In this thesis the synthesis, structure, and ionic conductivity of the sodium and lithium lanthanide pyrosilicates, A3LnSi2O7 (A = Li, Na; Ln = Gd-Er, Y), are reported. The Na- silicates, Na3LnSi2O7, were made by the ceramic method. The meta-stable Li-silicates, Li3LnSi2O7, were formed by a molten salt ion exchange of Li+ for Na+ in Na3LnSi2O7. Structural models for the two compounds were refined against powder X-ray and neutron diffraction data using the Rietveld method, and Fourier difference maps were used to identify the location of the lithium ions in Li3LnSi2O7. The refined structural models show that both alkali compounds are characterised by the P63/m space group and exhibit the same LnSi2O7 framework but possess a remarkably different arrangement of alkali ions. The Na-compounds exhibit four crystallographically distinct alkali-sites whereas the Li-compounds exhibit two such sites, only one of which is equivalent to that in the Na-structure. The new lithium site inhabits an atypical square planar environment. These structural models were corroborated by pair distribution function analysis, bond length analysis, and bond-valence calculations. Potential conduction pathways within the A3LnSi2O7 structures were identified with bond-valence methods and used to propose conduction mechanisms for alkali ion trans- port. Variable temperature impedance spectroscopy measurements were used to show the Li-silicates exhibit an ionic conductivity significantly greater than that of the Na-silicates. The maximum ionic conductivity measured in the Na-compounds was 1.08(7)×10−7 S/cm at 285.5(3) ◦C in Na3HoSi2O7, while that measured in the Li-compounds was 6.7(6)×10−6 S/cm at 286.17(14) ◦C in Li3HoSi2O7. A variable temperature impedance control system was developed to enable these measurements. A microcontroller was used to automate the measurement process; a custom software library was written to allow a user to orchestrate the process with a simple program; and a least squares fitting algorithm was implemented to allow for batch data analysis. This work may be further adapted to accelerate the discovery of new solid state electrolytes for application in all solid state batteries.