Al 2p Research Articles

Intense green‐, red‐emitting Tb3+, Tb3+/Bi3+‐doped and Sm3+, Sm3+/La3+‐doped Ca2Al2SiO7 phosphors

This paper focuses on an optical study of a Tb3+ /Bi3+ -doped and Sm3+ /La3+ - doped Ca2 Al2 SiO7 phosphor synthesized using combustion methods. Here, Ca2 Al2 SiO7 :Sm3+ showed a red emission band under visible light excitation but, when it co-doped with La3+ ions, the emission intensity was further enhanced. Ca2 Al2 SiO7 :Tb3+ shows the characteristic green emission band under near-ultraviolet light excitation wavelengths, co-doping with Bi3+ ions produced enhanced photoluminescence intensity with better colour tunable properties. The phosphor exhibited better phase purity and crystallinity, confirmed by X-ray diffraction. Binding energies of Ca(2p), Al(2p), Si(2p), O(1s) were studied using X-ray photoelectron spectroscopy. The reported phosphor may be a promising visible light excited red phosphor for light-emitting diodes and energy conversion devices.

Insight of the doping mechanism of F and Al co-doped ZnO transparent conductive films

Transparent conductive oxide (TCO) films, as transparent electrodes, are widely used in thin-film solar cells. The performance of TCO film has a significant influence on the conversion efficiency of the film solar cell fabricated byusing it. Although the conductivity can be improved by increasing the carrier concentration, the transmittance in the long wave will be sacrificed. Therefore, the only feasible method is to increase the carrier mobility within a certain carrier concentration range, rather than increase the mobility by reducing carrier concentration. In this paper, the F and Al co-doped ZnO (FAZO) films are deposited on glass substrates (Corning XG) by an RF magnetron sputtering technique with using a small amount of ZnF<sub>2</sub> (1 wt.%) and Al<sub>2</sub>O<sub>3</sub> (1 wt.%) dopant. The influences of sputtering pressure on the structure, morphology and photoelectric characteristics of the films are respectively investigated by X-ray diffraction analysis, scanning electron microscope, Hall effect measurement, and ultraviolet–visible–near infrared spectrophotometry. All the thin films show typical wurtzite structure with the <i>c</i> axis preferentially oriented perpendicular to the substrate. With the increase of sputtering pressure, the deposition rate of FAZO film decreases, the crystallization quality is deteriorated, surface topography changes gradually from “crater-like” to co-existent “crater-like” and “granular-like”, and the surface roughness increases. The FAZO film deposited at 0.5 Pa presents the optimal performance with a mobility of 40.03 cm<sup>2</sup>/V·s, carrier concentration of 3.92 × 10<sup>20</sup> cm<sup>–3</sup>, resistivity of 3.98 × 10<sup>–4</sup> Ω·cm, and about 90% average transmittance in a range of 380－1200 nm. The theoretical result shows that the co-doping of F and Al takes the advantages of single F and Al doped ZnO films, and overcomes the shortcoming of metal elements doping, which donates the carriers just from doped metal elements. Furthermore, the co-doping of F and Al not only increases the carriers but also reduces the scatterings caused by the inter-orbital interaction of doped atoms. The doped F 2p electron orbitals repel the O 2p and Zn 4s electron orbitals, making them move down and donate electrons. At the same time, the orbitals of Al 3s and Al 3p also make a contribution to the conductivity. After co-doping of F and Al, both the carrier concentration and conductivity increase significantly.

Protective Fabrics: Metal-Organic Framework Textiles for Rapid Photocatalytic Sulfur Mustard Simulant Detoxification

Summary Metal-organic frameworks (MOFs) can catalyze toxic chemical decontamination, but new MOF materials and synthesis strategies are needed to improve performance, particularly in field-usable MOF-textile formats. This article reports for the first time the exceptional photocatalytic reactivity of Al-PMOF (Al-porphyrin-based MOF), composed of an earth-abundant metal-containing Al(OH)O4 cluster bridged by H2TCPP (5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin) chromophores, against the toxic sulfur mustard simulant 2-chloroethyl ethyl sulfide (CEES) under visible-light irradiation. Furthermore, Al-PMOF is strongly immobilized into polymeric fibers via well-controlled Al2O3 solid film conversion using dimethylformamide/water cosolvent. The approach enables a secure integration of conformal Al-PMOF films onto polymer fibers at a relatively low synthesis temperature (120°C). In addition, on a per-unit mass of MOF basis, the surface-bound Al-PMOF films enable extremely rapid CEES detoxification turnover frequency, up to 170 mol CEES mol chromophore − 1 min − 1 , more than 10-fold faster than the best MOF powders and 2-fold better than MOF films reported to date.

