La_（0.67）Ca_（0.33）MnO_3/Sr_2FeMoO_6复合材料的制备和物性及Zn_2SnO_4的高压结构研究
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摘要
最近,在具有钙钛矿结构和双钙钛矿结构的氧化物中观测到了庞巨磁电阻效应(CMR)——材料的电阻随磁场的增大而巨大地减小的现象。由于这种效应可以用于磁存储、磁记录等领域,引起了科学家们的极大兴趣。双钙钛矿结构化合物Sr2FeMoO6具有较高的居里温度(420 K),在室温表现出了巨大的CMR效应,而这种CMR效应是由晶粒间的隧穿效应产生的。La0.67Ca0.33MnO3化合物的CMR效应大多发生在室温之下(高的也只250 K),而CMR效应是电子在Mn3+和Mn4+之间转移造成的,可以用双交换作用进行解释。为了得到具有各自优点、性能互补的室温之上较大CMR效应的新材料,我们使用高分子网络溶胶—凝胶技术制备了晶粒度小于50 nm的La0.67Ca0.33MnO3和Sr2FeMoO6细颗粒,并对这两种材料进行了纳米级尺度的复合。
我们对样品进行了X射线衍射实验,发现所有样品中参与复合的相都保持很好的单相性。根据系列样品的SEM形貌图,发现样品的结合性均比较好,通过对应微区的原位X射线能散谱EDX分析,证明了我们采取的复合实验线路的合理性和可行性。此外,对样品进行了扫描和透射电镜观察以及电、磁性质的测量。
目前,越来越多的研究人员对透光率高、电阻率低的金属二元或三元氧化物产生了浓厚的兴趣。这些材料可以统称为透明导体氧化物(TCO)。由于其优异、独特的光、电特性,在光敏玻璃、平板显示器等领域得到了广泛而深入的应用。因为高压可以影响这些材料的结构和性质,因此,对基本物性尚未完全清楚的三元TCO材料——锡酸锌(Zn2Sn04)进行高压下的结构稳定性研究是很有意义的。
常压下,Zn2SnO4(ZTO)所有的衍射峰都可以指标化为面心立方反尖晶石结构,其空间群为Fd3m,晶格常数α=8.61士0.01 A。原位高压同步辐射X射线角散(ADXD)实验中最高压力值为35.1 GPa。在压力范围12.9-26.0 GPa内,发生晶体结构的畸变,形成具有正交结构的中间过渡相。当压力升至32.7 GPa时,发生了高压相变,形成另一种正交结构高压相。当卸压到0.5 GPa时,样品的衍射谱图几乎又恢复到了原反尖晶石相的谱图。
与ADXD的研究结果相同,不同压力下ZTO纳米线的拉曼谱图表明,反尖晶石结构的ZTO纳米线在我们的实验压力范围内发生了可逆的晶体结构相变,在15.5 GPa附近开始形成中间过渡相,在大约32.8 GPa时,出现高压相。
Colossal magnetoresistance (CMR)-a huge decrease in resistance in response to a magnetic field-has recently been observed in materials with perovskite structure and ordered double perovskite structure. This effect gives rise to considerable interests from both fundamental and practical points of view. Sr2FeMoO6 with ordered double perovskite structure exhibits intrinsic tunnelling-type magnetoresistance at room temperature. Its fairly high magnetic transition temperature (Tc) is about 420 K. La0.67Ca0.33MnO3 with a perovskite structure has CMR effect around Tc (the maximum Tc is about 250 K). Its CMR effect can be explained by the Zener's Double Exchange mechanism. In order to get novel materials having CMR effect, we prepared for the series of composites of Lao.67Cao.33Mn03 and Sr2FeMoO6, which have sizes below 50 nm, using polymer-network sol-gel technique.
We performed x-ray diffraction experiments and scanning electron microscope (SEM) observations. Through x-ray diffraction, we have characterized the structures of the composites. We find from SEM observations that the composites have compacted connection among grains. High resolution transmission electron microscopy and in situ SEM energy dispersive x-ray analysis was also been carried out. All the results we obtained prove the feasibility of the complex route. The measurements for magnetic and electrical properties were also taken.
Optically transparent and electrically conducting oxides (TCO) have attracted many scientists due to their unique optical and electronic properties and widespread and successful applications in recent years, such as smart windows, thin-film photovoltaic, flat-panel displays, polymer-based electronics and architectural windows.
