B. C. Sales

Oak Ridge National Laboratory, Oak Ridge, Florida, United States

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Publications (560)1675.87 Total impact

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    ABSTRACT: The interactions between electrons and phonons drive a large array of technologically relevant material properties including ferroelectricity, thermoelectricity, and phase-change behaviour. In the case of many group IV-VI, V, and related materials, these interactions are strong and the materials exist near electronic and structural phase transitions. Their close proximity to phase instability produces a fragile balance among the various properties. The prototypical example is PbTe whose incipient ferroelectric behaviour has been associated with large phonon anharmonicity and thermoelectricity. Experimental measurements on PbTe reveal anomalous lattice dynamics, especially in the soft transverse optical phonon branch. This has been interpreted in terms of both giant anharmonicity and local symmetry breaking due to off-centering of the Pb ions. The observed anomalies have prompted renewed theoretical and computational interest, which has in turn revived focus on the extent that electron-phonon interactions drive lattice instabilities in PbTe and related materials. Here, we use Fourier-transform inelastic x-ray scattering (FT-IXS) to show that photo-injection of free carriers stabilizes the paraelectric state. With support from constrained density functional theory (CDFT) calculations, we find that photoexcitation weakens the long-range forces along the cubic direction tied to resonant bonding and incipient ferroelectricity. This demonstrates the importance of electronic states near the band edges in determining the equilibrium structure.
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    ABSTRACT: Within the BaFe2As2 crystal lattice, we partially substitute thallium for barium and report the effects of interlayer coupling in Ba1-xTlxFe2As2 crystals. We demonstrate the unusual effects of magneto-elastic coupling and charge doping in this iron-arsenide material, whereby Neel temperature rises with small x, and then falls with additional x. Specifically, we find that Neel and structural transitions in BaFe2As2 (TN =Ts= 133 K) increase for x=0.05 (TN = 138 K, Ts = 140 K) from magnetization, heat capacity, resistivity, and neutron diffraction measurements. Evidence from single crystal X-ray diffraction and first principles calculations attributes the stronger magnetism in x=0.05 to magneto-elastic coupling related to the shorter intraplanar Fe-Fe bond distance. With further thallium substitution, the transition temperatures decrease for x = 0.09 (TN = Ts = 131 K), and this is due to charge doping. We illustrate that small changes related to 3d transition-metal state can have profound effects on magnetism.
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    ABSTRACT: Single crystals of (Nd1-xCex)2Fe14B are grown out of Fe-(Nd,Ce) flux. Chemical and structural analysis of the crystals indicates that (Nd1-xCex)2Fe14B forms a solid solution until at least x = 0.38 with a Vegard-like variation of the lattice constants with x. Refinements of single crystal neutron diffraction data indicate that Ce has a slight site preference (7:3) for the 4g rare earth site over the 4f site. Magnetization measurements show that for x = 0.38 the saturation magnetization at 400 K, a temperature important to applications, falls from 29.8 for the parent Nd2Fe14B to 27.6 (mu)B/f.u., the anisotropy field decreases from 5.5 T to 4.7 T, and the Curie temperature decreases from 586 to 543 K. First principles calculations carried out within density functional theory are used to explain the decrease in magnetic properties due to Ce substitution. Though the presence of the lower-cost and more abundant Ce slightly affects these important magnetic characteristics, this decrease is not large enough to affect a multitude of applications. Ce-substituted Nd2Fe14B is therefore a potential high-performance permanent magnet material with substantially reduced Nd content.
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    ABSTRACT: We investigate the magnetic properties of the series Sr2Ir1-xRuxO4 with neutron, resonant x-ray and magnetization measurements. The results indicate an evolution and coexistence of magnetic structures via a spin flop transition from ab-plane to c-axis collinear order as the 5d Ir4+ ions are replaced with an increasing concentration of 4d Ru4+ ions. The magnetic structures within the ordered regime of the phase diagram (x<0.3) are reported. Despite the changes in magnetic structure no alteration of the Jeff=1/2 ground state is observed. The behavior of Sr2Ir1-xRuxO4 is consistent with electronic phase separation and diverges from a standard scenario of hole doping. The role of lattice alterations with doping on the magnetic and insulating behavior is considered. The results presented here provide insight into the magnetic insulating states in strong spin-orbit coupled materials and the role perturbations play in altering the behavior.
