Showing papers on "Lattice constant published in 2000"

Origin of anomalous lattice expansion in oxide nanoparticles

Abstract: Anomalous lattice expansions have been measured for the first time in monodisperse ${\mathrm{CeO}}_{2\ensuremath{-}x}$ nanoparticles and in ${\mathrm{BaTiO}}_{3}$ single nanoparticles by electron diffraction. X-ray photoelectron spectroscopy studies on ${\mathrm{CeO}}_{2\ensuremath{-}x}$ nanoparticles and ab initio computer simulation on ${\mathrm{BaTiO}}_{3}$ clusters show that the origin of expansion is the decrease of electrostatic force caused by valence reduction of Ce ions and the increase in ionicity of Ti ions, respectively. The lattice constant change of oxide (ionic) nanoparticles with the increase in ionicity would depend on the structure of the particles. Hence, first-principles calculations of large ionic clusters are indispensable.

Structure of laser-crystallized Ge2Sb2+xTe5 sputtered thin films for use in optical memory

Abstract: The structure of laser-crystallized thin films of Ge2Sb2+xTe5 (0.0

Microwave properties of tetragonally distorted (Ba0.5Sr0.5)TiO3 thin films

Abstract: A strong correlation is observed between the structure and the microwave dielectric properties of epitaxial Ba0.5Sr0.5TiO3 (BST) thin films deposited onto (001) MgO by pulsed laser deposition. Films were deposited at 750 °C in an oxygen pressure that was varied from 3 to 1000 mTorr. The tetragonal distortion (ratio of in-plane and surface normal lattice parameters, D=a/c) of the films depends on the oxygen deposition pressure. D varied from 0.996 at 3 mTorr to 1.003 at 800 mTorr. At microwave frequencies (1–20 GHz), BST films with low distortion have higher dielectric constants (e∼500), and lower dielectric loss (tan δ∼0.02) compared to films with higher distortion. The correlation of the microwave properties with the film structure can be attributed to stresses and polarizability in the film. The BST film grown at the oxygen deposition pressure of 50 mTorr exhibits a large dielectric constant change and a low dielectric loss at the same time, which corresponds to the film in low stress (D=1.0004). For tu...

Photofabrication of three-dimensional photonic crystals by multibeam laser interference into a photopolymerizable resin

Abstract: Three-dimensional (3D) photonic crystal structures can be fabricated into photopolymerizable resins by using laser beam interference with high precision. Three laser beams interfere into a glass cell filled with a liquid photopolymerizable resin to form a hexagonal periodic structure. Rods are formed in a hexagonal arrangement after being photopolymerzed according to the 3D periodic light distribution which results from the laser’s interference. Two beams of another laser also interfere to form layers which cross perpendicular to the rod array. After photofabrication, the nonsolidified resin is removed by ethanol. The lattice constant can be selected by tuning the angles of the incident beams and the laser wavelength. We have fabricated a 500 μm×500 μm×150 μm photonic crystal structure, the lattice constant of which is 1 μm and contains 150 lateral layers.

Effect of Si doping on strain, cracking, and microstructure in GaN thin films grown by metalorganic chemical vapor deposition

Abstract: The effect of Si doping on the strain and microstructure in GaN films grown on sapphire by metalorganic chemical vapor deposition was investigated. Strain was measured quantitatively by x-ray diffraction, Raman spectroscopy, and wafer curvature techniques. It was found that for a Si concentration of 2×1019 cm−3, the threshold for crack formation during film growth was 2.0 μm. Transmission electron microscopy and micro-Raman observations showed that cracking proceeds without plastic deformation (i.e., dislocation motion), and occurs catastrophically along the low energy {11_00} cleavage plane of GaN. First-principles calculations were used to show that the substitution of Si for Ga in the lattice causes only negligible changes in the lattice constant. The cracking is attributed to tensile stress in the film present at the growth temperature. The increase in tensile stress caused by Si doping is discussed in terms of a crystallite coalescence model.

