The perovskites and perovskite-related structures exhibit several features of technical as well as fundamental interest. Technically useful properties include oxide-ion conduction with/without electronic conduction, oxidation catalysis, ferroic displacements in classic and relaxor ferroelectrics, half-metallic ferromagnetism and high-temperature superconductivity. Of more fundamental interest is the ability to tune, by chemical substitution on the large-cation subarray, transition-metal oxides through the crossover on the transition-metal array from localized dn configurations to itinerant d-electron behaviour without/with changing the valence state of that array. The localized-electron configurations may exhibit cooperative Jahn–Teller distortions that introduce anisotropic exchange interactions. At crossover, bond-length fluctuations may segregate into an ordered array of alternating covalent and ionic bonding in a single-valent perovskite; multicentre polarons or correlation bags may replace small polarons in a mixed-valent system. Bond-length fluctuations at crossover give vibronic conduction and suppression of the phonon contribution to the thermal conductivity; the fluctuations may order, to give high-temperature superconductivity, or transform to quantum–critical-point behaviour at lowest temperatures. Crossover of σ-bonding electrons in the presence of localized spins associated with π-bonding electrons gives rise to the colossal magnetoresistance phenomenon above a ferromagnetic Curie temperature.

https://iopscience.iop.org/article/10.1088/0034-4885/67/11/R01/pdf

Electronic and ionic transport properties and other physical aspects of perovskites

The electron-lattice interaction of BaPb1-xBixO3 (BPB) and BaxK1-xBiO3 (BKB) is studied microscopically by using the realistic electronic bands of BaBiO3 reproduced by the tight-binding model. It is found that the electron-lattice coupling coefficients have strong wavevector and mode dependences. The electron-lattice interaction causes a remarkable renormalization of the longitudinal oxygen stretching and/or breathing mode vibration. Superconductivity is discussed in the framework of the strong-coupling theory of the phonon mechanism. The spectral function alpha 2F( omega ) has some prominent structures in the frequency range of the oxygen stretching/breathing mode. As x increases, some of the main peaks in alpha 2F( omega ) shift to the lower-frequency side, reflecting the phonon frequency renormalisation. The transition temperature Tc and the energy gap function Delta ( epsilon ) at T=0 K have been evaluated by solving the Eliashberg equations. The calculated Tc increases rapidly with increasing x, and reaches about 30 K for x=0.7. The oxygen isotope shift of Tc in BKB is calculated and the characteristic exponent alpha defined by Tc varies as MOalpha (MO is oxygen atomic mass) is evaluated to be 0.35-0.45. The superconducting energy gap Delta 0 is evaluated to be 4.8 meV for x=0.7. The ratio 2 Delta 0/kBTc is found to have a value close to that predicted by the Bardeen-Cooper-Schrieffer weak-coupling theory (2 Delta 0/kBTc=3.5). However, the calculated tunnelling differential conductance dI/dV and its derivative d2I/dV2 show behaviours that are characteristic to the strong-coupling superconductor.

https://iopscience.iop.org/article/10.1088/0953-8984/2/15/012/pdf

Electron-lattice interaction and superconductivity in BaPb1-xBixO3 and BaxK1-xBiO3

Electrodeposition of Ba.6K.4BiO3

A new solid solution Pb 0.5 Ca 0.5 Sr 2 Ca x Y 1− x Cu 2 O 7−gd which belongs to the “1212” family of superconducting cuprates has been isolated for 0⪯ x ⪯0.60. These materials exhibit superconducting properties for 0.50⪯ x ⪯0.60, with T c 's ranging from 50 to 80 K, and diamagnetic volume fractions of 20–25%. X-ray diffraction study of this phase shows that its double rock salt layers are built up from a (Ca 0.5 Pb 0.5 O) ∞ monolayer sandwiched between two (SrO) ∞ layers. The electron diffraction (ED) study of the x =0.5 and x =0.60 samples shows the existence of incommensurate satellites but which are considerably weaker than those of the superconductors Pb 0.5 Sr 2.5 Ca x Y 1− x Cu 2 O 7−δ . The important difference with those latter oxides deals with the absence of superstructures in the (001) ∗ plane. This existence of satellites and the absence of (001) ∗ superstructures are discussed in terms of distortion of the rock salt layers and of disordering phenomena between lead and calcium in the (Ca 0.5 Pb 0.5 O) ∞ layer.

