Anne Marie Flank

Evidence of 3d9-ligand hole states in the superconductor La1.85Sr0.15CuO4 from L3 X-ray absorption spectroscopy

The Cu L3 X-ray absorption spectra of La2−xSrxCuO4−y (x=0, 0.15 and 0.88) have been measured with high resolution. For comparison the L3 X-ray absorption spectrum of the formally trivalent Cu compound La2Li0.5Cu0.5O4 is reported. The semiconductor La2CuO4, with Cu formal valence v=2, shows a single white line due to the 2p3d10 final state arising from the Cu ed9 initial state. The superconducting sample with x=0.15 (Tc=37 K) shows the additional 2p3d10L final states arising from the initial states 3d9L (where L is a hole in the oxygen derived band, ligand hole). The characteristic feature of the superconducting material is that there is no energy gap between the final states 2p3d10 and 2p3d10L. In the semiconducting compound, formed by increasing Sr concentration (x=0.88), a red shift of 0.2 eV of the 2ped10 final state is enough to create a gap between the two states. The conductivity in these ceramic materials is assigned to the itinerant 3d9L states (probed by the 2p 3d10L final state) which indicate the presence of oxygen hole carriers. These results support a microscopic mechanism for high Tc superconductivity due to pairing of oxygen holes coupled with localized Cu 3d9 localized states i.e. of two itinerant 3d9L states.

Siloxane–polypropyleneoxide hybrid ormolytes: structure–ionic conductivity relationships

Siloxane-polypropyleneoxide (PPO) hybrids doped with sodium perchlorate (NaClO 4 ) obtained by the sol gel process were prepared with two PPO molecular weights (2000 and 4000 g/mol) and two sodium concentrations such as [O]/[Na] = 4 and 15 (O being the ether-type oxygen of PPO chains). The structure of these hybrids was investigated by 23 Na nuclear magnetic resonance (NMR) and X-ray absorption spectroscopy at the sodium K-edge (1071.8 eV) whereas complex impedance spectroscopy was used to determine their ionic conductivity. Three sodium sites were determined by NMR. The conjunction of NMR and X-ray absorption results allows us to identify one site in which Na is in a NaCI structure, a second one in which Na is in contact with perchlorate anions. The third site is attributed to mobile sodium species in interaction with the polymeric chain. The relative proportion of the different sites in the materials determines the ionic conductivity of the materials at room temperature: the largest ionic conductivity is 8.9 × 10 -6 Ω -1 cm -1 and is observed on the material with the larger amount (at least 85%) of sites in which sodium interacts with the polymer.

Cu L3 x-ray absorption of formally trivalent Cu compounds

Abstract The Cu L 3 X-ray Absorption Spectra (XAS) of formally trivalent Cu compounds NaCuO 2 , La 2 Li 0.5 Cu 0.5 O 4 , and KCu(III)(biuret) 2 have been measured. The spectra of trivalent systems exhibit two white lines. The low energy white line is found to be at about the same energy, between 930.9 eV and 931.2 eV on the contrary the high energy white line in the range between 933 eV and 933.9 eV is dependent on the chemical bond and it is assigned to the 3d 9 L initial state. The joint analysis of the Cu 2p XPS and XAS spectra of NaCuO 2 shows that both white lines are below the XPS main line 3d 10 L 2 and the energy separation between the XAS(3d 10 L ) and the XPS (3d 10 L 2 ) final state Δ = 1.7 ± 0.2eV is related to the energy for excitation of a ligand hole.

Hole density as evidenced by XAS and critical temperature in the TIBa2Ca1−xNdxCu2O7−δ series

Abstract To estimate the variations of the doping hole density in a series of thallium cuprates, TIBa 2 Ca 1− x Nd x Cu 2 O 7−δ with the substitution amount x , we used XAS at the copper L 3 -edge. The spectra were recorded on powder samples with the total electron yield method. From the simulations of the spectra with two pseudo-Voigt functions, the |;3d 9 L > relative intensity, with respect to the |3 d 9 > one, has been calculated and compared to the variations of T c s and to the copper formal charge deduced from previous neutron diffraction experiments. A correlation between the doping hole density in the [CuO 2 ] ∞ planes, n h ( x , y ), and T c s has been clearly established. The difference between the copper formal charge as deduced from neutron diffraction and the equivalent one deduced from our experiments has been assigned to extra holes likely located in the 2 p z orbitals of the apical oxygen of the copper square pyramids.

Applications in materials science of combining Raman and X-rays at the macro- and micrometric scale

Simultaneous combination of Raman with X-ray Absorption Spectroscopy (XAS) has become of great interest in Materials Science. Four applications are reviewed in this article that reflect the large range of possibilities offered by such coupling at the macro and micrometer scales. Special emphasis was laid on the macrometer scale on time-resolved studies of physico-chemical transformations, either for providing more complete analysis of structural changes or for looking at the sample integrity. The capability of Raman and XAS to provide complementary chemical and structural information was also pointed out for discriminating and characterizing chemical phases in heterogeneous materials at the micrometer scale.