Dielectric properties in heteroepitaxial Ba0.6Sr0.4TiO3 thin films: Effect of internal stresses and dislocation-type defects

Abstract: A series of heteroepitaxial Ba0.6Sr0.4TiO3 were grown on 0.29(LaAlO3):0.35(Sr2TaAlO6) substrates using pulsed-laser deposition. X-ray characterization revealed compressive in-plane stresses in the thinnest films, which were relaxed in a continuous fashion with increasing thickness. A theoretical treatment of the misfit strain was in good agreement with the measured out-of-plane lattice parameter. The low-frequency dielectric constant was measured to be significantly less than the bulk value and found to decrease rapidly for films less than 100 nm. A thermodynamic model was developed to understand the reduction in dielectric constant. By observing the microstructure using plan-view and cross-section transmission electron microscopy, we identified local strain associated with a threading dislocation density on the order of 1011 cm−2 as a possible mechanism for dielectric degradation in these films.

The Structure of the Passive Film That Forms on Iron in Aqueous Environments

Abstract: In situ surface X‐ray diffraction was used to identify the detailed structure of the passive film that forms on (000)− and (110)‐oriented iron single crystals in a borate buffer solution at +04 V vs mercurous sulfate reference electrode, a high passive potential The passive film is a new phase: a spinel with a fully occupied oxygen lattice, octahedral site occupancy of 80 ± 10%, tetrahedral site occupancy of 66 ± 10%, and an octahedral interstitial site occupancy of 12 ± 4% The passive film forms with an epitaxial relationship to the substrate iron; for growth on Fe(001), film(001)||Fe(001) and , while for growth on Fe(110), film(111)||Fe(110) and The in‐plane lattice parameter for the passive film (the LAMM phase) is 839 ± 001 A for growth on both faces, and the out‐of‐plane lattice parameter is 825 ± 01 A [Fe(001)] and 842 ± 01 A [Fe(110)] The passive film forms a nanocrystalline microstructure with numerous defects Specifically, the grain size is 50–80 A in‐plane and about 30 A out‐of‐plane There is a small mosaic spread of 25 to 41° and a high density of antiphase boundaries and stacking faults The structure of the film determined in situ was found to be identical to that found for an emersed sample, indicating that the high potential film studied here is stable on removal from the electrolyte Some of the implications of the film structure on passivity are discussed © 2000 The Electrochemical Society All rights reserved

Relationship between optical properties and crystallinity of nanometer Y2O3: Eu phosphor

Abstract: Y2O3-doped Eu3+ (Y2O3:Eu) nanometer particles (NPs) were synthesized via a chemical route The particle size estimated by x-ray diffractometry and transmission electron microscopy was about 61 nm Two photoluminescence peaks were observed at 582 and 587 nm, being attributed to the transition of 5D0→7F1a of Eu3+ in the S6 and C2 symmetry sites of Y2O3, respectively The intensity ratio of the photoluminescence peaks at 582 nm to at 587 nm was larger for NPs than for micrometer particles (MPs) The excitation peak of NPs due to the charge-transfer band shifted toward the high-energy side as compared with that of MPs According to x-ray diffractometry, the lattice distortion and the lattice constant were larger for NPs than for MPs, showing the restructure at the near surface and the increase in ionicity of the Eu–O bond with decreasing particle size This explains the blueshift in the excitation peak

Characterisation of Ti1−xSixNy nanocomposite films

Abstract: Ti 1− x Si x N y films were synthesised by RF reactive sputtering from Ti and Si elemental targets, in an Ar/N 2 gas mixture. XRD results revealed the development of a two-phase system, composed of a nanocrystalline f.c.c. TiN (phase 1: B1 NaCl type) and a second one (phase 2), where Si atoms replaced some of the Ti ones, inducing a structure that we may call a solid solution. An amorphous phase, supposed to be of silicon nitride, within grain boundaries seems to be also present, especially for high Si contents. TEM experiments confirmed the f.c.c.-type structure for phase 2, which is the only phase that develops without ion bombardment. The higher lattice parameter of phase 1 (∼0.429 nm compared to 0.424 nm for bulk TiN) may be explained by the residual stress effect on peak position. The Ti replacement by Si would explain the low value of the lattice parameter for phase 2 (∼0.418 nm). All samples showed good results for hardness (Hv≥30 GPa), and Ti 0.85 Si 0.15 N 1.03 at a deposition temperature of 300°C showed a value of approximately 47 Gpa, which is approximately double that of pure TiN. For higher deposition temperatures, an increase in hardness is observed, as demonstrated by this same sample, which at 400°C reveals a value of approximately 54 GPa. Similar behaviour was observed in adhesion, where this same sample revealed a critical load for adhesive failure of approximately 90 N. In terms of oxidation resistance, a significant increase has also been observed in comparison with TiN. At 600°C, the oxidation resistance of Ti 0.70 Si 0.30 N 1.10 is already 100 times higher than that of TiN. For higher temperatures this behaviour tends to be even better when compared with other nitrides.