A “1212” superconductor involving mixed calcium lead (Pb0.5Ca0.5O)∞ monolayers: Pb0.5Ca0.5Sr2CaxY1−xCu2O7−δ

The phonon densities of states (DOS) of insulating ${\mathrm{BaBiO}}_{3}$ and superconducting ${\mathrm{Ba}}_{0.6}$${\mathrm{K}}_{0.4}$${\mathrm{BiO}}_{3}$ and the variation of the phonon spectrum of the superconducting compound upon oxygen-isotope ${(}^{16}$O, $^{18}\mathrm{O}$) substitution are determined by inelastic neutron scattering (INS) and molecular-dynamics (MD) simulations. The MD simulations are carried out with an effective interaction potential which includes steric effects, Coulomb interactions, and the charge-dipole interactions due to the electronic polarizability of ${\mathrm{O}}^{2\mathrm{\ensuremath{-}}}$. The MD results are in good agreement with the INS experiments and electron-tunneling measurements. Partial DOS of Ba, K, Bi, and O in ${\mathrm{BaBiO}}_{3}$ and ${\mathrm{Ba}}_{0.6}$${\mathrm{K}}_{0.4}$${\mathrm{BiO}}_{3}$ are also determined from MD simulations. In the superconducting material, the phonon spectrum softens and is comprised of broad bands around 15, 30, and 60 meV. The partial DOS reveal that phonons above 20 meV are due to oxygen vibrations, whereas phonons below 20 meV are due to Ba, K, and Bi. The reference oxygen-isotope-effect exponent, ${\mathrm{\ensuremath{\alpha}}}_{\mathrm{O}}$r=-\ensuremath{\partial} ln〈\ensuremath{\omega}〉/\ensuremath{\partial} ln${\mathit{M}}_{\mathrm{O}}$, of ${\mathrm{Ba}}_{0.6}$${\mathrm{K}}_{0.4}$${\mathrm{BiO}}_{3}$ is determined to be ${\mathrm{\ensuremath{\alpha}}}_{\mathrm{O}}$r=0.42\ifmmode\pm\else\textpm\fi{}0.05 from the mass (${\mathit{M}}_{\mathrm{O}}$) variation of the first moment of the phonon DOS, $_{\mathrm{O}}^{16}\mathrm{〉}$ and $_{\mathrm{O}}^{18}\mathrm{〉}$.This value is close to the oxygen-isotope-effect exponent, ${\mathrm{\ensuremath{\alpha}}}_{\mathrm{O}}$, determined from the variation of ${\mathit{T}}_{\mathit{c}}$ (0.41\ifmmode\pm\else\textpm\fi{}0.03 by Hinks et al. and 0.35\ifmmode\pm\else\textpm\fi{}0.05 by Kondoh et al.), indicating that ${\mathrm{Ba}}_{0.6}$${\mathrm{K}}_{0.4}$${\mathrm{BiO}}_{3}$ is a weak- to moderate-coupling BCS-like superconductor and that the high ${\mathit{T}}_{\mathit{c}}$ (\ensuremath{\sim}30 K) results from large electron-phonon matrix elements involving high-energy oxygen-related phonons.

Phonon density of states and oxygen-isotope effect in Ba1-xKxBiO3.

Experimental results of Hall effect, thermoelectric power and magnetic susceptibility of the Cu-free high-Tc oxides, (Ba,K)BiO3 are presented. It is shown that the electronic nature of (Ba,K)BiO3 is quite different from those of high-Tc oxides with CuO2 planes. One of the remarkable differences between the Bi and Cu systems is that the charge carriers of the former are electrons with small effective mass, while the charge carriers of the latter are holes with large effective mass. We also measured the effect of oxygen isotope substitution on its Tc, where we first synthesized BaO2, KO2 and Bi2O3 by using 16O and 18O in parallel , and then the superconducting oxide (Ba,K)BiO3 was synthesized from these oxides, which guaranteed the complete substitution of 16O by 18O. A large oxygen isotope effect (α≅0.35) suggests that the usual phonon mediated pairing mechanism is working in the present system.

M. Sera

Papers

Normal state properties and oxygen isotope effect of (Ba,K)BiO3