Showing papers on "Lattice constant published in 1988"

Analytic nearest-neighbor model for fcc metals

Abstract: The implications of the mathematical format of the embedded-atom method of computer modeling of metals have been studied with use of a simple nearest-neighbor analytic model for the fcc lattice. The physical inputs into the model are the atomic volume, the cohesive energy, the bulk modulus, the average shear modulus, the vacancy-formation energy, and the slope at the nearest-neighbor distance of the spherically averaged free-atom electron density calculated with Hartree-Fock theory. The model employs an exponential repulsion between nearest-neighboring atoms, an exponentially decreasing function for the free-atom electron density, and a universal equation relating the crystal energy and the lattice constant. The anisotropy ratio of the cubic shear moduli is constrained to be 2 with this model. The dependence of the energies for unrelaxed configurations for vacancy formation, divacancy binding, and low-index plane surfaces on the model parameters has been analyzed. The average shear modulus plays a dominant role in determining these energies relative to the bulk modulus or the cohesive energy because the slope of the embedding function at the equilibrium electron density is linear in the average shear modulus. Embedding functions are not uniquely determined in specific models, and it is shown that the embedding functions used inmore » several models are essentially equivalent.« less

Low‐loss substrate for epitaxial growth of high‐temperature superconductor thin films

Abstract: A perovskite‐like single‐crystal substrate material has been investigated that simultaneously permits epitaxial growth of 1‐2‐3 superconductor films and possesses desirable rf properties of low dielectric constant and loss tangent. The lattice constant of 3.792 A provides a lattice match to within 1% of the a axis of 1‐2‐3. Sputtered films of erbium‐barium‐copper‐oxide have been produced on (100) LaAlO3 substrates that exhibit sharp resistive transitions at 90 K (ΔT=1K), bulk superconductivity as determined by ac susceptibility measurements, and nearly single‐crystal growth as evidenced by x‐ray diffraction and high‐resolution scanning electron microscopy. The high‐frequency dielectric properties of LaAlO3 were experimentally investigated at several temperatures. The low‐frequency dielectric constant was measured to be 15 and the microwave loss tangent ranged from 6×10−4 at room temperature to 5×10−6 at 4 K.

Growth and Optical Properties of Li13Nd(GeO4)4 Single Crystal

Abstract: In this paper, single crystals of Li13Nd(GeO4)4 are grown by the Czochralski method. The unit cell dimensions and the crystal system are determined by an X-ray diffraction method. The principal refractive indices, na, nb and nc and the temperature dependence of birefringences, nc-nb, nc-na and na-nb, over the temperature range from room temperature to 500degC are measured. A fluorescence spectrum at around 1.06 µm is observed, and strong fluorescence is detected.

Role of strain and growth conditions on the growth front profile of InxGa1−xAs on GaAs during the pseudomorphic growth regime

Abstract: Theoretical and experimental studies are presented to understand the initial stages of growth of InGaAs on GaAs. Thermodynamic considerations show that, as strain increases, the free‐energy minimum surface of the epilayer is not atomically flat, but three‐dimensional in form. Since by altering growth conditions the strained epilayer can be grown near equilibrium or far from equilibrium, the effect of strain on growth modes can be studied. In situ reflection high‐energy electron diffraction studies are carried out to study the growth modes and surface lattice spacing before the onset of dislocations. The surface lattice constant does not change abruptly from that of the substrate to that of the epilayer at the critical thickness, but changes monotonically. These observations are consistent with the simple thermodynamic considerations presented.

Formation and magnetic properties of the compounds R2Fe14C

Abstract: It was shown by X-ray diffraction that rare earth compounds of the type R2Fe17 can dissolve appreciable amounts of carbon up to x = 0.6 in R2Fe17Cx at 900 °C. The increase in the lattice constants is accompanied by a strong increase in Curie temperature. From a structure determination (R = 7%) it was found that the carbon atoms dissolve interstitially and occupy the 9e position in the Th2Zn17 structure type (R3m). This ternary carbide is present as the main phase when compounds of the type R2Fe14C are heated above their decomposition temperatures. The decomposition proceeds by means of a solid state transformation and the corresponding transformation temperatures Tt vary strongly with the R component. In several of the light rare earth systems Tt becomes rather low and makes an annealing treatment ineffective with regard to the formation of R2Fe14C phases. The magnetic properties of the R2Fe14C phases are compared with those of R2Fe14B. It is shown that the former follow the magnetic behaviour of the latter almost completely.

