Synthesis and physical properties of perovskite Sm$_{1-x}$Sr$_x$NiO$_3$ (x = 0, 0.2) and infinite-layer Sm$_{0.8}$Sr$_{0.2}$NiO$_2$ nickelates
Recently, superconductivity at about 9-15K was discovered in Nd$_{1-x}$Sr$_x$NiO$_2$ infinite-layer thin films, which has stimulated enormous interests in related rare-earth nickelates. Usually, the first step to synthesize this 112 phase is to fabricate the RNiO$_3$ phase, however, it was reported that the 113 phase is very difficult to be synthesized successfully due to the formation of unusual Ni$^{3+}$ oxidation state. And the difficulty of preparation is enhanced as the ionic radius of rare-earth element decreases. In this work, we report the synthesis and investigation on multiple physical properties of polycrystalline perovskites Sm$_{1-x}$Sr$_x$NiO$_3$ (x = 0, 0.2) in which the ionic radius of Sm$^{3+}$ is smaller than that of Pr$^{3+}$ and Nd$^{3+}$ in related superconducting thin films. The structural and compositional analyses conducted by X-ray diffraction and energy dispersive X-ray spectrum reveal that the samples mainly contain the perovskite phase of Sm$_{1-x}$Sr$_x$NiO$_3$ with small amount of NiO impurities. Magnetization and resistivity measurements indicate that the parent phase SmNiO$_3$ undergoes a paramagnetic-antiferromagnetic transition at about 224K on a global insulating background. In contrast, the Sr-doped sample Sm$_{0.8}$Sr$_{0.2}$NiO$_3$ shows a metallic behavior from 300K down to about 12K, while below 12K the resistivity exhibits a slight logarithmic increase. Meanwhile, from the magnetization curves, we can see that a possible spin-glass state occurs below 12K in Sm$_{0.8}$Sr$_{0.2}$NiO$_3$. Using a soft chemical reduction method, we also obtain the infinite-layer phase Sm$_{0.8}$Sr$_{0.2}$NiO$_2$ with square NiO$_2$ planes. The compound shows an insulating behavior which can be described by the three-dimensional variable-range-hopping model. And superconductivity is still absent in the polycrystalline Sm$_{0.8}$Sr$_{0.2}$NiO$_2$.