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1.
We investigated the upconversion (UC) emissions and their ambient dependences of SrTiO 3 polycrystals co-doped in Er 3+ and Yb 3+ at different substitution sites. i.e., the A-site and B-sites in ABO 3-type perovskite, and its response to H 2 and O 2 ambient annealing. Under near-infrared excitation at 980 nm, the as-synthesized samples exhibited strong UC emission features in the green (525 and 550 nm) and red (660 nm) region from Er 3+ ions owing to sensitization by Yb 3+; the emission was much stronger for A-site doping than for B-site doping. Interestingly, annealing in the H 2 atmosphere to increase the oxygen vacancies suppressed the photoluminescence and UC emission of the A-site doped samples, but enhanced the emission signals of the B-site doped samples. After subsequent annealing in the O 2 atmosphere to decrease the oxygen vacancies, the emission intensities showed a tendency to return to those in the as-synthesized A-site doped and B-site doped samples. These intriguing behaviors were explained in terms of the relationship between the substitution site and charge compensation. We performed the temperature dependent UC emissions and found that the intensity ratio between two green emissions changed significantly with temperature. This strong fluorescence intensity ratio could be used for optical thermometry. 相似文献
2.
The local chemical environment of the trivalent lanthanide cations in Nb 2O 5 nanopowders doped with 1 mol% of Eu 3+ and Er 3+, prepared via a Pechini approach, has been studied by means of EXAFS at the Ln-K edge. It can be demonstrated that the lanthanide ions enter the Nb 2O 5 structure as substitutional defects with respect to Nb, giving rise to a very large amount of disorder: both Eu 3+ and Er 3+ ions substitute Nb in the nine-fold coordinated site, with clustering of oxygen vacancies around the substitutional defects. Valence bond calculations have been used to validate the Ln-O distances obtained by the EXAFS fitting. The Er 3+-doped nanocrystalline Nb 2O 5 sample shows efficient luminescence in the near infrared region around 1.5 μm. The emission and excitation spectra are affected by significant inhomogeneous broadening, in agreement with the presence of strong disorder around the dopant ions in nanosized Nb 2O 5. 相似文献
3.
Thin film of both A- and B-site co-substituted Sr 0.8Bi 2.2Ta 2O 9 (SBT) by Pr 3+ and Nb 5+, i.e. Sr 0.8Pr 0.1Bi 2.1Ta 1.5Nb 0.5O 9 (SPBTN) was fabricated on Pt/Ti/SiO 2/Si substrates by metalorganic decomposition method. The Nb 5+ substitution at B-site and Pr 3+ substitution at A-site enhanced the remanent polarization and reduced the coercive field of the films, respectively. The remanent polarization (2 Pr) value of the SPBTN film was 22 μC/cm 2. The coercive field (2 Ec) value of the SPBTN film was 102 kV/cm, which was much lower than that of SBTN (165 kV/cm). The effects of substitution on structural and ferroelectric properties of SBT were discussed in detail. As a result, the A- and B-sites co-substitution may be one of the promising ways to improve ferroelectric properties of SBT. 相似文献
4.
The A-site substituted BaTiO 3 ceramics were prepared by solid-state reaction via partial substitution of Fe for Ba 2+. By comparison with the B-site substituted sample made under similar conditions, the effect of Fe doping site on microstructure and magnetism was investigated using X-ray diffraction, Mössbauer spectroscopy and vibrating sample magnetometer. It is found that A-site substitution can be realized to a certain extent at 7 at% Fe addition, whereas impurities are observed at higher Fe concentrations. In the nominal (Ba 0.93Fe 0.07)TiO 3 sample, the Fe ions are present as Fe 2+ and Fe 3+, respectively, replacing A-site Ba 2+ and octahedral B-site Ti 4+ in hexagonal perovskite lattice. The double-exchange Fe 2+-O 2−-Fe 3+ interactions produce ferromagnetism well above room temperature, but the saturation magnetization and the Curie temperature are both obviously lower than those for B-site substitution due to different magnetic exchange mechanisms. In the B-site substituted sample Ba(Ti 0.93Fe 0.07)O 3, the super-exchange interactions between Fe 3+ on pentahedral and octahedral Ti 4+ sites are responsible for ferromagnetism. These results mean that B-site substitution is a better way for Fe-doped BaTiO 3 system to obtain high-Curie-temperature ferromagnetism. Moreover, increasing pre-sintering time can further improve the magnetism of B-site substituted samples, through which the saturation magnetization for Ba(Ti 0.93Fe 0.07)O 3 is enhanced ∼6 times. 相似文献
5.
