The zinc-blende (ZB) and wurtzite (W) structures are the most common crystal forms of binary octet semiconductors. In this work we have developed a simple scaling that systematizes the T=O energy difference bE~z a between W and ZB for all simple binary semiconductors. We have first calculated the energy difference hE "za( AB) for A1N, GaN, InN, A1P, A1As, GaP, GaAs, ZnS, ZnSe, ZnTe, CdS, C, and Si using a numerically precise implementation of the first-principles local-density formalism (LDF), including structural relaxations. We then find a linear scaling between AE~"za(AB) and an atomistic orbital-radii coordinate R( A, B) that depends only on the properties of the free atoms A and B making up the binary compound AB. Unlike classical structural coordinates (electronegativity, atomic sizes, electron count), R is an orbital-dependent quantity; it is calculated from atomic pseudopotentials.The good linear fit found between hE za and R {rms error of -3 meV/atom) permits predictions of the W -ZB energy difference for many more AB compounds than the 13 used in establishing this fit. We use this model to identify chemical trends in hE~z a in the IV-IV, III-V, II-VI, and I-VII octet compounds as either the anion or the cation are varied. W'e further find that the ground state of Mg Te is the NiAs structure and that CdSe and HgSe are stable in the ZB form. These compounds were previously thought to be stable in the W structures.
While as elemental solids, Al, Ni, Cu, Rh, Pd, Pt, and Au crystallize in the face-centered-cubic (fcc) structure, at low temperatures, their 50%-50% compounds exhibit a range of structural symmetries: CuAu has the fcc-based L1o structure, CuPt has the rhombohedral L1& structure, and CuPd and A1Ni have the body-centered-cubic B2 structure, while CuRh does not exist (it phase separates into Cu and Rh). Phenomenological approaches attempt to rationalize this type of structural selectivity in terms of classical constructs such as atomic sizes, electronegativities, and electron/atom ratios. More recently, attempts have been made at explaining this type of selectivity in terms of the (quantum-mechanical) electronic structure, e.g. , by contrasting the self-consistently calculated total electron+ion energy of various ordered structures. Such calculations, however, normally select but a small, O(10) subset of "intuitive structures" out of the 2 possible configurations of two types of atoms on a fixed lattice with X sites, searching for the lowest energy. We use instead first-principles calculations of the total energies of O(10) structures to define a multispin Ising Hamiltonian, whose ground-state structures can be systematically searched by using methods of lattice theories. Extending our previous work on semiconductor alloys [S.-H. Wei, L. G. Ferreira, and A. Zunger, Phys. Rev. B 41, 8240 (1990)], this is illustrated here for the intermetallic compounds A1Ni, CuRh, CuPd, CuPt, and CuAu, for which the correct ground states are identified out of-65000 configurations, through the combined use of the densityfunctional formalism (to extract Ising-type interaction energies) with a simple configurational-search strategy (to find ground states). This establishes a direct and systematic link between the electronic structure and phase stability. 'References 3-7. Reference 1. 'Reference 7(b). Reference 5 at x =0.4. 'Reference 14. Reference 8. 512 1991 The American Physical Society 'Calculated results of Ref. 86 at the experimental lattice constant, using the linear augmented Slater-type-orbital method and the Hedin-Lundqvist exchange-correlation. "Experimental results of Ref. 5. 'Calculated results for the unrelaxed structures of Ref. 62(a) using the augmented spherical-spherical wave (ASW) method and the von Barth-Hedin exchange correlation. Calculated results for the unrelaxed structures of Ref. 62(b) using the ASW method and the von Barth-Hedin exchange correlation. 'Calculated results of Ref. 87 using the ASW method and the von Barth-Hedin exchange correlation. Calculated results of Ref. 88 using the LMTO method and the Hedin-Lundqvist exxhange correlation gExperimental result of Ref. 7(b).
