  In-situ high pressure single crystal X-ray diffraction study reveals that the
quantum material CaMn$_2$Bi$_2$ undergoes a unique plane to chain structural
transition between 2 and 3 GPa, accompanied by a large volume collapse.
CaMn2Bi2 displays a new structure type above 2.3 GPa, with the puckered Mn
honeycomb lattice of the trigonal ambient-pressure structure converting to
one-dimensional (1D) zigzag chains in the high-pressure monoclinic structure.
Single crystal measurements reveal that the pressure-induced structural
transformation is accompanied by a dramatic two order of magnitude drop of
resistivity; although the ambient pressure phase displays semiconducting
behavior at low temperatures, metallic temperature dependent resistivity is
observed for the high pressure phase, as, surprisingly, are two resistivity
anomalies with opposite pressure dependences. Based on the electronic structure
calculations, we hypothesized that the newly emerged electronic state under
high pressure is associated with a Fermi surface instability of the quasi-1D Mn
chains, while we infer that the other is a magnetic transition. Assessment of
the total energies for hypothetical magnetic structures for high pressure
CaMn$_2$Bi$_2$ indicates that ferrimagnetism is thermodynamically favored.
