Pressure-Induced transitions in amorphous TlxSe100−x alloys

Abstract

The electrical resistivity of bulk semiconducting amorphous Tl_xSe_100?x alloys with 0 ≤ x ≤ 25 has been investigated up to a pressure of 14 GPa and down to liquidnitrogen temperature by use of a Bridgman anvil device. All the glasses undergo a discontinuous pressure-induced semiconducting-to-metal transition. X-ray diffraction studies on the pressure-recovered samples show that the high-pressure phase is the crystalline phase. The pressure-induced crystalline products are identified to be a mixture of Se having a hexagonal structure with a = 4·37 Å and c = 4·95 Å and TlSe having a tetragonal structure with a = 8·0 Å and c = 7·0 Å     

High-pressure phase transition and lattice dynamics of rock-salt and FeSi-type phases of MgS

A phase transition of MgS under high pressure is investigated using a first-principles method. It is found from energy-volume calculations that the rock-salt (B1) phase of MgS transforms into a FeSi-type (B28) phase at 143?GPa. The calculated ground-state parameters in the B1 phase are in excellent agreement with available experimental and theoretical data. Ab initio phonon calculations are also performed to investigate the structural behaviour of MgS under high pressure. An unstable transverse acoustic mode and a phase transition from B1 to B28 phase at ~143.7?GPa driven by this soft mode are predicted. The B28 structure of MgS is stable up to 350?GPa according to lattice dynamics calculations.     

Pressure-induced phase transition in nanocrystalline La0.9Mn0.8Fe0.2O3.15 perovskite studied by Mössbauer spectroscopy

We report here on a pressure-induced phase transition in nanocrystalline La-deficient perovskite oxide, synthesised through the sol–gel technique, using Mössbauer spectroscopy, electrical resistivity and X-ray diffraction measurements at room temperature. At ambient pressure, the high-spin Fe³⁺ ion is distributed in two different environments. Below 0.6 GPa, a sudden dip in the resistivity curve and a single-environment Fe³⁺ doublet in the Mössbauer pattern are indicative of lattice compaction. A high-spin to low-spin Fe³⁺ transition at 2.1 GPa, and an orthorhombic to monoclinic structural transformation at 4.9 GPa, as revealed through Mössbauer patterns, are also associated with sudden dips in the electrical resistivity. X-ray diffraction patterns support the results.     

The order–dsorder transition at twin boundaries in Cu3Au as studied by TEM

Abstract

The order-disorder phase transition at ∑ = 3{111}- and {211}-type twin boundaries has been studied in the L1²-ordered alloy Cu³Au employing in situ heating in a transmission electron microscope. Evidence is presented for an order-disorder phase transition occurring in these boundaries prior to the bulk transition. The temperature difference ΔT between the transition temperature of both boundary types and the bulk is estimated as 0.5K <ΔT<2K. No difference in T _c for the twin boundaries can be established as yet. The nature of the order-disorder transition in both twin boundaries is presumably a second-order phase transition.     

Fluctuation-driven first-order crystal to rotator phase transition

This article develops renormalization-group techniques to analyze the phase transition between the crystal and the rotator-V (Rv), phase in n-alkanes. An explicit free energy is constructed and it is verified that the unstable fixed point corresponds to a fluctuation-driven first-order crystal to Rv phase transition. Available experimental data are consistent with our model.     

Neck formation processes of nanocrystalline silicon carbide: A tight-binding molecular dynamics study

Atomistic and electronic structures in the process of intergranular neck formation of nanocrystalline silicon carbide (SiC) have been investigated by a tight-binding molecular dynamics method. An order-N parallel algorithm is employed for efficient calculations of electronic energy and forces. We find that a defect-free neck is formed between SiC nanocrystals aligned along [112] axis at T     

Structure of a phase induced with Xe-ion irradiation-implantation in γ-TiAl

A phase transformation in γ-TiAl intermetallic alloy was found to be induced with 50keV Xe-ion irradiation-implantation at doses larger than 2.2 x 10¹⁸ ions m^-2 at room temperature. The structure and the chemical composition of the induced phase were investigated with high-resolution transmission electron microscopy and energy-dispersive X-ray spectroscopy. The zones of the induced phase have sizes up to about several tens of nanometres. The phase has a hexagonal structure with a = 0.286 nm and c = 0.462nm. The crystallographic orientation relationship between the phase (P) and the gamma-TiAl matrix is (001)_P//(111)_γ and \[100]_P//[011]_γ. The \[Al]/[Ti] atomic composition ratio in the phase is analysed to be 56/44, slightly different from that of the matrix, 51/49. These results suggest that the induced phase is an Al solid solution of α-Ti alloy phase, which has different structural parameters and chemical composition from those of the reported phase. It is suggested that the size of the ions is important in the phase transformation.     