Formation of Al + (C 6 H 6 ) 13 : The Origin of Magic Number in Metal–Benzene Clusters Determined by the Nature of the Core

The identification of highly abundant, “magic” species in the mass spectra of clusters have proven to be valuable in nanoscience, leading to the discovery of new stable species such as fullerenes a...

Preparation of Al2O3 coating on TiN coating by polymer-assisted deposition to improve oxidation resistance in coking inhibition applications

In order to inhibit the metal catalytic coking and improve oxidation resistance of single TiN coating, the TiN/Al2O3 double layer coatings were designed as a chemically inert coating for methylcyclohexane supercritical pyrolysis. Internal TiN coatings were prepared by atmospheric pressure chemical vapor deposition using TiCl4–H2–N2 system. The external Al2O3 coatings with different thicknesses were prepared on the TiN surface by polymer-assisted deposition, and the coating with the most suitable thickness was further annealed at different temperatures of 600, 700, 800 and 900 °C. The morphology, elemental and phase composition of TiN/Al2O3 coatings were characterized by SEM, EDX and XRD respectively. The chemical state information of the coating elements was based on Ti 2p, Al 2p core level X-ray photoelectron spectroscopy (XPS) spectra. The results indicated that the external Al2O3 coating will partially peel off at 900 °C annealing temperature. The thermogravimetric analysis results indicated that all TiN/Al2O3 coatings show better oxidation resistance than single-layer TiN coating. The anti-coking test with methylcyclohexane supercritical pyrolysis showed that the TiN/Al2O3 coatings can effectively cover the metal catalytic sites and eliminate metal catalytic coking. However, the acid sites of external Al2O3 coating slightly promoted coking, so the anti-coking ratios of TiN/Al2O3 coatings were smaller than that of TiN. Thus, the addition of external Al2O3 coating can greatly improve the oxidation resistance of TiN coatings with little loss of coking resistance.

An all-electron study of the low-lying excited states and optical constants of Al2O3 in the range 5–80 eV

This paper reports calculated energies and electronic structures of O(2p), O(2s) and Al(2p) excited states in bulk -Al2O3, at the and surfaces and in the presence of O vacancy defects, obtained from all-electron HF, B3LYP, GGA and LDA calculations based on a recently described direct -SCF approach (Mackrodt et al 2018 J. Phys.: Condens. Matter 30 495901). The closely related frequency-dependent optical constants derived from B3LYP calculations within the CPHF/DF framework are also reported, where both sets of results are shown to compare favourably with the experimental spectra. The differences between the directly calculated excited state energies, which in -Al2O3 are equal to the leading excitation edges, based on the four functionals, are substantially less than the differences between the corresponding (ground state) band gaps, as reported previously for AFII NiO (Mackrodt et al 2018 J. Phys.: Condens. Matter 30 495901). For the B3LYP functional, these energies are 8.7 eV, 12.5 eV and 73.7 eV for the O(2p), O(2s) and Al(2p) excitations respectively. The O(2p) edge is predicted to be degenerate, with distinct excitations from O(2p) states that are parallel to and perpendicular to the c-axis, in agreement with the reported spectra (Tomiki et al 1993 J. Phys. Soc. Japan 62 573). Detailed analyses of the charge and spin distributions in the four bulk excited states indicate that these are essentially charge-transfer excitonic, with acceptor sites at the nearest neighbour positions. Despite the close proximity of the O() and O(2p) excited state energies, the charge and spin distributions are predicted to be quite different.

