B. G. Aitken

Lawrence Berkeley National Laboratory, Berkeley, CA, United States

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Publications (73)121.9 Total impact

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    ABSTRACT: The structures of Ge-doped arsenic selenide glasses with Se contents varying between 25 and 90 at.% are studied using a combination of high-resolution, two-dimensional 77Se nuclear magnetic resonance (NMR) and Raman spectroscopy. The results indicate that, in contrary to the conventional wisdom, the compositional evolution of the structural connectivity in Se-excess glasses do not follow the chain-crossing model and chemical order is likely violated with the formation of a small but significant fraction of As-As bonds. The addition of As to Se results in a nearly random crosslinking of Se chains by AsSe3 pyramids and a highly chemically ordered network consisting primarily of corner-shared AsSe3 pyramids is formed at the stoichiometric composition. Further increase in As content, up to 40 at. % Se, results in the formation of a significant fraction of As4Se3 molecules with As-As homopolar bonds and consequently the connectivity and packing efficiency of the network decreases and anharmonic interactions increase. Finally, in the highly As-rich region with <40 at.% Se, the relative concentration of the As4Se3 molecules decrease rapidly and large clusters of As atoms connected via Se-Se-As and As-Se-As linkages dominate. These three composition regions with distinct structural characteristics and the corresponding mixing entropy of the Se environments are reflected in the appearance of multiple extrema in the compositional variation of a wide range of physical properties of these glasses, including density, glass transition temperature, thermal expansivity and fragility.
    The Journal of Physical Chemistry B 02/2014; · 3.61 Impact Factor
  • A.W. Mao, S.C. Currie, B.G. Aitken, S. Sen
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    ABSTRACT: Chalcogenide glasses in the system Na2Se-Ga2Se3-GeSe2 (NGGS) have been synthesized and characterized using Raman spectroscopy as well as density and glass transition temperature measurements. Results are compared to those from the analogous system BaSe-Ga2Se3-GeSe2 (BGGS) in an attempt to shed light on the comparative roles of Na and Ba as modifying elements in chalcogenide networks. In general, both NGGS and BGGS glasses exhibit the corner-sharing network of (Ga/Ge)Se4 tetrahedra that is characteristic of glasses containing Ga–Ge–Se. Some amount of tetrahedral edge-sharing and Ge–Ge bonding exists, but the concentration of Ge–Ge bonds in the network is heavily dependent on both the ratio R = Na2Se(BaSe)/Ga2Se3 and the type of the modifying element. The addition of Na2Se to an NGGS glass with R < 1 efficiently removes Ge–Ge bonding such that their concentration essentially goes to zero by the chemical threshold R = 1. In contrast, the addition of BaSe to a BGGS glass with R < 1 removes the Ge–Ge bonds less efficiently, such that a non-zero concentration still exists at R = 1. On the other hand, compared to BGGS glasses, increasing R results in a rapid drop in the glass transition temperature of NGGS glasses with R > 1 indicating that Na2Se is a much more efficient modifier that lowers the connectivity of the Ga–Ge–Se tetrahedral network via the formation of non-bridging Se atoms.
    Journal of Non-Crystalline Solids 01/2014; 404:91–97. · 1.72 Impact Factor
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    ABSTRACT: Structure of glasses in the pseudobinary system Ga2Se3-GeSe2 with Ga2Se3 content ranging from 6.3 to 30 mol % is investigated using a combination of Raman and multinuclear ((71)Ga, (77)Se) solid state nuclear magnetic resonance (NMR) spectroscopy. The results indicate that the structure of these glasses consists primarily of a corner sharing network of (Ge/Ga)Se4 tetrahedra with some fraction of edge-sharing GeSe4 tetrahedra and of ethane-like (Se3)Ge-Ge(Se3) units, in which the Ga, Ge, and Se atoms adopt coordination numbers of 4, 4, and 2, respectively. As expected, the concentration of metal-metal bonds increases with addition of Ga2Se3 as the glass structure becomes too deficient in Se to satisfy the tetrahedral coordination of both Ga and Ge by Se atoms alone. These metal-metal bonds are mostly limited to Ge-Ge homopolar bonds, indicating a violation of chemical order. At relatively high degrees of Se-deficiency, however, spectroscopic evidence suggests the formation of triply coordinated Se atoms as an alternate mechanism to accommodate the tetrahedral coordination of Ga and Ge atoms. This observation indicates a violation of the 8-N coordination rule and is reminiscent of oxygen triclusters in isoelectronic Al2O3-SiO2 glasses. Compositional variation of physical properties such as density, molar volume, optical band gap, glass transition temperature, and fragility are shown to be consistent with the proposed structural model.
