Average A-site Cation Radius Research Articles

Ground state structural, electronic and magnetic properties of Mg-doped LaMnO[formula omitted] perovskite: GGA+U study

The effect of Mg doping on the ground state structural, electronic and magnetic properties has been investigated by means of GGA+U calculations. The inspection of the structure, magnetic moment data and density plots, show that by substituting La by Mg at the La site at a concentration of 0.25, the compound exhibits a Pbnm orthorhombic structure and a half metallic ferromagnetic behaviour with degenerated Mn-eg states. The corresponding average A-site cation radius 〈rA〉 value does not allow metallic-insulator phase transition in the compound. However, by doping Mg on Mn site with the same proportion, the resulting compound shows a Pbnm orthorhombic structure and a half metallic behaviour with degeneracy lifting in Mn-eg states. Our results obtained for LaMn0.75Mg0.25MnO3 agree more with experimental data than those obtained for La0.75Mg0.25MnO3, suggesting that Mg will prefer to substitute at the Mn site due to their close ionic radii.

A new class of high-entropy fluorite oxides with tunable expansion coefficients, low thermal conductivity and exceptional sintering resistance

High-temperature thermal barrier coating (TBC) materials are desired for the development of high-efficient gas turbines and diesel engines. Herein, to meet up with this requirement, a new class of high-entropy fluorite-type oxides (HEFOs) has been synthesized via a solid-state reaction method. Comparing to La2Ce2O7, a promising TBC material, the HEFOs exhibit similar high thermal expansion coefficients (TECs) of 11.92×10−6∼12.11×10−6 K-1 at temperatures above 673 K but a better TEC matching performance at the temperature range of 473–673 K. It is also found that through tuning the average A-site cation radius, the TEC of the HEFOs could be tailored efficiently. The HEFOs also possess low thermal conductivities of 1.52-1.55 W∙m-1∙K-1 at room temperature, which is much lower than that of La2Ce2O7 and comparable to pyrochlores as Gd2Zr2O7. Moreover, the HEFOs display good sintering resistance and phase stability even at temperatures as high as 1873 K. The combination of these fascinating properties makes the HEFOs good candidates for thermal barrier coating and thermal insulating materials.

High magnetoresistance in La0.5Nd0.15Ca0.25A0.1MnO3 (A = Ca, Li, Na, K) CMR manganites: Correlation between their magnetic and electrical properties

The influence of A-site cation disorder of monovalent alkali ions on structural and magneto-transport properties of colossal magnetoresistive (CMR) materials has been reported. All the compounds crystallize in Pbnm space group with orthorhombic symmetry. At low temperature, the samples exhibit a paramagnetic (PM) to ferromagnetic (FM) transition and a semiconductor to metallic one. The variation in both Curie temperature (Tc) and metal-semiconductor transition temperature (TMS) can be explained by the change in Mn4+ content and average A-site cation radius. A decrease in resistivity in the vicinity of TMS has been observed with an increase in the applied magnetic field. The values of Tc are close to TMS, signifying strong correlations between the magnetic and electrical properties. The electrical resistivity data in paramagnetic-semiconducting region (T >TMS) may be described by the adiabatic small polaron hopping (ASPH) model. Maximum magnetoresistance (MR%) has been observed in the vicinity of TMS.

Compositional control of tunnel features in hollandite-based ceramics: structure and stability of (Ba,Cs)1.33(Zn,Ti)8O16

