Rock Salt Region Research Articles

Electron microscopy and spectroscopic study of structural changes, electronic properties, and conductivity in annealed LixCoO2

Chemically exfoliated nanoscale few-layer thin Li$_x$CoO$_2$ samples are studied as function of annealing at various temperatures, using transmission electron microscopy (TEM) and Electron Energy Loss Spectroscopies (EELS), probing the O-K, Co-L$_{2,3}$ spectra along with low energy interband transitions. These spectra are compared with first-principles DFT calculations of -Im$[\varepsilon^{-1}(q,\omega)]$ and O-2p Partial Densities of States weighted by dipole matrix elements with the core wavefunction and including the O-1s core-hole and with known trends of the L$_2$/L$_3$ peak ratio to average Co valence. Trends in these spectra under the annealing procedures are established and correlated with the structural phase changes observed from diffraction TEM and High Resolution TEM images. The results are also correlated with conductivity measurements on samples subjected to the same annealing procedures. A gradual disordering of the Li and Co cations in the lattice is observed starting from a slight distortion of the pure LiCoO$_2$ $R\bar{3}m$ to $C2/m$ due to the lower Li content, followed by a $P2/m$ phase forming at 200$^o$C indicative of Li-vacancy ordering, formation of a spinel type $Fd\bar{3}m$ phase around 250$^o$C and ultimately a rocksalt type $Fm\bar{3}m$ phase above 350$^o$C. This disordering leads to a lowering of the band gap as established by low energy EELS. The O-K spectra of the rocksalt phase are only reproduced by a calculation for pure CoO and not for a model with random distribution of Li and Co. This indicates that there may be a loss of Li from the rocksalt regions of the sample at these higher temperatures. The conductivity measurements indicate a gradual drop in conductivity above 200$^o$C, which is clearly related to the more Li-Co interdiffused phases, in which a low-spin electronic structure is no longer valid and stronger correlation effects are expected.

The Role of Intragranular Nanopores in Capacity Fade of Nickel-Rich Layered Li(Ni1-x-yCoxMny)O2 Cathode Materials.

Ni-rich layered LiNi1-x-yCoxMnyO2 (NCM, x + y ≤ 0.2) is an intensively studied class of cathode active materials for lithium-ion batteries, offering the advantage of high specific capacities. However, their reactivity is also one of the major issues limiting the lifetime of the batteries. NCM degradation, in literature, is mostly explained both by disintegration of secondary particles (large anisotropic volume changes during lithiation/delithiation) and by formation of rock-salt like phases at the grain surfaces at high potential with related oxygen loss. Here, we report the presence of intragranular nanopores in Li1+x(Ni0.85Co0.1Mn0.05)1-xO2 (NCM851005) and track their morphological evolution from pristine to cycled material (200 and 500 cycles) using aberration-corrected scanning transmission electron microscopy (STEM), electron energy loss spectroscopy, energy dispersive X-ray spectroscopy, and time-of-flight secondary ion mass spectrometry. Pores are already found in the primary particles of pristine material. Any potential effect of TEM sample preparation on the formation of nanopores is ruled out by performing thickness series measurements on the lamellae produced by focused ion beam milling. The presence of nanopores in pristine NCM851005 is in sharp contrast to previously observed pore formation during electrochemical cycling or heating. The intragranular pores have a diameter in the range between 10 and 50 nm with a distinct morphology that changes during cycling operation. A rock-salt like region is observed at the pore boundaries even in pristine material, and these regions grow with prolonged cycling. It is suggested that the presence of nanopores strongly affects the degradation of high-Ni NCM, as the pore surfaces apparently increase (i) oxygen loss, (ii) formation of rock-salt regions, and (iii) strain-induced effects within the primary grains. High-resolution STEM demonstrates that nanopores are a source of intragranular cracking during cycling.

