Roughening of a Si(100) surface induced by the adsorption of oxygen near the solid-oxide nucleation threshold

An investigation of the processes on a Si(100) surface interacting with oxygen near the solid-oxide nucleation threshold using x-ray photoelectron spectroscopy and atomic-force microscopy is described. The nucleation threshold is the boundary between the temperature and oxygen pressure regions where a phase transition with the formation of a submonolayer oxide and a roughening transition caused by oxygen adsorption occur. Near the nucleation threshold, either a random rough relief or a quasiperiodic structure is formed on a surface coated with chemisorbed oxygen. The formation of the rough relief due to oxygen adsorption has been interpreted within the theory of phase transitions as a result of vacancy clustering. A model that allows one to describe the dynamics of processes on the surface near the nucleation threshold in qualitative and in some cases in quantitative terms has been suggested. Zh. éksp. Teor. Fiz. 114, 239–262 (July 1998)     

Evidence for a magnetic collapse in the epsilon phase of solid oxygen

Solid oxygen is the only elementary molecular magnet. Under the very high pressure of 96 GPa oxygen transforms into a metal and a superconductor. Theory predicts a nonmagnetic state occurring before the transition into the superconducting xi phase. Nevertheless, until now there was no direct evidence of a magnetic collapse in high-pressure oxygen. For the first time direct information is provided on magnetic properties of the epsilon phase, which is sandwiched between the antiferromagnetic delta phase and the superconducting xi phase. We used magnetic neutron diffraction. The data show that the long-range magnetic order disappears at the delta-epsilon transition. The magnetic collapse occurs at P approximately equal to 8 GPa, far below the pressure of the insulator-metal (superconductor) transition. The collapse is preceded by a decrease in temperature of transition towards the long-range magnetically ordered state (T(LRO)) in the delta phase, at P = 7.6 GPa.     

Lattice dynamics,ferroelectricity and raman spectrum of BaTiQ3 in cubic phase

A dynamical model involving non linear anisotropic polarisability of oxygen is used to calculate the phonon dispersion curves and the Raman spectrum of barium titanate at 421 K near the paraelectric-ferroelectric phase transition. The temperature phase dependence of the dispersion curves is studied and the ferroelectric soft mode is described.     

A point‐defect model of structural phase transitions in the ferroelastic superconductor Y-Ba-Cu-O

A point-defect model of structural phase transitions in YBa₂Cu₃O_{6 + x} is proposed. The model treats the basal-plane oxygen atoms as point defects. The treatment parallels the work of Alefeld and coworkers on ferroelasticity due to hydrogen in metals. The model predicts that the tetragonal-to-orthorhombic phase transition should be accompanied by precipitation into regions of high and low oxygen density, as well as high and low oxygen ordering. This is in agreement with experiment, as well as with the predictions of Khachaturyan's concentration-wave model. A new feature of the present model is the prediction of two characteristic temperatures related, respectively, to Snoek-type and Gorsky-type diffusive relaxations. Practical implications of the high diffusivity of the basal-plane oxygens include the easy fabricability of a bicrystal of Y-Ba-Cu-O such that the two components of the bicrystal are rotated by 90° with respect to each other, and are separated by a non-superconducting layer. Such a bicrystal can find applications as a Josephson-tunneling device.     

Oxygen pressure induced structure,morphology and phase-transition for VO2/c-sapphire films by PLD

To investigate the effects of oxygen pressure on the structural and phase transition properties for VO₂/c-sapphire, highly orientated VO₂ thin films were grown on (0001) sapphire substrates by pulsed laser deposition (PLD) with different oxygen pressures. The crystal structure, morphology and component of the films were systematically investigated. The temperature-dependent resistance (R-T) measurement was conducted, which showed the distinct phase transition characteristic for the prepared films. The results indicate that the oxygen pressure plays an important role for the VO₂ film preparation. The film grown at 1.7 Pa has lower phase transition temperature, higher film strain, and smaller grain size than that at 5.4 Pa, while no obvious crystal phase transition is observed. The experiment suggests that even a small change in oxygen pressure can influence the structure, morphology and phase-transition behavior of VO₂ films obviously, and its potential causes are mainly attributed to the reduction of the kinetic energy to the substrate for target atoms caused by the oxygen pressure, the resulting grain aggregation and interfacial stress.     

