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1.
Nat Mater ; 14(11): 1142-9, 2015 Nov.
Artículo en Inglés | MEDLINE | ID: mdl-26322717

RESUMEN

Self-assembled nanostructures with periodic phase separation hold great promise for creating two- and three-dimensional superlattices with extraordinary physical properties. Understanding the mechanism(s) driving the formation of such superlattices demands an understanding of their underlying atomic structure. However, the nanoscale structural fluctuations intrinsic to these superlattices pose a new challenge for structure determination methods. Here we develop an optimized atomic-level imaging condition to measure TiO6 octahedral tilt angles, unit-cell-by-unit-cell, in perovskite-based Li(0.5-3x)Nd(0.5+x)TiO3, and thereby determine the mathematical formula governing this nanoscale superstructure. We obtain a direct real-space correlation of the octahedral tilt modulation with the superstructure geometry and lattice-parameter variations. This reveals a composition-dependent, self-ordered octahedral superlattice. Amazingly, we observe a reversible annihilation/reconstruction of the octahedral superlattice correlated with the delithiation/lithiation process in this promising Li-ion conductor. This approach to quantify local octahedral tilt and correlate it with strain can be applied to characterize complex octahedral behaviours in other advanced oxide systems.

2.
Phys Chem Chem Phys ; 17(33): 21547-54, 2015 Sep 07.
Artículo en Inglés | MEDLINE | ID: mdl-26220067

RESUMEN

For structures that can be treated as networks of rigid, corner-connected polyhedra, the dominant distortion modes can be described by so-called rigid unit modes that are close to zero frequency. This type of behaviour is common in zeolitic/zeotypic materials such as the AlPO4 family of compounds and has been suggested by some authors to play a significant role in molecular diffusion within the pores of such compounds. We explore the energy and temperature dependence of these modes in AlPO4-5 using inelastic neutron scattering and heat capacity measurements. Ab initio based computational modelling is also used to assign the observed dynamic behaviour to rigid unit modes. We observe that these rigid unit modes persist down to very low temperatures and show no signs of freezing out.

3.
Nat Mater ; 12(9): 821-6, 2013 Sep.
Artículo en Inglés | MEDLINE | ID: mdl-23812129

RESUMEN

The immense potential of colossal permittivity (CP) materials for use in modern microelectronics as well as for high-energy-density storage applications has propelled much recent research and development. Despite the discovery of several new classes of CP materials, the development of such materials with the required high performance is still a highly challenging task. Here, we propose a new electron-pinned, defect-dipole route to ideal CP behaviour, where hopping electrons are localized by designated lattice defect states to generate giant defect-dipoles and result in high-performance CP materials. We present a concrete example, (Nb+In) co-doped TiO2 rutile, that exhibits a largely temperature- and frequency-independent colossal permittivity (> 10(4)) as well as a low dielectric loss (mostly < 0.05) over a very broad temperature range from 80 to 450 K. A systematic defect analysis coupled with density functional theory modelling suggests that 'triangular' In2(3+)Vo(••)Ti(3+) and 'diamond' shaped Nb2(5+)Ti(3+)A(Ti) (A = Ti(3+)/In(3+)/Ti(4+)) defect complexes are strongly correlated, giving rise to large defect-dipole clusters containing highly localized electrons that are together responsible for the excellent CP properties observed in co-doped TiO2. This combined experimental and theoretical work opens up a promising feasible route to the systematic development of new high-performance CP materials via defect engineering.


Asunto(s)
Modelos Teóricos , Titanio , Conductividad Eléctrica , Indio/química , Ensayo de Materiales , Temperatura , Difracción de Rayos X
4.
J Am Chem Soc ; 135(17): 6477-84, 2013 May 01.
Artículo en Inglés | MEDLINE | ID: mdl-23570580

RESUMEN

The high-temperature cubic form of bismuth oxide, δ-Bi2O3, is the best intermediate-temperature oxide-ionic conductor known. The most elegant way of stabilizing δ-Bi2O3 to room temperature, while preserving a large part of its conductivity, is by doping with higher valent transition metals to create wide solid-solutions fields with exceedingly rare and complex (3 + 3)-dimensional incommensurately modulated "hypercubic" structures. These materials remain poorly understood because no such structure has ever been quantitatively solved and refined, due to both the complexity of the problem and a lack of adequate experimental data. We have addressed this by growing a large (centimeter scale) crystal using a novel refluxing floating-zone method, collecting high-quality single-crystal neutron diffraction data, and treating its structure together with X-ray diffraction data within the superspace symmetry formalism. The structure can be understood as an "inflated" pyrochlore, in which corner-connected NbO6 octahedral chains move smoothly apart to accommodate the solid solution. While some oxide vacancies are ordered into these chains, the rest are distributed throughout a continuous three-dimensional network of wide δ-Bi2O3-like channels, explaining the high oxide-ionic conductivity compared to commensurately modulated phases in the same pseudobinary system.

