A-site Substitution Research Articles

Effect of A site and B site doping on structural, thermal, and dielectric properties of BiFeO 3 ceramics

Polycrystalline Bi 1− x Ba x Fe 1− y M y O 3 (M = Co and Mn; x = 0.1, y = 0.1) were synthesized by solid-state route method to study the compositional driven structural transformations in multiferroics. Room temperature X-ray diffraction patterns confirmed the formation of perovskite structure. Rietveld-refined crystal structure parameters revealed the existence of rhombohedral R3 c symmetry for both the samples. The samples were found to be nearly free from any other secondary phases. Raman analysis reveals that Ba atom substitutes Bi site and Mn and Co atom substitutes Fe site into the BiFeO 3 with the shifting of phonon modes. The red shift is attributed to Co or Mn doping where as blue shift occurs from Ba doping. The differential scanning calorimetry reveals the corresponding Neel temperature 370 °C and 326 °C for Co and Mn doped samples. Ba and Co substitution with x = 0.1 and y = 0.1 has not affected the Neel temperature of the parent BiFeO 3 as well of Ba and Mn substitution. The variation of frequency dispersion in permittivity and loss pattern due to A-site and B-site substitution in BiFeO 3 was observed in the dielectric response curve.

Properties of KNTN [1 0 0] surfaces: A first-principles study

The properties of potassium natrium tantalite niobate (KNTN) surfaces are discussed, A-site substitution with potassium tantalite niobate (KTN) and the use of Na cations to replace K ones. After geometrical structure optimization, band structure and density of states (DOS) are analyzed for KNTN [1 0 0] surface. Furthermore, we evaluate the optical and physical properties of KNTN surfaces using first-principles calculations such as dielectric function, refractive index, and energy-loss function and compare the results with KNTN bulk and KTN surfaces; there are some similar and different properties with these materials.

Tunability of third order nonlinear absorption in (Pb,La)(Zr,Ti)O3 thin films

This paper reports the demonstration of the tunability of the third order nonlinear optical absorption in (Pb1−3x/2Lax)(Zr0.53Ti0.47) (PLZT) thin films. The optical nonlinearity is investigated employing the open aperture z-scan technique at the off-resonant wavelength of 532 nm. By altering the defect chemistry in PZT through A-site aliovalent lanthanum substitution, the nonlinear optical (NLO) absorption can be effectively tuned. The results of the influence of lanthanum concentration on leakage current and microstructure have also been combined to confirm the defect-mediated mechanism of NLO absorption in PLZT thin films.

Effects of A-site equivalence and non-equivalence substitution on polarization properties of K0.5Na0.5NbO3 lead-free piezoelectric ceramics

The effects of A-site equivalence and non-equivalence substitution on polarization temperature and electric field of K0.5Na0.5NbO3 lead-free piezoelectric ceramics are studied, and the results show that the ceramics sample of A-site equivalence is independent of polarization temperature and electric field, which can improve the piezoelectric constant. On the contrary, the ceramics sample of A-site non-equivalence substitution is sensitive to polarization temperature and electric field, constraining the piezoelectric properties.

Magnetic and Static Nonlinear Dielectric Response in Doped CdCr2S4

The influence of cation substitution on the magnetic and static nonlinear dielectric response in the doped magnetic relaxor ferroelectrics CdCr2S4 is investigated within the framework of the spherical random-bond-random-field model (SRBRF) and the site-dilution Heisenberg model by adding an appropriate coupling term between the magnetic and ferroelectric subsystems. A-site substitution of the Cd by Fe in Cd1-xFexCr2S4 is found to reduce significantly the magnetization, which well agrees with the experimental results. Furthermore, the doping Fe ions play a crucial role in the spin-pair correlation and the nonlinear dielectric properties.

