Showing papers by "Alex Zunger published in 1996"
TL;DR: Using first-principles supercell calculations, a giant (7{endash}16 eV) and composition-dependent optical bowing coefficient in GaAs{sub 1{minus}{ital x}N{sub {ital x}} alloys} is found and it is suggested that in semiconductor alloys band gap variation as a function of {italx} can be divided into two regions.
Abstract: Using first-principles supercell calculations we find a giant (7{endash}16 eV) and composition-dependent optical bowing coefficient in GaAs{sub 1{minus}{ital x}}N{sub {ital x}} alloys. We show that both effects are due to the formation in the alloy of spatially separated and sharply localized band edge states. Our analysis suggests that in semiconductor alloys band gap variation as a function of {ital x} can be divided into two regions: (i) a bandlike region where the bowing coefficient is relatively small and nearly constant, and (ii) an impuritylike region where the bowing coefficient is relatively larger and composition dependent. For GaAs{sub 1{minus}{ital x}}N{sub {ital x}} the impuritylike behavior persists even for concentrated alloys. {copyright} {ital 1996 The American Physical Society.}
467 citations
TL;DR: A plane-wave semiempirical pseudopotential method with nonlocal potentials and spin-orbit coupling is used to calculate the electronic structure of surface-passivated wurtzite CdSe quantum dots with up to 1000 atoms, finding the correct form of Coulomb interaction energy with size-dependent dielectric constant is found to be essential.
Abstract: A plane-wave semiempirical pseudopotential method with nonlocal potentials and spin-orbit coupling is used to calculate the electronic structure of surface-passivated wurtzite CdSe quantum dots with up to 1000 atoms. The calculated optical absorption spectrum reproduces the features of the experimental results and the exciton energies agree to within \ensuremath{\sim}0.1 eV over a range of dot sizes. The correct form of Coulomb interaction energy with size-dependent dielectric constant is found to be essential for such good agreement. \textcopyright{} 1996 The American Physical Society.
414 citations
TL;DR: In this paper, first principles electronic structure calculations on wurtzite AlN, GaN, and InN reveal crystal field splitting parameters ΔCF of −217, 42, and 41 meV, respectively.
Abstract: First‐principles electronic structure calculations on wurtzite AlN, GaN, and InN reveal crystal‐field splitting parameters ΔCF of −217, 42, and 41 meV, respectively, and spin–orbit splitting parameters Δ0 of 19, 13, and 1 meV, respectively. In the zinc blende structure ΔCF≡0 and Δ0 are 19, 15, and 6 meV, respectively. The unstrained AlN/GaN, GaN/InN, and AlN/InN valence band offsets for the wurtzite (zinc blende) materials are 0.81 (0.84), 0.48 (0.26), and 1.25 (1.04) eV, respectively. The trends in these spectroscopic quantities are discussed and recent experimental findings are analyzed in light of these predictions.
288 citations
TL;DR: It is shown that if the dilute alloy (e.g., Ga-N-Ga-N...) first forms in the alloy, then there will be a composition domain in the concentrated alloy where its electronic properties become irregular: unusually large and composition dependent.
Abstract: Tradition has it that in the absence of structural phase transition, or direct-to-indirect band-gap crossover, the properties of semiconductor alloys (bond lengths, band gaps, elastic constants, etc.) have simple and smooth (often parabolic) dependence on composition. We illustrate two types of violations of this almost universally expected behavior. First, at the percolation composition threshold where a continuous, wall-to-wall chain of given bonds (e.g., Ga-N-Ga-N...) first forms in the alloy (e.g., $\mathrm{Ga}{\mathrm{As}}_{1\ensuremath{-}x}{\mathrm{N}}_{x}$), we find an anomalous behavior in the corresponding bond length (e.g., Ga-N). Second, we show that if the dilute alloy (e.g., $\mathrm{Ga}{\mathrm{As}}_{1\ensuremath{-}x}{\mathrm{N}}_{x}$ for $x\ensuremath{\rightarrow}1$) shows a localized deep impurity level in the gap, then there will be a composition domain in the concentrated alloy where its electronic properties (e.g., optical bowing coefficient) become irregular: unusually large and composition dependent. We contrast $\mathrm{Ga}{\mathrm{As}}_{1\ensuremath{-}x}{\mathrm{N}}_{x}$ with the weakly perturbed alloy system $\mathrm{Ga}{\mathrm{As}}_{1\ensuremath{-}x}{\mathrm{P}}_{x}$ having no deep gap levels in the impurity limits, and find that the concentrated $\mathrm{Ga}{\mathrm{As}}_{1\ensuremath{-}x}{\mathrm{P}}_{x}$ alloy behaves normally in this case.