Commensurate-incommensurate phase transition in muthmannite,AuAgTe2: first evidence of a modulated structure at low temperature

To study the temperature-dependent structural changes and to analyze the crystal chemical behavior of silver as a function of temperature, a crystal of muthmannite, AuAgTe₂, has been investigated by X-ray single-crystal diffraction methods at 300 K and 110 K. At room temperature, muthmannite was confirmed as belonging to the space group P2/m, while at low temperature (110 K) it undergoes a reversible commensurate–incommensurate phase transition with a modulation wave vector q = 0.215(1)a* + 0.379(2)c*. Muthmannite reconverts to the commensurate type upon returning to room temperature, thus indicating that the phase transition is completely reversible in character. The average structure of the low-temperature muthmannite remains monoclinic, space group P2/m, and shows only normal thermal compression over the entire temperature range investigated. Crystal-chemical characteristics are compared with published data on the other members of the system Au–Ag–Te. Speculations on the possible origin of the modulated structure at low temperature are also given.     

A connectionist model of a continuous developmental transition in the balance scale task

A connectionist model of the balance scale task is presented which exhibits developmental transitions between ‘Rule I’ and ‘Rule II’ behavior [Siegler, R. S. (1976). Three aspects of cognitive development. Cognitive Psychology,8, 481–520.] as well as the ‘catastrophe flags’ seen in data from Jansen and van der Maas [Jansen, B. R. J., & van der Maas, H. L. J. (2001). Evidence for the phase transition from Rule I to Rule II on the balance scale task. Developmental Review, 21, 450–494]. The model extends a connectionist model of this task [McClelland, J. L. (1989). Parallel distributed processing: Implications for cognition and development. In R. G. M. Morris (Ed.), Parallel distributed processing: Implications for psychology and neurobiology (pp. 8–45). Oxford: Clarendon Press] by introducing intrinsic variability into processing and by allowing the network to adapt during testing in response to its own outputs. The simulations direct attention to several aspects of the experimental data indicating that children generally show gradual change in sensitivity to the distance dimension on the balance scale. While a few children show larger changes than are characteristic of the model, its ability to account for nearly all of the data using continuous processes is consistent with the view that the transition from Rule I to Rule II behavior is typically continuous rather than discrete in nature.     

Stability of the F2-(Al-Pd-Mn) phase

The stability of the F2-(Al-Pd-Mn) phase has been studied by in-situ neutron diffraction on a single quasicrystal with composition Al_69.8Pd_21.4Mn_8.8. We find that the F2 phase is not stable and corresponds to a transient state in the process of the transformation of the icosahedral phase to the F2_M phase. The icosahedral-to-F2_Mphase transition occurs at around 715^oC. In the F2 phase a large amount of diffuse scattering is located close to the icosahedral Bragg reflection in place of the S ₁ superstructure reflections characteristic of the F2_m phase.     

FOCUS: European Views on Complexity,Recursion, and Planetary Ethics Preface to the FOCUS Section

Global geographical patterns of repressive, painful, traumatic, and violent patrist behaviors and social institutions, which thwart materal‐infant and male‐female bonds, were correlated and developed through a systematic analysis of anthropological data on 1170 subsistence‐level cultures. When the behavior data were mapped, the hyperarid desert belt encompassing North Africa, the Near East, and Central Asia, which I call Saharasia, was found to possess the greatest areal extent of the most extreme patrist behaviors and social institutions on Earth. Regions farthest removed from Saharasia, in Oceania and the New World, were found to possess the most extreme matrist behaviors, which support and protect maternal‐infant and male‐female bonds. A systematic review of archaeological and historical materials suggests that patrism first developed in Saharasia after c.4000 BCE, the time of a major ecological transition from relatively wet grassland‐forest conditions to arid desert conditions. Settlement and migration patterns of patrist peoples were traced, from their earliest homelands in Saharasia, to explain the later appearance of patrism in regions outside of Saharasia. Prior to the onset of dry conditions in Saharasia, evidence for matrism is widespread, but evidence for patrism is generally nonexistent. It is argued that matrism constitutes the earliest, original, and innate form of human behavior and social organization, while patrism, perpetuated by trauma‐inducing social institutions, first developed among Homo Sapiens in Saharasia, under the pressures of severe desertification, famine, and forced migrations.     

On Iachello's model of cuprate superconductors

A U(3) model proposed by Iachello for superconductivity in cuprate materials is analyzed. The model consists of s and d pairs (approximated as bosons) in a two-dimensional Fermi system with a surface. The transition occurs between a phase in which the system is a condensate of one type of boson, and a phase which is a mixture of two types of bosons. In the current work we have investigated the validity of the Bogoliubov approximation, and we used a reduced Hamiltonian to determine a phase diagram, the symmetry of the phases, and the temperature dependence of the heat capacity.     