The impact of composition on the tunnel features of hollandite materials for the purpose of radioactive cesium (Cs) immobilization was evaluated. The barium (Ba) to cesium (Cs) ratio was varied in the tunnel sites referred to as the A-site of the hollandite structure. Zinc (Zn) was substituted for titanium (Ti) on the B-site to achieve the targeted stoichiometry with a general formula of BaxCsyZnx+y/2Ti8−x−y/2O16 (0 < x < 1.33; 0 < y <1.33). The tunnel cross-section depended on the average A-site cation radius, while the tunnel length depended on the average B-site cation radius. Substitution of Cs resulted in a phase transition from a monoclinic to a tetragonal structure and an increase in unit cell volume of 1.8% across the compositional range. Cs loss due to thermal evaporation was found to decrease in compositions with higher Cs content. The enthalpies of formation from binary oxides of Zn-doped hollandite measured using high-temperature oxide melt solution calorimetry were strongly negative, indicating thermodynamic stability with respect to their parent oxides. The formation enthalpies became more negative, indicating hollandite formation is more energetically favorable, when Cs was substituted for Ba across the range of Zn-doped compositions investigated in this study. Compositions with high Cs content exhibited lower melting points of approximately 80 °C. In addition, high Cs content materials exhibited a significant reduction in Cs release from the solid to liquid phase by leaching or aqueous corrosion as compared to low Cs content materials. These property changes would be beneficial for applications in radioactive cesium immobilization in a multi-phase ceramic by allowing for decreased processing temperatures and higher cesium weight loadings. More broadly, these results establish the link between composition, structural symmetry, and thermodynamic stability for tunnel structured ceramics with implications in the design of new energy conversion and storage materials.

Effect of La-doping on magnetic properties of Bi0.6−xLaxCa0.4MnO3 (0.0⩽x⩽0.6) perovskite manganites

In this paper, we report the synthesis of polycrystalline samples with nominal compositions Bi0.6−xLaxCa0.4MnO3 (0.0⩽x⩽0.6) via the solid state reaction route and our study of their structural, magnetic and electrical transport properties with the variation of doping concentration of La. Magnetic measurements of the parent Bi0.6Ca0.4MnO3 compound reveal the presence of the robust charge-ordered antiferrromagnetic (COAFM) phase with charge-ordering temperature (TCO) ∼289 K and antiferromagnetic Néel temperature (TN) ∼136 K. The COAFM phase disappears when this compound is doped with La. Furthermore, the samples with x=0.2–0.6 exhibit paramagnetic-to-ferromagnetic (PM–FM) transition and PM–FM transition temperature (TC) decreases progressively from 270 to 203 K as x increases from 0.2 to 0.5. However, the samples with x=0.0 and 0.1 do not show PM–FM transition. The temperature-dependent resistance measurements of samples with x=0.0–0.4 exhibit an insulating behaviour, whereas samples with x=0.5 and 0.6 exhibit metal–insulator transition at ∼163 and 198 K, respectively. The experimental results are discussed on the basis of the character of the Bi3+ lone pair electron and the average A-site cation radius (⟨rA⟩).

Effect of Divalent Ions (A = Ca, Ba and Sr) Substitution in La-A-Mn-O Manganite on Structural, Magnetic and Electrical Transport Properties

Problem statement: Microstructure of the grain will influence the properties of a polycrystalline manganites when different dopant is introduced. In this work, an effort had been made to investigate the influence of Ca, Ba and Sr substitution in La site. Approach: Polycrystalline manganites compound of La0.67A0.33MnO3 where A = Ba, Sr and Ca had been prepared via conventional solid-state reaction method. The structure, microstructure, magnetic and electrical properties had been investigated using XRD, SEM, VSM and four-point probe techniques. Results: XRD spectrums showed that LBMO and LCMO were in single-phase orthorhombic structure whereas LSMO was rhombohedral structure. Scanning electron micrographs showed that LSMO had smaller average grain size as compared to LBMO. However, for LCMO, there was no clear grain boundary that can be observed and all the grains were well connected. The difference in the microstructure image might be due to the variance A-site cation that differs in grain growth. The Curie temperature, Tc of LBMO and LSMO was 343.0 and 363.5 K, respectively. But the Tc for LCMO was lower then 300 K. The Tp for LCMO was 215 K while it was near or above 300K for LBMO and LSMO. All samples showed extrinsic Magnetoresistance (MR) effect. Higher MR was observed well below Tc or Tp where MR value increased monotonically with the decrease of temperature. This was associated with the different grain and grain boundary formation when different cation was substituted. LCMO displayed the highest low-field MR value (-15.82% at 0.2 T, 90 K) and high-field MR (-25.60% at 1 T, 300 K). Conclusion: Hence, electrical and magnetic transition temperature (Tp and Tc) were affected by the average A-site cation radius.

