Figure 2
From: Topological defects at octahedral tilting plethora in bi-layered perovskites
(a) Structural phase diagram of Ca3−xSrxTi2O7 with the angles of octahedral rotation (θR, empty squares) and tilt (θT, cyan spheres) as a function of Sr doping, x. The black and cyan curves are guides for the eyes. Two structural phase transition temperatures (T1=473 K and T2=710 K) at x=0.9, marked with red spheres, are obtained from in situ TEM heating experiments. (b) Dielectric constant versus Sr content (x), measured at 350 K and 44 kHz. Red triangles and blue circles are from measurements on polycrystalline and single-crystalline (electric field applied along the in-plane direction) specimens, respectively. The dielectric constant peaks at the O–T′ structural boundary of x=0.9. (c) Thermal sequence of selected area electron diffraction patterns on x=0.9, showing two phase transitions upon in situ heating: O (300 K)→T′ (473 K)→ T (713 K) states. S1=½(130)T-type superlattice spots (cyan triangles) are allowed in the O state. When temperatures reaches 473 K, additional S2=½(200)T-type and S3=½(−130)T-type superlattice spots appear (yellow and green triangles), indicating the appearance of a tetragonal T′ state. Above 713 K, all superlattice spots disappear, consistent with the presence of the T state at very high temperatures.