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    B. C. Sales · M. A. Susner · B. S. Conner · J. Q. Yan · A. F. May ·
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    ABSTRACT: Compounds based on the Fe2P structure have continued to attract interest because of the interplay between itinerant and localized magnetism in a non-centrosymmetric crystal structure, and because of the recent developments of these materials for magnetocaloric applications. Here we report the growth and characterization of mm size single crystals of FeMnP0.8Si0.2. Single crystal x-ray diffraction, magnetization, resistivity, Hall and heat capacity data are reported. Surprisingly, the crystals exhibit itinerant antiferromagnetic order below 158 K with no hint of ferromagnetic behavior in the magnetization curves and with the spins ordered primarily in the ab plane. The room temperature resistivity is close to the Ioffe-Regel limit for a metal. Single crystal x-ray diffraction indicates a strong preference for Mn to occupy the larger pyramidal 3g site. The cation site preference in the as-grown crystals and the antiferromagnetism are not changed after high temperature anneals and a rapid quench to room temperature.
    Physical Review B 07/2015; 92(10). DOI:10.1103/PhysRevB.92.104429 · 3.74 Impact Factor
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    ABSTRACT: A high-pressure study of the lattice dynamics in the filled skutterudite Eu0.84Fe4Sb12 was carried out by means of x-ray powder diffraction and nuclear inelastic scattering. The anharmonicity of particular phonon modes was characterized by mode and element specific Grüneisen parameters. The large anharmonicity of the rattling optical mode that is hybridized with the acoustical phonons at ambient pressure is reduced at high pressure as the phonon modes decouple. This result suggests that anharmonic coupling between acoustic and optical phonon modes plays a central role in the reduced thermal conductivity.
    Physical Review B 06/2015; 91(22):224304. DOI:10.1103/PhysRevB.91.224304 · 3.74 Impact Factor

  • Chemistry of Materials 05/2015; 27(11):150522062627000. DOI:10.1021/acs.chemmater.5b01781 · 8.35 Impact Factor
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    ABSTRACT: Recent investigations have shown that Fe$_{1+y}$Te$_{1-x}$Se$_{x}$ can be made superconducting by annealing it in Se and O vapors. The current lore is that these chalcogen vapors induce superconductivity by removing the magnetic excess Fe atoms. To investigate this phenomenon we performed a combination of magnetic susceptibility, specific heat and transport measurements together with scanning tunneling microscopy and spectroscopy and density functional theory calculations on Fe$_{1+y}$Te$_{1-x}$Se$_{x}$ treated with Te vapor. We conclude that the main role of the Te vapor is to quench the magnetic moments of the excess Fe atoms by forming FeTe$_{m}$ (m $\geq$ 1) complexes. We show that the remaining FeTe$_{m}$ complexes are still damaging to the superconductivity and therefore that their removal potentially could further improve superconductive properties in these compounds.
    Physical Review B 05/2015; 91(6). DOI:10.1103/PhysRevB.91.060513 · 3.74 Impact Factor
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    ABSTRACT: The high temperature magnetic order in SrRu$_2$O$_6$ was studied by measuring magnetization and neutron powder diffraction with both polarized and unpolarized neutrons. SrRu$_2$O$_6$ crystallizes into the hexagonal lead antimonate (PbSb$_2$O$_6$, space group \textit{P}$\overline{3}$1\textit{m}) structure with layers of edge-sharing RuO$_6$ octahedra separated by Sr$^{2+}$ ions. SrRu$_2$O$_6$ orders at $T_N$=565\,K with Ru moments coupled antiferromagnetically both in-plane and out-of-plane. The magnetic moment is 1.30(2) $\mu_\mathrm{B}$/Ru at room temperature and is along the crystallographic \textit{c}-axis in the G-type magnetic structure. We performed density functional calculations with constrained RPA to obtain the electronic structure and effective intra- and inter-orbital interaction parameters. The projected density of states show strong hybridization between Ru 4$d$ and O 2$p$. By downfolding to the target $t_{2g}$ bands we extracted the effective magnetic Hamiltonian. We performed Monte Carlo simulations to determine the transition temperature as a function of inter- and intra-plane couplings and find weak inter plane coupling, 3\% of the intra-plane coupling, permits three dimensional magnetic order at $T_N$. As suggested by the magnetic susceptibility, two-dimensional correlations persist above $T_N$ due to the strong intra-plane coupling.
