Synthesis and physical properties of perovskite Sm _1−x Sr _x NiO ₃ (x = 0, 0.2) and infinite-layer Sm _0.8Sr _0.2NiO ₂ nickelates

Abstract

Recently, superconductivity at about 9–15 K was discovered in Nd _{1− x} Sr _x NiO ₂ (Nd-112, x ≈ 0.125–0.25) 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 ( R-113, R: rare-earth element) 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 224 K on a global insulating background. In contrast, the Sr-doped sample Sm _0.8Sr _0.2NiO ₃ shows a metallic behavior from 300 K down to about 12 K, while below 12 K the resistivity exhibits a slight logarithmic increase. Meanwhile, from the magnetization curves, we can see that a possible spin-glass state occurs below 12 K in Sm _0.8Sr _0.2NiO ₃. Using a soft chemical reduction method, we also obtain the infinite-layer phase Sm _0.8Sr _0.2NiO 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 _0.8Sr _0.2NiO ₂.