J. A. Kafalas

Fast Na+-ion transport in skeleton structures☆

Skeleton structures have been explored experimentally for fast Na+-ion transport. A skeleton structure consists of a rigid skeletal array of atoms stabilized by electrons donated by alkali ions partially occupying sites in a three dimensionally linked interstitial space. Fast Na+-ion transport was demonstrated in several structures, and the system Na1+xZr2P3−xSixO12 has a Na+-ion resistivity at 300°C of ϱ300 ≲ 5Ω-cm for x ≈ 2, which is competitive with the best β″-alumina. An activation energy ea ≈ 0.29 eV is about 0.1 eV larger than that of β″-alumina.

Photoelectrolysis of water in cells with SrTiO3 anodes

The photoelectrolysis of water in cells with SrTiO3 anodes has been confirmed. The maximum external quantum efficiency at zero bias voltage is 10% (at hν=3.8 eV), about an order of magnitude higher than the maximum value obtained with anodes of TiO2, the only other material so far known to catalyze photoelectrolysis. In agreement with the energy‐level model proposed previously, the efficiency is increased because the band bending at the anode surface is about 0.2 eV larger for SrTiO3 than for TiO2, as a result of the smaller electron affinity of SrTiO3.

Photoelectrolysis of water in cells with TiO2 anodes

Abstract The photoelectrolysis of water has been investigated by experiments on cells consisting of an illuminated nTiO 2 (rutile) anode, an aqueous electrolyte, and a platinized-Pt cathode. It has been found that such cells operate either in the photogalvanic mode (no H 2 evolved) or in the photoelectrolytic mode (H 2 evolved at the cathode by decomposition of water), depending on whether or not the electrolyte surrounding the cathode contains dissolved oxygen. In both cases, current flows through the external circuit and O 2 is evolved at the anode. For operation in the photogalvanic mode, maximum values of 80–85% for the external quantum efficiency (η) for current production have been measured at h v ≈4 eV with both single-crystal and polycrystalline TiO 2 anodes. Similar results have been obtained in preliminary experiments with SrTiO 3 anodes. The internal quantum efficiencies, corrected for reflection and absorption losses, are close to 100%, indicating that the band bending in TiO 2 under photogalvanic conditions is sufficient to separate the electron-hole pairs generated by photon absorption and also that the oxygen over-voltage for charge transfer at the semiconductor-electrolyte interface is negligible for illuminated anodes. For operation in the photoelectrolytic mode, η is only 1–2% if the anode and cathode are shorted together, but the efficiency can be greatly increased by applying a bias voltage. By using a photogalvanic TiO 2 -Pt cell to supply this voltage, it has been possible to obtain η values of ∼20%, computed on the basis of the total number of photons incident on the anodes of both cells. All the observations can be given a straightforward explanation in terms of the energy levels of the electrodes and the electrolyte.

Pressure-induced structural changes in the system Ba1−xSrxRuO3

Abstract As part of a continuing study to demonstrate that there are only three intermediate structures between the all hexagonal stacking of AX 3 layers in CsNiCl 3 to the all cubic stacking in the perovskite structure, we report a structural investigation of the Ba 1−x Sr x RuO 3 system as a function of pressure and the composition x. It has been found that both pressure and increasing Sr 2+ increase the proportion of cubic close packed layers. The metastable phases prepared at high pressure are stable to 1100°C at atmospheric pressure.

Structure and properties of the high and low pressure forms of SrIrO3

Abstract The structure of the atmospheric pressure form of SrIrO3 is shown to be a monoclinic distortion of the hexagonal BaTiO3 structure ( a = 5.604 A , b = 9.618 A , c = 14.17 A , β = 93.26°). The cell dimensions have been studied to 1000°C and the coefficients of thermal expansion given. The structure transforms at 40 kbar and 1000°C to an orthorhombic perovskite ( a = 5.60 A , b = 5.58 A , c = 7.89 A ) with a 3% decrease in volume. This high pressure phase only retransforms slowly at atmospheric pressure and 1200°C and exhibits metallic conductivity and Pauli paramagnetism.