259 citations
TL;DR: In this article, a pseudopotential plane wave theory is used to study the electronic and optical properties of a thousand atom hydrogen passivated Si quantum dots, wires and films, including band gap vs. shape, total electronic density of state and optical absorption spectra.
122 citations
TL;DR: In this article, the authors have calculated band offsets between ordered and disordered Ga 0.5In0.5P and GaAs using the first-principles pseudopotential method.
Abstract: Using the first‐principles pseudopotential method we have calculated band offsets between ordered and disordered Ga0.5In0.5P and between ordered GaInP2 and GaAs. We find valence band offsets of 0.10 and 0.27 eV for the two interfaces with the valence band maximum on ordered GaInP2 and GaAs, respectively. Using experimental band gaps these offsets indicate that the ordered/disordered Ga0.5In0.5P interface has type I band alignment and that the ordered GaInP2/GaAs interface has type II alignment. Assuming transitivity of the band offsets, these results suggest a type I alignment between disordered Ga0.5In0.5P and GaAs and a transition from type I to type II as the GaInP side becomes more ordered. Our calculations also show that ordered GaInP2 has a strong macroscopic electric polarization. This polarization will generate electric fields in inhomogeneous samples, strongly affecting the electronic properties of the material.
101 citations
TL;DR: This approach compares band energies, dispersion, and wave functions for GaAs/AlAs superlattices and quantum wells as computed directly from a pseudopotential band structure and using eight-band k\ensuremath{\cdot}p.
Abstract: The k\ensuremath{\cdot}p method combined with the envelope-function approximation is the tool most commonly used to predict electronic properties of semiconductor quantum wells and superlattices. We test this approach by comparing band energies, dispersion, and wave functions for GaAs/AlAs superlattices and quantum wells as computed directly from a pseudopotential band structure and using eight-band k\ensuremath{\cdot}p. To assure equivalent inputs, all parameters needed for the k\ensuremath{\cdot}p treatment are extracted from calculated bulk GaAs and AlAs pseudopotential band structures. Except for large exchange splittings in the in-plane dispersion for thin superlattices, present in pseudopotential calculations but absent from the k\ensuremath{\cdot}p results, we find generally good agreement for heterostructure hole bands within \ensuremath{\sim}200 meV of the GaAs valence-band maximum. There are systematic errors in band energies and dispersion for deeper hole bands (all other than hh1 and lh1) and significant qualitative and quantitative errors for the conduction bands. Errors for heterostructure conduction states which are derived from the zinc-blende \ensuremath{\Gamma} point diminish as length scales increase beyond \ensuremath{\sim}20 ML, while significant errors persist for L- and X-derived states.For bulk GaAs and AlAs, eight-band k\ensuremath{\cdot}p bands agree well with pseudopotential results very near the zinc-blende \ensuremath{\Gamma} point (where k\ensuremath{\cdot}p parameters are fit) but the first GaAs X point conduction band is \ensuremath{\simeq}26 eV too high with respect to the pseudopotential result. We show that this inadequate description of the bulk band dispersion is the principal source of k\ensuremath{\cdot}p errors in these heterostructures. A wave-function projection analysis shows that k\ensuremath{\cdot}p errors for heterostructures simply reflect corresponding errors for the bulk constituents, weighted by the amount that such bulk states participate in heterostructure states. \textcopyright{} 1996 The American Physical Society.
74 citations
TL;DR: The total energy of a given surface is expressed as a sum of the energies of the motifs, and an electrostatic term representing the Madelung energy of point charges, which are derived by fitting them to a set of pseudopotential total-energy calculations for flat GaAs(001) surfaces and for point defects in bulk GaAs.
