Solid state chemistry of some polar intermetallic tetrelides of the rare-earth and alkaline-earth metals

Syntheses of both binary M[subscript]5X[subscript]3 hosts and M[subscript]5X[subscript]3Z interstitial compounds with the same Mn[subscript]5Si[subscript]3 structure type was extended to rare-earth metal tetrelides. The variety of interstitials Z that were incorporated into these host binaries allowed the modulation of the bonding and properties of the host compound without greatly altering or breaking up the structure. Investigations on the binaries also revealed that both La[subscript]5Ge[subscript]3 and La[subscript]5Pb[subscript]3 exist in the Mn[subscript]5Si[subscript]3-type, and La[subscript]5Sn[subscript]3 exists only in the W[subscript]5Si[subscript]3-type contrary to previous reports. Exploratory work on La[subscript]5Ge[subscript]3Z, La[subscript]5Pb[subscript]3Z, and La[subscript]5Sn[subscript]3Z resulted in the synthesis of more than 30 new interstitial compounds with structures related to the filled-Mn[subscript]5Si[subscript]3 and Cr[subscript]5B[subscript]3 structure types. The novel transition-metal-stabilized analogs Ca[subscript]5Pb[subscript]3Z were also synthesized and characterized. Based on XPS, results these compounds contain cationic interstitials in contrast to the La[subscript]5Ge[subscript]3Z and La[subscript]5Pb[subscript]3Z phases. Application of the Zintl concept to the bonding interpretation of these compounds was fairly successful;Although a large number of interstitial compounds having the filled-Mn[subscript]5Si[subscript]3 structure are known, ordered but partially filled examples have not been reported. A new class of compounds La[subscript]15Ge[subscript]9Z (Z = Mn-Cu, P, C, O) with a hexagonal √3 x √3 superstructure of the Mn[subscript]5Si[subscript]3 structure type was synthesized and characterized. Structural investigations also revealed that Ca[subscript]5.67Pb[subscript]3 had a related √3 x √3 superstructure with calcium atoms occupying two-thirds of the interstitial sites in an ordered manner. Physical property measurements and band structure calculations indicated that it was a semiconductor. The apparent electron-deficiency was resolved by the formation of Pb-Pb dimers;The compounds La[subscript]5Ge[subscript]3Z (Z = group 13 and 14 elements) were found to contain dimers in structures related to Zr[subscript]5Si[subscript]4, Sm[subscript]5Ge[subscript]4 and Gd[subscript]5Si[subscript]4 types. The structural relationships between the various 5:4 structures were discussed. Similarly, a novel compound RE[subscript]5Ru[subscript]2 (RE = La, Nd) was synthesized and characterized. The structure contains chains of Ru-centered edge-sharing La octahedra similar to those found in Pr[subscript]4I[subscript]5Ru. RE[subscript]5RuGe[subscript]2 (RE = La, Nd) become antiferromagnetic around 45°K;Physical property measurements, XPS experiments and extended-Huckel calculations on selected compounds provided information on the nature of their chemical bonding. ftn*DOE Report IS-T-1573. This work was performed under contract No. W-7405-Eng-82 with the U.S. Department of Energy

Correlated Quantum Transport of Density Wave Electrons

Recently observed Aharonov-Bohm quantum interference of period h/2e in charge density wave rings strongly suggest that correlated density wave electron transport is a cooperative quantum phenomenon. The picture discussed here posits that quantum solitons nucleate and transport current above a Coulomb blockade threshold field. We propose a field-dependent tunneling matrix element and use the Schrodinger equation, viewed as an emergent classical equation as in Feynman's treatment of Josephson tunneling, to compute the evolving macrostate amplitudes, finding excellent quantitative agreement with voltage oscillations and current-voltage characteristics in NbSe3. A proposed phase diagram shows the conditions favoring soliton nucleation versus classical depinning. (Published in Phys. Rev. Lett. 108, 036404 (2012).)Comment: 9 pages, 4 figures, (5 pages & 3 figures for main article), includes Supplemental Material with 1 figure. Published version: Physical Review Letters, vol. 108, p. 036404 (2012