Microstructure of cement paste incorporating high volume of low-grade metakaolin

The influence of low-grade metakaolin (MK) on the mechanical and microstructural properties of cement paste was studied through compressive strength test and microstructural analyses, including thermogravimetric analysis (TGA), X-ray diffraction (XRD), scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) and X-ray photoelectron spectroscopy (XPS). The results indicated that cement paste incorporating MK with around 40 wt.% kaolinite content in raw clays achieved comparable 28-day strength to that of the reference even at 40 wt.% replacement level. Microstructural studies showed a considerable amount of portlandite consumption after 3 d and a refinement of pore structure in backscattered micrographs. The Al/Ca ratio of calcium-aluminosilicate-hydrate (C-A-S-H) increased upon the increased replacement levels, while Ca/Si decreased due to the introduction of MK, but it did not further reduce with the increased MK content. The binding energy of Si 2p, Al 2p and Ca 2p spectra increased due to the incorporation of MK.

Effect of Complex Modification on Microstructure and Mechanical Properties of Hypoeutectic Al–Si

Hypoeutectic Al–Si alloy was modified by Al–5Ti and Al–3P alloys. The microstructure, fracture morphology, and mechanical properties of the alloys were characterized using optical microscopy, scanning electron microscopy, and a universal electronic testing machine. The results show that there was more primary α-Al phase in the Al–7Si alloy modified with 0.5 wt.% Al–5Ti and Al–3P compared to that modified with 0.5 wt.% Al–5Ti alone. With increased Al–3P, the area fraction of α-Al phase in the complex modified Al–7Si alloy decreased. For 0.1 wt.% Al–3P, eutectic silicon appeared as short rods or flakes, distributed uniformly in the Al–7Si alloy. Compared with hypoeutectic Al–Si alloy modified with 0.5 wt.% Al–5Ti, the relative amount of α-Al phase decreased and the amount of eutectic microstructure increased in the complex modified alloy with increased Si content. The complex modified Al–7Si alloy exhibited 10% higher tensile strength, 21% higher elongation, and presented ductile fracture.

Immobilization efficiency and mechanism of metal cations (Cd2+, Pb2+ and Zn2+) and anions (AsO43- and Cr2O72-) in wastes-based geopolymer

In this study, a composite geopolymer based on solid wastes (drinking water treatment residue (DWTR) and granulated blast furnace slag (GBFS)) were used in immobilization of heavy metals cations (Cd2+, Pb2+ and Zn2+) and anions (AsO43− and Cr2O72-). For evaluating the immobilization effect for heavy metals, the mechanical strength and leaching properties of geopolymers were investigated. Meanwhile, different characterization methods were used to research the immobilization mechanisms. The results indicated that the mechanical strength of geopolymers containing heavy metals was effectively improved by 37.11% with addition of DWTR. The stability of metals in geopolymer were increased and the release risk was decreased under the effect of DWTR. Characterization results verified the uniform distribution of heavy metals in geopolymer, and the adding of Pb2+, AsO43− and Cr2O72- caused the formation of crystalline phases. After the adding of heavy metals, the chemical environment change of Al 2p is more significant than Si 2p. Based on 29Si and 27Al MAS-NMR results, the different metals show various influences on the silicon and aluminum species in geopolymer matrix, and the strength of polymerized structure is mainly based on the Q4(mAl).

Point Defect-Mediated Interface Formation and Appearance of a Cooper Minimum for AlOx Atomic-Layer-Deposited Films on CH3NH3PbI3

We report on the interaction of CH3NH3PbI3 substrates with AlOx films prepared by atomic layer deposition at room temperature. We use synchrotron radiation-based photoemission spectroscopy and study the Pb 4f, I 3d, and Al 2p core levels as well as the corresponding valence band states (Pb 5d and O 2p). A Cooper minimum is observed for both the Pb 5d and O 2p states, and it indicates that the bonding at the interface must be covalent. We focus on the electronic properties of the substrate and its interface and identify a delicate charge balance between polaronic and excitonic states in MAPI and AlOx. The perovskite vacancy sites, identified by changes in the core-level intensities, mediate the charge balance and covalent interaction.