Dynamical mean-field theory of nickelate superlattices

Dynamical mean-field methods are used to calculate the phase diagram, many-body density of states, relative orbital occupancy, and Fermi-surface shape for a realistic model of LaNiO(3)-based superlattices. The model is derived from density-functional band calculations and includes oxygen orbitals. The combination of the on-site Hunds interaction and charge transfer between the transition metal and the oxygen orbitals is found to reduce the orbital polarization far below the levels predicted either by band-structure calculations or by many-body analyses of Hubbard-type models which do not explicitly include the oxygen orbitals. The findings indicate that heterostructuring is unlikely to produce one band-model physics and demonstrate the fundamental inadequacy of modeling the physics of late transition-metal oxides with Hubbard-like models.     

高氧压合成产物La2CuO4+δ的相变研究

 本文采用超高氧压2.3 GPa合成方法，得到超导相含量大于30%的超导化合物La₂CuO_4+δ所采用的压力是目前所见报导中对La₂CuO₄化合物施加的最高氧压。湿化学方法测量给出过量氧含量δ=0.05±0.01，而TGA（氮气中）给出δ=0.143。高氧压合成产物经在空气中、不同温度下加热，然后淬火到室温，结果发现超导体在200~300 ℃的加热处理中存在着一个一级相变。经此相变后，超导体转变成反铁磁半导体。TGA及DSC证实了此相变的存在。该相变前后所联系的氧含量变化恰好为湿化学方法给出的结果，这部分与超导电性存在联系的过量氧，实际上引起了Cu²⁺向Cu^(2+2δ)+的转变。该结果强有力地排除了所谓“超氧化物”引起超导电性的可能性。     

Temperature dependence of oxygen content and phase transition in Y1Ba2Cu3O9−x compounds

Systematic TGA (Thermogravimetric analysis), magnetic susceptibility measurements and X-ray diffraction investigations have been made on the temperature dependence of oxygen content and phase transition in Y₁Ba₂Cu₃O_9–x compounds. The experimental results show that reversible oxygen relief and absorption appeared during heating and cooling in the temperature range of 400–950°C. A low cooling rate leads to higher oxygen content and quenching can suppress oxygen absorption. The deoxidation process is accompanied by a change of the cell constants,a andb increase whilec decreases with reduced oxygen content while the phase transition from the ortorhombic to the tetragonal system appeares. Nevertheless, this transition is a gradually changing process, no critical point of transition exists. Quenching stress and defects have a pronounced influence on the absorption and relief of oxygen. Maximum rates of oxygen relief and absorption were obtained at about 650°C. Oxygen absorption and desorption can be obtained only in an athermal process. Magnetic susceptibility measurements show that diamagnetic and paramagnetic suceptibilities increase obviously with increasing oxygen content. Deoxidation and oxygen absorption can be restrained by adding Fe to the Y₁Ba₂Cu₃O_9–x phase, which suggests that deoxidation and oxygen absorption are very sensitive to crystal structure and lattice distortions.     

Electric field induced oscillations in the catalytic water production on rhodium: A theoretical analysis

Methods are developed to describe catalytic reactions on multi-facetted surfaces in high electric fields in the conditions of field emission microscopy. These methods are applied to the hydrogen–oxygen–rhodium system for which a mean-field kinetic model is established. This model is shown to reproduce not only the nonequilibrium regimes of bistability and oscillations and their nanopatterns, but also the temperature-programmed desorption spectra of hydrogen and oxygen on rhodium, as well as the equilibrium phase diagram of oxygen on rhodium with the transition to the trilayer surface oxide ORhO. The dependence of the kinetic constants on the surface orientation of the facets is taken into account by expanding them in kubic harmonics suitable for the fcc rhodium crystal. The electric field modifies the gas pressure as well as the activation energies of the different kinetic processes. The tip of the field emission microscope is shown to behave as a nanoreactor.     

