A-site Disorder Research Articles

High stability of electro-transport and magnetism against the A-site cation disorder in SrRuO3.

It is known that the electro-transport and magnetism of perovskite alkaline-earth ruthenate oxides are sensitive to the lattice distortion associated with the A-site cation size. Orthorhombic CaRuO3 and cubic BaRuO3 exhibit distinctly different electro-transport and magnetic properties from orthorhombic SrRuO3. It has been suggested that SrRuO3 can be robust against some intrinsic/external perturbations but fragile against some others in terms of electro-transport and magnetism, and it is our motivation to explore such stability against the local site cation disorder. In this work, we prepare a set of SrRuO3-based samples with identical averaged A-site size but different A-site cation disorder (size mismatch) by Ca and Ba co-substitution of Sr. It is revealed that the electro-transport and magnetism of SrRuO3 demonstrate relatively high stability against this A-site cation disorder, characterized by the relatively invariable electrical and magnetic properties in comparison with those of SrRuO3 itself. A simple electro-transport network model is proposed to explain quantitatively the measured behaviors. The present work suggests that SrRuO3 as an itinerant electron ferromagnetic metal possesses relatively high robustness against local lattice distortion and cation occupation disorder.

Synthesis method dependence of the lattice effects in Ln0.5M0.5FeO3 perovskites (Ln = La and (Nd or Gd); M = Ba and (Ca or Sr))

A new series of cubic iron perovskites with the composition Ln0.5M0.5FeO3 (Ln = La and Nd (or Gd); M = Ba and Sr (or Ca)) with the same average A-site radius (〈rA〉 = 1.3 Å) but different A-site size disorder, σ2(rA), from 0.0114 to 0.0230 Å2, has been prepared by three different synthesis routes: (a) ceramic method, (b) combustion method and (c) the Pechini method. A strong correlation of the structural parameters with σ2(rA) and the synthesis method is observed. The unit cell volume increases with σ2(rA) in all the cases but the overall isotropic displacement factor of the atoms and the lattice microstrain do not show the same trends. Samples synthesised by the low temperature routes show a σ2(rA)-dependence of the microstrain whereas in the oxides prepared by the ceramic method microstrain seems to be σ2(rA)-independent.

Inducement of Itinerant Electron Transport in Charge-Ordered Pr0.6Ca0.4MnO3 by Ba Doping

The effects of Ba 2+ doping on the electrical and magnetic properties of charge-ordered Pr0.6Ca0.4MnO3 were investigated through electrical resistivity and AC susceptibility measurements. X-ray diffraction data analysis showed an increase in unit cell volume with increasing Ba 2+ content indicating the possibility of substituting Ba 2+ for the Ca-site. Electrical resistivity measurements showed insulating behavior and a resistivity anomaly at around 220 K. This anomaly is attributed to the existence of charge ordering transition temperature, $T^{\mathrm {R}}_{\text {CO}}$ for the x = 0 sample. The Ba-substituted samples exhibited metallic to insulator transition (MI) behavior, with transition temperature, T MI, increasing from ∼98 K (x = 0.1) to ∼122 K (x = 0.3). AC susceptibility measurements showed ferromagnetic to paramagnetic (FM-PM) transition for Ba-substituted samples with FM-PM transition temperature, T c, increasing from ∼121 K (x = 0.1) to ∼170 K (x = 0.3), while for x = 0, an antiferromagnetic to paramagnetic transition behavior with transition temperature, T N, ∼170 K was observed. In addition, inverse susceptibility versus T plot showed a deviation from the Curie–Weiss behavior above T c, indicating the existence of the Griffiths phase with deviation temperature, T G, increasing from 160 K (x = 0.1) to 206 K (x = 0.3). Magnetoresistance, MR, behavior indicates intrinsic MR mechanism for x = 0.1 which changed to extrinsic MR for x > 0.2 as a result of Ba substitution. The weakening of charge ordering and inducement of ferromagnetic metallic (FMM) state as well as increase in both T c and T MI are suggested to be related to the increase of tolerance factor, τ, and increase of e g −electron bandwidth as average ionic radius at A-site, <r A> increased with Ba substitution. The substitution may have reduced MnO6 octahedral distortion and changed the Mn–O–Mn angle which, in turn, promotes itinerancy of charge carrier and enhanced double exchange mechanism. On the other hand, increase in A-site disorder, which is indicated by the increase in σ 2 is suggested to be responsible for the widening of the difference between T c and T MI.

