Showing papers on "Valency published in 1980"

Mixed valency of TmSe

Abstract: Vacuum-cleaved single crystals of mixed valence TmSe were studied by x-ray photoemission The divalent and trivalent part of the Tm $4f$ spectrum were fitted using optical multiplet energies and fractional parentage intensities From the areas of the divalent and trivalent components we obtained the average valences for samples with four different stoichiometries Comparison of these results with other estimates of the average valence suggest that the x-ray photoemission spectroscopy data are significantly affected by a divalent surface layer However, attempts to demonstrate the reality of this layer by varying the take-off angle were inconclusive It was verified that the ${\mathrm{Tm}}^{2+}$ ground state lies at the Fermi energy, giving a direct proof of the homogeneous character of the mixed valency The location of this multiplet can therefore be used in general to distinguish between homogeneous and inhomogeneous mixed valency

Dimensional analysis of interstitial phases with the AlCr2C structure

Abstract: A dimensional analysis is reported of interstitial phases with the hexagonal AlCr2C, NM2C, structure, which is based on examining the variation of the unit-cell parameters a and c as functions of DM and DN, the diameters of the M and N components for CN 12. The observed behaviour is uniquely different to that of phases with all other structures so examined, there being a drastic change of the DN dependences of a and c in the region where a ≃ DN. Nevertheless, equations are derived which reproduce the a and c dependences on DM, DN and the valency of the M component to better than 0.5% and which indicate the atomic arrays that control the cell dimensions. It is found that the M-M distances that do not lie in (0001) planes and which form half of the sides of the M octahedra surrounding the metalloid atoms, are an invariant feature of the structure for phases with the same M component. This invariant distance couples a, c and z, the atomic parameters of the M atoms, and it causes e dependences on 6.5DN (a > DN) and -1.65DN (a < DN), which could not possibly arise through the intrinsic contacts of the N atoms. The electronic distribution from the M atoms to M-M and M-N contacts and also to bands derived from the metalloid 2p states and transition-metal d states, is further analysed to account for the apparent dependence of a and c on the M-atom valency. The cell dimensions of SnM2C phases with the AlCr2C structure do not follow the systematic variations discussed above; reasons are advanced for this.

Introduction to Mixed-Valence Chemistry

Abstract: To begin with, some definitions: ‘mixed valency’ is one of several names, such as ‘mixed oxidation state’ or ‘non-integral oxidation state’, used to describe inorganic or metal-organic compounds in which an element is present in more than one level of oxidation. Since the conventional definition of ‘oxidation state’ implies that we can define the number of valence shell electrons associated with each atom in a compound we shall exclude compounds in which covalency or electron delocalization between the constituent atoms is so great that we cannot judge, even approximately, how many valence shell electrons should be assigned to each kind of atom. Thus Fe3O4 comes within our compass but Cu3Si does not. Without any doubt in Fe3O4. the most loosely bound electrons are mainly localized on the Fe atoms. The choice then is to say that each Fe has 5.33 d-electrons on average, or that one Fe per formula unit has 6 while two others have__5. The same argument applies to Prussian Blue, Fe4 /Fe(CN)6_/314H2O, or the Creutz-Taube ion (NH3)5Ru (pyrazine) Ru (NH3) 5 5+ .

"Wandering" valency in organophosphorus free radicals

Abstract: Phosphorus—contain.ing radicals of the type P')where OO stands or phen1enedioxi group, are desc?ibed. The new radicals have been generated bi photoche— mical addition of o—quinones to phosphorus or b' photoreac-.. tions o: o—quinones with b1rogen spirophoaphoranea. Oxda— tion of corresponding o-.oxyspirophosphoranea with Pb02 leads to the same radicals. The ESR spectra of the radicals ob-. tamed show a characteristic temperature dependence. per— fine splittings observed at low temperatures correspond to unpaired electron interactions with protons of only one aro— matic ring, whereas at higher temperatures, protons of two or all the three aromatic uganda are involved. In the latter case, the radical has an octahedral symmetry on the ESR time scale, and the unpaired electron \"wanders\" over six oxygen atoms (\"wandering valency\"). The number of o:xygen atoms participating in spin delocalization may be lower than 6 depending on the radical structure. The phenomenon described is a new type of phosphorotropic tautomerism. Temperature—dependent variations of the ESR spectra studied include characteristic linewidth alternations which have been used to calculate exchange frequencies. A possible mechanism of valency wandering is discussed. New radicals have been generated by reduction of phosphors— te anions containing hexacoordinate phosphorus. Salts con-. taming these anions have been prepared from ortho—quinones and phosphorus in high yields. The properties of the new compounds are described. The wandering valency phenomenon has also been observed with radicals containing other than phosphorus elements, such as boron, aluminium, silicon, arsenic, etc. • Temperature van— atious of ESR spectra of these radicals have been studied. By analogy with ionotropic tautomerism,we use the term tautoxnerism of free radicals to describe their spontaneous and reversible isomerization: One of the possible isomerization mechanisms is free valency and bond transfer within the radical: x-1x • The process should be regarded as intramolecular radical substitution at X. With Symmetrical radicals (R is identical to R'), the equilibrium is degenerate Which implies a double—well potential with two minima of the same depth. The group X undergoing migration may be hydrogen. Aurik and Stork were the first to observe such radical H—tropy as degenerate tautomerism of iminol radical (I) in 1972 [11. Somewhat later we have studied phenoyl radical (II) and its tautomerism [21.

