Ideal Stoichiometry Research Articles

Energy of free excitons in CuInSe2 single crystals

CuInSe 2 single crystals grown by the vertical Bridgman method were studied using photoluminescence (PL) and optical reflection (OR) at temperatures 4.2 and 78 K. A and B free- exciton peaks were well resolved in both the PL and OR spectra measured at a low temperature from a number of samples. The energy, spectral separation, and the width of the excitonic lines varied considerably for samples with different elemental compositions. The closer to the ideal stoichiometry, the higher the energies and the smaller the A–B separation. Similar changes in the excitonic spectra were revealed for a temperature rise from 4.2 up to 78 K. These measurements explain the scatter in the excitonic parameters and the band-gap values measured in the present work and reported in the literature.

Structure and magnetic properties of rf thermally plasma synthesized Mn and Mn–Zn ferrite nanoparticles

Plasma synthesis has previously been shown to be a viable route to producing nanocrystalline magnetite and Ni ferrite nanoparticles. In this work nanocrystalline powders of Mn and Mn–Zn ferrites have been synthesized using a 50 kW–3 MHz rf (radio frequency) induction plasma torch. We investigate these materials for soft magnetic applications. High-energy ball milled Mn + Fe powders and (Mn+Zn) +Fe powders (<10 μm) in the stoichiometric ratio of 1:2 were used as precursors for the ferrite synthesis. Compressed air was used in the oxygen source for oxidation of metal species in the plasma. X-ray diffraction patterns for the plasma-torched Mn ferrite and MnZn ferrite powders were indexed to the spinel ferrite crystal structure. An average grain size of ∼20 nm was determined from Scherrer analysis confirmed by transmission electron microscopy studies. The particles also exhibited faceted polygonal growth forms with the associated truncated cuboctahedral shapes. Room-temperature vibrating sample magnetometer measurements of the hysteretic response revealed saturation magnetization Ms and coercivity Hc of Mn ferrite are 23.65 emu/g and 20 Oe, respectively. The Néel temperatures of Mn ferrite powders before and after annealing (500 °C, 30 min) were determined to be 200 and 360 °C, respectively. Inductively coupled plasma chemical analysis and energy dispersive x-ray analysis data on the plasma-torched powders indicated deviations in the Mn or Zn contents than the ideal stoichiometry. MnZn ferrite was observed to have a Néel temperature increased by almost 400 °C as compared with as-synthesized Mn ferrite but with a larger coercivity of ∼35 Oe.

Segregation in strongly ordering compounds: a key role of constitutional defects.

For the example of the B2 CoAl(100) surface, we demonstrate that even slight deviations from an ordered alloy's ideal stoichiometry in a subsurface region or in the bulk can drastically affect its surface composition. By experimental surface analysis and first-principles calculations, we show that Co antisite atoms segregate to the very surface, driven by the same strong interactions which enforce order in the bulk. Our findings are consistent with the lack of antisite segregation we found earlier for the much weaker ordering FeAl(100), and resolve contradictory reports for NiAl(100).

Nonstoichiometry and chemical purity effects in thermoelectric Ba8Ga16Ge30 clathrate

Zone melting purification experiments have been carried out on the clathrate, Ba8Ga16Ge30. The impurities present have been identified and their approximate concentrations measured. Trace impurities were determined to be approximately 240 parts per million (ppm) in the most impure sample to 17 ppm in the most pure sample. The temperature-dependent Seebeck coefficient, thermal conductivity, and electrical conductivity are reported as a function of sample purity as well as the room-temperature Hall coefficient. Microprobe analysis suggests that the samples are nonstoichiometric with excess Ge relative to Ga, and there are indications of the presence of defects. Single-crystal x-ray investigations as well as synchrotron powder diffraction measurements support the presence of defects, but the x-ray data cannot accurately determine the relative amounts of Ga and Ge. Band-structure calculations in the generalized gradient approximation show that the measured Hall and Seebeck coefficients are consistent with a defect lattice of approximate stoichiometry Ba8Ga14Ge31. Although the figure of merit (ZT) is found to be the highest for the purest sample, the dominant contribution to transport is conjectured to arise from deviations from the ideal stoichiometry and not impurities.