5.
Nat Mater ; 9(7): 559-64, 2010 Jul.
Artículo en Inglés | MEDLINE | ID: mdl-20526323

RESUMEN

The search for active semiconductor photocatalysts that directly split water under visible-light irradiation remains one of the most challenging tasks for solar-energy utilization. Over the past 30 years, the search for such materials has focused mainly on metal-ion substitution as in In(1-x)Ni(x)TaO(4) and (V-,Fe- or Mn-)TiO(2) (refs 7,8), non-metal-ion substitution as in TiO(2-x)N(x) and Sm(2)Ti(2)O(5)S(2) (refs 9,10) or solid-solution fabrication as in (Ga(1-x)Zn(x))(N(1-x)O(x)) and ZnS-CuInS(2)-AgInS(2) (refs 11,12). Here we report a new use of Ag(3)PO(4) semiconductor, which can harness visible light to oxidize water as well as decompose organic contaminants in aqueous solution. This suggests its potential as a photofunctional material for both water splitting and waste-water cleaning. More generally, it suggests the incorporation of p block elements and alkali or alkaline earth ions into a simple oxide of narrow bandgap as a strategy to design new photoelectrodes or photocatalysts.


Asunto(s)
Oxígeno/química , Fosfatos/química , Fotoquímica/métodos , Semiconductores , Compuestos de Plata/química , Catálisis , Cristalografía por Rayos X/métodos , Electroquímica/métodos , Electrodos , Diseño de Equipo , Iones , Luz , Ensayo de Materiales , Metales/química
6.
Inorg Chem ; 49(5): 2290-301, 2010 Mar 01.
Artículo en Inglés | MEDLINE | ID: mdl-20131830

RESUMEN

We report structural investigations into (MoO(2))(2)P(2)O(7) using a combination of X-ray, neutron and electron diffraction, and solid-state NMR supported by first principles quantum chemical calculations. These reveal a series of phase transitions on cooling at temperatures of 377 and 325 K. The high temperature gamma-phase has connectivity consistent with that proposed by Kierkegaard at room temperature (but with improved bond length distribution), and contains 13 unique atoms in space group Pnma with lattice parameters a = 12.6577(1) A, b = 6.3095(1) A, c = 10.4161(1) A, and volume 831.87(1) A(3) from synchrotron data at 423 K. The low temperature alpha-structure was indexed from electron diffraction data and contains 60 unique atoms in space group P2(1)/c with cell parameters a = 17.8161(3) A, b = 10.3672(1) A, c = 17.8089(3) A, beta = 90.2009(2) degrees, and volume 3289.34(7) A(3) at 250 K. First principles calculations of (31)P chemical shift and J couplings were used to establish correlation between local structure and observed NMR parameters, and 1D and 2D (31)P solid-state NMR used to validate the proposed crystal structures. The intermediate beta-phase is believed to adopt an incommensurately modulated structure; (31)P NMR suggests a smooth structural evolution in this region.

7.
Inorg Chem ; 48(2): 414-6, 2009 Jan 19.
Artículo en Inglés | MEDLINE | ID: mdl-19072122

RESUMEN

Crystals of a new family of lanthanide-containing platinum oxides with a unique framework structure were grown out of molten hydroxide fluxes. The structure consists of a crystallographically well-behaved [Ln(6)Pt(2)O(15)](4-) framework permeated by channels filled with disordered atoms along the [100] and equivalent directions. Open-channel structures are rare in oxides and apparently unknown in platinate chemistry.

8.
Adv Mater ; 30(44): e1801619, 2018 Nov.
Artículo en Inglés | MEDLINE | ID: mdl-30589463

RESUMEN

The anomolous photovoltaic (APV) effect is an intriguing phenomenon and rarely observed in bulk materials that structurally have an inversion symmetry. Here, the discovery of such an APV effect in a centrosymmetric vanadate, BiVO4, where noticeable above-bandgap photovoltage and a steady-state photocurrent are observed in both ceramics and single crystals even when illuminated under visible light, is reported. Moreover, the photovoltaic voltage can be reversed by the stress modulation, and a sine-function relationship between the photovoltage and stress directional angle is derived. Microstructure and strain-field analysis reveal localized asymmetries that are caused by strain fluctuations in bulk centrosymmetric BiVO4. On the basis of the experimental results, a flexoelectric coupling via a strain-induced local polarization mechanism is suggested to account for the APV effect observed. This work not only allows new applications for BiVO4 in optoelectronic devices but also deepens insights into the mechanisms underlying the APV effect.