Phase Transition Behaviors of Lead-Free (Na1/2Bi1/2)TiO3-BaTiO3 Single Crystals Studied by Inelastic Light Scattering Spectroscopy

Acoustic anomalies of (1-x)(Na1/2Bi1/2)TiO3-xBaTiO3 (NBT-xBT) with x = 0.05 were investigated over a wide temperature range of from 900°C to −200°C by using Brillouin light scattering. The sound velocity of the longitudinal acoustic mode propagating along the [100] direction showed a substantial softening, while the acoustic attenuation coefficient of this mode exhibited a large increase in the paraelectric phase. These anomalies were attributed to the formation and growth of noncentrosymmetric polar regions in the paraelectric phase, which was supported by the appearance of the strong polarized central peak. The absence of any sharp or discontinuous changes in the acoustic properties seems to suggest that the polar regions remain short-ranged and do not form macroscopic polar domains at low temperatures, which may be attributed to the inherent A-site disorder induced by random substitutions of Bi, Na and Ba cations.

The “self spin valve” in oxygen stoichiometric SrRu1−Fe O3−δ epitaxial thin films

Abstract SrRu 1− x Fe x O 3−δ ( x = 0.00, 0.05, 0.10, and 0.20) thin films were fabricated to study the intrinsic aspects of a “self spin valve”. Using epitaxial strain and high oxygen partial pressure during thin film growth, single phase thin films with negligible oxygen vacancies were successfully grown, and problems related to A-site disorder and grain boundaries were minimized. Under application of an external magnetic field of up to 9 T, the resistivity of all films decreased, resulting in large negative magnetoresistance (up to ∼14.4%), which was stronger at temperatures in the range 10–30 K. An abrupt metal-insulator transition at T ∼ 43 K was found in the x = 0.20 film, which was explained using a two-fluid model related to electron–electron interactions. From the model, two fitting parameters were found to be necessary for in-situ and homogenous defects, while three or unphysical fitting parameters were necessary for ex-situ and inhomogeneous defects.

Influence of A-site cation disorder on structural and magnetocaloric properties of Nd0.7–xLaxSr0.3MnO3 (x=0.0, 0.1, 0.2 & 0.3)

In the present research, the influence of A-site ionic size disorder on structural, magnetic and magnetocaloric properties of Nd0.7–xLaxSr0.3MnO3 (for x=0.0, 0.1, 0.2 & 0.3) compounds were investigated. These compounds were synthesized by solid state reaction route and crystallized in orthorhombic structure with Pnma space group. An average ionic radius <rA> increased with La-substitution which influenced the physical properties such as the magnetic entropy varying from 4.78 J/kg·K (for x=0.0) to 3.55 J/kg·K (for x=0.3) and the maximum magnetization at 5 K changing from 3.92 μB (for x=0.0) to 3.59 μB (for x=0.3). The change in maximum magnetic entropy decreased with increase of A-site average ionic radius. The decrease in magnetic entropy indicated the suppression of lattice disorder that influenced the spin-lattice coupling. The ferromagnetic transition temperature shifted toward room temperature with La substitution. As <rA> increased, the phase transition changed from first order (for x=0.0) to second order (for x≥ 0.1) and the refrigeration capacity increased to a significant level. The modest change in magnetic entropy over a wide range of transition temperature might be useful for future active magnetic refrigeration technology.

