Interstitial Oxide Ion Conductivity in the Langasite Structure: Carrier Trapping by Formation of (Ga,Ge)₂O₈ Units in La₃Ga_5–xGe_1+xO_14+x/2 (0 < x ≤ 1.5)

Abstract

Framework oxides with the capacity to host mobile interstitial oxide anions are of interest as electrolytes in intermediate temperature solid oxide fuel cells (SOFCs). High performance materials of this type are currently limited to the anisotropic oxyapatite and melilite structure types. The langasite structure is based on a corner-shared tetrahedral network similar to that in melilite but is three-dimensionally connected by additional octahedral sites that bridge the layers by corner sharing. Using low-temperature synthesis, we introduce interstitial oxide charge carriers into the La3Ga5–x Ge1+x O14+x/2 langasites, attaining a higher defect content than reported in the lower dimensional oxyapatite and melilite systems in La3Ga3.5Ge2.5O14.75 (x = 1.5). Neutron diffraction and multinuclear solid state 17O and 71Ga NMR, supported by DFT calculations, show that the excess oxygen is accommodated by the formation of a (Ge,Ga)2O8 structural unit, formed from a pair of edge-sharing five-coordinated Ga/Ge square-based pyramidal sites bridged by the interstitial oxide and a strongly displaced framework oxide. This leads to more substantial local deformations of the structure than observed in the interstitial-doped melilite, enabled by the octahedral site whose primary coordination environment is little changed by formation of the pair of square-based pyramids from the originally tetrahedral sites. AC impedance spectroscopy on spark plasma sintered pellets showed that, despite its higher interstitial oxide content, the ionic conductivity of the La3Ga5–x Ge1+x O14+x/2 langasite family is lower than that of the corresponding melilites La1+y Sr1–y Ga3O7+y/2. The cooperative structural relaxation that forms the interstitial-based (Ga,Ge)2O8 units stabilizes higher defect concentrations than the single-site GaO5 trigonal bipyramids found in melilite but effectively traps the charge carriers. This highlights the importance of controlling local structural relaxation in the design of new framework electrolytes and suggests that the propensity of a framework to form extended units around defects will influence its ability to generate high mobility interstitial carriers.

La2Ga3O7.5: A Metastable Ternary Melilite with a Super-Excess of Interstitial Oxide Ions Synthesized by Direct Crystallization of the Melt

“…Recent studies suggest that non-equilibrium synthesis methods can be used to exceed such compositional limits. For example, a sol-gel approach with annealing at moderate temperatures drastically extends the accessible interstitial oxide concentration range in langasite-type La3Ga5-xGe1+xO14+x/2; 21 whilst the synthesis of metastable melilites including the nonstoichiometric aluminates La1+xCa1-xAl3O7+x/2 (x ≤ 0.6) and a range of stoichiometric and nonstoichiometric small-A-site compositions, which cannot be achieved by high temperature ceramic routes, is made possible by crystallization-fromglass. 8,10,22 A recent study of (1-y)La2O3 -(y)Ga2O3 glasses by ADL-laser heating found that y = 0.40 was difficult to vitrify despite lying well within the limits of the glass-forming region (0.20 ≤ y ≤ 0.55), hinting at the presence of an unknown metastable compound.…”

“…Recent studies suggest that non-equilibrium synthesis methods can be used to exceed such compositional limits. For example, a sol–gel approach with annealing at moderate temperatures drastically extends the accessible interstitial oxide concentration range in langasite-type La 3 Ga 5– x Ge 1+ x O 14+ x /2 , while the synthesis of metastable melilites including the non-stoichiometric aluminates La 1+ x Ca 1– x Al 3 O 7+ x /2 ( x ≤ 0.6) and a range of stoichiometric and nonstoichiometric small-A-site compositions, which cannot be achieved by high-temperature ceramic routes, is made possible by crystallization from glass. ,, …”

Principal Vibration Modes of the La2O3–Ga2O3 Binary Glass Originated from Diverse Coordination Environments of Oxygen Atoms

“…According to the 71 Ga MAS NMR study reported by Lopez et al, the chemical shifts of 4-, 5-, and 6-coordinated Ga of La 3 Ga 5−x Ge 1+x O 14+x/2 crystals were confirmed at ∼250, ∼150, and ∼30 ppm, respectively. 35 Therefore, the broad signals at 100−300 ppm observed in both 3L7G and 5L5G are estimated to be an overlap of the two signals due to 4-and 5-coordinated Ga. The above results suggest that the La 2 O 3 −Ga 2 O 3 glass has 4−6coordinated Ga and that the Ga-rich glass has a higher fraction of the six-coordinated Ga than La-rich glass.…”

“…This indicates that the fraction of 6-coordinated Ga in La-rich glass is small. According to the 71 Ga MAS NMR study reported by Lopez et al, the chemical shifts of 4-, 5-, and 6-coordinated Ga of La 3 Ga 5– x Ge 1+ x O 14+ x /2 crystals were confirmed at ∼250, ∼150, and ∼30 ppm, respectively . Therefore, the broad signals at 100–300 ppm observed in both 3L7G and 5L5G are estimated to be an overlap of the two signals due to 4- and 5-coordinated Ga.…”

Stabilization of the Trigonal Langasite Structure in Ca3Ga2–2xZnxGe4+xO14 (0 ≤ x ≤ 1) with Partial Ordering of Three Isoelectronic Cations Characterized by a Multitechnique Approach

“…The langasite structure, of general formula A 3 BC 3 D 2 O 14 , is a layered framework structure type constructed from corner-sharing (C,D)­O 4 tetrahedra and BO 6 octahedra; the BO 6 octahedra also bridge adjacent layers, forming 8-coordinate interlayer sites for A cations, which are arranged in channels along the stacking axis (Figure ). The structure type, which is of interest for properties arising from its polarity, ,, optical transparency, , or ability to host interstitial oxide ions, can accommodate multiple cations with different coordination preferences: the A sites tend to host alkaline- and rare-earth cations, while transition metal and p -block cations tend to be distributed across the corner-linked B, C, and D sites. Ca 3 Ga 2 Ge 4 O 14 is readily synthesized in bulk and adopts the common trigonal langasite structure (space group P 321).…”