Dissolution kinetics of NbC particles in the heat-affected zone of type 347 austenitic stainless steel

Dissolution kinetics of NbC particles in the heat-affected zone (HAZ) of Type 347 austenitic stainless steel were experimentally studied by a rapid cooling method. Coupons with 1.5-mm thickness were water quenched on a GLEEBLE thermomechanical simulator at various instances during the heating portion of a welding thermal cycle. Particle dissolution kinetics data were obtained by statistical analysis of digital images of resulting microstructure. For most of the test alloys, a good correlation exists between the dissolution kinetics and the susceptibility to cracking in the HAZ. The faster the Nb-carbide particles dissolve, the more resistant the alloy is to constitutional liquation cracking. The rate at which particles dissolve seems to be affected by their stoichiometry. If Nb:C ratios in bulk alloys are closer to the ideal stoichiometry of 7.7, NbC particles formed in the alloy are more likely to be stable and, hence, slower to dissolve.

Atomic structure of the Ag(001)c(2×2) Mn surface alloy

The structure of the unstable $c(2\ifmmode\times\else\texttimes\fi{}2)$ Mn surface alloy that is formed on Ag(001) upon deposition of an Mn monolayer at room temperature is determined by low-energy electron diffraction (LEED). The optimized structure model is basically a two-monolayer-thick ${\mathrm{Mn}}_{x}{\mathrm{Ag}}_{100\ensuremath{-}x}$ alloy that is ordered in a checkerboard arrangement in the first layer and random in the second layer, with substantial deviations from ideal order and stoichiometry $x=50.$ The surface alloy essentially prolongs the face-centered-cubic Ag lattice, with interlayer distances ${d}_{12}=1.985\ifmmode\pm\else\textpm\fi{}0.010\AA{}$ and ${d}_{23}=2.030\ifmmode\pm\else\textpm\fi{}0.008\AA{}$ only slightly reduced with respect to Ag bulk, and a buckling of 0.07 \AA{} (Mn displaced outwards) in the top layer. The present work clearly confirms the unusually large atomic volume of Mn in this environment as found in previous surface-extended x-ray absorption fine structure work [P. Schieffer, M.-H. Tuilier, M.-C. Hanf, C. Krembel, G. Gewinner, D. Chandesris, and H. Magnan, Phys. Rev. B 57, 15 507 (1998)] and disproves the geometry inferred from an earlier LEED study [Wondong Kim, Wookje Kim, S.-J. Oh, J. Seo, J.-S. Kim, H.-G. Min, and S.-C. Hon, Phys. Rev. B 57, 8823 (1998)].

Synthesis of ferrite and nickel ferrite nanoparticles using radio-frequency thermal plasma torch

Nanocrystalline (NC) ferrite powders have been synthesized using a 50 kW–3 MHz rf thermal plasma torch for high-frequency soft magnet applications. A mixed powder of Ni and Fe (Ni:Fe=1:2), a NiFe permalloy powder with additional Fe powder (Ni:Fe=1:2), and a NiFe permalloy powder (Ni:Fe=1:1) were used as precursors for synthesis. Airflow into the reactor chamber was the source of oxygen for oxide formation. XRD patterns clearly show that the precursor powders were transformed into NC ferrite particles with an average particle size of 20–30 nm. SEM and TEM studies indicated that NC ferrite particles had well-defined polygonal growth forms with some exhibiting (111) faceting and many with truncated octahedral and truncated cubic shapes. The Ni content in the ferrite particles was observed to increase in going from mixed Ni and Fe to mixed permalloy and iron and finally to only permalloy starting precursor. The plasma-torch synthesized ferrite materials using exclusively the NiFe permalloy precursor had 40%–48% Ni content in the Ni-ferrite particle, differing from the NiFe2O4 ideal stoichiometry. EXAFS was used to probe the cation coordination in low Ni magnetite species. The coercivity and Neel temperature of the high Ni content ferrite sample were 58 Oe and ∼590 °C, respectively.

New barium manganese titanates prepared under reducing conditions

We report the synthesis and initial characterization of BaMnTi5O12, the first example of an oxide displaying an interesting tripled hollandite-type structure. Shiny black crystals which crystallize in the monoclinic space group, P2/n, with a=9.921(3) Å, b=8.8716(22) Å, c=10.309(3) Å, and β=91.24(3)°, were grown from a borate-based melt in vacuo. The structure contains layers made up of edge-shared pairs of TiO6 octahedra forming a hollandite framework. The Ba2+ ions occupy large high coordination number channel sites. Oxidation states of the transition metal ions are such that a charge balance of one half barium ion per three framework units with an ideal stoichiometry Ba0.5M3O6, produces the tripling. In addition, crystals of Ba2MnTi5O13, monoclinic, space group C2/m, a=15.075(1) Å, b=3.9470(2) Å, c=9.0985(5) Å, and β=98.23(1)°, which is isostructural with the mineral jeppeite, K2Ti6O13, were also grown.

