Novel SrTi1-xFexO3 nanocubes synthesized by microwave-assisted hydrothermal method

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Novel SrTi1-xFexO3 nanocubes synthesized by microwave-assisted hydrothermal method

We report herein for the first time a facile synthesis method to obtain SrTi1-xFexO3 nanocubes by means by a microwave-assisted hydrothermal (MAH) method at 140 degrees C. The effect of iron addition on the structural and morphological properties of SrTiO3 was investigated. X-ray diffraction measurements show that all STFO samples present a cubic perovskite structure. X-ray absorption spectroscopy at Fe absorption K-edge measurements revealed that iron ions are in a mixed Fe2+/Fe3+ oxidation state and preferentially occupy the Ti4+-site. UV-visible spectra reveal a reduction in the optical gap (E-gap) of STFO samples as the amount of iron is increased. An analysis of the data obtained by field emission scanning electron microscopy points out that the nanoparticles present a cubic morphology independently of iron content. According to high-resolution transmission electron microscopy results, these nanocubes are formed by a self-assembly process of small primary nanocrystals.

Insight into the Effects of Fe Addition on the Local Structure and Electronic Properties of SrTiO3

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Ozone and nitrogen dioxide gas sensor based on a nanostructured SrTi0.85Fe0.15O3 thin film

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Low-temperature electrical conductivity of LaNi1-xFexO3

We report the electrical conductivity between 2 and 300 K for LaNi1-xFexO3 across the composition-controlled metal-insulator (m-i) transition. Using a method first suggested by Mobius, we identify the critical concentration x(c) to be 0.3 for the m-i transition. The negative temperature coefficient of resistivity observed at low temperatures in the metallic phase follows a temperature dependence characteristic of disorder effects. The semiconducting compositions (x greater than or equal to 0.3) do not show a simple activation energy but exhibit variable-range hopping at high temperatures confirming that the m-i transition in this system is driven by increasing disorder effects.

Studies on BiCoO3 and BiCo1−xFexO3

BiCoO3 and BiCo1−xFexO3 (x ≤ 0.7) prepared at atmospheric pressure are found to possess a cubic structure probably involving the Bi12O36 network. BiCoO3 is paramagnetic above 100 K, with a negative paramagnetic Curie temperature. BiCo1−xFexO3 (x = 0.5 or 0.7) is ferromagnetic with Tc ≈ 600K.

Influence of Fe3+ on perovskite oxides: La2/3Ca1/3Mn1-xFexO3

The structure and magnetoresistance properties in sintered samples of La-2/3 Ca-1/3 Mn1-x FexO3 (0 less than or equal to x less than or equal to 0.84) are studied by using Mossbauer spectroscopy, XRD and magnetic measurement. There are antiferromagnetic interactions between Fe and its nearest neighbors (Fe, Mn) when 0 less than or equal to x less than or equal to 0.67, which are important factors influencing the double-exchange between Mn3+ and Mn4+, Curie temperature, magnetic moment and GMR. It is suggested that the Mn3+(Fe3+)/Mn4+ system also consists of magnetic clusters with different sizes.

X-Ray Photoelectron Spectroscopy Study of LaMn1-xFexO3 (x=0-1) Oxides

The redox potential, surface composition and oxygen species of a series of complex oxides LaMn1-xFexO3 (x=0-1) having perovskite structure (ABO(3)) have been investigated by means of XI'S. The variation of binding energies referring to Mn2p and Fe 2p under different treatment offerred an obvious evidence of redox between Mn and Fe, which could be expressed as Mn4+ + Fe(3-delta)+ Mn(4-delta)+ Fe3+ Feat Through computer fit three kinds of adsorbed oxygen species (O-I, O-II, O-III) have been evaluated based on the XPS spectra of O1s. From the variation of contents of different oxygen species, it could be concluded that. the redox occuring in the surface might be related with the adsorbed oxygen species O-I and O-II, furthermore the possibility of transfer of electron between adsorption site and oxygen was also discussed.

