Single-source-precursor synthesis and high-temperature evolution of a boron-containing SiC/HfC ceramic nano/micro composite

A boron-containing SiHfC(N,O) amorphous ceramic was synthesized upon pyrolysis of a single-source-precursor at 1000 °C in Ar atmosphere. The high-temperature microstructural evolution of the ceramic at high temperatures was studied using X-ray powder diffraction, Raman spectroscopy, solid-state nuclear magnetic resonance spectroscopy and transmission electron microscopy. The results show that the ceramic consists of an SiHfC(N,O)-based amorphous matrix and finely dispersed sp²-hybridized boron-containing carbon (i.e. B_yC). High temperature annealing of B_yC/SiHfC(N,O) leads to the precipitation of HfC_xN_1-x nanoparticles as well as to β-SiC crystallization. After annealing at temperatures beyond 1900 °C, HfB₂ formation was observed. The incorporation of boron into SiHfC(N,O) leads to an increase of its sintering activity, consequently providing dense materials possessing improved mechanical properties as compared to those of boron-free SiC/HfC. Thus, hardness and elastic modulus values up to 25.7 ± 5.3 and 344.7 ± 43.0 GPa, respectively, were measured for the dense monolithic SiC/HfC_xN_1-x/HfB₂/C ceramic nano/micro composite.     

Synthesis and pyrolysis of single-source precursor for HfCxN1-x-SiC ceramic with different SiC contents

A novel single-source precursor was synthesized to prepare HfC_xN_1-x/SiC multiphase ceramics by using hafnium chloride (HfCl₄), diallylamine (DAA) and polycarbosilane (PCS). We conducted an investigation of the synthesis process, polymer-to-ceramic conversion, as well as the microstructure and phase evolution of HfC_xN_1-x/SiC multiphase ceramics with different levels of SiC content. The results showed that the core-shell particles of HfC_xN_1-x-carbon were embedded homogeneously in the β-SiC matrix which is beneficial for preventing grain growth and improving oxidation resistance. Based on data from oxidation tests, the ceramics improved the oxidation temperature and remained stable at a high temperature (1500 °C) with oxidation layer formation on the surface. Due to the highly cross-linked structure without oxygen, high ceramic yield, homogeneous composition and excellent oxidation resistance of the pyrolysis product, the as-prepared precursor is a promising material for making high-performance composite ceramics.     

Microwave Absorption of SiC/HfCxN1−x/C Ceramic Nanocomposites with HfCxN1−x‐Carbon Core–Shell Particles

The dielectric properties of high‐temperature stable single‐source precursor‐derived SiC/HfC_xN_1?x/C ceramic nanocomposites are determined by microwave absorption in the X‐band (8.2–12.4 GHz) at room temperature. The samples synthesized at 1700°C, denoted as SiC/5HfC_xN_1?x/C‐1700°C and SiC/15HfC_xN_1?x/C‐1700°C ceramics, comprising ≈ 1.3 and ≈ 4.2 vol% HfC_xN_1?x, respectively, show enhanced microwave absorption capability superior to hafnium‐free SiC/C‐1700°C. The minimum reflection loss of SiC/5HfC_xN_1?x/C‐1700°C and SiC/15HfC_xN_1?x/C‐1700°C are ?47 and ?32 dB, and the effective absorption bandwidth amount to 3.1 and 3.6 GHz, respectively. Segregated carbon, including graphitic carbon homogeneously dispersed in the SiC matrix and less ordered carbon deposited as a thin film on HfC_xN_1?x nanoparticles, accounts for the unique dielectric behavior of the SiC/HfC_xN_1?x/C ceramics. Due to their large reflection loss and their high chemical and temperature stability, SiC/5HfC_xN_1?x/C‐1700°C and SiC/15HfC_xN_1?x/C‐1700°C ceramics are promising candidate materials for electromagnetic interference applications in harsh environment.     