Abstract: First-principles calculations of the atomic structure and formation energies of semiconductor surfaces and surface steps are often complicated by the existence of complex structural patterns. We suggest here a simpler, algebraic (not differential) approach that is based on two observations distilled from previous first-principles calculations. First, a relatively large collection of equilibrium structures of surfaces and bulk point defects can be built from a limited number of recurring local ``structural motifs,'' including for GaAs tetrahedrally bonded Ga and As and miscoordinated atoms such as threefold-coordinated pyramidal As. Second, the structure is such that band-gap levels are emptied, resulting in charged miscoordinated atoms. These charges compensate each other. We thus express the total energy of a given surface as a sum of the energies of the motifs, and an electrostatic term representing the Madelung energy of point charges. The motif energies are derived by fitting them to a set of pseudopotential total-energy calculations for flat GaAs(001) surfaces and for point defects in bulk GaAs. This set of parameters is shown to suffice to reproduce the energies of other (001) surfaces, calculated using the same pseudopotential approach. Application of the ``linear combination of structural motif'' (LCSM) method to flat GaAs(001) surfaces reveals the following: (i) The observed h(2\ifmmode\times\else\texttimes\fi{}3) surface may be a disordered c(8\ifmmode\times\else\texttimes\fi{}6) surface. (ii) The observed (2\ifmmode\times\else\texttimes\fi{}6) surface is a metastable surface, only 0.03 eV/(1\ifmmode\times\else\texttimes\fi{}1) higher than the \ensuremath{\alpha}(2\ifmmode\times\else\texttimes\fi{}4) surface having the same surface coverage. (iii) We confirm the recent suggestion by Hashizume et al. that the observed \ensuremath{\gamma}(2\ifmmode\times\else\texttimes\fi{}4) phase of the (2\ifmmode\times\else\texttimes\fi{}4) surface is a mixture of the \ensuremath{\beta}2(2\ifmmode\times\else\texttimes\fi{}4) and c(4\ifmmode\times\else\texttimes\fi{}4) surfaces. In particular, we examined an 8\ifmmode\times\else\texttimes\fi{}7 surface structure which has a lower energy than the earlier proposed \ensuremath{\gamma}(2\ifmmode\times\else\texttimes\fi{}4) structure. Application of the LCSM method to prototype steps on the GaAs(001)-(2\ifmmode\times\else\texttimes\fi{}4) surface is illustrated, comparing the LCSM results directly to pseudopotential results. \textcopyright{} 1996 The American Physical Society.
66 citations
TL;DR: An efficient method for electronic-structure calculations of nanostructures in which multiband coupling is included throughout the Brillouin zone and the underlying bulk band structure is described correctly even for far-fromG states, and a previously neglected interband overlap matrix appears in the k–p formalism.
Abstract: The electronic structure of quantum wells, wires, and dots is conventionally described by the envelope-function eight-band k\ensuremath{\cdot}p method (the ``standard k\ensuremath{\cdot}p model'') whereby coupling with bands other than the highest valence and lowest conduction bands is neglected. There is now accumulated evidence that coupling with other bands and a correct description of far-from-\ensuremath{\Gamma} bulk states is crucial for quantitative modeling of nanostructure. While multiband generalization of the k\ensuremath{\cdot}p exists for bulk solids, such approaches for nanostructures are rare. Starting with a pseudopotential plane-wave representation, we develop an efficient method for electronic-structure calculations of nanostructures in which (i) multiband coupling is included throughout the Brillouin zone and (ii) the underlying bulk band structure is described correctly even for far-from-\ensuremath{\Gamma} states. A previously neglected interband overlap matrix now appears in the k\ensuremath{\cdot}p formalism, permitting correct intervalley couplings. The method can be applied either using self-consistent potentials taken from ab initio calculations on prototype small systems or from the empirical pseudopotential method. Application to both short- and long-period (GaAs${)}_{\mathit{p}}$/(AlAs${)}_{\mathit{p}}$ superlattices (SL) recovers (i) the bending down (``deconfinement'') of the \ensuremath{\Gamma}\ifmmode\bar\else\textasciimacron\fi{}(\ensuremath{\Gamma}) energy level of (001) SL at small periods p; (ii) the type-II--type-I crossover at p\ensuremath{\approxeq}8 SL, and (iii) the even-odd oscillation of the energies of the R\ifmmode\bar\else\textasciimacron\fi{}/X\ifmmode\bar\else\textasciimacron\fi{}(L) state of (001) SL and \ensuremath{\Gamma}\ifmmode\bar\else\textasciimacron\fi{}(L) state of (111) SL. Introducing a few justified approximations, this method can be used to calculate the eigenstates of physical interest for large nanostructures. Application to spherical GaAs quantum dots embedded in an AlAs barrier (with \ensuremath{\sim}250 000 atoms) shows a type-II--type-I crossover for a dot diameter of 70 \AA{}, with an almost zero \ensuremath{\Gamma}-X repulsion at the crossing point. Such a calculation takes less than 30 min on an IBM/6000 workstation model 590. \textcopyright{} 1996 The American Physical Society.