Fluorine insertion into the Ruddlesden–Popper phase La2BaFe2O7: the structure and magnetic properties of La2BaFe2O5F4

Fluorination of the n = 2 Ruddlesden-Popper phase La2BaFe2O7 occurs at ~300 °C in flowing 10% F2 in N2 to form La2BaFe2O5F4. This oxide fluoride contains 2F(-) ions in interstitial sites within the rocksalt regions and 2F(-) ions that have substituted for O(2-) ions in apical sites within the rocksalt layers. The fluorination results in an expansion along c of 7.6% to yield a tetragonal unit cell of dimensions a = 3.96237(7) Å, c = 22.3972(5) Å. The structure and magnetic properties have been examined by Mössbauer spectroscopy, neutron powder diffraction and magnetic susceptibility measurements. La2BaFe2O5F4 becomes antiferromagnetically ordered at temperatures below ~500 K, and the magnetic order shows a striking resemblance to that observed in La2BaFe2O7. The magnetic moments on Fe(3+) are perpendicular to [001] and aligned along ±{100} directions above 300 K, but at temperatures below 200 K, they rotate by 45° to lie along ±{110}. Mössbauer spectroscopy suggests the presence of Fe(3+) within the primary phase, but also indicates that fluorination results in some particle fragmentation to form a paramagnetic component of the fluorinated material.

The spinel and cubic rocksalt solid-solutions in the Li–Mn–Ni oxide pseudo-ternary system

Abstract Solution-based combinatorial samples of lithium manganese nickel oxide were synthesized and studied by X-ray diffraction in order to map out the pseudo-ternary system over the entire metal composition ranges. More than 300 compositions were prepared and many duplicates were made to test reproducibility. The samples were made by heating to 800 °C in an oxygen atmosphere and two cooling methods were tested: quenching as well as a slower cooling rate. A partial phase diagram in air was also determined wherein regions showing extreme lithium loss were avoided. This article focuses on two solid solutions: the spinel and rocksalt regions. Both are larger than previously suspected which is of significance for composite electrode materials of Li-ion batteries Part of the rocksalt solid solution region shows a new structure with a cubic lattice, similar to that of NiO rocksalt but with ordering of manganese, lithium and metal site vacancies such that the scattering pattern resembles that of Ni 6 MnO 8 . The results obtained here confirm that the spinel structures have oxygen site vacancies when quenched even when synthesized in pure oxygen, while slow cooling relieves these vacancies. The boundaries are found to move during slow cooling in such a way that is consistent with the reaction equilibrium favoring the high oxygen content spinel phases. The high level of precision obtained in drawing the solid solution boundaries and the tie-lines in co-existence regions using quantitative phase analysis establishes the use of combinatorial samples in mapping complex pseudo-ternary diagrams.

Thermodynamics of Solid‐Solution Formation in NiO‐CuO

Samples in the (NixCu1‐x)O system, 0.5 ≤x≤ 1.0, have been examined using powder X‐ray diffraction and high‐temperature Solution calorimetry. According to the X‐ray characterization samples form a one‐phase rock salt solid solution up to x= 0.30, and crystallize with a tetragonal structure at x= 0.35. The enthalpies of mixing in the rock salt region of the system show large negative deviations from ideal mixing. The tetragonal compound does not show a large increase in enthalpic stability relative to the rock salt solid solutions. The variation of stability with composition is complex and suggests that substantial stabilization occurs at Ni‐rich compositions. The thermodynamic mixing properties have been interpreted in terms of cation order between the Ni and Cu ions. The degree of cation order appears to increase up to the composition x= 0.65 at which a compound with substantial cation order crystallizes with a tetragonally distorted structure. The calorimetric results correlate well with TEM observations of a complex microstrueture.

The Electronic States of Tetracyanoethylene and Its Anion Radical as Studied by Their Infrared Spectra

Abstract The infrared spectra of tetracyanoethylene (TCNE) and its anion radical in the rock-salt region were examined in order to observe any appreciable frequency differences between their corresponding bands. The vibrational assignments were made, and the basic Urey-Bradley force field was determined for the TCNE anion radical. By comparing them with those of the neutral TCNE, we discussed the difference in the electronic states between the neutral TCNE and its anion radical. The half-occupied molecular orbital of the TCNE anion radical was found to belong to the B2g irreducible representation.

