Phys. Rev. Lett., submitted.
O. Delaire, M.G. Kresch, M.S. Lucas, J. Munoz, J.Y.Y. Lin and B. Fultz
California Institute of Technology, W. M. Keck Laboratory, mail 138-78, Pasadena CA 91125
Inelastic neutron scattering was used to measure the phonon densities of states for pure V and solid solutions of V with 6 to 7at\% of Co, Nb, and Pt, at temperatures from 10 K to 1323 K. Pure V exhibits an anomalous anharmonic stiffening of phonons with increasing temperature. This anharmonicity is suppressed by Co and Pt, but not by iso-electronic Nb solutes. The changes of phonon frequency with alloying and with temperature both correlate to the decrease in electronic DOS at the Fermi level as calculated from first-principles DFT methods The effects of both temperature and alloying can be understood by the same systematics of how the electron-phonon interaction (EPI) decreases with d-band filling. These results show that the adiabatic EPI can influence the phonon thermodynamics at temperatures up to 1000 K, and that the simple Fermi description of metals can be insufficient when the EPI is significant.
J. Phys. Chem. B, 110 (45): 22732-22735 Nov. 16 2006.
J. L. Dodd, I. Halevy. and B. Fultz
California Institute of Technology, W. M. Keck Laboratory, mail 138-78, Pasadena CA 91125
The local electronic structure around iron ions in Li_{0.6}FePO_4 was studied by ^{57}Fe Mossbauer spectrometry at temperatures from 25-240 C. The equilibrium two-phase, triphylite plus heterosite, material was compared to a disordered solid solution that was obtained by quenching from a high temperature. Substantial electronic relaxations were found in the disordered sample compared to the two-phase sample at temperatures of 130 C and above. Fluctuations in the electric field gradient showed an approximately Arrhenius behavior, with an activation energy of 335 meV, and a prefactor of 5 x 10^{11} Hz. It is suggested that the spectral relaxations are caused by the motions of Li+ ions. A slight relaxation at 180 C in 10% of the two-phase material can be attributed to defects in the heterosite and triphylite phases.
Phys. Rev. Lett., in press.
O. Delaire and B. Fultz
California Institute of Technology, W. M. Keck Laboratory, mail 138-78, Pasadena CA 91125
Effects of alloying on the lattice dynamics of vanadium were investigated using inelastic neutron scattering. Phonon densities-of-states (DOS) were obtained for bcc solid solutions of V with 3d, 4d, and 5d transition metal solutes, from which vibrational entropies of alloying were obtained. A good correlation is found between the vibrational entropy of alloying and the electronegativity of the transition metal solutes. This trend is successful both across the 3d row and down columns of the periodic table. First-principles calculations on supercells matching the experimental compositions predicted a systematic charge redistribution in the nearest-neighbor shell around the solute atoms, also following the Pauling and Watson electronegativity scales. The systematic stiffening of the phonons is interpreted in terms of the modified screening properties of the electron density around the solutes.
Phys. Rev. Lett., 93 (18): Art. No. 185704 Oct. 29, 2004.
O. Delaire, T. Swan-Wood, B. Fultz
California Institute of Technology, W. M. Keck Laboratory, mail 138-78, Pasadena CA 91125
The phonon densities of states for pure vanadium and the solid solutions V-6.25% Ni, V-6.25% Pd, and V-6.25% Pt were determined from inelastic neutron scattering measurements. These solutes caused large stiffenings of the phonons compared to the phonons of the elemental constituents. The vibrational entropies of mixing 6.25% substitutional solutes into vanadium were, for Ni, Pd and Pt solutes, -0.082, -0.185 and -0.272 k_B/atom, respectively. For V-6.25% Pt, the negative vibrational entropy of mixing exceeds the conventional positive chemical entropy of mixing. This negative total entropy of mixing should extend to lower concentrations of Pt, and the effect on the BCC solvus line of the V-Pt phase diagram is discussed. The experimental data were inverted to obtain inter-atomic force constants by using a Born--von Karman model with an iterative optimization algorithm. The stiffening of bonds responsible for the decrease of entropy was mainly in first-nearest-neighbor solute-host bonds, and correlates in part with the solute metallic radius.
758 pages, 478 figures, 1,300 equations (Springer-Verlag, 2007).
Brent Fultz and James Howe
This book explains concepts of transmission electron microscopy (TEM) and x-ray diffractometry (XRD) that are important for the characterization of materials. The third edition has been updated to cover important technical developments, including the remarkable recent advances in resolution of the TEM. This edition is not substantially longer than the second, but all chapters have been updated and revised for clarity. A new chapter on high resolution STEM methods has been added. The book explains the fundamentals of how waves and wavefunctions interact with atoms in solids, and the similarities and differences of diffraction measurements with x-rays, electrons, or neutrons. Diffraction effects of crystalline order, defects, and disorder in materials are explained in detail. Both practical and theoretical issues are covered. This textbook can be used in an introductory-level or advanced-level course, since sections are identified by difficulty. Each chapter includes a set of problems to illustrate principles, and the extensive Appendix includes laboratory exercises.
Excerpts from the book in Adobe Acrobat .pdf format can be found
here.