Evolution of A-site disorder-dependent structural and magnetic transport properties in La 2/3− xEu xCa 1/3− ySr yMnO 3

We systematically investigate the local disorder effect induced by size mismatch of A-site cation ions on crystallographic structure and magnetic transport behaviors of co-doped La 2/3− x Eu x Ca 1/3− y Sr y MnO 3. We have prepared a series of Eu and Sr co-doped samples by keeping the average A-site cation radius preserved at 1.204 Å and varying A-site cation disorder σ 2 = ∑ i x i r i 2 − r A 2 , where x i is the fractional occupancy and r i is the ionic radii of i cation, respectively. Rietveld refinements show that with increasing doping level, the averaged lattice parameters vary slightly while the local atomic configurations in the unit cell vary obviously. This confirms that with doping single orbital disordered state is extended which is homogenous on macroscales, but inhomogenous at microscales due to lattice strain, as a consequence of random distribution of ions locating at A-site. Further resistance measurements show that the metal–insulator transition temperature ( T MI) decreases linearly with the change of A-site disorder, whereas, larger and more broadening magnetoresistance occur. Analysis of inverse magnetic susceptibility has identified the coexistence of FM metallic and insulating clusters in compounds, thus, contributing to the magnetic inhomogeneity when disorder is present. In addition, magnetocaloric effect is greatly suppressed as a function of disorder. This is ascribed to the gradual loss of first-order magnetic transition. Our experimental findings could be coherently explained in the framework of phase separation.

The Feshbach resonance and nanoscale phase separation in a polaron liquid near the quantum critical point for a polaron Wigner crystal

The additional long range order parameter that competes with the high Tc superconductivity long range order is identified as an electronic crystal of pseudo Jahn-Teller polarons beyond the critical value of the electron lattice interaction. We show that the region of quantum critical fluctuations in the two variables phase diagram of cuprates: the doping δ and the chemical pressure (i.e., the tolerance factor, or the average ionic radius of A-site cations) can be measured via the microstrain ϵ of the Cu-O length in the CuO2 lattice. The fluctuating order in the proximity of the microstrain quantum critical point that competes with the superconducting long range order is the polaron electronic crystalline phase called a Wigner polaron crystal and the variation of the spin gap energy as a function of microstrain provides a strong experimental support for this proposal.

Colossal magnetoresistance properties of Nd-doped Bi0.5Sr0.5MnO3

We have studied structure, transport, and magnetic properties of NdxBi0.5−xSr0.5MnO3 (NBSMO) (x=0.2 and 0.4) through x-ray diffraction, magnetoresistance, and magnetization measurements. The Rietveld analysis of the x-ray diffraction data shows that the NBSMO crystallizes in an orthorhombic perovskite structure with Pbnm space group for x=0.2 and Imma space group for x=0.4. The lattice variations indicate that the highly polarizable 6s2 lone pair of Bi ion is screened and the structural changes occur in accordance with change in the average radius of A-site cations. Magnetotransport studies reveal that both samples are variable range hopping semiconductors and the hopping depends on the orientation of localized spin moment of the electrons. Magnetic studies confirm that substituting Bi with Nd collapses the charge ordered antiferromagnetic (AFM) state of Bi0.5Sr0.5MnO3 to ferromagnetic state, with TC’s∼276 and ∼278K, for x=0.2 and 0.4, respectively. The observed enhancement in colossal magnetoresistance effect in NBSMO compared to Bi0.5Sr0.5MnO3 is attributed to the presence of competing ferromagnetic and charge ordered AFM state.

Fabrication of polycrystalline La0.67(Sr1−x Cd x )0.33MnO3 thin films on Si(100) substrates by sol–gel process

Polycrystalline La0.67 (Sr1-xCdx)0.33MnO3 (LSCMO x = 0, 1/8, 2/8, 3/8, 4/8) films were fabricated on Si(100) single crystal substrates by the sol–gel and the rapid annealing process. According to the field-emission scan electron microscopy (FE-SEM) observation and X-ray (XRD) diffraction studies, all the samples are single phase. We also prepared a series of samples by fixing the ratio of La at 0.67. It is shown that with increase of the doping amount x in La0.67(Sr1-xCdx)0.33MnO3, the average A-site cation radius 〈A〉 decreases, the temperature of metal-insulator transition (TMI) decreases monotonically, ρ and magnetoresistance (MR) values increase dramatically, which can be explained by the lattice effects. The enhanced MR effect at low temperature for the LSCMO film is attributed to the suppression of the spin dependent scattering of polarized electrons at the grain boundaries. The low-field magnetoresistance values under 0.3 T are observed to be 2.48% (x = 0) at 300 K and 37.5% (x = 4/8) at 85 K, respectively.