Multi-junction, monolithic solar cell using low-band-gap materials lattice matched to GaAs or Ge

Abstract: A multi-junction, monolithic, photovoltaic solar cell device is provided for converting solar radiation to photocurrent and photovoltage with improved efficiency. The solar cell device comprises a plurality of semiconductor cells, i.e., active p/n junctions, connected in tandem and deposited on a substrate fabricated from GaAs or Ge. To increase efficiency, each semiconductor cell is fabricated from a crystalline material with a lattice constant substantially equivalent to the lattice constant of the substrate material. Additionally, the semiconductor cells are selected with appropriate band gaps to efficiently create photovoltage from a larger portion of the solar spectrum. In this regard, one semiconductor cell in each embodiment of the solar cell device has a band gap between that of Ge and GaAs. To achieve desired band gaps and lattice constants, the semiconductor cells may be fabricated from a number of materials including Ge, GaInP, GaAs, GaInAsP, GaInAsN, GaAsGe, BGaInAs, (GaAs)Ge, CuInSSe, CuAsSSe, and GaInAsNP. To further increase efficiency, the thickness of each semiconductor cell is controlled to match the photocurrent generated in each cell. To facilitate photocurrent flow, a plurality of tunnel junctions of low-resistivity material are included between each adjacent semiconductor cell. The conductivity or direction of photocurrent in the solar cell device may be selected by controlling the specific p-type or n-type characteristics for each active junction.

Fragility of photonic band gaps in inverse-opal photonic crystals

Abstract: Inverse-opal techniques provide a promising routine of fabricating photonic crystals with a full band gap in the visible and infrared regimes. Numerical simulations of band structures of such systems by means of a supercell technique demonstrate that this band gap is extremely fragile to the nonuniformity in crystals. In the presence of disorder such as variations in the radii of air spheres and their positions, the band gap reduces significantly, and closes at a fluctuation magnitude as small as under 2% of the lattice constant. This imposes a severe requirement on the uniformity of the crystal lattice. The fragility can be attributed to the creation of this band gap at high-frequency bands (eight to nine bands) in inverse-opal crystals.

Symmetry-adaptive ferroelectric mesostates in oriented Pb(BI1/3BII2/3)O3–PbTiO3 crystals

Abstract: An intermediate orthorhombic ferroelectric phase has been found in oriented crystals of (0.92)Pb(Zn1/3Nb2/3)O3–0.08PbTiO3 (PZN-PT 92/8). Investigations have been performed by electrically induced polarization and strain methods, and reciprocal phase space mapping. The lattice parameters of this intermediate ferroelectric state have been shown to be equal to those predicted by the adaptive martensite theory. A hierarchy of symmetries is believed to exist on the mesoscale which are due to symmetry reductions by domain averaging.

Electronic structure of three-dimensional graphyne

Abstract: Graphyne is a hypothetical carbon allotrope with a layered structure. We calculated the optimized geometries and electronic structures of three-dimensional graphyne in some possible stacking arrangements from symmetry considerations. The optimized lattice constants and the binding energy of graphyne are given in comparison with graphite. The binding energy of graphyne is about 90% of that of graphite, and graphyne will be stable when it is synthesized. The electronic structures are classified into two types, metallic and semiconducting, according to the stacking arrangements. The most stable graphyne is expected to be a semiconductor with a moderate band gap.

Superconductivity at 52 K in hole-doped C60.

Abstract: Superconductivity in electron-doped C60 was first observed almost ten years ago. The metallic state and superconductivity result from the transfer of electrons from alkaline or alkaline-earth ions to the C60 molecule, which is known to be a strong electron acceptor. For this reason, it is very difficult to remove electrons from C60—yet one might expect to see superconductivity at higher temperatures in hole-doped than in electron-doped C60, because of the higher density of electronic states in the valence band than in the conduction band. We have used the technique of gate-induced doping in a field-effect transistor configuration to introduce significant densities of holes into C60. We observe superconductivity over an extended range of hole density, with a smoothly varying transition temperature Tc that peaks at 52 K. By comparison with the well established dependence of Tc on the lattice parameter in electron-doped C60, we anticipate that Tc values significantly in excess of 100 K should be achievable in a suitably expanded, hole-doped C60 lattice.