Size-induced structural phase transitions and hyperfine properties of microcrystalline Fe2O3

Abstract: Microcrystalline particles of Fe2O3 having different sizes (varying between 70 and 5 nm) have been synthesised using a novel three-component micro-emulsion technique. A succession of crystal-size-induced structural transitions was observed. While alpha -Fe2O3 was found to nucleate for a particle size above 30 nm, gamma -Fe2O3 was preferentially formed for a size below 30 nm, whereas amorphous Fe2O3 was formed at a particle size of 5 nm. These structural transformations have been related to the increase in the unit-cell volume that occurs as the particle size is decreased. The size dependence of the lattice parameter is shown to arise from a coupling of the surface energy to the dilatational lattice mode. A model Hamiltonian which incorporates this interaction and displays size-induced phase transitions is defined. The Mossbauer hyperfine field in the microcrystalline samples at 4.2 K was found to be substantially smaller than in the 'bulk'. The hyperfine parameters of the amorphous sample were found to be similar to those pertaining to samples prepared by conventional techniques such as melt quenching. A large anisotropy in the ionic vibrational amplitudes was detected in samples with particles smaller than about 10 nm.

Structural anomalies at the disappearance of superconductivity in Ba2YCu3O7−δ: Evidence for charge transfer from chains to planes

Abstract: In systematic studies of oxygen-deficient Ba 2 YCu 3 O 7−δ , we have found that a structural anomaly occurs at the composition where bulk superconductivity disappears. The observed step-like increase in the c lattice parameter, when associated with changes in the atomic positions within the unit cell, indicates that superconductivity is lost due to a charge redistribution from the copper-oxygen chains to the copper-oxygen planes. This is microscopic experimental evidence establishing the role of secondary structural elements in the layered copper-oxide high- T c superconductors.

A new high- T c TlBa 2 Ca 3 Cu 4 O 11 superconductor with T c >120K

Abstract: Three superconducting compounds are known at present in the Tl–Ba–Ca–Cu–O system: Tl2Ba2CuO6+y, Tl2Ba2CaCu2O8+y, and Tl2Ba2Ca2Cu3O10+y, known by their cation numbers as '2201', '2212' and '2223', respectively1–3. The transition temperature Tc increases in this series, from 80 to 110 to 120 K, as the number of CuO2 layers in the unit cell increases from one to three. Here we report the synthesis and characterization of a new Tl–Ba-Ca–Cu–O superconductor which differs from these three in having only a single thallium layer (instead of two) per unit cell, but which has four CuO2 layers. The composition of the phase is TlBa2Ca3Cu4O11 (1234) and its structure is tetragonal, with lattice constants a = 3.85 A and c = 19.1 A.

Magnetism of Epitaxial fcc-Fe(100) Films on Cu(100) Investigated in Situ by Conversion-Electron Mössbauer Spectroscopy in Ultrahigh Vacuum

Abstract: Ultrathin films (10 to 17 monolayers) of fcc Fe(100) grown epitaxially on Cu(100) are unambiguously found to be paramagnetic at 295 K by $^{57}\mathrm{Fe}$ conversion-electron M\"ossbauer spectroscopy in UHV. Below ${T}_{\mathrm{N}}=65\ifmmode\pm\else\textpm\fi{}5$ K, fcc-Fe films are magnetically ordered and show a small average hyperfine field, e.g., 1.1 T at 29 K for 10 monolayers, indicating a small atomic magnetic moment. This demonstrates that fcc Fe(100) stabilized at a lattice constant close to that of Cu has an antiferromagnetic ground state.

Characterization of p‐type GaAs heavily doped with carbon grown by metalorganic molecular‐beam epitaxy

Abstract: p‐type GaAs with doping levels of up to 5.8×1020 cm−3 has been grown by metalorganic molecular‐beam epitaxy (MOMBE) using carbon (C) as a dopant. The mobility and minority‐carrier diffusion length of the C‐doped MOMBE layers were comparable to those of Be‐doped MBE layers. The diffusion coefficient of C at 900 °C was estimated to be 6×10−15 cm2 /s which is about two orders of magnitude less than that of Be (1×10−12 cm2 /s). In addition, the lattice constant of C‐doped GaAs was found to be 5.6533 A which completely matches that of the substrate, while the lattice constant of Be‐doped GaAs decreases to 5.6467 A at a doping level of 2×1020 cm−3 as reported by Lievin et al. [Inst. Phys. Conf. Ser. No. 79, 595 (1985)].

