High-pressure crystal structure investigation of the cage compound Fe1+δGa3

The pristine compound FeGa3 crystalizes in a tetragonal P42/mnm crystal symmetry, where Fe atoms surrounded by two non-equivalent Ga sites feature a cage-type structure. It has been shown in single crystal FeGa3 that an intrinsic energy gap ~ 0.4 eV develops, establishing a semiconducting and diamagnetic ground state [1]. However, at low temperatures, additional in-gap states related to the presence of intrinsic disorder induces a small intrinsic energy gap Δe, followed by putative weak ferromagnetism (below Tm) [2], which open a debate about the role of disorder in FeGa3 ground state.

In order to investigate the role of disorder, we synthesize FeGa3 with a delicate inclusion of Fe-antisite disorder, named as Fe1+δGa3 (δ ~ 0.16). Our initial electrical transport and magnetization studies under pressure (up to 2 GPa) reveals a non-canonical phase diagram for semiconducting electronic correlated material: Δe decreases while Tm increases with pressure. These findings call for X-ray diffraction (XRD) experiments under high-pressure to demystify whether this unusual phase diagram is due either to Anderson localization or to structural phase transition.

In this work, we present XRD results of Fe1+δGa3 up to 38 GPa and at 294 K, performed at HPCAT (Sector 16), Advanced Photon Source (APS), Argonne National Laboratory. Preliminary XRD data analysis distinguishes two different pressure ranges separated by critical pressure Pc ~ 18 GPa. In particular at Pc, the average volume grain size reduces ~ 30%, the microstrain increases ~ 50% drastically, accompanied by strong broadening in the XRD pattern. We infer that the system undergoes a structural transition from crystalline to amorphous phase around Pc.

Our findings are of extremely relevance to understand an intriguing metalization reported for the pristine single crystal FeGa3 at very high pressure [3].