Magnetically induced metal-insulator transition in Pb2CaOsO6

TL;DR

This study investigates how structural distortions in Pb2CaOsO6 induce a spin-driven metal-insulator transition, contrasting it with Pb2ZnOsO6 which remains metallic and paramagnetic due to its different crystal structure.

Contribution

It reveals the role of crystal structure distortion in enabling magnetic order and a metal-insulator transition in Pb2CaOsO6, a novel example of a spin-driven transition.

Findings

Pb2CaOsO6 undergoes a metal-insulator transition below 80 K with antiferromagnetic order.

Pb2ZnOsO6 remains a paramagnetic metal down to 2 K due to its cubic lattice.

Structural distortion in Pb2CaOsO6 relieves magnetic frustration, enabling magnetic order.

Abstract

We report on the structural, magnetic, and electronic properties of two new double-perovskites synthesized under high pressure; Pb2CaOsO6 and Pb2ZnOsO6. Upon cooling below 80 K, Pb2CaOsO6 simultaneously undergoes a metal--insulator transition and develops antiferromagnetic order. Pb2ZnOsO6, on the other hand, remains a paramagnetic metal down to 2 K. The key difference between the two compounds lies in their crystal structure. The Os atoms in Pb2ZnOsO6 are arranged on an approximately face-centred cubic lattice with strong antiferromagnetic nearest-neighbor exchange couplings. The geometrical frustration inherent to this lattice prevents magnetic order from forming down to the lowest temperatures. In contrast, the unit cell of Pb2CaOsO6 is heavily distorted up to at least 500 K, including antiferroelectric-like displacements of the Pb and O atoms despite metallic conductivity above 80 K. This distortion relieves the magnetic frustration, facilitating magnetic order which in turn drives the metal--insulator transition. Our results suggest that the phase transition in Pb2CaOsO6 is spin-driven, and could be a rare example of a Slater transition.