Showing papers on "Tetragonal crystal system published in 2011"

Ferroelectricity in hafnium oxide thin films

Abstract: We report that crystalline phases with ferroelectric behavior can be formed in thin films of SiO2 doped hafnium oxide. Films with a thickness of 10 nm and with less than 4 mol. % of SiO2 crystallize in a monoclinic/tetragonal phase mixture. We observed that the formation of the monoclinic phase is inhibited if crystallization occurs under mechanical encapsulation and an orthorhombic phase is obtained. This phase shows a distinct piezoelectric response, while polarization measurements exhibit a remanent polarization above 10 μC/cm2 at a coercive field of 1 MV/cm, suggesting that this phase is ferroelectric. Ferroelectric hafnium oxide is ideally suited for ferroelectric field effect transistors and capacitors due to its excellent compatibility to silicon technology.

Structure and dynamics of the fast lithium ion conductor “Li7La3Zr2O12”

Abstract: The solid lithium-ion electrolyte “Li7La3Zr2O12” (LLZO) with a garnet-type structure has been prepared in the cubic and tetragonal modification following conventional ceramic syntheses routes. Without aluminium doping tetragonal LLZO was obtained, which shows a two orders of magnitude lower room temperature conductivity than the cubic modification. Small concentrations of Al in the order of 1 wt% were sufficient to stabilize the cubic phase, which is known as a fast lithium-ion conductor. The structure and ion dynamics of Al-doped cubic LLZO were studied by impedance spectroscopy, dc conductivity measurements, 6Li and 7Li NMR, XRD, neutron powder diffraction, and TEM precession electron diffraction. From the results we conclude that aluminium is incorporated in the garnet lattice on the tetrahedral 24dLi site, thus stabilizing the cubic LLZO modification. Simulations based on diffraction data show that even at the low temperature of 4 K the Li ions are blurred over various crystallographic sites. This strong Li ion disorder in cubic Al-stabilized LLZO contributes to the high conductivity observed. The Li jump rates and the activation energy probed by NMR are in very good agreement with the transport parameters obtained from electrical conductivity measurements. The activation energy Ea characterizing long-range ion transport in the Al-stabilized cubic LLZO amounts to 0.34 eV. Total electric conductivities determined by ac impedance and a four point dc technique also agree very well and range from 1 × 10−4 Scm−1 to 4 × 10−4 Scm−1 depending on the Al content of the samples. The room temperature conductivity of Al-free tetragonal LLZO is about two orders of magnitude lower (2 × 10−6 Scm−1, Ea = 0.49 eV activation energy). The electronic partial conductivity of cubic LLZO was measured using the Hebb–Wagner polarization technique. The electronic transference number te− is of the order of 10−7. Thus, cubic LLZO is an almost exclusive lithium ion conductor at ambient temperature.

Phase transitions in ferroelectric silicon doped hafnium oxide

Abstract: We investigated phase transitions in ferroelectric silicon doped hafnium oxide (FE-Si:HfO2) by temperature dependent polarization and x-ray diffraction measurements. If heated under mechanical confinement, the orthorhombic ferroelectric phase reversibly transforms into a phase with antiferroelectric behavior. Without confinement, a transformation into a monoclinic/tetragonal phase mixture is observed during cooling. These results suggest the existence of a common higher symmetry parent phase to the orthorhombic and monoclinic phases, while transformation between these phases appears to be inhibited by an energy barrier.

Morphology-Controlled Synthesis and Novel Microwave Absorption Properties of Hollow Urchinlike α-MnO2 Nanostructures

Abstract: Three types of hollow urchinlike α-MnO2 nanostructures, namely, columnar nanorod clusters, tetragonal nanotube clusters, and tetragonal nanorod clusters, have been synthesized through a facile hydrothermal method The microstructure and morphologies of the resulting materials were investigated by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and selected-area electron diffraction, and the microwave absorption properties of these nanostructures were investigated in terms of complex permittivity and permeability The results indicate an obvious magnetic loss in the manganese oxide/paraffin wax composites The tetragonal nanorod clusters exhibit enhanced microwave absorption properties compared with columnar nanorod clusters and tetragonal nanotube clusters, which result from proper electromagnetic impedance matching These urchinlike manganese oxide nanostructures are considered to have great potential applications as microwave absorbents