Thermopower studies of Pr0.67D0.33MnO3 manganite system

With a view to explaining the structural and electrical transport behaviour of some rare earth manganites, having the general formula Pr0.67D0.33MnO3 (D = Ca, Sr, Pb and Ba), a systematic investigation of the electrical resistivity, thermopower and magnetic properties has been undertaken. These materials were prepared using the citrate–gel route by sintering at 900 °C. All the materials were characterized by x-ray diffraction, scanning electron microscopy, etc measurements. The x-ray data were analysed using the Rietveld method and the variation of various parameters involved in the process as explained. The average crystallite size of the materials has been estimated using the peak broadening method, while TC and TP values were determined from ac magnetic susceptibility and electrical resistivity measurements, respectively. Finally, the magnetoresistance (MR) measurements were also carried out over a magnetic field of 1–7 T and the data clearly exhibit the extrinsic MR in ferromagnetic metallic region.The influence of average A-site cation radius ⟨rA⟩ and cation mismatch parameter σ2 on thermoelectric power (TEP), MR, electrical and magnetic transitions has been analysed. The TEP data at low temperatures (T < TP) have been analysed by considering the magnon concepts, while the high temperature (T > TP) data were explained by the Mott's small polaron conduction mechanism. The effects of ⟨rA⟩ and σ2 on polaron activation and hopping energies were also studied systematically.

極低温電顕法による電荷・軌道整列状態の層状ペロブスカイト型マンガン酸化物の超構造解析

The variation of the superstructure related to the charge/orbital ordering in layered manganites RE0.5 (Ca1-ySry) 1.5MnO4 (RE = Pr or Eu) has been systematically investigated by lowtemperature transmission electron microscopy (TEM), It is found that the superstructure is collapsed as increasing (1) the average ionic radius (rA) of A-site cations or enhancing (2) the quenched disorder by the substituting Sr for Ca. The results indicate that the electronic phase of the single layered manganites is controlled not only by the average ionic radius of A-site, but also by the quenched disorder ascribed as the variance in A-site ionic radii (σ2 = Σixiri2-rA2) .

Bandwidth control effects in electron doped manganite La 0.7− xY xCe 0.3MnO 3 thin films

We report the effect of average A-site cation radius on the structural, magnetic and electrical properties of electron doped manganite La 0.7Ce 0.3MnO 3 thin films. A site cation radius 〈 r A〉 is varied systematically by partially replacing La +3 ions by smaller Y +3 ions in the parent compound. The carrier doping, i.e. the fraction of tetravalent Ce atoms at the A-site was kept at 30%. A series of La 0.7− x Y x Ce 0.3MnO 3 ( x=0, 0.05, 0.1, 0.15 and 0.25) thin films were prepared under identical conditions by using pulsed laser deposition technique. Metal–insulator transition temperature ( T p) and the ferromagnetic Curie temperature ( T c) are found to be decreasing significantly with increasing yttrium concentration, i.e. decreasing 〈 r A〉. The resistivity ( ρ) at T p increases by one order of magnitude when the yttrium concentration is increased from x=0 to 0.25, while magnetization decreases with decreasing 〈 r A〉. Magnetoresistance as measured under field of 1 T is significant near T c. Structural analysis reveals the films are having single phase and c-axis lattice parameter decreasing linearly as 〈 r A〉 decreases. There is no marked difference in surface morphology of samples with different yttrium concentration. From EDX the Y:Ce:Mn ratio were consistent within 5% of the nominal composition.