Structure of Spinel

Abstract: This paper reviews the crystal structure of compounds with the general formula AB 2 X 4 , which crystallize with the same atomic structure as the mineral spinel, MgAl 2 O 4 . Three degrees of freedom associated with the detailed atomic arrangements of spinels are considered here: (i) the lattice parameter, a; (ii) the anion parameter, u; and (iii) the cation inversion parameter, i. Oxide spinels are used as examples to explore the interrelationships between these parameters.

Transport properties of strongly correlated metals: A dynamical mean-field approach

Abstract: The temperature dependence of the transport properties of the metallic phase of a frustrated Hubbard model on the hypercubic lattice at half-filling is calculated. Dynamical mean-held theory, which maps the Hubbard model onto a single impurity,Anderson model that is solved self-consistently, and becomes exact in the limit of large dimensionality, is used. As the temperature increases there is a smooth crossover from coherent Fermi liquid excitations at low temperatures to incoherent excitations at high temperatures. This crossover leads to a nonmonotonic temperature dependence for the resistance, thermopower, and Hall coefficient, unlike in conventional metals. The resistance smoothly increases from a quadratic temperature dependence at low temperatures to large values which can exceed the Mott-Ioffe-Regel value ha/e(2) (where a is a lattice constant) associated with mean free paths less than a lattice constant. Further signatures of the thermal destruction of quasiparticle excitations are a peak in the thermopower and the absence of a Drude peak in the optical conductivity. The results presented here are relevant to a wide range of strongly correlated metals, including transition metal oxides, strontium ruthenates, and organic metals.

Structural characterization of vacuum evaporated ZnSe thin films

Abstract: Thermally evaporated ZnSe thin films deposited on glass substrates within substrate temperatures (T s)at 303 K-623 K are of polycrystalline nature having f.c.c. zincblende structure. The most preferential orientation is along [111] direction for all deposited films together with other abundant planes [220] and [311]. The lattice parameter, grain size, average internal stress, microstrain, dislocation density and degree of preferred orientation in the film are calculated and correlated with T s.

Critical size and anomalous lattice expansion in nanocrystalline BaTiO 3 particles

Abstract: Nanocrystalline barium titanate particles are prepared by the alkoxide method. The lattice constants are obtained from the electron diffraction patterns for various orientations of single particles in the size range of 15--250 nm in diameter. The present result indicates that the structural change from a tetragonal (ferroelectric) phase to a cubic (paraelectric) one occurs around 80 nm in diameter, which is in good agreement with a critical diameter recently reported. Large lattice expansions of more than 2.5% are detected in the particles down to 15 nm in diameter. The origin of the expansion is discussed on the basis of x-ray photoelectron spectroscopic analyses and a computer simulation.

Hard nanocomposite Ti–Si–N coatings prepared by DC reactive magnetron sputtering

Abstract: Films resulting from Si additions to TiN matrix were prepared with Si contents in the range 0–19 at.%, using a closed field unbalanced DC magnetron sputtering system. Transmission Electron Microscopy (TEM) analyses revealed the nanocrystalline nature of these coatings, confirming the results of grain size evaluation from X-ray diffraction (XRD) patterns. Nanoindentation tests and scratch tests were carried out for the mechanical characterisation. Regarding the results, the samples show hardness values as high as 45 GPa. Best hardness values were found for Si content in the range 4–10 at.%. Almost all samples showed high critical loads for total adhesion failure, generally higher than 80 N, although the critical load for the first adhesion failure was found to be lower than 20 N for several samples. XRD patterns revealed the presence of only one phase that can be assigned to a cubic B1 NaCl structure, typical for TiN, with a lattice parameter of approximately 0.430 nm. The preferential growth, as a function of Si content, changes from a strong (111) orientation at the lowest Si additions to a weak (200) orientation at the highest Si content. Density values in the range 3.0–3.7 g/cm3 were obtained for most of the samples prepared with deposition rates between 0.5 and 1.1 μm/h, although higher density values were obtained for higher Ti deposition rates, with maximum of approximately 4.7 g/cm3 for the case of samples with low Si content.

