Synthesis and characterization of bulk ${\mathrm{Nd}}_{1-x}{\mathrm{Sr}}_x\mathrm{Ni}{\mathrm{O}}_2$ and ${\mathrm{Nd}}_{1-x}{\mathrm{Sr}}_x\mathrm{Ni}{\mathrm{O}}_3$

The recent reports of superconductivity in ${\mathrm{Nd}}_{1-x}{\mathrm{Sr}}_x\mathrm{Ni}{\mathrm{O}}_2/\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ heterostructures have reinvigorated interest in potential superconductivity of low-oxidation state nickelates. Synthesis of ${\mathrm{Ni}}^{1+}$-containing compounds is notoriously difficult. In the current work, a combined sol-gel combustion and high-pressure annealing technique was employed to prepare polycrystalline perovskite ${\mathrm{Nd}}_{1-x}{\mathrm{Sr}}_x\mathrm{Ni}{\mathrm{O}}_3$ $(x=0, 0.1, \mathrm{and} 0.2)$. Metal nitrates and metal acetates were used as starting materials, and the latter were found to be superior to the former in terms of safety and reactivity. The ${\mathrm{Nd}}_{1-x}{\mathrm{Sr}}_x\mathrm{Ni}{\mathrm{O}}_3$ compounds were subsequently reduced to ${\mathrm{Nd}}_{1-x}{\mathrm{Sr}}_x\mathrm{Ni}{\mathrm{O}}_2$ using calcium hydride in a sealed, evacuated quartz tube. To understand the synthesis pathway, the evolution from $\mathrm{Nd}\mathrm{Ni}{\mathrm{O}}_3$ to $\mathrm{Nd}\mathrm{Ni}{\mathrm{O}}_2$ was monitored using in situ synchrotron x-ray diffraction during the reduction process. Electrical transport properties were consistent with an insulator-metal transition occurring between $x=0$ and 0.1 for ${\mathrm{Nd}}_{1-x}{\mathrm{Sr}}_x\mathrm{Ni}{\mathrm{O}}_3$. Superconductivity was not observed in our bulk samples of ${\mathrm{Nd}}_{1-x}{\mathrm{Sr}}_x\mathrm{Ni}{\mathrm{O}}_2$. Neutron diffraction experiments at 3 and 300 K were performed on ${\mathrm{Nd}}_{0.9}{\mathrm{Sr}}_{0.1}\mathrm{Ni}{\mathrm{O}}_2$, in which no magnetic Bragg reflections were observed, and the results of structural Rietveld refinement are provided.

Figure 1

X-ray powder diffraction patterns of combusted ${\mathrm{Nd}}_{1-x}{\mathrm{Sr}}_x\mathrm{Ni}{\mathrm{O}}_3$ ($x=0$) powder from nitrate method and after annealing in ${\mathrm{O}}_2$ at ambient pressure for 12 and 84 h, respectively.

Figure 2

Room temperature X-ray powder diffraction patterns of ${\mathrm{Nd}}_{1-x}{\mathrm{Sr}}_x\mathrm{Ni}{\mathrm{O}}_3$ ($0.0\le x\le 0.2$) and corresponding Rietveld fitting results using Pbnm space group. The black dots are the observed data, the red line is the calculated fit, and gray line shows the difference between the two. Positions of allowed reflections are indicated by vertical black (${\mathrm{Nd}}_{1-x}{\mathrm{Sr}}_x\mathrm{Ni}{\mathrm{O}}_3$) and red (NiO) lines.

Figure 3

Variation of the (a) unit-cell parameters and (b) cell volume of ${\mathrm{Nd}}_{1-x}{\mathrm{Sr}}_x\mathrm{Ni}{\mathrm{O}}_3$ as a function of the Sr content ($x$). Solid squares: this work; open squares from Ref. [26]. Uncertainties represent one standard deviation.

Figure 4

X-ray powder diffraction patterns for samples made from acetates (red) and nitrates (black). (a) Combusted powders, (b) powders annealed at 800 °C for 12 h under ambient pressure, and (c) annealed at 1000 °C for 12 h under oxygen pressure of 150 bar. (d) Enlarged view of the region marked by dashed rectangle in (c).

Figure 5

High-energy in situ x-ray data showing the evolution from (a) $\mathrm{Nd}\mathrm{Ni}{\mathrm{O}}_3$ to ${\mathrm{Nd}}_3{\mathrm{Ni}}_3{\mathrm{O}}_7$ and (b) ${\mathrm{Nd}}_3{\mathrm{Ni}}_3{\mathrm{O}}_7$ to $\mathrm{Nd}\mathrm{Ni}{\mathrm{O}}_2$. The 113, 112, and 337 denote $\mathrm{Nd}\mathrm{Ni}{\mathrm{O}}_3$, $\mathrm{Nd}\mathrm{Ni}{\mathrm{O}}_2$, and ${\mathrm{Nd}}_3{\mathrm{Ni}}_3{\mathrm{O}}_7$ phases, respectively.

Figure 6

Powder x-ray diffraction of ${\mathrm{Nd}}_{0.9}{\mathrm{Sr}}_{0.1}\mathrm{Ni}{\mathrm{O}}_2$ (a) and ${\mathrm{Nd}}_{0.8}{\mathrm{Sr}}_{0.2}\mathrm{Ni}{\mathrm{O}}_2$ (b) and the corresponding Pawley refinement. The black dots are the observed data, the red line is the calculated fit, and gray line shows the difference between the two. Positions of allowed reflections are indicated by vertical black (${\mathrm{Nd}}_{1-x}{\mathrm{Sr}}_x\mathrm{Ni}{\mathrm{O}}_2$) and red (Ni) lines.

Figure 7

Temperature-dependent resistivity of the ${\mathrm{Nd}}_{1-x}{\mathrm{Sr}}_x\mathrm{Ni}{\mathrm{O}}_3$ ($x=0.0$, 0.1, and 0.2) and ${\mathrm{Nd}}_{1-x}{\mathrm{Sr}}_x\mathrm{Ni}{\mathrm{O}}_2$ ($x=0.2$) samples.

Figure 8

Neutron diffraction and the corresponding Rietveld refinement of ${\mathrm{Nd}}_{0.9}{\mathrm{Sr}}_{0.1}\mathrm{Ni}{\mathrm{O}}_2$ at 3 K (a) and at 300 K (b). The black dots are the observed data, the red line is the calculated fit, and gray line shows the difference between the two. Positions of allowed reflections are indicated by vertical black (${\mathrm{Nd}}_{1-x}{\mathrm{Sr}}_x\mathrm{Ni}{\mathrm{O}}_2$) and red (Ni) lines.