  We studied the crystal structure and spin state of MnP under high pressure
with synchrotron X-ray diffraction and X-ray emission spectroscopy. MnP has an
exceedingly strong anisotropy in compressibility, with the primary compressible
direction along the b axis of the Pnma structure. X-ray emission spectroscopy
reveals a pressure-driven quenching of the spin state in MnP. Firstprinciples
calculations suggest that the strongly anisotropic compression behavior
significantly enhances the dispersion of the Mn d-orbitals and the splitting of
the d orbital levels compared to the hypothetical isotropic compression
behavior. Thus, we propose spin quenching results mainly from the significant
enhancement of the itinerancy of d electrons and partly from spin rearrangement
occurring in the split d-orbital levels near the Fermi level. This explains the
fast suppression of magnetic ordering in MnP under high pressure. The spin
quenching lags behind the occurrence of superconductivity at ~8 GPa implying
that spin fluctuations govern the electron pairing for superconductivity.