Regular ArticleRedox Mechanisms and Density of Holes by XAS in the Compensated Series Bi2-xPbxSr2Ca1-xYxCu2O8+δ (0 ≤ x ≤ 1)

Bi[sub 2[minus]x]Pb[sub x]Sr[sub 2]Ca[sub 1[minus]x]Y[sub x]Cu[sub 2]O[sub 8+[delta]] (0 [le] x [le] 1) of structural type Bi(2212) is a very interesting solid solution since one is able to adjust the critical temperature over a large range of x by means of a mere formal exchange of charge between cations which do not belong to the essential [CuO[sub 2]][sub [infinity]] planes. X-ray absorption spectroscopy was performed at bismuth, lead, and copper L[sub 3]-edges on the Bi[sub 2[minus]x]Pb[sub x]Sr[sub 2]Ca[sub 1[minus]x]Y[sub x]Cu[sub 2]O[sub 8+[delta]] (0 [le] x [le] 1) solid solution in order to investigate the formal valence states of the elements and the correlation between hole density on the [CuO[sub 2]][sub [infinity]] planes and superconducting properties. It is shown that lead substitution does affect the role of the [BiO][sub [infinity]] planes which exchange electrons (holes) with the [CuO[sub 2]][sub [infinity]] planes. The decrease of the hole density in the [CuO[sub 2]][sub [infinity]] planes with increasing x has been correlated to the decrease of [Tc]s, whereas the rapid decrease of the diamagnetic volume for 0.3 [le] x [le] 0.7 has been understood on the basis of a segregation between bismuth- and lead-rich phases. 37 refs., 11 figs., 1 tab.

Redox mechanisms and density of holes by XAS in the compensated series Bi[sub 2[minus]x]Pb[sub x]Sr[sub 2]Ca[sub 1[minus]x]Y[sub x]Cu[sub 2]O[sub 8+[delta]] ( 0 [le] x [le] 1)

Bi[sub 2[minus]x]Pb[sub x]Sr[sub 2]Ca[sub 1[minus]x]Y[sub x]Cu[sub 2]O[sub 8+[delta]] (0 [le] x [le] 1) of structural type Bi(2212) is a very interesting solid solution since one is able to adjust the critical temperature over a large range of x by means of a mere formal exchange of charge between cations which do not belong to the essential [CuO[sub 2]][sub [infinity]] planes. X-ray absorption spectroscopy was performed at bismuth, lead, and copper L[sub 3]-edges on the Bi[sub 2[minus]x]Pb[sub x]Sr[sub 2]Ca[sub 1[minus]x]Y[sub x]Cu[sub 2]O[sub 8+[delta]] (0 [le] x [le] 1) solid solution in order to investigate the formal valence states of the elements and the correlation between hole density on the [CuO[sub 2]][sub [infinity]] planes and superconducting properties. It is shown that lead substitution does affect the role of the [BiO][sub [infinity]] planes which exchange electrons (holes) with the [CuO[sub 2]][sub [infinity]] planes. The decrease of the hole density in the [CuO[sub 2]][sub [infinity]] planes with increasing x has been correlated to the decrease of [Tc]s, whereas the rapid decrease of the diamagnetic volume for 0.3 [le] x [le] 0.7 has been understood on the basis of a segregation between bismuth- and lead-rich phases. 37 refs., 11 figs., 1 tab.

Electronic Structure and Symmetry of the Itinerant States in High Tc Superconductors

The linearly polarized Cu L3 XANES (X-ray Absorption Near Edge Structure) spectra of several high Tc superconductor single crystals and of their parent insulating compounds have been measured. The spectra have been interpreted in the frame of the full multiple scattering (MS) theory in the real space. Over a range of 40 eV, the polarized Cu L3 XANES spectra have been predicted in term of a one-electron dipole (Δl=+1) transitions, probing the partial density of states of the conduction bands projected on the Cu site with the selected orbital angular momentum l=2 and their projections ml. The oscillator strenght for the dipole allowed transitions Δl=-1 has been found to be a factor 100 weaker than the Cu 2p to 3d transitions. The physical origin of all features in the continuum XANES spectra and the effect of the electron-hole Coulomb interaction in the final states have been identified. The difference spectra between the metal and the insulating phase allows to identify the symmetry of the itinerant 3d9L states induced by doping, that is deduced to be a mixture of Cu 3d holes with ml=0, and 2 orbital angular momentum and oxygen 2p holes on planar oxygen in the molecular orbital combination of local a1 and b1 symmetry. Evidence for the more delocalized character of (3d9x2-y2 ) L(b1) states versus the more localized character of (3d3z2-r2 ) L(a1) states has been found.