Exploring the A+B5+O3 compounds

The several structures exhibited by A + B 5+ O 3 compositions having B = Nb, Ta, Sb or Bi are summarized. New phases, prepared either by high-pressure techniques or by ion exchange, are included. Their stability is interpreted qualitatively in terms of four principal factors: relative ionic sizes, Madelung energy, polarizability of A -cation cores, and the covalent contribution to the B -0 bonds. This latter contribution inhibits the formation of 180° Sb-O-Sb or Bi-O-Bi bond angles, whereas electrostatic forces inhibit the formation of Sb-Sb or Bi-Bi pairs in octahedral sites sharing a common face. A strong π-bond contribution to the B -O covalency weakens both these constraints for Nb 5+ and Ta 5+ ions. A criterion for ferroelectric distortions of the cubic-perovskite phase is presented, and evidence for polarizability of the 3 d 10 and 4 d 10 cores of Cu + and Ag + ions is cited.

High pressure synthesis of (ABX3) (AX)n compounds

Abstract When ABX 3 compounds form the perovskite structure at atmospheric pressure (i.e., SrTiO 3 , SrZrO 3 , CaTiO 3 ), the other members of the (ABX 3 )(AX) n series ( n = 0, 1 3 , 1 2 , 1 ) can generally be formed also. However, if the relative size of the A cation is too large, then the ABX 3 end members form a hexagonal polytype that transforms to the perovskite structure only at elevated pressures. In such cases, pressure is often needed for the synthesis of the interlayer compounds (ABX 3 ) (AX) n . In this paper we report the high pressure synthesis of several (ABX 3 ) (AX) n compounds having ABX 3 hexagonal polytype counterparts at atmospheric pressure with B = Ir, Ru, and Cr.

High Na+-ion conductivity in Na5YSi4O12

Ceramic specimens of Na5YSi4O12 with densities up to 95% of theoretical and Na+-ion conductivities at 300°C up to 0.15 Ω−1 cm−1 have been fabricated from powders synthesized by solid-state reaction of Na2C2O4, Y2(C2O4)3, and SiO2. Conductivity values obtained by complex impedance measurements on one specimen between 25 and 320°C gave an activation energy of 0.15 eV above 175°C, but the ln (σ T) versus 1/T plot was not linear at the lower temperatures.

Crystal chemistry in the system MSbO3

Abstract Cubic, disordered phases of the compounds MSbO 3 (M = Li, Na, K, Rb, Tl, and Ag) have been investigated. KSbO 3 is readily synthesized in the disordered, cubic structure at high pressure, and the other isomorphic compounds were obtained by ion exchange. The structures of NaSbO 3 and AgSbO 3 , which have space group Im 3, were solved by X-ray single-crystal analysis. The structures contain an essentially rigid SbO 3 subarray consisting of pairs of edge-shared octahedra sharing common corners. Within this subarray, face-shared octahedra form 〈111〉 tunnels that intersect at the origin and body center of the unit cell, and the M + ions are randomly distributed over two positions within these tunnels. Ordered, cubic phases have the primitive-cubic space group Pn 3. The two M positions are different for Na + and for Ag + ions. At one of the Ag + -ion positions, the AgO bond length is only 2.26 A, consistent with the gray-black color of AgSbO 3 . Deformation of the 4 d 10 Ag + -ion core by 4 d -5 s hybridization appears to be induced by AgO covalent bonding. This conclusion is compatible with the observation that ion exchange is reversible for all compounds but AgSbO 3 . Several properties of these compounds are compared with the super ionic conductors M 2 O·11Al 2 O 3 β-alumina.

The effect of pressure and B-cation size on the crystal structure of CsBF3 compounds (B=Mn, Fe, Co, Ni, Zn, Mg)☆

Abstract The compounds CsBF3, B = Mn, Fe, Co, Ni, Zn, Mg, occur with four related structures that are known, in other compounds, to transform into one another with increasing pressure. In this paper a phase diagram is presented showing the combined effect of pressure and B-cation size on the occurrence of these related structures. These structures have different proportions of cubic and hexagonal stacking of close-packed CsF3 layers, and in all cases the proportion of cubic stacking increased with both pressure and B-cation size. This result is shown to be consistent with a stabilization mechanism for the intermediate structures involving displacement by electrostatic forces of the B cations from the centers of symmetry of their interstices. The high-pressure phases, identified by powder X-ray diffraction, were retained by quenching from 700C to room temperature before reducing the pressure. The transition pressures were relatively insensitive to preparation temperature. The retained phases required heating to 200–500C for a sluggish retransformation to their atmospheric-pressure form.