Preparation, structures, and multiferroic properties of single-phase BiRFeO 3, R = La and Er ceramics

A simple and effective method of solid state reaction followed by quenching process is used to prepare multiferroic Er 3+ and La 3+ doped BiFeO 3 ceramics. X-ray diffraction patterns indicated that these ceramics were of single-phase perovskite structure. Doping with La enhanced the dielectric, ferroelectric and ferromagnetic properties of BiFeO 3. A-site substitution with the La and Er ions has been found to suppress the spiral spin structure of BiFeO 3 giving rise to the appearance of room-temperature weak ferromagnetism. The enhanced multiferroic properties are attributed to the enhanced magnetoelectric interaction, which results from the La and Er substitution-induced destruction of the spin cycloid.

Synthesis and Thermal Stability of the Solid Solution AFeO2 (A = Ba, Sr, Ca)

We have studied the A-site substitution effect on the structural, thermal, and magnetic properties of the infinite layer iron oxide AFeO(2) (A = alkali-earth elements) with an FeO(4) square-planar coordination. Together with the previous study showing a total substitution by Ca, Ba substitution is found to be tolerable up to 30%, presenting almost the same substitutional range as that found in ACuO(2) under high pressure. Notably, Ba substitution shows little influence on the magnetic properties, in contrast to expectations from first principles calculations. The temperature at which oxidation to an AFeO(2.5) phase occurs and its transformation rate show a wide variation tunable solely by the out-of-plane distance.

ELECTRICAL AND MAGNETIC BEHAVIOR OF TRANSITION METAL OXIDES Ln0.7A0.3TMO3, Ln= La, Pr; A = Ca, Sr AND TM = Mn, Co

We present a detailed magnetic study of the polycrystalline transition metal oxides Ln 0.7 A 0.3 TMO 3, Ln = La , Pr ; A = Ca , Sr and TM = Mn , Co at low temperatures including electrical resistance, magnetization and ac susceptibility measurements. The effect of the A-site and B-site substitutions on the properties of these compounds is discussed. The results of this analysis indicate that the electrical and magnetic behavior of these compounds have some reminiscences of the manganites having similar formula, but the spin disorder and the grain boundary effects control the properties of these materials.

Interstitial Oxide Ion Order and Conductivity in La1.64Ca0.36Ga3O7.32 Melilite

Solid oxide fuel cells (SOFCs) are a major candidate technology for clean energy conversion because of their high efficiency and fuel flexibility.1 The development of intermediate-temperature (500–750 °C) SOFCs requires electrolytes with high oxide ion conductivity (exceeding 10−2 S cm−1 assuming an electrolyte thickness of 15 μm1). This conductivity, in turn, necessitates enhanced understanding of the mechanisms of oxide ion charge carrier creation and mobility at an atomic level. The charge carriers are most commonly oxygen vacancies in fluorites2, 3 and perovskites.3, 4 There are fewer examples of interstitial-oxygen-based conductors such as the apatites5, 6 and La2Mo2O9-based materials,7–9 so information on how these excess anion defects are accommodated and the factors controlling their mobility is important.

Influence of non-isovalent ion substitution at A site on microstructure and magnetic properties of Ba(Ti 0.3Fe 0.7)O 3 ceramic

The (Ba 0.8K 0.2)(Ti 0.3Fe 0.7)O 3 ceramic was prepared by solid-state reaction, and post-annealed in oxygen ambient. By comparison with Ba(Ti 0.3Fe 0.7)O 3 made under identical conditions, the effect of non-isovalent A-site substitution of K + on microstructure and magnetism of as-prepared and annealed Ba(Ti 0.3Fe 0.7)O 3 samples was investigated using X-ray diffraction, Mössbauer spectroscopy, vibrating sample magnetometer and iodometric titration. It is found that all samples have a single 6H-BaTiO 3-type hexagonal perovskite structure without any impurities detected, regardless of A-site K + substitution or annealing. In the as-prepared state, non-isovalent A-site substitution of K + induces the variation in Fe occupational site, resulting in the disappearance of room-temperature ferromagnetism. The super-exchange interactions of Fe 3+ at tetrahedral and octahedral Ti sites determine the paramagnetism of (Ba 0.8K 0.2)(Ti 0.3Fe 0.7)O 3. During the O 2 annealing process, the presence of Fe 4+, an unusual valence for iron, besides Fe 3+ is observed, both distributed over octahedral Ti site. By A-site substitution of K + with a lower valence than Ba 2+, the charge compensation mechanism is further enhanced, and thus more Fe 3+ ions are oxidized to Fe 4+ in annealed (Ba 0.8K 0.2)(Ti 0.3Fe 0.7)O 3. Consequently, the ferromagnetic Fe 4+–O 2−–Fe 4+ super-exchange interactions are strengthened, which leads not only to a paramagnetism–ferromagnetism transition but also to a higher saturation magnetization compared with annealed Ba(Ti 0.3Fe 0.7)O 3.