The structural and magnetic properties of the mixed spinel Mg 1+xMn xFe 2-2xO 4 system for 0.1<= x <= 0.9 have been studied by means of X-ray diffraction, magnetization, a.c. susceptibility and Mössbauer spectroscopy measurements. X-ray intensity calculations indicate that Mn 4+ ions occupy only octahedral (B) sites replacing Fe 3+ ions and the added Mg 2+ ions substitute for A-site Fe 3+ ions. All samples are magnetic at 12 K displaying Mössbauer spectra that have magnetic sextets coexisting with a central doublet that increases in population with increasing Mn concentration, indicating the presence of short range ordering (clustering). The Mössbauer intensity data show that Mn possesses a preference for the B-site of the spinel over the whole range of concentration. As expected, the hyperfine field and Curie temperature determined from a.c.susceptibility data decrease with increasing Mn content. Magnetization results indicate that on increasing dilution x, the collinear ferrimagnetic phase breaks down at x = 0.3 before reaching the ferrimagnetic percolation limit (x=0.6), as a result of the presence of competing exchange interactions, which is well supported by Mössbauer results. From all the above results, it is proposed that with increasing Mn content from x=0.6 to 0.9, the frustration and disorder increase in the system suppressing the ferrimagnetic ordering, and the system approaches to a cluster spin glass type of ordering at x=0.8 as reflected in the a.c.susceptibility and Mössbauer spectrum. 相似文献
6.
We studied by Mössbauer spectroscopy the Na 0.82CoO 2 compound using 1% 57Fe as a local probe which substitutes for the Co ions. Mössbauer spectra at T=300 K revealed two sites which correspond to Fe 3+ and Fe 4+. The existence of two distinct values of the quadrupole splitting instead of a continuous distribution should be related with the charge ordering of Co +3, Co +4 ions and ion ordering of Na(1) and Na(2). Below T=10 K part of the spectrum area, corresponding to Fe 4+ and all of Fe 3+, displays broad magnetically split spectra arising either from short-range magnetic correlations or from slow electronic spin relaxation. 相似文献
7.
A single-crystal TlGaSe 2 doped by paramagnetic Fe ions has been studied at room temperature by electron paramagnetic resonance (EPR) technique. The fine structure of EPR spectra of paramagnetic Fe 3+ ions was observed. The spectra were interpreted to correspond to the transitions among spin multiplet ( S=5/2, L=0) of Fe 3+ ion, which are splitted by the local ligand crystal field (CF) of orthorhombic symmetry. Four equivalent Fe 3+ centers have been observed in the EPR spectra and the local symmetry of crystal field at the Fe 3+ site and CF parameters were determined. Experimental results indicate that the Fe ions substitute Ga at the center of GaSe 4 tetrahedrons, and the rhombic distortion of the CF is caused by the Tl ions located in the trigonal cavities between the tetrahedral complexes. 相似文献
8.
The solid solution between the antiferroelectric PbZrO 3 (PZ) and relaxor ferroelectric Pb(Co 1/3Nb 2/3)O 3 (PCoN) was synthesized by the columbite method. The phase structure and thermal properties of (1− x)PZ– xPCoN, where x=0.0–0.3, were investigated. With these data, the ferroelectric phase diagram between PZ and PCoN has been established. The
crystal structure data obtained from XRD indicates that the solid solution PZ–PCoN, where x=0.0–0.3, successively transforms from orthorhombic to rhombohedral symmetry with an increase in PCoN concentration. The AFE→FE
phase transition was found in the compositions of 0.0≤ x≤0.10. The AFE→FE phase transition shift to lower temperatures with higher compositions of x. The width of the temperature range of FE phase was increased with increasing amount of PCoN. It is apparent that the replacement
of the Zr 4+ ion by (Co 1/3Nb 2/3) 4+ ions would decrease the driving force for antiparallel shift of Pb 2+ ions, because they interrupt the translational symmetry. This interruption caused the appearance of a rhombohedral ferroelectric
phase when the amount of PCoN was more than 10 mol%. 相似文献
9.
