  Neutron and X-ray diffraction, magnetic susceptibility, and specific heat
measurements have been used to investigate the magnetic and structural phase
transitions of the spinel system Fe1+xCr2-xO4(0.0<=x<=1.0). The temperature
versus Fe concentration (x) phase diagram features two magnetically ordered
states and four structural states below 420 K. The complexity of the phase
diagram is closely related to the change in the spin and orbital degrees of
freedom induced by substitution of Fe ions for Cr ions. The systematic change
in the crystal structure is explained by the combined effects of Jahn-Teller
distortion, spin-lattice interaction, Fe2+-Fe3+ hopping, and disorder among
Fe2+, Fe3+, and Cr3+ ions.