Large piezoelectric effect in Pb-free Ba(Ti,Sn)O3-x(Ba,Ca)TiO3 ceramics

Abstract: We designed a Pb-free pseudo-binary system, Ba(Sn012Ti088)O3-x(Ba07Ca03)O3 (BTS-xBCT), characterized by a phase boundary starting from a tricritical triple point of a paraelectric cubic phase, ferroelectric rhombohedral, and tetragonal phases The optimal composition BTS-30BCT exhibits a high piezoelectric coefficient d33 � 530 pC/N at room temperature In view of the recent report of high piezoelectricity in another Pb-free system BZT-BCT (Liu and Ren, Phys Rev Lett 103, 257602 (2009)), which possesses a similar tricritical triple point in the phase diagram, it seems that forming a suitable phase boundary starting from a tricritical triple point could be an effective way to develop high-performance Pb-free piezoelectrics V C 2011 American Institute of Physics

Microstructure basis for strong piezoelectricity in Pb-free Ba(Zr0.2Ti0.8)O3-(Ba0.7Ca0.3)TiO3 ceramics

Abstract: In this letter, we use transmission electron microscopy to study the microstructure feature of recently reported Pb-free piezoceramic Ba(Zr0.2Ti0.8)O3-x(Ba0.7Ca0.3)TiO3 across its piezoelectricity-optimal morphotropic phase boundary (MPB) by varying composition and temperature, respectively. The domain structure evolutions during such processes show that in MPB regime, the domains become miniaturized down to nanometer size with a domain hierarchy, which coincides with the d33-maximum region. Further convergent beam electron diffraction measurement shows that rhombohedral and tetragonal crystal symmetries coexist among the miniaturized domains. Strong piezoelectricity reported in such a system is due to easy polarization rotation between the coexisting nano-scale tetragonal and rhombohedral domains.

Stripe order in superconducting La 2-x Ba x CuO 4 (0.095≤x≤0.155)

Abstract: The correlations between stripe order, superconductivity, and crystal structure in La{sub 2-x}Ba{sub x}CuO{sub 4} single crystals have been studied by means of x-ray and neutron diffraction as well as static magnetization measurements The derived phase diagram shows that charge stripe order (CO) coexists with bulk superconductivity in a broad range of doping around x=1/8, although the CO order parameter and correlation length fall off quickly for x {ne} 1/8 Except for x=0155, the onset of CO always coincides with the transition between the orthorhombic and the tetragonal or less orthorhombic low-temperature structures The CO transition evolves from a sharp drop at low x to a more gradual transition at higher x, eventually falling below the structural phase boundary for optimum doping With respect to the interlayer CO correlations, we find no qualitative change of the stripe stacking order as a function of doping, and in-plane and out-of-plane correlations disappear simultaneously at the transition Similarly to the CO, the spin stripe order (SO) is also most pronounced at x=1/8 Truly static SO sets in below the CO and coincides with the first appearance of in-plane superconducting correlations at temperatures significantly above the bulk transition to superconductivity (SC) Indications that bulk SCmore » causes a reduction of the spin or charge stripe order could not be identified We argue that CO is the dominant order that is compatible with SC pairing but competes with SC phase coherence Comparing our results with data from the literature, we find good agreement if all results are plotted as a function of x' instead of the nominal x, where x' represents an estimate of the actual Ba content, extracted from the doping dependence of the structural transition between the orthorhombic phase and the tetragonal high-temperature phase« less

Stress-induced R − M A − M C − T symmetry changes in BiFeO 3 films

Abstract: Recent reports on epitaxial BiFeO{sub 3} films show that the crystal structure changes from nearly rhombohedral (''R like'') to nearly tetragonal (''T like'') at strains exceeding {approx}-4.5%, with the T-like structure being characterized by a highly enhanced c/a ratio. While both the R-like and the T-like phases are monoclinic, our detailed x-ray diffraction results reveal a symmetry change from M{sub A} and M{sub C} type, respectively, at this R-like-to-T-like transition. Therefore, the ferroelectric polarization is confined to different (pseudocubic) planes in the two phases. By applying additional strain or by modifying the unit-cell volume of the film by substituting Ba for Bi, the monoclinic distortion in the T-like M{sub C} phase is reduced, i.e., the system approaches a true tetragonal symmetry. Therefore, in going from bulk to highly strained films, a phase sequence of rhombohedral (R)-to-monoclinic (R-like M{sub A})-to-monoclinic (T-like M{sub C})-to-tetragonal (T) is observed. This sequence is otherwise seen only near morphotropic phase boundaries in lead-based solid-solution perovskites (i.e., near a compositionally induced phase instability), where it can be controlled by electric field, temperature, or composition. Our results now show that this evolution can occur in a lead-free, stoichiometric material and can be induced by stress alone.