Stereochemistries of three types of bi-nuclear copper(II) chloride complexes with N-substituted salicylideneimines

The preparation of some bi-nuclear copper(II) chloride complexes with N-substituted salicylideneimines, [ Cu( O 2. C 6 H 4.( 5X|5). C 1 H − R) Cl] 2, where R = alkyl, substituted phenyl groups and (CH 2) 3OH; and X = H, Cl, CH 3 and NO 2; and some addition compounds of these with two moles of dimethylformamide (DMF) and dimethylsulfoxide (DMSO) per one dimeric molecule are reported. Measurements of magnetic moments at room temperatures or for some of them temperature variations of magnetic susceptibilities over a temperature range 76°–320°K, reflection spectra at 2000–2340 mμ, and i.r. spectra in rock salt region were recorded for these complexes. These complexes are classified into three types of different stereochemistries based on color and reflection spectra: (1) B-type, brown in color, pseudo-tetrahedral; (2) YG-type, yellow-green, square pyramidal; and (3) Y-type yellow-orange, trigonal-bipyramidal. The empirical rule, “higher energies of d- d bands — lower magnetic moments”, was valid in the B-type complexes. For the Y-type complexes, the existence of electronegative atom or groups seems to be required.

Electrical and Optical Properties of the Iodine Complexes of Phenoxazine, Phenoselenazine, and Benzophenothiazines

Abstract The electrical resistivities and Seebeck coefficients of the 2 : 3 iodine complexes of phenoxazine, phenoselenazine, and benzophenothiazines were measured as a function of the temperature. They have resistivity values at 20°C ranging from 10 ohm·cm for the phenoxazine complex to 34 ohm·cm for the benzo[a]phenothiazine complex. Employing the equation of Johnson and Lark-Horowitz for the Seebeck coefficient of an intrinsic semiconductor, the ratio of the electron-to-hole mobility in the phenoselenazine complex was found to be 1.07 and the electron mobility was estimated as 0.26 cm2/V sec. By an examination of the vibrational spectra, the expected cation-radical character was confirmed for the complexes of phenoselenazine and benzophenothiazines. The presence of a broad electronic absorption band in the rock-salt region characteristic of low-resistivity organic semiconductors was noted in every case.

Approximate activity-composition relations in the system MgOZnO at 1205±5°C

Activity-composition relations in the system MgOZnO were derived from the direction of conjugation lines between oxide and spinel phases in the ternary system MgOZnOAl 2O 3 at 1205±5°C in air, under the assumption that the spinel phase behaves ideally. The activity curves in the rocksalt region appear more complex than for the CoOZnO and NiOZnO systems studied previously. For the transformation: MgO (rocksalt) = MgO (zincite), ΔG = 9·6 kcal.

Semiconducting Anion-radical Salts Derived from 3,3′5,5′-Tettrahalo-p-diphenoquinones

Abstract 3,3′5,5′-Tettrahalo- and 3,3′,5,5′-tetrabromo-p-diphenoquinones were found to form series of complex anion-radical salts of the M+(Q2)− or M+2(Q3)2− type with various onium cations. These complex salts are darkcolored, almost black in many cases, and show electronic absorptions extending into the rock-salt region. In addition, bluish green-colored simple salts could be isolated with a few cations. The complex salts exhibit electrical resistivities in orders from 10 to 104 ohm cm at room temperature. Neither the pattern due to the quinone molecule nor that due to the semiquinone ion is present in their vibrational spectra. A single carbonyl stretching vibration band appears around 1600 cm−1, intermediate between that in the quinone and that in the semiquinone salt.

