Showing papers on "Tetragonal crystal system published in 2008"
TL;DR: In this article, a huge enhancement of the superconducting transition temperature Tc was observed in a tetragonal FeSe superconductor under high pressure and the onset temperature was as high as 27K at 1.48GPa and the pressure coefficient showed an extremely high value of 9.1K∕GPa.
Abstract: A huge enhancement of the superconducting transition temperature Tc was observed in a tetragonal FeSe superconductor under high pressure. The onset temperature was as high as 27K at 1.48GPa and the pressure coefficient showed an extremely high value of 9.1K∕GPa. The upper critical field Hc2 was estimated to be ∼72T at 1.48GPa. Because of the high Hc2, the FeSe system can be applied to superconducting wire rods.
655 citations
TL;DR: Although structural distortions are robust to changes in particle size, what is affected is the coherency of the distortions, which is decreased in the smaller particles, in contrast to the sharp transition that is found for the bulk sample.
Abstract: We have investigated the paraelectric-to-ferroelectric phase transition of various sizes of nanocrystalline barium titanate (BaTiO3) by using temperature-dependent Raman spectroscopy and powder X-ray diffraction (XRD). Synchrotron X-ray scattering has been used to elucidate the room temperature structures of particles of different sizes by using both Rietveld refinement and pair distribution function (PDF) analysis. We observe the ferroelectric tetragonal phase even for the smallest particles at 26 nm. By using temperature-dependent Raman spectroscopy and XRD, we find that the phase transition is diffuse in temperature for the smaller particles, in contrast to the sharp transition that is found for the bulk sample. However, the actual transition temperature is almost unchanged. Rietveld and PDF analyses suggest increased distortions with decreasing particle size, albeit in conjunction with a tendency to a cubic average structure. These results suggest that although structural distortions are robust to cha...
480 citations
TL;DR: In this article, it was shown that CaFe2As2 undergoes a pressure-induced transition to a nonmagnetic volume "collapsed" tetragonal phase, which becomes superconducting at lower temperature.
Abstract: Recent investigations of the superconducting iron-arsenide families have highlighted the role of pressure, be it chemical or mechanical, in fostering superconductivity. Here we report that CaFe2As2 undergoes a pressure-induced transition to a nonmagnetic volume "collapsed" tetragonal phase, which becomes superconducting at lower temperature. Spin-polarized total-energy calculations on the collapsed structure reveal that the magnetic Fe moment itself collapses, consistent with the absence of magnetic order in neutron diffraction. © 2008 The American Physical Society.
280 citations
Abstract: The size and crystal orientation of Sn grains in Pb-free, near eutectic Sn-Ag-Cu solder joints were examined. A clear dependence of the thermomechanical fatigue response of these solder joints on Sn grain orientation was observed (Sn has a body centered tetragonal crystal structure). Fabricated joints tend to have three orientations in a cyclic twin relationship, but among the population of solder balls, this orientation triplet appears to be randomly oriented. In thermally cycled joints, solder balls with dominant Sn grains having the particular orientation with the c-axis nearly parallel to the plane of the substrate were observed to fail before neighboring balls with different orientations. This results from the fact that the coefficient of thermal expansion of Sn in the basal plane (along the alpha-axis) is half the value along the c-axis; joints observed to be damaged had the maximum coefficient of thermal expansion mismatch between solder and substrate at the joint interface, as well as a tensile stress modes during the hot part of the thermal cycle. Localized recrystallization was observed in regions of maximum strain caused by differential expansion conditions, and its connection with crack nucleation is discussed.
266 citations
TL;DR: The crystal structure in the superconducting state shows remarkable similarities to that of the REFeAsO(1-x)F(x) (RE = rare earth) superconductors.
262 citations
TL;DR: In this article, a series of structure transformations depend upon the doping level of Bi1−xLaxFeO3 ceramics with x=0, 0.8La0.2 and Bi0.7La 0.3 have been synthesized by solid state reaction starting from metal oxides.