Effect of average A-site cation radius on complex permeability of perovskite manganite La 0.65Nd 0.05(Sr, Ba) 0.3MnO 3

A series of polycrystalline perovskite samples La 0.65Nd 0.05(Sr, Ba) 0.3MnO 3 were prepared by a sol–gel technique. Their permeabilities have been measured at different fields and temperatures. A pronounced dispersion of the relaxation was observed near the Curie temperature T C, which was obtained from the permeability vs. temperature curves. The effects of the average A-site cation radius 〈 r A〉 and the A-site cation disorder parameter σ 2( r A) on the permeability were investigated. With increasing 〈 r A〉 and σ 2( r A), the permeability of La 0.65Nd 0.05(Sr, Ba) 0.3MnO 3 increases linearly.

Investigation of activated transport in hole doped rare earth manganites in the high temperature paramagnetic regime

Electronic transport in the high temperature paramagnetic regime of the colossal magnetoresistive oxides, La 1− x A x MnO 3, A=Ca, Sr, Ba, x≃0.1–0.3, has been investigated using resistivity measurements. The main motivation for this work is to relook into the actual magnitude of the activation energy for transport in a number of manganites and study its variation as a function of hole doping ( x), average A-site cation radius (〈 r A〉), cationic disorder ( σ 2) and strain ( ε zz ). We show that contrary to current practice, the description of a single activation energy in this phase is not entirely accurate. Our results clearly reveal a strong dependence of the activation energy on the hole doping as well as disorder. Comparing the results across different substituent species with different 〈 r A〉 reveals the importance of σ 2 as a metric to qualify any analysis based on 〈 r A〉.

Synthesis and Dielectric Properties of Layer-structured Compounds An−3Bi4TinO3n+3 (A = Ba, Sr, Ca) with n &gt; 4

Several bismuth layer-structured compounds An−3Bi4TinO3n+3 (A = Ba, Sr, Ca) with n &gt; 4 were synthesized and investigated. The average radius of A-site cations was found to be closely related to the lattice parameters in the a–b plane and, therefore, closely related to the stability of the layer structure. For Ba2Bi4Ti5O18 ceramics, there was a diffused phase transition centered at approximately 330 °C, which should be due to the cation redistribution of A-site Ba2+ and Bi3+ in the (Bi2O2)2+ layer. For Sr2Bi4Ti5O18 ceramics, a sharp dielectric constant peak was found at 325 °C. Ceramics of five-layered Ca2Bi4Ti5O18 and six-layered Ca3Bi4Ti6O21 showed temperature-stable dielectric constants up to 800 °C, and ferroelectricity was observed. In addition, as ferroelectrics, these Ca-containing bismuth layer-structured compounds showed frequency-independent dielectric constants &gt;150 and small tan δ on the order of 10−3–10−4 in the frequency range of 1 kHz to 1 MHz.

Insulator–metal transition and the magnetic phase diagram of La1−xTexMnO3 (0.1 ≤ x ≤ 0.6)

The structural, electrical transport and magnetic properties of perovskite oxides La1−xTexMnO3 (0.1≤x≤0.6) have been investigated and thus the magnetic phase diagram of La1−xTexMnO3 (0.1≤x≤0.6) compounds as a function of temperature and the doping level x has been obtained. All samples have rhombohedral structure and undergo paramagnetic–ferromagnetic (PM–FM) transition accompanied with metal–insulator transition (MIT). A charge ordering (CO) transition begins to appear at TCO∼250K for the sample with x=0.60. Moreover, the variation of the Curie temperature Tc and the MIT temperature Tp is quite complex and the results are discussed in terms of three factors including the average A-site cation radius 〈rA〉, the size mismatch σ2 and the Te content. In addition, there is an evidence for magnetoresistance (MR) at low-temperatures for all samples.