The preparation and spectroscopic studies of morpholine complexes with nickel(II), copper(II) and zinc(II) sulphates

The preparation of morpholine complexes of nickel(II), copper(II) and zinc(II) sulphates is described. U.v., visible reflectance and i.r. spectra in the rock salt and low frequency region have been recorded for these complexes in the solid state. Possible structures for these complexes in the solid state are discussed. I.r. spectra in the rock salt region indicate coordinated sulphate groups and that the morpholine is coordinated via the nitrogen and not the oxygen atom. It is suggested that (a) Ni(morpholine) 2SO 4 is a polymeric structure with bridging sulphate groups, (b) Cu(morpholine) 2SO 4 is square planar with sulphate group acting as a chelate, and (c) Zn(morpholine) 2SO 4 and Zn(morpholine)SO 4 are polymeric structures with the sulphate group functioning probably as chelate and of symmetry lower than C 2v Allocation for the possible metal-oxygen and metal-nitrogen modes have also been made.

The preparation and spectroscopic studies of morpholine complexes with some transition metal (II) halides

The preparation of morpholine complexes of manganese (II), cobalt (II), copper (II), zinc (II) and cadmium (II) halides is described. Infrared spectra in the rock salt region indicates that the morpholine is co-ordinated via the nitrogen and not the oxygen atom. Ultraviolet and visible reflectance spectra and the far i.r. spectra have been measured and indicate and essentially octahedral structure for the manganese (II) and cadmium (II) complexes, and a tetrahedral structure for the cobalt (II) and zinc (II) complexes. Possible metal-nitrogen (350–310 cm −1) and metal-halogen modes have been assigned.

Structure of the Aqueous Pertechnetate Ion by Raman and Infrared Spectroscopy. Raman and Infrared Spectra of Crystalline KTcO4, KReO4, Na2MoO4, Na2WO4, Na2MoO4·2H2O, and Na2WO4·2H2O

The Raman crystal spectra of KTcO4, KReO4, Na2MoO4, Na2WO4, Na2MoO4·2H2O, and Na2WO4·2H2O, and the Raman aqueous spectra of TcO4— and MoO42— have been observed. Infrared spectra of Nujol mulls of all the salts were measured in the rock salt region. Infrared spectra in the CsBr region were also obtained for Na2MoO4 and Na2WO4. For the tetrahedral ions (Td symmetry) of the latter two salts v2(e)>v4(f2), which represents an inversion of the usual order. Examination of all the spectra leads to the conclusion that the Raman spectrum of aqueous pertechnetate ion, which shows only two lines, represents the spectrum of tetrahedral TcO4— ion perturbed by the close association of water molecules. The appearance of two instead of four Raman lines in the spectrum is shown to result from the accidental degeneracy of v1(a1) and v3(f2) vibrations. The v2(e) line is not detected probably because of its low intensity and broadness. The observation of three rather than the expected four Raman lines for molybdate ion is similarly believed to be due to the low intensity and broadness of the v2(e) line. This explanation is also offered as an alternative explanation for the absence of the fourth line in the Raman spectrum of ReO4—(aq) and WO42—(aq) to that previously given, namely, that v2(e) and v4(f2) are sufficiently close together that they are unresolved.

The Infrared Spectra of Some Chlorinated Hydrocarbons in the Cesium Bromide Region

The infrared spectra of 54 C1 to C4 chlorinated hydrocarbons in the cesium bromide region are presented Rough frequency correlations are seen for various structural features in the compounds studied However, these correlations are not sufficiently definitive for structure determination. Many of the compounds give unique absorption bands which would permit their detection and determination in the presence of very closely related compounds This is especially important in a mixture of compounds such as cis- and trans-1-chloro-1,3-butadiene where a determination of the ratio of the isomers is difficult in the rocksalt region.

