Experimental confirmation of a p-p intraband gap in Sr2FeO4

The electrical-transport, magnetic, and structural properties of the antiferromagnetic semiconductor Sr2FeO4(Fe4+,d(4)) have been probed by resistance, Fe-57 Mossbauer spectroscopy, and x-ray-diffraction measurements to similar to 30 GPa in a diamond-anvil cell. The pertinent results of this work are that a semiconductor-metal transition occurs at P(C)similar to 18(1) GPa, while the Fe(IV) 3d magnetic moments remain unaltered across the transition. To the highest pressure of 30 GPa, the relative volume of the unit cell decreases to V/V(0)similar to 0.82, with no structural symmetry modification of the K2NiF4-type unit cell. At and above P-C, a magnetic hyperfine spectrum comprised of a single component with a non-negligible quadrupole interaction is evident below spin-ordering temperatures T<T-N. No drastic or discontinuous change occurs in both the hyperfine field (B-hf = 23 T) and isomer shift at pressures encompassing P-C, suggesting that the Fe-3d electronic state is unchanged upon metallization. This is considered compelling experimental evidence for closure of a p-p gap within the ligand band manifold, and therefore is the mechanism responsible for the onset of a metallic state. [S0163-1829(98)06540-0].