Electronic Phase Segregation Research Articles

Wide-Band-Gap Mixed-Halide 3D Perovskites: Electronic Structure and Halide Segregation Investigation.

Mixed-halide organolead perovskites ( ) are of great interest for both single-junction and tandem solar cells because of their wide band gap. In this study, we investigate the family of mixed iodide/bromide (I/Br) and bromide/chloride (Br/Cl) perovskites, revealing the strong influence of halide substitution on electronic properties, morphology, film composition, and phase segregation. A qualitative blue shift with the I → Br → Cl series was observed, with the resulting optical absorption ranging from 420 to 800 nm covering nearly the entire visible region. The ionization potential increases from ≈6.0 to ≈7.0 eV as the halide composition changes from I to Br. However, with Cl components, the valence band position shows little variation, while the conduction band minimum shifts to a lower value with increasing Cl concentration. By collecting XPS spectra as a function of the sputtering depth, we observed halide segregation in both I/Br and Br/Cl mixed-halide perovskite films, where the large halide ion (I in the I/Br mix or Br in the Br/Cl mix) is preferentially found on the surface of the film and the smaller halide ion (Br in the I/Br mix or Cl in the Br/Cl mix) accumulates at the bottom of the film. These differences in the band structure, electronic properties, morphology, and film composition impacted the device performance: a decreased short-circuit current density and increased open-circuit voltage were observed with the I → Br → Cl series. This study highlights the role of halides in the band structure and phase segregation in mixed-halide perovskite solar cells and provides a foundational framework for future optoelectronic applications of these materials.

Electronic Traps and Phase Segregation in Lead Mixed-Halide Perovskite

An understanding of the factors driving halide segregation in lead mixed-halide perovskites is required for their implementation in tandem solar cells with existing silicon technology. Here we report that the halide segregation dynamics observed in the photoluminescence from CH3NH3Pb(Br0.5I0.5)3 is strongly influenced by the atmospheric environment, and that encapsulation of films with a layer of poly(methyl methacrylate) allows for halide segregation dynamics to be fully reversible and repeatable. We further establish an empirical model directly linking the amount of halide segregation observed in the photoluminescence to the fraction of charge carriers recombining through trap-mediated channels, and the photon flux absorbed. From such quantitative analysis we show that under pulsed illumination, the frequency of the modulation alone has no influence on the segregation dynamics. Additionally, we extrapolate that working CH3NH3Pb(Br0.5I0.5)3 perovskite cells would require a reduction of the trap-related c...

Investigation of electronic phase segregation in La0.75Ca0.15Sr0.10MnO3 manganite

The effect of electronic phase segregation in a broad metal-Insulator transition (MIT) observed in La0.75Ca0.25-xSrxMnO3 (x=0.1) composition is investigated using heat capacity, magnetization, electrical resistivity and magnetoresistance measurements. The negative magnetoresistance of 65% in an applied magnetic field of 12T and 15% in 1T with a broad working range of 18K around 300K which is beneficial for room temperature colossal magnetoresistance (CMR) applications. The broad transition in temperature dependent zero field resistivity measurement is analyzed in the light of percolation model indicates the abundance of insulating/metallic clusters in metallic/insulating region. A significant difference between the metallic fraction around the MIT and the ferromagnetic phases observed around the Curie temperature demonstrates the interplay between volume of itinerant and polaronic electronic phases.

75As NMR study of overdoped CeFeAsO0.8F0.2

We report on the results from a 75As nuclear magnetic resonance (NMR) study of the overdoped iron pnictide superconductor CeFeAsO0.8F0.2. We find two As sites with different shifts at temperatures as high as 100 K, which is above the superconducting transition temperature of 39 K, and hence they cannot be attributed to the effect of vortices in the superconducting state as previously suggested (Ghoshray et al 2009 Phys. Rev. B 79 144512). The much larger spin–lattice relaxation rate compared with that found in other pnictides without magnetic rare earth ions, and the temperature dependence of the 75As NMR shifts for the two central lines, are consistent with the hyperfine coupling from magnetic Ce to As. The low temperature spectra indicate that there are As ions with two different quadrupole splittings. Our findings appear to be consistent with an electronic phase segregation into regions with two different F dopings or the presence of a correlated spatial charge and spin density variations.

Random resistivity network calculations for cuprate superconductors with an electronic phase separation transition

The resistivity as a function of temperature for high temperature superconductors is very unusual and, despite its importance, lacks a unified theoretical explanation. It is linear with the temperature for overdoped compounds but it falls more quickly as the doping level decreases. The resistivity of underdoped cuprates increases like that of an insulator below a characteristic temperature where it shows a minimum. We show that this overall behavior can be explained by calculations using an electronic phase segregation into two main component phases with low and high electronic densities. The total resistance is calculated from the various contributions through several processes of random picking of the local resistivities and using a common statistical random resistor network approach.

