In recent years, great progress has been made in the lead-free conversion of electronic materials. Sodium potassium niobate (KNN) and bismuth sodium titanate (BNT) are the two most competitive components that are expected to partially replace the currently used lead zirconate titanate (PZT) component. KNN undergoes cubic, tetragonal, orthogonal and tripartite phase transitions from high temperature to low temperature; The phase transition behavior of BNT is relatively complex. From high temperature to low temperature, BNT experiences cubic, tetragonal and tripartite phases in turn, and there is a tetragonal and tripartite coexistence phase in a temperature region. However, many studies have shown that BNT at room temperature is a monoclinic phase, and with the increase of temperature, orthogonal and tetragonal phases will appear. There is no oxygen octahedral tilt in the tetragonal and tripartite phases of KNN, while the oxygen octahedral is tilted in the tetragonal and tripartite phases of BNT. The solid solution of KNN-Bnt has attracted a lot of attention for a long time, but the change of its structure with its composition has not been clearly understood.
We found that the main problems in this system are as follows: 1) the position of phase boundary and phase composition are not clear. On the KnN-rich side, most people believe that with the increase of BNT, the tebral-orthogonal coexistence rapidly moves to room temperature and then appears cubic phase, but the electrical properties are not consistent with the macroscopic structure. On the BNT-rich side, there exists a ferroelectric-antiferroelectric phase boundary, the structural composition of which remains unclear. 2) macrostructure and microstructure, with the increase of BNT, XRD diffraction pattern of solid solution soon present cubic structure, but the electrical properties show ferroelectricity, in addition, even if the temperature dielectric spectrum is displayed on the suitable electrical phase, but still there is a stronger Raman spectrum peak, it shows that the micro structure and macro structure is inconsistent.
The phase structure of (1-X) KNN-XBNT was studied in detail by synchrotron radiation XRD. It can be seen from the XRD pattern that the whole solid solution can be divided into 5 regions. In the range of x≤0.005, Bragg diffraction peak is the same as pure KNN; With the increase of BNT, the diffraction peak starts to have tetragonal distortion in the region 0.02 When the BNT is further increased, the diffraction peak exhibits cubic symmetry in the range of 0.20≤x≤0.50, and the solid solution exhibits three-way symmetry in the range of 0.90≤x on the BNt-rich side, and the superlattice diffraction caused by the tilt of the oxygen octahedron is clearly visible.
The exact phase composition of each component was obtained by Rietveld refinement of each map. For components with x≤0.005, the structures are all orthogonal structure Amm2 point groups. For the components around x~0.02, they are mainly composed of two phases, namely the Pm point group with monoclinic structure and the P4mm point group with tetragonal structure. The P4mm point group is similar to the P4bm point group, but the latter has an oxyoctahedral tilt. Compared with other heavy metal ions, oxygen atoms are not strong enough to X-ray diffraction, so its weak octahedral tilt can not be well reflected in the XRD pattern. Therefore, neutron diffraction is used in the experiment, and the results show that the component with x=0.02 exhibits a single-phase normal phase Amm2 point group at low temperature. Most literatures believe that the structure of the components at x=0.20 must be cubic phase. Through the analysis of the atlas and other auxiliary technologies, we found that the main composition of the components is cubic phase ~85%, and the rest is tetragonal phase with oxygen octahedral tilt. The point group of tetragonal phase is P4bm rather than P4mm. The tilt of the oxygen octahedron is mainly due to the introduction of BNT. With the further increase of BNT, the transition peak of the ferroelectric – paramoelectric phase transition moves to the room temperature, but the diffraction peak of Raman spectrum is still very clear, showing a relatively complex structure. On the one hand, there is a cubic phase, on the other hand, there must be weak distortion of the quadripartite or tripartite phase, or even a normal phase. The tetragonal phase with weak distortion may belong to P4mm or P4bm point groups. Therefore, synchrotron radiation XRD test and neutron diffraction test were carried out on this component. Through combination of various models, the composition of this component was finally determined as the combination of P4bm and PM-3M, which accounted for 45% and 55% respectively at room temperature, and this proportion changed slightly with temperature. The teteal phase can even stay above 300 degrees Celsius, while the cubic phase can stay around -270 degrees.
In the KNN-Bnt solid solution, from x=0.90, the diffraction peak of the weak superlattice begins to appear in the XRD pattern, which means that the tilted tripartite phase R3c of the oxygen octahedron begins to appear. The ratio of P4bm and R3c was determined by Rietveld refinement. In order to further see the tilt of oxygen octahedron, neutron diffraction was also carried out on this component, and the superlattice peak of 1/2(OOE) corresponding to P4bm and 1/2(OOO) corresponding to R3c could be seen. To further confirm the microstructure, the samples were examined by transmission electron microscopy. Sample x=0.5 has obvious 1/2(OOE) superlattice spots on the  C band axis, but no superlattice diffraction spots on the  C direction, which is consistent with the XRD and neutron diffraction refinement results. Sample x=0.9 has obvious 1/2(OOE) superlattice spots on the band axis of  C, and 1/2(OOO) superlattice diffraction spots on the band axis of  C, indicating the coexistence of tetraval phase P4bm and tripartite phase R3c, which is consistent with the results of XRD and neutron diffraction refinement.
The structure evolution of KnN-Bnt Solid solution is obviously different from that of KnN-BT [Solid State Commun. 2010(150), 1497 — 1500], and has an important relationship with Bi3+ containing lone pair electrons. During electron diffraction of BNT, localized in-phase octahedral tilts are shown in the macroscopic structure of a-A-a -/a-a-a- at room temperature. We analyzed the possible reasons as follows: 1) sheet P4bm was mixed in the main phase of R3c at room temperature; 2) The long-range ordered a-A-a -/ a-a-a-a-a-a-tilt (10-40nm) is inlaid with A-A-C + tilt (1-3 nm). In this tilted sequence, the short-range order appears on the 100C twin boundary. Such a disordered combination of oxyoctahedra along the oxyoctahedral chain leads to an average out-of-phase tilting, resulting in a pseudotripartite or monoclinic structure, which will be observed in a later spherical aberration corrected electron microscope.