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Abstract:
We report on the microscopic magnetic modeling of the spin-1/2 copper
mineral malachite at ambient and elevated pressures. Despite the layered
crystal structure of this mineral, the ambient-pressure susceptibility
and magnetization data can be well described by an unfrustrated
quasi-one-dimensional magnetic model. Weakly interacting
antiferromagnetic alternating spin chains are responsible for a large
spin gap of 120 K. Although the intradimer Cu-O-Cu bridging angles are
considerably smaller than the interdimer angles, density functional
theory (DFT) calculations revealed that the largest exchange coupling of
190 K operates within the structural dimers. The lack of the inversion
symmetry in the exchange pathways gives rise to sizable
Dzyaloshinskii-Moriya interactions which were estimated by
full-relativistic DFT + U calculations. Based on available high-pressure
crystal structures, we investigate the exchange couplings under pressure
and make predictions for the evolution of the spin gap. The calculations
evidence that intradimer couplings are strongly pressure dependent and
their evolution underlies the decrease of the spin gap under pressure.
Finally, we assess the accuracy of hydrogen positions determined by
structural relaxation within DFT and put forward this computational
method as a viable alternative to elaborate experiments.
