Modeling complex diffusion mechanisms in L1 2 -structured compounds

We report on a procedure developed to create stochastic models of hyperfine interactions for complex diffusion mechanisms and demonstrate its application to simulate perturbed angular correlation spectra for the divacancy and 6-jump cycle diffusion mechanisms in L1₂-structured compounds.     

Topologically appropriate coordinates for (Vzz, η) joint probability distributions

Inhomogeneous broadening (IHB) of hyperfine interactions in materials arises from a distribution of electric field gradients (EFGs) due to randomly distributed defects contributing non-uniformly to the EFG at probe sites. Hyperfine experiments reflect the inhomogeneous distribution of defects through the joint probability distribution function (PDF) of V_zz and η determined by the defect concentration, crystal structure, and defect sites in the crystal. Czjzek showed how to choose coordinates in the (V_zz, η) plane that are consistent with the physical constraints and ordering convention for these EFG parameters. Here we show how to transform to a new set of coordinates that decreases the distortion inherent in Czjzek’s representation. These new coordinates allow one to express the joint PDF for random distributions of defects in a form reasonably approximated by the product of two independent marginal distributions. This paper focuses on these topologically appropriate coordinates, with simple examples drawn from Czjzek’s work and from our simulations of point defects in cubic lattices as well as random amorphous distributions of defects. Detailed simulations have been carried out for IHB in cubic structures and point charge models relevant to perturbed angular correlation (PAC) experiments.     

Inhomogeneous broadening of PAC spectra with V zz and η joint probability distribution functions

The perturbed angular correlation (PAC) spectrum, G ₂(t), is broadened by the presence of randomly distributed defects in crystals due to a distribution of electric field gradients (EFGs) experienced by probe nuclei. Heuristic approaches to fitting spectra that exhibit such inhomogeneous broadening (ihb) consider only the distribution of EFG magnitudes V _zz , but the physical effect actually depends on the joint probability distribution function (pdf) of V _zz and EFG asymmetry parameter η. The difficulty in determining the joint pdf leads us to more appropriate representations of the EFG coordinates, and to express the joint pdf as the product of two approximately independent pdfs describing each coordinate separately. We have pursued this case in detail using as an initial illustration of the method a simple point defect model with nuclear spin I?=?5/2 in several cubic lattices, where G ₂(t) is primarily induced by a defect trapped in the first neighbor shell of a probe and broadening is due to defects distributed at random outside the first neighbor shell. Effects such as lattice relaxation are ignored in this simple test of the method. The simplicity of our model is suitable for gaining insight into ihb with more than V _zz alone. We simulate ihb in this simple case by averaging the net EFGs of 20,000 random defect arrangements, resulting in a broadened average G ₂(t). The 20,000 random cases provide a distribution of EFG components which are first transformed to Czjzek coordinates and then further into the full Czjzek half plane by conformal mapping. The topology of this transformed space yields an approximately separable joint pdf for the EFG components. We then fit the nearly independent pdfs and reconstruct G ₂(t) as a function of defect concentration. We report results for distributions of defects on simple cubic, face-centered cubic, and body-centered cubic lattices. The method explored here for analyzing ihb is applicable to more realistic cases.     

Comments on “CARS probe of RDX decomposition”

Recent CARS data from a solid propellant flame that purported to show resonances from the bending modes of HCN are analyzed to demonstrate that the spectral features in question are vibrational transitions of CO₂.     

Heavy Ion Beam Probe Plasma Diagnostic System for the Deep Magnetic Well Lite Device

A heavy ion beam probe plasma diagnostic system has been developed for the Laser Initiated Target Experiment (LITE) at UTRC. This is the first application of ion beam probing to a plasma confined by a strongly three dimensional magnetic field. The deep magnetic well, minimum-B field produced by the "baseball" magnet coil results in complex trajectories and severe defocusing of both the injected primary beam and detected secondary beam. Spatial resolution can be maintained by aperturing the entrance slit to the detector or installing compensating ion optics. The system is capable of space and time resolved measurements of plasma density and space potential near the central region of the mirror-confined plasma.     

Relating PAC damping to EFG fluctuation rates through the PAC relaxation peak

A perturbed angular correlation (PAC) experiment that measures dynamic damping also needs information about the fundamental quadrupole frequency to relate the damping as a function of temperature to the EFG fluctuation rate. When the experiment is unable to access slow electric field gradient (EFG) fluctuations that show the fundamental quadrupole frequency directly, one needs additional information to determine the hyperfine field parameters and thereby the connection between observed damping and EFG fluctuation rates. One way to solve this problem is to estimate the hyperfine parameters from the fluctuation rate for maximum damping (i.e. at the relaxation peak) or from the rate of maximum damping. This work relates both the maximum damping rate and the fluctuation rate at the relaxation peak to EFG magnitudes (or quadrupole frequencies) for five dynamic N-state symmetric models of fluctuating EFGs.