Yildirim, Taner
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Publication Neutron Vibrational Spectroscopy and First-Principles Calculations of the Ternary Hydrides Li4Si2H(D) and Li4Ge2H(D): Electronic Structure and Lattice Dynamics(2007-12-17) Wu, Hui; Zhou, Wei; Udovic, Terrence J.; Rush, John J.; Yildirim, Tanner; Hartman, Michael R.; Bowman, Robert C.; Vajo, John J.Using combined neutron spectroscopy and first-principles calculations, we investigated the electronic structure and vibrational dynamics of the recently discovered class of ternary hydrides Li4Tt2H (Tt=Si and Ge). In these compounds, all hydrogen atoms are located in a single type of Li6-defined octahedral site. The Tt atoms form long-range Tt-Tt chains sandwiched between each Li6-octahedra layer. The Li-H interactions are strongly ionic, with bond lengths comparable to those in LiH. Our density functional theory calculations indicate that Li atoms transfer their electrons to both H and Tt atoms. Tt atoms within the Tt-Tt chain are bonded covalently. The electronic density of states reveals that both hydrides exhibit metallic behavior. The observed vibrational spectra of these hydrides are in good overall agreement with the calculated phonon modes. There is evidence of dispersion induced splitting in the optical phonon peaks that can be ascribed to the coupling of H vibrations within the Li6-octahedra layers.Publication Origin of the Exceptional Negative Thermal Expansion in Metal-Organic Framework-5 Zn4O(1,4-benzenedicarboxylate)3(2008-08-20) Zhou, Wei; Wu, Hui; Yildirim, Tanner; Simpson, Jeffrey R; Hight Walker, Angela R.Metal organic framework-5 (MOF-5)was recently suggested to possess an exceptionally large negative thermal-expansion coefficient. Our direct experimental measurement of the thermal expansion of MOF-5 using neutron powder diffraction, in the temperature range of 4 to 600 K, shows that the linear thermal-expansion coefficient is ≈−16×10−6 K−1. To understand the origin of this large negative thermal-expansion behavior, we performed first-principles lattice dynamics calculations. The calculated thermal-expansion coefficients within quasiharmonic approximation agree well with the experimental data. We found that almost all lowfrequency lattice vibrational modes (below ∼23 meV) involve the motion of the benzene rings and the ZnO4 clusters as rigid units and the carboxyl groups as bridges. These so-called “rigid-unit modes” exhibit various degrees of phonon softening (i.e., the vibrational energy decreases with contracting crystal lattice) and thus are directly responsible for the large negative thermal expansion in MOF-5. Initial efforts were made to observe the phonon softening experimentally.Publication Transition-Metal-Ethylene Complexes as High-Capacity Hydrogen-Storage Media(2006-11-30) Durgun, Engin; Ciraci, Salim; Zhou, Wei; Yildirim, Tanerwith two transition metals (TM) such as Ti. The resulting TM-ethylene complex then absorbs up to ten hydrogen molecules, reaching to gravimetric storage capacity of ~ 14 wt%. Dimerization, polymerizations, and incorporation of the TM-ethylene complexes in nanoporous carbon materials are also discussed. Our results are quite remarkable and open a new approach to high-capacity hydrogen-storage materials discovery.Publication Rotational and Vibrational Dynamics of Interstitial Molecular Hydrogen(2002-12-06) Yildirim, Taner; Harris, A. BrooksThe calculation of the hindered roton-phonon energy levels of a hydrogen molecule in a confining potential with different symmetries is systematized for the case when the rotational angular momentum J is a good quantum number. One goal of this program is to interpret the energy-resolved neutron time-of-flight spectrum previously obtained for H2C60. This spectrum gives direct information on the energy-level spectrum of H2 molecules confined to the octahedral interstitial sites of solid C60. We treat this problem of coupled translational and orientational degrees of freedom (i) by construction of an effective Hamiltonian to describe the splitting of the manifold of states characterized by a given value of J and having a fixed total number of phonon excitations, (ii) by numerical solutions of the coupled translation-rotation problem on a discrete mesh of points in position space, and (iii) by a group theoretical symmetry analysis. Results obtained from these three different approaches are mutually consistent. The results of our calculations explain several aspects of the experimental observations, but show that a truly satisfactory orientational potential for the interaction of an H2 molecule with a surrounding array of C atoms has not yet been developed.Publication Electronic, dynamical, and thermal properties of ultra-incompressible superhard rhenium diboride: A combined first-principles and neutron scattering study(2007-11-19) Zhou, Wei; Wu, Hui; Yildirim, TanerRhenium diboride is a recently recognized ultra-incompressible superhard material. Here we report the electronic (e), phonon (p), e-p coupling, and thermal properties of ReB2 from first-principles density-functional theory calculations and neutron scattering measurements. Our calculated elastic constants (c11 =641 GPa, c12=159 GPa, c13=128 GPa, c33=1037 GPa, and c44=271 GPa), bulk modulus (B≈350 GPa) and hardness (H≈46 GPa) are in good agreement with the reported experimental data. The calculated phonon density of states agrees very well with our neutron vibrational spectroscopy result. Electronic and phonon analysis indicates that the strong covalent B-B and Re-B bonding is the main reason for the super incompressibility and hardness of ReB2. The thermal expansion coefficients, calculated within the quasiharmonic approximation and measured by neutron powder diffraction, are found to be nearly isotropic in a and c directions and only slightly larger than that of diamond in terms of magnitude. The excellent agreement found between calculations and experimental measurements indicate that first-principles calculations capture the main interactions in this class of superhard materials, and thus can be used to search, predict, and design new materials with desired properties.Publication Structure and Interstitial Deuterium site of ß-phase ZrNi Deuteride(2007-02-09) Wu, Hui; Zhou, Wei; Udovic, Terrence J.; Rush, John J.; Yildirim, Tanner; Huang, Qingzhen; Bowman, Robert C.