Abstract: Bi1−xLaxFeO3 ceramics with x=0, 0.1, 0.2, and 0.3 have been synthesized by solid state reaction, starting from metal oxides. A series of structure transformations is found to depend upon the doping level. Below 10% La doping, Bi1−xLaxFeO3 maintains the rhombohedral structure of BiFeO3. However, for Bi0.8La0.2FeO3 and Bi0.7La0.3FeO3, the structures change to the orthorhombic and tetragonal, respectively. La doping significantly reduces electric leakage and leads to successful observation of electrical polarization hysteresis loops. Doping with La also enhances the ferromagnetic moment, due to the broken cycloid spin structure caused by the changes in the crystalline structure.
258 citations
TL;DR: Based on density functional methods, relative stabilities between monoclinic, tetragonal, and cubic phases with cation dopants or oxygen vacancies are investigated in this article, where it is found that dopants such as Si, Ge, Sn, P, Al or Ti with ionic radii smaller than Hf stabilize the tetragonal phase but destabilize the cubic phase.
Abstract: Based on density functional methods, relative stabilities between monoclinic, tetragonal, and cubic phases of ${\text{HfO}}_{2}$ with cation dopants or oxygen vacancies are investigated. It is found that dopants such as Si, Ge, Sn, P, Al or Ti with ionic radii smaller than Hf stabilize the tetragonal phase but destabilize the cubic phase. In contrast, larger dopants such as Y, Gd or Sc favor the cubic phase. The ionized oxygen vacancies compensating trivalent dopants greatly stabilize both cubic and tetragonal phases. Microscopic explanations on the results are also given. The metastable phase favored by each dopant is in good agreement with experimental data. Our results can serve as a useful guide in selecting dopants to stabilize a specific phase.
254 citations
TL;DR: In this article, the results of X-ray diffraction analysis revealed a MPB composition range of x ǫ = 0.06-0.10 for (Na 0.5 Bi 0.10 ) 1− x Ba x TiO 3 system at room temperature, where the relative content of the tetragonal phase is substantially higher than that of the rhombohedral phases.
Abstract: (Na 0.5 Bi 0.5 ) 1− x Ba x TiO 3 ceramics were synthesized by the citrate method and their structure and electrical properties were systematically investigated. The results of X-ray diffraction analysis revealed a MPB composition range of x = 0.06–0.10 for (Na 0.5 Bi 0.5 ) 1− x Ba x TiO 3 system at room temperature. It was found that the piezoelectric and ferroelectric properties of (Na 0.5 Bi 0.5 ) 1− x Ba x TiO 3 compositions near the MPB are rather sensitive to the phase composition and reach preferred values at x = 0.07, where the relative content of the tetragonal phase is substantially higher than that of the rhombohedral phases. (Na 0.5 Bi 0.5 ) 1− x Ba x TiO 3 ceramics present a decrease diffusive factor ( δ ) with increasing BaTiO 3 content, implying a degradation of the relaxor feature and a transition from relaxor ferroelectrics to normal ferroelectrics.
237 citations
TL;DR: In this paper, single-crystal tetragonal α-MnO2 nanotubes have been successfully synthesized by a facile hydrothermal treatment of KMnO4 in the hydrochloric acid solution.
Abstract: Single-crystal tetragonal α-MnO2 nanotubes have been successfully synthesized by a facile hydrothermal treatment of KMnO4 in the hydrochloric acid solution. The sample has been characterized by X-ray powder diffraction, field emission scanning electron microscopy, transmission electron microscopy, and orientation dependent Raman spectroscopy, which indicates the nanotubes have high-quality crystalline and shape-dependent optical properties. The morphology evolution of the sample reveals that the nanotubes are formed via the solid nanorod by a chemical etching process.
232 citations
TL;DR: In this article, the authors examined the crystal structures, magnetic properties and conductivity of undoped (normal conductor) and 14.5% F-doped LaFeAsO (Tc = 20?K) by synchrotron x-ray diffraction (XRD), DC magnetic measurements, and ab?initio calculations demonstrated that the anomaly is associated with a phase transition from tetragonal (P4/nmm) to orthorhombic (Cmma) phases at ~160?K as well as an antiferromagnetic spin ordering transition at
Abstract: Undoped LaFeAsO, the parent compound of the newly found high-Tc superconductor, exhibits a sharp decrease in the temperature-dependent resistivity at ~160?K. The anomaly can be suppressed by F doping with simultaneous appearance of superconductivity appears correspondingly, suggesting a close association of the anomaly with the superconductivity. We examined the crystal structures, magnetic properties and conductivity of undoped (normal conductor) and 14?at.% F-doped LaFeAsO (Tc = 20?K) by synchrotron x-ray diffraction (XRD), DC magnetic measurements, and ab?initio calculations demonstrated that the anomaly is associated with a phase transition from tetragonal (P4/nmm) to orthorhombic (Cmma) phases at ~160?K as well as an antiferromagnetic spin ordering transition at ~140?K. These transitions can be explained by spin configuration-dependent potential energy surfaces derived from the ab?initio calculations. The suppression of the transitions is ascribed to interrelated effects of geometric and electronic structural changes due to doping by F? ions.
