Impact of Li Doping on the Structure and Phase Stability in AgNbO₃

The impact of Li doping on the temperature-induced phase transitions in silver niobates Ag_1-LiNbO₃ has been investigated using a combination of high-resolution powder neutron diffraction and synchrotron X-ray diffraction. Considering both the cell metric and distortions of the NbO₆ octahedra, estimated by Rietveld refinements, it is shown that the sequence of temperature-induced phases in AgNbO₃ is P2₁am → Pcam → Cmcm → P4/mbm → Pm3̅m. This sequence is simpler than that proposed in earlier studies. Evidence is presented for a second-order Jahn-Teller distortion in the Pcam phase. At x > 0.05, Li doping favors the formation of a rhombohedral phase in space group R3c, and such samples display the temperature-induced sequence R3c → Pbnm → Cmcm → P4/mbm → Pm3̅m. Unusual volume changes associated with the phase transitions point to the potential importance of lattice matching in optimizing the properties of thin films of doped AgNbO₃.