# From Oxidized PrNi0.9Al0.1O3 to Reduced PrNi0.9Al0.1O2+δ Perovskite Nickelates: Stabilization of Infinite-Layer Specimens with Monovalent Ni in the Bulk Polycrystalline Form

## TL;DR

Researchers synthesized a new nickelate material with an infinite-layer structure and confirmed hydrogen incorporation, which could be important for understanding high-temperature superconductivity.

## Contribution

Bulk polycrystalline infinite-layer PrNi0.9Al0.1O2+δ was synthesized and hydrogen incorporation was confirmed using neutron diffraction.

## Key findings

- Bulk PrNi0.9Al0.1O2+δ with infinite-layer structure was successfully synthesized via topotactic reduction.
- Neutron diffraction confirmed hydride ions in the IL lattice of PrNi0.9Al0.1O2.10H0.16.
- Spectroscopic and magnetometric analyses confirmed Ni3+ to Ni+ reduction.

## Abstract

Recently, a new class
of high-temperature superconductors, RNiO2 (where R represents rare-earth
elements) with infinite-layer (IL) structure, has been identified.
They possess the same structural framework as the renowned high-T
c cuprates but with nickel replacing copper
as the central element. In this study, we successfully synthesized
infinite-layer samples of PrNi0.9Al0.1O2+δ in the bulk polycrystalline form through topotactic
reduction of the PrNi0.9Al0.1O3 orthorhombic
perovskite, via treatment with CaH2. The incorporation
of aluminum at the octahedral sites promotes the stabilization of
bulk derivatives of the infinite-layer structure since unreduced [AlO6] octahedra keep the layers together and prevent their decomposition.
The lack of superconductivity in bulk samples has been a subject of
intense debate in recent literature. One major theoretical question
concerns whether hydrogen becomes incorporated into the structure
during the reduction from RNiO3 to RNiO2as
suggested by theory. Here, we present neutron powder diffraction data
demonstrating that hydride ions indeed reside within the IL lattice
in samples of stoichiometry PrNi0.9Al0.1O2.10H0.16. Additional crystallographic analyses
were carried out using temperature-dependent synchrotron X-ray diffraction
on both reduced and oxidized phases. Furthermore, spectroscopic analysis
via XAS and magnetometry confirms the reduction of Ni3+ to the Ni+ oxidation state, aligning with the crystallochemical
evidence.