231 citations
TL;DR: In this article, the full set of relaxor-based ternary single crystals Pb(In(12)Nb(12))O(3)-Pb(Mg(13) Nb(23)) O(3)PbTiO(4) (PIN-PMN-PT) was determined and compared to binary PbMgNgNbNb (13),O(23),PbNg(23)),O( 3)-PmNb((23))O((3)-pbTiNb
Abstract: The full set of material constants for relaxor-based ternary single crystals Pb(In(12)Nb(12))O(3)-Pb(Mg(13)Nb(23))O(3)-PbTiO(3) (PIN-PMN-PT) were determined and compared to binary Pb(Mg(13)Nb(23))O(3)-PbTiO(3) (PMNT) crystals The Curie temperature for rhombohedral compositions of PIN-PMN-PT was found to be in the range of 160-200 degrees C with ferroelectric rhombohedral to tetragonal phase transition on the order of 120-130 degrees C, more than 30 degrees C higher than that found for PMNT The piezoelectric coefficients (d(33)) were in the range of 1100-1500 pCN, with electromechanical coupling factors (k(33)) about 89%-92% comparable to PMNT crystals The coercive field of the ternary crystal was found to be 55 kVcm, double the value of the binary counterparts The dielectric behavior under varying dc bias exhibited a similar trend as observed in PMNT with a much broader usage temperature range Together with its enhanced field induced phase transition level, the ternary PIN-PMN-PT crystals are promising candidates for high temperature and high drive transducer applications
TL;DR: In this paper, the magnetic phase diagram of the multiferroic solid solution system with morphotropic phase boundary (MPB) was established and the significant difference in antiferromagnetic ordering temperatures of the rhombohedral and tetragonal phases was attributed to the different structural effects on the magnetic interactions between rhombohedra and the tetragonal phases, and the effect of the magnetic dilution on the ordering strength.
Abstract: A series of multiferroic $(1\ensuremath{-}x){\text{BiFeO}}_{3}\ensuremath{-}x{\text{PbTiO}}_{3}$ solid solution ceramics were prepared by solid-state reactions. Structural characterization by x-ray diffraction reveals the existence of a morphotropic phase boundary (MPB) region in this system in which a tetragonal, a rhombohedral, and an orthorhombic phase exist simultaneously with a large tetragonality in the tetragonal phase region. The temperature variation of magnetic moment of the samples with MPB compositions, measured under zero-field cooling (ZFC) mode, shows three anomalies arising from the antiferromagnetic orderings of the rhombohedral, tetragonal, and orthorhombic phases, respectively. The significant difference in antiferromagnetic ordering temperatures of the rhombohedral and tetragonal phases is attributed to the different structural effects on the magnetic interactions between the rhombohedral and the tetragonal phases, and the effect of the magnetic dilution on the magnetic ordering strength. The magnetic phase diagram of the $(1\ensuremath{-}x){\text{BiFeO}}_{3}\ensuremath{-}x{\text{PbTiO}}_{3}$ solid solution system was established.
TL;DR: Comprehensive structural analyses throughout this transformation, from primitive orthorhombic to body-centered tetragonal (I4/mcm), reveal a flexing of the framework and a dilation of the channels, with an accompanying subtle distortion of the iron(II) coordination geometry.