Controlled nitrogen content synthesis of hafnium carbonitride powders by carbonizing hafnium nitride for enhanced ablation properties

In this study, a new method of carbonizing hafnium nitride was proposed to synthesize ultrahigh-temperature hafnium carbonitride (HfC_xN_y) powders. The new method helps to maintain both the purity of phases and control the content of nitrogen in the HfC_xN_y. The results show that the as-prepared HfC_xN_y powders have a single phase, with an average particle size of approximately 2 $\mu m$, and Hf, C and N are evenly distributed. Moreover, the microstructures, phase compositions, ablation properties and mechanism of the HfC_0.62N_0.38 composites under a plasma ablation environment were studied in detail. The results show that the HfC_0.62N_0.38 composites exhibited excellent ablation resistance at 3073 K for 60 s and the ablation mechanism of HfC_0.62N_0.38 can be identified as HfC_0.62N_0.38→HfC_xO_y→HfO₂. The mass ablation rate of the HfC_0.62N_0.38 composite is evaluated to be 1.36 mg/cm²∙s, which is lower than that of HfC ceramics. Our work is intended to provide new insight regarding the development of ultrahigh-temperature ceramics and widen their applications.     

Effects of HfC/PyC core-shell structure nanowires on the microstructure and mechanical properties of Hf1-xZrxC coating

To elevate the mechanical and anti-ablation properties of Hf_1-xZr_xC coating on C/C composites, HfC/PyC core-shell structure nanowires (HfCnw/PyC) with different PyC layer thickness were synthesized by two steps of CVD. Influences of HfCnw/PyC on the microstructure and mechanical properties of Hf_1-xZr_xC coating were researched. Toughening mechanism of HfCnw/PyC was also investigated. PyC layer exhibited a lamellar structure and combined well with HfCnw. After incorporating HfCnw/PyC, Hf_1-xZr_xC coating structure converted from columnar crystal to isometric crystal. HfCnw improved H, E, K_c and bonding strength of Hf_1-xZr_xC coating, which is ascribed to the nanowire pullout, debonding, bridging and crack deflection mechanism. With the PyC layer thickness increasing, H and E of the coating reduced, K_c and bonding strength of the coating increased. Because of the moderate bonding strength between HfCnw/PyC and coating matrix, lamellar structure of PyC layer and higher K_c of PyC, toughening effectiveness of the core-shell structures gradually enhanced with the PyC layer thickness increasing.     

Influence of ordered carbon‐vacancy networks on the electronic structures and elastic properties of Nb4AlC3−x

Carbon‐vacancy‐bearing Nb₄AlC_3?x has the best high‐temperature mechanical robustness among MAX phases. The existing form of the vacancies has been long overlooked. Recently, the vacancies in Nb₄AlC_3?x have been identified to be ordered, establishing an ordered compound Nb₁₂Al₃C₈. Here, the spatial distribution of the ordered vacancies and their influences on bonding characteristics and elastic properties are unraveled by thoroughly comparing Nb₁₂Al₃C₈ and vacancy‐free Nb₄AlC₃. In Nb₁₂Al₃C₈, the carbon vacancies break only relatively weak Nb–C bonds and form ordered equilateral triangular carbon‐vacancy networks (OETCVNs) to maximize the bond strengthening effect. The networks slightly shift partial and total density of states toward the Fermi energy level, and bring about a feature of “de‐metallization”. Meanwhile, the presence of OETCVNs results in the softening of elastic modulus, decreasing of the anisotropy of Young's modulus, yet increasing that of shear modulus. These results shed lights on the carbon‐vacancy ordering behavior of MAX phases, and provide an opportunity to tailor their electronic structures and elastic properties through defect engineering.     

Electronic structure and magnetic properties of Ba1-xSrxCoFe11O19 hexaferrites

We have prepared Ba_1-xSr_xCoFe₁₁O₁₉ hexaferrite nanoparticles (NPs) by using a co-precipitation method. The crystal/electronic structures and magnetic properties were then studied. Results revealed that all Ba_1-xSr_xCoFe₁₁O₁₉ NPs with particle sizes of 100–300?nm crystallized in a hexagonal structure. Both the particle shape and the unit-cell parameters are changed when Sr content (x) increases. The analysis of the electronic structure based on the Fe and Co K-edge XAS spectra proved the oxidation states of Fe and Co to be 3?+?and 2?+?, respectively, which are stable versus an x change in Ba_1-xSr_xCoFe₁₁O₁₉. Local-structural studies also revealed the average bond length between Fe and O of 1.89–1.91?Å less changed by Sr doping. Though the electronic structures of Fe and Co were unchanged, the studies about the magnetic property demonstrated a strong dependence of M_s and H_c on Sr doping. While M_s decreases from 46.1?emu/g for x?=?0–34.2?emu/g for x?=?1, H_c tends to increase from 1630?Oe for x?=?0 to ~ 2200?Oe for x?=?0.5, but slightly decreases to 2040?Oe for x?=?1. We think that the addition of the exchange interaction between Fe³⁺ and Co²⁺ ions and the changes of local-geometric structures and microstructures influenced directly M_s and H_c of NPs.     