Optical and structural properties of GaAs grown on (100) Si by molecular‐beam epitaxy

Abstract: GaAs epitaxial layers (0.5 μm

Saturation magnetisation, coercivity and lattice parameter changes in the system Fe3O4-γFe2O3, and their relationship to structure

Abstract: This study has characterised the oxidation products of a fine-grained single domain magnetite which was made synthetically by a colloidal method. Changes in the intrinsic magnetic properties (saturation magnetisation, saturation remanence, and coercive force) during progressive oxidation are correlated with lattice parameter changes and an oxidation mechanism. It is proposed that magnetite oxidises to hematite via at least two metastable maghemites. The first of these, formed on low temperature oxidation by the formation of a magnetite/maghemite solid solution, is a face centered maghemite with lattice parameter a= 8.3419±0.0006 A. A second maghemite, produced on oxidation at higher temperatures, has a primitive cubic structure and a lattice parameter a = 8.3505±0.0005 A. Maghemite cation distributions are derived to explain the reduced saturation magnetisations of between 56 and 74 Am2 kg-1 observed, and a maghemite structure containing an increase in tetrahedral Fe3+ ions and up to 3 octahedral vacancies per 32 oxygen unit cell is proposed.

Theory of dielectric breakdown in metal-loaded dielectrics

Abstract: We have examined a very simple model of dielectric breakdown in random mixtures of metal and dielectric1,2,3 We expect this analysis to be relevant for an class of materials which are composed of a random mixture of metallic particles embedded in a dielectric matrix An example is solid fuel rocket propellant4 which is a mixture of microscopic aluminum particles (the fuel) in a dielectric matrix composed of oxidizer and rubber binder The model we analyze is a percolation model in which the bonds of a d-dimensional lattice with lattice spacing a are occupied by conductors with probability p and by capacitors with probability 1-p The probability p is chosen to be less than the percolation threshold pc so that no conducting path traverses the entire system The breakdown process is modeled by assuming that the capacitors can withstand a maximum voltage drop of 1 volt The entire lattice has a size of L lattice spacings A macroscopic voltage is applied across the lattice This voltage is raised until the voltage drop across one of the capacitors exceeds 1 volt This macroscopic voltage is called V1 the initial breakdown voltage The capacitor which fails is replaced by a conducting element The process of failing one of the capacitors is repeated until a conducting path is formed across the sample The maximum value of the applied voltage during this procedure is called the complete breakdown voltage and is denoted Vb

On epitaxial growth of diamond films on (100) silicon substrates

Abstract: We have investigated characteristics of polycrystalline diamond thin films formed by plasma‐enhanced chemical vapor deposition method on silicon substrates using Raman spectroscopy, analytical and high‐resolution transmission electron microscopy techniques. Grains with average size 1 μm in diameter were observed in these films. The Raman spectra from these films contain the strongest peak at 1335 cm−1, providing the characteristic signature for sp3 (diamond) bonding. The broad peak centered around 1550 cm−1 is believed to be due to some graphitic bonding. From detailed high‐resolution images and microdiffraction, films were characterized to be cubic diamond with a lattice parameter of 3.56 A. Diamond crystallites with fivefold external morphologies were also observed. The large crystallites in the films exhibited preferential texture in 〈011〉 type orientations. These crystallites were found to be twinned in {111} planes. The large 〈011〉 crystallites exhibited matching in {111} or {200} lattice planes of diamond with {022} planes of silicon. This is in agreement with our previous work on the growth of Ni on MgO, which showed that textured growth can occur by matching a set of lattice planes in the absence of matching of lattice constants.

Heavy quarks on the lattice

Abstract: A method of treating heavy quarks is applied to lattice Q.C.D. for heavy quar masses (mH) and lattice spacing (a) satisfying the condition mHa > 1. Explicit applications to the measurement of heavy-light meson masses, decay constants, and mixing parameters are presented. Numerical results for B mesons are obtained on a 82 × 16 × 24 lattice with β = 5.7.

