| Sumario: | The effects of reaction temperature and pO were investigated on a series of (Ba,Ca,Nd)FeO perovskite systems in order to isolate phases containing ordered arrangements of the distinct vacancy and cation ordering patterns identified in less compositionally complex iron oxide systems. Initial synthesis in air at high temperature yields cubic perovskite phases (I) with average iron oxidation states higher than 3; selected area electron diffraction together with diffuse features observed in the synchrotron X-ray diffraction (SXRD) patterns of these materials show evidence of small domains of short-range cation and vacancy order. Annealing these materials in nitrogen or in a sealed tube in the presence of an NiO/Ni buffer yielded the Fe phase CaBaNdFeO (II), closely related to SrLaFeO but with partial cation order as well as anion order present-the larger Ba cations are largely present in the 12-coordinate site between the octahedral iron layers, and Ca is largely present in 10-coordinate sites between octahedral and tetrahedral sites. Further reduction of CaBaNdFeO using a Zr getter yields the mixed-valence phase CaBaNdFeO (III). The structure of III was solved by maximum entropy analysis of XRD data coupled with analysis of high-temperature neutron diffraction data and refined against combined SXRD and high-Q ambient-temperature neutron data. This material crystallizes in a 20-fold perovskite super cell (Imma, a ~ √2×a, b ~ 10×a, c ~ √×2a) and can be visualized as an intergrowth between brownmillerite (CaFeO) and the YBaFeO structure. There are three distinct iron coordination environments, octahedral (O), square-pyramidal (Sp), and trigonal planar (Tp, formed by distorting the tetrahedral site in brownmillerite), which form a Sp-O-Tp-O-Sp repeat. Bond valence calculations indicate that Tp is an Fe site, while the O and Sp sites are Fe. The A-site cations are also partially ordered over three distinct sites: 8-coordinate between the Sp layers, 10-coordinate between Tp and O layers, and 12-coordinate between Sp and O layers. Mössbauer spectroscopy, magnetometry, and variable-temperature neutron diffraction show that the material undergoes two magnetic transitions at ~700 and 255 K.
|
|---|