b-direction anomalous thermal expansion behavior in SmCrO3

The ceramic composites of Sm1-xBixCrO3 (x = 0, 0.1, 0.2, 0.3) have been prepared by solid state reaction. X-ray diffraction patterns demonstrate that Bi-doped polycrystalline samples match with standard pattern of SmCrO3. After Bi doping, the lattice constants in three directions increase slightly. The thermal expansion properties were studied by X-ray powder diffraction over the temperature range from 100 to 400 K. The results showed that the compound SmCrO3 exhibits positive thermal expansion behavior in the a and c directions in the whole measurement temperature range. The anomaly is that in the b direction the lattice exhibits positive thermal expansion behavior at low temperatures, while above 200 K it exhibits near zero thermal expansion behavior. The thermal expansion behaviors of all Bi-doped SmCrO3 are similar to that of SmCrO3, but unexpectedly, negative thermal expansion behaviors were observed in the b direction above 200 K in the highly Bi doped samples. The dielectric measurement shows that there are two thermal activation processes in all Sm1-xBixCrO3 (x = 0, 0.1, 0.2, 0.3) respectively, with the low-temperature activation corresponding to the grain itself, while the high-temperature activation comes from the grain boundary. After low-temperature thermal activation, with increasing temperature, the enhanced vibration of the corner oxygen atoms of the CrO6 octahedron is mainly perpendicular to the b axis, which leads to the near zero thermal expansion behavior in the b direction. Moreover, after high-temperature thermal activation, as the temperature increases, the enhanced interaction between Bi3+ and the free oxygen atom (space oxygen atom) at the grain boundary will further lead to the negative thermal expansion behavior in the b-direction of the high concentration doped samples.