Nominal compositions Nd_(6)MoO_(12)(3:1)and Nd_(10)Mo_(2)O_(21)(5:2)were prepared by high-temperature synthesis in air from a mechanically activated mixture of Nd_(2)O_(3)and MoO_(3).Phase-pure Nd_(6)MoO_(12-δ)and Nd...Nominal compositions Nd_(6)MoO_(12)(3:1)and Nd_(10)Mo_(2)O_(21)(5:2)were prepared by high-temperature synthesis in air from a mechanically activated mixture of Nd_(2)O_(3)and MoO_(3).Phase-pure Nd_(6)MoO_(12-δ)and Nd_(6)MoO_(12)-αhave been obtained at 1600 and 1650℃,respectively.They are both slightly rhombohedrally distorted derivatives from the cubic fluorite structure.Nd_(10)Mo_(2)O_(21) with a lower content of Nd_(2)O_(3)was shown to be a more complex phase based on the rhombohedral phase(R3)in the 1600-1650℃temperature range.As a result of the formation of a more complex rhombohedral phase,the conductivity of Nd_(10)Mo_(2)O_(21) changes dramatically in comparison with Nd_(6)MoO_(12-δ).In wet air Nd_(6)MoO_(12-δ)is a p-type electronic conductor,whereas proton conductivity dominates in Nd_(10)Mo_(2)O_(21) over the entire temperature range studied.The electrical conductivity dependence of Nd_(6)MoO_(12-δ)on the oxygen partial pressure shows a V-type behaviour typical of a transition from a p-type to n-type conductivity mechanism at 800≤T≤1000℃.There is no p-type conductivity contribution in Nd_(10)Mo_(2)O_(21) in the same temperature range.The prevalence of electronic conductivity in the samples with nominal composition Nd_(6)MoO_(12)in a wide temperature range is due to the fact that Nd and Mo in the fluorite materials are readily reduced.Predominantly Nd^(3+)and Mo6+forms exist in more complex rhombohedral phase Nd_(10)Mo_(2)O_(21) and it has proton conductivity~8.5×10^(-3) S cm^(-1) at 800℃.Thus,the loss of dimensional stability is more characteristic of fluorites and rhombohedral phases with small rhombohedral distortion(Nd_(5.4)Zr0._(6)MoO_(12).3,Nd_(6)MoO_(12-δ))than more complex rhombohedral phases based on(R3)(Nd_(10)Mo_(2)O_(21)).A comparative high-temperature in situ neutron diffraction study under high vacuum of rhombohedral Nd_(6)MoO_(12-δ)and cubic fluorite Nd_(5.4)Zr0._(6)MoO_(12).3 showed that the former transforms to the high-temperature cubic fluorite type above~1140℃while the latter retains its cubic structure in the studied range up to 1350℃.展开更多
基金supported by the Russian Foundation for Basic Research(grants no.16-03-00143,19-03-00358)supported into frameworks of the state task for the ICP RAS no.0082-2014-0011State registration number AAAA-A17-111711600093-8。
文摘Nominal compositions Nd_(6)MoO_(12)(3:1)and Nd_(10)Mo_(2)O_(21)(5:2)were prepared by high-temperature synthesis in air from a mechanically activated mixture of Nd_(2)O_(3)and MoO_(3).Phase-pure Nd_(6)MoO_(12-δ)and Nd_(6)MoO_(12)-αhave been obtained at 1600 and 1650℃,respectively.They are both slightly rhombohedrally distorted derivatives from the cubic fluorite structure.Nd_(10)Mo_(2)O_(21) with a lower content of Nd_(2)O_(3)was shown to be a more complex phase based on the rhombohedral phase(R3)in the 1600-1650℃temperature range.As a result of the formation of a more complex rhombohedral phase,the conductivity of Nd_(10)Mo_(2)O_(21) changes dramatically in comparison with Nd_(6)MoO_(12-δ).In wet air Nd_(6)MoO_(12-δ)is a p-type electronic conductor,whereas proton conductivity dominates in Nd_(10)Mo_(2)O_(21) over the entire temperature range studied.The electrical conductivity dependence of Nd_(6)MoO_(12-δ)on the oxygen partial pressure shows a V-type behaviour typical of a transition from a p-type to n-type conductivity mechanism at 800≤T≤1000℃.There is no p-type conductivity contribution in Nd_(10)Mo_(2)O_(21) in the same temperature range.The prevalence of electronic conductivity in the samples with nominal composition Nd_(6)MoO_(12)in a wide temperature range is due to the fact that Nd and Mo in the fluorite materials are readily reduced.Predominantly Nd^(3+)and Mo6+forms exist in more complex rhombohedral phase Nd_(10)Mo_(2)O_(21) and it has proton conductivity~8.5×10^(-3) S cm^(-1) at 800℃.Thus,the loss of dimensional stability is more characteristic of fluorites and rhombohedral phases with small rhombohedral distortion(Nd_(5.4)Zr0._(6)MoO_(12).3,Nd_(6)MoO_(12-δ))than more complex rhombohedral phases based on(R3)(Nd_(10)Mo_(2)O_(21)).A comparative high-temperature in situ neutron diffraction study under high vacuum of rhombohedral Nd_(6)MoO_(12-δ)and cubic fluorite Nd_(5.4)Zr0._(6)MoO_(12).3 showed that the former transforms to the high-temperature cubic fluorite type above~1140℃while the latter retains its cubic structure in the studied range up to 1350℃.
