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In this work, GdMnO 3 ceramics were synthesized by solid state reaction and sol-gel methods, and the structure, defects and optical, dielectric and magnetic properties of the synthesized samples were comparatively investigated. The samples synthesized by different methods show a single phase structure without any detectable impurities. The SEM results suggest that the particle size of the specimen obtained by the solid phase route is on the micron scale, while that of the specimen fabricated by the sol-gel route is on the nanometer scale. Compared with the ceramic fabricated by solid-state reaction technology, the specimen synthesized by sol-gel technique possesses lower oxygen vacancies and Mn2+ concentration, and Mn3+ concentration. The positron annihilation analyses show that the cation vacancy concentration of the specimen synthesized by the solid phase approach is higher than that of the specimen synthesized via the sol-gel approach. The compound obtained by the solid phase reaction has better dielectric properties than that obtained with the sol-gel method. The magnetic transition temperature and the effective magnetic moment are influenced by the Mn ion valence state in GdMnO 3. The stronger magnetization of the ceramic synthesized via the sol-gel approach is associated with the lower concentration of cation vacancies. [ABSTRACT FROM AUTHOR]

In this paper, M (Sc3+ and Zr4+)-doped CuFeO2 ceramics were prepared by using a solid-state reaction method. The effects of isovalent Sc3+ and nonisovalent Zr4+ substitution on the structure, defects, optical, and magnetic properties of CuFeO2 ceramics are comparatively investigated. The results demonstrated that all of the obtained samples are well crystallized, and M substitution leads to lattice structure distortion. The average grain sizes reduce with the addition of M. Positron annihilation lifetime results indicate that the vacancy clusters are present in all samples, and the Zr-substituted CFO sample has the maximum open volume and concentration of the defect among those three samples. Furthermore, the bandgap of the M-substituted samples becomes wider due to the broad bandgap of Sc2O3 and ZrO2. M substitution enhances the Urbach energy (EU) values in CFO system, which is in good consistent with the dependences of the defect concentration on the substitution elements. The magnetic measurements reveal that Sc and Zr substitution decrease the magnetic temperatures TN1 and TN2, which may be attributed to the consequence of the magnetic dilution effect and the change in the strength of exchange interaction. In addition, the defects are of major importance for regulating the magnetic properties of CFO ceramics. [ABSTRACT FROM AUTHOR]

GdMnO3 (GMO) ceramics were prepared by solid-state reaction method, the structure, defects, dielectric, and magnetic properties of GMO ceramics sintered at different temperatures have been studied. Characterization by XRD indicates that impurity phase-free GMO can be synthesized at 1350 °C. SEM measurements show that the grain size of GMO increases and the microstructure becomes denser with increasing sintering temperature. Positron annihilation results reveal that the vacancies concentration decreases when sintering temperature increases from 1300 to 1350 °C, and then increases when sintering temperature increases from 1350 to 1400 °C. Dielectric measurements indicate that the GMO ceramic sintered at 1350 °C presents higher dielectric constant and smaller dielectric loss. Magnetic properties measurements show that the magnetic properties of GMO can be modulated by sintering temperature. The relation between structure and properties implies that the cation vacancy defect has remarkable effects on the dielectric and magnetic properties of GMO. [ABSTRACT FROM AUTHOR]

• GdMn 1- x Al x O 3 nano ceramics were synzhesized by sol–gel method. • The defects of GdMn 1- x Al x O 3 were studied by positron annihilation technology. • Al doping influences the optical and magnetic properties of GdMnO 3. • The magnetic properties are related to the vacancy concentration. The evolution of Al substitution on the structure, optical and magnetic properties of GdMnO 3 was investigated and GdMn 1- x Al x O 3 (x = 0.00–0.30) nano ceramics were prepared by sol–gel method. All ceramics exhibit the single phase and orthorhombic structure. Al doping causes the change of Raman mode and microscopic morphology of GdMn 1- x Al x O 3 ceramics. Vacancy concentration firstly decreases and then increases when the Al substitution amount in the range of 0.00–0.15. Vacancy concentration decreases in the Al substitution amount range of 0.15–0.20, and increases in the Al substitution amount range of 0.20–0.30. Optical results indicate that Al doping has an impact on the band gap of GdMnO 3 ceramics. Al substitution can adjust the long-range ordering of Gd3+ magnetic moments and magnetic transition temperature. The magnetic transition temperature depends on the local electron density by Al substitution while the magnetization is obviously related to cation vacancy concentration and dilution effect. [ABSTRACT FROM AUTHOR]

• The microstructral defects of GdMnO 3 were studied by positron annihilation technology. • Zr doping can adjust the vacancy defects concentration. • The magnetic properties of GdMnO 3 can be improved by Zr doping. • The magnetic properties seem related to the cation vacancy defects concentration. The influences of the Zr doping on the structure, vacancy defects, and magnetic properties of GdMn 1- x Zr x O 3 (x = 0.00–0.20) ceramic samples synthesized using a solid-state reaction method were investigated. All the GdMn 1- x Zr x O 3 samples formed a single-phase structure, and no phase transformation occurred within the experimental doping range; however, Zr doping caused lattice distortion. The microstructure of the GdMn 1- x Zr x O 3 samples was compact, and Zr doping reduced the grain size. The positron annihilation spectrum (PAS) measurements showed that Zr doping had a great influence on the cation vacancy concentration of GdMn 1- x Zr x O 3 samples. The vacancy concentration increased with increasing Zr doping content from 0.00 to 0.10, and then decreased with the further increase in Zr doping content. XPS results showed that the charge compensation induced by Zr doping was achieved mainly by the decrease of cation valence (forming Mn2+) in the x = 0.00–0.10 ceramics. Appropriate Zr doping obviously improved the transition temperatures and magnetization of the GdMn 1- x Zr x O 3 samples. This investigation demonstrated that the cation vacancy concentration is one of the important factors that could tailor the magnetic properties of GdMn 1- x Zr x O 3 ceramics. [ABSTRACT FROM AUTHOR]