    Physical Review B 04/2015; 92(10). DOI:10.1103/PhysRevB.92.100404 · 3.74 Impact Factor
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    ABSTRACT: CrSiTe3 has attracted recent interest as a candidate single-layer ferromagnetic semiconductor, but relatively little is known about the bulk properties of this material. Here, we report single-crystal X-ray diffraction, magnetic properties, thermal conductivity, vibrational, and optical spectroscopies and compare our findings with complementary electronic structure and lattice dynamics principles calculations. The high temperature paramagnetic phase is characterized by strong spin-lattice interactions that give rise to glassy behavior, negative thermal expansion, and an optical response that reveals that CrSiTe3 is an indirect gap semiconductor with indirect and direct band gaps at 0.4 and 1.2 eV, respectively. Measurements of the phonons across the 33 K ferromagnetic transition provide additional evidence for strong coupling between the magnetic and lattice degrees of freedom. The Si-Te stretching and Te displacement modes are sensitive to the magnetic ordering transition, a finding that we discuss in terms of the superexchange mechanism. Spin-lattice coupling constants are also extracted.
    APL Materials 04/2015; 3(4):041515. DOI:10.1063/1.4914134 · 2.79 Impact Factor
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    ABSTRACT: Lattice distortions corresponding to Ba displacements with respect to the FeAs sublattice are revealed to break the room-temperature tetragonal symmetry in Ba(Fe1-x Cox )2 As2. The displacements yield twin domains of the size of ≈10 nm. The domain size correlates with the magnitude of the local Fe magnetic moment and its non-monotonic dependence on Co concentration. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    Advanced Materials 03/2015; 27(17). DOI:10.1002/adma.201404079 · 17.49 Impact Factor
  • M J Neish · M P Oxley · J Guo · B C Sales · L J Allen · M F Chisholm ·
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    ABSTRACT: MnFePSi compounds are promising materials for magnetic refrigeration as they exhibit a giant magnetocaloric effect. From first principles calculations and experiments on bulk materials, it has been proposed that this is due to the Mn and Fe atoms preferentially occupying two different sites within the atomic lattice. A recently developed technique was used to deconvolve the obscuring effects of both multiple elastic scattering and thermal diffuse scattering of the probe in an atomic resolution electron energy-loss spectroscopy investigation of a MnFePSi compound. This reveals, unambiguously, that the Mn atoms preferentially occupy the 3g site in a hexagonal crystal structure, confirming the theoretical predictions. After deconvolution, the data exhibit a difference in the Fe L_{2,3} ratio between the 3f and 3g sites consistent with differences in magnetic moments calculated from first principles, which are also not observed in the raw data.
    Physical Review Letters 03/2015; 114(10):106101. DOI:10.1103/PhysRevLett.114.106101 · 7.51 Impact Factor
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    ABSTRACT: High quality single crystals of BaFe12O19 were grown using the floating zone technique in 100 atm of flowing oxygen. Single crystal neutron diffraction was used to determine the nuclear and magnetic structures of BaFe12O19 at 4 K and 295 K. At both temperatures, there exist local electric dipoles formed by the off-mirror-plane displacements of magnetic Fe3+ ions at the bipyramidal sites. The displacement at 4 K is about half of that at room temperature. The temperature dependence of the specific heat shows no anomaly associated with long range polar ordering in the temperature range from 1.90 to 300 K. The inverse dielectric permittivity, 1/ε, along the c-axis shows a T 2 temperature dependence between 10 K and 20 K, with a significantly reduced temperature dependence displayed below 10 K. Moreover, as the sample is cooled below 1.4 K there is an anomalous sharp upturn in 1/ε. These features resemble those of classic quantum paraelectrics such as SrTiO3. The presence of the upturn in 1/ε indicates that BaFe12O19 is a critical quantum paraelectric system with Fe3+ ions involved in both magnetic and electric dipole formation.
    APL Materials 03/2015; 3(6). DOI:10.1063/1.4922934 · 2.79 Impact Factor
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    ABSTRACT: The vibrational behavior of heavy substitutional impurities $(M=\phantom{\rule{4.pt}{0ex}}\text{Ir},\phantom{\rule{4.pt}{0ex}}\text{Os})$ in ${\text{Fe}}_{1$-${}x}{M}_{x}\text{Si}\phantom{\rule{4.pt}{0ex}}(x=0,0.02,0.04,0.1)$ was investigated with a combination of inelastic neutron scattering (INS), transport measurements, and first-principles simulations. Our INS measurements on single crystals mapped the four-dimensional dynamical structure factor, $S(\mathbf{Q},E)$, for several compositions and temperatures. Our results show that both Ir and Os impurities lead to the formation of a weakly dispersive resonance vibrational mode, in the energy range of the acoustic phonon dispersions of the FeSi host. We also show that Ir doping, which introduces free carriers, leads to softened interatomic force constants compared to doping with Os, which is isoelectronic to Fe. We analyze the phonon $S(\mathbf{Q},E)$ from INS through a Green's-function model incorporating the phonon self-energy based on first-principles density functional theory simulations, and we study the disorder-induced lifetimes on large supercells. Calculations of the quasiparticle spectral functions in the doped system reveal the hybridization between the resonance and the acoustic phonon modes. Our results demonstrate a strong interaction of the host acoustic dispersions with the resonance mode, likely leading to the large observed suppression in lattice thermal conductivity.