    The Journal of Physical Chemistry B 12/2013; · 3.61 Impact Factor
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    ABSTRACT: The structure of chalcogenide glasses in the system BaSeGa2Se3GeSe2 is investigated using a combination of Raman spectroscopy, 71Ga and 77Se NMR spectroscopy and neutron scattering. The results of these experiments, when taken together, indicate that the structure of these glasses consists primarily of a corner-shared network of (Ga/Ge)Se4 tetrahedra with minor edge-sharing. Homopolar metal–metal bonds are detected in glasses even with BaSe:Ga2Se3 > 1, implying violation of chemical order; however, such bonding is primarily limited to Ge atoms, in the form of GeGe bonds. A comparison between these glasses and crystalline Ba2GeSe4, of their Raman and NMR spectroscopic signatures, suggest the formation of a modified network with non-bridging Se atoms upon increasing the BaSe content. Therefore, the structure of BGGS glasses undergoes network modification characteristic of oxides as well as continuous alloying and homopolar bond formation characteristic of non-oxide chalcogenides.
    Journal of Non-Crystalline Solids 09/2013; 375:40–46. · 1.72 Impact Factor
  • A.W. Mao, B.G. Aitken, S. Sen
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    ABSTRACT: Chalcogenide glasses in the system BaSe–Ga2Se3–GeSe2 have been synthesized and their optical band gap, density, glass transition temperature and fragility have been measured. The band gap and density exhibit nearly linear variation with Ba:Ga ratio at a fixed GeSe2 content. On the other hand, the glass transition temperature and fragility display either a clear maximum or a break in slope near 2Ba/Ga = 1. The compositional dependence of these physical properties are consistent with a scenario where the atomic structure of these glasses transforms across 2Ba/Ga = 1 that behaves as a chemical threshold between a Se-poor network with Ge/Ga–Ge/Ga bonding and a network characterized by Se–Se bonding and/or the formation of non-bridging Se atoms.
    Journal of Non-Crystalline Solids 06/2013; 369:38–43. · 1.72 Impact Factor
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    ABSTRACT: Pressure induced densification in a molecular arsenic sulfide glass is studied at ambient temperature using x-ray scattering, absorption and Raman spectroscopic techniques in situ in a diamond anvil cell. The relatively abrupt changes in the position of the first sharp diffraction peak, FSDP, and the pressure-volume equation of state near ∼2 GPa suggest a phase transition between low- and high-density amorphous phases characterized by different densification mechanisms and rates. Raman spectroscopic results provide clear evidence that the phase transition corresponds to a topological transformation between a low-density molecular structure and a high-density network structure via opening of the constituent As(4)S(3) cage molecules and bond switching. Pressure induced mode softening of the high density phase suggests a low dimensional nature of the network. The phase transformation is hysteretically reversible, and therefore, reminiscent of a first-order phase transition.
    The Journal of Chemical Physics 12/2012; 137(22):224503. · 3.12 Impact Factor
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    ABSTRACT: The crystalline, liquid and amorphous phase stabilities and transformations of the Ge1Sb2Te4 (GST124) alloy are investigated as a function of pressure and temperature using synchrotron diffraction experiments in a diamond anvil cell. The results indicate that the solid-state amorphization of the cubic GST124 phase under high pressure may correspond to a metastable extension of the stability field of the GST124 liquid along a hexagonal crystal-liquid phase boundary with a negative P-T slope. The internal pressures generated during phase change are shown to be too small to affect phase stability. However, they may be important in understanding reliability issues related to thermomechanical stress development in phase change random access memory structures.