52 citations
TL;DR: A strong dependence of surface segregation on the surface reconstruction is found, and a Ga enrichment of the surface in the moderate cation-rich limit and In enrichment in the anion-rich region is predicted, which suggests a way to achieve abrupt interfaces in semiconductor heterostructures.
Abstract: Using the first-principles total-energy pseudopotential method, we have studied the formation energy of the (001) ${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{In}}_{\mathit{x}}$P alloy surface as a function of composition and reconstruction The results are presented as T=0 surface stability diagrams that show the lowest energy reconstruction and cation occupation pattern as functions of the chemical potentials Slightly different stability diagrams emanate depending on whether or not there is equilibrium between the surface and the bulk The stability diagrams show a pronounced asymmetry between the Ga- and In-rich regions The asymmetry is interpreted in terms of the size difference between In and Ga and the effect of this size difference on the bonding geometry For surfaces in equilibrium with the bulk, we find a strong dependence of surface segregation on the surface reconstruction, and we predict a Ga enrichment of the surface in the moderate cation-rich limit and In enrichment in the anion-rich region This result suggests a way to achieve abrupt interfaces in semiconductor heterostructures For surfaces not in equilibrium with the bulk, we identify regions in the stability diagram where surface-induced CuPt ordering (both type A and type B) occurs \textcopyright{} 1996 The American Physical Society
49 citations
TL;DR: It is shown that the atomic geometry of the surface {ital V}{sub As}({ital q}) depends critically on the charge state {ital q} in sharp contrast with {ital bulk} vacancy, and it is predicted that the STM images will depend on the applied bias voltage.
Abstract: We report a comprehensive study of the As vacancies ( ${V}_{\mathrm{As}}$) in the GaAs(110) surface via ab initio total energy minimization. Previous scanning tunneling microscopy (STM) images of the ${V}_{\mathrm{As}}$ in $p$-type GaAs(110) were interpreted with a structure with outward movement of the Ga next to the vacancy. While our simulation of the STM images, using ab initio wave functions, agrees with experiment, our total energy minimization suggests, however, inward movement of Ga. We explain this apparent conflict as a charge induced band bending effect. As a consequence, we predicted that the STM images will depend on the applied bias voltage. We show that the atomic geometry of the surface ${V}_{\mathrm{As}}(q)$ depends critically on the charge state $q$ in sharp contrast with bulk vacancy.
TL;DR: In this paper, a plane-wave pseudopotential method was used to investigate the electronic structure of free standing and embedded GaAs quantum dots, wires, and films, and it was shown that the confinement energy of the valence-band maximum (VBM) is larger in AlAs-embedded than in free standing quantum structures, because of the zero-confinement character of the VBM wave function.
Abstract: Using a plane‐wave pseudopotential method we investigate the electronic structure of free‐standing and of AlAs‐embedded GaAs quantum dots, wires, and films. We predict that (i) the confinement energy of the valence‐band maximum (VBM) is larger in AlAs‐embedded than in free‐standing quantum structures, because of the zero‐confinement character of the VBM wave function in the latter case; (ii) small GaAs quantum structures have an indirect band gap, whereas large GaAs quantum structures have a direct band gap; (iii) the conduction‐band minimum of small free‐standing quantum structures originates from the GaAs X1c valley, while it derives from the AlAs X1c state in AlAs‐embedded quantum structures; (iv) the critical size for the direct/indirect crossover is larger in embedded quantum structures than in free‐standing quantum structures.
TL;DR: In this article, the authors compared single-band truncated-crystal approximation with a direct pseudopotential bandstructure calculation for free-standing hydrogen-passivated GaAs quantum films, wires, and dots.
Abstract: A single‐band approach for semiconductor clusters which accounts for the nonparabolicity of the energy bands was recently used by Rama Krishna and Friesner [M.V. Rama Krishna and R.A. Friesner, Phys. Rev. Lett. 67, 629 (1991)]. We compare the results of this method (denoted here as single‐band truncated‐crystal, or SBTC, approximation) with a direct pseudopotential band‐structure calculation for free‐standing hydrogen‐passivated GaAs quantum films, wires, and dots. The direct pseudopotential calculation, which includes coupling between all bands, shows that isolated GaAs quantum films, wires, and dots have an indirect band gap for thicknesses below 16, 28, and at least 30 A (8, 14, and at least 15 ML), respectively; beyond these critical dimensions the transition becomes direct. A comparison of the SBTC approximation with the direct pseudopotential calculation shows that (i) the confinement energy of the valence‐band maximum is overestimated by the SBTC method, because the zero‐confinement character of th...
