Abstract
The infinite layer structure type has been known to host high-temperature superconductivity since the discovery of ${\mathrm{Ca}}_{0.86}{\mathrm{Sr}}_{0.14}{\mathrm{CuO}}_{2}$, yet little progress has been made to synthesize many analogs. Here, using ${\mathrm{SrFeO}}_{x}$ as a prototype system, we explore the thermodynamic obstacles behind the scarcity of $3d$ elements adopting the infinite layer structure type. In this context, synthetic considerations to achieve the $AB{\mathrm{O}}_{3}$ to $AB{\mathrm{O}}_{2}$ transformation are discussed. Specifically, we demonstrate that the conventionally reported topochemical reduction can result in hydride incorporation into $\mathrm{SrFe}{\mathrm{O}}_{2}$, causing a decrease in the magnetic ordering temperature of the infinite layered oxide. First-principles simulations further confirm that the incorporation of H is necessary for stabilizing the $\mathrm{SrFe}{\mathrm{O}}_{2}$ phase by decreasing the thermodynamic cost of individual steps required to transform $\mathrm{SrFe}{\mathrm{O}}_{3}$ into $\mathrm{SrFe}{\mathrm{O}}_{2}$, and is the driving factor behind the changes in magnetic exchange interactions that ultimately change the N\'eel temperature (${T}_{\mathrm{N}}$). Additionally, inspired by recent reports of superconductivity in another low-dimensional oxide ${\mathrm{Nd}}_{0.8}{\mathrm{Sr}}_{0.2}{\mathrm{NiO}}_{2}$, ${\mathrm{Sr}}_{0.95}{\mathrm{Nd}}_{0.05}{\mathrm{FeO}}_{2}$ was synthesized via a more traditional topochemical reduction procedure. Both physical characterization and accompanying density-functional theory calculations show that this $A$-site doping can have similar effects on $A{\mathrm{FeO}}_{2}$ stability and magnetic ordering temperatures as with the incorporation of hydrogen. Ultimately, these results suggest that charge doping either through the incorporation of H or $A$-site substitution may be fruitful routes in tuning stability and magnetic properties, with direct consequences on superconducting behavior.
Highlights
The interest in a class of materials known as infinite layer oxides, of the general formula ABO2, first began with the discovery of the high-temperature superconductor Ca0.86Sr0.14CuO2 [1]
To produce SrFeO2−xHx, a likely hydrogen-doped system, the above method was modified such that the molar ratio of CaH2:SrFeO3 was increased to 3:1, the reaction temperature was increased to 300 °C, and a second open alumina crucible was added to the sealed evacuated quartz tube containing 1 millimole well-ground CaH2 to provide a stronger reducing atmosphere within the tube
We first examine the energetics for reducing SrFeO3 successively to SrFeO2
Summary
The interest in a class of materials known as infinite layer oxides, of the general formula ABO2, first began with the discovery of the high-temperature superconductor Ca0.86Sr0.14CuO2 [1]. By reducing the metastable sublattice, the thermodynamic barrier associated with the direct synthesis of such a phase is effectively circumvented Examples of this can be found in the syntheses of LaNiO3 [5,6], NdNiO3 [7], and SrFeO3 [14,15,16,17,18,19] and their subsequent reduction to ABO2 with the use of an alkali or alkaline earth metal hydride reductant to cleave axial oxygens from BO6 units, producing stable sublattices composed entirely of square planar BO4 units (Fig. 1). Together these results help us understand the thermodynamic limitations in the formation of ABO2 compounds while presenting routes to tuning the magnetic (and potentially superconducting) properties of related compounds
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