Phase transition and superconductivity of (Ba,Sr,La)2YCu3Oz (z>6.9)

A series of (Ba,Sr,La)₂YCu₃O_z (z>6.9) samples is studied to elucidate the relationship between structure, cation composition and oxygen content. The concentration of La plays an important role in the orthorhombic to tetragonal phase transition. The tetragonal structure is stabilized by increasing oxygen content and decreasing average ionic radius of cations on the Ba site. The phase transition is explained in terms of the random distribution of CuO₅ pyramids in the BaO–CuO_x slab. Superconductivity does not depend significantly on the phase transition.     

Nd0.9Ca0.1Ba2Cu3O6＋x超导体系的红外光谱研究

用固相反应法制备、并通过没条件的热处理得到了一组同氧含量的Ｎｄ０．９Ｃｄ０．１ｂａ２Ｃｕ３Ｏ６＋ｘ样品。用Ｘ射线衍射确定了样品的相结构。随着氧含量的增大，结构发生从四方到正交相变。进一步用红外吸收谱研究了体系的结构变化特征。谱图上６４２ｃｍ＾－１，５７８ｃｍ＾－１，５３８ｃｍ＾－１峰的出现及位移，与氧含量引起的结构变化是一致的，并讨论结构与超导电性的关系。     

Éléments d'un modèle d'interactions oxygène sous basse pression-métaux de transition à haute température

This paper presents a general model for the interaction of low pressure oxygen with transition metals at high temperatures, when all products are volatile. It is based on the following major hypotheses: (a) adsorption of molecular oxygen is the only direct process between the gas phase and the surface; (b) all chemical reactions occur in the adsorbed layer. This model is in agreement not only with our experimental results on platinum and rhenium but also with those of other authors on different metals (W, Mo. Ta, Nb). It becomes identical to Stickney's quasi-equilibrium theory when assuming that desorption activated complexes are identical to molecules moving freely in a plane parallel to the surface.     

Phase diagram of oxygen on Ni(100)

We have studied the coverage-temperature phase diagram of chemisorbed oxygen on Ni(100) with LEED and AES. We find that the oxygen p(2 × 2) structure undergoes a nearly reversible order- disorder phase transition for coverages between 0.15 and 0.30 monolayer (T_c ≈ 560 K for θ = 0.25). For these coverages, it is also found that the chemisorbed oxygen begins dissolving into the bulk nickel as the overlayer disorders. A reversible order-disorder transition is not seen for the oxygen c(2 × 2) structure, but there is evidence that bulk dissolution here also occurs coincident with the overlayer disordering.     

Ca$lt;sub$gt;0.64$lt;/sub$gt;WO$lt;sub$gt;4$lt;/sub$gt;:Eu$lt;sub$gt;0.24$lt;/sub$gt;陶瓷的高温相变机理及其对发光性能的影响

纯的CaWO₄具有优异的耐压、耐热稳定性,化学组成为Ca_0.64WO₄:Eu_0.24的陶瓷也具有CaWO₄结构,但Ca²⁺晶格位置含有12 mol%的肖特基缺陷. 这种缺陷浓度高的CaWO₄ 相是否具有良好的高温稳定性还有待研究. 本文探讨了过度烧结对Ca_0.64WO₄:Eu_0.24陶瓷相结构的影响,揭示了在高温下产生相变的可能原因,并研究了该相变对材料发光性能的影响. 研究表明,当烧结温度超过1100 ℃时,被肖特基缺陷束缚的部分氧离子会解离,造成Ca_0.64WO₄:Eu_0.24陶瓷体相中氧元素含量严重不足,诱发CaWO₄相发生相变,析出单斜晶系的Eu₂WO₆;研究还发现,CaWO₄相的晶面间距在高温相变后会增大;这可能是导致Ca_0.64WO₄:Eu_0.24陶瓷发光强度显著降低的一个重要原因. 关键词： 相变 钨酸钙 铕 发光     