Insight into the structures,melting points,and mechanical properties of NbSi2 from first-principles calculations

Studies are carried out on the equilibrium structural, mechanical properties, and melting points of NbSi₂ with four ground-state crystal structures (C40, C11_b, C54, and C49) using first-principles approach. By means of the calculated formation enthalpies and phonon dispersion, it is found that these NbSi₂ phases are thermodynamically and dynamically stable. C54-NbSi₂ is uncovered to possess the lowest energy and formation enthalpy, implying that it is expected to be the most favorite structure for NbSi₂. The results of the calculated elastic constants reveal that four NbSi₂ phases are mechanically stable. We further find that the mechanical properties of C54-NbSi₂ are superior to those of the other NbSi₂ phases. The melting points of these NbSi₂ phases are calculated to examine their thermal stability. The elastic anisotropy is calculated and discussed using three patterns. The results prove that C54- and C40-NbSi₂ have good elastic isotropy, as confirmed by the given three-dimensional plots of elastic moduli. Analyzing the difference charge density and Mulliken overlap population provides the explanation about the relationship between bonding characteristics and mechanical properties.     

Crystal structure,magnetic properties,and magnetization variation in Bi0.84La0.16Fe1-xTixO3 polycrystalline ceramic

The crystal structure, microstructure, and magnetic properties of Bi_0.84La_0.16Fe_1-xTi_xO₃ (x?=?0.1, 0.14, 0.16, 0.2) ceramic samples prepared by solid-state reaction are investigated by combining X-ray diffraction, Raman scattering, scanning electron microscope, and magnetic measurements. The analysis of crystal structure reveals the presence of multiphase structures and the gradual formation of Aurivillus Bi₅FeTi₃O₁₅ phase when increasing Ti concentration. The microstructure study shows several grain shapes, corresponding to the different crystal structures and phases present in the sample. The competition in lattice strain of the coexisting phases leads to the isothermal structural transition and the self-change in magnetization. A drastic decrease in coercivity and an increase in magnetization are observed with increasing ferromagnetic-like hysteresis loops. The change in magnetic properties of the samples is strongly dependent on the coexisting phases in the samples.     

Phase stability,electronic structures,and superconductivity properties of the BaPb1−xBixO3 and Ba1−xKxBiO3 perovskites

With extensive first‐principles calculations, we investigate the phase stability, electronic structures, and superconductivity properties of the BaPb_1?xBi_xO₃ (BPBO) and Ba_1?xK_xBiO₃ (BKBO) perovskites with the cubic (C), tetragonal (T), and orthorhombic (O) phases. Our calculations show that the tetragonal superconducting phases of both perovskites are metastable. However, the orthorhombic phase of the BPBO perovskite in the superconductivity region is only slightly more stable than the tetragonal phase. The small energy difference between the T and O phases and the discontinuous T‐to‐O phase transition account for the experimentally observed coexistence of the T and O phases. On the other hand, the BKBO perovskite involves a large energy difference between the T and O phases, which induces a low equilibrium temperature of the discontinuous T‐to‐O phase transition, in agreement with the experimental observation that the tetragonal BKBO is maintained down to low temperatures. Moreover, the electronic structures of both BPBO and BKBO superconductors show a flat band near the Fermi level, which is favorable for superconductivity. Furthermore, we find that the longer the total length of the flat band segment is, the higher the critical temperature of the BPBO or BKBO perovskite is. This key finding could be generalized straightforwardly to other unconventional superconductors and can be used to design and find optimal composition with maximum T_c for new unconventional superconductors.     