Cation effects in new superconductors Sr2−xBaxCuO3+δ

A series of new superconductors Sr2−xBaxCuO3+δ (x⩽0.6) were prepared under high pressure high temperature. The superconductors crystallize into K2NiF4-type structure of I4/mmm space group similar to the classical La2CuO4 but with partially occupied apical oxygen. It is found that Tc of this barium doped Sr2CuO3+δ superconductor is associated with the ratio of A-site cation substitution while keeping at optimal carrier doping level. The superconducting transition temperature with Tcmax=98K which represents the record high Tc in the single-layer copper oxide superconductors was achieved in this material.

Systematic variations in structural and electronic properties of BiFeO3 by A-site substitution

Systematic variations in the structural and electronic properties of BiFeO3 with A-site substitutions were studied using first-principles density functional theory calculations. It is found that the ferroelectric distortion of BiFeO3 with group IIIA element (Sc3+, Y3+, and La3+) substitutions is significantly affected by the hybridization between substitute d states and oxygen 2p states, while that with group VB element (Sb3+) substitution is stabilized by the s2 lone pair electrons. For both groups, the substitute with smaller ionic size and larger electronegativity causes more significant off-center displacement and narrower band gap.

A-site substitution in Fe1.1Te: synthesis, structure and properties

A-site substitution in Fe1.1Te: synthesis, structure and properties

Doping effect on the magnetocapacitance in Cd1−xFexCr2S4

The influence of cation substitution on the Curie temperature and the magnetocapacitance in the doped magnetic relaxor ferroelectrics CdCr2S4 is investigated within the framework of the spherical random-bond-random-field model for the relaxor ferroelectric sublattice and the Heisenberg model for the magnetic sublattice. The site-dilution model is also introduced to describe the doping effect. A-site substitution of the Cd by Fe in Cd1−xFexCr2S4 is found to increase significantly the value of the Curie temperature, which well agrees with the experimental results. Additionally, the doping Fe ions play a crucial role in the fluctuation of spin-pair correlation and magnetocapacitive response.

THE EFFECT OF A-SITE La AND Nd SUBSTITUTION ON THE RESIDUAL STRESS OF Bi4Ti3O12 THIN FILMS FABRICATED BY SOL-GEL METHOD

ABSTRACT In this paper, thin films of Bi4Ti3O12 (BTO), Bi3.25La0.75Ti3O12 (BLT) and Bi3.15Nd0.85Ti3O12 (BNT) were fabricated on Pt/Ti/SiO2/Si substrates by sol-gel method. The structures and ferroelectric properties of the polycrystalline films are studied and the correlation between substitution and residual stress are discussed in detail. The results show that, the ferroelectric and fatigue properties of the BLT and BNT films are improved greatly, compared with those of the BTO thin film. The Pr values of BLT and BNT are 13.14 and 21.23 μC/cm2, respectively. X-ray diffraction result shows that the measured residual stress is tensile stress for all films. After A-site substitution, the value of tensile stress for the BTO films has increased and the stress value is maximal for the BNT film. The calculated intrinsic residual stress values are 288.4, 418.4 and 608.2 MPa for BTO, BLT and BNT, respectively.