Nuclear microprobe analysis of U-doped (Bi,Pb) 2Sr 2Ca 2Cu 3O y/Ag superconducting tapes

Superconducting (Bi,Pb) 2Sr 2Ca 2Cu 3O y /Ag tapes are doped with uranium compounds to introduce flux pinning defects from neutron-induced fission. The composition and distribution of elements in cross sections of the tapes were probed with a scanned 3 MeV proton microbeam using proton induced X-ray emission (PIXE). Distributions of the constituent elements were found to be heterogeneous on a scale of 10 μm. By combining the PIXE analysis with Rutherford backscattering spectrometry, the stoichiometry of the superconductor within the tape was measured, including oxygen from elastic scattering, revealing a departure from the desired 2223 composition. In one of the tapes, PIXE elemental maps of the Ag distribution showed diffusion of Ag into the superconductor from the enclosing jacket. Crystals of the same material, not fabricated into tapes, did not contain the contaminants and had a more ideal stoichiometry. Correlation maps between the constituent elements, deduced from the elemental maps, indicate the presence of secondary or unreacted phases.

Formation Enthalpies of Tetravalent Lanthanide Perovskites by High Temperature Oxide Melt Solution Calorimetry

AbstractHigh-temperature oxide melt solution calorimetry was used to measure formation enthalpies for several compositions of perovskites of nominal stoichiometry BaPrO3 and BaCeO3. Samples were synthesized from chemical solution methods followed by calcination at 1100-1300°C. PrO2 was synthesized by oxidation of Pr6O11 in an oxygen flow at 280°C. The samples were characterized by microprobe, thermogravimetric and differential thermal analyses. Cell parameters were refined by the Rietveld method. Barium excess in the samples with respect to ideal stoichiometry was detected. Drop solution enthalpies were measured in a Calvet type twin microcalorimeter, using 3Na2O·4MoO3 solvent at 702°C. Preliminary values of the formation enthalpy of BaPrO3 and BaCeO3 from oxides were -70 ±10 kJ/mol and -51 ±10 kJ/mol, respectively. They fall on the normal trend of energetics versus Goldschmidt tolerance factor and do not show any special stabilization of BaPrO3 relative to other MLnO3 perovskites.

Chemical Self-Diffusion in CdTe

High-temperature in-situ galvanomagnetic measurements on CdTe are performed at temperatures T = 500-700 °C in the interval of Cd pressures (PCd) one order of magnitude below and above the ideal stoichiometry line. The time evolution of samples after step-like change of PCd is analyzed and the chemical diffusion coefficient D = 5 exp (-1.12eV/kBT) (cm2/s) is evaluated. Neither magnitude of PGa nor direction of the step of PCd (increased/decreased) have manifested a distinguishable effect on D. Surface conduction below 600 °C dependent on PCd is reported.

Phase Relations at 1500°C in the Ternary System ZrO2–Gd2O3–TiO2

Phase relations at 1500°C in the ternary system ZrO2–Gd2O3–TiO2 have been determined by the powder X-ray diffraction of samples prepared by standard solid state reaction. A large area of this ternary oxide system centered on the Gd2Ti2O7–Gd2Zr2O7 join was shown to exhibit the pyrochlore and defect fluorite structures. The pyrochlore structure was observed for stoichiometries as far from the ideal M4O7 as M4O6.7 and M4O7.4, although the degree of disorder seemed much higher at these stoichiometries. On further deviation from the ideal M4O7 stoichiometry a smooth transition to fluorite average structure was observed for Zr-rich compositions. None of the other binary phases were observed to show significant extent of solid solution into the ternary region.

