Polarized Neutron Diffraction Study of a CuCr₂Se₄Single Crystal

The electronic state of CuCr 2 Se 4 is investigated by means of the polarized neutron diffraction technique. From the measured form factors of copper and chromium atoms, the magnetic moments are determined to be -0.07±0.02 (µ B /Cu) and 2.64±0.04 (µ B /Cr), respectively. Further, the uniform polarization existent throughout the crystal is estimated to be -0.20±0.11 µ B per formula unit, using the total moment 5.01±0.01 (µ B /CuCr 2 Se 4 ) obtained from the magnetization measurement. This result which differs from those reported by other authors with neutron diffraction, shows that neither of the ionic configuration models of Cu + [Cr 3+ Cr 4+ ]Se 4 and Cu 2+ [Cr 3+ 2 ]Se 4 gives a satisfactory explanation of CuCr 2 Se 4 , although the former is closer to the experimental data. The form factor of the chromium atom agrees well with the calculated free ion form factor for Cr + , but not for Cr 3+ , and this fact indicates that the unpaired electron density on the chromium atom is more spread than that for free Cr 3+ . It is found that 84±16% of the unpaired 3 d electrons occupy t 2g orbitals.