Microstructure and magnetic properties of strained La0.7Sr0.3MnO3 thin films

Abstract: The lattice deformation of dense strained La0.7Sr0.3MnO3 (LSMO) films is shown to control the easy direction of the magnetization. Optimized pulsed laser deposited conditions allow the fabrication of dense LSMO thin films which present an exceptional flatness with a peak–valley roughness (Rp–v) of 1 A, associated to epitaxial grains as large as 1 μm. Electron microscopy coupled with x-ray diffraction have been used to study the unit cell distortion of both tensile and compressive dense LSMO films as a function of the thickness. No relaxation of the lattice distortion imposed by substrate has been observed in the thickness range 10–60 nm. The Curie temperature is not significantly affected by the nature of the substrate: a TC of 350 K is observed for both SrTiO3 (STO) and LaAlO3 (LAO) substrates, i.e., close to the bulk material (369 K). In contrast, the easy direction of magnetization depends on the substrate. For tensile films deposited on the STO substrate, the unit cell is elongated along the film’s pl...

Measurement of internal stresses via the polarization in epitaxial ferroelectric films

Abstract: The relations between electrical and mechanical properties of constrained ferroelectric films are analyzed. It is shown that the internal stresses and the elastic constants can be determined through the measurement of the electrical response. The change in the polarization is proportional to internal stresses due to film-substrate misfit, whereas the linear electrical and electromechanical responses to external field do not depend on the misfit and are determined by the film constraint. The theoretical results are successfully applied to PbZr0.2Ti0.8O3 films on (001) LaAlO3 substrate which exhibit a considerable increase in the saturation polarization due to epitaxial stresses. Significant recovery in the piezoelectric constant and susceptibility is theoretically predicted and experimentally verified for specific film configurations which reduce the degree of constraint. The concept presented in this Letter can be expanded to constrained ferromagnetic and superconductor films. PACS numbers: 77.22.Ch, 77.22.Ej, 77.55. + f, 77.65. – j Our recent experimental work has demonstrated that there is a considerable increase in the spontaneous polarization in thin films of epitaxial PbZr 0.2Ti0.8O3 (PZT) grown by pulsed laser deposition on (001) LaAlO3 (LAO) substrates with La0.5Sr0.5CoO3 (LSCO) electrodes in comparison with relatively thicker films [1]. This effect is accompanied with an increase in the out-of-plane (c-axis) lattice parameter. Both observations can be interpreted as an effect of internal stresses due to lattice mismatch which for the first time allow us to determine internal stresses using the polarization. In this Letter we present an analysis of these facts. Our goal is to formulate an approach to determine internal stresses in the constrained ferroelectric (FE) films using its electrical and electromechanical characteristics. Furthermore, this analysis leads to the engineering of the constraint to govern and enhance the electrical and electromechanical properties of FE films. To start with, we clarify the theoretical background for electromechanical properties of constrained FE films. Afterwards, we analyze our experimental work from this point of view and discuss perspectives. The dependence of the polarization on internal stresses follows directly from the Helmholtz free energy of a constrained film with top and bottom electrodes:

High‐pressure elasticity of stishovite and the P42/mnm ⇌ Pnnm phase transition

Abstract: A Landau free energy expansion has been developed to describe the elastic constant variations of stishovite (SiO2) associated with the P42/mnm (rutile-type) ⇌ Pnnm (CaCl2-type) phase transition as a function of pressure. The transition appears to display classical second-order character, with an equilibrium transition pressure which is renormalized by coupling of the soft optic mode with spontaneous strain. Lattice parameter data from the literature show that the symmetry breaking strain is large, while the nonsymmetry breaking strains are small. These have been used to constrain the numerical values of the strain/order parameter coupling coefficients, which in turn have been used to constrain values for the Landau coefficients. When substituted into expressions for all the individual elastic constants, taking values for experimental and calculated values of the bare elastic constants from the literature, a clear view of the pattern of elastic constant variations is produced. Variations of P and S wave velocities derived from the calculated elastic constants for a second-order transition show a dip as the transition point is approached from high pressure and from low pressure but do not show a discontinuity. The velocity anomaly provides a signature for the presence of free silica in the lower mantle.

