A catastrophic charge density wave in BaFe$_2$Al$_9$

TL;DR

BaFe₂Al₉ exhibits an unusual first-order charge density wave transition below 100 K, causing significant lattice strain and crystal shattering, contrasting with typical CDW behavior.

Contribution

This study reports the discovery of a catastrophic, first-order CDW transition in BaFe₂Al₉, with detailed structural and electronic analysis highlighting its unique properties.

Findings

First-order CDW transition causes crystal shattering.

Super-lattice peaks indicate lattice modulation.

No similar transition observed in BaCo₂Al₉.

Abstract

Charge density waves (CDW) are modulations of the electron density and the atomic lattice that develop in some crystalline materials at low temperature. We report an unusual example of a CDW in BaFe$_2$Al$_9$ below 100 K. In contrast to the canonical CDW phase transition, temperature dependent physical properties of single crystals reveal a first-order phase transition. This is accompanied by a discontinuous change in the size of the crystal lattice. In fact, this large strain has catastrophic consequences for the crystals causing them to physically shatter. Single crystal x-ray diffraction reveals super-lattice peaks in the low-temperature phase signaling the development of a CDW lattice modulation. No similar low-temperature transitions are observed in BaCo$_2$Al$_9$. Electronic structure calculations provide one hint to the different behavior of these two compounds; the d-orbital states in the Fe compound are not completely filled. Iron compounds are renowned for their magnetism and partly filled d-states play a key role. It is therefore surprising that BaFe$_2$Al$_9$ develops a structural modulation instead at low temperature instead of magnetic order.