Critical behavior and long-range ferromagnetic order in perovskite manganite Nd0.55Sr0.45MnO3

The critical behavior of perovskite manganite Nd0.55Sr0.45MnO3 has been investigated based on the static magnetization measurement around the paramagnetic-ferromagnetic transition temperature 273 K. A large critical exponent and a small one have been obtained by calculating the magnetic-field dependence of the magnetic-entropy change and the Widom scaling relation. These critical exponents not only obey the scaling hypothesis, but also corroborate the results obtained from the Kouvel-Fisher method. In comparison with the values given by standard models, these obtained exponents are very close to those expected from the mean-field model ( and ) and its magnetic-coupling type belongs to nearly long-range interaction. We suggest that the A-site spin disorder and localized magnetic phase competition are the main reasons for the actual critical exponents to show a slight deviation from the theoretical model.

Effect of local strain fields on the structural, Néel transition temperature and long range ferroelectric ordering in rare earth substituted Bi0.9R0.1FeO3 multiferroic ceramics (where, R = Gd3+, Tb3+, Dy3+)

X-ray diffraction analysis of BiFeO3 (BFO) and rare earth (R) substituted BiFeO3 (BRFO) ceramics revealed the pervoskite phase formation. Raman scattering measurements elucidated the weakening of Bi–O covalent bond and destabilization in FeO6 octahedral chains in BRFO ceramics. A variation in Fe–O bond length and modulation in FeO6 local environment were evidenced from UV–visible spectroscopic studies. A weak ferromagnetism along with high coercivity (H C ) values was observed for BRFO ceramics. Heat flow measurements revealed a reduction in Neel transition temperature (T N ) and a slight increase in Curie transition temperature (T C ). The suppression of long range ferroelectric ordering in BRFO ceramics were probed from the nature of endotherm corresponding to T C . The variation and nature of phase transitions in BRFO ceramics were explained based on a structural model by considering the local strain fields created by ordered oxygen displacements due to A-site cation disorder and size variance effects.

Elucidating the real effect of Ba and Co doping on the magnetic and optical properties of BiFeO3

We present Ba/Co co-doping effects on BiFeO3 structure, optical, ferromagnetic, and ferroelectric properties at room temperature. XRD indicates a high distortion of the rhombohedral symmetry, confirmed by Raman spectroscopy, where a change in the intensity ratio of E-1/A1-1 vibrational modes is a result of the partial substitution of Fe3+ by Co3+. This induces an A-site disorder in the FeO6 octahedra. Co2+ acts as grain growth inhibitor reducing average grain sizes from 304nm (without cobalt) to 200nm (with cobalt) after sintering. The magnetization results show that the Co-cation impregnated samples have higher magnetization (31%) than doped and undoped samples.

Incommensurate and commensurate modulations of Ba5RTi3Nb7O30 (R = La, Nd) tungsten bronzes and the ferroelectric domain structures

Incommensurate and commensurate structural modulations of Ba5RTi3Nb7O30 (R = La, Nd) tungsten bronze ceramics were investigated by using a cooling holder equipped transmission electron microscopy in the temperature range from 100 K to 363 K. The incommensurate modulation was observed in both Ba5LaTi3Nb7O30 and Ba5NdTi3Nb7O30 at room temperature, while there was a transition from incommensurate tilted structure to commensurate superstructure for Ba5NdTi3Nb7O30 with decreasing temperature. The incommensurate and commensurate modulations were determined by the A-site occupancy of Ba and R cations. The A-site disorder resulted in larger incommensurability parameter δ and the diffusion of the satellite reflection spots. The effect of A-site disorder on the coupling between long-range dipolar order and the commensurate modulation was also discussed. The obvious ferroelectric 180° domains with spike-like shape parallel to c axis were observed for Ba5NdTi3Nb7O30, while no macro ferroelectric domain was determined for Ba5LaTi3Nb7O30.