On the basis of the 120×120 complete energy matrices for a d 3 configuration ion in a trigonal ligand field, for Cr 3+ ions doped in MgTiO 3 and LiTaO 3, the local structures and EPR g factors of the octahedral (CrO 6) 9− clusters have been studied, respectively. By simulating the calculated optical spectra and the EPR spectra data to the experimental results, local structure parameters are obtained. The calculated results show that although the local lattice structures around the M (M=Mg 2+, Ta 5+) ions are obviously different, after Cr 3+ replacing the M, the local lattice structures around the Cr 3+ ions are quite similar and close to those of the Cr 2O 3. This may be ascribed to the fact that the octahedral Cr 3+ center in MgTiO 3:Cr 3+ and LiTaO 3:Cr 3+ systems and that in Cr 2O 3 exhibit similar octahedral (CrO 6) 9− clusters. Moreover, the corresponding theoretical values of the optical spectra have been reported. It is also found that the orbital reduction factor k is very important to understand the EPR g factors for Cr 3+ ions doped in MgTiO 3 and LiTaO 3. 相似文献
10.
The effects of A-site cation size disorder in ABO 3 type charge-ordered and antiferromagnetic Pr 0.5Ca 0.5MnO 3 system have been studied by substituting La 3+, Sr 2+ or Ba 2+, while keeping the valency of Mn ions and the tolerance factor ( t=0.921) constant in the substituted compounds. We find that the substitutions by these larger cations induce successive sharp step-like metamagnetic transitions at 2.5 K. The critical field for metamagnetism is the lowest for 3% Ba substituted compound, which has the largest A-site cation size disorder and the least distorted MnO 6 octahedra, among the compounds reported here. These cation substitutions give rise to ferromagnetic clusters within antiferromagnetic matrix, indicating phase-separation at low temperatures. The growth of the clusters is found to vary with the substitution amount. The local lattice distortion of MnO 6 octahedra enhances the charge ordering temperature and reduces the magnetization at high fields (>1 T) in these manganites. 相似文献
11.
The magnetic properties of Co-ferrite-doped hydroxyapatite (HAP) nanoparticles of composition Ca 10−3xFe 2xCo x(PO 4) 6(OH) 2 (where x=0, 0.1, 0.2, 0.3, 0.4 and 0.5% mole) are studied. Transmission electron microscope micrograms show that the 90 nm size nanoparticles annealed at 1250 °C have a core/shell structure. Their electron diffraction patterns show that the shell is composed of the hydroxyapatite and the core is composed of the Co-ferrite, CoFe 2O 4. Electron spin resonance measurements indicate that the Co 2+ ions are being substituted into the Ca(1) sites in HAP lattice. X-ray diffraction studies show the formation of impurity phases as higher amounts of the Fe 3+/Co 2+ ions which are substituted into the HAP host matrix. The presence of two sextets (one for the A-site Fe 3+ and the other for the B-site Fe 3+) in the Mössbauer spectrum for all the doped samples clearly indicates that the CoFe 2O 4.cores are in the ferromagnetic state. Evidence of the impurity phases is seen in the appearance of doublet patterns in the Mössbauer spectrums for the heavier-doped ( x=0.4 and 0.5) specimens. The decrease in the saturation magnetizations and other magnetic properties of the nanoparticles at the higher doping levels is consistent with some of the Fe 3+ and Co 2+ which being used to form the CoO and Fe 2O 3 impurity phase seen in the XRD patterns. 相似文献
12.
用正电子湮没寿命谱技术对有意掺杂的或被杂质替代的钙钛矿结构压电陶瓷中的点缺陷进行了研究.掺杂了La 3+的PbTiO 3,掺杂了Sr 2+,Cd 2+和La 3+的Pb(Mg 1/3Nb 2/3) 0.375Ti 0.375Zr 0.25O 3,掺杂了Ca 2+的BaT
关键词: 相似文献
13.