Preferentially oriented epitaxial Y‐Ba‐Cu‐O films prepared by the ion beam sputtering method

Abstract: Preferentially oriented epitaxial Y‐Ba‐Cu‐O films were prepared on (100) SrTiO3 substrates by oxygen reactive ion beam sputtering. The epitaxial orientations were varied by controlling both substrate temperature and oxygen parital pressure. c‐axis oriented films tended to be formed at higher substrate temperatures (>620 °C) and lower oxygen pressures ( 3×10−3 Torr). The best Tc (end) of 82 K was observed in one of the c‐axis oriented film without post‐annealing. The tendency for preferential orientation can be well understood in terms of the lattice mismatch between the substrate and the film, the lattice constants of which depend on oxygen deficiency.

Spatial localization of impurities in δ‐doped GaAs

Abstract: Capacitance‐voltage profiles on δ‐doped GaAs grown by molecular beam epitaxy reveal extremely narrow widths of ≲40 A at room temperature. Subband structure and capacitance‐voltage (C‐V) profiles of δ‐doped GaAs are calculated self‐consistently. Experimental C‐V profiles agree with self‐consistent results, only if we assume that Si impurities are localized on the length scale of the lattice constant in the host GaAs zinc‐blende lattice.

Epitaxial growth of CO on NaCl(100) studied by infrared spectroscopy

Abstract: Vibrational spectra of CO physisorbed onto well defined NaCl(100) surfaces were studied using a Fourier transform infrared interferometer. Structures of CO starting from the monolayer to multilayers were explored. At 31.5 K and a CO pressure of 1×10−6 mbar only the monolayer is formed. Polarization measurements confirm our earlier study that the monolayer CO molecules are aligned perpendicular to the NaCl(100) surface. Increasing the CO pressure to 7×10−6 mbar produces multilayer adsorption. The multilayer spectra closely resemble that of α‐CO absorption previously reported. The near perfect match of crystal structures and lattice constants of α‐CO and NaCl is reasoned to force the epitaxial growth of single crystal multilayers in our experiments. At 22 K the monolayer absorption is at 2155.01 cm−1 with a bandwidth (FWHH) of 0.26 cm−1. The two prominent features in the multilayer spectra at 22 K are assigned to the longitudinal optical (LO) mode at 2142.54 cm−1 and the transverse optical (TO) mode at 2138...

Scanning tunneling microscopy of the local atomic structure of two-dimensional gold and silver islands on graphite.

Abstract: Small, two-dimensional islands of silver and gold formed in situ by evaporation onto graphite cleaved in ultrahigh vacuum are imaged atom by atom with a scanning tunneling microscope. These islands contain ordered regions of roughly 50 atoms in rectangular lattices, incommensurate with the substrate lattice, that are not close packed as in the bulk fcc structure. In one series of images, the shorter lattice spacing remained constant at 2.35 +- 0.15 A whereas the longer decreased from 4.05 +- 0.1 to 3.5 +- 0.1 A over a period of roughly 10 min.

Rietveld analysis of Tl2Ba2Can−1CunO4+2n (n=1, 2 and 3) by powder x-ray diffraction

Abstract: Crystal structures of the phases in the Tl-Ba-Ca-Cu-O system determined by powder X-ray diffraction were refined using Rietveld analysis. Three samples having the general composition Tl 2 Ba 2 Ca n −1 Cu n O 4+2 n where synthesized. These were Tl 2 Ba 2 Ca 2 Cu 3 O 10 ( n =3,2223 phase ) with a T c of 116 K, Tl 2 Ba 2 CaCu 2 O 8 ( n =2,2212 phase ) with a T c of 85 K, and Tl 2 Ba 2 CuO 6 ( n =1,2201 phase ) which did not exhibit superconductivity. The results of the Rietveld analysis revealed that as n increased the lattice parameter, a , and the interatomic distance Cu-O(2) became shorter, and the Tl-O and Ba-O layers showed a tendency to become flatter. Possible substitution of about 10% of the Ca-sites with Tl ions in the 2212 and 2223 phases was also suggested.

X-ray diffraction and electron microscopy studies of the room-temperature structure of Cu2Se

Abstract: The structural phase transition of cuprous selenide has been studied by means of X-ray diffraction, electron diffraction and high-resolution transmission electron microscopy. The room-temperature phase is pseudo-monoclinic (a=c=7.14 AA approximately=1/2ac(112), b=81.9 AA approximately=8ac(111) and beta =120 degrees , where ac is the lattice constant in the high-temperature cubic phase). The structure is characterised by a periodic array of vacant tetrahedral and nearby interstitial copper atoms. A simplified structural model is presented for the arrangement of vacant copper sites; the vacancies occupy the 312/*31/2 units in the (111)c basal planes, and these vacancy layers are stacked every four copper layers, forming a ladder-like structure.

