Isolated Al Atoms Research Articles

Impacts of framework Al distribution and acidic properties of Cu-exchanged CHA-type zeolite on catalytic conversion of methane into methanol followed by lower hydrocarbons

This work highlighted the significance of the Al distribution in the CHA-type zeolite in the formation of active Cu species for the oxidative conversion of CH4; isolated Al atoms in the zeolite framework are found to be advantageous for producing CH3OH.

Chemical bonding and dynamic structural fluxionality of a boron-based Al2B8 binary cluster: the robustness of a doubly 6π/6σ aromatic [B8]2- molecular wheel.

Despite the isovalency between Al and B elements, Al-doping in boron clusters can deviate substantially from an isoelectronic substitution process. We report herein on a unique sandwich di-Al-doped boron cluster, Al2B8, using global structural searches and quantum chemical calculations. The cluster features a perfectly planar B8 molecular wheel, with two isolated Al atoms symmetrically floating above and below it. The two Al atoms are offset from the center of the molecular wheel, resulting in a C 2v symmetry for the cluster. The Al2B8 cluster is shown to be dynamically fluxional even at far below room temperature (100 K), in which a vertical Al2 rod slides or rotates freely within a circular rail on the B8 plate, although there is no direct Al-Al interaction. The energy barrier for intramolecular rotation is only 0.01 kcal mol-1 at the single-point CCSD(T) level. Chemical bonding analysis shows that the cluster is a charge-transfer complex and can be formulated as [Al]+[B8]2-[Al]+. The [B8]2- molecular wheel in sandwich cluster has magic 6π/6σ double aromaticity, which underlies the dynamic fluxionality, despite strong electrostatic interactions between the [Al]+, [B8]2-, and [Al]+ layers.

The proximity of aluminium atoms influences the reaction pathway of ethanol transformation over zeolite ZSM-5

The organization of aluminium atoms in zeolites affects their catalytic properties. Here we demonstrate that the aluminium distribution is a key parameter controlling the reaction pathway of acid catalysed reactions over ZSM-5 zeolites. We study ethanol transformation over two ZSM-5 samples with similar Si/Al ratios of ~15, and with aluminium atoms located mainly at the channel intersections but differently distributed in the framework. One of the samples contains mostly isolated aluminium atoms while the other has a large fraction of two aluminium atoms located in one ring. The FT-IR time-resolved operando study, supported by catalytic results, reveals that the reaction pathway in ethanol transformation over ZSM-5 is controlled by the proximity of aluminium atoms in the framework. ZSM-5 containing mostly isolated Al atoms transforms ethanol in the associative pathway, and conversely ZSM-5 containing a dominating fraction of two aluminium atoms in one ring transforms ethanol in the dissociative pathway.

Catastrophic Recombination/Reaction of Aluminum Atoms in Cryogenic Parahydrogen Solids.

We report the results from experiments testing the limits of chemical energy storage in Al-atom-doped cryogenic parahydrogen (pH2) solids produced by codeposition of Al vapor and pH2 gas. These results map out three regimes of behavior. As described in the immediately preceding companion manuscript, for target Al atom concentrations, [Al]target ≲ 200 parts-per-million (ppm), the Al/pH2 solids are optically transparent and primarily contain isolated Al atoms, with a small admixture of AlxHy molecules formed by Al atom recombination/reaction. For [Al]target ≳ 500 ppm, the depositions fail immediately, producing highly optically scattering solids with evidence of extensive Al atom recombination/reaction. For intermediate [Al]target concentrations, near-threshold metastable transparent solids are formed which suffer catastrophic recombination/reaction as they grow past some critical thickness. Analysis of these catastrophic events using a minimal two parameter model [Jackson. J. Chem. Phys. 1959, 31, 722-729] yields a critical Al atom concentration near [Al] ≈ 280 ppm and an Al atom diffusion length ≈ 100 μm. While this diffusion length appears unphysically large, it is roughly compatible with even larger 300-900 μm wavelengths of spatial inhomogeneities visible in images of a remnant postrecombination sample. This spatial pattern is evocative of Turing structures or perhaps the structures sometimes formed upon phase separation; however, the actual mechanism for morphogenesis remains undetermined. The largest achieved Al atom concentrations of [Al] ≈ 300 ppm are roughly two orders-of-magnitude lower than those required for advanced chemical propulsion applications.

