Origin of abnormal grain growth in tungsten bronze structured ferroelectric Sr_xBa_1-xNbiOo ceramics

Green pellets of Sr_xBa_1-xNb₂O₆ (SBN, 0.3 ≤ x ≤ 0.7) and SrNb₂O₆ (SN) were fabricated and put together in order to form a diffusion couple. After sintering, the microstructure development at the interface of the diffusion couple was inspected. When the diffusion couple was sintered at a temperature over 1350°C, abnormal grain growth was seen at the interface of SBN and SN. The cation ratio of (Sr + Ba)/Nb for both SBN and SN is 1 : 2. However, because the concentrations of Sr and Ba are different in SBN and SN, Sr diffusion from SN to SBN and Ba diffusion from SBN to SN occurs concurrently at the interface during sintering. Since the diffusion coefficients of Sr and Ba are different, an area with a Nb-rich composition may be generated near the interface during sintering. The Nb-rich composition development, which has a low melting point, is believed to be the reason for the abnormal grain growth.