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

Abstract

Recently, superconductivity at about 9-15 K was discovered in Nd1−xSrxNiO2 (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 RNiO3 (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 Sm1−xSrxNiO3 (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 Sm1−xSrxNiO3 with small amount of NiO impurities. Magnetization and resistivity measurements indicate that the parent phase SmNiO3 undergoes a paramagneticantiferromagnetic transition at about 224 K on a global insulating background. In contrast, the Sr-doped sample Sm0.8Sr0.2NiO3 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 Sm0.8Sr0.2NiO3. Using a soft chemical reduction method, we also obtain the infinite-layer phase Sm0.8Sr0.2NiO2 with a square NiO2 plane. 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 Sm0.8Sr0.2NiO2.

Universal spin-glass behaviour in bulk LaNiO2, PrNiO2 and NdNiO2

“…It is worth noting that there are no observable nickel impurities in these samples. The nickel was one of the prime suspects responsible for the FM-like transition at low temperature as well as the weak magnetic hysteresis in bulk Nd 0.8 Sr 0.2 NiO 2 and Sm 0.8 Sr 0.2 NiO 2 [3][4][5]. To further exclude the possibility of nickel impurities, we re-oxidized the samples back to RNiO 3 and did not observe any NiO.…”

“…* Electronic address: hailin@physik.uzh.ch First, superconductivity is absent in bulk samples, e.g. in bulk Nd 1−x Sr x NiO 2 [3,4] and Sm 1−x Sr x NiO 2 [5], and no signs of long-range magnetic orders have been observed. This discrepancy between non-superconducting bulk samples and superconducting thin films has not been well understood so far.…”

Universal spin-glass behaviour in bulk LaNiO2, PrNiO2 and NdNiO2

“…First, superconductivity is absent in bulk samples, e.g. in bulk Nd 1−x Sr x NiO 2 [5,6] and Sm 1−x Sr x NiO 2 [7], and no signs of long-range magnetic orders have been observed. This discrepancy between non-superconducting bulk samples and superconducting thin films has not been well understood so far.…”

Geometric effects in the infinite-layer nickelates

“…This circumstance has raised the question of the role of the 4f electrons [9] as well as of extrinsic factors such as the unintentional presence of topotatic hydrogen or apical oxygens, which have been argued to correlate with the structure [10,11]. Examining the RNiO 2 series for R from La to Lu as well as additional candidate materials for nickelate superconductivity has thus emerged as a consequential outlook [12][13][14][15][16][17][18][19][20][21][22][23][24]. At the same time, in analogy with their perovskite counterparts [25][26][27], the appearance of structural instabilities associated to the relative size of the R atom may also come into play as an additional key ingredient.…”