Abstract: The porous coordination framework material, Fe(NCS)2(bped)2·3EtOH, SCOF-3(Et) (where bped is dl-1,2-bis(4‘-pyridyl)-1,2-ethanediol), displays a spin-crossover (SCO) transition that has been stimulated both thermally and by light irradiation. The one-step thermal SCO (70−180 K) is sensitive to the presence of molecular guests, with a more gradual transition (70−225 K) apparent following the desorption of ethanol molecules that hydrogen bond to the spin centers. Additional intraframework hydrogen-bonding interactions stabilize the vacant one-dimensional pore structure of the apohost, SCOF-3, despite a dramatic single-crystal to single-crystal (SC−SC) structural change upon removal of the guests. Comprehensive structural analyses throughout this transformation, from primitive orthorhombic (Pccn) to body-centered tetragonal (I4/mcm), reveal a flexing of the framework and a dilation of the channels, with an accompanying subtle distortion of the iron(II) coordination geometry. Photomagnetic measurements of the ...
TL;DR: In this article, the binary lead-free piezoelectric ceramics with the composition of (1 − ǫ x )Bi 0.5 K 0.3 Na 0.4 O 3, x Bi 0.1 O 3 O 4, x 0.20 O 4 O 3 were synthesized by conventional mixed-oxide method.
TL;DR: In this paper, the structural and magnetic phase transitions of the ternary iron arsenides SrFe2As2 and EuFe 2As2 were studied by temperature-dependent x-ray powder diffraction and 57Fe M?ssbauer spectroscopy.
Abstract: The structural and magnetic phase transitions of the ternary iron arsenides SrFe2As2 and EuFe2As2 were studied by temperature-dependent x-ray powder diffraction and 57Fe M?ssbauer spectroscopy. Both compounds crystallize in the tetragonal ThCr2Si2-type structure at room temperature and exhibit displacive structural transitions at 203?K (SrFe2As2) and 190?K (EuFe2As2), respectively, to orthorhombic lattice symmetry in agreement with the group?subgroup relationship between I4/mmm and Fmmm (?-SrRh2As2-type). 57Fe M?ssbauer spectroscopy experiments with SrFe2As2 show full hyperfine field splitting below the phase transition temperature (8.91(1)?T at 4.2?K). Order parameters were extracted from detailed measurements of the lattice parameters and fitted to a simple power law. We find a relation between the critical exponents and the transition temperatures for AFe2As2 compounds, which shows that the transition of BaFe2As2 is indeed more continuous than the transition of SrFe2As2, but it remains second order even in the latter case.
TL;DR: In this article, the antiferroelectric-paraelectric phase transition is achieved by progressive canceling of the antiphase octahedra tilting prevailing within a - b + a - orthorhombic structure, whereas the in-phase tilting is maintained.
Abstract: NBT presents an orthorhombic to tetragonal second order phase transition that occurs near 320 °C. It corresponds to the so-called antiferroelectric-paraelectric phase transition. A model is presented in which the diffuse phase transition is achieved by the progressive canceling of the antiphase octahedra tilting prevailing within a - b + a - orthorhombic structure, whereas the in-phase tilting is maintained. This transformation gives finally rise to the a 0 a 0 c + octahedra tilting system of the tetragonal phase. Electron diffraction experiments show that a long-range ordering occurs within the tetragonal phase. It probably develops as P4 2 /mnm ordered nanoregions disseminated within a P4-/mbm disordered matrix. The order is still visible in the temperature domain of the cubic phase.
TL;DR: In this article, a transmission electron microscopy study of Na 0.5 Bi0.5 TiO 3 (NBT) crystals shows two types of ferroelectric domains characterized by interface boundaries lying in the {1, 1/0/0} C and {1, 0/1/0] C planes.
TL;DR: In this article, a composite structure of barium titanate (BaTiO3) nanoparticles was analyzed using a composite model and it was found that the size effect of nanoparticles originated from the composite structure and the crystal structure of each region did not depend on particle size.
Abstract: Nanostructures of barium titanate (BaTiO3) nanoparticles were analyzed using a composite structure model. It was found that BaTiO3 nanoparticles had a composite structure consisting of (i) inner tetragonal core, (ii) gradient lattice strain layer (GLSL), and (iii) surface cubic layer. The crystal structure of each region did not depend on particle size while the volume fraction of the GLSL and the surface cubic layer increased with decreasing the particle size. These results suggested that the size effect of BaTiO3 nanoparticles originated from the composite structure.
TL;DR: The obtained tetragonal samples were found to possess superior hydrothermal stability compared to those reported previously, which provides the possibility for systematically studying the effects of ZrO(2) phases on many catalytic reactions involving water as a reactant or product.