The purity of the synthesized Zn2Sn04 (ZTO) nanowires was checked by XRD and all the sharp diffraction peaks can be indexed to a fcc inverse spinel structure with a space group of Fd3m and lattice parameter of a=8.61±0.01 A. We utilized in situ high-pressure ADXD experiments and in situ high-pressure Raman scattering investigation to study the phase transition of ZTO nanowires under high pressure. With increasing pressure, an intermediate phase with an orthorhombic structure appears at 15.5 GPa and a high-pressure phase occurs at about 32.8 GPa. The high-pressure phase of ZTO is considered to possess the ambient pressure structure of CaFe2O4.
我们对样品进行了X射线衍射实验,发现所有样品中参与复合的相都保持很好的单相性。根据系列样品的SEM形貌图,发现样品的结合性均比较好,通过对应微区的原位X射线能散谱EDX分析,证明了我们采取的复合实验线路的合理性和可行性。此外,对样品进行了扫描和透射电镜观察以及电、磁性质的测量。
目前,越来越多的研究人员对透光率高、电阻率低的金属二元或三元氧化物产生了浓厚的兴趣。这些材料可以统称为透明导体氧化物(TCO)。由于其优异、独特的光、电特性,在光敏玻璃、平板显示器等领域得到了广泛而深入的应用。因为高压可以影响这些材料的结构和性质,因此,对基本物性尚未完全清楚的三元TCO材料——锡酸锌(Zn2Sn04)进行高压下的结构稳定性研究是很有意义的。
常压下,Zn2SnO4(ZTO)所有的衍射峰都可以指标化为面心立方反尖晶石结构,其空间群为Fd3m,晶格常数α=8.61士0.01 A。原位高压同步辐射X射线角散(ADXD)实验中最高压力值为35.1 GPa。在压力范围12.9-26.0 GPa内,发生晶体结构的畸变,形成具有正交结构的中间过渡相。当压力升至32.7 GPa时,发生了高压相变,形成另一种正交结构高压相。当卸压到0.5 GPa时,样品的衍射谱图几乎又恢复到了原反尖晶石相的谱图。
与ADXD的研究结果相同,不同压力下ZTO纳米线的拉曼谱图表明,反尖晶石结构的ZTO纳米线在我们的实验压力范围内发生了可逆的晶体结构相变,在15.5 GPa附近开始形成中间过渡相,在大约32.8 GPa时,出现高压相。
Colossal magnetoresistance (CMR)-a huge decrease in resistance in response to a magnetic field-has recently been observed in materials with perovskite structure and ordered double perovskite structure. This effect gives rise to considerable interests from both fundamental and practical points of view. Sr2FeMoO6 with ordered double perovskite structure exhibits intrinsic tunnelling-type magnetoresistance at room temperature. Its fairly high magnetic transition temperature (Tc) is about 420 K. La0.67Ca0.33MnO3 with a perovskite structure has CMR effect around Tc (the maximum Tc is about 250 K). Its CMR effect can be explained by the Zener's Double Exchange mechanism. In order to get novel materials having CMR effect, we prepared for the series of composites of Lao.67Cao.33Mn03 and Sr2FeMoO6, which have sizes below 50 nm, using polymer-network sol-gel technique.
We performed x-ray diffraction experiments and scanning electron microscope (SEM) observations. Through x-ray diffraction, we have characterized the structures of the composites. We find from SEM observations that the composites have compacted connection among grains. High resolution transmission electron microscopy and in situ SEM energy dispersive x-ray analysis was also been carried out. All the results we obtained prove the feasibility of the complex route. The measurements for magnetic and electrical properties were also taken.
Optically transparent and electrically conducting oxides (TCO) have attracted many scientists due to their unique optical and electronic properties and widespread and successful applications in recent years, such as smart windows, thin-film photovoltaic, flat-panel displays, polymer-based electronics and architectural windows.
The purity of the synthesized Zn2Sn04 (ZTO) nanowires was checked by XRD and all the sharp diffraction peaks can be indexed to a fcc inverse spinel structure with a space group of Fd3m and lattice parameter of a=8.61±0.01 A. We utilized in situ high-pressure ADXD experiments and in situ high-pressure Raman scattering investigation to study the phase transition of ZTO nanowires under high pressure. With increasing pressure, an intermediate phase with an orthorhombic structure appears at 15.5 GPa and a high-pressure phase occurs at about 32.8 GPa. The high-pressure phase of ZTO is considered to possess the ambient pressure structure of CaFe2O4.
引文
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[2]戴道生,熊光成,吴思成.RE1-xTxMnO3氧化物的结构,电磁特性和巨磁电阻.物理学进展(Progress In Physics),1997,17(2)：201-222.
[3]Yuan C L, Wang S G, Song W H, et al. Enhanced intergrain tunneling magnetoresistance in double perovskite Sr2FeMoO6 polycrystals with nanometer-scale particles [J]. Appl Phys Lett,1999,75(24):3853-3855.