Studies in IVth group organometallic chemistry. XVI: Synthesis of organogermanium compounds containing the p‐phenylene group. Some infrared characteristics of p‐phenylene derivatives of silicon, germanium, tin and lead

AbstractA few oligomeric and polymeric p‐phenylenegermanes have been prepared by Wurtz‐type condensation reactions from p‐chlorophenyl‐ and chlorogermanes. The oligomeric and polymeric products obtained have been characterized by infrared spectroscopy, elementary analysis and molecular weight determinations.Infrared spectra in the rocksalt region of phenyl and p‐phenylene derivatives of silicon, germanium, tin and lead are discussed. The spectra of the p‐phenylene compounds show some characteristic absorption bands shifting to lower frequencies as the weight of the metal atoms in the molecule increases.

Infrared Absorption Spectra of Hydrazides. III. Hydrazides of Aliphatic Acids

Abstract The infrared absorption spectra of the hydrazides of aliphatic acids have been examined in the rock salt region. The N-H stretching frequencies are found near 3200 and 3030 cm−1 in the solid state and near 3320 cm−1 in dilute solution. The amide I band is seen near 1600 cm−1 in the solid state, while in solution there are two bands, near 1720 and 1690 cm−1. In the solid state the amide II band appears near 1490 cm−1. The amide III band is found near the lower limit of the normal range of 1305∼1200 cm−1 for the secondary amides.

Infrared Absorption Spectra of Hydrazides. I. Hydrazides of Aromatic Acids

Abstract The infrared absorption spectra of acetylbenzoylhydrazine, trichloroacetylbenzoylhydrazine and dibenzoylhydrazine have been studied in the rock salt region. The assignment of the amide frequencies characteristic to the hydrazides RCONHNHCOR′ and the coupling between vibrations due to two CONH groups have been discussed.

Infrared Absorption Spectra of Hydrazides. II. Sodium Hydrazides

Abstract The infrared absorption spectra of HCONH·NCOH· Na, CH3CONHNCOCH3· Na, CH3CON·HNCOC6H5· Na, and C6H5CONHNCOC6H5· Na and their N-deuterated derivatives have been examined in the rock salt region. The characteristic frequencies of the ions (RCON·HNCOR′)- have been obtained, and the influence of an electronic charge on the amide frequencies has been discussed.

Molecular Vibrations and Structures of High Polymers. I. General Method of Normal Coordinate Treatment by Internal Coordinates and Infrared Frequencies and Conformations of (—CH2—)n, (—CH2–O—)n, and (—CH2–O–CH2—)n

The normal coordinate treatment of the (—A—)n chain was made in terms of internal symmetry coordinates and optically active frequencies of (—CH2—)n were calculated for various chain conformations. The calculated frequencies of polyethylene and cyclopentane agreed with the observed values. The low infrared frequency was found to be structure sensitive. The infrared band of polytetrafluoroethylene at approx 100 cm—1 may be primarily due to this mode. The low infrared frequencies of polyoxymethylene were compared with the corresponding frequencies of (—A—)n calculated for various conformations and were found to be in accord with Huggins' model but not with the planar zigzag structure. The normal coordinate treatment of (—A—B—)n was made taking into account the torsional potential as well as the stretching and bending potentials, and vibrational assignments of polyoxymethylene were made. For poly-(ethylene glycol) a structure model was proposed. This model is made up of only the gauche configuration throughout the helical chain and contains seven chemical units and five turns of the helix per fiber period. The polarized infrared spectra of this polymer were measured in the region 800–400 cm—1 and the observed skeletal frequencies were compared with the corresponding frequencies of (—A—)n calculated for various conformations. The infrared spectra were found to be in accord with the model proposed here. The infrared spectra in the rocksalt region were also reasonably assigned. Finally a general method of treating any infinite helical chain belonging to dihedral group is presented in terms of real internal symmetry coordinates. The G or F matrix of an infinite order is factored into the set of matrices G(δ) or F(δ) associated with the phase difference δ. The method is explained for the cases of polyoxymethylene and poly-(ethylene glycol).

Infrared Spectra of Water of Crystallization in Some Inorganic Chlorides and Sulfates

Abstract The characteristic bands of water of crystallization in various chlorides and sulfates were determined in the rock salt region. The results were classified and explained inclusively by the effects of coördination and hydrogen bonding.