Flux-line shear through narrow constraints in superconducting films.

Abstract: Flux-line-lattice flow has been studied in a novel superconducting device containing straight, nanometer-scale, weak-pinning channels in a strong-pinning environment. The shear strength of a two-dimensional, triangular lattice has been directly probed on length scales of order of the lattice parameter by variation of the magnetic field. Good agreement is found with a continuum-approximation model for shear flow. Unusual oscillations in shear strength and history effects were observed and explained by the (in)commensurability between flux-line lattice and channel.

Coarsening and morphology of β′ particles in Fe-Ni-Al-Mo ferritic alloys

Abstract: Studies have been carried out of the effect of aging at 973 K on the ordered NiAl β′ precipitate particles in a series of Fe-Ni-Al alloys with and without the addition of Mo. The lattice parameter disregistry between matrix and precipitates is affected by the Mo content. The rate of precipitate coarsening during isothermal aging is slowest when the misfit is minimized. The dependence of coarsening upon misfit is more pronounced in alloys with low precipitate volume fractions. Raising the precipitate volume fraction increases β′ coarsening rates more than predicted by theory. The lattice parameters of both the β′ particles and the matrix are functions of aging time. Compositional changes, segregation of Mo to the particle-matrix interface and, at long aging times, loss of coherency are proposed as causes of the observed lattice parameter changes. The addition of Mo to the Fe-Ni-Al alloys causes the β′ particles to remain spherical even after extended periods of aging whereas the particles become somewhat cubic or rectangular parallelepiped in form in the Mo-free alloys. The size distributions of the β′ particles agree in a general way with the predictions of the Brailsford and Wynblatt and Voorhees and Glicksman models, but are somewhat less skewed and do not show the predicted sharp cutoff in particle size on the high radius side.

Superstructure of the superconductor Bi2Sr2CaCu2O8 by high-resolution electron microscopy

Abstract: The structure of the 80-K superconductor Bi2Sr2CaCu2O8 (ref. 1) determined by X-ray and neutron diffraction2–4 is an Aurivillius-related phase as suggested by Michel et al.5, with approximate unit-cell dimensions a = √2ap, b = 5√2ap and c = 8ap, where the perovskite lattice parameter ap = 3.8 A. It consists of layers of perovskite sandwiched between BiO layers. Although electron microscopy does not have sufficient resolution to unambiguously determine the basic structural unit of such a compound, it is an invaluable tool in determining the superstructure6. Here we present a high-resolution electron microscope study which reveals waves of distortion along the b-axis, giving a superlattice slightly larger or smaller than 5√2ap. The superlattice is composed of building blocks four, five, and six times √2ap/2 in approximately periodic combinations. The very low 'twin' density in Bi2Sr2CaCu2O8 and the nature of the 'twins' (successive planes with the a and b axes interchanged, also called twist boundaries) implies that twinning does not play an essential role in the superconducting phenomenon.

Magnetism and crystal structure of Laves phase compounds

Abstract: To investigate the magnetism and the stability of the crystal structure of the Laves phase compounds AB2, the total energy was systematically calculated by the LMTO method within the framework of the LDS approximation. For the non-magnetic compounds the crystal structure is determined by the B atom: the stable structure is the C15 structure when the B atom is Cr, Co or Ni(Mo, Rh or Pd) and the C14 structure when the B atom is Mn or Fe (Tc or Ru). In the Fe Laves phase compounds AFe2 the electronic structures change slightly with a change in the lattice spacing, thus inducing a varied magnetic behaviour. The theoretical predictions about the stability of the crystal structure and the magnetism are in good agreement with the experimental results.

Structural properties of two superconducting phases in the Bi−Sr−Ca−Cu−O system

Abstract: X-ray powder diffraction patterns and transmission electron diffraction patterns of superconductors with the nominal composition of BiSrCa3Cu4Ox have been observed and compared with the results of the magnetic susceptibility. Unit cell dimensions of high-Tc phase (Tc~107 K) are a=5.40 A, b=27.0 A, c=36.8 A, indicating a pseudotetragonal symmetry. High resolution structure image of the high-Tc phase shows that there are layer structural units with 15.4 A and 22.5 A in thickness, in addition to the main layer structural unit with a thickness of 18.4 A.