9.
ACS Appl Mater Interfaces ; 10(35): 29786-29794, 2018 Sep 05.
Artículo en Inglés | MEDLINE | ID: mdl-30088753

RESUMEN

In this report, a ferroelectric-luminescent heterostructure is designed to convert infrared light into electric power. We use BiFeO3 (BFO) as the ferroelectric layer and Y2O3:Yb,Tm (YOT) as the upconversion layer. Different from conventional ferroelectric materials, this heterostructure exhibits switchable and stable photovoltaic effects under 980 nm illumination, whose energy is much lower than the band gap of BFO. The energy transfer mechanism in this heterostructure is therefore studied carefully. It is found that a highly efficient nonradiative energy transfer process from YOT to BFO plays a critical role in achieving the below-band-gap photon-excited photovoltaic effects in this heterostructure. Our results also indicate that by introducing asymmetric electrodes, both the photovoltage and photocurrent are further enhanced when the built-in field and the depolarization field are aligned. The construction of ferroelectric-luminescent heterostructure is consequently proposed as a promising route to enhance the photovoltaic effects of ferroelectric materials by extending the absorption of the solar spectrum.

10.
ACS Appl Mater Interfaces ; 10(15): 12781-12789, 2018 Apr 18.
Artículo en Inglés | MEDLINE | ID: mdl-29521082

RESUMEN

The effect of above-band gap photons on the domains of the BiFeO3 (BFO) thin film was investigated via piezoresponse force microscopy and Kelvin probe force microscopy. It is found that under above-band gap illumination, the relaxation time of the polarization state was significantly extended, while the effective polarizing voltage for the pristine domains was reduced. We propose that this photoinduced domain stabilization can be attributed to the interaction between photogenerated surface charges and domains. Importantly, a similar phenomenon is observed in other ferroelectric (FE) materials with an internal electric field once they are illuminated by above-band gap light, indicating that this photoinduced stabilization is potentially universal rather than specific to BFO. Thus, this study will not only contribute to the knowledge of photovoltaic (PV) phenomena but also provide a new route to promote the stability of PV and FE materials.

11.
J Phys Chem Lett ; 8(14): 3249-3255, 2017 Jul 20.
Artículo en Inglés | MEDLINE | ID: mdl-28661671

RESUMEN

Ionic codoping offers a powerful approach for modifying material properties by extending the selection of potential dopant ions. However, it has been a major challenge to introduce certain ions that have hitherto proved difficult to use as dopants (called "difficult-dopants") into crystal structures at high concentrations, especially through wet chemical synthesis. Furthermore, the lack of a fundamental understanding of how codopants are incorporated into host materials, which types of defect structures they form in the equilibrium state, and what roles they play in material performance, has seriously hindered the rational design and development of promising codoped materials. Here we take In3+ (difficult-dopants) and Nb5+ (easy-dopants) codoped anatase TiO2 nanocrystals as an example and investigate the doping mechanism of these two different types of metal ions, the defect formation, and their associated impacts on high-pressure induced structural transition behaviors. It is experimentally demonstrated that the dual mechanisms of nucleation and diffusion doping are responsible for the synergic incorporation of these two dopants and theoretically evidenced that the defect structures created by the introduced In3+, Nb5+ codopants, their resultant Ti3+, and oxygen vacancies are locally composed of both defect clusters and equivalent defect pairs. These formed local defect structures then act as nucleation centers of baddeleyite- and α-PbO2-like metastable polymorphic phases and induce the abnormal trans-regime structural transition of codoped anatase TiO2 nanocrystals under high pressure. This work thus suggests an effective strategy to design and synthesize codoped nanocrystals with highly concentrated difficult-dopants. It also unveils the significance of local defect structures on material properties.

12.
Adv Mater ; 29(11)2017 Mar.
Artículo en Inglés | MEDLINE | ID: mdl-28112832

RESUMEN

Highly efficient visible-light catalysts are achieved through forming defect-pairs in TiO2 nanocrystals. This study therefore proposes that fine-tuning the chemical scheme consisting of charge-compensated defect-pairs in balanced concentrations is a key missing step for realizing outstanding photocatalytic performance. This research benefits photocatalytic applications and also provides new insight into the significance of defect chemistry for functionalizing materials.