Quantitative Absorption Spectroscopy of Aluminum Atoms in Cryogenic Parahydrogen Solids.

We report results of quantitative ultraviolet (UV) and infrared (IR) absorption spectroscopy experiments on Al-atom-doped cryogenic parahydrogen (pH2) solids produced by codeposition of Al vapor and pH2 gas. For Al-atom concentrations [Al] ≲200 parts-per-million (ppm), the Al/pH2 solids are optically transparent and primarily contain isolated Al atoms, with a small admixture of AlH, Al2H2, and Al2H4 molecules formed by UV irradiation and Al-atom recombination/reaction. We assign the Al/pH2 UV absorption spectrum by invoking a large (≈0.6 eV) gas-to-matrix blue shift to accompany the increase in principal quantum number in the 4s 2S ← 3p 2P1/2 transition, as previously discussed for boron-atom-doped pH2 solids. We assign a series of sharp features observed in the 4140-4155 cm-1 IR region to Al-atom-induced Q1(0) and Q1(1) absorptions of the pH2 solid. We use the solid pH2 Q1(0) + S0(0) absorption to determine the sample thickness and to establish a constant pH2 deposition efficiency independent of the flow rate. Using all of these absorption features in concert, we show that the Al-atom flux delivered by the effusive source is well described by the Knudsen-Langmuir equation, calculate an absolute Al-atom deposition yield per mass of aluminum evaporated, and demonstrate both constant Al-atom deposition and isolation efficiencies for [Al] ≲200 ppm. We discuss this unexpected constant Al-atom isolation efficiency in detail and speculate that it indicates nonuniform Al-atom recombination/reaction on the surface of the accreting sample, perhaps dominated by processes occurring near pH2 crystallite grain boundaries. We demonstrate "control" over the deposition process, which we define as the ability to set, achieve, and verify "targets" for the final Al-atom concentrations and pH2 solid thicknesses. This ability is key to sorting out the very different phenomena observed in samples targeting [Al] ≳300 ppm, which are described in the immediately following companion manuscript.

Atomistic Investigations of the Effects of Si/Al Ratio and Al Distribution on the Adsorption Selectivity of n-Alkanes in Brønsted-Acid Zeolites

The adsorption of n-alkanes onto Brønsted-acid sites is a key step in the catalytic cracking of alkanes. Employing configurational-bias Monte Carlo simulations, we have investigated how the ratio of equilibrium adsorption constants for central C–C bonds relative to terminal bonds of n-alkanes (i.e., the adsorption selectivity ratio) in Brønsted-acid zeolites is influenced by the Si/Al ratio and the Al distribution. A new computational approach was implemented, and the developed force field was validated by a comprehensive comparison between simulation results and experimental data for a number of Brønsted-acid zeolites. While the adsorption selectivity seems to be relatively insensitive to the Si/Al ratio, our results reveal that the Al distribution plays a crucial role in determining the adsorption selectivity. Changes in the Al distribution result in a change of as much as 2-fold in the adsorption selectivity ratio for n-hexane. The selectivity generally shows larger variations with respect to Al distribution in zeolites with a larger Si/Al ratio. The two factors identified by this work that substantially influence the selectivity ratio are the siting of Al atoms among T-sites and their spatial proximity, and an atomic-level understanding of each of these effects was achieved. The siting of Al atoms at more or less selective T-sites significantly influences the overall selectivity ratio, and Al atoms in close proximity can synergistically enhance the adsorption of central C–C bonds, leading to a higher selectivity ratio relative to isolated Al atoms. We anticipate that these results will have important implications for future large-scale computational screenings and the development of advanced synthesis approaches to target certain Al distributions in zeolites.

Location of Framework Al Atoms in the Channels of ZSM‐5: Effect of the (Hydrothermal) Synthesis

(27) Al 3Q MAS NMR and UV/Vis spectroscopy with bare Co(II) ions as probes of Al pairs in the zeolite framework were employed to analyze the location of framework Al atoms in the channel system of zeolite ZSM-5. Furthermore, the effect of Na(+) ions together with tetrapropylammonium cation (TPA(+)) in the ZSM-5 synthesis gel on the location of Al in the channel system was investigated. Zeolites prepared using exclusively TPA(+) as a structure-directing agent (i.e., in the absence of Na(+) ions) led to 55-90% of Al atoms located at the channel intersection, regardless the presence or absence of Al pairs [Al-O-(Si-O)2 -Al sequences in one ring] in the zeolite framework. The presence of Na(+) ions in the synthesis gel did not modify the Al location at the channel intersection (55-95% of Al atoms) and led only to changes in i) the distribution of framework Al atoms between Al pairs (decrease) and single isolated Al atoms (increase), and ii) the siting of Al in distinguishable framework tetrahedral sites.