Single‐Source Sol‐Gel Synthesis of Nanocrystalline ZnAl2O4: Structural and Optical Properties

Nanometer‐sized zinc aluminate (ZnAl2O4) particles were synthesized from heterometal alkoxides, [ZnAl2(OR)8], possessing an ideal cation stoichiometry for the ZnAl2O4 spinel. ZnAl2O4 is formed at 400°C, which is the lowest temperature reported for the formation of monophasic ZnAl2O4. 27Al magic‐angle spinning nuclear magnetic resonance spectroscopy revealed that ZnAl2O4 possesses an inverse structure at <900°C, while the normal spinel phase is observed at higher temperatures. The homogeneity of the in‐depth composition and Zn:Al stoichiometry (1:2) was confirmed by electron spectroscopy for chemical analysis. Evaluation of the valence‐band spectra of ZnAl2O4 and ZnS suggested that the hybridization of O 2p and Zn 3d orbitals is responsible for lowering the bandgap in the latter. The average crystallite size showed an exponential relationship to the calcination temperature (X‐ray diffractometry and transmission electron microscopy data). The optical spectra of different spinel powders (average particle sizes, 20–250 nm) showed that the absorption edge exhibits a blue shift as particle size decreases.

Fe SPECIATION IN WEATHERED PYROCHLORE-GROUP MINERALS FROM THE LUESHE AND ARAXA (BARREIRO) CARBONATITES BY 57Fe MOSSBAUER SPECTROSCOPY

The minerals of the pyrochlore group exhibit a large range of chemical composition. The general structural formula can be written as A 2− m B 2 X 6− w Y 1− n · p H 2 O. The occupancy of the eight-fold-coordinated A site can reflect the degree of alteration of the pyrochlore. In unaltered pyrochlore-group minerals, this site is completely filled by Na and Ca, whereas in the altered equivalents, the A site is only partly filled by cations as K, Ba, Sr and Ce. In the literature, the structural formula is systematically calculated assuming an ideal stoichiometry, i.e. , a total of two six-fold-coordinated B -site cations per formula unit, dominantly Nb, with subordinate Ti and Ta in certain rock-types, e.g. , carbonatites. Fe is systematically considered as Fe 3+ and assigned to the B site. The present 57 Fe Mossbauer study focuses on altered pyrochlore-group minerals from two Nb ore deposits in laterite: Lueshe in the Democratic Republic of Congo, and Araxa in Brazil. In contrast to the common assumption that all Fe is present as Fe 3+ , 47 ± 2% of the total Fe is in the Fe 2+ oxidation state in the kalipyrochlore sample from Lueshe. Spectra of both samples are consistent with the presence of all the Fe 3+ at the B site, but with two different coordination spheres. Fe 3+ coordinated by 6 O 2− is observed in both cases. In the Lueshe sample, it is also coordinated by 5 O 2− and 1 OH − , whereas in bariopyrochlore from Araxa, the unusually large value of the quadrupole splitting, 3 mm/s, affecting ~19 ± 2% of the Fe 3+ , suggests that this Fe 3+ is coordinated by only 5 O 2− owing to the presence of anion vacancies at the X site.

Structure of the clean NiAl(1 1 0) surface and the Al 2O 3/NiAl(1 1 0) interface by measurements of crystal truncation rods

The clean NiAl(1 1 0) surface and the Al 2O 3/NiAl(1 1 0) interface have been investigated by synchrotron X-ray diffraction experiments. In the case of the oxide surface the analysis of the NiAl(1 1 0) crystal truncation rods (CTR) provide information about the interface between the Al 2O 3 film and the NiAl substrate. The analysis of the CTR-data shows clearly a rippled Ni–Al topmost surface with an amplitude value of R Ni/ Al =0.16±0.01 A ̊ for the clean surface and R Ni/ Al =0.18±0.02 A ̊ for the oxide covered surface. On the clean surface the Al sites are expanded by +3.8% (outwards) and the Ni sites are contracted by −3.2% (inwards) respect to the unrelaxed interlayer separation. For the oxide covered surface an increase of the expansion of the outermost Al atoms (+7.3%) relative to their bulk positions has been found, while the Ni atoms remain (−0.9%) at the bulk position. On both cases, an ideal surface stoichiometry (1:1) was obtained. However, some intermixing (chemical disorder) of one specimen in the sites of the other and vice versa was present (less than 4%). This chemical disorder was not enhanced by the presence of the Al 2O 3 overlayer. Neither rippling nor oscillatory relaxation in deeper layers was detectable.