Facile synthesis of silver chalcogenide (Ag2E; E=Se, S, Te) semiconductor nanocrystals.

Abstract: A general, one-pot, single-step method for producing colloidal silver chalcogenide (Ag2E; E = Se, S, Te) nanocrystals is presented, with an emphasis on Ag2Se. The method avoids exotic chemicals, high temperatures, and high pressures and requires only a few minutes of reaction time. While Ag2S and Ag2Te are formed in their low-temperature monoclinic phases, Ag2Se is obtained in a metastable tetragonal phase not observed in the bulk.

Unprecedented anisotropic metallic state in undoped iron arsenide BaFe2As2 revealed by optical spectroscopy

Abstract: An ordered phase showing remarkable electronic anisotropy in proximity to the superconducting phase is now a hot issue in the field of high-transition-temperature superconductivity. As in the case of copper oxides, superconductivity in iron arsenides competes or coexists with such an ordered phase. Undoped and underdoped iron arsenides have a magnetostructural ordered phase exhibiting stripe-like antiferromagnetic spin order accompanied by an orthorhombic lattice distortion; both the spin order and lattice distortion break the tetragonal symmetry of crystals of these compounds. In this ordered state, anisotropy of in-plane electrical resistivity is anomalous and difficult to attribute simply to the spin order and/or the lattice distortion. Here, we present the anisotropic optical spectra measured on detwinned BaFe2As2 crystals with light polarization parallel to the Fe planes. Pronounced anisotropy is observed in the spectra, persisting up to an unexpectedly high photon energy of about 2 eV. Such anisotropy arises from an anisotropic energy gap opening below and slightly above the onset of the order. Detailed analysis of the optical spectra reveals an unprecedented electronic state in the ordered phase.

Transparent Tetragonal Yttria-Stabilized Zirconia Ceramics: Influence of Scattering Caused by Birefringence

Abstract: The correlation between grain size, optical birefringence, and transparency is discussed for tetragonal zirconia (ZrO2) ceramics using the Mie, Rayleigh, and Rayleigh–Gans–Debye scattering models. Our results demonstrate that at the degree of mean birefringence in the range (0.03–0.04) expected for tetragonal ZrO2, only the Mie theory provides reasonable results. At small particle size (o50 nm) the more straightforward Rayleigh approximation correlates with the Mie model. A real in-line transmission of B50% at visible light and 1 mm thickness is expected at a mean grain size o40 nm and B70% at a mean grain size o20 nm. At an infrared (IR) wavelength of 5 lm there should not be any scattering caused by birefringence for grain sizes o200 nm. Our simulations were validated with experimental data for tetragonal ZrO2 (3 mol% Y2O3) ceramics made from a powder with an initial particle size of B10 nm by sintering in air and using hot-isostatic pressing. The maximum in-line transmission of about 77% was observed at IR wavelengths of 3–5 lm.

Structural diversity and electronic properties of Cu2SnX3 (X = S, Se): A first-principles investigation

Abstract: The ternary semiconductors Cu(2)SnX(3) (X = S, Se) are found frequently as secondary phases in synthesized Cu(2)ZnSnS(4) and Cu(2)ZnSnSe(4) samples, but previous reports on their crystal structures and electronic band gaps are conflicting. Here we report their structural and electronic properties as calculated using a first-principles approach. We find that (i) the diverse range of crystal structures such as the monoclinic, cubic, and tetragonal phases can all be derived from the zinc-blende structure with tetrahedral coordination. (ii) The energy stability of different structures is determined primarily by the local cation coordination around anions, which can be explained by a generalized valence octet rule. Structures with only Cu(3)Sn and Cu(2)Sn(2) clusters around the anions have low and nearly degenerate energies, which makes Cu and Sn partially disordered in the cation sublattice. (iii) The direct band gaps of the low-energy compounds Cu(2)SnS(3) and Cu(2)SnSe(3) should be in the range of 0.8-0.9 and 0.4 eV, respectively, and are weakly dependent on the long-range structural order. A direct analogy is drawn with the ordered vacancy compounds found in the Cu(In,Ga)Se(2) solar-cell absorbers.

