Topological Reduction Of Perovskite-like Oxides

Perovskite materials application areas cover ferroelectric,paraelectric,piezoelectric,antiferroelectric,insulation,semiconductors,negative thermal expansion and other fields.Based on their excellent performance,we performed to the low-temperature topochemical manipulation to perovskite-related complex oxides by reductive anion deintercalation,which offers many opportunities for the rational synthesis of metastable materials with novel structure and properties.In this study,PbTiO₃ with negative thermal expansion（NTE）property and mixed conducting LnBaCo₂O5+δ（Ln=Eu,Tb）with adjustable oxygen vacancy were used as the parent material for topological reduction.The main results are described as the following.1)PbTiO₃ was synthesized as parent phase by solid state reaction method and was reduced by topological method.The oxygen content of the reduced product was 2.825（1）per formula calculated by thermogravimetric analysis.The NTE property for the parent phase and the reduced product were studied by the method of variable-temperature diffraction.The results show that the Curie temperature of the reduced product is about 50℃lower,and the thermal expansion coefficient is higher than those for the parent compound.Under 400℃the conductivtity of the reduced product is 1-2 orders of magnitude higher than that of the parent compound,but with the temperature increase,their electrical conductivities tend to be close.2)LnBaCo₂O5+δ（Ln=Eu,Tb）were synthesized by solid state reaction method and was reduced by the topological reduction method,which resulted in formation ofnew phases on the reduced products.The O content of the parent compound and the reduced product was 5.5and 4.5,which are determined by thermogravimetry analyses.The X-ray diffraction patterns for the reduced product EuBaCo₂O_4.5.5 and TbBaCo₂O_4.5 were indexed using monoclinic unit cells with spacegroup P12/m1(EuBaCo₂O_4.5:a=15.3701（2）?,b=8.2301（1）?,c=3.7843（2）?,β=89.1943（3）°;TbBaCo₂O_4.5:a=15.235（2）?,b=8.3165（4）?,c=3.6985（1）?,β=89.976（3）°).Magnetic hysteresis loop at room temperature reveals ferromagnetic behaivor for the reduced products.The magnetic moment of EuBaCo₂O_4.5 is 6.11μ_B which fitted by Curie-Weiss law.The X-ray photoelectron spectroscopy（XPS）measurement confirmed the valence increase of cobalt in the reduced productrs,which could be responsible for the magnetic behvior.