    Physical Review B 03/2015; 91(9). DOI:10.1103/PhysRevB.91.094307 · 3.74 Impact Factor
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    ABSTRACT: We have examined the crystallographic and magnetic properties of single crystals of CrI3, an easily cleavable, layered and insulating ferromagnet with a Curie temperature of 61 K. Our X-ray diffraction studies reveal a first-order crystallographic phase transition occurring near 210-220 K upon warming, with significant thermal hysteresis. The low-temperature structure is rhombohedral (R3̅, BiI3-type) and the high-temperature structure is monoclinic (C2/m, AlCl3-type). We find evidence for coupling between the crystallographic and magnetic degrees of freedom in CrI3, observing an anomaly in the interlayer spacing at the Curie temperature and an anomaly in the magnetic susceptibility at the structural transition. First-principles calculations reveal the importance of proper treatment of the long-ranged interlayer forces, and van der Waals density functional theory does an excellent job of predicting the crystal structures and their relative stability. Calculations also suggest that the ferromagnetic order found in the bulk material may persist into monolayer form, suggesting that CrI3 and other chromium trihalides may be promising materials for spintronic and magnetoelectronic research.
    Chemistry of Materials 01/2015; 27(2):612-620. DOI:10.1021/cm504242t · 8.35 Impact Factor
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    ABSTRACT: The pressure induced crystalline collapse at 14.7 GPa of the semiconductor clathrate Sr8Ga16 Ge 30 and its polyamorphic structures are reported up to 35 GPa. In-situ total scattering measurements under pressure allow the direct microscopic inspection of the mechanisms associated with the pressure induced amorphization in these systems, as well as the structure of the recovered phase. It is observed that, between 14.7 and 35 GPa, the second peak in the structure factor function gradually disappears. Analysis of the radial distribution function extracted from those data indicates a systematic lengthening of the nearest-neighbor framework bonds. This feature is associated with gradual cage collapse and breakdown of the tetrahedral structure. This suggests a change in the local bonding in the high density amorphous form, similarly to that observed in other semiconductor clathrates and elemental silicon. Upon recovery from high pressure, the sample remains amorphous and, while there is some indication of the guest-host cage reforming, it does not seem that the tetrahedral coordination is fully reestablished. As such, the compression-decompression process in these systems gives rise to three distinct amorphous forms.
    Applied Physics Letters 01/2015; 106(2):021911. DOI:10.1063/1.4905224 · 3.30 Impact Factor
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    ABSTRACT: La$_{0.4}$Na$_{0.6}$Fe$_2$As$_2$ single crystals have been grown out of an NaAs flux in an alumina crucible and characterized by measuring magnetic susceptibility, electrical resistivity, specific heat, as well as single crystal x-ray and neutron diffraction. La$_{0.4}$Na$_{0.6}$Fe$_2$As$_2$ single crystals show a structural phase transition from a high temperature tetragonal phase to a low-temperature orthorhombic phase at T$_s$\,=\,125\,K. This structural transition is accompanied by an anomaly in the temperature dependence of electrical resistivity, anisotropic magnetic susceptibility, and specific heat. Concomitant with the structural phase transition, the Fe moments order along the \emph{a} direction with an ordered moment of 0.7(1)\,$\mu_{\textup{B}}$ at \emph{T}\,=\,5 K. The low temperature stripe antiferromagnetic structure is the same as that in other \emph{A}Fe$_{2}$As$_{2}$ (\emph{A}\,=\,Ca, Sr, Ba) compounds. La$_{0.5-x}$Na$_{0.5+x}$Fe$_2$As$_2$ provides a new material platform for the study of iron-based superconductors where the electron-hole asymmetry could be studied by simply varying La/Na ratio.
