Comparison of XYZ Model Fitting Functions for 111Cd in In3La

The XYZ model describes the interaction between nuclear probes and an electric field gradient that fluctuates among three orthogonal directions. The model presents a means to calculate the perturbation function that represents spectra obtained using perturbed angular correlation spectroscopy. Three analytic approximations of the perturbation function have been developed previously, and they are evaluated in the present paper in the context of Cd jumping among In-lattice sites in In₃La.     

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.     

Stochastic PAC models for vacancy motions with trapping and detrapping

In order to explain PAC data for tetragonal zirconia at temperatures between 900 and 1300oC, we have developed a four-state stochastic model. The model simulates vacancies which trap and detrap at a PAC probe nucleus. While trapped, the vacancies hop around the probe in equivalent sites. The four states in this Winkler–Gerdau stochastic theory are three trapped states with equivalent electric field gradients (EFGs) of different orientations and a detrapped state with a weaker EFG whose axis of symmetry is oriented along the diagonal between the three trapped EFGs. There are three hopping rates in this model: w, the rate a trapped vacancy hops around the probe, w_D, the detrapping rate, and w_t, the trapping rate. We report results of calculations for values of these hopping rates implied by our tetragonal zirconia data, and we report heuristic fitting functions which summarize the computer results and can be used to fit data efficiently for a wide range of parameters. This revised version was published online in August 2006 with corrections to the Cover Date.     

Site Preferences of Hyperfine Impurities in Ni2Al3 Phases

The site occupation of indium solutes in Ni₂Al₃-type compounds was determined using perturbed angular correlation of gamma rays. Measurements were made for compositions on both sides of the stoichiometric composition. Sites were identified through their quadrupole interactions. For transition-metal (TM) rich aluminides and galliumides, indium solutes occupy only one of two inequivalent trivalent-metal sites (Al or Ga). For TM-poor aluminides, indium solutes tend to locate at noncrystallographic sites such as in grain boundaries. For TM-poor galliumides, the solutes occupy nickel sites while close to the stoichiometric composition they also occupy sites on an empty Ni-sublattice. Composition dependences of site preference for indium solutes in Pt₂Al₃ and Ni₂Ga₃ are examined in terms of a recently developed thermodynamic model. This revised version was published online in September 2006 with corrections to the Cover Date.     

Site Preference Model for Hyperfine Impurities in Compounds

A thermodynamic model for site preferences of solute atoms in ordered binary phases is applied in this paper for substitutional and interstitial sites of the Cu₃Au, or L1₂, structure. Site preferences are found to depend on formation energies of combinations of elementary point defects and on energies for transfer of solutes among different sites. The composition dependence in compounds having a wide phase field is examined in detail. A phenomenology of site preference behavior is outlined for regular lattice sites as well as for non-lattice sites such as grain boundaries. This revised version was published online in September 2006 with corrections to the Cover Date.     

Copper‐Catalyzed Deaminative Difluoromethylation

The difluoromethyl group (CF₂H) is considered to be a lipophilic and metabolically stable bioisostere of an amino (NH₂) group. Therefore, methods that can rapidly convert an NH₂ group into a CF₂H group would be of great value to medicinal chemistry. We report herein an efficient Cu‐catalyzed approach for the conversion of alkyl pyridinium salts, which can be readily synthesized from the corresponding alkyl amines, to their alkyl difluoromethane analogues. This method tolerates a broad range of functional groups and can be applied to the late‐stage modification of complex amino‐containing pharmaceuticals.     

Segregation of Solutes in Two-Phase Mixtures

Fractions of indium solutes in each phase of a mixture of two binary phases were measured using perturbed angular correlation of gamma rays. Measurements of phase fractions were made on Pd₃Ga₇–PdGa, PdGa–Pd₅Ga₃, and FeAl₂–FeAl mixtures as a function of composition. The phase fractions were analyzed using a thermodynamic model that takes into account differences between energies of solute atoms in the two phases. From the model, segregation coefficients were obtained for the systems studied. Also, earlier measurements on Ni₂Al₃–NiAl were reanalyzed. Large differences are found among the segregation coefficients. This revised version was published online in September 2006 with corrections to the Cover Date.     

Dynamics and thermodynamics of tetragonal zirconia determined by 111In/Cd TDPAC

For several years this research group has been using ¹¹¹In/Cd TDPAC to study oxygen vacancies in pure and lightly doped zirconia powders at temperatures up to 1400°C. This paper includes a brief survey of important results from those studies and some recent results. In particular, our new measurement showing negligible relaxation due to vacancy hopping above 600°C provides a major breakthrough in our ability to analyze PAC data and to determine accurately hopping and trapping rates and enthalpies for vacancies in tetragonal zirconia. This revised version was published online in August 2006 with corrections to the Cover Date.     

Polymorphic Phase Transformation in In2La and CeIn2

Nuclear quadrupole interactions of ¹¹¹Cd probes in In₂La and CeIn₂ were measured using perturbed angular correlation of gamma rays (PAC). Near room temperature, a single non-axially symmetric quadrupole interaction was observed in each compound, with ω ₀ = eQV _zz /ℏ = 78.8(2) Mrad/s and η = 0.312(1) for In₂La at 11°C and ω ₀ = 80(1) Mrad/s and η = 0.29(2) for CeIn₂ at 34°C. The observed non-axial symmetry is consistent with the reported CeCu₂ structure of the phases. The non-axially symmetric interactions were replaced completely by axially symmetric interactions (η = 0) at 524°C, with ω ₀ = 101.0(1) Mrad/s in In₂La and ω ₀ = 96.9(4) Mrad/s in CeIn₂. The change in symmetry is attributed to a polymorphic phase transformation. Based on symmetry information from the quadrupole interaction and on chemical arguments, it is proposed that high-temperature phases of In₂La and CeIn₂ both have the AlB₂ (C32) structure.