Destabilization of the Zhang-Rice singlet at optimal doping

The construction of the Zhang-Rice singlet is revisited in the light of the recent understanding of high-temperature superconductors at optimal doping. A minimal local model is derived that contains the physical regime found relevant for ARPES experiments and characterized by significant direct oxygen-oxygen hopping. For the values of orbital parameters indicated by experiment, the Zhang-Rice singlet is strongly mixed with a pure oxygen singlet of the same symmetry. The Zhang-Rice ground state is destabilized because the coherence factor of the oxygen singlet with respect to oxygen-oxygen hopping is twice as large. An analogous quantum phase transition is identified in the t-t′-J model. The orbital-antisymmetric copper-oxygen singlet is confirmed to be irrelevant, as found originally. The usual symmetry analysis is extended to include dynamical symmetries.     

Structural phase transitions in CdTiO3

The reconstructive phase transition from the ilmenite-like CdTiO₃ modification to the perovskite modification is investigated thoroughly. It is revealed that the reconstructive transition is determined by size effects, results in the formation of the closest packing of the ilmenite CdTiO₃ structure, and is irreversible with a decrease in temperature. The perovskite CdTiO₃ modification undergoes displacive structural phase transitions at temperatures of 110, 220, and 380°C. The first displacive phase transition is isostructural, whereas the second and third transitions are associated with rotations of oxygen octahedra.     

Load-induced increase in the concentration and chemical potential of oxygen atoms in YBa2Cu3O6+x near the order-disorder phase transition

It is found that YBa₂Cu₃O_6+x samples subjected to a load absorb oxygen. The phenomenon is associated with a load-induced shift in the phase transition point. In addition, this shift increases the chemical potential of the oxygen atoms in the loaded sample and is a reason for the increased compressibility of the material near the phase transition. Fiz. Tverd. Tela (St. Petersburg) 39, 240–242 (February, 1997)     

Optical Tomograms of Multiple-Photon-Added Gaussian States via the Intermediate State Representation Theory

The oxygen isotope effect in PrBaMn₂^16-18 O_5.97 manganite with an ordered cation arrangement is studied. The field dependences of magnetic susceptibility and magnetization are measured in the temperature range 100–270 K and magnetic fields up to 32 T. A significant increase in the temperature of the spin-reorientation antiferromagnet–ferromagnet phase transition is detected in samples enriched in heavy oxygen ¹⁸O (negative isotope effect). The transition temperature and the isotope effect depend strongly on the magnetic field. An H–T phase diagram is plotted for samples with various isotope compositions. An analysis of the experimental results demonstrates that the detected negative isotope effect and the giant positive isotope effect revealed earlier in doped manganites have the same nature. The mechanisms of appearance of isotope effects are discussed in terms of the double exchange model under a polaron narrowing of the free carrier band.     

Ignition-to-spread transition of externally heated electrical wire

Ignition of electrical wires by external heating is investigated in order to gain a better understanding of the initiation of electrical-wire fires. An ignition-to-spread model is developed to systematically explain ignition and the following transition to spread. The model predicts that for a higher-conductance wire it is more difficult to achieve ignition and the weak flame may extinguish during the transition phase because of a large conductive heat loss along the wire core. An experimental study was performed using several sample wires with different core metals, diameters and coating thicknesses of polyethylene. A coil heater was adopted as the ignition source, and both the heat flux and heating time were selected as the main parameters to identify the flashpoint and spread point of wire fires. Experiments show that additional heating times after flash are required in order to fully pass the transition and achieve a spreading flame, agreeing with model predictions. Finally, the effects of different heating lengths, environmental pressures, and oxygen concentration on wire ignition are discussed, which may be useful for upgrading the design and standards of future fire-safe wires.