The Strength of Brønsted Acid Sites in Microporous Aluminosilicates

Ensemble-averaged deprotonation energies (DPE) derived from periodic density functional theory models are insensitive to the location of isolated Al atoms and associated protons and similar among microporous aluminosilicates (i.e., zeolites) with different crystalline frameworks (MFI, BEA, FER, MOR, CHA, FAU). These DPE values are 1201 ± 11 kJ mol–1 after correcting for systematic artifacts of periodic DFT methods, which vary with framework density, and averaging over the four distinct proton locations at each Al atom. These energies rigorously reflect the strength of the acid sites in these important catalytic solids. Thus, the stability of the conjugate anions and the acid strength of these materials merely reflect the presence of Al atoms within the silicate framework, and not their specific siting or local confining environment. DPE values did not show any systematic trends with the vibrational frequency or length of O–H bonds, with Si–O–Al bond angles, or with NH3 adsorption enthalpies, properties th...

Aluminum coprecipitates with Fe (hydr)oxides: Does isomorphous substitution of Al 3+ for Fe 3+ in goethite occur?

Iron (hydr)oxides are common in natural environments and typically contain large amounts of impurities, presumably the result of coprecipitation processes. Coprecipitation of Al with Fe (hydr)oxides occurs, for example, during alternating reduction-oxidation cycles that promote dissolution of Fe from Fe-containing phases and its re-precipitation as Fe–Al (hydr)oxides. We used chemical and spectroscopic analyses to study the formation and transformation of Al coprecipitates with Fe (hydr)oxides. In addition, periodic density functional theory (DFT) computations were performed to assess the structural and energetic effects of isolated or clustered Al atoms at 8 and 25 mol% Al substitution in the goethite structure. Coprecipitates were synthesized by raising the pH of dilute homogeneous solutions containing a range of Fe and Al concentrations (100% Fe to 100% Al) to 5. The formation of ferrihydrite in initial suspensions with ⩽20 mol% Al, and of ferrihydrite and gibbsite in initial suspensions with ⩾25 mol% Al was confirmed by infrared spectroscopic and synchrotron-based X-ray diffraction analyses. While base titrations showed a buffer region that corresponded to the hydrolysis of Fe in initial solutions with ⩽25 mol% Al, all of the Al present in these solutions was retained by the solid phases at pH 5, thus indicating Al coprecipitation with the primary Fe hydroxide precipitate. In contrast, two buffer regions were observed in solutions with ⩾30 mol% Al (at pH ∼2.25 for Fe 3+ and at pH ∼4 for Al 3+), suggesting the formation of Fe and Al (hydr)oxides as two separate phases. The Al content of initial coprecipitates influenced the extent of ferrihydrite transformation and of its transformation products as indicated by the presence of goethite, hematite and/or ferrihydrite in aged suspensions. DFT experiments showed that: (i) optimized unit cell parameters for Al-substituted goethites (8 and 25 mol% Al) in clustered arrangement (i.e., the formation of diaspore-like clusters) were in good agreement with available experimental data whereas optimized unit cell parameters for isolated Al atoms were not, and (ii) Al-substituted goethites with Al in diaspore-like clusters resulted in more energetically favored structures. Combined experimental and DFT results are consistent with the coprecipitation of Al with Fe (hydr)oxides and with the formation of diaspore-like clusters, whereas DFT results suggest isomorphous Al for Fe substitution within goethite is unlikely at ⩾8 mol% Al substitution.

