Fabrication and ionic conductivity of oriented lanthanum silicate films with apatite-type structure

• We have fabricated c-axis oriented films of apatite-type lanthanum silicate.
• The synthesis was possible with lower temperature and shorter time than so far.
• Bulk ionic conductivity was about 2 × 10− 5 S cm− 1 at 600 °C.

We have successfully fabricated c-axis oriented films of apatite-type lanthanum silicate (LSO) using sputtering technique. Two kinds of thin films, La2SiO5 and La2Si2O7, were successively deposited on yttria-stabilized zirconia (YSZ) or La-doped SrTiO3 (STO) single crystal substrates by RF magnetron sputtering in pure Ar atmosphere at room temperature. Annealing the films in air at 1100 °C promoted the crystal formation. A surface layer of the obtained specimens were c-axis oriented apatite-type LSO, which was confirmed by in-plane and out-of-plane X-ray diffraction patterns. An activation energy of the ionic conductivity of the LSO sample using STO substrate was 0.74 eV, which agrees well with that of polycrystalline La9.33Si6O26 though the conductivity is as low as 2 × 10− 5 S cm− 1 at 600 °C. Using the laminated thin film method, we have accomplished to fabricate c-axis oriented apatite-type LSO with shorter time and lower temperature than the bulk diffusion synthesis between La2SiO5 and La2Si2O7.