Effect of Fe-doping on the structural, microstructural, optical, and ferroeletric properties of Pb1/2Sr1/2Ti1-xFexO3 oxide prepared by spin coating technique

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Reduction behaviour of Fe3+/Al2O3 obtained from the mixed oxalate precursor and the formation of the Fe0-Al2O3 metal-ceramic composite

Reduction behaviour of Fe3+/Al2O3 obtained by the decomposition of the oxalate precursor has been investigated by employing X-ray diffraction (XRD), Mössbauer spectroscopy and electron paramagnetic resonance (EPR) spectroscopy. Calcination of Fe3+/Al2O3 at or below 1070 K yields mainly a poorly ordered, fine particulate form of ?-Al2�xFexO3. Calcination at or above 1220 K yields ?-Al2�xFexO3. Reduction of Fe3+/Al2O3 samples calcined at or below 1070 K gives the FeAl2O4 spinel on reduction at 870 K; samples calcined at or above 1220 K give Al2-xFexO3 with a very small proportion of metallic iron. Fe3+/Al2O3 samples calcined at 1220 K or above yield metallic iron and a very small proportion of the spinel on reduction below 1270 K. In the samples reduced at or above 1270 K, the main product is metallic iron in both ferromagnetic and superparamagnetic forms. The oxalate precursor route yields more metallic iron than the sol�gel route.

Investigations of the reduction behavior of Iron-impregnated alumina gels (Fe/AlOOH) and the formation of Fe0-Al2O3 metal-ceramic composites

Fe/AlOOH gels calcined and reduced at different temperatures have been investigated by a combined use of Mossbauer spectroscopy, x-ray diffraction, and electron microscopy in order to obtain information on the nature of the iron species formed as well as the various reduction processes. Calcination at or below 1070 K mainly gives reducible Fe3+ while calcination at higher temperatures gives substitutional Fe3+ in the form of Al2-xFexO3. The Fe3+ species in the calcined samples are, by and large, present in the form of small superparamagnetic particles. Crystallization of Al2O3 from the gels is catalyzed by Fe2O3 as well as FeAl2O4. Fe (20 wt. %)/AlOOH gels calcined at or below 870 K give FeAl2O4 when reduced in hydrogen at 1070 K or lower and a ferromagnetic Fe0-Al2O3 composite (with the metallic Fe particles >100 angstrom) when reduced at 1270 K. Samples calcined at 1220 K or higher give the Fe0-Al2O3 composite when reduced in the 870-12,70 K range, but a substantial proportion of Fe3+ remains unreduced in the form of Al2-xFexO3, showing thereby the extraordinary stability of substitutional Fe3+ to reduction even at high temperatures. Besides the ferromagnetic Fe0-Al2O3 composite, high-temperature reduction of Al2-xFexO3 yields a small proportion of superparamagnetic Fe0-Al2O3 wherein small metallic particles (<100 angstrom) are embedded in the ceramic matrix. In order to preferentially obtain the Fe0-Al2O3 composite on reduction, Fe/AlOOH gels should be calcined at low temperatures (less-than-or-equal-to 1100 K); high-temperature calcination results in Al2-xFexO3. Several modes of formation of FeAl2O4 are found possible during reduction of the gels, but a novel one is that involving the reaction, 2Fe3+ + Fe0 --> 3Fe2+.

Role of interface states associated with transitional nanophase precipitates in the photoluminescence enhancement of SrTiO3 : Pr3+,Al3+

SrTiO3:Pr3+,Al3+ phosphor samples with varying ratios of Sr/Ti/Al were prepared by the gel-carbonate method and the mechanism of enhancement of the red photoluminescence intensity therein was investigated. The photoluminescence (PL) spectra of SrTiO3:Pr3+ show both D-1(2) --> H-3(4) and P-3(0) --> H-3(4) emission in the red and blue spectral regions, respectively, with comparable intensity. The emission intensity of D-1(2) --> H-3(4) is drastically enhanced by the incorporation of Al3+ and excess Ti4+ in the compositional range Sr(Ti,Al-y)(O3+3y/2):Pr3+ (0.2 less than or equal to y less than or equal to 0.4) and SrTi1+xAlyO3+z:Pr3+ (0.2 less than or equal to x less than or equal to 0.5; 0.05 less than or equal to y less than or equal to 0.1; z = 2x + 3y/2) with the complete disappearance of the blue band. This cannot be explained by the simple point defect model as the EPR studies do not show any evidence for the presence of electron or hole centers. TEM investigations show the presence of exsolved nanophases of SrAl12O19 and/or TiO2 in the grain boundary region as well as grain interiors as lamellae which, in turn, form the solid-state defects, namely, dislocation networks, stacking faults and crystallographic shear planes whereby the framework of corner shared TiO6 octehedra changes over to edge-sharing TiO5-AlO5 strands as indicated from the Al-27 MAS NMR studies. The presence of transitional nanophases and the associated defects modify the excitation-emission processes by way of formation of electronic sub-levels at 3.40 and 4.43 eV, leading to magnetic-dipole related red emission with enhanced intensity. This is evidenced by the fact that SrAl12O19:Pr3+,Ti4+ shows bright red emission whereas SrAl12O19:Pr3+ does not show red photoluminescence.