Spin glass-like behaviour in Fe-rich phases of Sr 2Fe 1− xMn xMoO 6 (0.1 [formula omitted]0.4)

Below their respective ferromagnetic Curie temperatures ( T C ), Sr 2Fe 1− x Mn x MoO 6 (0.1 ≤ x ≤ 0.4) undergo a spin glass-like transition owing to the electronic phase segregation process that creates highly spin disordered regions. At low temperatures, an additional short range magnetic interaction is observed due to large differences in volumes of FeO 6 and MnO 6 octahedra. The magnetoelastic structural change is also reflected in the lattice parameters.

Competing orders and the resistivity curves of cuprate superconductors

The resistivity as function of temperature of high temperature superconductors (HTSC) is an important property of cuprates but it still lacks a widely accepted explanation. Here we show that the overall behavior of an entire series can be explained by calculations using an electronic phase segregation into two main component phases.

Phase segregation as origin of magnetoresistance in La0.85Sr0.15CoO3

Abstract The La0.85Sr0.15CoO3 compound has been synthesized under different annealing temperatures. It has been observed that the transport properties and magnetoresistance of this compound depend on such temperatures. This response is discussed in the framework of a model of electronic phase segregation.

Co-dopedSr2FeMoO6double perovskites: A plausible scenario for phase segregation

The structural, magnetic, and transport properties of Co-doped ${\mathrm{Sr}}_{2}{\mathrm{FeMoO}}_{6}$ in a series of nominal composition ${\mathrm{Sr}}_{2}{\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}{\mathrm{MoO}}_{6}$ $(x=0$, 0.05, 0.1, 0.2, 0.4, 0.7, and 0.9) are studied. At room temperature, the crystal structure is tetragonal with space group I4/m. The unit cell and the $\mathrm{Fe}∕\mathrm{Co}$ octahedra volumes increase while the Mo octahedra and $\mathrm{Fe}∕\mathrm{Co}\text{\ensuremath{-}}\mathrm{O}\text{\ensuremath{-}}\mathrm{Mo}$ bond angles tend to decrease with increasing ${\mathrm{Co}}^{2+}$ content. Antisite disorder clearly decreases along the series and the lattice expands along the $c$ direction while contracting in the basal $ab$ plane. As expected, it is observed that the ferromagnetic ordering temperature decreases and antiferromagnetic correlations set in with Co doping. Also a spin-glass-like behavior, stemming from the presence of competing ferromagnetic patches in the antiferromagnetic ${\mathrm{Co}}^{2+}$ matrix, is observed for intermediate doping levels. The conductivity exhibits a semiconducting behavior for all samples, although for low Co contents a metallic regime is recovered upon application of an external magnetic field. A maximum in the magnetoresistance is observed for ${\mathrm{Sr}}_{2}{\mathrm{Fe}}_{0.95}{\mathrm{Co}}_{0.05}{\mathrm{MoO}}_{6}$ and ${\mathrm{Sr}}_{2}{\mathrm{Fe}}_{0.9}{\mathrm{Co}}_{0.1}{\mathrm{MoO}}_{6}$, but surprisingly enough, as an antiferromagnetic and insulating behavior becomes apparent, the heavily doped samples ${\mathrm{Sr}}_{2}{\mathrm{Fe}}_{0.3}{\mathrm{Co}}_{0.7}{\mathrm{MoO}}_{6}$ and ${\mathrm{Sr}}_{2}{\mathrm{Fe}}_{0.1}{\mathrm{Co}}_{0.9}{\mathrm{MoO}}_{6}$ exhibit magnetoresistive ratios almost the same as those of the corresponding pure compound ${\mathrm{Sr}}_{2}{\mathrm{FeMoO}}_{6}$. The observed phenomenology could be compatible with an electronic phase segregation scenario as has recently been predicted to occur.

Electrodynamics of high-Tcsuperconductors

Recent studies of the electromagnetic response of high-Tc superconductors using terahertz, infrared, and optical spectroscopies are reviewed. In combination these experimental techniques provide a comprehensive picture of the low-energy excitations and charge dynamics in this class of materials. These results are discussed with an emphasis on conceptual issues, including evolution of the electronic spectral weight in doped Mott-Hubbard insulators, the d-wave superconducting energy gap and the normal-state pseudogap, anisotropic superfluid response, electronic phase segregation, emergence of coherent electronic state as a function of both temperature and doping, the vortex state, and the energetics of the superconducting transition. Because the theoretical understanding of these issues is still evolving the review is focused on the analysis of the universal trends that are emerging out of a large body of work carried on by many research teams. Where possible data generated by infrared/optical techniques are compared with the data from other spectroscopic and transport methods.

