High-conductivity oxide ion electrolytes are needed to reduce the operating temperature of solid-oxide fuel cells. Oxide mobility in solids is associated with defects. Although anion vacancies are the charge carriers in most cases, excess (interstitial) oxide anions give high conductivities in isolated polyhedral anion structures such as the apatites. The development of new families of interstitial oxide conductors with less restrictive structural constraints requires an understanding of the mechanisms enabling both incorporation and mobility of the excess oxide. Here, we show how the two-dimensionally connected tetrahedral gallium oxide network in the melilite structure La(1.54)Sr(0.46)Ga(3)O(7.27) stabilizes oxygen interstitials by local relaxation around them, affording an oxide ion conductivity of 0.02-0.1 S cm(-1) over the 600-900 degrees C temperature range. Polyhedral frameworks with central elements exhibiting variable coordination number can have the flexibility needed to accommodate mobile interstitial oxide ions if non-bridging oxides are present to favour cooperative network distortions.

Tetrahedral units can transport oxide anions via interstitial or vacancy defects owing to their great deformation and rotation flexibility. Compared with interstitial defects, vacancy-mediated oxide-ion conduction in tetrahedra-based structures is more difficult and occurs rarely. The isolated tetrahedral anion Scheelite structure has showed the advantage of conducting oxygen interstitials but oxygen vacancies can hardly be introduced into Scheelite to promote the oxide ion migration. Here we demonstrate that oxygen vacancies can be stabilized in the BiVO4 Scheelite structure through Sr2+ for Bi3+ substitution, leading to corner-sharing V2O7 tetrahedral dimers, and migrate via a cooperative mechanism involving V2O7-dimer breaking and reforming assisted by synergic rotation and deformation of neighboring VO4 tetrahedra. This finding reveals the ability of Scheelite structure to transport oxide ion through vacancies or interstitials, emphasizing the possibility to develop oxide-ion conductors with parallel vacancy and interstitial doping strategies within the same tetrahedra-based structure type.

It is of great difficulty to obtain deep-UV transparent materials with enhanced second harmonic generation (SHG), mainly limited by the theoretically poor transparency of these materials in the deep-UV spectral region. Here we report a new noncentrosymmetric, deep-UV transparent phosphate RbNaMgPO, which undergoes a thermo-induced reversible phase transition (at a high temperature of 723 K) and correspondingly an evident SHG enhancement up to ∼1.5 times. The phase transition is aroused by the twist of [PO] dimers with deviation from the P-O-P equilibrium positions. Theoretical analyses reveal that the enhanced SHG can be ascribed to the thermo-induced collective alignment of SHG-active [PO] dimers along the polar axis of high-temperature phase. This work provides an unprecedented physical routine (to SHG-enhanced materials) that is distinguished from the traditional one by chemical design and synthesis.

Oxide ion conductors are technologically important materials because of their potential applications in oxygen sensors and pumps, as dense membranes for oxygen permeation, catalysts, and as electrolytes for solid oxide fuel cells (SOFCs). [1][2][3][4] To be efficient in various applications, candidate materials should possess a conductivity of at least 10 À2 S cm À1 at deviceoperating temperatures; currently commercially used yttriastabilized zirconia (YSZ) reaches this target at 700 8C.[1]Given the drive towards lowering device-operating temperatures, there is a strong impetus and a great challenge for materials chemists to develop materials with enhanced ionic mobility and superior low-temperature oxide ion conductivity. [5,6] A better understanding of generic structural features and pathways which facilitate ionic mobility at lower temperature is a key step in reaching this goal.Here we report a remarkably high oxide ion conductivity at low temperatures (300-500 8C) in an ordered pseudo-cubic 3 3 3 [8,9] By contrast and unusually, our materials crystallize as stable ordered superstructures, and do not undergo phase transitions to lower symmetry and lower conductivity polymorphs. Our ab initio molecular dynamics (AIMD) simulations reveal the structural features and mechanisms which facilitate the high oxide ion mobility at low temperatures, and provide conceptual insight readily applicable to other materials and structure types.The high-temperature cubic fluorite-type bismuth oxide, d-Bi 2 O 3 , with intrinsic oxygen vacancies, shows the highest oxide ion conductivity measured in any material (around 1 Scm À1 at 750 8C); [10] however, it is only thermodynamically stable in the narrow range between 730 and 824 8C.[11] There has been considerable interest in stabilizing the highly conducting d-Bi 2 O 3 phase by isovalent or aliovalent cation substitution to preserve oxide ion conductivity at lower temperatures. For example, 20 % substitution of Er into Bi 2 O 3 results in oxide ion conductivity of 2 10 À2 S cm À1 at 500 8C and 0.4 S cm À1 at 700 8C.[12] Double cation substitution has yielded even higher conductivities at low temperatures (300-500 8C); the best examples include Dy-W, [13] Pr-V, [7] and the recently reported La-Re [8] À3 -10 À2 S cm À1 at 300-400 8C, approaching the Cu-doped layered Bi 2 VO 5.5 (BICUVOX), which itself has the disadvantage of two-dimensional, anisotropic conductivity. Although the relative chemical instability of Bi oxides under reducing conditions has so far hampered their applications in SOFCs, the use of bilayer electrolytes can overcome this issue. [14] In addition to high oxide ion conductivity, bismuthbased oxides show electrocatalytic activity and therefore also have great potential for applications in electrochemical oxygen separation. [15,16] A common structural feature in the best d-Bi 2 O 3 -based oxide ion conductors reported so far is that doping stabilizes simple cubic structures with a % 5.5 and space group Fm " 3m. [7][8]13] By comparison, doped d-Bi 2 O 3 material...

No abstract