Complex Analysis of the Aluminum Siting in the Framework of Silicon-Rich Zeolites. A Case Study on Ferrierites

We present a multistep method combining multispectroscopic experiments with DFT calculations to determine the complete Al distribution in silicon-rich zeolites, independent of the presence of Al–O–(Si–O)n–Al (n = 1, 2) sequences in their frameworks. 29Si MAS NMR spectroscopy is employed to confirm the absence of Al–O–Si–O–Al in the framework of silicon-rich zeolites while 27Al 3Q MAS NMR spectroscopy and DFT computations of 27Al isotropic chemical shifts serve to determine the locations of isolated Al atoms. The maximum ion-exchange capacity of zeolites for [Co2+(H2O)6]2+ reveals the presence of close Al atoms (i.e., those Al atoms which are able to balance [Co2+(H2O)6]2+ ions). Then visible spectroscopy of the bare Co(II) ion in the dehydrated zeolite of the samples with close Al is utilized to identify the locations of the corresponding Al–O–(Si–O)2–Al pairs in a ring. Subsequently, their 27Al isotropic chemical shifts are evaluated at DFT and the complete Al distribution is determined. The complete Al siting in three ferrierite samples with only isolated framework Al atoms and two ferrierites with Al–O–(Si–O)2–Al sequences was determined. Our results reveal that the Al siting in the former samples varies with the conditions of the zeolite synthesis; Al is present in three or four sites (T1b, T2, T3, and T4) depending on the sample while T1a is never occupied by Al and the concentrations of Al atoms in various T sites are very diverse. For ferrierites with both isolated and close Al atoms, isolated Al atoms occupy the T2, T3, and T4 sites and the close Al atoms are arranged in Al(T1a)–O–(Si–O)2–Al(T1a) and Al(T2)–O–(Si–O)2–Al(T2) sequences forming the α and β – 2 cationic sites, respectively. Isolated Al atoms do not occupy the T1b site and close Al atoms do not form Al(T4)–O–(Si–O)2–Al(T4) sequences of the β – 1 site. The differences between the concentrations of Al in T sites are not as pronounced as those for the ferrierite samples with only isolated framework Al atoms. In addition, our results reveal that the Al siting in ferrierite is not random and depends on the conditions of the zeolite synthesis.

Control of metal ion species in zeolites by distribution of aluminium in the framework: From structural analysis to performance under real conditions of SCR-NO x and NO, N 2O decomposition

The structure, properties and activity of the counter Co, Cu and Fe ion species depending on the distribution of aluminium atoms in the framework between Al pairs of Al–O–(Si–O) 2–Al sequences in one ring and isolated Al atoms (Al–O–(Si–O) n>2 –Al sequences) in two rings are analyzed. The effect of the Al distribution on the activity of metal ion species is investigated for reactions of technology importance, i.e. SCR-NO x by propane, and decomposition of NO and N 2O under real conditions of NO x streams. Zeolite samples of BEA, MFI and FER structures, of similar composition for each topology, but with basically different distribution of Al atoms in the framework, were either selected from commercial samples or prepared by the developed synthesis. The concentration of Al pairs and isolated Al atoms in the framework was determined by the combination of 29Si MAS NMR and quantitative analysis of the Vis spectra of dehydrated Co-zeolites with Co(II) ions exchanged to a maximum degree. The structure and reactivity of the metal sites was described by UV–Vis, FTIR of skeletal vibrations and adsorbed NO, TPR-H 2 and reactions of C 3H 8-SCR-NO x/H 2O, and NO and N 2O decomposition. It is concluded that the local negative framework charge, originated from Al pairs and isolated Al atoms, constitutes a decisive parameter controlling the structure and reactivity of the counter metal ion species. The bare Co(II), Cu(II) and Fe(II) ions are balanced by Al pairs, while Co-oxo and Cu-oxo or Cu(I) species are adjacent to isolated Al atoms. The Cu(I) ions in vicinity of isolated Al atoms in Cu-MFI zeolites are the most active sites in NO decomposition. Co-oxo species attached to isolated Al atoms in Co-BEA zeolites, in contrast to bare Co(II) ions, exhibit high activity in C 3H 8-SCR-NO x/H 2O in the presence of water vapor. N 2O decomposition to elements over Fe-FER requires two Fe ions at a defined distance of β cationic sites, with each Fe(II) cation charge-balanced by an Al pair.