Abstract: Pure monoclinic (m) and tetragonal (t) zirconia nanoparticles were readily synthesized from the reaction of inorganic zirconium salts (e.g., hydrated zirconyl nitrate) and urea in water and methanol, respectively, via a facile solvothermal method. The role of the solvents was crucial in the formation of the pure ZrO(2) phases, whereas their purity was essentially insensitive to other variables, including reaction temperature, reactant concentration, pH, and zirconium salts. Water as the solvent led to the transformation of hydrous ZrO(2) precipitates initially formed with tetragonal structures to thermodynamically more stable m-ZrO(2) via the dissolution-precipitation process, whereas methanol favored the removal of water molecules from the precursors via their reaction with urea, consequently maintaining the tetragonal structures. The obtained tetragonal samples were found to possess superior hydrothermal stability compared to those reported previously, which provides the possibility for systematically studying the effects of ZrO(2) phases on many catalytic reactions involving water as a reactant or product. Using these pure m- and t-ZrO(2) phases as supports, dispersed MoO(x) catalysts were synthesized at MoO(x) surface densities of approximately 5.0 Mo/nm(2), which is close to one monolayer of coverage. Characterization by X-ray diffraction and Raman spectroscopy confirmed that the pure ZrO(2) phases remained unchanged in the presence of the MoO(x) domains and the MoO(x) domains existed preferentially as 2D polymolybdate structures. The catalysts were subsequently examined for selective methanol oxidation as a test reaction. m-ZrO(2) support led to 2-fold greater oxidation rates than for t-ZrO(2) support, reflecting the higher intrinsic reactivity of the MoO(x) domains on m-ZrO(2). This is consistent with their higher reducibility probed by temperature-programmed reduction with H(2) (H(2) TPR). These observed effects of the ZrO(2) phases provide the basis for designing catalysts with tunable redox properties and reactivity.
TL;DR: The dielectric response to infrared waves polarized along the tetragonal axis of a ferroelectric single-domain crystal of BaTiO3 was determined by time-domain THz spectroscopy and Fourier-transform infrared reflectivity techniques, showing an additional mode of the relaxation type in the THz spectral region, which accounts for the Curie-Weiss behavior of the c-axis dielectrics constant.
Abstract: The dielectric response to infrared waves polarized along the tetragonal axis of a ferroelectric single-domain crystal of ${\mathrm{BaTiO}}_{3}$ was determined by time-domain THz spectroscopy and Fourier-transform infrared reflectivity techniques. In addition to the three well-known polar lattice modes, the experiment shows an additional mode of the relaxation type in the THz spectral region, which accounts for the Curie-Weiss behavior of the $c$-axis dielectric constant. A comparison of experimental results with ab initio based effective-Hamiltonian simulations allows us to elucidate its relation to the order-disorder model of Comes, Lambert, and Guinier [Solid State Commun. 6, 715 (1968)].
TL;DR: In this paper, the structure and dielectric behavior of Ba1−−xNdxTiO3 perovskites fabricated by conventional solid state mixed method were investigated and the phase transition behavior appeared from tetragonal to cubic structure with increasing Nd-doping level by XRD results.
TL;DR: In this article, a new route was used to synthesize pure BaTiO3 (BT) nanoparticles at 140°C for 10 min under rapid reacting with stoichiometric Ba/Ti ratio.
Abstract: Hydrothermal microwave method was used as a new route to synthesize pure BaTiO3 (BT) nanoparticles at 140°C for 10 min under rapid reacting with stoichiometric Ba/Ti ratio. The crystalline products were characterized by X-ray powder diffraction (XRD) and the structure was refined by the Rietveld method from the tetragonal structure, which was supported by the Ti K-edge X-ray absorption near-edge structure (XANES). The pre-edge of Ti in the XANES spectra indicated that titanium ions are localized in a nonregular octahedron. Typical FT-Raman spectra for tetragonal BaTiO3 nanoparticles presented well-defined peaks, indicating a substantial short-range order in the system. However, a scattering peak at 810 cm−1 was attributed to the presence of lattice OH− groups, commonly found in materials obtained by hydrothermal process. Besides, the peak at 716 cm−1 can be related to eventual Ba2+ defects in the BaTiO3 lattice. BaTiO3 (BT) nanoparticles presented spherical morphology with a non-uniform distribution of pa...