TL;DR: In this article, the physical properties of alloys are compared as computed from ''direct'' and ''inverse'' procedures, where the direct procedure involves Monte Carlo simulations of a set of LDA-derived pair and multibody interactions, generating short-range order (SRO), ground states, order disorder transition temperatures, and structural energy differences.
Abstract: Physical properties of alloys are compared as computed from ``direct'' and ``inverse'' procedures. The direct procedure involves Monte Carlo simulations of a set of local density approximation (LDA)-derived pair and multibody interactions {
u_f}, generating short-range order (SRO), ground states, order- disorder transition temperatures, and structural energy differences. The inverse procedure involves ``inverting'' the SRO generated from {
u_f} via inverse-Monte-Carlo to obtain a set of pair only interactions {\tilde{
u}_f}. The physical properties generated from {\tilde{
u}_f} are then compared with those from {
u_f}. We find that (i) inversion of the SRO is possible (even when {
u_f} contains multibody interactions but {\tilde{
u}_f} does not) but, (ii) the resulting interactions {\tilde{
u}_f} agree with the input interactions {
u_f} only when the problem is dominated by pair interactions. Otherwise, {\tilde{
u}_f} are very different from {
u_f}. (iii) The same SRO pattern can be produced by drastically different sets {
u_f}. Thus, the effective interactions deduced from inverting SRO are not unique. (iv) Inverting SRO always misses configuration-independent (but composition- dependent) energies such as the volume deformation energy G(x); consequently, the ensuing {\tilde{
u}_f} cannot be used to describe formation enthalpies or two-phase regions of the phase diagram, which depend on G(x).
TL;DR: Generalizing the model to include a dependence of the charge on the atoms in the first three (two) shells in bcc (fcc) - rather than the first shell only - removes the fluctuations, in complete agreement with the LDA data.
Abstract: A simple analytic model of point-ion electrostatics has been previously proposed in which the magnitude of the net charge q_i on each atom in an ordered or random alloy depends linearly on the number N_i^(1) of unlike neighbors in its first coordination shell. Point charges extracted from recent large supercell (256-432 atom) local density approximation (LDA) calculations of Cu-Zn random alloys now enable an assessment of the physical validity and accuracy of the simple model. We find that this model accurately describes (i) the trends in q_i vs. N_i^(1), particularly for fcc alloys, (ii) the magnitudes of total electrostatic energies in random alloys, (iii) the relationships between constant-occupation-averaged charges and Coulomb shifts (i.e., the average over all sites occupied by either $A$ or $B$ atoms) in the random alloy, and (iv) the linear relation between the site charge q_i and the constant- charge-averaged Coulomb shift (i.e., the average over all sites with the same charge) for fcc alloys. However, for bcc alloys the fluctuations predicted by the model in the q_i vs. V_i relation exceed those found in the LDA supercell calculations. We find that (a) the fluctuations present in the model have a vanishing contribution to the electrostatic energy. (b) Generalizing the model to include a dependence of the charge on the atoms in the first three (two) shells in bcc (fcc) - rather than the first shell only - removes the fluctuations, in complete agreement with the LDA data. We also demonstrate an efficient way to extract charge transfer parameters of the generalized model from LDA calculations on small unit cells.
TL;DR: All three effects have a simple and common physical origin and unlike the case for conventional common-anion II-VI superlattices, the mixed offset {Delta}{ital E}{sub {ital V}/Mn{ital X}) depends strongly on {ital X}.
Abstract: Calculation of the unstrained band offsets between conventional zinc-blende II-VI superlattices (ZnS/ZnSe/ZnTe), or between magnetic II-VI superlattices (MnS/MnSe/MnTe) or combinations thereof ($\frac{\mathrm{Mn}X}{\mathrm{Zn}X}$) show that (i) the range of offsets spanned by different magnetic II-VI superlattices is compressed by a factor of 2 relative to the range of offsets spanned by conventional II-VI superlattices, (ii) the distance between the Mn $d$ band and the valence-band maximum in $\mathrm{Mn}X$ depends weakly on $X$, while in conventional II-VI superlattices (e.g., Zn $3d$ in $\mathrm{Zn}X$) there is a wider spread, and (iii) unlike the case for conventional commonanion II-VI superlattices, the mixed offset $\ensuremath{\Delta}{E}_{V}(\frac{\mathrm{Zn}X}{\mathrm{Mn}X})$ depends strongly on $X$. We show that all three effects have a simple and common physical origin.