The decisive role of the distribution of Al in the framework of beta zeolites on the structure and activity of Co ion species in propane–SCR–NO x in the presence of water vapour

The role of the distribution of aluminium in the framework of beta zeolites in the formation of various types of counter Co species of different activity in C 3H 8–SCR–NO in the absence/presence of 10% water vapour was investigated. Different concentrations of Al atoms in the Al–O–(Si–O) 2–Al sequences in one ring (Al pairs) and isolated Al atoms in Al–O–(Si–O) n >2–Al sequences located in two framework rings of parent BEA* zeolites were obtained by variation of the zeolite synthesis and by post-synthesis dealumination. Al atoms in the framework and their distribution were characterized by the combination of 29Si MAS NMR of the parent BEA* zeolites and UV–Vis spectroscopy of the maximum-Co(II)-loaded BEA*. The concentration of Al pairs corresponded to the concentration of bare Co(II) ions in dehydrated maximum exchanged-Co(II)-BEA* calculated from the Vis spectra. Co-BEA* zeolites prepared by ion exchange from Co nitrate solution contained bare Co(II) ions balanced by Al pairs, which were formed preferentially, and “monovalent” Co-oxo species balanced by the isolated Al atoms. The relative concentration of bare Co(II) ions and Co-oxo species depends on the degree of Co(II) ion exchange and the concentration of Al pairs and isolated Al atoms in the zeolite framework. Superior activity was found with the counter Co-oxo species balancing isolated Al atoms, surviving even in wet (10% H 2O) NO x streams. The bare Co(II) ions were active in dry NO x streams, but their activity was negligible in wet NO x streams. A highly active catalyst (more than twice than the other Co-BEA*) can be prepared by using a BEA zeolite containing a predominant concentration of isolated Al atoms in the framework.

The interaction of Al atoms with water molecules: A theoretical study

The interaction of individual Al atoms with water molecules has been studied from the point of view of the energy by means of ab initio and DFT calculations in order to find the pathways for the generation of HAlOH(H(2)O)(n) or AlOH(H(2)O)(n)+H. The potential energy surface of the Al(H(2)O)(n+1) (n=0-3) systems has been explored for local minima and the relevant saddle points. The interaction of Al and several water molecules tends to produce low-lying local minima in which (for n=2-3) Al is "inserted" into relatively compact structures of water molecules, so typically a ring is formed containing an AlOH(2) moiety. Isomerization of such moiety into HAlOH(H(2)O)(n) may take place through saddle points of the "relay" type; however those saddle points lie very close to AlOH(2)(H(2)O)(n-1)+H(2)O, which means that such isomerization process has to compete with a water-elimination process. Larger systems Al(H(2)O)(n+1) (n=4-7) have also been computed by means of a DFT method, as a first step to predict the behavior of isolated Al atoms in even larger clusters or in the bulk. Finally, an effective fragment potential method has been employed to simulate those large clusters together with a polarizable continuum model to take into account the effect of the bulk. Our results suggest that the reaction should take place in inert matrices and in the bulk. We also conclude that HAlOH(H(2)O)(n) and AlOH(H(2)O)(n) could be the intermediates involved in the generation of hydrogen by the interaction of Al atoms with water, so their production would be the critical step of such process.

Acid sites topology in aluminas and zeolites from high-resolution solid-state NMR

High-resolution solid-state nuclear magnetic resonance (NMR) spectroscopy is recognized as one of the most powerful tools in characterization of acid catalysts. Hundreds of research papers and tens of reviews and monographs appear each year. It is evidently an impossible task not only to discuss all recent contributions but even to mention each of them. Therefore, we restricted ourselves to a rather narrow (but fast developing) field, namely, the characterization of surface topology of real catalysts by heteronuclear dipolar interaction between magnetic nuclei. The common desire of all spectroscopists to obtain spectra with the best possible resolution competes with the general trend to obtain the greatest amount of information on systems. In this review we outline experimental approaches to this problem and describe several editing techniques which introduce new dimension into conventional NMR. The discussion is furnished with examples from current literature, however the emphasis is given to our recent publications in which the introduction of new experimental ideas resulted in deeper insight into the nature of surface acidity. In particular, two problems are discussed in detail, namely, how to observe selectively surface species (cross-polarization experiments) and how to retrieve dipolar contribution from the overall lineshape (double-resonance spin-echo experiments). The results obtained from these experiments allowed us to reach important conclusions concerning the nature of Lewis acidity in aluminas and the role of isolated Al atoms in formation of Bronsted sites in zeolites.