    Phys. Rev. B. 07/2011; 84(1).
  • E L Gjersing, S Sen, B G Aitken
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    ABSTRACT: The structure of phosphorus selenide glasses with compositions close to the P(4)Se(3) stoichiometry with and without doping with a few atom % Ge has been investigated with Raman and (31)P NMR spectroscopic techniques. The results indicate that the structure of these glasses consists predominantly of P(4)Se(3) cage molecules. However, in spite of this structural similarity, doping with Ge results in a remarkably large increase in T(g). The dynamical behavior of the constituent P(4)Se(3) molecules in the Ge-free composition is investigated with a (31)P NMR hole-burning technique in the supercooled liquid state. These molecules perform large angle rotational reorientations near and above the glass transition with time scales similar to those expected for shear relaxation. Such coupling between molecular rotation and shear relaxation processes near T(g) is reminiscent of the dynamical behavior of organic molecular glass-forming liquids. However, this behavior is in stark contrast with the large temporal decoupling between molecular rotation and shear relaxation previously reported for a Ge-doped arsenic sulfide liquid that contained similarly structured As(4)S(3) cage molecules.
    The Journal of Physical Chemistry B 03/2011; 115(12):2857-63. · 3.61 Impact Factor
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    ABSTRACT: The crystalline, liquid and amorphous phase stabilities and transformations of the Ge1Sb2Te4 (GST124) alloy are investigated as a function of pressure and temperature using synchrotron diffraction experiments in a diamond anvil cell. The results indicate that the solid-state amorphization of the cubic GST124 phase under high pressure may correspond to a metastable extension of the stability field of the GST124 liquid along a hexagonal crystal-liquid phase boundary with a negative P-T slope. The internal pressures generated during phase change are shown to be too small to affect phase stability. However, they may be important in understanding reliability issues related to thermomechanical stress development in phase change random access memory structures.
    Physical review. B, Condensed matter 01/2011; 84(1). · 3.77 Impact Factor
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    ABSTRACT: The local coordination environments of Te atoms have been investigated in crystalline and glassy binary and ternary tellurides in the system Ge–As–Sb–Te using 125Te solid-state wideline nuclear magnetic resonance (NMR) spectroscopy. The average 125Te NMR chemical shifts in these materials range from 300 to 1050, 90 to 700 and − 2000 to − 4100 ppm for 2, 3 and 6-coordinated environments, respectively. Te atoms are predominantly 2-coordinated in binary Ge–Te, As–Te and ternary Ge–As–Te glasses. The 125Te NMR spectrum of the cubic Ge1Sb2Te4 phase with rock salt structure is consistent with a random distribution of Ge/Sb vacancies in the lattice. Besides the coordination number, the 125Te chemical shifts in these materials are also found to be sensitive to the chemical identity of the nearest neighbors. 125Te NMR spectroscopy shows significant future promise in its application as a technique complementary to diffraction and EXAFS in understanding the short-range structure of amorphous Ge–As–Sb tellurides.
    Journal of Non-Crystalline Solids 01/2011; 357:3036-3041. · 1.72 Impact Factor
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    ABSTRACT: High-energy x-ray diffraction is employed to study the atomic structure of bulk Ge(x)As(2x)Te(100-3x) glasses with compositions in the range 25 ≤ 3x ≤ 70. The coordination environments of Te atoms suggest significant violation of chemical order in these glasses. Analyses of the nearest-neighbor coordination environments and the parameters for the first sharp diffraction peak indicate that these telluride glasses are structurally and chemically more disordered as compared with their sulfide or selenide analogs. The compositional evolution of the structural parameters is shown to be consistent with the corresponding variation in molar volume and glass transition temperature.