Evidence of ferromagnetic short-range correlations in cubic La1−xSrxMnO3−δ(x = 0.80, 0.85) above antiferromagnetic ordering

We report the existence of ferromagnetic correlations (FMC) in paramagnetic (PM) matrix of cubic La1-xSrxMnO3-{\delta} (x = 0.80, 0.85) well above its coupled structural, magnetic and electronic phase transitions. The dc-magnetization vs temperature [M(T)] behaviour under different magnetic fields (from 100 Oe to 70 kOe) shows the presence of short range magnetic correlations up to (TFMC ~) 365 K, far above the antiferromagnetic ordering temperatures (TN =) 260 K and 238 K for x=0.80 and 0.85, respectively. More importantly the observed short-range FMC survive even up to 70 kOe, which indicates their robust nature. The temperature region between TN to TFMC is dominated by the presence of correlated ferromagnetic (FM) entities within the PM matrix and stabilized due to A-site chemical disorder. Our results further illustrate that for the studied compositions, the oxygen off-stoichiometry does not have any significant effect on the nature and strength of these FM entities; however, FM interactions increase in the oxygen deficient samples. These compositions are the unique examples, where the presence of FMC is observed in an undistorted basic cubic perovskite lattice well above TN and therefore are novel to understand the physics behind the colossal magneto-resistance effect.

Effect of A-site average radius and cation disorder on magnetism and electronic properties in manganite $$\hbox {La}_{0.6}\hbox {A}_{0.1}\hbox {Sr}_{0.3}\hbox {MnO}_{3}$$ La 0.6 A 0.1 Sr 0.3 MnO 3 (A = Sm, Dy, Er)

The magnetism and electronic transport properties of $$\hbox {La}_{0.6}\hbox {A}_{0.1}\hbox {Sr}_{0.3}\hbox {MnO}_{3}$$ (A = Sm, Dy, Er) are studied by the measurements of magnetization and resistivity. Infrared spectra reveal that two typical absorbed peaks occur in their corresponding positions. However, the stretching mode $$\nu _{3}$$ exhibits an obvious shift to low frequency and its intensity gets enhanced in Dy- and Er-doping samples. In Raman spectra, the stretching mode shows the same shift, while the Jahn–Teller mode remains invariant. The A-site substitution with other elements changes the average ionic radius and induces cation disorder, causing a decrease of the Curie temperature and the formation of spin–cluster-glass. Except for Sm-doping sample, the field dependence of magnetization at different temperatures shows a similar magnetization process indicating that the low concentration substitution with 10 % magnetic ions cannot signally influence the magnetic exchange interaction on B-site sublattice. Based on the analysis of electronic transport in metallic regime, we find that the main factor for impacting the electronic transport is from the variation of crystal lattice and the induced cationic disorder, regardless of magnetic or non-magnetic substitution.

A real space description of magnetic field induced melting in the charge ordered manganites: II. The disordered case

We study the effect of A site disorder on magnetic field induced melting of charge order (CO) in half doped manganites using a Monte-Carlo technique. Strong A-site disorder destroys CO even without an applied field. At moderate disorder, the zero field CO state survives but has several intriguing features in its field response. Our spatially resolved results track the broadening of the field melting transition due to disorder and explain the unusual dependence of the melting scales on bandwidth and disorder. In combination with our companion paper on field melting of charge order in clean systems we provide an unified understanding of CO melting across all half doped manganites.

Effects of A-site disorder in the properties of A2CoMnO6 (A = La, Tb)

We have studied the structural and physical properties of the La2-xTbxCoMnO6 series. The crystal and magnetic structures of these compounds were determined by x-ray and neutron diffraction techniques. All samples belong to the family of double perovskites with space group P21/n, but the Co/Mn ordering is not perfect, and antisite defects are formed. The concentration of these defects increases for intermediate compositions, indicating that La/Tb disorder influences the Co/Mn arrangement. A ferromagnetic ground state is established due to the strong Mn4+-O-Co2+ superexchange interaction. For the intermediate compositions and at low temperature, the Co/Mn ordering is accompanied by the ordering of Tb3+ moments in the ab-plane, indicating a mutual polarization between both sublattices. Macroscopic magnetic properties reveal that Curie temperature decreases as Tb content increases in correlation with the increase of the structural distortion. All samples show semiconducting behaviour, and overall the electrical resistivity increases with decreasing La-content. The dielectric constant (ε’) has a value of around 12 at low temperatures for all samples, revealing the lack of permanent dipoles. The temperature dependence of ε’ on warming exhibits a strong increase that depends heavily on the frequency of the electric field. This effect is ascribed to non-intrinsic effects such as contacts or internal barrier-layers.

