High-pressure Mossbauer study of perovskite iron oxides

One of the most interesting problems in condensed-matter systems is metal-insulator transition driven by the correlation effects associated with electron-electron interaction [I]. A charge ordering in perovskite compounds often occurred with their metal-insulator transitions. The high valence state of Fe in SrFeO3, CaFeO3 and La1/3Sr2/3FeO3 among the perovskite iron oxides is quite important to study, because they show a charge disproportionation (CID) and a maunctic order (MO). The crystal structure of CaFeO3 is of GaFeO3 type [2-5]. CaFeO3 shows a metal-insulator transition at 290 K. At the same temperature the Mossbauer spectrum begins to split gradually into two paramagnetic absorption lines suggesting a CD (2Fe(4+) --> Fe3+ + Fe5+) CD (2Fe(4+) --> Fe3+ + Fe5+ with two different valence states with decreasing temperature in a second-order transition. CaFe03 below 125 K shows an antiferromagnetic long-range order with a helical spin structure. An x-ray photoemission study of CaFeO3 deduces that the Fe4+ and Fe5+ states may consequently be considered as Fe3+ states accompanied by a single and double oxygen holes, d(5)L and d(5)L(2), respectively. The CD may be written as 2d(5)(L) under bar --> d(5) + d(5) (L-2) under bar [6]. La1/3Sr2/3FeO3 in the solid solution system shows metal-insulator transition with charge order and antiferromagnetic order at the same temperature of 207 K with narrow thermal hysteresis, suggesting the occurrence of a first-order phase transition. Such an anomalous valence state as well as the real-space ordering of valence-skipping, sites is confirmed by neutron scattering measurements [7]. The disproportionated charges are condensed within C! the (I 11) plane of the cubic perovskite, From the magnitudes of the respective staggered moments on the Fe sites, estimated from neutron diffraction, the Fe valence is estimated to be +3.4 and +4.2, respectively; these may correspond to the Fe3+ and Fe5+ detected by Mossbauer spectroscopy. Photoemission spectroscopy and unrestricted Hartree-Fock band structure calculation of La1/3Sr2/3FeO3 Suggest that the ordering of holes on the oxygen sites plays an essential role in realizing the CD ground state [8]. In this investigation, we examined the effects of pressure on the CD, the MO and the crystal structure of CaFeO3 and La1/3Sr2/3FeO3 by "Fe Mossbauer spectroscopy. The results obtained are summarized as P-T magnetic phase diagrams of CaFeO3 and La1/3Sr2/3FeO3.