Charge inhomogeneities in La 2/3Ca 1/3MnO 3 epitaxial thin films

We present and compare here transport measurements on epitaxial thin films of La 2/3Ca 1/3MnO 3 grown by sputtering and MOCVD on SrTiO 3 substrates. As-grown sputtered films exhibit depressed magnetic and transport properties when compared with MOCVD samples, but both magnetic transition temperature, T C, and saturation magnetization, M S, may be substantially improved by annealing samples in air. A transverse voltage drop is measured across a patterned measuring track with a temperature dependence that, particularly around T C, cannot be explained in terms of the longitudinal resistance component alone. High temperature annealing substantially suppress these features as well as the application of high magnetic fields. We argue that these effects reflect inhomogeneous current flow, likely to be related with charge inhomogeneities and electronic phase segregation.

Novel structures and electromagnetic properties of the A-site-ordered/disordered manganitesRBaMn2O6/R0.5Ba0.5MnO3 (R = Y and rare earth elements)

The A-site-ordered/disordered manganese perovskites,RBaMn2O6/ R0.5Ba0.5MnO3 (R = Y and rare earthelements), are reviewed. RBaMn2O6 displays remarkable features: (1) the charge/orbital order (CO) transitionat relatively high temperatures far above 300 K, (2) a new stackingvariation of the CE-type CO with a fourfold periodicity along thec-axis, (3) the presence of structural transition possibly accompanied bydx2−y2 orbital order and (4) electronic phase segregation. These novel structural andelectromagnetic properties are discussed in terms of the structural characteristic that theMnO2 square sublattice is sandwiched by two types of rock-salt layer,R O and BaO, with different lattice sizes. Such structure introduces a strong frustration to theMnO2 sublattice and gives a new perturbation to the competition of multipledegrees of freedom among charge, orbital, spin and lattice. InR0.5Ba0.5MnO3 with a primitive cubic perovskite cell, on the other hand, the magnetic glassystates are dominant as the ground state. A peculiar behaviour, steplikeultrasharp magnetization and resistivity changes, has been observed inPr0.5Ba0.5MnO3.

NMR study of layered manganite La1.4Sr1.6Mn2O7

A 55 Mn nuclear magnetic resonance study of a layered perovskite La 1.4 Sr 1.6 Mn 2 O 7 is reported. Between 4.2 K and the magnetic ordering temperature, 100 K , zero field spin echo spectra consist of signals corresponding to the double-exchange (DE) states and charge localized Mn 3+ , Mn 4+ states. This indicates occurrence of electronic and magnetic phase segregation into ferromagnetic metallic and ferromagnetic insulating regions. Above 100 K a DE line is observed, revealing the presence of long lived metallic clusters. The relation of results to magnetoresistive properties of the compound is discussed.

Localized and itinerant electronic states at the insulator–metal transition in Y1−xCaxVO3+δ: evidence from electric transport, magnetic properties and XAS spectroscopy

Electron transport and macroscopic magnetic properties of the perovskite mixed-valent system Y1−xCaxVO3+δ were investigated by means of resistivity and magnetic susceptibility measurements. As is shown by transport data, the system undergoes an insulator–metal transition on doping with Ca beyond the threshold concentration x≈0.35. However, magnetic response data evidence a more complex scenario in which localized-electrons giving rise to magnetism coexist with itinerant electrons, well beyond the metal–insulator (M–I) transition. In the insulating compounds, susceptibility data suggest the occurrence of short-range order slightly above T≈114 K, which is reminiscent of the temperature at which the parent compound YVO3 undergoes a transition to an antiferromagnetic long-range order. In the metallic samples, evidence of a possible electronic phase segregation are found.

Spin-state ordered clusters in the perovskiteNdBaCo2O5.47

${\mathrm{NdBaCo}}_{2}{\mathrm{O}}_{5.47}$ cobaltite has been investigated using neutron and synchrotron powder diffraction. At ${T}_{\mathrm{MI}}\ensuremath{\sim}360\mathrm{K},$ the compound exhibits an associated metal-insulator and spin-state transition commonly observed in these systems. At ${T}_{N}\ensuremath{\sim}275\mathrm{K},$ the $\mathrm{IS}\ensuremath{-}{\mathrm{Co}}^{3+}$ species order in a AFM G-type structure. At ${T}_{\mathrm{SSO}}\ensuremath{\sim}230\mathrm{K},$ the AFM G-type volume fraction is gradually transformed into spin-state ordered clusters having a distinct magnetic structure. These clusters result from a selective $\mathrm{IS}\ensuremath{-}{\mathrm{Co}}^{3+}$ ${(t}_{2g}^{5}{e}_{g}^{1})$ to $\mathrm{LS}\ensuremath{-}{\mathrm{Co}}^{\mathrm{III}}$ ${(t}_{2g}^{6}{e}_{g}^{0})$ crossover and the ordering of the diamagnetic ions over $\ensuremath{\sim}350\mathrm{\AA{}}$ correlation lengths. The same electronic phase segregation is observed in ${\mathrm{TbBaCo}}_{2}{\mathrm{O}}_{5.46}$ and seems to occur only for a narrow range of oxygen stoichiometry.

