261 research outputs found
Coherent view of crystal chemistry and ab initio analyses of Pb(II) and Bi(III) Lone Pair in square planar coordination
The stereochemistry of 6s2 (E) lone pair of divalent Pb and trivalent Bi
(PbII and BiIII designated by M*) in structurally related PbO, PbFX (X= Cl, Br,
I), BiOX (X= F, Cl, Br, I) and Bi2NbO5F is rationalized. The lone pair LP
presence determined by its sphere of influence E, equal to those of oxygen or
fluorine anions, was settled by its center then giving M*-E directions and
distances. Detailed description of structural features of both elements in the
title compounds characterized by [PbEO]n and [BiEO]n layers allowed to show the
evolution of M*-E distance versus the changes with the square pyramidal SP
coordination polyhedra. All are different, in red PbO one finds {PbEO4E4}
square antiprism, a {[Bi.E]O4X4Xapical} monocapped square antiprism in PbFX and
BiOX and {BiEO4F4}square antiprism in Bi2NbO5F. To analyze the crystal
chemistry results, the electronic structures of these compounds were calculated
within density functional theory DFT. Real space analyses of electron
localization illustrate a full volume development of the lone pair on PbII
within {PbEO4E4} in PbOE, {PbEF4X4} in PbFXE and Bi(III) within {BiEO4X4}
square antiprisms, contrary to Bi(III) within {[Bi.E]O4F4Fapical} monocapped
square antiprism. Larger hardness (larger bulk modules B0) and band gap
characterize BiOF versus PbO due to the presence of F which brings antibonding
Bi-F interactions oppositely to mainly bonding Bi-O. In PbFX and BiOX series
there is a systematic decrease of B0 with the increasing volume following the
nature and size of X which is decreasingly electronegative and increasingly
large. The electronic densities of states mirror these effects through the
relative energy position and relative electronegativities of F/X and O/X
leading to decrease the band gap.Comment: 20 text pages/ 10 multifigures/large review article, J. Progress
Solid State Chemistry, under production MAY 201
Novel Carbon allotropes with mixed hybridizations: ene-C10, and ene-yne-C14. Crystal chemistry and first principles investigations
Based on C8, carbon 4C, with cfc topology, two hybrid carbon allotropes
generated by inserting C(sp2) and C(sp1) carbon atoms into C8 diamond-like
lattice were identified and labeled ene-C10 containing C(sp2) and ene-yne-C14
containing C(sp2 and sp1). The introduced double and triple chemical
descriptions were illustrated from the projected charge densities. The crystal
density and the cohesive energy were found to decrease due to the enhanced
openness of the structures from inserted sp2/sp1 carbons, with a resulting
decrease of the hardness along the series C8, C10, C12, and C14. The novel
hybrid allotropes were found stable mechanically (elastic constants and their
combinations) and dynamically (phonons band structures). The thermal properties
from the temperature dependence of the heat capacity CV were found to
increasingly depart from diamond-like C8 to higher values. From the electronic
band structures, the inserted carbons were found to add up bands rigidly to
diamond-like C8 while being characterized by metallic-like behavior for ene-C10
and ene-yne-C14.Comment: 18 pages, 32 references, 2 tables, 6 figure
Evolution from quartz (qtz) to diamond (dia) carbon allotropes: Crystal engineering and DFT investigations
Based on crystal engineering and density functional theory DFT calculations a
transformational pathway is proposed from qtz (quartz-based) topology
characterized by distorted tetrahedra to dia (diamond-like) regular tetrahedra
topology. The protocol consists of carbon insertions into orthorhombic (space
group P222, No. 16) within C5, C6, and C7, leading to ultimate C8 identified as
diamond-like. The induced structural and physical changes are assessed with
elastic properties pointing to ultra hardness, larger for qtz-C6 than dia-C8,
whilst intermediate C7 is compressible due to its diamond-defective structure.
The dynamic stability was shown from the phonons, and thermodynamic quantities
as the specific heat CV was addressed in comparison with diamond experimental
data. The electronic band structures reveal semi-conducting C6, metallic C7
characterized by diamond-defect structure, and insulating C8.Comment: 15 pages, 4 figures, 2 tables, 27 reference
C5 as simplest ultrahard allotrope with mixed sp2/sp3 carbon hybridizations from first principles
From crystal chemistry rationale and density functional DFT calculations,
novel tetragonal carbon C5 is proposed as simplest ultrahard allotrope with
mixed hybrid carbon hybridizations (sp2 - sp3). Novel pentacarbon is identified
as cohesive and stable both dynamically and mechanically. Whereas charge
density is localized about tetrahedral C-sp3, it is found delocalized around
trigonal C-sp2 resulting in metallic behavior. The anisotropic structure
characteristics caused by the aligned trigonal carbon along the tetragonal
c-axis provide high Vickers hardness with a magnitude close to diamondComment: 15 pages, 26 refs, 2 Tables, 5 figs. arXiv admin note: text overlap
with arXiv:2206.0591
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