Neutron diffraction studies of Ce0.33U0.67C2 and Ce0.67U0.33C2 at 296–4 °K

Abstract: Neutron diffraction measurements have shown that in the ternary CeC2‐type compounds, Ce0.33U0.67C2 and Cs0.67U0.33C2, the cubic NaCl‐type structure, which is usually stable above about 1000 °C, can be arrested down to 4 °K due to a large strain energy of the disordered lattice. Also retained down to 4 °K are a random distribution of Ce and U atoms, as well as the orientational disordering of C2 molecules. The lattice parameters at 296–4 °K indicate no valency change, such as Ce3+→Ce4+. Ce0.33U0.67C2 showed no magnetic ordering at 296–4 °K in 0–21 kOe. Ce0.67U0.33C2 becomes a weak ferromagnet below 17 °K in 21 kOe, with the moment direction parallel to the [111] direction.

Dimensional analysis of the structures of metallic phases: Bonding information derived from valency effects that appear as artefacts

Abstract: It is shown to be a good prediction that the unit cell dimensions of metallic phases M x N y with a given crystal structure vary linearly with D M for a given N component, and with D N at a given D M value, where D M and D N are the atomic diameters for coordination number (CN) 12. This permits explicit determination of the atomic arrays that control the structural dimensions and other interesting structural information but it depends on the assumptions (1) that the valency and bonding orbitals of the atoms in the intermetallic phases are the same as in the elemental structures from which the CN 12 diameters are determined and (2) that the bonding electrons are distributed to the neighbours of an atom according to Pauling's equation R (1)− R ( n ) = 0.3 log n and the relative distances between the atoms. If either of these conditions is unsatisfied and if the appropriate arrays of interatomic contacts control the structural dimensions, “valency effects” will appear in the dimensional analyses. These can be interpreted, often quantitatively, to give information about details of the bonding and electron distribution in the structures considered. This is illustrated by examples of phases with (1) the AlB 2 and similar structures, (2) the NaCl and Mn 5 Si 3 structures and (3) interstitial phases of the transition metals obeying Hagg's rule and having a radius ratio of metalloid to transition metal atom of less than 0.59.

57Fe and 125Te Mössbauer Study of LiFeCo3TeO8 and LiFeNi3TeO8

Abstract: 57Fe and 125Te Mossbauer data on the spinels LiFeNi3TeO8 and LiFeCo3TeO8 are presented. Structural information relating to site occupations by Fe, valency of Fe and Te and magnetic hyperfine fields are obtained from this study.

The molecular structure and conformations of oxepane as determined by gas electron diffraction

Abstract: The electron-diffraction data for gaseous oxepane, collected at 310 K, can be explained in terms of a 53:47% mixture of two twist-chair conformations. Using the nomenclature of Crerner and Pople [1], the conformations are characterised by q 2 = 0.579 a, q 3 = 0.685 A, φ 2 = 13.3°, φ 3 , = 63.0° and q 2 = 0.511 A. q 3 = 0.588 A, o 2 = 116.1°, o 3 = 217.6°. The other structural parameters ( r a -structure) are r CO = 1.419 A, r cc = 1.531 A, r CH = 1.105 A, ∠H-C-H = 106.0°, with a mean ring valency angle of 112-0° for the former conformation, and of 116.2° for the latter. There is a good agreement between the experimental geometries and the results from molecular mechanics calculations.

Mixed valency and phase transitions at the (001) SmS surface

Abstract: A model for mixed valency and phase transitions at (001) SmS is suggested. Focussing on the electron-lattice coupling as a mechanism potentially responsible for both phenomena, the problem of the semi-infinite crystal is factorized and reduced to the investigation of a low-dimensional effective Hamiltonian matrix. Surface geometry and electronic structure are obtained simultaneously by numerical minimization of the free energy. The possible appearance of mixed valency and of phase transitions confined to the surface region and involving relaxed or reconstructed geometry is demonstrated.