13.
Sci Rep ; 6: 23659, 2016 Mar 30.
Artículo en Inglés | MEDLINE | ID: mdl-27025685

RESUMEN

Electric-field-induced, antiferroelectric-ferroelectric (AFE-FE) phase transitions are common for AFE materials. To date, the strain and preferred orientation evolution as well as the role of the intermediate FE state during the successive AFE-FE-AFE phase transitions has not been clear. To this end, we have herein studied a typical AFE Pb0.97La0.02(Zr0.56Sn0.33Ti0.11)O3 (PLZST) material using in-situ neutron diffraction. It is striking that the AFE-FE phase transition is not fully reversible: in the electric-field-induced FE state, the induced strain exhibits an elliptical distribution, which in turn leads to significant preferred orientation in the final AFE state after withdrawal of the applied electric-field. The ω-dependent neutron diffraction patterns show clear evidence of the induced strain distribution and associated preferred orientation arising from the AFE-FE phase transition. The current work also provides an explanation for several temperature and electric-field dependent dielectric anomalies as well as unrecovered strain change which appear in AFE materials after exposure to sufficiently high electric fields.

14.
ACS Appl Mater Interfaces ; 8(23): 14313-7, 2016 Jun 15.
Artículo en Inglés | MEDLINE | ID: mdl-27214052

RESUMEN

This work systematically investigated the structure and property of the near-surface and bulk regions of Pb0.99(Nb0.02Zr0.73Sn0.21Ti0.04)O3 ceramics using a combination of X-ray and neutron diffraction, piezoresponse force microscopy, and conventional ferroelectric/piezoelectric characterization. It is found that mechanical force can induce an antiferroelectric/ferroelectric phase transition within micrometers of the surface. Such a phase transition is strongly dependent on the processing scenario, leading to differences from the bulk region. This work provides crucial insights into the sensitivity of this class of AFE materials. Clearly, surface processing conditions must be taken into account for both accurate structural determination and practical applications.

15.
Acta Crystallogr B Struct Sci Cryst Eng Mater ; 71(Pt 6): 679-87, 2015 Dec 01.
Artículo en Inglés | MEDLINE | ID: mdl-26634724

RESUMEN

The Type II phase in the Bi1 - xWxO1.5 + 1.5x system is shown to have a (3 + 3)-dimensional modulated δ-Bi2O3-related structure, in which the modulation vector ℇ `locks in' to a commensurate value of 1/3. The structure was refined in a 3 × 3 × 3 supercell against single-crystal Laue neutron diffraction data. Ab initio calculations were used to test and optimize the local structure of the oxygen sublattice around a single mixed Bi/W site. The underlying crystal chemistry was shown to be essentially the same as for the recently refined (3 + 3)-dimensional modulated structure of Type II Bi1 - xNbxO1.5 + x (Ling et al., 2013), based on a transition from fluorite-type to pyrochlore-type via the appearance of W4O18 `tetrahedra of octahedra' and chains of corner-sharing WO6 octahedra along 〈110〉F directions. The full range of occupancies on this mixed Bi/W site give a hypothetical solid-solution range bounded by Bi23W4O46.5 (x = 0.148) and Bi22W5O48 (x = 0.185), consistent with previous reports and with our own synthetic and analytical results.

16.
ACS Appl Mater Interfaces ; 7(45): 25321-5, 2015 Nov 18.
Artículo en Inglés | MEDLINE | ID: mdl-26512874

RESUMEN

Stimulated by the excellent colossal permittivity (CP) behavior achieved in In+Nb co-doped rutile TiO2, in this work we investigate the CP behavior of Ga and Nb co-doped rutile TiO2, i.e., (Ga(0.5)Nb(0.5))(x)Ti(1-x)O2, where Ga(3+) is from the same group as In(3+) but with a much smaller ionic radius. Colossal permittivity of up to 10(4)-10(5) with an acceptably low dielectric loss (tan δ = 0.05-0.1) over broad frequency/temperature ranges is obtained at x = 0.5% after systematic synthesis optimizations. Systematic structural, defect, and dielectric characterizations suggest that multiple polarization mechanisms exist in this system: defect dipoles at low temperature (∼10-40 K), polaronlike electron hopping/transport at higher temperatures, and a surface barrier layer capacitor effect. Together these mechanisms contribute to the overall dielectric properties, especially apparent observed CP. We believe that this work provides comprehensive guidance for the design of new CP materials.