Effects of Cr3+, Cu2+, and Pb2+ on Fly Ash based Geopolymer

The effects of Cr3+, Cu2+, and Pb2+ on compressive strength, reaction products, and pore structures of fly ash based geopolymer were studied. In addition, the immobilization and bonding interaction between heavy metal and fly ash based geopolymers were investigated by X-ray photoelectron spectroscopic (XPS) and environmental scanning electron microscope (ESEM) techniques. The experimental results showed that the incorporation of Cr3+, Cu2+, and Pb2+ had a great effect on the later compressive strength and resulted in producing reinhardbraunsite in the solidified body. Moreover, the Pb2+ reduced the total pore volume of the solidified body, while Cr3+ and Cu2+ increased it. The XPS results indicated that O(1s), Si(2p), and Al(2p) bind energy increased due to Cr3+ and Cu2+ addition, but it did not change significantly due to Pb2+ addition. The microstructure of calcium silicate hydrate (C-S-H) gel and sodium silicoaluminate hydrate (N-A-S-H) gel changed in different degree according to the ESEM results. The immobilization of Cr3+, Cu2+, and Pb2+ using fly ash based geopolymer is attributed not only to the physical encapsulation, but also to the chemical bonding interaction.

Physical and Chemical Characterization of Chinese Fallen Poplar Leaf Ash: Effects of the Calcining Temperature and Aqueous Solution

This study focused on the physical and chemical characterization of Chinese poplar leaf ash (PLA) with 500, 700 and 850 °C calcination temperatures and residual PLA leaching from aqueous solution. The grain size distribution, chemical composition and microstructure of PLA were investigated by the laser granulometric distribution, X-ray fluorescence (XRF) and scanning electron microscopy (SEM). The ash samples obtained before and after aqueous dissolution were analyzed using X-ray powder diffraction (XRD) to identify the mineral components. X-ray photoelectron spectroscopy (XPS) was used to illustrate the Si 2p and Al 2p transformation behaviors in the PLA samples. The zeta potentials, conductivities and pH values of hybrid-solutions were tested at different dissolution times. Silica, sulfur, calcium and potassium were the dominant components observed in the PLA. The conductivities and pH values were nearly stable with an increasing dissolution time. The zeta potential of PLA was calculated to be a negative value. Calcite and potassium sulfate were found in the PLA-500 and PLA-700 samples, whereas magnesite and lime were easily identified in PLA-850. The Si 2p peak shifted a lower position because of the additional synthesis of Si-OH with the increasing calcination temperature. These conclusions could help investigations into the possibility of using PLA in cement systems.

XPS, 29Si, 27Al, 11B MAS -NMR, ATR-IR and FESEM characterization of geopolymer based on borax modified water glass activated Fly ash-GGBS blend

The present work focused on preparing Geopolymer using water glass (Na2SiO3, W) and anhydrous borax (Na2SiO3 + B4Na2O7, B) as an activation media for FA/GGBS blend as a source of silica and alumina was studied. A shift in Geopolymer fingerprint, Si–O–T band in Bp to lower wave number at 987 cm−1 compared to Wp (1008 cm−1) is noteworthy. High strength mixes with improved workability achieved in Bp through more Q3 and Q4 cross-polymeric link network with more Al(IV) incorporation. Transformations of 3T ring form of boron to Q4 (tetrahedral) site confirmed from B11NMR. FESEM/EDAX showed denser matrix in Bp confirming fully condensed geopolymer. XPS lines of O(1s), Si(2p), Al(2p), and Ca(2p) used for bond identification, and results showed that the enhancement of non-bridging oxygen sites in Bp as well the reduction in binding energy of all elemental lines. The combined spectral date showed that boron is part of the geopolymer network structure.

Structural Characterizations of Aluminosilicates in Two Types of Fly Ash Samples from Shanxi Province, North China