Nematic spin fluid in the tetragonal phase of BaFe2As2

Abstract: We use inelastic neutron scattering to study spin waves below and above T{sub N} in iron-arsenide BaFe{sub 2}As{sub 2}. In the low-temperature orthorhombic phase, we find highly anisotropic spin waves with a large damping along the antiferromagnetic a-axis direction. On warming the system to the paramagnetic tetragonal phase, the low-energy spin waves evolve into quasi-elastic excitations, while the anisotropic spin excitations near the zone boundary persist. These results strongly suggest the presence of a spin nematic fluid in the tetragonal phase of BaFe{sub 2}As{sub 2}, which may cause the electronic and orbital anisotropy observed in these materials.

Structure, Dielectric Properties and Temperature Stability of BaTiO3–Bi(Mg1/2Ti1/2)O3 Perovskite Solid Solutions

Abstract: (1−x)BaTiO3–xBi(Mg1/2Ti1/2)O3 [(1−x)BT–xBMT] polycrystalline ceramics were obtained via solid-state processing techniques. The solubility limit for (1−x)BT–xBMT was determined to be about x=0.07. A systematic structural change from the ferroelectric tetragonal phase to pseudocubic phase was observed at about x≥0.05 at room temperature. Dielectric measurements revealed a gradual change from normal ferroelectric of pure BaTiO3 to highly dispersive relaxor-like characteristics in the solid solution with 30–60 mol% Bi(Mg1/2Ti1/2)O3, showing low-temperature coefficients of capacitance over a wide temperature range. The properties of Nb2O5-doped 0.85BT–0.15BMT ceramics were investigated to better understand the formation mechanism of core-shell structure, for further improving the temperature stability of the dielectric behavior.

Preparation and structure refinement of Eu3+ doped CaMoO4 nanoparticles

Abstract: The nanoparticles of CaMoO4 : Eu3+ (Eu3+ = 0, 1, 3, 5, 7, 10 at. %) are prepared at low temperature (150 °C for 3 h) using urea hydrolysis in ethylene glycol. These are characterized by X-ray diffraction (XRD), infrared spectroscopy (IR) and transmission electron microscopy (TEM). From XRD study, it was found that the solubility limit of Eu3+ ions at the Ca2+ sites is up to 3 at. % and above this, phase segregation occurs. In combination with Rietveld analysis, its crystal structure was found to be tetragonal phase (space groupI41/a (88) and Z = 4 (number of CaMoO4 formula units per unit cell). Unit cell parameters and bond distances are calculated. The average crystallite sizes of as-prepared, 500 and 900 °C heated samples are found to be 20, 35 and 70 nm, respectively. The lattice strain is found to be 0.003–0.005. From IR study, the bands at 820 and 441 cm−1 are assigned to asymmetric stretching and bending vibrations of the MoO42− tetrahedron, respectively. From TEM study, the shape of particle was found to be spherical. The high resolution TEM suggests a change in orientation of the crystal on annealing up to 900 °C.

Optical and photocatalytic properties of heavily F(-)-doped SnO2 nanocrystals by a novel single-source precursor approach.

Abstract: Heavily F-doped SnO2 nanocrystals were successfully prepared by a novel synthetic approach involving low-temperature oxidation of a Sn2+-containing fluoride complex KSnF3 as the single-source precursor with H2O2. The F-doped SnO2 powder was characterized by powder X-ray diffraction, TG-MS, BET surface area, diffuse reflectance spectroscopy, XPS, PL, FTIR spectroscopy, Raman spectroscopy, EPR spectroscopy, SEM, and TEM. Broadening of the diffracted peaks, signifying the low crystallite size of the products, was quite evident in the powder X-ray diffraction pattern of SnO2 obtained from KSnF3. It was indexed in a tetragonal unit cell with lattice constants a = 4.7106 (1) A and c = 3.1970 (1) A. Agglomeration of particles, with an average diameter of 5–7 nm, was observed in the TEM images whose spotwise EDX analysis indicated the presence of fluoride ions. In the core level high-resolution F 1s spectrum, the peak observed at 685.08 eV was fitted by the Gaussian profile yielding the fluoride ion concentration...