Theoretical investigation of thermodynamic and optoelectronic properties of Ce4+ doped SrZrO3 ceramics: A DFT study

Theoretical insight into the thermodynamic, structural, electronic and optical properties of SrZr_1-xCe_xO₃ (x = 0, 0.037 and 0.125) ceramics is presented using DFT calculations for predicting their potential applications as photocatalysts. The stable incorporation of Ce⁴⁺ dopant at Zr site of SrZrO₃ is examined in terms of enthalpies of formation and defect formation energies using valid limits of atomic chemical potentials of the species involved in substitutional doping. Our results indicate expansion in the SrZrO₃ lattice and decrease of band gap with increasing cerium doping concentration. Significant differences are observed in the electronic and optical properties of SrZr_1-xCe_xO₃ ceramics due to different nature of unoccupied Ce-4f states above the Femi level when cerium doping concentration increases from x = 0.037 to x = 0.125. Inclusion of the spin-orbit coupling in our DFT calculations are found to cause splitting of the unoccupied Ce-4f states above Fermi level for high concentration of doping. The trends observed in the structural and electronic properties of SrZr_1-xCe_xO₃ ceramics with increasing cerium doping concentration are found to be qualitatively similar for calculations performed without and with the inclusion of spin-orbit coupling. We show that the near UV absorption of SrZrO₃ can be considerably enhanced by increasing cerium doping concentration beyond x = 0.037; making Ce⁴⁺ doped SrZrO₃ potential heterogeneous photocatalysts.     

A computational examination of the zeta and eta phases in the hafnium nitrides

The hafnium-rich portion of the of the hafnium-nitrogen phase diagram is dominated by a substoichiometric rocksalt HfN_1-x, the ζ-Hf₄N_3−x, the η-Hf₃N_2−x, and the elemental Hf phase. The zeta and eta nitride phases have a close packed metal atom stacking sequence but their nitrogen atom ordering has yet to be concretely identified. With respect to the composition of these phases, recent computational studies of their phase stability using density functional theory (DFT) are not in agreement with reported experimental observations. In this work, we re-examine the phase stability of the zeta and eta phases using DFT combined with enumerated searches using the known metal atom stacking sequences of these phases but with variable carbon concentration and ordering. We have found new structures for the zeta and eta phases that are now in better agreement with experimental findings. Furthermore, we report a new eta phase, -Hf₁₂N₇, which lies on the convex hull and has a nitrogen atom ordering that is substantially different from the zeta phase. This work also demonstrates the importance of configurational entropy in dictating the finite temperature phase diagrams in this system.     

Fabrication of ultra-high-temperature nonstoichiometric hafnium carbonitride via combustion synthesis and spark plasma sintering

In this study, nonstoichiometric hafnium carbonitrides (HfC_xN_y) were fabricated via short-term (5 min) high-energy ball milling of Hf and C powders, followed by combustion of mechanically induced Hf/C composite particles in a nitrogen atmosphere (0.8 MPa). The obtained HfC_0.5N_0.35 powder exhibited a rock-salt crystal structure with a lattice parameter of 0.4606 nm. The melting point of this synthesized ceramic material was experimentally shown to be higher than that of binary hafnium carbide (HfC). The nonstoichiometric hafnium carbonitride was then consolidated under a constant pressure of 50 MPa at a temperature of 2000 °C and a dwelling time of 10 min, through spark plasma sintering. The obtained bulk ceramic material had a theoretical material density of 98%, Vickers hardness of 21.3 GPa, and fracture toughness of 4.7 MPa m^1/2.     

The mechanical properties and electronic structures of V,Cr, Mn,Fe, Ni atoms doped MoCoB by first-principles calculation

MoCoB-based cermets have been regarded as the potential substitution of WC cermets with high hardness, high melting point and high oxidation resistance. Ternary borides-based cermets are widely used in extreme environment, such as high-pressure environment. Therefore, it is significant to explore the mechanical properties and electronic structures of transition elements X (X = V, Mn, Fe, Ni) atoms doped MoCoB under high pressure, which are performed by first-principles calculations to provide guidance for industry applications. The analysis of cohesive energy and formation enthalpy indicates high pressure leads to unstable states with lower lattice constants and crystal volumes. The deviation of cohesive energy and formation enthalpy indicate Mo₄Co₃FeB₄ and Mo₄Co₃NiB₄ have similar stability. The shear modulus, Young's modulus and bulk modulus increase under high pressure, which consists with the increasing of covalence. The variation of ductility and anisotropy indicate similar upward trend, which is verified by Poisson's ratio, B/G ratio and anisotropy index A^U. The analysis of overlap population indicates high pressure leads to the increasing of covalence of B-Co covalent bonds and the decreasing of the covalence of B-Mo covalent bonds. The analysis of electronic structures indicates the high pressure leads to higher hybridization and lower density of states of metallic bonds. The analysis of charge density difference consists with the variation of mechanical properties, implying shorter bond length and higher bonds strength under high pressure.     

