Pb Lone Pair Research Articles

Identification of the consequences of Pb-O chemical bond modification, Pb lone pair and Pb volatilization on the intrinsic and extrinsic physical properties of Ba1-xPbxTiO3 (x ≤ 0–0.5)

In classical ferroelectric BaTiO3, inclusion of Pb at A-site, forms a unique system in terms of improvement in the physical properties and for investigation of several interesting aspects associated with the physical chemistry and materials science. Manipulation of ferroelectric instability with high tetragonality arises due to Pb-O bond which exhibits strong covalent nature along with the presence of the stereochemically active lone pair of Pb (i.e. 6s2). In contrast, Ba-O chemical bond is ionic in nature. In the present work, Pb is progressively substituted with Ba, to observe the intrinsic effects on the physical properties such as tetragonality, phase transition temperature, vibrational modes, piezoelectric coefficient, ferroelectric domain configuration etc. Phase transition temperature significantly increases with substitution of Pb, which is a manifestation of the effect of chemical bond modification. Sintering temperature effects both intrinsic as well as extrinsic contributions to the physical properties of the system.

Bis(quinolin-8-olato-κ2N,O)lead(II)

The PbII atom in the title compound, [Pb(C9H6NO)2], is N,O-chelated by two organic ligands, and two mol­ecules are linked across a centre of inversion to form a dinuclear entity. These dinuclear entities are linked by somewhat weaker Pb⋯O bonds into a chain. There are two independent mol­ecules in the asymmetric unit with similar geometry around the Pb atoms, which have a pseudo-penta­gonal–bipyramidal coordination with one of the axial positions occupied by the Pb lone pair. Some distances involving the Pb atoms are significantly different between the two mol­ecules.

A new lead Mo(IV) phosphate with a tunnel structure: Pb 2Mo 2O(PO 4) 2P 2O 7

A molybdenum (IV) phosphate containing lead, Pb 2Mo 2(PO 4)2P 2O 7, has been synthesized for the first time. It crystallizes in the space group C2/ c with a=14.098(1) Å, b=14.187(2) Å, c=6.5592(4) Å and β=102.08(1)°. Its original tunnel structure, built up of Mo 2O 11 bioctahedra, P 2O 7 and PO 4 phosphate groups can be described from the assemblage of [Mo 4P 4O 24] ∞ ribbons interconnected through monophosphate groups. The stereoactivity of the 6 s 2 lone pair of Pb 2+, which is surrounded by nine oxygen atoms, is discussed.

A Short Pb.Pb Separation in the Polymeric Compound Bis(pyrrolidinecarbodithioato)lead(II) and a Conformational Pathway Interconversion for the "Pb(II)S(4)" Framework.

The X-ray structure of bis(pyrrolidinecarbodithioato)lead(II), Pb(II)(pdtc)(2), C(10)H(16)N(2)S(4)Pb, shows the metal to be on a crystallographic 2-fold axis. The primary Pb-S bonds are equal, 2.89 Å; this is a unique feature of this compound compared to other Pb(II) dithiocarbamate structures. The crystal structure consists of polymeric Pb(pdtc)(2) units stacked along the c axis in two antiparallel columns; successive Pb(pdtc)(2) units are staggered. There are weaker Pb-S bonding interactions with the unit above (secondary Pb-S bonds) such that the coordination polyhedron formed by the eight S atoms bound to the Pb atom is a distorted square antiprism. Successive antiprisms share the pyramidal base, and the Pb.Pb distance between neighboring Pb(pdtc)(2) units is 3.886(1) Å. We analyze the structures of other PbS(4) compounds and find that the Pb lone pair is important in defining their geometry. The large variety of Pb configurations results from peripheral changes in the ligand. This is due to a compromise between the efficient use of space in the crystal and the tendency of lead to have different coordination numbers. These configurations can be described by a pathway of conformational interconversion from a 4-sided pyramidal geometry to an octahedral system. The title compound crystallizes in the orthorhombic space group Pccn with z = 4; cell constants are a = 13.839(14) Å, b = 14.058(16) Å, c = 7.763(6) Å.