Defect structure in homoepitaxial non-stoichiometric strontium titanate thin films

Abstract: The defect structure of non-stoichiometric strontium titanate thin films epitaxially grown on SrTiO3(100) substrates by pulsed-laser deposition was studied using transmission electron microscopy and X-ray diffraction. The A-site-excess SrTiO3 thin film adopted a completely unrelaxed pseudomorphic perovskite structure with an enlarged lattice parameter normal to the surface. Excess SrO was accommodated as Ruddlesden-Popper planar faults showing a peculiar three-dimensional mosaic structure, without forming secondary phases or dislocations due to non-stoichiometry. Biaxial compressive stress induced by lattice mismatch between the film and substrate was estimated at 9,7 GPa. Such a highly strained film is attributable to planar fault formation, which increases the cell volume and suppresses misfit dislocations. The B-site-excess SrTiO3 thin-film microstructure consisted of crystalline SrTiO3 and amorphous TiO2-rich phases. The slight increase in the lattice parameter normal to the surface and cryst...

The order of the magnetic phase transitions in RCo2 (R = rare earth) intermetallic compounds

Abstract: It has been found experimentally that the order of the magnetic phase transitions in RCo2 compounds (R standing for rare-earth metals) at Tc changes from second order for the light-rare-earth series up to TbCo2 to first order for the heavier-rare-earth compounds DyCo2, HoCo2 and ErCo2. On the basis of results of fixed-spin-moment band-structure calculations for the isostructural compound YCo2 at different lattice constants, we propose an explanation for this behaviour. In contrast to the widely accepted Inoue-Shimizu theory for this class of compounds, our explanation also includes Pr, Nd which were thought to behave differently due to the influence of crystal-field effects. We show that an itinerant-electron metamagnetic transition in these compounds can occur only over a certain range of lattice constants and that the possibility of a first-order phase transition is connected to features of the electronic structure rather than to the magnitude of the transition temperature as conjectured earlier. The influence of the latter is only important if the transition takes place at elevated temperatures, where effects of spin fluctuations can suppress a first-order transition.

Thickness and oxygen pressure dependent structural characteristics of BaTiO3 thin films grown by laser molecular beam epitaxy

Abstract: A series of BaTiO3 thin films with various thicknesses from 10 to 400 nm were epitaxially grown under various oxygen pressures from 2×10−4 to 12 Pa on SrTiO3 (001) substrates using laser molecular beam epitaxy. Being confirmed by reflection high energy electron diffraction, atomic force microscopy, x-ray diffraction, and high resolution transmission electron microscopy the epitaxial single crystal BaTiO3 thin films are highly c-axis or a-axis oriented with a root-mean-square surface roughness of 0.14 nm. The observed thickness and oxygen pressure dependent structural characteristics of the BaTiO3 thin films are discussed by taking into account both the misfits in thermal expansion and lattice constants between BaTiO3 films and SrTiO3 substrates, and the effect of the energy of the sputtered particles, which is consistent with the established strain relaxation mechanism. An abnormal expansion of lattice volume of a BaTiO3 unit cell is found and attributed to the effect of oxygen vacancies in the BaTiO3 films.

Linear and nonlinear optical properties of group-III nitrides

Abstract: Linear and nonlinear [second harmonic generation (SHG)] optical functions of cubic and hexagonal BN, AlN, GaN, and InN have been studied by using the first-principles full-potential linearized augmented plane-wave method. Equilibrium lattice constants are determined from the total-energy minimization method. The calculated spectra of the second-order optical susceptibility show pronounced structures related to the two-photon resonances. In materials with heavy metals there are remarkable contributions from the single-photon transitions. Line shapes of the linear and particularly the nonlinear optical spectra of GaN and InN crystals are very sensitive to the interactions between the conduction bands and metallic d states. Studies of the nonlinear optical susceptibilities in both wurtzite and cubic crystals show high sensitivity of the SHG spectra to the changes of atomic structure.

Trends in band-gap pressure coefficients in boron compounds BP, BAs, and BSb

Abstract: We have performed an ab initio investigation for a series of boron compounds, BP, BAs, and BSb, and have compared their structural and electronic properties with those of c-BN. The calculations are performed using a plane-wave expansion within the local density approximation and the pseudopotential approximation. Results are given for lattice constants, bulk moduli, band structures, and band-gap pressure coefficients. The electronic properties of these compounds are shown to have features that differ from those of other III-V materials. We found that the direct-band-gap pressure coefficient in boron compounds is nearly independent of the anion substitutions. As a result, this trend is similar to the one resulting from cation substitutions in other zinc-blende compounds. This is another anomalous behaviour which can be characterized by reversing the standard assignments for the anion and cation in these compounds.