Magnetic study of CaMn 0.96Mo 0.04O 3, canting vs. phase separation

CaMn 0.96Mo 0.04O 3 is an example of Mn 4+ rich perovskite manganites, which exhibits a net ferromagnetic component at low temperature, observed by dc magnetization and ac susceptibility. To characterize the magnetic state of this compound, neutron powder diffraction was carried out in the 2–400 K temperature range, showing that it is necessary to use three components (ferromagnetic and G- and A-type antiferromagnetic) to describe it. This particular state is in agreement with the unusual magnetic behaviour observed by macroscopic measurements and is compared to the one observed for manganites with similar Mn valence but obtained by A-site substitution.

Effect of Gd substitution on the crystal structure and multiferroic properties of BiFeO 3

The crystal structure, magnetic and local ferroelectric properties of polycrystalline Bi 1− x Gd x FeO 3 ( x = 0.1, 0.2, 0.3) samples were investigated at room temperature. Gadolinium substitution was found to induce a polar-to-polar R3 c → Pn2 1 a structural phase transition at x ≈ 0.1. Increasing the content of the substituting element suppressed the spontaneous polarization in Bi 1− x Gd x FeO 3, resulting in a ferroelectric–paraelectric Pn2 1 a → Pnma phase transition at 0.2 < x < 0.3. The substitution caused the appearance of a weak ferromagnetic moment. The data presented are consistent with the hypothesis that the most effective way to induce spontaneous magnetization in antiferromagnetic BiFeO 3 seems to be related to A-site substitution by rare-earth ions with a large difference in ionic radius with respect to Bi 3+. The effect of the magnetically active Gd 3+ substitution on the low-temperature magnetic properties of Bi 1− x Gd x FeO 3 samples is also discussed.

Investigation of proton conductivity in Sm 1.92Ca 0.08Ti 2O 7 − δ and Sm 2Ti 1.92Y 0.08O 7 − δ pyrochlores

The results of the synthesis and characterisation of pyrochlore systems Sm 1.92Ca 0.08Ti 2O 7 − δ and Sm 2Ti 1.92Y 0.08O 7 − δ are reported. The electrical conductivity of the materials was studied using impedance spectroscopy under wet and dry oxygen and argon. Enhancements of the bulk conductivity at temperatures up to 500 °C were observed for wet conditions indicative of significant proton conductivity. The presence of dissolved protons in the materials is supported by thermogravimetric analysis and infrared spectroscopy. Proton conduction was confirmed by measurements in O 2/D 2O and Ar/D 2O. The results reveal the importance of the correct choice of dopant site for the pyrochlore structure, with A-site substitution yielding the highest levels of proton, as well as oxide ion, conduction. For both samples bulk, rather than grain boundary, conduction is found to be dominant.

Oxygen-vacancy concentration in A2MgMoO 6−δ double-perovskite oxides

Accurate oxygen-content analysis by means of a coulometric redox titration method specially devised for the purpose shows that as-synthesized (in 5% H 2/Ar) samples of the recently reported novel solid oxide fuel cells anode material Sr 2MgMoO 6− δ contain oxygen vacancies with a concentration of δ≈0.05. Oxygen contents and the resultant Mo-valence values are also analyzed for various A 2MgMoO 6− δ samples in order to reveal both the isovalent and aliovalent A-site cation substitution effects.

Correlation between Ionic Radius of Substituting Element and Magnetic Properties of Bi&lt;sub&gt;1-x&lt;/sub&gt;A&lt;sub&gt;x&lt;/sub&gt;FeO&lt;sub&gt;3-x/2&lt;/sub&gt; (A= Ca, Sr, Pb, Ba) Multiferroics

Investigation of crystal structure and magnetic properties of the diamagnetically- substituted Bi1-xAxFeO3-x/2 (A= Ca, Sr, Pb, Ba; x= 0.2, 0.3) polycrystalline samples has been carried out. It has been shown that the heterovalent A2+ substitution result in the formation of oxygen vacancies in the host lattice. The solid solutions have been found to possess a rhombohedrally distorted perovskite structure described by the space group R3c. Magnetization measurements have shown that the magnetic state of these compounds is determined by the ionic radius of the substituting elements. A-site substitution with the biggest ionic radius ions has been found to suppress the spiral spin structure of BiFeO3 giving rise to the appearance of weak ferromagnetism.