Pressure-induced orbital reordering in Na$_2$CuF$_4$

Craig I. Hiley; Catriona A. Crawford; Craig L. Bull; Nicholas P. Funnell; Urmimala Dey; Nicholas C. Bristowe; Richard I. Walton; Mark S. Senn

arXiv · 2504.01120

Pressure-induced orbital reordering in Na$_2$CuF$_4$

Craig I. Hiley·Catriona A. Crawford·Craig L. Bull·Nicholas P. Funnell·Urmimala Dey·Nicholas C. Bristowe·Richard I. Walton·Mark S. Senn

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Abstract

The high-pressure behaviour of Na$_2$CuF$_4$ is explored by powder neutron diffraction and density functional theory (DFT) calculations. A first-order phase transition is observed to take place between 2.4 - 2.9 GPa, involving a reorientation of the Jahn-Teller (JT) long axes of the (CuF6) octahedra (and therefore the d$_{z^2}$ Cu orbitals), in agreement with our DFT calculations which suggest a transition at 2.8 GPa. The transition can be described as being between a state of ferro-orbital order and one of A-type antiferro-orbital order, reflecting a shift in the associated electronic instability from being in the zone-center to zone boundary of the first Brillouin zone of the parent structure, with pressure. This change results in a decoupling of magnitude of the associated Jahn-Teller distortion of the Cu-F bond lengths from the lattice strain. This scenario is supported by our observations that the compressibility of the pre-transition phase is highly anisotropic, whilst in the post-transition phase it becomes almost isotropic, and that we observed no further decrease of the magnitude the JT distortion up to 5 GPa, or melting of the OO in our DFT calculations up to at least 5 GPa.

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Craig I. Hiley, Catriona A. Crawford, Craig L. Bull, Nicholas P. Funnell, Urmimala Dey, Nicholas C. Bristowe, Richard I. Walton, Mark S. Senn. 2025-04-01. Pressure-induced orbital reordering in Na$_2$CuF$_4$. https://arxiv.org/abs/2504.01120

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