Nine Hexagonal Ca5Pb3Z Phases in Stuffed Mn5Si3-Type Structures with Transition Metal Interstitial Atoms Z. Problems with Classical Valence States in Possible Zintl Phases

Ternary hexagonal Ae5Tt3Z phases have been obtained from high-temperature reactions (1000−1300 °C in Ta) only for Ae (alkaline-earth metal) = Ca, Tt (tetrel) = Pb, and Z = V, Cr, Mn, Fe, Co, Ni, Zn, Ru, or Cd. The hexagonal crystal structures (stuffed Mn5Si3-type, P63/mcm, Z = 2) were refined for Z = Mn and Fe (a = 9.3580(3), 9.3554(5) Å, c = 7.009(1), 7.009(1) Å, respectively). In contrast, Ca5Pb3Z for Z = Cu or Ag form only with a trigonal structure (P3̄c1, Z = 2, a = 9.4130(3) Å, c = 7.052(1) Å for Cu) in which regular displacements of only the linear strings of Ca1 atoms occur. The existence of these compounds stands in contrast to the nonexistence of all binary Ae5Tt3 products from Ca to Ba (Ae) and Si to Pb (Tt) with a Mn5Si3-type structure. Therefore, it once seemed attractive to consider the Z elements in these Ca5Pb3Z compounds as reducing agents (electron donors). The Mn and Fe structures appropriately exhibit greatly enlarged antiprismatic calcium cavities about Z. Other indications of relatively electron-poor environments around Fe are found in its properties, which include soft ferromagnetism with an elevated magnetic moment (6.3 μB) and a large Fe 3p3/2 binding energy relative to that in La5Ge3Fe, La15Ge9Fe, etc. The Ca5Pb3Mn phase exhibits metallic behavior (ρ295 = 135 μΩ cm) and temperature-independent Pauli paramagnetism. These properties are supported by ab initio band structure calculations for Ca5Pb3Mn, which show strong Ca−Pb bonding and a broad Pb-based band, with appreciable Ca−Mn and Ca−Pb bonding states at and above EF. Distortion of the Cu analogue gives strengthened Ca−Pb bonding and reduced Cu−Ca1 repulsions. A Zintl phase description of these compounds and some releated compounds in terms of closed Pb bands is not appropriate

A call to ownership : three volunteer experiences within a religious nongovernmental organization (RNGO)

This qualitative research study explores the transition from member to leader made by three volunteers of the same religious nongovernmental organization (RNGO), which is characterized by a community-based, grassroots structure, but is also part of a global organization. Three different volunteers, from diverse backgrounds within this group, underwent this leadership transition process within either the Montreal or Ottawa area. This study explored how each volunteer perceived and constructed meaning from his or her leadership experiences. Multiple case studies were used to uncover these perspectives. Over the course of the study, the question of how a new leader decides to commit to his or her new role emerged. A grounded theory was constructed from the cross-case analysis that revealed the phenomenon of a new leader's decision to take ownership of the RNGO's mission. While each participant was a member of the same religious nongovernmental organization, their individual experiences were studied from their own personal viewpoints within a naturalistic context. Through studying the individual experiences of new volunteer leaders, insight was gained on how religious organizations can enhance their leadership selection and development to increase the likelihood of leader commitment beyond the period of transition

Raman scattering study of (Kx_xSr1−x_{1-x})Fe2_2As2_2 (xx = 0.0, 0.4)

Polarized Raman spectra of non-superconducting SrFe$_2$As$_2$ and superconducting K$_{0.4}$Sr$_{0.6}$Fe$_2$As$_2$ ($T_c = 37$ K) are reported. All four phonon modes (A$_{1g}$ + B$_{1g}$ + 2E$_g$) allowed by symmetry, are found and identified. Shell model gives reasonable description of the spectra. No detectable anomalies are observed near the tetragonal-to-orthorhombic transition in SrFe$_2$As$_2$ or the superconducting transition in K$_{0.4}$Sr$_{0.6}$Fe$_2$As$_2$.Comment: 4 pages, 4 figures, 2 table