Lattice effects on ferromagnetism in perovskite ruthenates

Ferromagnetism and its evolution in the orthorhombic perovskite system Sr(1-x)Ca(x)RuO3 have been widely believed to correlate with structural distortion. The recent development of high-pressure synthesis of the Ba-substituted Sr(1-y)Ba(y)RuO3 makes it possible to study ferromagnetism over a broader phase diagram, which includes the orthorhombic Imma and the cubic phases. However, the chemical substitutions introduce the A-site disorder effect on Tc, which complicates determination of the relationship between ferromagnetism and structural distortion. By clarifying the site disorder effect on Tc in several unique series of ruthenates in which the average bond length <A-O> remains the same but the bond-length variance varies, we are able to demonstrate a parabolic curve of Tc versus mean bond length <A-O>. A much higher Tc ∼ 177 K than that found in orthorhombic SrRuO3 can be obtained from the curve at a bond length <A-O>, which makes the geometric factor t = <A-O>/(√2<Ru-O>) ∼ 1. This result reveals not only that the ferromagnetism in the ruthenates is extremely sensitive to the lattice strain, but also that it has an important implication for exploring the structure-property relationship in a broad range of oxides with perovskite or a perovskite-related structure.

Relaxor ferroelectric behavior ofBaBi4Ti4O15aurivillius ceramic

Aurivillius type compound BaBi4Ti4O15(BBT) ceramic was prepared by solid state reaction route and its electrical properties were studied. X-ray diffraction (XRD) pattern showed orthorhombic structure with space group A21am confirming it to be an m = 4 member of the Aurivillius oxide. The scanning electron microscope of the sintered pellet showed a plate-like morphology of grains which is a characteristic feature of the Aurivillius compounds. The temperature and frequency dependence dielectric study showed a shifting of transition temperature with frequency indicating relaxor type behavior in the material. The diffuseness parameter (γ) established the relaxor nature and it was attributed to the A-site cationic disorder. The dielectric relaxation obeyed the Vogel–Fulcher (VF) relation and various parameters like activation energy for relaxation, freezing temperature, relaxation frequency were determined after nonlinear curve fitting. The temperature dependence of dielectric constant at temperatures much higher and lower than Tmwas analyzed by two exponential functions, which gives an idea about the production of polar clusters at high temperature and the distribution of freezing temperatures at lower temperature. Various other associated parameters were calculated by nonlinear curve fitting and their significance has been explained.

Raman- and Brillouin-scattering studies on lead-free piezoelectric Bi0.5(Na0.78K0.22)0.5−x TiO3 ceramics with A-site vacancies

The Raman and Brillouin spectra of lead-free piezoelectric Bi0.5(Na0.78K0.22)0.5−x TiO3 ceramics with A-site vacancies were investigated for x = 0 ∼ 0.05. The Raman spectra measured at room temperature showed seven broad bands over the range of 50–1000 cm−1, reflecting strong anharmonicity, inherent A-site disorder and the resulting breakdown of the Raman selection rule. The splitting of the A1 mode near 270 cm−1 indicated that the tetragonal symmetry was maintained over the whole x range, consistent with the X-ray diffraction results. The Brillouin spectra exhibited distributed longitudinal and transverse acoustic modes, in addition to central peaks. The distributed acoustic modes were attributed to the scattering events at all scattering angles from 0 to 180° due to multiple reflections and refractions at the grain boundaries. The relaxation time obtained from fitting the broad central peak by using a Debye function exhibited a slowing-down behavior in the paraelectric phase. The formation of the central peak was attributed to the existence of nano-sized polar regions due to the anharmonic vibrations and the hopping motions of the A-site cations.