Magnetization in electron- and Mn- doped SrTiO3

Mn- doped SrTiO3.0, when synthesized free of impurities, is a paramagnetic insulator with interesting dielectric properties. Since delocalized charge carriers are known to promote ferromagnetism in a large number of systems via diverse mechanisms, we have looked for the possibility of any intrinsic, spontaneous magnetization by simultaneous doping of Mn ions and electrons into SrTiO3 via oxygen vacancies, thereby forming SrTi1-xMnxO3-delta, to the extent of making the doped system metallic. We find an absence of any enhancement of the magnetization in the metallic sample when compared with a similarly prepared Mn doped, however, insulating sample. Our results, thus, are not in agreement with a recent observation of a weak ferromagnetism in metallic Mn doped SrTiO3 system.

稀土锰氧化物和双钙钛矿氧化物的合成及电磁性能研究

本文对钙钛矿锰氧化物La0.67Sr0.33MnO3、La0.67Sr0.33Mn1-xFexO3和双钙钛矿氧化物Sr2－xLaxMnMoO6、Sr2－xLaxMnWO6和Sr2(Ni，Co)WO6的结构、磁性和磁电阻效应进行了系统地研究。 利用溶胶-凝胶方法制得La0.67Sr0.33MnO3粉体，通过不同的烧结温度，可以对样品的微观结构进行调控。其中，晶界的状态对自旋电子的输运产生了明显的影响。900℃和1300℃烧结的样品，都出现了远低于居里温度的金属－半导体导电行为转变，表明自旋电子的传输不仅与晶粒的大小有关，而且与晶界状态密切联系。合适的烧结条件，可以得到大的，使用温区较宽的磁电阻效应。 在La0.67Sr0.33Mn1-xFexO3(0≤x≤0.2)体系中，Fe3＋的掺入没有明显改变La0.67Sr0.33MnO3的结构和晶胞参数，但却极大地降低了样品的居里温度。在x≥0.08时同样出现了低于居里温度的金属－半导体导电行为转变，这是由体系的铁磁态磁不均匀性导致的。电子顺磁共振研究表明，La0.67Sr0.33Mn1-xFexO3(0≤x≤0.20)的EPR谱中峰对峰宽（ΔHpp）在Tmin前后两种温区的线宽展宽机制不同。随着掺杂量的增加，在Tmin处的线宽逐渐变宽并且出现低场顺磁信号。x=0.20的样品，低场顺磁信号在380K左右开始衰减，与La0.67Sr0.33MnO3的顺磁信号具有类似的行为。我们利用相分离模型对系列样品的电子顺磁共振信号的变化规律进行了解释。 在Sr2－xLaxMnMoO6 (0≤x≤1)系列样品中，所有的样品都具有双钙钛矿有序结构。磁化强度、居里温度随掺杂量的增加而增大，但都存在磁受挫行为。在5T的外场下磁化强度依然没有饱和，实际测得的磁矩远远小于理论预测的结果。在低掺杂状态下出现分步磁化。La3+的掺入，不仅改变了体系电子能带的状况，而且极大的改变了体系的结构因素，两者竞争的结果，使体系的电磁性能产生了复杂的变化。

复合氧化物LaMn_(1-x)Fe_xO_3(x=0-1)的XPS研究

用XPS方法研究了具有ABO3结构的LaMn1-xFexO3（x＝0－1）氧化物的氧化还原性能、表面组成和吸附氧种．样品经还原和再氧化处理后，Mn2p和Fe2p结合能的变化对Fe和Mn之间发生的氧化还原提供了明显的证据．可表示如下：Me4＋＋Fe（3－δ）→Mn（4－δ）＋＋Fe3＋通过计算机用三种氧物种（OⅠ，OⅡ和OⅢ）对O1s峰进行拟合，确定了每种氧物种的状态．同时，以氧物种含量随还原、再氧化的变化，确定了发生在表面上的氧化还原反应同OⅠ和OⅡ吸附氧物种有关．在此基础上，对吸附位与氧之间的电子转移过程进行了讨论.