In order to determine the structural characterization of aluminosilicates in two types of fly ashes, two samples from Shanxi Province, China were selected for study. One was from a pulverized coal boiler (FA-1), and the other from a circulating fluidized bed boiler (FA-2). FA-1 had a much higher content of silicon dioxide (SiO2) (70.30%) than FA-2(42.19%), but aluminum oxide (Al2O3) was higher in FA-2 (25.41%) than in FA-1 (17.04%). The characterizations were investigated using various methods including X-ray diffraction (XRD), Fourier transform infrared spectrometry (FTIR), magic angle spinning nuclear magnetic resonance (MAS–NMR) spectrometry, and X-ray photoelectron spectroscopy (XPS). The XRD analysis showed that FA-1 contained aluminosilicate glass, quartz and mullite, while FA-2 contained significant amounts of amorphous aluminosilicate, quartz and gypsum. The FTIR results showed an increased substitution of Al3+ for Si4+ as the band of asymmetric stretching vibrations Si–O(Si) (1100 cm−1) moved to 1090 cm−1 for FA-2, much lower than for FA-1(1097 cm−1). Moreover, the sharpness of the bands in the 1250–1000 cm−1 region for FA-2 indicates that the silicate structure of FA-2 was more ordered than for FA-1. It can be understood from the 29Si MAS–NMR results that Q4(mAl) (Q4 are connected via 4 bridging O atoms to mAl) is the main structural type in FA-1 and FA-2, and that FA-2 contains more Al, which substitutes for Si in the Q4 structure. 27Al MAS–NMR demonstrated that a combination of tetrahedral, pentahedral, and octahedral Al existed in FA-1 and FA-2. The Si 2p XPS spectra suggested that there were three forms of Si, including bridging Si (Si–O2), non–bridging Si (Si–O), and SiO2 gel. The content of Si–O2 for FA-1 was 37.48% higher than Si–O (28.57%), while the content of Si–O2 was 30.21% lower than Si–O (40.15%) for FA-2. The Al 2p XPS spectra showed that octahedral Al was the dominant form for FA-1 with a content of 40.25%, while the main phase was tetrahedral Al for FA-2 with a proportion of 37.36%, which corresponds well with the 27Al MAS–NMR results.

Effect of annealing on the surface characteristics of α-Al2O3(0001) probed by XPS

α-Al2O3(0001) is a technologically important metal oxide substrate used in current solid state electronic devices. This report presents the effect of heat treatment on the surface electronic structure characteristics of α-Al2O3(0001) substrates examined by x-ray photoelectron spectroscopy. Survey spectra, O 1s, Al 2p, Al 2s core levels, and valence band spectra are presented for the as received, vacuum annealed, and oxygen annealed substrates. Annealing removes various contaminants such as C, Zn, and OH groups from the surface, and a sharp (1 × 1) low energy electron diffraction pattern confirms the ordered hexagonal periodicity of the surface. No substantial differences in the valence band spectra are observed due to annealing and suggest that the (1 × 1) surface characteristics of α-Al2O3(0001) are preserved during the surface cleaning procedures.

Anisotropic elasticity, electronic structure and thermodynamic properties of Al-Fe-Si intermetallic compounds from first principles calculations

In this paper, structural stability, elastic properties, electronic structure and thermodynamic properties of Al-Fe-Si compounds are systematically investigated by first-principles calculations. The negative formation enthalpy indicates that all Al-Fe-Si compounds are thermodynamically stable, and the Al-Fe-Si compounds are determined to be mechanically stable by the stability criteria. The bulk, shear and Young's moduli as well as Poisson's ratio of Al-Fe-Si compounds are also calculated. The results show that Al2Fe3Si3 has the highest shear modulus and Young's modulus, which leads to stronger shear deformation resistance and stiffness. The elastic anisotropy and shear anisotropic factors of Al-Fe-Si compounds are analyzed in this study, and the anisotropy of Al3Fe2Si3 is the greatest among them. Al3Fe2Si3 and Al2Fe3Si4 show metallic character and Al2Fe3Si3 is classified as direct band gap semiconductors. The DOS of Al-Fe-Si compounds are found mainly from Al 3p, Si 3p and Fe 4d states and the Fe-Al/Si bonding is much stronger than that of Al-Si. According to the results, Al2Fe3Si3 has the highest Debye temperature, indicating strong chemical bonding and higher thermal conductivity, which are consistent with the results of Young's modulus and minimum thermal conductivity.

Lithium-based transition-metal oxides for battery electrodes analyzed by x-ray photoelectron spectroscopy. I. LiNi0.8Co0.15Al0.05O2

X-ray photoelectron spectroscopy (XPS) and x-ray-induced Auger electron spectroscopy were used to analyze untreated LiNi0.8Co0.15Al0.05O2 powder obtained from TODA America. X-ray photoelectron and x-ray-induced Auger electron spectra were obtained using incident monochromatic Al Kα radiation at 0.83401 nm. A survey spectrum together with Ni 2p, Co 2p, Ni LMM, O 1s, C 1s, Al 2s, Ni 3s, Co 3s, Al 2p, Ni 3p, Co 3p, and Li 1s are presented. The spectra indicate the principal core level photoelectron and Auger electron signals and show carbon with only minor sulfur contamination. Making use of the O 1s, Al 2s, Ni 3p, Co 3p, and Li 1s lines and neglecting the components related to surface contaminants, XPS quantitative analysis reveals an altered stoichiometry of the air-exposed powder surface of LiNi0.2Co0.03Al0.1O0.7.