Molecular dynamics study of high-pressure polymorphs of BaCO3

We report a computational study of BaCO₃ at high pressure using molecular dynamics simulation based on a recent potential model. The first phase transition from the witherite to the post-aragonite phase is observed at 9.67 GPa with a resistance to shear along the [0 0 1] direction and a high compressibility in a direction perpendicular to the layers. This stable phase is also found through the variation of the bulk modulus, the shear moduli, and the compressional and shear wave velocities with pressure. A softening mode is noticed at a pressure around 10 GPa. The computed X-ray diffraction pattern of the post-aragonite phase at 16 GPa shows favorable agreement with the available experimental data.     

Pressure dependence of the electrical resistivity of natural cassiterite SnO2 and of undoped and Co-doped nanocrystalline SnO2

The pressure dependence of the electrical resistivity of three different samples of cassiterite, namely natural cassiterite SnO₂, synthetic nanocrystalline SnO₂ (with crystallite size 46?nm) and nanocrystalline Co-doped SnO₂ (with crystallite size 32?nm), has been measured up to 7?GPa at room temperature. The resistivity of natural cassiterite SnO₂ decreases from 2.5?×?10⁴?Ωm at normal pressure and temperature to 1.7?×?10⁴?Ωm at 7.0?GPa. The nanocrystalline SnO₂ has a high resistivity 6.0?×?10⁵?Ωm at normal pressure and temperature and decreases with pressure reaching a value of 2.98?×?10⁵?Ωm at 7?GPa. The activation energy of the electrical conduction of the studied samples were found to be 0.32?eV for the natural SnO₂, 0.40?eV for the nanocrystalline SnO₂ sample and 0.28?eV for the nanocrystalline Co-doped SnO₂. Measurements of the pressure dependence of the electrical resistivity of the Co-doped SnO₂ showed a decrease from 3.60?×?10⁵ to 5.4?×?10⁴?Ωm at 7.0?GPa. We did not observe any pressure-induced phase transition in SnO₂ up to 7?GPa. This study of the high-pressure phase stability of cassiterite corroborates the experimental findings of SnO₂ nanoinclusions in diamonds.     

Single-particle excitations in a two-dimensional strong-coupling superconductor

We present calculations of the single-particle excitation spectrum for a twodimensional strong-coupling superconductor in a conserving approximation. Spectral weight at low frequency is substantially reduced as the superconducting transition is approached from the normal state. The suppression of low-energy excitations is a consequence of Cooper-pair fluctuations that are described self-consistently in the fluctuation exchange approximation. The static and uniform electromagnetic response provides a measure of the super fluid density and a fully self-consistent indication of the superconducting transition temperature.     

Intentional Switching Between Patterns of Bimanual Coordination Depends on the Intrinsic Dynamics of the Patterns

Two predictions arising from previous theoretical and empirical work which demonstrated that spontaneous changes of bimanual coordination patterns result from a loss of pattern stability (i.e., a nonequilibrium phase transition) were tested: (a) that the time it takes to intentionally switch from one pattern to another depends on the differential stability of the patterns themselves; and (b) that an intention, defined as an intended behavioral pattern, can change the dynamical characteristics, e.g., the overall stability of the behavioral patterns. Subjects moved both index fingers rhythmically at one of six movement frequencies while performing either an in-phase or antiphase pattern of finger coordination. On cue from an auditory signal, subjects switched from the ongoing pattern to the other pattern. The relative phase of movement between the two fingers was used to characterize the ongoing coordinative pattern. The time taken to switch between patterns, or switching time, and relative phase fluctuations were used to evaluate the modified pattern dynamics resulting from a subject's intention to change patterns. Switching from the in-phase to the antiphase pattern was significantly slower than switching in the opposite direction for all subjects. Both the mean and distribution of switching times in each direction were found to be in agreement with model predictions. Movement frequency had little effect on switching time, a finding that is also consistent with the model. Relative phase fluctuations were significantly larger when moving in the antiphase pattern at the highest movement frequencies studied. The results show that, although intentional influences act to modify a coordinative pattern's intrinsic dynamics, the influence of these dynamics on the resulting behavior is always present and is particularly strong at high movement frequencies.     

First steps in developing the Dutch version of the Bayley III: Is the original Bayley III and its item sequence also adequate for Dutch children?

The Bayley Scales of Infant and Toddler Development [Bayley III, Bayley, N. (2006). The Bayley scales of infant and toddler development. San Antonio, TX: The Psychological Corporation] is currently developed and exclusively normed for American children. In this study, the appropriateness of the Bayley III item content and item sequence for Dutch children was evaluated. The translated version, the Bayley-III-NL was evaluated in two phases. In phase 1 (N = 100), analyses showed that overall the item content seemed to be appropriate for Dutch children. In addition, the item sequence was found to be suitable for Dutch children. After phase 1, small adaptations were made to the instructions of a few items based on the experiences of the examiners to improve standardization in performance. In phase 2 (N = 400), the findings of phase 1 were confirmed in a larger sample. It is concluded that Dutch norms can be based upon the current version of the Bayley-III-NL.