Distinctive finite size effects on the phase diagram and metal–insulator transitions of tungsten-doped vanadium(IV) oxide

Abstract: The influence of finite size in altering the phase stabilities of strongly correlated materials gives rise to the interesting prospect of achieving additional tunability of solid–solid phase transitions such as those involved in metal–insulator switching, ferroelectricity, and superconductivity. We note here some distinctive finite size effects on the relative phase stabilities of insulating (monoclinic) and metallic (tetragonal) phases of solid-solution WxV1−xO2. Ensemble differential scanning calorimetry and individual nanobelt electrical transport measurements suggest a pronounced hysteresis between metal → insulator and insulator → metal phase transformations. Both transitions are depressed to lower critical temperatures upon the incorporation of substitutional tungsten dopants but the impact on the former transition seems far more prominent. In general, the depression in the critical temperatures upon tungsten doping far exceeds corresponding values for bulk WxV1−xO2 of the same composition. Notably, the depression in phase transition temperature saturates at a relatively low dopant concentration in the nanobelts, thought to be associated with the specific sites occupied by the tungsten substitutional dopants in these structures. The marked deviations from bulk behavior are rationalized in terms of a percolative model of the phase transition taking into account the nucleation of locally tetragonal domains and enhanced carrier delocalization that accompany W6+ doping in the WxV1−xO2 nanobelts.

Quaternary CuIn(S1−xSex)2 Nanocrystals: Facile Heating-Up Synthesis, Band Gap Tuning, and Gram-Scale Production

Abstract: Tetragonal chalcopyrite CuIn-(S1−xSex)2 (0 ≤ x ≤ 1) nanocrystals were synthesized by reacting a mixture of CuCl, InCl3, S, and Se in the presence of oleylamine at 265 °C. The S/Se composition ratio in the CuIn(S1−xSex)2 could be tuned across the entire composition range of x from 0 to 1 by modulating the S/Se reactant mole ratio. The tetragonal lattice constants, that is, a and c, increase linearly with the increase of Se content, following Vegard’s law. The band gap energies of CuIn(S1−xSex)2 nanocrystals could be tuned in the range between 0.98 and 1.46 eV and change nonlinearly with respect to x, deriving a bowing parameter of 0.17 eV. In addition, the method developed in this study was scalable to achieve gram-scale production of stoichiometry-controlled CuIn(S1−xSex)2 and CuIn1−xGaxSe2 nanocrystals.

Surface structure sensitivity of manganese oxides for low-temperature selective catalytic reduction of NO with NH3

Abstract: Hollandite-type manganese oxides with K+ or H+ cations in the tunnels (K-Hol or H-Hol) were successfully synthesized and investigated in low-temperature selective catalytic reduction of NO by NH3. The results of the catalytic tests revealed that both K-Hol and H-Hol with almost same catalytic activities had much higher reaction rates than β-MnO2 under the same conditions, and even at a high gas hourly space velocity of 160,000 h−1, K-Hol obtained more than 90% conversions of NO in a wide temperature window of 100–300 °C. The high resolution transmission electron microscopy observations showed that both K-Hol and β-MnO2 were tetragonal prism-shaped nanorods with same exposed {1 1 0} planes, and the atoms in the {1 1 0} planes of K-Hol arranged to form semitunnel structures, while the {1 1 0} planes of β-MnO2 were relatively smooth surfaces. The temperature-programmed reduction by H2 and thermal gravimetric analyses indicated that active surface lattice oxygen atoms of K-Hol were around 1.6% with respect to total lattice oxygen atoms. The transition reactions of NH3 demonstrated that K-Hol with special semitunnel structured surface and active surface lattice oxygen showed much stronger ability to efficiently adsorb and activate NH3 molecules than β-MnO2. Hence, both efficient semitunnel structured external surfaces and high active surface lattice oxygen atoms predominantly accounted for the high catalytic activities.

