Electronic and crystal structures of ( Na 1 − x Ca x ) Cr 2 O 4 with anomalous colossal magnetoresistance

The electronic and crystal structures of $({\\mathrm{Na}}_{1\\ensuremath{-}x}{\\mathrm{Ca}}_{x}){\\mathrm{Cr}}_{2}{\\mathrm{O}}_{4}$ have been studied in detail by combining photoelectron spectroscopy (PES), x-ray absorption spectroscopy (XAS), and x-ray diffraction (XRD). The PES results suggest a gap opening at the Fermi level with decreasing temperature and/or increasing the Ca concentration. The XAS spectra at the O $K$-absorption edge suggest a slight increase of the O $2p$--Cr $3d$ hybridization at low temperatures in ${\\mathrm{NaCr}}_{2}{\\mathrm{O}}_{4}$ and ${\\mathrm{Na}}_{0.8}{\\mathrm{Ca}}_{0.2}{\\mathrm{Cr}}_{2}{\\mathrm{O}}_{4}$, which corresponds to the appearance of the antiferromagnetic order. However, XRD showed no corresponding structural transition. The experimental results were compared with spin-resolved density functional theory (DFT) calculations. In ${\\mathrm{Na}}_{1\\ensuremath{-}x}{\\mathrm{Ca}}_{x}{\\mathrm{Cr}}_{2}{\\mathrm{O}}_{4}$ the pre-edge intensity of the XAS spectra at the O $K$-absorption edge is strongly suppressed with increasing $x$, in accordance with the gap opening by Ca doping. This observation is consistent with the DFT calculation, where the density of states just above and below the Fermi level diminishes as the electron is doped into ${\\mathrm{NaCr}}_{2}{\\mathrm{O}}_{4}$.