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Size dependence of the pressure-induced gamma to alpha structural phase transition in iron oxide nanocrystals

机译:氧化铁纳米晶体中压力诱导的γ到α结构相变的尺寸依赖性

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The size trend for the pressure-induced gamma-Fe_2O_3 (maghemite) to alpha-Fe_2O_3 (haematite) structural phase transition in nanocrystals has been observed. The transition pressure was found to increase with decreasing nanocrystal size: 7 nm nanocrystals transformed at 27 +- 2 GPa, 5 nm ones at 34 +- 3 GPa and 3 nm ones at 37 +- 2 GPa. Annealing of a bulk sample of gamma-Fe_2O_3 was found to reduce the transition pressure from 35 +- 2 to 24 +- 2 GPa. The bulk modulus was determined as 262 +- 6 GPa for 7 nm nanocrystals of gamma-Fe_2O_3, which is significantly higher than the value of 190 +- 6 GPa that we measured for bulk samples. For alpha-Fe_2O_3, the bulk moduli for 7 nm nanocrystals (336 +- 5) and the bulk (300 +- 30) were found to be almost the same within error. The bulk modulus for the gamma phase was found to decrease with decreasing particle size between 10 and 3.2 nm particle size. Values for the ambient pressure molar volume were found within 1 percent to be: 33.0 cm~3 mol~(-1) for bulk gamma-Fe_2O_3; 32.8 cm~3 mol~(-1) for 7 nm diameter gamma-Fe_2O_3 nanocrystals; 30.7 cm~3 mol~(-1) for bulk alpha-Fe_2O_3; and 30.6 cm~3 mol~(-1) for alpha-Fe_2O_3 nanocrystals.
机译:已经观察到压力诱导的纳米晶体中的γ-Fe_2O_3(赤铁矿)到α-Fe_2O_3(赤铁矿)结构相变的尺寸趋势。发现转变压力随着纳米晶体尺寸的减小而增加:7nm纳米晶体在27±2GPa下转变,5nm在34±3GPa下转变,在37±2GPa下转变为3nm。发现对大量的γ-Fe_2O_3样品进行退火可以将转变压力从35 +-2 GPa降低到24 +-2 GPa。对于γ-Fe_2O_3的7 nm纳米晶体,其体积模量确定为262±6 GPa,这明显高于我们对体积样品测得的190±6 GPa的值。对于α-Fe_2O_3,发现7 nm纳米晶体的体积模量(336 + -5)和体积的模量(300 + -30)在误差范围内几乎相同。发现γ相的体积模量随着粒径在10至3.2nm之间的减小而降低。发现环境压力摩尔体积的值在1%以内:对于本体γ-Fe_2O_3为33.0 cm〜3 mol〜(-1);直径7 nm的γ-Fe_2O_3纳米晶体为32.8 cm〜3 mol〜(-1);对于本体α-Fe_2O_3为30.7 cm〜3 mol〜(-1); α-Fe_2O_3纳米晶体的30.6 cm〜3 mol〜(-1)。

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