Low-temperature mean valence of nickel ions in pressurized La$_3$Ni$_2$O$_7$

TL;DR

This study investigates how pressure affects the valence state of nickel ions in La$_3$Ni$_2$O$_7$ at low temperatures, revealing that the mean valence remains stable up to high pressures and correlates with structural and superconducting phase transitions.

Contribution

The paper provides the first detailed analysis of pressure effects on Ni valence in La$_3$Ni$_2$O$_7$ using synchrotron X-ray absorption spectroscopy, linking valence stability to phase transitions.

Findings

Mean Ni valence remains stable from 1 atm to 40 GPa at 20 K.

Structural and superconducting phase transitions occur at a critical pressure.

Pressure-induced structural transition aligns with X-ray diffraction results.

Abstract

The discovery of high critical temperature (Tc) superconductivity in pressurized La$_3$Ni$_2$O$_7$ has ignited renewed excitement in the search of novel high-Tc superconducting compounds with 3d transition metals. Compared to other ambient-pressure superconductors, such as copper-oxide and iron-oxypnictides, unraveling the mechanisms of the pressure-induced superconductivity poses significant and unique challenges. A critical factor in this phenomenon seems to be related to the electronic configuration of 3d orbitals, which may play a fundamental role in driving high-Tc superconductivity. However, the pressure effects on the mixed-valence states of 3d-orbital cations and their influence on the emergence of high-Tc superconductivity remain poorly understood. Here, we use high-pressure (P) and low-temperature synchrotron X-ray absorption spectroscopy to investigate the influence of pressure on the mean valence change of Ni ions in La$_3$Ni$_2$O$_7$. Our results demonstrate that at a low-temperature of 20 K, the mean valence remains relatively stable across the pressures range from 1 atm to 40 GPa. Based on analyzing the absorption data, we find that, at a critical pressure, the ambient-pressure ordered phases disappear and both the structural and the superconducting phase transition occur. The pressure-induced structural phase transition revealed by our absorption results is consistent with that determined by X-ray diffraction, offering new information for a comprehensive understanding on the pressure-induced superconductivity in La$_3$Ni$_2$O$_7$.