TL;DR: Gao et al. as discussed by the authors presented a Vickers hardness expression for the covalency-dominant crystals such as transition-metal carbides and nitrides, and calculated hardness using their expression has a good agreement with the experimental values for known monocarbides, mononitrides of transition metals and cubic Zr3N4 with Th3P4 structure.
Abstract: Based on the detailed analysis of chemical bonds, we present a Vickers hardness expression for the covalency-dominant crystals such as transition-metal carbides and nitrides. Hardness is dependent not only on bond length, bond density, and ionicity of bond [F. M. Gao et al., Phys. Rev. Lett. 91, 015502 (2003)] but also on the metallicity of bond and orbital form in the crystal structure of a compound, and all of these parameters can be determined by first-principles calculations. The calculated hardness using our expression has a good agreement with the experimental values for known monocarbides, mononitrides of transition metals, and cubic Zr3N4 with Th3P4 structure. In addition, we have predicted the Vickers hardness of the recently predicted tetragonal BC3 and tetragonal B2CN, and the recently synthesized pyrite PtN2 and marcasite OsN2. Our method offers one useful technique to search for superhard materials in transition-metal carbides and nitrides.
TL;DR: LiNbO3-doped lead-free piezoceramics were prepared by conventional sintering at a temperature as low as 950°C using excess Na2O additives as discussed by the authors.
Abstract: LiNbO3-doped (Na, K)NbO3 lead-free piezoceramics were prepared by conventional sintering at a temperature as low as 950 °C using excess Na2O additives. The crystal structure changed from orthorhombic to tetragonal with increasing LiNbO3 amount since the phase transition temperature TO−T shifted downward. In the region of two-phase coexistence, enhanced piezoelectric constant d33 (280 pC/N) and electromechanical coupling factor kp (48.3%) with a high Curie temperature TC (475 °C) were obtained in the nominal composition 0.92(Na0.535K0.48)NbO3–0.08LiNbO3. Our results open up the way to low-temperature sintering of (Na, K)NbO3-based lead-free piezoceramics with high performance.
TL;DR: In this article, the influence of dopants on the dielectric properties of the high-k materials HfO2 and ZrO2 was elucidated based on first-principles calculations.
Abstract: Based on first-principles calculations, we elucidate the influence of dopants on the dielectric properties of the high-k materials HfO2 and ZrO2. Our calculations demonstrate that by doping the tetragonal phase can become energetically more favorable than the monoclinic phase present at ambient conditions. The stabilization of the tetragonal phase increases the dielectric constant significantly. A series of dopants was investigated to understand the efficiency and the mechanism of the stabilization process. The calculations reveal that at a moderate doping level (∼12%) only some of the dopants stabilize the tetragonal phase and that Si is the most efficient stabilizer atom.
TL;DR: The local structure of the Sc(BH4)4(-) complex is refined as a distorted form of the theoretical structure, and the Li ions are found to be disordered along the z axis.
Abstract: LiSc(BH4)4 has been prepared by ball milling of LiBH4 and ScCl3. Vibrational spectroscopy indicates the presence of discrete Sc(BH4)4(-) ions. DFT calculations of this isolated complex ion confirm that it is a stable complex, and the calculated vibrational spectra agree well with the experimental ones. The four BH4(-) groups are oriented with a tilted plane of three hydrogen atoms directed to the central Sc ion, resulting in a global 8 + 4 coordination. The crystal structure obtained by high-resolution synchrotron powder diffraction reveals a tetragonal unit cell with a = 6.076 A and c = 12.034 A (space group P-42c). The local structure of the Sc(BH4)4(-) complex is refined as a distorted form of the theoretical structure. The Li ions are found to be disordered along the z axis.
TL;DR: In this article, a morphotropic phase boundary between tetragonal and rhombohedral perovskite phases was observed at x≈0.9, for compositions rich in BaTiO3.
Abstract: (1−x)Bi(Zn1/2Ti1/2)O3−xBaTiO3 polycrystalline ceramics were obtained via solid state processing techniques. The perovskite structure was found to be stable for compositions containing x=0.66 or greater. Based on x-ray diffraction data, a morphotropic phase boundary between tetragonal and rhombohedral perovskite phases was observed at x≈0.9. For compositions rich in BaTiO3, the symmetry of the perovskite phase was tetragonal but with increased Bi(Zn1/2Ti1/2)O3 content, the rhombohedral phase appeared. Dielectric characterization revealed that as Bi(Zn1/2Ti1/2)O3 content increased, the transition temperature decreased and the transition peak became very diffused. A comparison of the dielectric behavior with other Bi(Zn1/2Ti1/2)O3-based solid solutions is also discussed.