Investigation of fast and slow traps in atomic layer deposited AlN on 4H-SiC

We investigated the electrical and interfacial properties of AlN/4H-SiC interfaces prepared by atomic layer deposition (ALD). The conductance–voltage (G/ω–V) characteristics were identified based on two types of traps: fast and slow. Fast traps located in the semiconductor energy gap close to the AlN/SiC interface showed both narrow and wide energy distributions. Slow traps with defects in the AlN layer exhibited a narrow energy distribution. According to X-ray photoelectron spectroscopy analysis, the formation of AlN during the initial ALD deposition was unclear and some of the oxygen atoms present on the SiC surface were converted into Al2O3. Both O 1s and Al 2p core level spectra shifted to lower energies as the AlN thickness increased. This was associated with the strain relaxation originating from the lattice mismatch between AlN and SiC.

Modified band alignment at multilayer MoS2/Al2O3 heterojunctions by nitridation treatment

Interface engineering plays a decisive role in the two-dimensional material/high-κ oxide heterojunction-based devices. In this work, the effects of NH3 plasma interface treatment on the energy band alignment at multilayer MoS2/Al2O3 heterostructures were explored using x-ray photoelectron spectroscopy. A type-Ⅰ band alignment was formed with valence band offset (VBO) and conduction band offset (CBO) about 3.67 eV and 2.33 eV, respectively. Significantly, the CBO was enlarged by about 0.45 eV after the plasma treatment, which was mainly attributed to the increased separation between the Al 2p core-level energy and valence band maximum. This interesting finding provides a significant guidance for the band adjustment at the interface and further applications of multilayer MoS2 in electronic devices.

Characterization of Rh-Al Bond in Rh(PAlP) (PAlP = Pincer-type Diphosphino-Aluminyl Ligand) in Comparison with Rh(L)(PMe3)2 (L = AlMe2, Al(NMe2)2, BR2, SiR3, CH3, Cl, or OCH3): Theoretical Insight.

The unique Rh-Al bond in recently synthesized Rh(PAlP) 1 {PAlP = pincer-type diphosphino-aluminyl ligand Al[NCH2(P iPr2)]2(C6H4)2NMe} was investigated using the DFT method. Complex 1 has four doubly occupied nonbonding d orbitals on the Rh atom and one Rh d orbital largely participating in the Rh-Al bond which exhibits considerably large bonding overlap between Rh and Al atoms like in a covalent bond. Interestingly, Rhδ--Alδ+ polarization is observed in the bonding MO of 1, which is reverse to Rhδ+-Eδ- (E = coordinating atom) polarization found in a usual coordinate bond. This unusual polarization arises from the presence of the Al valence orbital at significantly higher energy than the Rh valence orbital energy. Characteristic features of 1 are further unveiled by comparing 1 with similar Rh complexes RhL(PMe3)2 (2 for L = AlMe2, 3 for L = Al(NMe2)2, 4 for L = BMe2, 5 for L = SiMe3, 6 for L = SiH3, 7 for L = CH3, 8 for L = OMe, and 9 for L = Cl). As expected, 7, 8, and 9 exhibit usual Rhδ+-Eδ- polarization (E = coordinating atom) in the Rh-E bonding MO. On the other hand, the reverse Rhδ--Eδ+ polarization is observed in the Rh-E bonding MOs of 2-5 like in 1, while the Rh-Si bond is polarized little in 6. These results are clearly understood in terms of the valence orbital energy of the ligand. Because the LUMO of 1 mainly consists of the Rh 4dσ, 5s, and 5p orbitals and the Al 3s and 3p orbitals, both Rh and Al atoms play the role of coordinating site for a substrate bearing a lone pair orbital. For instance, NH3 and pyridine coordinate to both Al and Rh atoms with considerably large binding energy. PAlP exhibits significantly strong trans influence, which is as strong as that of SiMe3 but moderately weaker than that of BMe2. The trans influence of these ligands is mainly determined by the valence orbital energy of the ligand and the covalent bond radius of the coordinating E atom.