Unveiling the strong coulomb interaction effect on electronic structures and mechanical properties of C4AF

There are works have reported the crystal structures and mechanical properties of ferrite cement (C₄AF) at the atomic scale with deviation owing to the omission of the Coulomb interaction effect (U_eff) between 3d electrons of Fe in C₄AF. In this work, the DFT+U method was used to evaluate its effect on their electronic structures and mechanical properties of C₄AF with two different phases I2mb (C₄AF-I) and Pnma (C₄AF-P). The Fe O bonds of the two phases are all weaker and display U_eff due to the presence of Fe ions. The mechanical properties of C₄AF calculated using DFT+U method significantly differ from those obtained without considering U_eff, in which the former shows lower inferior mechanical properties than the latter. This work presents a comparative study the effect of Coulomb interaction to the internal electronic structures and mechanical properties, which will pave the way for designing high hydration reaction cement and high toughness materials.     

First-principles calculations on structural energetics of Cu-Ti binary system intermetallic compounds in Ag-Cu-Ti and Cu-Ni-Ti active filler metals

Structural, mechanical and thermodynamic properties, as well as the electronic structures of Cu-Ti binary system intermetallic compounds in Ag-Cu-Ti and Cu-Ni-Ti active filler metals were calculated systematically using a first-principles density functional theory (DFT). The calculated formation enthalpy index that all the Cu-Ti intermetallic compounds are thermodynamic stable from degradation to pure metals and the relationship between Cu content (x) and formation enthalpy (y) for tetragonal structure meets the function y=0.572+(−1.005/(0.048*sqrt(3.142/2)))*exp(−0.5*((x−47.167)/13.533)^2). The mechanical properties, including bulk modulus B, shear modulus G, Young's modulus E and Poisson's ratio v, and elastic anisotropy were derived from the elastic data C_ij. For the tetragonal Cu-Ti intermetallic compounds, the shear modulus G and Young's modulus E are negatively related to the formation enthalpy, while for the orthorhombic Cu-Ti intermetallic compounds, G and E are positively related to the formation enthalpy. Moreover, the elastic anisotropy increases in the following order: Cu₄Ti<CuTi₃<Cu₄Ti₃<Cu₂Ti<CuTi<CuTi₂<Cu₃Ti₂. The thermodynamic properties were estimated from the electronic structures and elastic constants simultaneously, and the results found that Cu₄Ti possess the best thermal conductivity and heat capacity among all the Cu-Ti intermetallic compounds, while CuTi₃ shows the worst ones. Finally, the relationship between electronic structures and physical properties was discussed, and get the inference that for the Cu-Ti intermetallic compounds, the mechanical properties are positively related to the strength of the covalent bond, while the thermophysical properties are influenced by the ionic character and covalent character simultaneously and the ionic character shows the dominant role, therefore, CuTi and Cu₄Ti₃ show the strongest mechanical properties due to the strongest covalent character, while Cu₄Ti shows the strongest thermal conductivity and heat capacity due to the strongest ionic character.     

DETERMINATION OF DILUTE SLURRY DENSITIES IN A VERTICAL ANNULUS USING ISOKINETIC SAMPLING

The effects of coal properties on N₂O and NO_x formation from circulating fluidized bed combustion of coal was examined through burning nine typical coals and a coal shale, widely used in China over a wide range of coal ranks, in a bench-scale circulating fluidized bed. It was found that N₂O and NO_x formation had similar dependence on coal rank. Fixed carbon content and nitrogen content were the most important coal properties to influence N₂O and NO_x emissions from circulating fluidized bed combustion of coal. A coal with high fixed carbon content had high conversion ratio of fuel-N into N₂O and NO_x. The conversion ratio of fuel-N into N₂O or NO_x increased with nitrogen content of coal, whereas it decreased with O/N ratio. No significant correlation between conversion ratio of fuel-N into N₂O or NO_x and C/N ratio was identified. To clarify the coal property effect, investigation of a wide range of coal rank, is important.     