Brillouin optical time-domain analysis (BOTDA) requires frequency mapping of the Brillouin spectrum to obtain environmental information (e.g., temperature or strain) over the length of the sensing fiber, with the finite frequency-sweeping time-limiting applications to only static or slowly varying strain or temperature environments. To solve this problem, we propose the use of an optical chirp chain probe wave to remove the requirement of frequency sweeping for the Brillouin spectrum, which enables distributed ultrafast strain measurement with a single pump pulse. The optical chirp chain is generated using a frequency-agile technique via a fast-frequency-changing microwave, which covers a larger frequency range around the Stokes frequency relative to the pump wave, so that a distributed Brillouin gain spectrum along the fiber is realized. Dynamic strain measurements for periodic mechanical vibration, mechanical shock, and a switch event are demonstrated at sampling rates of 25 kHz, 2.5 MHz and 6.25 MHz, respectively. To the best of our knowledge, this is the first demonstration of distributed Brillouin strain sensing with a wide-dynamic range at a sampling rate of up to the MHz level.
Total-energy local-density calculations on approximately 20 periodic crystal structures of a given AB compound are used to define a long-range Ising Hamiltonian which correctly represents atomic relaxations.This allows us to accurately calculate structural energies of relaxed substitutional Aj B systems containing thousands of transition-metal atoms, simply by adding up spin products in the Ising Hamiltonian. The computational cost is thus size independent. We then apply Monte Carlo and simulated-annealing techniques to this Ising Hamiltonian, finding (i) the T = 0 ground-state structures, (ii) the order-disorder transition temperatures T"and (iii) the T ) T, short-range-order parameters. The method is illustrated for a transition-metal alloy (Cui Pd ) and a semiconductor alloy (Gai, In P). It extends the applicability of the local-density method to 6nite temperatures and to huge substitutional supercells. We 6nd for Cup. 75Pdp. 25 a characteristic fourfold splitting of the difFuse scattering intensity due to short-range order as observed experimentally.
A ground-state search of a generalized, many-body Ising Hamiltonian whose interaction energies are determined from first-principles local-density calculations reveals that PtY intermetallics for A' = Ni, Cu, Rh, and Pd will form stable ordered structures at low temperatures. In contrast, */-band tight-binding models universally predict phase separation in all late-transition-metal alloys. It is shown that the previously neglected s-electron cohesion is responsible for this phase stability.PACS numbers: 71.45.Nt, 61.55.Hg Voluminous catalogs of phase diagrams of binary transition-metal (TM) alloys 1,2 have instigated many attempts to identify global phenomenological trends and explain them in terms of microscopic constructs. 3 " 7 One such well-known 4 " 7 regularity is that systems where both constituents have nearly filled d shells ("late TM's"), have positive mixing enthalpies AH, and should show, at low temperatures, phase separation rather than longrange ordering. This has been explained in terms of tight-binding -band-filling arguments: 4 " 7 It was found that even in the absence of size mismatch between the constituents, 6 occupation of the upper "antibonding" part of the d band leads universally to AH > 0 for all late TM's with an average rf-electron count N > 8.The actual situation appears to be more complex, as illustrated, for example, by the phase behavior of binary alloys of Pt with its neighboring elements in the periodic table. 1,2 First, even discarding for a moment "special cases" such as the ordering Pt-Cu and Pt-Ag intermetallics 1 that contain a noble metal, or the ordering 1,2 Pt-Co system which is complicated by a magnetic behavior over a wide composition range, the fact that even the nonmagnetic Pto.5Nio.5 alloy orders defies all current rf-band theories. 5,7 While it is certainly possible to fit the observed Pt-Ni phase diagram with an Ising model, 8 attempts to explain even the sign of the Ising interaction energies required to produce the fit have all failed. 7 Second, while Pt-Rh and Pt-Pd were surmised 1,4 to phase separate, examination of the original data 9 shows that no evidence exists to this effect (they were measured only at very high temperatures where solid solutions exist 1 ), except for a suggestive extrapolation from the known behavior of Pd-Rh and Pd-Ir. In fact, measurements on Pt-Pd have shown negative mixing enthalpies 10 and clear evidence in x-ray diffuse scattering 11 for a substantial degree of short-range order which remains unexplained. Such structural preferences in intermetallic compounds can be addressed by highly precise totalenergy calculations based on the local-density approximation (LDA). 12,13 There, one selects for each intermetallic compound a few intuitively appealing candidate crystal structures and computes for each the equilibrium total energy, selecting the lowest. However, one is left to
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