We report that ferroelectric-relaxor behavior is induced by doping of SrO and TiO 2, or BaO and TiO 2 into classic ferroelectric (Na 0.5K 0.5)NbO 3. It is found that [(Na 0.5K 0.5) 0.9Sr 0.1](Nb 0.9Ti 0.1)O 3 ceramics exhibit a pronounced ferroelectric-relaxor behavior, comparable to that of [(Na 0.5K 0.5) 0.9Ba 0.1](Nb 0.9Ti 0.1)O 3 ceramics. Our results indicate that the relaxor behavior is closely related to the appearance of micropolar regions in these systems. The relaxor behavior should arise from the dynamic response of micropolar clusters. Raman spectra of [(Na 0.5K 0.5) 1−xSr x](Nb 1−xTi x)O 3 ceramics measured in the wavenumber range from 100 to 1200 cm −1 confirm that the first order scattering is dominant in phonon bands should result from both short-range ordered region (micropolar regions) and disordered matrix. The frequency dependence of dielectric permittivity measurements show that the relaxor behavior of SrO and TiO 2, or BaO and TiO 2 doped (Na 0.5K 0.5)NbO 3 ceramics is not a Debye type in the radio frequency range. 相似文献
14.
Mg 2+-, Ca 2+-, Sr 2+- and Ba 2+-doped silica glasses have been prepared using sol-gel processing by employing Si(OC 2H 5) 4, MgCl 26H 2O, CaCl 22H 2O, SrCl 26H 2O and BaCl 22H 2O as precursors, with HCl as a catalyst. The UV–visibleabsorption spectra of the doped samples are almost the same as those
of the undoped sample. The absorption bands of alkaline earth metallic ions have not been observed in the doped samples. Strong
visible light has been observed from sol- gel silica glasses doped with alkaline earth metallic ions. The relative fluorescence
intensity of the Sr 2+-doped (the impurity mole ratio of Sr 2+ was 0.268%) and the Ba 2+-doped (the impurity mole ratio of Ba 2+ was 0.448%) samples was about 4 times that of the undoped sample. The relative fluorescence intensity of the Mg 2+-doped (the impurity mole ratio of Mg 2+ was 0.069%) sample was about 2.5 times that of the pure glass sample. The relative fluorescence intensity of the Ca 2+- doped (the impurity mole ratio of Ca 2+ was 0.179%) sample was about 3 times that of the pure glass sample. Alkaline earth metallic ions affect the formation and
conversion of luminescent defects in sol-gel silica glass. Thus, the relative fluorescence intensity of the doped samples
increases more than that of the undoped sample.
Received: 17 April 2001 / Accepted: 6 June 2001 / Published online: 30 August 2001 相似文献
15.
This paper reports on the spectroscopy properties, absorption and luminescence, of Cr 3+ ions in singly doped, ZnO-codoped, and Zn in-diffused LiNbO 3:Cr crystals. In addition to the broad absorption, inter-ionic transitions ascribed to Cr 3+ ions located in Li + and Nb 5+ sites; [Cr] Li and [Cr] Nb centres two absorption bands at higher energy are reported and ascribed to the charge transfer transitions of the Cr 3+ ions of the two defect centres.
The charge transfer transitions are used as optical probe to study the role of the Zn ions in the Zn in-diffused LiNbO 3:Cr samples. It has been observed that the Zn-in-diffused processes created [Cr] Nb centres in the diffusion zone. The location of the diffused Zn 2+ ions is considered to be in Li + site, displacing the Cr 3+ ions from the Li + sites, [Cr] Li, to the Nb 5+ positions, [Cr] Nb. 相似文献
16.
The dielectric properties of Ba(Zr 0.25Ti 0.75)O 3+ xY 2O 3 ceramics are investigated. We believe that, integrating with the lattice parameters, there is an alternation of substitution preference of yttrium ions for the host cations in perovskite lattice that is responsible for the Curie point. The Tc rises with the increase of Y 3+ doping when the doping content is less than 0.05 at%, owing to the replacement of Y 3+ ions for Ba 2+ ions at the A-site; when the Y 3+ content is more than 0.05 at%, Y 3+ ions tend to occupy the B-site in perovskite lattice, causing a drop of Tc. Owing to the modifications of Y 3+ doping, the loss tangent of BZT ceramics is depressed remarkably, making it a superior candidate to replace widely used lead-contained ceramics. 相似文献
17.