Electronic properties and stability of M2O3 (M = Al,Ga, In) and alloy (MxGa1-x)2O3 in α and β phases: A theoretical study

Ga₂O₃ is clearly emerging as an important wide band-gap semiconductor. Band-gap engineering is now highly demanded for expanding its applications. Alloying with the same group of metal oxides is a straightforward and effective way. In this work, by using hybrid density functional theory calculations, the structural, electronic properties, and phase stability of group IIIA (Al, Ga, In) metal oxides and their ternary alloys (M_xGa_1-x)₂O₃ (M = Al, In) in the corundum and monoclinic phases are systematically investigated. The lattice constants, elastic constants, modulus, formation energies, band-gaps, band-gap deformation potentials, band-edge alignments, band-gap bowing, and ternary alloy formation energies are obtained. The basic relations between the geometric structure and electronic properties are discussed. It is found that the cation ordered structure is the most stable alloy structure in the monoclinic phase, rather than the random alloy structure as is commonly thought. A phase stability diagram of the (M_xGa_1-x)₂O₃ alloys is established, showing that the stable phase of the alloy changes from the monoclinic phase to the corundum phase when the incorporation of Al₂O₃ (In₂O₃) is greater than 69% (76%). These results can be used to understand the relative experimental data and shed some light on the synthesis and device design efforts of Ga₂O₃.     

First-principles study of the stability,electronic and mechanical properties of Cr2N and CrN with the variation of Ni doping concentration

First-principles approach was applied to investigate the stability, electronic and mechanical properties of Cr_2-xNi_xN (x = 0, 0.083, 0.167,0.250, 0.333) and Cr_1-xNi_xN (x = 0,0.125,0.25,0.375, 0.5). The calculated formation enthalpy and mechanical stability results show that Cr_2-xNi_xN and Cr_1-xNi_xN are all stable. The bulk, shear and Young's modulus results indicate that different variation trend is observed in Cr_2-xNi_xN and Cr_1-xNi_xN with the increase of x. Base on Pugh and Pettifor criteria, Cr₂N belongs to the brittle area and the ductility of Cr_2-xNi_xN increases with the increment of x, obtain the maximum results when x = 0.333. However, CrN, which belongs to the ductile area, alloying with Ni decreases its ductility and increases its brittleness, reach the maximum brittleness when x = 0.5. The charge density difference study reveals that the doped Ni atom affects the interaction between Cr and N in Cr_2-xNi_xN and Cr_1-xNi_xN differently. Furthermore, the stress-strain curve of Cr₂N, Cr_1.833Ni_0.167N, and Cr_1.667Ni_0.333N under shear and tensile deformation shows that the ultimate stress of Cr₂N is decreased and its ductility increased. Nevertheless, the stress-strain curve of CrN, Cr_0.75Ni_0.25N, and Cr_0.5Ni_0.5N under shear and tensile deformation indicates that the strength of CrN can be enhanced and its deformation process is significantly changed when x = 0.25.     

Temperature effect on the nitrogen insertion in carbon nitride films deposited by ECR

The impact of the temperature on the local structure of carbon nitride coating a-C_1-x N_x:H was investigated by spectroscopic analysis. A set of carbon nitride films were deposited at several substrate temperatures (77 K, 300 K, 673 K and 900 K) by electron cyclotron resonance (ECR) ion gun technique fed of CH₄/N₂ plasma.The films were in situ characterized by X-ray photoelectron spectroscopy (XPS). A drastic decrease of the nitrogen content was observed when increasing the deposition temperature from 77 K to 900 K. Qualitative structural and electronic changes were followed after air exposure by infrared (FTIR), near-edge X-ray absorption fine structure (NEXAFS) and Ultraviolet photoelectron (UPS) spectroscopy. Below 300 K, the films are hydrogenated with aliphatic structure and nitrogen is bonded to carbon in many kind of configuration. Between 300 K and 600 K, the nitrogen amount is reduced while both the aromatic and the aliphatic carbons increase. The local structure of the films radically changes at 900 K, whereas the nitrogen surrounding is the same at 673 K. In that case the hydrogen fraction into the films is reduced to zero. The increase of the sp³ carbon as well as the ratio π?/σ? on the nitrogen K edge can be observed. This behaviour may be explain by nitrogen substituted to sp² carbon which induces local changes in the distribution of the π? states.