Stacking disorder in novel ABAC-stacked brochantite, Cu$_4$SO$_4$(OH)$_6$

TL;DR

This study reports the discovery of a new ABAC-stacked polytype of brochantite with stacking and anion disorder, revealing insights into how disorder affects its magnetic properties and identifying a double magnetic transition.

Contribution

We synthesized single crystals of brochantite with a new ABAC stacking polytype and analyzed how stacking and anion disorder influence its magnetic behavior.

Findings

Identified a new ABAC-stacked brochantite polytype.

Discovered cation disorder impacts magnetism more than anion or stacking disorder.

Observed a double magnetic transition in specific heat measurements.

Abstract

In geometrically frustrated magnetic systems, weak interactions or slight changes to the structure can tip the delicate balance of exchange interactions, sending the system into a different ground state. Brochantite, Cu$_4$SO$_4$(OH)$_6$, has a copper sublattice composed of distorted triangles, making it a likely host for frustrated magnetism, but exhibits stacking disorder. The lack of synthetic single crystals has limited research on the magnetism in brochantite to powders and natural mineral crystals. We grew crystals which we find to be a new polytype with a tendency toward ABAC stacking and some anion disorder, alongside the expected stacking disorder. Comparison to previous results on natural mineral specimens suggests that cation disorder is more deleterious to the magnetism than anion and stacking disorder. Our specific heat data suggest a double transition on cooling into the magnetically ordered state.