Structural Study of the Oxide Pb 2Sr 2PrCeCu 3O 10, Intergrowth of 0201- and 0222-Type Structures

The structure of the oxide Pb 2Sr 2LnCeCu 3O 10 was determined from powder X-ray and neutron diffraction data in the orthorhombic cell a = 5.4522(2) Å, b = 5.4807(2) Å and c = 37.012(1) Å, space group Fmmm. This study is completed by an H.R.E.M. study. The structure consists of an intergrowth of 0201 and 0222 structures, i.e., it derives from the Pb 2Sr 2YCu 3O 8 structure by replacing one [YO 2]∞ simple fluorite layer by a [ Ln 2O 4]∞ double fluorite layer. The main feature deals with the anionic disordering in the [PbO]∞ layers, leading to the tetrahedra PbO 3 L, in which L characterizes the 6 5 2 lone pair of Pb(II). The twofold coordination of CU (1) is also shown but no conclusion about the linearity of the Cu 1O 2 group can be drawn owing to the existence of a mirror plane at the level of CU (1). The CuO 5 pyramids corresponding to Cu (2) exhibit a regular basal plane and the lanthanides are off center in the fluorite cages, having a 4 + 4 coordination. In spite of the existence of diffuse streaks and additional weak spots on the E.D. patterns, the H.R.E.M. images exhibit, for most of them, a very regular sequence in agreement with the N.D. and X.R.D. study; only some extended defects forming triple fluorite layers are observed in the bulk, whereas large fluorite domains appear on thin edges of the crystals.

Neutron diffraction and EM structure determination of the oxide PbBa 0.8Sr 1.2PrCeCu 3O 9, a perfectly ordered intergrowth of rock salt, oxygen-deficient perovskite, and fluorite-type structures

The structure of PbBa 0.8Sr 1.2PrCeCu 3O 9 has been determined with powder neutron diffraction techniques and profile analysis. The symmetry of this oxide is pseudotetragonal P4 mm, but ED study shows that it is orthorhombic, space group Cmm2, with cell parameters a = 5.4816(1) Å, b = 5.4703(1) Å, and c = 16.4017(4) Å. The structure of this oxide corresponds to a triple intergrowth of single and double oxygen-deficient perovskite layers, with single rock salt and double fluorite-type layers. The analysis of the structure does not show any statistical distribution of copper and lead in the same sites, contrary to what was found for similar compositions. The main original structural features are: (a) the disordered distribution of oxygen in the PbO layers and the distorted tetrahedral coordination of lead PbO 3L due to the 6 s 2 lone pair of Pb(II)(L); (b) the angle of the OCuO bond of univalent copper (164°); (c) the off-centering of rare earth elements in the “O 8” cages, which move away from the copper layers; and (d) the displacement of Cu(II) inside the CuO 5 pyramids toward the apical oxygen. A comparison with Pb 2Sr 2YCu 3O 8-type structures is made.

Superconductivity up to 100 K in lead cuprates: A new superconductor Pb 0.5Sr 2.5Y 0.5Ca 0.5Cu 2O 7−δ

A new superconductor Pb 0.5Sr 2.5Y 0.5Ca 0.5Cu 2O 7−δ has been isolated whose superconducting properties depend upon the thermal treatments, i.e. are closely related to the oxygen stoichiometry. This material exhibits a T on c=100 K whereas the T off c ( R=0) ranges from 45 K to 53 K. The magnetic measurements using a SQUID magnetometer show superconducting volume fractions ranging from 14% to 72%. Anomalies appear in the magnetization curves at low field for some samples. This effect and the broad transition are interpreted in terms of an inhomogeneous distribution of oxygen in the matrix. The structure of this tetragonal phase ( a=3.81 Å, c=11.908 Å) belongs to the “1212” type observed for thallium cuprates, i.e. consists of an intergrowth of double rock salt-type layers involving lead and strontium and of double copper pyramidal layers interleaved with yttrium and calcium. This oxide differs also from the classical intergrowth structures by the existence of numerous oxygen vacancies in the rock salt type layers (δ≅0.7−0.5); moreoer satellites are observed on E.D. diffraction patterns. These two effects are explained by the presence of the 6s 2 lone pair of Pb(II).