Character of the structural and magnetic phase transitions in the parent and electron-doped BaFe2As2 compounds

Abstract: We present a combined high-resolution x-ray diffraction and x-ray resonant magnetic scattering (XRMS) study of as-grown BaFe2As2. The structural/magnetic transitions must be described as a two-step process. At T_S = 134.5 K we observe the onset of a second-order structural transition from the high-temperature paramagnetic tetragonal structure to a paramagnetic orthorhombic phase, followed by a discontinuous step in the structural order parameter that is coincident with a first-order antiferromagnetic (AFM) transition at T_N = 133.75 K. These data, together with detailed high-resolution x-ray studies of the structural transition in lightly doped Ba(Fe{1-x}Co{x})2As2 and Ba(Fe{1-x}Rh{x})2As2 compounds, show that the structural and AFM transitions do, in fact, occur at slightly different temperatures in the parent BaFe2As2 compound, and evolve towards split secondorder transitions as the doping concentration is increased. We estimate the composition for the tricritical point for Co-doping and employ a mean-field approach to show that our measurements can be explained by the inclusion of an anharmonic term in the elastic free energy and magneto-elastic coupling in the form of an emergent Ising-nematic degree of freedom.

Structural transitions and enhanced ferroelectricity in Ca and Mn co-doped BiFeO3 thin films

Abstract: Ca and Mn co-doped BiFeO3 (BFO) thin films were fabricated on (111) Pt/Ti/SiO2/Si substrates via a simple solution approach. Enhanced ferroelectric properties were obtained in these Ca and Mn co-doped BFO films, e.g., large remnant polarization value (Pr ∼ 89 μC/cm2) and large remnant polarization to saturated polarization ratio (Pr/Ps ∼ 0.84) for Bi0.90Ca0.10Fe0.90Mn0.10O3 film. X-ray diffraction and Raman spectra of these films showed that B-site Mn doping causes substantial structural transition toward orthorhombic phase and A-site Ca doping facilitates an evolution to tetragonal phase with higher crystal symmetry. The structural transitions are in favor of enhanced ferroelectric properties in the BFO films. The approach of A and B site co-doping is proved to be effective in enhancing ferroelectric performance in multiferroic BFO films.

Evidence for (Bi,Pb)–O Covalency in the High TC Ferroelectric PbTiO3–BiFeO3 with Large Tetragonality

Abstract: Electron-density analyses of tetragonal and cubic 0.4 PbTiO3-0.6 BiFeO3 (PTBF) have indicated that the (Bi,Pb)-O bond in tetragonal PTBF exhibits strong covalency compared with that in cubic phase. Compared with Pb−O, the Bi−O exhibits stronger covalency, which would be a key factor of extremely large tetragonality and high TC of PTBF.

Thickness-dependent structural phase transition of strained SrRuO 3 ultrathin films: The role of octahedral tilt

Abstract: We grew epitaxial SrRuO${}_{3}$ (SRO) films on SrTiO${}_{3}$ (STO) (001) substrates with SRO layer thicknesses ($t$) between 10 and 200 pseudocubic unit cells (uc). Using the square net of the cubic STO surface, we were able to epi-stabilize the tetragonal SRO phase at room temperature for ultrathin films with $t$ \ensuremath{\leqslant} 17 uc. On the other hand thicker films with $t$ \ensuremath{\geqslant} 19 uc have an orthorhombic crystal structure similar to that of bulk SRO at room temperature. With increasing temperature, the orthorhombic films undergo a structural transition to the tetragonal phase at ${T}_{\mathrm{OT}}$. The value of ${T}_{\mathrm{OT}}$ and the orthorhombicity factor at room temperature are reduced with decreasing film thickness. We also observed half-order Bragg reflections, indicating that the tetragonal structure arises from the suppression of the tilt angle of RuO${}_{6}$ octahedra. The observed critical thickness around ${t}_{c}$ \ensuremath{\sim} 18 uc is much larger than the recent theoretical prediction (i.e., less than 2 uc) [J. He, A. Borisevich, S. V. Kalinin, S. J. Pennycook, and S. T. Pantelides, Phys. Rev. Lett. 105, 227203 (2010)]. This work thus demonstrates that the lattice symmetry mismatch at the interface plays an important role in determining the structural properties of perovskite films.