    Journal of Physics Condensed Matter 10/2010; 22(40):405401. · 2.22 Impact Factor
  • E. L. Gjersing, S. Sen, B. G. Aitken
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    ABSTRACT: High-resolution 77Se MAS NMR spectroscopy has been conducted at 11.7 T to investigate the short-and intermediate- range structure and chemical order in binary GexSe100−x glasses with 5 ≤ x ≤ 33.33. Four distinct Se environments are observed for the first time, corresponding to Se−Se−Se and Ge−Se−Se linkages as well as Ge−Se−Ge sites where the Se atom is shared by two GeSe4 tetrahedra in either corner-sharing or edge-sharing configuration. Assignments of corner and edge-shared tetrahedra were made based on the 77Se MAS NMR spectrum of crystalline β-GeSe2. Analysis of the compositional variation of the relative concentrations of these Se sites indicates that the structure of GexSe100−x glasses in this composition range can be described as a randomly interconnected network of GeSe4 tetrahedra and chains of Se atoms. The implications of this structural model are discussed in relation to the composition dependence of the glass-forming ability and kinetic fragility of the corresponding parent liquids.
    Journal of Physical Chemistry C - J PHYS CHEM C. 01/2010; 114(18):8601-8608.
  • S. Sen, E.L. Gjersing, B.G. Aitken
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    ABSTRACT: Ternary GexAs2xTe100 − 3x glasses with 20 ≤ 3x ≤ 70 have been investigated to determine the compositional dependence of select physical properties. Molar volume, viscosity, optical band gap and thermophysical properties display a monotonic and often linear variation with the Te content in these glasses. Such compositional dependence is hypothesized to be due to the progressive replacement of a three-dimensional network of corner-shared Ge and As centered coordination polyhedra by –Te–Te–Te– chains as the Te content increases. This topologic change arises as Ge/As–Ge/As bonds in Te-deficient glasses are gradually replaced by Ge/As–Te bonds and ultimately the Te–Te bonds in Te-excess glasses. The compositional evolution of the Raman spectra of these glasses is shown to be consistent with this structural scenario. Furthermore, detailed analysis of the Raman spectra shows significant violation of chemical order in the form of the presence of Te–Te (Ge/As–Ge/As) bonds in Te-deficient (-excess) glasses. Such behavior in ternary Ge–As telluride glasses is unique in comparison with their sulfide and selenide counterparts.
    Journal of Non-Crystalline Solids 01/2010; 356:2083-2088. · 1.72 Impact Factor
  • E L Gjersing, S Sen, H Maekawa, B G Aitken
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    ABSTRACT: The dynamics of As2P2S8 quasi-molecular units caged in an As-S network in the supercooled chalcogenide liquid of composition (As2S3)90(P2S5)10 have been studied near the glass transition region (Tg=468<or=T<or=628 K) using 31P NMR line shape analysis and spin-lattice relaxation techniques. 31P NMR line shape analysis indicates the presence of isotropic rotational reorientation of As2P2S8 quasi-molecular units at frequencies on the order of tens of kilohertz at T<540 K. At higher temperatures, the time scale of intramolecular bond-breaking and rearrangement is coupled to that of shear/structural relaxation of the surrounding network. On the other hand, over the entire temperature range, the 31P NMR spin-lattice relaxation results from fast cage-rattling dynamics of the same molecules at frequencies in the megahertz to gigahertz range. When taken together, these results imply the presence of serial hierarchical dynamics in which the fast rattling of As2P2S8 quasi-molecular units trapped in their cages coexists with slower isotropic rotational reorientation. The shear or alpha-relaxation involves cooperative rearrangement of the surrounding As-S network and, consequently, relaxation of the cages that provides feedback to the fast rattling dynamics over the entire temperature range.
