  Application of high pressure (HP) induces evolution from metallic to
semi-metallic state in the layered superconducting LaO1-xFxBiSe2 phase, as
concluded from ab initio calculations based on experimental P-dependent
structural data. These changes in conduction properties are associated with a
reported decrease in the superconducting transition temperature, Tc. However,
further increase in pressure induces structural transformation in LaO1-xFxBiSe2
which results in a strong enhancement of its two-dimensionality with a related
improvement in superconducting performance. In addition, the HP structural
anisotropy induces a loss of long-range order correlations along the stacking
direction of the LaO/F and BiSe2 layers yielding formation of stacking faults.
Similar but even more pronounced structural transition was observed in the
related LaO1-xFxBiS2 phase which is also accompanied by an increase in Tc. This
demonstrates that enhancement of two-dimensionality in layered superconducting
systems acts in favor of their superconducting performance.