Nanocrystalline acceptor (Pr3+) and donor (Nb5+) doped cerium oxide are synthesized by sol-gel method via hydrolysis process and evaluated for the suitability of applying as an electrolyte for the intermediate temperature solid oxide fuel cells. Phase purity and crystallite size of the synthesized materials are ascertained by powder X-ray diffraction studies. Introduction of Pr3+ ions in the cerium lattice exhibited a lower crystallite size than the Nb5+, which exposes the probability to attain high oxide ion conductivity of Pr3+ doped cerium oxide. Fourier transform infrared and Raman spectra confirm the functional groups and formation of Pr3+ and Nb5+ ions in the cerium lattice. Absorbance spectra exhibit the charge-transfer transition from O2− (2p) to Ce4+ (4f) orbital in cerium oxide. Pr3+ and Nb5+ ions doped cerium lattice create the oxygen vacancies and favor the formation of Ce3+ from Ce4+. Valence band transition of Ce3+ ions from the 5d to 4f levels are examined by photoluminescence studies. The morphological features of Ce-Pr-O and Ce-Nb-O are investigated by scanning and transmission electron microscopy. Electrochemical impedance spectroscopy is used to analyze the conductivity properties of solid electrolytes. Ce-Pr-O shows the high oxide ion conductivity of 0.1 S/cm at 600 °C with an activation energy of 0.73 eV. Electrolytes with specific conductivities higher than 10−2 S/cm at intermediate temperatures (∼400–600 °C) are required for solid oxide fuel cells to operate with less maintenance. Hence, Ce-Pr-O can be a suitable electrolyte material for intermediate temperature solid oxide fuel cells. 相似文献
18.
Rare-earth-doped polycrystalline Ca 3(PO 4) 2:Eu, Ca 3(PO 4) 2:Dy and Ca 3(PO 4) 2:Eu,Dy phosphors prepared by a modified solid-state synthesis has been studied for its X-ray diffraction, thermoluminescence (TL) and photoluminescence (PL) characteristics. The PL emission spectra of the phosphor suggest the presence of Eu 3+ ion in Ca 3(PO 4) 2:Eu and Dy 3+ ion in Ca 3(PO 4) 2:Dy lattice sites. The TL glow curve of the Ca 3(PO 4) 2:Eu compounds has a simple structure with a prominent peak at 228 °C, while Ca 3(PO 4) 2:Dy peaking at 146 and 230 °C. TL sensitivity of phosphors are compared with CaSO 4: Dy and found 1.52 and 1.20 times less in Ca 3(PO 4) 2:Eu and Ca 3(PO 4) 2:Dy phosphors, respectively. The Ca 3(PO 4) 2:Eu,Dy phosphors shows switching behavior under two different excitation wavelengths and enhancement in PL intensity of Dy 3+ ions were reported. The paper discusses the photoluminescence and thermoluminescence behavior of Eu 3+ and Dy 3+ ion in Ca 3(PO 4) 2 hosts, it may be applicable to solid-state lighting as well as thermoluminescence dosimetry applications. 相似文献
19.
The magnetic property of double doped manganite Nd 0.5(1+x)Ca 0.5(1−x)Mn (1−x)Cr xO 3 with a fixed ratio of Mn 3+:Mn 4+=1:1 has been investigated. For the undoped sample, it undergoes one transition from charge disordering to charge ordering (CO) associated with paramagnetic (PM)-antiferromagnetic (AFM) phase transition at T<250 K. The long range AFM ordering seems to form at 35 K, rather than previously reported 150 K. At low temperature, an asymmetrical M- H hysteresis loop occurs due to weak AFM coupling. For the doped samples, the substitution of Cr 3+ for Mn 3+ ions causes the increase of magnetization and the rise of Tc. As the Cr 3+ concentration increases, the CO domain gradually becomes smaller and the CO melting process emerges. At low temperature, the FM superexchange interaction between Mn 3+ and Cr 3+ ions causes a magnetic upturn, namely, the second FM phase transition. 相似文献
20.
TlGaS 2 single crystal doped by paramagnetic Fe 3+ ions has been studied by electron paramagnetic resonance (EPR) technique. The fine structure of EPR spectra of paramagnetic Fe 3+ ions was observed. The spectra reveal a nearly orthorhombic symmetry of the crystal field (CF) on the Fe 3+ ions. Two groups each consisting of four equivalent Fe 3+ centers were observed in the EPR spectra. The local symmetry of the crystal field on the Fe 3+ centers and CF parameters were determined. Experimental results indicate that the Fe ions substitute Ga at the center of the GaS 4 tetrahedrons. The rhombic distortion of the sulfur ligand CF is attributed to the effect of Tl ions located in the trigonal cavities between the tetrahedral complexes. The observed twinning of the resonance lines indicates a presence of two non-equivalent positions of Tl ions that confirms their zigzag alignment in the TlGaS 2 crystal structure. 相似文献
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