Polycrystalline CuFeO 2 (CFO) and CuFe 0.99 R 0.01 O 2 (R –CFO, R = La, Sm, Eu, Gd, Dy, and Ho) were prepared via solid-phase synthesis to investigate the effects of rare-earth (R) ion addition on the microstructures and magnetism. Positron annihilation lifetime spectroscopy (PALS) experimental results demonstrated that the size, concentration, and defect types were altered by R substitution due to the competition between the agglomeration of small-sized defects and the decomposition of vacancy clusters, which resulted in the redistribution of electrons around annihilation sites. The magnetic measurements revealed that the antiferromagnetic stability was influenced by R substitution, which was ascribed to the effect of magnetic moments, variations in exchange interactions, and the partial reduction of spin frustration. The magnetization behaviors of the R –CFO samples were also influenced by doping ions, which was mainly explained by the different magnitudes of the R ions' magnetic moments. Ferromagnetism and antiferromagnetism were present in all of the samples. The local electron density n e was associated with the maximum magnetization (M m). The correlative physical mechanisms are discussed in detail. The results demonstrated that the n e plays an important role in regulating the magnetic properties of CFO ceramics. [ABSTRACT FROM AUTHOR]

Microstructures, defect characteristics, optoelectronic, and magnetic properties of polycrystalline delafossite Cu 1- x Ca x FeO 2 (x = 0.00–0.08) samples have been systemically investigated. Our investigation results distinctly show that divalent Ca2+ ions can successfully substitute partial Cu+ ions and prompt the grain size and induce the partial Fe3+ valence variation to Fe2+. The positron annihilation lifetime spectra signifies that the cation vacancy clusters exist in all samples, the size and concentration of vacancy defects are redistributed with increasing Ca2+ ions content, while the overall defect environment of CuFeO 2 system is not changed. The Hall-effect measurement results indicate the transition of semiconductor type from p-type to n-type. The UV–Visible absorption spectroscopy results indicate that the calculated band gap of the prepared samples is estimated to be 3.42–3.58 eV, indicating the series can be applied to the transparent conductive oxide materials. The magnetic measurements reveal that the transition temperature T N1 and T N2 are independent of Ca2+ content x , while the coexistence of antiferromagnetism and weak ferromagnetism for all the doped samples are observed. A moderate Ca2+ content, i.e. x = 0.02, can distinctly promote weak ferromagnetism. The above internal mechanism is probably relevant to the evolution of lattice structure and defect characteristics caused by Ca2+ ions doping. • Polycrystalline Cu 1- x Ca x FeO 2 samples have been synthesized successfully by direct solid state reaction. • PALS is employed to reveal electron structure and defect evolution for Cu 1- x Ca x FeO 2 ceramics. • The Cu 1- x Ca x FeO 2 series can be applied to the transparent conductive oxide materials. • A moderate Ca content x can distinctly promote weak ferromagnetism. [ABSTRACT FROM AUTHOR]

• Fe vacancies were introduced into BiFeO 3 by nonstoichiometric Zr substitution. • The microstructral defects were studied by positron annihilation technology. • Nonstoichiometric Zr substitution could effectively enhance the multiferroic properties of BiFeO 3. • Fe vacancies concentration was a key factor that modulated the magnetic properties of BiFeO 3. • Fe2+ concentration was the main source of the leakage current in BiFeO 3. Nonstoichiometric BiFe 1−4 x /3 Zr x O 3 (0 ≤ x ≤ 15%) multiferroic ceramics with Fe vacancies were synthesized by using rapid liquid phase sintering process. It was found that nonstoichiometric Zr substitution induced structure distortion, and an appropriate amount of Zr could remove impurity phases. XPS and PAS studies indicated that there was no obvious change in Bi vacancies concentration, but the Fe vacancies concentration increased with increasing substitution amount of Zr. The Fe2+ concentration first decreased and then increased with the increase in substitution amount of Zr, while the oxygen vacancies concentration decreased with increasing substitution amount of Zr. Nonstoichiometric Zr substitution could effectively enhance the multiferroic behaviors of BiFeO 3. The evolution of structure and properties induced by nonstoichiometric Zr substitution indicated that Fe2+ concentration was the main source of the leakage current, while Fe vacancies concentration was a key factor that modulated the magnetic performances of BiFeO 3. Furthermore, the applied magnetic field dependence of dielectric constant suggested that the magnetodielectric effect increased with increasing Zr substitution amount. The results above imply that nonstoichiometric BiFeO 3 composition with Fe vacancies has great potential for exploiting high quality BiFeO 3 -based ceramics. [ABSTRACT FROM AUTHOR]