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Layered materials form a wide range of technologically important ceramics. Within the class of layered materials, ion-exchangeable tantalates have shown great promise. However, the majority of ion exchange methods require large molar excesses of reagents, high temperatures, and by-products or undesired second phases that have to be removed in subsequent reaction steps. Here we show how direct reaction of H 2 SrTa 2 O 7 with varying quantities of LiOH.H 2 O at room temperature can exchange protons for Li+. This process does not form a continuous series such as that observed in H 2-x Li x La 2 Ti 3 O 10 , instead forming LiHSrTa 2 O 7 balanced with either unreacted H 2 SrTa 2 O 7 or LiOH.H 2 O depending on the initial ratio of H 2 SrTa 2 O 7 to LiOH.H 2 O. When H 2 SrTa 2 O 7 is mixed at room temperature with less than one equivalent of LiOH.H 2 O it reacts to produce a mixture of H 2 SrTa 2 O 7 and LiHSrTa 2 O 7. Mixing equimolar amounts of H 2 SrTa 2 O 7 and LiOH.H 2 O produces a single phase of LiHSrTa 2 O 7. Room temperature mixing with x LiOH.H 2 O (1 < x < 2) followed by heating to 120 °C gives a two-phase product mixture of LiHSrTa 2 O 7 and Li 2 SrTa 2 O 7 and a single phase Li 2 SrTa 2 O 7 from a reaction with x = 2. LiHSrTa 2 O 7 can be further dehydrated at 360 °C to form the defect layered perovskite, ☐LiSrTa 2 O 6.5. Materials with Li+ and vacancies on the same crystallographic site often show fast ion conduction but controllable synthesis has previously proved challenging. The synthesis and degree of exchange is investigated using X-ray powder diffraction data, thermogravimetric analysis and the ionic mobility assessed via a.c. impedance spectroscopy. As use of LiOH.H 2 O has now been shown to directly control ion exchange in both n = 1 and n = 3 (ideal) and n = 2 (distorted) Ruddlesden Popper phases the application of this methodology generally to layered materials is discussed. Intralayer ion exchange leads to large adjustments in the layered perovskite structures H 2 SrTa 2 O 7, LiHSrTa 2 O 7 and Li 2 SrTa 2 O 7 via direct acid/base reaction between solids at near ambient temperatures. [Display omitted] • Direct room temperate ion exchange possible allowing the synthesis of, LiHSrTa 2 O 7 from H 2 SrTa 2 O 7 and LiOH.H 2 O. • Full exchange to form Li 2 SrTa 2 O 7 possible at relatively low temperature of 120 °C. • The defect phase ☐LiSrTa 2 O 6.5 can be formed by dehydrating LiHSrTa 2 O 7. [ABSTRACT FROM AUTHOR]

Layered and defect layered titanates are a wide class of ion exchangeable and technologically important ceramics. The majority of ion exchange methods employ excess reagents at elevated temperatures often leading to complete exchange. Control over the degree of ion exchange is desirable as this allows composition, structure and properties to be continuously tuned. Here, direct control over the degree of lithiation in the Ruddlesden-Popper n ​= ​3 derived phase H 2 La 2 Ti 3 O 10 , at room temperature using LiOH.H 2 O, is demonstrated. The resulting series H 2-x Li x La 2 Ti 3 O 10 (x ​= ​0, 0.125, 0.25, 0.5, 0.75, 1) can then be dehydrated allowing defect tuning in Li 2x Vac 2-2x La 2 Ti 3 O 9+x defect layered perovskites. Materials containing a mixture of lithium cations and vacant sites often show fast-ion conducting but controlling the ratio has previously proved challenging. The synthesis and degree of exchange is investigated using X-ray powder diffraction data, thermogravimetric analysis and the ionic mobility assessed via a.c. impedance spectroscopy. As use of LiOH.H 2 O has now been shown to directly control ion exchange in both n ​= ​1 and n ​= ​3 phases the possibility of the methodology being applied more generally to layered materials is discussed. Multi-step reactions at ambient temperature afford a quantitative ion-exchange route to new crystalline compounds in the layered perovskite Ruddlesden-Popper family. [Display omitted] • Room temperature, direct & tuneable control of lithiation in protonated layered titanate perovskites and RP3 phases. • Dehydration of partly lithiated compounds allows for control of lithium to vacancy ratio. • Lithium ion conductivity is shown to be affected by A site vacancy: lithium ratio. [ABSTRACT FROM AUTHOR]

Abstract: The structures of new phases Li6CaLa2Sb2O12 and Li6.4Ca1.4La2Sb2O12 have been characterised using neutron powder diffraction. Rietveld analyses show that both compounds crystallise in the space group la3̄d and contain the lithium cations in a complex arrangement with occupational disorder across oxide tetrahedra and distorted oxide octahedra, with considerable positional disorder in the latter. Variable temperature neutron diffraction experiments on Li6.4Ca1.4La2Sb2O12 show the structure is largely invariant with only a small variation in the lithium distribution as a function of temperature. Impedance spectroscopy measurements show that the total conductivity of Li6CaLa2Sb2O12 is several orders of magnitude smaller than related lithium-stuffed garnets with σ=10−7 Scm−1 at 95°C and an activation energy of 0.82(3)eV. The transport properties of the conventional garnets Li3Gd3Te2O12, Li3Tb3Te2O12, Li3Er3Te2O12 and Li3Lu3Te2O12 have been evaluated and consistently show much lower values of conductivity, σ≤4.4×10−6 Scm−1 at 285°C and activation energies in the range 0.77(4)≤E a /eV≤1.21(3). [Copyright &y& Elsevier]

Abstract: The garnets Li3Nd3W2O12 and Li5La3Sb2O12 have been prepared by heating the component oxides and hydroxides in air at temperatures up to 950°C. Neutron powder diffraction has been used to examine the lithium distribution in these phases. Both compounds crystallise in the space group with lattice parameters a=12.46869(9)Å (Li3Nd3W2O12) and a=12.8518(3)Å (Li5La3Sb2O12). Li3Nd3W2O12 contains lithium on a filled, tetrahedrally coordinated 24d site that is occupied in the conventional garnet structure. Li5La3Sb2O12 contains partial occupation of lithium over two crystallographic sites. The conventional tetrahedrally coordinated 24d site is 79.3(8)% occupied. The remaining lithium is found in oxide octahedra which are linked via a shared face to the tetrahedron. This lithium shows positional disorder and is split over two positions within the octahedron and occupies 43.6(4)% of the octahedra. Comparison of these compounds with related d 0 and d 10 phases shows that replacement of a d 0 cation with d 10 cation of the same charge leads to an increase in the lattice parameter due to polarisation effects. [Copyright &y& Elsevier]