[4]Kobayashi K L, Kimura T, Sawada H, et al. Room-temperature magnetoresistance in an oxide material with an ordered double-perovskite structure [J]. Nature,1998,395(6703):677-680.
[5]赵旭.双钙钛矿结构化合物Sr2FeMo1-xNbxO6的磁、电性质及高压下结构稳定性的研究：[河北师范大学硕士学位论文].河北省石家庄市：河北师范大学物理科学与信息工程学院,2003,11-23.
[6]Zener C. Interaction between the d-shells in the transition metals.2. ferromagnetic compounds of manganese with perovskite structure [J]. Phys Rev, 1951,82(3):403-405.
[7]Yoshinori T. Colossal Magnetoresistive Oxides. Gordon and breach science publishers,2000.
[8]Anderson P W. Antiferromagnetism-theory of superexchange interaction [J]. Phys Rev,1950,79(2):350-356.
[9]Millis A J, Littlewood P B, Shraiman B I. Double exchange alone does not explain the resistivity of La1-xSrxMnO3 [J]. Phys Rev Lett,1995,74(25): 5144-5147.
[10]Ashcroft N W, Mermin N D. Solid State Physics. New York:Holt, Reinhart and Winston,1976.
[11]Mcguire T R, Potter R I. Anisotropic magnetoresistance in ferromagnetic 3D alloys [J]. IEEE Trans Magn,1975,11(4):1018-1038.
[12]Vanelst H C. The anisotropy in the magneto-resistance of some nickel alloys [J]. Physica,1959,25(8):708-720.
[13]Baibich M N, Broto J M, Fert A, et al. Giant magnetoresistance of (001)Fe/(001)Cr magnetic superlattices [J]. Phys Rev Lett,1988,61(21): 2472-2475.
[14]Parkin S S P, Bhadra R, Roche K P. Oscillatory magnetic exchange coupling through thin copper layers [J]. Phys Rev Lett,1991,66(16):2152-2155.
[15]Pratt W P, Lee S F, Slaughter J M, et al. Perpendicular giant magnetoresistances of Ag/Co multilayers [J]. Phys Rev Lett,1991,66(23):3060-3063.
[16]Berkowitz, A E, Mitchell J R, Carey M J, et al. Giant magnetoresistance in heterogeneous Cu-Co alloys [J]. Phys Rev Lett,1992,68(25):3745-3748.
[17]Chien C L, Xiao J Q, Jiang J S. Giant negative magnetoresistance in granular ferromagnetic systems [J]. J Appl Phys,1993,73(10):5309-5314.
[18]Shapira Y, Foner S, Reed T B. EuO.1. resistivity and hall-effect in fields up to 150 kOe [J]. Phys Rev B,1973,8(5):2299-2315.
[19]Fujimori H, Mitani S, Ohnuma S. Tunnel-type GMR in Co-Al-O insulated granular system-Its oxygen-concentration dependence [J]. J Magn Mater,1996,156(1-3): 311-314.
[20]Coey J M D, Berkowitz A E, Balcells L, et al. Magnetoresistance of chromium dioxide powder compacts [J]. Phys Rev Lett,1998,80(17):3815-3818.
[21]Xiao J Q, Jiang J S, Chien C L. Giant magnetoresistance in nonmultilayer magnetic systems [J]. Phys Rev Lett,1992,68(25):3749-3752.
[22]Fujimori H, Mitani S, Ohnuma S. Tunnel-type GMR in metal-nonmetal granular alloy thin-films [J]. Mater. Sci. Eng. B,1995,31(1-2):219-223.
[23]Hwang H Y, Cheong S W, Ong N P, et al. Spin-polarized intergrain tunneling in La2/3Sr1/3MnO3 [J]. Phys Rev Lett,1996,77(10):2041-2044.
[24]Hwang H Y, Cheong S W. Low-field magnetoresistance in the pyrochlore Tl2Mn207 [J]. Nature,1997,389(6654):942-944.
[25]Sanchez R D, Rivas J, Vazquez Vazquez C, et al. Giant magnetoresistance in fine particle of Lao.67Cao.33Mn03 synthesized at low temperatures [J]. Appl Phys Lett,1996,68(1):134-136.
[26]Wang L S, Zhang X Z, Liao X, et al. A simple method to synthesize single-crystalline Zn2SnO4 (ZTO) nanowires and their photoluminescence properties [J]. Nanotech,2005,16(12):2928-2931.
[27]Segev D, Wei S. Structure-derived electronic and optical properties of transparent conducting oxides [J]. Phys Rev B,2005,71(12):125129(1-11).
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