinic ${C}_{2h}^{2}$ in the sequence of decreasing temperature. These are consistent with the previous experimental results on nuclear-quadrupole-resonance spectra of ${\mathrm{Cl}}^{\ensuremath{-}}$ ion. A phenomenological theory has been developed to explain these successive phase transitions caused by the condensation of the ${M}_{3}$ and ${R}_{25}$ zone-boundary phonons. The observed phase-transition schemes can be qualitatively explained by a suitable choice of the parameters in the free-energy expansion with respect to order parameters.

Neutron-scattering study on phase transitions of CsPb Cl 3

CsCl type NH 4 Br crystal has been treated as an Ising spin-phonon coupled system to explain the phase transitions associated with the orientational order of NH 4 + ions. The appearance of two ordered phases is explained as essentially due to the fact that the direct interaction between NH 4 + ions stabilizes parallel ordering while the indirect interaction through phonons stabilizes the antiparallel ordering. Based on a microscopic hamiltonian describing the spin-phonon coupled system, several experimental results have been analized such as (i) anomalous lattice expansions (ii) orientational order parameters (iii) spontaneous displacement of Br - ions (iv) P - T phase diagram (v) X-ray critical diffuse scattering. Semiquantitative agreements are obtained. Especially the property of correlated fluctuations of displacement of Br - ions seems to give a support on the validity of the microscopic model used.

A Microscopic Theory on the Phase Transitions in NH 4 Br —An Ising Spin Phonon Coupled System—

The critical dynamical behavior of the pseudospin-phonon coupled system has been investigated in connection with the structural phase transition of molecular crystals. Based on Onsager's equation of motion, explicit expressions for three correlation functions, spin-spin correlation, phonon-phonon correlation and spin-phonon correlation, are obtained. It has been shown that the characteristic of the spectra of these correlation functions shows wide variation depending on the values of two parameters: The ratio of the relaxation rate of reorientational motion of an isolated spin and the frequency of the phonon, and the ratio of the interaction parameter of direct spin-spin coupling and that of spin-phonon coupling. Especially, in `fast relaxation case', the spectrum of phonon-phonon correlation shows critical softening, whereas, in `show relaxation case' the spectrum gives `triple peak' structure and only the central (relaxational) mode shows critical slowing down leaving phonon side peaks unchanged.

Critical Dynamical Phenomena in Pseudospin-Phonon Coupled Systems

In the shape-memory alloy, NiTi:Fe, the martensitic transformation is well separated from the premartensitic (PM) phase and the latter can be studied in detail. X-ray diffraction studies in the PM phase reveal satellites near ${\stackrel{\ensuremath{\rightarrow}}{q}}_{I}=\frac{1}{3}(0,\overline{1},1)$ and ${\stackrel{\ensuremath{\rightarrow}}{\mathrm{q}}}_{\mathrm{II}}=\frac{1}{3}(1,1,1)$. For both $q$ values, the satellites are incommensurate and the surprising feature observed is that the incommensurate wave vector $\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\delta}}$ and the width of the satellites depend upon the Brillouin zone studied. This contradicts a simple condensed charge-density and/or lattice-wave description of the PM phase. If the shift is neglected, the structure in the PM phase can be explained by a condensation of three [$\ensuremath{\zeta}\overline{\ensuremath{\zeta}}0$]${\mathrm{TA}}_{2}$ modes with $\ensuremath{\zeta}=\frac{1}{3}$. In addition we propose a relationship between the well-known $\ensuremath{\omega}$ phase and the PM phase observed in this experiment and for other alloys.

X-ray investigation of the premartensitic phase in Ni 46.8 Ti 50 Fe 3.2

The phase transition that KMnF 3 exhibits at T c =184°K has been studied in detail by X-ray scattering. The anomalous scattering that accompanies the transition near T c is found to be anisotropic for q ∼0.05A -1 . For these wave vectors therefore, the Ornstein-Zernike approximation for the density-density correlation function cannot describe the critical properties. The temperature dependence of the square of the frequency of the soft $\varGamma_{25}$ mode at R is found to be reasonably well described by a Curie-law divergence. The space group of the condensed phase of KMnF 3 has been reexamined. The space group is found to be the tetragonal I 4/ m c m ( D 4 h 18 ) rather than the orthorhombic P b n m ( D 2 h 16 ) proposed by Beckman and Knox. The space groups of the condensed phases of KMnF 3 and SrTiO 3 are therefore identical.

Yasusada Yamada

Papers

Neutron-scattering study on phase transitions of CsPb Cl 3

A Microscopic Theory on the Phase Transitions in NH 4 Br —An Ising Spin Phonon Coupled System—

Critical Dynamical Phenomena in Pseudospin-Phonon Coupled Systems

X-ray investigation of the premartensitic phase in Ni 46.8 Ti 50 Fe 3.2

X-Ray Scattering and the Phase Transition of KMnF 3 at 184°K