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Lanthanum silicate/germanate apatite materials have attracted significant interest as Solid Oxide Fuel Cell electrolytes due to their high oxide ionic conductivity at lower temperatures (500-800 ⁰C). In these structures, oxide ion conduction is due to interstitial pathways associated with the high oxygen excess within the structures. Therefore, cation doped La8+xA2-x(M6-xBxO4)O2+x/2 (A = Ca, Sr, Ba; M = Si and Ge; B = Mg, Ga, Al, Zn, B) have been reported to increase oxide ion conductivity via the introduction of excess oxide ions.[1-3] Whilst apatite silicate has higher conductivities at lower temperatures due to lower activation energies, apatite germanates can achieve higher oxygen excess and therefore higher conductivities at elevated temperatures. However, in increasing oxygen content there is a change in symmetry from hexagonal to triclinic leading to a subsequent reduction in conductivity.[4] Previous studies have shown the issue may be overcome by the incorporation of Y doping e.g La8Y2GeO6O27 which leads to stabilisation of the higher conducting hexagonal phase.[5] Herein, we demonstrate the successful incorporation of borate into La10-xYxGeO6O27 and show that it also stabilises the higher conducting hexagonal form. We show that B can be doped into both the Ge site and the oxide ion channels. Interestingly, the conductivity of samples with borate in the channels (which would be expected to block channel oxide ion conductivity along the c direction) is still reasonably high, which supports suggestions that there is significant conduction perpendicular to the channels in these apatite germanates. [1] A Orera and P R Slater, Chem. Mater 2010 22 675-690 [2] P M Panchmatia, A Orera, G J Rees, M E Smith, J. V. Hanna, P. R. Slater and M. S. Islam, Angew. Chem. Int. Ed, 2011 50 9328-9333 [3] E Kendrick, M S Islam and P R Slater, J. Mater. Chem 2007 17 3104-3111. [4] M S Chambers, P Chater, I R Evans and J S Evans, Inorg. Chem., 2019 58(21)14853-14862, [5] A Najib, J. E. H. Sansom, J. R. Tolchard, P. R. Slater and M. S. Islam, Dalton Trans., vol. 19, pp. 3106-3109, 2004.

In this paper we report the synthesis of the new layered perovskite oxide carbonate, Ba3Yb2O5CO3. This phase is formed when 3BaCO3 : 1Yb2O3 mixtures are heated in air at temperatures ≤1000 °C, while above this temperature the carbonate is lost and the simple oxide phase Ba3Yb4O9 is observed. The structure of Ba3Yb2O5CO3 was determined from neutron diffraction studies and consists of a tripled perovskite with double Yb–O layers separated by carbonate layers, the first example of a material with such a structure. Further studies showed that analogous Ba3Ln2O5CO3 phases could be formed for other rare earths (Ln = Y, Dy, Ho, Er, Tm and Lu). The results highlight the ability of the perovskite structure to accommodate carbonate groups, and emphasise the need to consider their potential presence particularly for perovskite systems prepared in lower temperature synthesis routes.

In this paper we revisit the borate systems, Sr4-xBaxNa(BO3)3 and illustrate their structural relationship with the perovskite structure. In these phases, the Na and B can be considered as forming an ordered arrangement on the perovskite small cation site, with the orientation of the oxygen atoms on the borate group such that the Na is octahedrally coordinated to O. In addition, we report the preparation of a new related solid solution series, Sr4-xBaxNa1-yLiy(BO3)3, showing a complete Sr/Ba solid solution series for y ​= ​0.5. Through further investigation into Ca doping into Sr4Li(BO3)3, we have identified a new A site (Ca/Sr) ordered variant of this structure with structure refinement indicating a composition, Sr2·2Ca1·8Li(BO3)3. Thus, these borate systems represent interesting perovskite-type variants, with large oxygen deficiencies compared to a traditional perovskite brought about by ordered oxygen vacancies, and whose existence is dependent on cation ordering features.