Observation of non-superconducting phase changes in LuH$_{2\pm\text{x}}$N$_y$

TL;DR

This study investigates nitrogen-doped lutetium hydride phases under pressure, revealing color changes due to nitrogen redistribution, and finds no evidence of superconductivity, clarifying previous claims and establishing key phase benchmarks.

Contribution

The paper provides the first detailed characterization of pressure-induced phase changes in nitrogen-doped lutetium hydride, refuting earlier superconductivity claims and explaining color variations.

Findings

No superconductivity observed between 1.8-300 K and 0-30 GPa.

Identified a new purple phase of LuH$_{2 ext{-} ext{x}}$N$_y$.

Color changes are due to nitrogen redistribution, not superconductivity.

Abstract

The recent report of near-ambient superconductivity in nitrogen doped lutetium hydride has triggered a worldwide fanaticism and raised major questions about the latest claims. An intriguing phenomenon of color changes in pressurized samples from blue to pink to red was observed and correlated with the claimed superconducting transition, but the origin and underlying physics of these color changes have yet to be elucidated. Here we report synthesis and characterization of high-purity nitrogen doped lutetium hydride LuH$_{2\pm\text{x}}$N$_y$ with the same structure and composition as in the main phase of near-ambient superconductor1. We find a new purple phase of LuH$_{2\pm\text{x}}$N$_y$ between blue and pink phase, and reveal that the sample color changes likely stem from pressure-driven redistribution of nitrogen and its interaction with the LuH$_2$ framework. No superconducting transition is found in all blue, purple, pink and red phases at temperatures 1.8-300 K and pressures 0-30 GPa. Instead, we identify a notable temperature-induced resistance anomaly of structural and/or electronic origin in LuH$_{2\pm\text{x}}$N$_y$, which is most pronounced in the pink phase and may have been erroneously interpreted as a sign of superconducting transition. This work establishes key benchmarks for nitrogen doped lutetium hydrides, allowing an in-depth understanding of the novel pressure-induced phase changes.