    Physical review. B, Condensed matter 01/2015; 91:024501. DOI:10.1103/PhysRevB.91.024501 · 3.66 Impact Factor
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    ABSTRACT: Atomic level spatial variability of electronic structure in Fe-based superconductor FeTe0.55Se0.45 (Tc = 15 K) is explored using current-imaging tunneling-spectroscopy. Multivariate statistical analysis of the data differentiates regions of dissimilar electronic behavior that can be identified with the segregation of chalcogen atoms, as well as boundaries between terminations and near neighbor interactions. Subsequent clustering analysis allows identification of the spatial localization of these dissimilar regions. Similar statistical analysis of modeled calculated density of states of chemically inhomogeneous FeTe1−x Sex structures further confirms that the two types of chalcogens, i.e., Te and Se, can be identified by their electronic signature and differentiated by their local chemical environment. This approach allows detailed chemical discrimination of the scanning tunneling microscopy data including separation of atomic identities, proximity, and local configuration effects and can be universally applicable to chemically and electronically inhomogeneous surfaces.
    APL Materials 12/2014; 2(12):120701. DOI:10.1063/1.4902996 · 2.79 Impact Factor
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    ABSTRACT: Hexagonal Fe3Sn has many of the desirable properties for a new permanent magnet phase with a Curie temperature of 725 K, a saturation moment of 1.18 MA/m. and anisotropy energy, K1 of 1.8 MJ/m(3). However, contrary to earlier experimental reports, we found both experimentally and theoretically that the easy magnetic axis lies in the hexagonal plane, which is undesirable for a permanent magnet material. One possibility for changing the easy axis direction is through alloying. We used first principles calculations to investigate the effect of elemental substitutions. The calculations showed that substitution on the Sn site has the potential to switch the easy axis direction. However, transition metal substitutions with Co or Mn do not have this effect. We attempted synthesis of a number of these alloys and found results in accord with the theoretical predictions for those that were formed. However, the alloys that could be readily made all showed an in-plane easy axis. The electronic structure of Fe3Sn is reported, as are some are magnetic and structural properties for the Fe3Sn2, and Fe5Sn3 compounds, which could be prepared as mm-sized single crystals.
    Scientific Reports 11/2014; 4:7024. DOI:10.1038/srep07024 · 5.58 Impact Factor
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    ABSTRACT: Structural and physical properties determined by measurements on large single crystals of the anisotropic ferromagnet MnBi are reported. The findings support the importance of magneto-elastic effects in this material. X-ray diffraction reveals a structural phase transition at the spin reorientation temperature $T_{SR}$ = 90 K. The distortion is driven by magneto-elastic coupling, and upon cooling transforms the structure from hexagonal to orthorhombic. Heat capacity measurements show a thermal anomaly at the crystallographic transition, which is suppressed rapidly by applied magnetic fields. Effects on the transport and anisotropic magnetic properties of the single crystals are also presented. Increasing anisotropy of the atomic displacement parameters for Bi with increasing temperature above $T_{SR}$ is revealed by neutron diffraction measurements. It is likely that this is directly related to the anisotropic thermal expansion in MnBi, which plays a key role in the spin reorientation and magnetocrystalline anisotropy. The identification of the true ground state crystal structure reported here may be important for future experimental and theoretical studies of this permanent magnet material, which have to date been performed and interpreted using only the high temperature structure.
    Physical Review B 11/2014; 90(17). DOI:10.1103/PhysRevB.90.174425 · 3.74 Impact Factor

Publication Stats

14k Citations
1,675.87 Total Impact Points


  • 1981-2015
    • Oak Ridge National Laboratory
      • • Materials Science and Technology Division
      • • Solid State Division
      • • Chemical Sciences Division
      Oak Ridge, Florida, United States
  • 2011
    • Los Alamos National Laboratory
      • Materials Science and Technology Division
      Лос-Аламос, California, United States
  • 2009
    • National High Magnetic Field Laboratory
      Tallahassee, Florida, United States
  • 2005-2009
    • University of Tennessee
      • • Department of Materials Science and Engineering
      • • Department of Physics & Astronomy
      Knoxville, TN, United States
  • 2006
    • American Physical Society
      Maryland, United States
  • 2003
    • North Carolina State University
      • Department of Materials Science and Engineering
      Raleigh, NC, United States
  • 2002
    • National Institute of Standards and Technology
      GAI, Maryland, United States
  • 1997-2002
    • The University of Tennessee Medical Center at Knoxville
      Knoxville, Tennessee, United States
  • 2000
    • University of California, San Diego
      • Department of Physics
      San Diego, California, United States
    • Iowa State University
      • Ames Laboratory
      Ames, Iowa, United States
  • 1998
    • Argonne National Laboratory
      • Division of Materials Science
      Lemont, Illinois, United States