Origin of high piezoelectric response in A-site disordered morphotropic phase boundary composition of lead-free piezoelectric 0.93(Na0.5Bi0.5)TiO3–0.07BaTiO3

Perovskite piezoelectric compositions near the morphotropic phase boundary (MPB) are known to exhibit high piezoelectric response. In lead-based ABO3 compound with B-site disorder, the origin of this enhancement has been associated with the presence of an intermediate monoclinic/orthorhombic state that bridges the adjacent ferroelectric rhombohedral and tetragonal phases. However, the origin of high piezoelectric response in lead-free ABO3 compounds with A-site disorder has not been conclusively established. We describe a microscopic model derived from comparative analyses of high resolution transmission electron microscopy and neutron diffraction that explains the origin of high piezoelectric response in lead-free MPB compositions of 0.93(Na0.5Bi0.5)TiO3–0.07BaTiO3. Direct observation of nanotwins with monoclinic symmetry confirmed the presence of an intermediate bridging phase that facilitates a pathway for polarization reorientation. Monoclinic distortions of an average rhombohedral phase are attributed to localized displacements of atoms along the non-polar directions.

Dielectric and piezoelectric properties of cerium modified BaBi 4 Ti 4 O 15 ceramics

Cerium modified barium bismuth titanate ceramics BaBi4Ti4O15-xCeO2 (wt%) were prepared by a solid-state reaction method. X-ray diffraction patterns showed that the ceramics have a four-layer Aurivillius-type structure with orthorhombic phase at room temperature. The plate-like morphology with average grain size about 2 μm was observed by scanning electron microscope (SEM). The effects of cerium modification on dielectric and piezoelectric properties of BaBi4Ti4O15 ceramics were studied. The results showed that cerium modification enhanced the piezoelectric properties, decreased the temperature of the dielectric maximum (Tm) slightly from 413 °C to 406 °C and the dielectric loss, tan δ, as well. The modified Curie–Weiss law was used to describe the relaxor behavior of the ceramics and the degree of dielectric relaxation (γ) firstly increases then decreases with the cerium increasing. The relaxor nature was attributed to the A-site cationic disorder.

Structure and properties of perovskites for SOFC cathodes as a function of the A-site cation size disorder

The structure, electronic conductivity and electrochemical properties of the oxide perovskites with the Ln0.5M0.5FeO3−δ composition (Ln=La, Sm; M=Ba, Sr) are studied in the context of their possible use as cathode materials for intermediate temperature solid oxide fuel cells (IT-SOFCs). The main objective of the work is to analyse the influence of the A-site cation size disorder (σ2(rA)) on the properties of these compounds. The A-site cation size disorder of these samples varies from 0.0286 to 0.089Å2, whilst the doping (x) and the average ionic radius of the A-site cations (<rA>) are fixed to 0.5 and ~1.30Å, respectively.The samples are prepared using the glycine-nitrate method. X-ray powder diffraction (XRD) shows that all of the compounds have rhombohedral symmetry (space group: R-3c). The structure is highly dependent on σ2(rA); the lattice parameters and unit cell volume decrease with decreasing σ2(rA). All of the samples have a porous microstructure with fine grain sizes (100–250nm).The electrical conductivity (σ) measurements on the bulk samples shows a strong dependence on the A-site cation size disorder. Electrochemical impedance spectroscopy (EIS) measurements of the cathodes on the yttria-stabilised zirconia (YSZ) electrolyte-based symmetrical cells indicate that the lowest polarisation resistance value is observed for the sample with the lowest σ2(rA). The effect of introducing a samarium doped ceria (SDC) interlayer between the best performing cathode and the electrolyte is also studied.