17.
Sci Rep ; 4: 6582, 2014 Oct 10.
Artículo en Inglés | MEDLINE | ID: mdl-25301286

RESUMEN

The structures of many important functional oxides contain networks of metal-oxygen polyhedral units i.e. MOn. The correlation between the configurations and connectivities of these MOn to properties is essentially important to be well established to conduct the design, synthesis and application of new MOn-based functional materials. In this paper, we report on an atomic-scale solution-chemistry approach that for the first time enables TiO6 octahedral network control starting from metastable brookite TiO2 through simultaneously tuning pH values and interfering ions (Fe(3+), Sc(3+), and Sm(3+)). The relationship between solution chemistry and the resultant configuration/connectivity of TiO6 octahedra in TiO2 and lepidocrocite titanate is mapped out. Apart from differing crystalline phases and morphologies, atomic-scale TiO6 octahedral control also endows numerous defect dipoles for giant dielectric responses. The structural and property evolutions are well interpreted by the associated H(+)/OH(-) species in solution and/or defect states associated with Fe(3+) occupation within TiO6 octahedra. This work therefore provides fundamental new insights into controlling TiO6 octahedral arrangement essential for atomic-scale structure-property design.

18.
Chem Mater ; 25(3): 496-502, 2013 Feb 12.
Artículo en Inglés | MEDLINE | ID: mdl-23503683

RESUMEN

Large whiskers of a new KAl9O14 polymorph with mullite-type structure were synthesized. The chemical composition of the crystals was confirmed by energy-dispersive X-ray spectroscopy, and the structure was determined using single-crystal X-ray diffraction. Nanosized twin domains and one-dimensional diffuse scattering were observed utilizing transmission electron microscopy. The compound crystallizes in space group P21/n (a = 8.1880(8), b = 7.6760(7), c = 8.7944(9) Å, ß = 110.570(8)°, V = 517.50(9) Å3, Z = 2). Crystals of KAl9O14 exhibit a mullite-type structure with linear edge-sharing AlO6 octahedral chains connected with groups of two AlO4 tetrahedra and one AlO5 trigonal bipyramid. Additionally, disproportionation of KAl9O14 into K ß-alumina and corundum was observed using in situ high-temperature optical microscopy and Raman spectroscopy.

19.
Chem Mater ; 25(21): 4436-4446, 2013 Nov 12.
Artículo en Inglés | MEDLINE | ID: mdl-24489435

RESUMEN

Elastic and anelastic properties of a member of the BiFeO3-CaFeO2.5 perovskite solid solution (BCFO), which is known to have multiple instabilities, have been investigated by resonant ultrasound spectroscopy. This phase, with 64% Bi and 36% Ca on the A site, is antiferromagnetic (TN ∼650 K) and has an ordered arrangement of oxygen vacancies with tetragonal lattice geometry. The inverse mechanical quality factor, Q-1, has a maximum near 100 K, correlating closely with a peak in dielectric loss, reported previously, consistent with a loss mechanism that involves the movement of oxygen vacancies accompanied by local lattice distortion. At higher temperature, there is a further acoustic loss peak that is correlated with complex impedance anomalies. There is no clear relationship to the magnetic transition, and the observations are interpreted as relating to ionic conductivity. A small stiffening, scaling with the square of the magnetic order parameter below TN, indicates that the main coupling with strain is biquadratic, confirming that conventional coupling of magnetic order with symmetry-breaking shear strains is weak in BCFO. Data from the literature for BCFO indicates that local strain fields are likely to be responsible for suppressing the spin cycloid present in BiFeO3.

20.
J Phys Condens Matter ; 24(49): 495301, 2012 Dec 12.
Artículo en Inglés | MEDLINE | ID: mdl-23137958

RESUMEN

We propose that systems exhibiting compositional patterning at the nanoscale, so far assumed to be due to some kind of ordered phase segregation, can be understood instead in terms of coherent, single phase ordering of minority motifs, caused by some constrained drive for uniformity. The essential features of this type of arrangement can be reproduced using a superspace construction typical of uniformity-driven orderings, which only requires the knowledge of the modulation vectors observed in the diffraction patterns. The idea is discussed in terms of a simple two-dimensional lattice-gas model that simulates a binary system in which the dilution of the minority component is favoured. This simple model already exhibits a hierarchy of arrangements similar to the experimentally observed nano-chessboard and nano-diamond patterns, which are described as occupational modulated structures with two independent modulation wavevectors and simple step-like occupation modulation functions.

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