Cation ordering in magnesioferrite, MgFe2O4 to 982 °C using in situ synchrotron X-ray powder diffraction
Cation ordering in magnesioferrite, MgFe2O4 to 982 °C using in situ synchrotron X-ray powder diffraction
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DOI：
10.2138/am.2005.1559
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发表时间：
2005-01-01
影响因子：
3.1
通讯作者：
Parise, JB
中科院分区：
文献类型：
--
作者：
Antao, SM;Hassan, I;Parise, JB
镁铁氧体尖晶石 MgFe2O4 是由等摩尔量的试剂级氧化物 MgO 和 Fe2O3 在 900 摄氏度下合成的，并在空气中淬火。镁铁氧体的结构行为由原位同步加速器 X 射线粉末衍射数据 [lambda = 0.92225(4) 埃] 在室温和 28 至 982 摄氏度的加热和冷却温度下确定。 a 晶胞参数随加热呈线性增加，但在 581°C 时出现偏差并出现不连续性。在 581 摄氏度以上以及从 982 摄氏度冷却时，a 参数呈线性变化。加热前 [8.39704(5) Angstrom] 和冷却至 47℃ [8.39514(4) Angstrom] 后 28°C 的 a 参数不同，因为结构中冻结的阳离子顺序不同。根据反转参数 x、{(iv)[Mg1-xFex](vi)[Mgx/2Fe1-x/2](2)O-4} 和有序参数 Q = 1-(3/2) x 分析的阳离子序数显示，加热时没有变化，直到温度足够高以引起八面体和四面体位点之间 Mg2+ 和 Fe3+ 阳离子的交换。这种激活势垒在 581 摄氏度时被克服，此时样品在加热时达到最大有序状态 [x(max) = 0.867(4)] 并开始趋向平衡。这种弛豫是朝着更有序的配置发展，并且是一个动力学控制的过程。在 581 'C 以上，阳离子沿着平衡路径连续无序到达所研究的最高温度 [T-max = 982 摄氏度，x = 0.769(3)]，并在冷却时沿着平衡路径反转。在 T-B 处，最大平衡级被冻结并维持在室温，其中 X-max = 0.895(4)。 O'Neill-Navrotsky、Landau 和 Ginzburg-Landau 模型很好地描述了 MgFe2O4 的有序过程。使用 Netzsch STA 449C 同步 TG-DSC 仪器获得同步差示扫描量热法 (DSC) 和热重分析 (TG) 数据。 MgFe3O4 的 DSC 曲线在第一次加热实验中在约 550°C = T-relax 处包含不可逆放热峰，与该峰相关的能量变化为 -162 J/g ( = -32 KJ/mol)，对应于阳离子弛豫。根据 Rietveld 的改进，T-relax 大约为 581°C。近似360℃的T-Curic是通过在磁场中进行的TG实验获得的。
Magnesioferrite spinel, MgFe2O4, was synthesized at 900degreesC from equimolar amounts of reagent-grade oxides, MgO and Fe2O3, and quenched in air. The structural behavior of magnesioferrite was determined from in situ synchrotron X-ray powder-diffraction data [lambda = 0.92225(4) Angstrom] at room pressure and temperatures from 28 to 982degreesC on heating and cooling. The a unit-cell parameter increases linearly on heating, but deviates to give a discontinuity at 581degreesC. Above 581degreesC and on cooling from 982degreesC, the a parameter varies linearly. The a parameter at 28degreesC before heating [8.39704(5) Angstrom] and after cooling to 47degreesC [8.39514(4) Angstrom] is different because the cation order frozen in the structure is not the same. Cation order, analyzed in terms of the inversion parameter, x, {(iv)[Mg1-xFex](vi)[Mgx/2Fe1-x/2](2)O-4}, and the order parameter, Q = 1-(3/2) x, show no change on heating until the temperature is high enough to cause exchange of Mg2+ and Fe3+ cations between the octahedral and tetrahedral sites. This activation barrier is overcome at 581degreesC, where the sample achieves the maximum ordered state on heating [x(max) = 0.867(4)] and begins to move toward equilibrium. This relaxation is toward a more ordered configuration and is a kinetically controlled process. Above 581 'C, the cations continuously disorder along the equilibrium pathway to the maximum temperature studied [T-max = 982degreesC, x = 0.769(3)] and reverse along the equilibrium pathway on cooling. At T-B, the maximum equilibrium order is frozen in, and maintained to room temperature, where X-max = 0.895(4). O'Neill-Navrotsky, Landau, and Ginzburg-Landau models give good descriptions of the ordering process in MgFe2O4. Simultaneous differential scanning calorimetry (DSC) and thermogravimetry (TG) data were obtained using a Netzsch STA 449C simultaneous TG-DSC instrument. The DSC curve for MgFe3O4 contains an irreversible exothermic peak at about 550degreesC = T-relax in the first heating experiment, and the energy change associated with this peak is -162 J/g ( = -32 KJ/mol), and corresponds to cation relaxation. From Rietveld refinements, T-relax approximate to 581degreesC. The T-Curic approximate to 360degreesC was obtained from TG experiments carried out in a magnetic field.