ß-ZrNiD1-x (for x≈0.1, near the ß-γ phase boundary) was found to possess a triclinic P1(overline) structure as determined by high-resolution neutron power diffraction. This is very different from the widely accepted orthohombic and distorted orthorhombic Cmcm structures previously proposed. In contrast to the single type of D site associated with these latter structures, the true ß-ZrNiD1-x structure contains two crystallographically distinct interstitial D sites: "Zr4Ni2" octahedral sites and "Zr4" tetrahedral sites, alternately ordered along the a direction. From first-principles calculations, the total energy of the P1(overline) structure was found to be ≈0.24 eV per unit cell lower than Cmcm-symmetry ZrNiD and could be rationalized in terms of different D local-bonding configurations and metal-deuterium interactions. Resultant phonon calculations based on this structure were also consistnet with the measured neutron vibrational spectrum.Publication Charge and Spin Ordering in the Mixed-Valence Compound LuFe2O4(2010-04-15) Harris, A Brooks; Yildirim, TannerLandau theory and symmetry considerations lead us to propose an explanation for several seemingly paradoxical behaviors of charge ordering (CO) and spin ordering (SO) in the mixed valence compound LuFe2O4. Both SO and CO are highly frustrated. We analyze a lattice gas model of CO within mean-field theory and determine the magnitude of several of the phenomenological interactions. We show that the assumption of a continuous phase transitions at which CO or SO develops implies that both CO and SO are incommensurate. To explain how ferroelectric fluctuations in the charge-disordered phase can be consistent with an antiferroelectric-ordered phase, we invoke an electron-phonon interaction in which a low-energy (20 meV) zone-center transverse phonon plays a key role. The energies of all the zone center phonons are calculated from first principles. We give a Landau analysis which explains SO and we discuss a model of interactions which stabilizes the SO state, if it is assumed commensurate. However, we suggest a high-resolution experimental determination to see whether this phase is really commensurate, as believed up to now. The applicability of representation analysis is discussed. A tentative explanation for the sensitivity of the CO state to an applied magnetic field in field-cooled experiments is given.Publication Spin Structures of Tetragonal Lamellar Copper Oxides(1994-06-06) Yildirim, Taner; Harris, A. Brooks; Entin-Wohlman, Ora; Aharony, AmnonThe spin Hamiltonian of tetragonal lamellar antiferromagnets is shown to contain several novel anisotropies. Symmetry allows bond-dependent anisotropic exchange interactions, which lead to (a) interplane mean-field coupling and (b) an in-plane anisotropy which vanishes classically but arises from quantum zero point energy (QZPE). A similar QZPE involving the interplane isotropic interaction prefers collinear spins. Adding also diploar anisotropy, the competition between all these effects explains for the first time the spin structures of many cuprates.Publication Three-Dimensional Ordering in bct Antiferromagnets Due to Quantum Disorder(1996-03-01) Yildirim, Taner; Harris, A. Brooks; Shender, Eugene FQuantum effects on magnetic ordering in body-centered-tetragonal antiferromagnets with only nearest-neighbor interactions are studied in detail using interacting spin-wave theory. The model consists of M noninteracting (in a mean-field sense) antiferromagnetic planes which together form a body-centered-tetragonal structure. We obtain the leading quantum correction of order 1/S from the zero-point energy for a system of M planes whose staggered moments have arbitrary orientations. The infinite degeneracy of the ground-state manifold of this system is partially removed by collinear ordering in view of effects previously calculated by Shender at relative order J2⊥/(J2S), where J, the antiferromagnetic in-plane exchange interaction, is assumed to dominate J⊥, the out-of-plane interaction which can be of either sign. We study the complete removal of the remaining degeneracy of the collinear spin structures by assigning an arbitrary sign σi (i=1,2,...M) to the staggered moment of the planes. Our result for the zero-point energy (for M>2) up to the sixth order in j=J⊥/J is E({σi}) =E1+CEG(j6/S)[-2σ1σ3-2σM−2σM+2∑i =1M-2σiσi+2-3∑i=1M-3σiσi+1σi+2σi+3], where C>0 and E1 are constants independent of the σ’s, and EG is the classical ground-state energy. (Here sums from i to j when j4 second-neighboring planes are antiferromagnetically coupled in the ground state and thus the three-dimensional spin structure cannot be described by a single wave vector, as is often assumed. At order j4, σ-dependent terms first appear at order 1/S3 and these also favor antiferromagnetic coupling of alternate planes.Publication Spin Dynamics of Trimers on a Distorted Kagome Lattice(2013-07-11) Harris, A. Brooks; Yildirim, TanerWe treat the ground state, elementary excitations, and neutron scattering cross section for a system of trimers consisting of three tightly bound spins 1/2 on a distorted kagome lattice, subject to isotropic nearest-neighbor (usually antiferromagnetic) Heisenberg interactions. The interactions between trimers are assumed to be weak compared to the intratrimer interactions. We compare the spin-wave excitation spectrum of trimers with that obtained from standard spin-wave theory and attribute the differences at low energy to the fact that the trimer formulation includes exactly the effects of intratrimer zero-point motion.