7Li NMR Studies of Chemically-Delithiated Li1-xCoO2

7Li magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy and Rietveld analysis of X-ray diffraction data have been used to study chemically delithiated Li1-xCoO2. Samples of LiCoO2 were prepared at 400 (LT), 600 (MT), and 800 °C (HT), chemically delithitated to Li0.5CoO2 composition, and heated at 200 °C. It was found that all HT materials and the as-prepared MT-Li0.5CoO2 displayed layered structure, whereas all LT materials and the 200 °C heated MT- Li0.5CoO2 displayed spinel structure. The NMR results suggest that the local atomic and electronic structures of the as-prepared MT-Li0.5CoO2 approach that of spinel phase although X-ray refinement results show the rhombohedral layer structure. The 7Li MAS NMR results provide evidence for electronic phase segregation in layered and spinel Li1-xCoO2 materials. Several samples showed coexisting NMR peaks arising from lithium in a diamagnetic Co3+ environment and in a paramagnetic mixed-valence Co3+/4+ environment. Li0.5CoO2 samples derived from LiCoO2 fired at 600 °C and 800 °C gave rise to only one NMR peak, associated with a mixed-valence cobalt environment, and produced a low NMR signal intensity, arising from an environment containing localized t2g holes (Co4+ ions). A large NMR shift was observed for lithium in the mixed-valence environment, attributed to a Knight shift for the as-prepared HT-Li0.5CoO2 and to a hyperfine shift in the other samples. Variable-field studies showed homonuclear dipolar coupling to be the dominant source of residual line broadening in Li1-xCoO2, and chemical shift dispersion to be a probable secondary source.

NMR probe of phase segregation in electron doped mixed valence manganites

A 55Mn and 139La NMR study of La0.35Ca0.65MnO3 is reported. The zero field 55Mn spectra consist of two lines centered at 290 and 375 MHz. Their behavior under an applied magnetic field makes it possible to attribute them to regions of antiferromagnetically and ferromagnetically coupled Mn spins, respectively. This gives evidence for the existence of electronic phase segregation of microscopic ferromagnetic regions of double exchange coupled Mn spins over a charge ordered antiferromagnetic background. The behavior of these ferromagnetic regions in the applied magnetic field is related to the magnetoresistive properties of the manganites.

Neutron scattering studies of phase segregation in Pr 0.7Ca 0.3MnO 3

By using high-resolution time-of-flight neutron powder diffraction, we have investigated phase segregation in the narrow-bandwidth manganite Pr 0.7Ca 0.3MnO 3. Unexpectedly, our data clearly demonstrate that phase segregation in Pr 0.7Ca 0.3MnO 3 is unrelated to electronic phase segregation, but is primarily driven by chemical disorder and intra-granular strains, similar to martensitic phase transitions.

Critical exponents of the ferromagnetic-paramagnetic phase transition ofLa1−xSrxCoO3(0.20<~x<~0.30)

The critical exponents of the second-order ferromagnetic-paramagnetic phase transition of ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CoO}}_{3}$ $(0.20<~x<~0.30)$ compounds are obtained. The results $0.43<~\ensuremath{\beta}<~0.46,$ $1.39<~\ensuremath{\gamma}<~1.43,$ and $4.02<~\ensuremath{\delta}<~4.38$ are qualitatively different than those reported in the literature for parent manganites, and are taken as the indication of the absence of long-range interactions below the Curie point. Electronic phase segregation in hole-rich ferromagnetic and hole-poor ${\mathrm{LaCoO}}_{3}$-like regions is supposed to explain the observations. The hole-poor regions would dilute the magnetic lattice and prevent the occurrence of long-range order. Also, a spin transition of the ${\mathrm{Co}}^{3+}$ ions at ${T}_{C}$ or the hole-poor regions (acting equivalently to missed spin clusters in amorphous ferromagnets) could be the reason for the high \ensuremath{\beta} value in the framework of a Heisenberg model.

Colossal Magnetoresistance in Manganites: Role of the Electronic Phase Segregation

Colossal Magnetoresistance in Manganites: Role of the Electronic Phase Segregation