    The Journal of Physical Chemistry B 06/2009; 113(25):8514-9. · 3.61 Impact Factor
  • E.L. Gjersing, S. Sen, B.G. Aitken
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    ABSTRACT: Raman spectroscopic measurements have been performed on Ge20Se80 glass and supercooled liquid at temperatures ranging between 298 and 500 K. Temperature dependent softening of vibrational mode frequencies has been used in conjunction with the available vibrational density of states data at ambient temperature to estimate the relative contributions of vibrational and configurational entropies across glass transition. Nearly 20% of the additional entropy above glass transition is estimated to be vibrational. Thermal expansion effect on vibrational mode softening is found to be insufficient to account for the anharmonic component of vibrational entropy implying possible coupling between the vibrational and configurational entropies at temperatures above Tg. These results may have important consequences in shaping our understanding of various aspects of glass transition.
    Journal of Non-Crystalline Solids 01/2009; · 1.72 Impact Factor
  • Randall E. Youngman, Bruce G. Aitken
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    ABSTRACT: Quaternary glasses containing only the glass forming oxides B2O3, Al2O3, P2O5 and SiO2, have been studied by multinuclear NMR as a function of B for Al substitution at 7·5 mol% P2O5 and 70 mol% SiO2. Clear glasses along this join can be obtained for all B:Al ratios, except near the 1:1 composition, where liquidus temperatures are unusually high. As B+Al is always in excess of P in these glasses, P was found almost exclusively in symmetric Q4 units with all bridging oxygen atoms and B or Al next nearest neighbours. 11B and 27Al NMR showed a clear preference for Al–P instead of B–P associations, regardless of the B:Al ratio, in contrast to the situation in AlBP ternary glasses where B has been determined to be more strongly associated with P.
    Physics and Chemistry of Glasses - European Journal of Glass Science and Technology Part B. 01/2009; 50(3).
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    ABSTRACT: New mixed-network glasses along the composition line xAl2O3−(30 − x)P2O5−70SiO2 have been prepared and characterized in terms of their density, thermal expansion coefficient, refractive index, and characteristic temperatures. The compositional changes in these macroscopic properties have been correlated with structural information, obtained via Raman spectroscopy and state-of-the-art solid state NMR techniques, including 27Al, 29Si, and 31P magic-angle spinning (MAS) NMR, 27Al triple quantum MAS NMR, as well as static 31P spin echo decay spectroscopy. In addition, the extent of P−O−Al connectivity has been quantified on the basis of 27Al{31P} rotational echo double resonance (REDOR) and 31P{27Al} rotational echo adiabatic passage double resonance (REAPDOR) measurements. Both the macroscopic and the structural properties show nonlinear dependences on x, including abrupt changes at a nominal Al/P ratio of 1 (x = 0.15), where no glasses can be formed by melt quenching under the conditions used in this study. The structure of phosphorus-rich glasses (Al/P < 1) is characterized by four-, five- and six-coordinated Al species, whose second coordination sphere is dominated by phosphorus. 31P static and MAS NMR spectra suggest the presence of at least three distinct phosphorus environments, corresponding to silicon-bonded P(3) units, anionic metaphosphate P(2) species interacting with octahedral aluminum, and tetrahedral PO4/2 groups (P(4) units) bonded similarly as in AlPO4. (In this P(n) nomenclature, the superscript denotes the number of bridging oxygen atoms attached to a P atom.) The latter species is also the dominant phosphorus environment in the Al-rich glasses (Al/P > 1), where the alumina component is involved in Al−O−P, Al−O−Si, and possibly also Al−O−Al linkages. All of these results indicate that the structure of these glasses is dominated by the strong mutual affinity of the phosphorus oxide and alumina components. To quantify this affinity, the experimental REDOR and REAPDOR results have been compared with a cluster model assuming that both components react completely under formation of aluminum phosphate-like domains, thereby maximizing the number of Al−O−P linkages. Both the REDOR and the REAPDOR results show, however, clear deviations from such a structural scenario, supporting a more homogeneous glass structure with a certain degree of connectivity randomization.