Crystal structure refinements of the three-layer Aurivillius ceramics Bi2Sr2$minus;xAxNb2TiO12 (A=Ca,Ba, x=0,0.5,1) using combined X-ray and neutron powder diffraction

Three-layer Aurivillius ceramics Bi 2 SrCaNb 2 TiO 12 , Bi 2 Sr 1.5 Ca 0.5 Nb 2 TiO 12 , Bi 2 Sr 2 Nb 2 TiO 12 , Bi 2 Sr 1.5 Ba 0.5 Nb 2 TiO 12 , and Bi 2 SrBaNb 2 TiO 12 were formed via solid-state synthesis and their structures characterized by combined Rietveld analysis of powder X-ray and neutron diffraction data. Static disorder was observed in the form of mixed cation occupancies between the Bi and the Sr, Ca, or Ba on the A sites in the perovskite block, as well as between the Nb and Ti sites. The degree of site mixing between the Bi site in the (Bi 2 O 2 ) 2+ layer and the perovskite-block A site increased with increasing average A site cation radius (ACR). Bi 2 SrBaNb 2 TiO 12 displayed the greatest degree of Bi–A site static disorder. Bond valence sum (BVS) calculations showed an increase in A site BVS with average A site cation radius. All compositions except Bi 2 SrCaNb 2 TiO 12 had overbonded A sites and the A site BVS increased nearly linearly with lattice parameter and ACR. A preference was observed for Ca 2+ to remain on the A site while Ba 2+ preferred to disorder to the Bi site, indicating that the cation site mixing occurs to reduce strain between the (Bi 2 O 2 ) 2+ layer and the perovskite block in the structure. Unusually large Ti site BVS and thermal parameter for the equatorial oxygen in the TiO 6 octahedra were observed in structural models that included full oxygen occupancy. However, excellent structure models and more reasonable BVS values were obtained by assuming oxygen vacancies in the TiO 6 octahedra. AC impedance spectroscopy performed on all samples indicate that the total electrical conductivity is on the order of 10 −4 S / cm at 900°C.

Crystal structure refinements of the three-layer Aurivillius ceramics Bi 2Sr 2− xAxNb 2TiO 12 ( A=Ca,Ba, x=0,0.5,1) using combined X-ray and neutron powder diffraction

Three-layer Aurivillius ceramics Bi 2SrCaNb 2TiO 12, Bi 2Sr 1.5Ca 0.5Nb 2TiO 12, Bi 2Sr 2Nb 2TiO 12, Bi 2Sr 1.5Ba 0.5Nb 2TiO 12, and Bi 2SrBaNb 2TiO 12 were formed via solid-state synthesis and their structures characterized by combined Rietveld analysis of powder X-ray and neutron diffraction data. Static disorder was observed in the form of mixed cation occupancies between the Bi and the Sr, Ca, or Ba on the A sites in the perovskite block, as well as between the Nb and Ti sites. The degree of site mixing between the Bi site in the (Bi 2O 2) 2+ layer and the perovskite-block A site increased with increasing average A site cation radius (ACR). Bi 2SrBaNb 2TiO 12 displayed the greatest degree of Bi–A site static disorder. Bond valence sum (BVS) calculations showed an increase in A site BVS with average A site cation radius. All compositions except Bi 2SrCaNb 2TiO 12 had overbonded A sites and the A site BVS increased nearly linearly with lattice parameter and ACR. A preference was observed for Ca 2+ to remain on the A site while Ba 2+ preferred to disorder to the Bi site, indicating that the cation site mixing occurs to reduce strain between the (Bi 2O 2) 2+ layer and the perovskite block in the structure. Unusually large Ti site BVS and thermal parameter for the equatorial oxygen in the TiO 6 octahedra were observed in structural models that included full oxygen occupancy. However, excellent structure models and more reasonable BVS values were obtained by assuming oxygen vacancies in the TiO 6 octahedra. AC impedance spectroscopy performed on all samples indicate that the total electrical conductivity is on the order of 10 −4 S/ cm at 900°C.

Suppression of the Curie temperature by cation disorder in doped manganites

The electrical and magnetic transport properties have been studied in a series of samples Nd0.7(Ca,Sr,Ba)0.3MnO3 with a fixed average A-site cation radius 〈rA〉 = 1.21 Å. The residual resistivity increases dramatically and the metal-transition and Curie temperatures decrease systematically with increasing A-site cation disorder. It has been observed that the ferromagnetic-insulator-like state can occur within the measured temperature range when the amount of A-site cation disorder is enhanced even in Nd0.7(Ca,Sr,Ba)0.3MnO3. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)