Al diffusion into GaAs from monatomic AlAs layers investigated by localized vibrational modes

Using infrared absorption spectroscopy, we investigate the thermal diffusion behavior of Al atoms from monatomic Al layers embedded in a GaAs epitaxial film. After thermal annealing, the absorption peak of the two-dimensionally localized vibrational modes at 358 cm−1 due to Al layers decreases, while the peak at 362 cm−1 due to isolated Al atoms increases. The 362 cm−1 peak height is compared with the fraction of isolated Al atoms calculated, assuming the second nearest neighbor hopping diffusion from a monatomic Al layer into GaAs matrix. We thus determine the diffusion coefficient of Al atoms in GaAs to be 2×10−19 cm2/s at 700 °C. The present study offers a simple and reliable method to investigate the impurity diffusion in crystals.

The chemical and electronic structure of the interface between aluminum and conjugated polymers

The chemical and electronic structure of the interface between aluminum and several proto-typical conjugated systems is investigated with a combined experimental and theoretical approach. The experiments consists of following the evolution of the polymer surface during the early stages of aluminum deposition, with X-ray and Ultraviolet Photoelectron Spectroscopies (XPS, UPS). In parallel, quantum chemical calculations are performed on model oligomer systems interacting with isolated Al atoms. Aluminum is found to interact strongly with the polymer chain. New covalent Al-carbon bonds are formed along the polymer backbone; the chain geometry is deeply modified and the π electron conjugation can be dramatically reduced.

Stoichiometric adsorption complexes in H-ZSM-5, H-ZSM-12, and H-mordenite zeolites

In order to understand the chemical properties of acid sites in zeolites, the authors have been examining the adsorption of simple molecules which can undergo classical acid-catalyzed chemistry in high-silica H-ZSM-5. For many molecules, including a series of alcohols, toluene, 2-propanamine, and 2-propanone, the authors have found that, following adsorption and evacuation, a well-defined adsorption complex corresponding to one molecule per Al site could be obtained. Furthermore, the unimolecular reactivities of the molecules in these 1 : 1 adsorption complexes were consistent with solution-phase, Bronsted acid-catalyzed chemistry and were unaffected by the Al concentration for SiO{sub 2}/Al{sub 2}O{sub 3} ratios from 38 to 520. These results imply that the active acid sites in H-ZSM-5 are identical and are present in a concentration of one/Al atom. This is consistent with conclusions reached from n-hexane cracking studies on H-ZSM-5. In order to examine the effect of solid-state structure on these acid sites, the authors extended their previous work to two other zeolites. H-ZSM-12 and H-mordenite. The adsorption of 2-propanol and 2-propanamine were chosen for study since the 1 : 1 complexes formed by these two molecules were the most distinctive in our previous adsorption studies. The authors show in this paper thatmore » 2-propanol and 2-propanamine also form clearly defined, 1 : 1 adsorption complexes with the Al site in H-ZSM-12 and H-mordenite. The reactivity of these adsorption complexes is identical to the reactivity of the 1 : 1 adsorption complexes formed by these molecules in H-ZSM-5. The complexes decompose at the same temperatures and form the same products on all three zeolites. This is strong evidence that the acid sites formed by isolated Al atoms in a zeolite lattice are chemically equivalent, independent of the structure of the lattice.« less

Electron scattering in semiconductor alloys

Suitability of the Born approximation and the Boltzmann equation is demonstrated for the scattering of free-carrier electrons by random-alloy atomic potentials in semiconductor alloys. Composition dependences of alloy-scattering potential strengths are hypothesized and electron scattering rates are derived. ‘‘Order parameters’’ are derived from scattering theory and compared to those derived previously from statistical and thermodynamic arguments by Warren and Cowley. The treatment is generalized to include ternary, quaternary, and lattice-matched alloys which, in general, show more complicated order-parameter dependencies than the previously known x(1−x) dependence for ternary zincblende alloys. Electron-momentum relaxation-rate expressions are given, including nonparabolic Kane bands and admixed wave functions appropriate to small energy-gap semiconductors. Electron drift mobility, as determined by alloy scattering, is derived in the effective-mass limit which shows that any short-range alloy potential yields the experimentally observed 1/(m*5T)1/2 dependence reported in the literature. An effective-charge model for alloy scattering is compared to experiments on AlxGa1−xAs. The magnitude of the effective charge on isolated Al atoms in GaAs is found to be 0.145 electron charges.