TL;DR: In this paper, the structural and magnetic phase transitions of the ternary iron arsenides SrFe2As2 and EuFe 2As2 were studied by temperature-dependent x-ray powder diffraction and 57-Fe Moessbauer spectroscopy.
Abstract: The structural and magnetic phase transitions of the ternary iron arsenides SrFe2As2 and EuFe2As2 were studied by temperature-dependent x-ray powder diffraction and 57-Fe Moessbauer spectroscopy. Both compounds crystallize in the tetragonal ThCr2Si2-type structure at room temperature and exhibit displacive structural transitions at 203 K (SrFe2As2) or 190 K (EuFe2As2) to orthorhombic lattice symmetry in agreement with the group-subgroup relationship between I4/mmm and Fmmm. 57-Fe Moessbauer spectroscopy experiments with SrFe2 As2 show full hyperfine field splitting below the phase transition temperature (8.91(1) T at 4.2 K). Order parameters were extracted from detailed measurements of the lattice parameters and fitted to a simple power law. We find a relation between the critical exponents and the transition temperatures for AFe2As2 compounds, which shows that the transition of BaFe2As2 is indeed more continuous than the transition of SrFe2As2 but it remains second order even in the latter case.
TL;DR: In this paper, the effect of substitution for Ba 2+ on the crystal chemistry, oxygen content, thermal expansion, electrical conductivity, and catalytic activity for oxygen reduction reaction (ORR) of the double perovskite oxides GdBa 1-x Sr x Co 2 O 5+δ has been investigated for 0 ≤ x ≤ 1.
Abstract: The effect of Sr2 + substitution for Ba 2+ on the crystal chemistry, oxygen content, thermal expansion, electrical conductivity, and catalytic activity for oxygen reduction reaction (ORR) of the double perovskite oxides GdBa 1-x Sr x Co 2 O 5+δ has been investigated for 0 ≤ x ≤ 1.0. The GdBa 1-x SrCo 2 O 5+δ system exhibits a structural change from orthorhombic (x = 0) to tetragonal (0.2 as x ≤ 0.6) to orthorhombic (x = 1) with increasing Sr content. The difference in ionic radii between (Ba 1-x Sr x ) 2+ and Gd 3+ plays a dominant role in determining the oxygen-content value in GdBa 1-x Sr X Co 2 O 5+δ , and the oxygen content and the oxidation state of cobalt increase with increasing Sr content. The electrical conductivity of the GdBa 1-x Sr x Co 2 O 5+δ system increases with Sr content due to an increasing oxygen content and a straightening of the O-Co-O bonds as evidenced by the structural change from orthorhombic to tetragonal. Sr substitution also improves the chemical stability of the GdBa 1-x Sr x Co 2 O 5+δ cathodes in contact with the Ce 0.9 Gd 0.1 O 1.95 and La 0.8 Sr 0.2 Ga 0.8 Mg 0.2 O 2.8 electrolytes at 1100°C. The x = 0.2 and 0.6 samples with a tetragonal structure exhibit higher catalytic activity for ORR in single-cell solid oxide fuel cell than the x = 0 and 1.0 samples with an orthorhombic structure.
TL;DR: In this paper, multiferroic BiFeO3 epitaxial films with thicknesses ranging from 40to960nm were grown by pulsed laser deposition on SrTiO3 (001) substrates with SrRuO3 bottom electrodes.
Abstract: Multiferroic BiFeO3 epitaxial films with thicknesses ranging from 40to960nm were grown by pulsed laser deposition on SrTiO3 (001) substrates with SrRuO3 bottom electrodes. X-ray characterization shows that the structure evolves from angularly distorted tetragonal with c∕a≈1.04 to more bulklike distorted rhombohedral (c∕a≈1.01) as the strain relaxes with increasing thickness. Despite this significant structural evolution, the ferroelectric polarization along the body diagonal of the distorted pseudocubic unit cells, as calculated from measurements along the normal direction, barely changes.