Synthesis of non-stoichiometric lithium nickel cobalt oxides and their structural and electrochemical characterization

Non-stoichiometric phases of lithium nickel cobalt oxides were synthesized by a sol–gel method using oxalic acid as a chelating agent. The structural properties have been examined using X-ray diffraction techniques. Electrochemical coin cell studies showed materials with excess lithium stoichiometry had interesting properties of improved capacity and cyclability. Of all the compositions with excess lithium stoichiometry, Li 1.1Ni 0.8Co 0.2O 2, showed better electrochemical characteristics with a first cycle discharge capacity of 182 mAh/g and a 10th cycle of 172 mAh/g than the ideal stoichiometry LiNi 0.8Co 0.2O 2. The structural and electrochemical properties of Li x Ni 0.8Co 0.2O 2 with x=1.00, 1.05, 1.10 and 1.15 are discussed in detail.

Evidence of Anionic Disorder in Fluoride Borate Eu3(BO3)2F3 from Eu3+ Luminescence: Comparison with Fluoride Carbonate Ba2Eu(CO3)2F3

The optical behavior of europium fluoride borate Eu3(BO3)2F3 and barium fluoride carbonates Ba2Eu(CO3)2F3 is investigated with Eu3+ as a local structural probe. Tentative correlation between the optical and structural data are discussed. Unexpected broad emission lines are observed for Eu3+ in fluoride borate and the separation of two crystallographic sites, predicted from the structure determination, is impossible. It is assumed that deviation from the ideal stoichiometry Eu3(BO3)2F3 occurs and is due to the substitution BO3−3↔3F−, leading to the formulation Eu3(BO3)2+xF3−3x. Crystal field analysis is performed on the 7FJ basis of the 4f6 configuration of Eu3+. The crystal field parameters (cfp) and crystal field strength parameter NV are similar for both compounds though slightly larger for Eu3(BO3)2F3 This difference is especially significant for two rank (cfp), indicative of a larger electrostatic field in Eu3(BO3)2F3.

COMPOSITION AND Th - U - TOTAL Pb AGES OF HUTTONITE AND THORITE FROM GILLESPIE'S BEACH, SOUTH ISLAND, NEW ZEALAND

The average result of 49 electron-microprobe spot analyses of huttonite (monoclinic ThSiO 4 ) from its type locality, Gillespie’s Beach, southern Westland, New Zealand, yields the following chemical composition (in wt.%, ± 1σ standard deviation in italics in parentheses): P 2 O 5 0.43 ( 0.13 ), SiO 2 18.78 ( 0.17 ), ThO 2 79.21 ( 0.53 ), UO 2 0.66 ( 0.07 ), Y 2 O 3 0.35 ( 0.15 ), La 2 O 3 0.02 ( 0.02 ), Ce 2 O 3 0.06 ( 0.07 ), Pr 2 O 3 0.02 ( 0.02 ), Nd 2 O 3 0.12 ( 0.08 ), Sm 2 O 3 0.06 ( 0.05 ), Gd 2 O 3 0.10 (0.05 ), Dy 2 O 3 0.06 ( 0.04 ), CaO 0.01 ( 0.01 ), FeO 0.01 ( 0.01 ), PbO 0.03 ( 0.01 ), total 99.94 ( 0.32 ). Huttonite is very close to the theoretical ideal stoichiometry, with the general formula (Th 0.96 U 0.01 Y 0.01 REE 0.01 ) ∑0.99 (Si 0.99 P 0.02 ) ∑1.01 O 4 . It displays little compositional heterogeneity and is near end-member composition (93.2 to 98.4 mol.% ThSiO 4 ). The chondrite-normalized REE patterns are umbrella-like, with Gd being the most prominent rare-earth element. The P, U, Fe, and total REE contents of the grains studied differ strongly from those of the type material published by Pabst & Hutton (1951). Thorite (tetragonal ThSiO 4 ) from Gillespie’s Beach displays the following average composition ( n = 30): P 2 O 5 0.12 ( 0.09 ), SiO 2 18.84 ( 0.18 ), ThO 2 73.46 ( 1.85 ), UO 2 5.90 ( 1.46 ), Y 2 O 3 0.21 ( 0.11 ), La 2 O 3 0.00 ( 0.01 ), Ce 2 O 3 0.08 ( 0.05 ), Pr 2 O 3 0.02 ( 0.04 ), Nd 2 O 3 0.07 ( 0.05 ), Sm 2 O 3 0.04 ( 0.04 ), Gd 2 O 3 0.04 (0.04 ), Dy 2 O 3 0.04 ( 0.04 ), CaO 0.13 ( 0.08 ), PbO 0.43 ( 0.20 ), total 99.42 ( 0.44 ). Like huttonite, it is virtually non-metamict, nearly stoichiometric [(Th 0.90 U 0.07 Y+ REE 0.01 Ca 0.01 Pb 0.01 ) ∑1.00 Si 1.01 O 4 ] and approaches end-member composition (86.3 to 96.7 mol.% ThSiO 4 ). Th – U – total Pb dating reveals a late Tertiary (Miocene) age for the huttonite. The thorite records older ages, from late Triassic–early Jurassic to early Tertiary. All ages coincide with major metamorphic or magmatic events on South Island. Rocks that formed in the experimentally derived P–T stability field of huttonite are unknown in the area. There thus are suggestions of discrepancies between the experimental data on huttonite–thorite phase relations and those observed in natural systems.

