A catastrophic charge density wave in BaFe2Al9

Abstract

Charge density waves (CDWs) are modulations of the electron density and the atomic lattice that develop in some crystalline materials at low temperatures. We report an unusual example of a CDW in BaFe2Al9 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 superlattice peaks in the low-temperature phase signaling the development of a CDW lattice modulation. No similar low-temperature transitions are observed in BaCo2Al9. 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 BaFe2Al9 develops a structural modulation at low temperature instead of magnetic order.