Examination of the bonding in binary transition-metal monophosphides MP (M = Cr, Mn, Fe, Co) by X-ray photoelectron spectroscopy

The binary transition-metal monophosphides CrP, MnP, FeP, and CoP have been studied with X-ray photoelectron spectroscopy. The shifts in phosphorus 2p(3/2) core line binding energies relative to that of elemental phosphorus indicated that the degree of ionicity of the metal-phosphorus bond decreases on progressing from CrP to CoP. The metal 2p(3/2) core line binding energies differ only slightly and show similar line shapes to those of the elemental metals, reaffirming the notion that these transition-metal phosphides have considerable metallic character. The satellite structure observed in the Co 2p(3/2) X-ray photoelectron spectra of Co metal and CoP was examined by reflection electron energy loss spectroscopy and has been attributed to plasmon loss, not final state effects as has been previously suggested. Valence-band spectra of the transition-metal phosphides agree well with the density of states profiles determined from band structure calculations. The electron populations of the different electronic states were extracted from the fitted valence-band spectra, and these confirm the presence of strong M-P and weak P-P bonding interactions. Atomic charges determined from the P 2p core line spectra and the fitted valence-band spectra support the approximate formulation M(1+)P(1-) for these phosphides.