Experimental study of Cr incorporation in pargasite

Pargasite compositions corresponding to the nominal structural formula NaCa 2 (Mg 4 Al 1-x Cr x )(Si 6 Al 2 )O 22 (OH) 2 , with O ≤ x ≤ 1, were investigated at 900 °C and 3 and 20 kbar P H₂O . The aims of this work were to determine the extent of the solid solution, i.e., the maximum x value, as a function of experimental conditions, and to characterize the M 3+ (Al 3+ and/or Cr 3+ ) distribution over the M sites. A first series of experiments at 900 ºC and 3 kbar showed the presence of large amounts of Cr 6+ as sodium chromate (Na 2 CrO 4 ) and dichromate (Na 2 Cr 2 O 7 ) in the final fluid phase, which promotes the extraction of Cr from the bulk solid (up to 45 mol%). The oxidizing agent was the oxygen from water, liberated by diffusion of hydrogen from water through the Pt tube wall. To prevent this oxidation, a new double-chamber tube was designed, one chamber contain mg powdered metallic Cr to trap the excess of oxygen and the second chamber initially containing the starting gel and water. Using this tube, (chroimum)-pargasites were produced with a maximum Cr content around 0.43 apfu from EMP analyses (based on 23 O atoms), close to the maximum Cr solubility observed in naturally occurring pargasites. In FTIR spectra of Cr-free pargasite, two intense OH-stretching bands are observed at 3714 and 3681 cm -1 , corresponding to OH groups adjacent to Mg 3 and to Mg 2 Al, respectively, and pointing toward a filled A-site. A third band is observed at 3656 cm -1 , which can be assigned to OH groups pointing toward vacant A-sites. Along the Al-Cr pargasite join, sig­nificant modifications are observed in the OH-stretching region: a new band appears at 3660 cm -1 , which can be assigned to OH groups adjacent to Mg 2 Cr, and pointing toward a filled A-site. Its relative intensity increases regularly with the Cr content, showing that Cr 3+ enters M site(s) adjacent to an OH group. Combining EMP and FTIR data, we conclude that up to 0.27 Cr per formula unit occupies the M3 site. However, these are more octahedral trivalent cations than expected, and these are balanced by lower tetrahedral Al and lower occupancies of the A and M4 sites. This charge arrangement, with important deviations from ideal stoichiometry, is apparently the only stable one under the conditions applied. Results were confirmed at 900 °C and 20 kbar P H₂O , indicating that increasing pressure does not affect Cr solubility in pargasite.

NiO2 Obtained by Electrochemical Lithium Deintercalation from Lithium Nickelate: Structural Modifications

A study of the system at high potential has shown the successive formation of two phases with O3 (AB CA BC) and O1 (AB) oxygen packings, respectively, near the composition. For the phases with z ≥ 0.07, only the O3 packing is observed at the end of lithium deintercalation: the extra nickel ions present in the interslab spaces destabilize the O1 type packing. The nonhomogeneous distribution of extra nickel ions from one interslab space to another leads to the presence of stacking faults in both phases. Lithium can be reversibly reintercalated into the phase. Good stability is observed at low charge‐discharge rates during long‐term cycling of the system over a large potential range. This study also provided evidence for inhomogeneity in the starting material at the crystallite scale. The material corresponds to a mixture of various phases with very similar stoichiometries. The crystallites which are closest to the ideal stoichiometry form an O1 type phase which is stable at high potential, whereas those which are farthest from stoichiometry lead to an O1 type phase which is slowly transformed into a new phase, characterized by an O3 type packing, through a migration from the slab to the interslab space. © 2000 The Electrochemical Society. All rights reserved.