    Journal of Physical Chemistry C - J PHYS CHEM C. 01/2009; 113(8):3322-3331.
  • S. Sen, S. Joshi, B.G. Aitken, S. Khalid
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    ABSTRACT: The nearest-neighbor coordination environments of Te atoms in GexTe100−x glasses with x = 15 and 20 and in AsxTe100−x glasses with 40 ⩽ x ⩽ 65 have been studied with Te K-edge EXAFS spectroscopy. The average coordination number of Te atoms in all glasses is found to be ∼2.0 and no violation of the 8-N rule is observed. The compositional makeup of the first coordination shell of Te atoms indicates that chemical order is largely preserved in both glass-forming binary systems. Sudden changes in the Te coordination environment and violation of chemical order are observed at the stoichiometric As40Te60 glass implying formation of a constrained network. The compositional dependence of the physical properties in both systems can be correlated to short-range chemical order.
    Journal of Non-Crystalline Solids 10/2008; 354:4620-4625. · 1.72 Impact Factor
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    ABSTRACT: A combination of neutron and x-ray diffraction has been employed to study the compositional dependence of the atomic structures of GexAsxS100−2x glasses with S concentration varying between 33.3 and 70.0 at.%. The nearest-neighbor coordination numbers of Ge and As atoms are always found to be 4 and 3, respectively, irrespective of the glass composition. Ge and As atoms have primarily heteropolar bonding to S atoms in stoichiometric and S-excess glasses with x≤18.2. Low and intermediate levels of deficiency of S (20≤x≤25) are accommodated via the formation of homopolar As–As bonds while Ge atoms remain primarily bonded to four S atoms, resulting in As-rich regions in the glass structure. Ge starts to participate in metal–metal bonding only in the highly S-deficient glasses with 27.5≤x≤33.3. The intermediate-range order and its topological influence on atomic packing in these three compositional regions, in the order of increasing deficiency in S, are controlled by (a) a mixed GeS2 and As2S3 network, (b) the coexistence of a GeS2 network and As clusters, and (c) large Ge–As metal-rich regions. This evolution of the intermediate-range structure with composition is consistent with the corresponding variation of the position, intensity and width of the first sharp diffraction peak in the structure factor.
    Journal of Physics Condensed Matter 07/2008; 20(33):335105. · 2.22 Impact Factor
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    ABSTRACT: The structures of chalcogenide glasses in the Ge−As−S system with Ge:As = 1:1 and with S concentration varying between 33.3 and 70.0 atom% have been studied using neutron diffraction. Ge and As atoms are primarily heteropolar bonded to S atoms in stoichiometric and S-excess glasses. Formation of homopolar As−As bonds at low and intermediate levels of S-deficiency results in violation of chemical order and in the formation of As-rich structural moieties. Ge takes part in metal−metal bonding, predominantly via the formation of Ge−As bonds, only in the highly S-deficient glasses once all the As atoms are used up in homopolar bonding. Incorporation of tetrahedrally coordinated Ge into the structure disrupts the topological continuity of the low-dimensional As-rich clusters and the GeS2 network. These intermediate-range order structural orderings are manifested in the compositional dependence of the intensity, position, and width of the first sharp diffraction peak in the structure factor.
    Journal of Physical Chemistry C - J PHYS CHEM C. 04/2008; 112(18).

Publication Stats

333 Citations
121.90 Total Impact Points

Institutions

  • 2012
    • Lawrence Berkeley National Laboratory
      • Advanced Light Source Facility
      Berkeley, CA, United States
  • 2009–2011
    • University of California, Davis
      • Department of Chemical Engineering and Materials Science
      Davis, CA, United States
  • 1995–2011
    • Corning Incorporated
      Corning, New York, United States
  • 2001–2002
    • Cornell University
      • School of Applied and Engineering Physics
      Ithaca, NY, United States
  • 1999
    • Worcester Polytechnic Institute
      • Department of Physics
      Worcester, MA, United States