TL;DR: The phonon-assisted-hopping probabilities between different localized electronic states are calculated from first principles, including contributions of polar optical, acoustic deformation-potential, and acoustic piezoelectrical effects, and the results agree qualitatively with recent experimental data.
Abstract: We have calculated from first principles the recently measured electron and hole transport of disordered AlAs/GaAs superlattices, in which the individual layer thicknesses n,m,${\mathit{n}}^{\ensuremath{'}}$,${\mathit{m}}^{\ensuremath{'}}$,... of sequence (AlAs${)}_{\mathit{n}}$(GaAs${)}_{\mathit{m}}$(AlAs${)}_{\mathit{n}}^{\ensuremath{'}}$(GaAs${)}_{\mathit{m}}^{\ensuremath{'}}$... are 1, 2, or 3, selected at random with equal probabilities. First, the near-edge electronic states are calculated using a three-dimensional pseudopotential representation for a \ensuremath{\sim}2000-ML cell. The results are then modeled by an effective-mass approximation, thus obtaining the electronic states in a wider energy range. All electronic states are found to be localized in the superlattice direction. Second, the phonon-assisted-hopping probabilities between different localized electronic states are calculated from first principles, including contributions of polar optical, acoustic deformation-potential, and acoustic piezoelectrical effects. Third, the master equation describing electron transport via phonon-assisted hopping is addressed using Monte Carlo simulations. The resulting transport properties versus temperature are analyzed according to dispersive transport theories, including the crossover from dispersive to equilibrium transport. A simple model for the photoluminescence process is proposed on the basis of the transport calculations. Our results agree qualitatively with recent experimental data.
TL;DR: In this paper, the relative stabilities of these two step structures and their stabilities with respect to other low energy A step structures are examined using a newly developed method of linear combination of structural motifs.
TL;DR: In this paper, the authors compared 8-band k¢p with pseudopotential results for (001) GaAs/AlAs superlattices and quantum wells with all input parameters directly computed from bulk GaAs and AlAs pseudoprocessor bands.
Abstract: T he k ¢ pmethod + envelope function combination used for semiconductor het- erostructures is based on approximations dubious under some conditions. We directly compare 8-band k¢p with pseudopotential results for (001) GaAs/AlAs superlattices and quantum wells with all k¢p input parameters directly computed from bulk GaAs and AlAs pseudopotential bands. We flnd generally very good agreement for zone-center hole states within» 200 meV of the GaAs valence band maximum, but i) systematic errors deeper in the valence band and ii) qualitative errors for even the lowest conduction bands with appreciable contributions from ofi-i zinc-blende states. We trace these errors to inadequate k¢p description of bulk GaAs and AlAs band dispersion away from the zone center.
01 Jan 1996
TL;DR: In this paper, Massal-ski et al. showed that the Pd-Pt and Rh-pt phase-separating alloys should phase separate or order, based on the maximum miscibility gap temperature and the difference in melting points of the constituents.
Abstract: In 1959, Raub1 suggested the existence of miscibility gaps in the Pd-Pt and Rh-Pt systems from the assumed correlation between the maximum miscibility gap temperature and the difference in melting points of the constituents, and from the experimentally observed miscibility gaps in “similar alloys” such as Pd-Rh, Ir-Pt, and Ir-Pd.1 Attempts to observe these miscibility gaps have, however, consistently failed.2–5 Phenomenological theories6,7 are not conclusive on whether Pd-Pt and Rh-Pt should phase-separate or order: Miedema’s model6 predicts ΔH ∼ ± (1–3) kJ/mol for Pd-Pt and Rh-’t, a value being close to the model’s limit of accuracy. Thus, neither experiment2–5 nor simple phenomenological theories6,7 shed light on whether these systems order or not. Yet, all phase-diagram compilations including the latest assessments by Massal-ski et al.,8 have adopted Raub’s view, labeling Pd-Pt and Rh-Pt as phase-separating (“miscibility gap”) systems.