Partial Cation Ordering Research Articles

Partial cation ordering, relaxor ferroelectricity, and ferrimagnetism in Pb(Fe1 − xYbx)2/3W1/3O3 solid solutions

The structural, magnetic, and dielectric properties of ceramic samples of Yb-doped PbFe2/3W1/3O3 have been investigated by a variety of methods including x-ray powder diffraction, magnetometry, and dielectric spectroscopy. In addition, theoretical investigations were made using first-principles density functional calculations. All the doped samples Pb(Fe1 − xYbx)2/3W1/3O3 (PFYWO) (0.1 ≤ x ≤ 0.5) were found to crystallize in an ordered cubic (Fm3¯m) structure with partial ordering in the B-perovskite sites. Observed changes in the cationic order were accompanied by differences in the dielectric and magnetic responses of the system. While pure PbFe2/3W1/3O3 is antiferromagnetic, the doped Pb(Fe1 − xYbx)2/3W1/3O3 PFYWO samples display excess moments and ferrimagnetic-like behavior, associated with differences in B′ and B″ site occupancies of the magnetic Fe3+ cations. The magnetic transition temperature of the ferrimagnetic phase is found to decrease with increasing Yb content, from TN ∼350 K of the undoped sample down to 137 K for x = 0.5. All PFYWO compounds display a ferroelectric relaxor behavior akin to that of PbFe2/3W1/3O3, albeit our results show significant changes of the frequency and temperature dependence of the dielectric properties. The changes of the properties of PFYWO with increasing Yb substitution can be explained by the changes in the cation size/charge mismatch and the size difference of the two ordered positions.

Structure of Pb(Fe2/3W1/3)O3 single crystals with partial cation order

Despite intensive studies on the complex perovskite Pb(Fe2/3W1/3)O3 (PFWO) relaxor, understanding the exact nature of its multifunctional properties has remained a challenge for decades. In this work we report a comprehensive structural study of the PFWO single crystals using a combination of synchrotron X-ray diffraction and high-resolution electron microscopy. The set of {h + ½, k + ½, l + ½} superlattice reflections was observed for the first time based on single-crystal synchrotron X-ray experiments (100–450 K) and transmission electron microscopy investigations, which indicates some kind of B-cation ordering in PFWO which had been thought to be totally disordered. It was found that (1) the crystal structure of PFWO should be described by a partly ordered cubic perovskite (i.e. Fm − 3m), (2) the weak ferromagnetic properties and excess magnetic moment of PFWO can be understood based on non-random distribution of Fe cations between the 4a and 4b sites, and (3) the Pb displacement disorder is present in this material and the cations are probably displaced along the <100> directions. The X-ray diffraction results of this investigation show that partial cation ordering indeed exists in PFWO, which makes it necessary to revisit the generally accepted interpretations of the results obtained up to date. In agreement with X-ray diffraction study the main results of TEM study include: (1) a long range order that can be described with the Fm − 3m symmetry is reliably detected, (2) the coherence length of that long range order is in the order of 1–2 nm and (3) no remarkable chemical inhomogeneity is found in the tested PFWO crystal, excluding the possibility of a compositional ordering arising from substitutional defects in the perovskite structure.

Carbocernaite from Bear Lodge, Wyoming: Crystal chemistry, paragenesis, and rare-earth fractionation on a microscale

Zoned crystals of carbocernaite occur in hydrothermally reworked burbankite-fluorapatite-bearing calcite carbonatite at Bear Lodge, Wyoming. The mineral is paragenetically associated with pyrite, strontianite, barite, ancylite-(Ce), and late-stage calcite, and is interpreted to have precipitated from sulfate-bearing fluids derived from an external source and enriched in Na, Ca, Sr, Ba, and rare-earth elements (REE) through dissolution of the primary calcite and burbankite. The crystals of carbocernaite show a complex juxtaposition of core-rim, sectoral, and oscillatory zoning patterns arising from significant variations in the content of all major cations, which can be expressed by the empirical formula (Ca 0.43–0.91 Sr 0.40–0.69 REE 0.18–0.59 Na 0.18–0.53 Ba 0–0.08 ) ∑1.96–2.00 (CO 3 ) 2 . Interelement correlations indicate that the examined crystals can be viewed as a solid solution between two hypothetical end-members, CaSr(CO 3 ) 2 and NaREE(CO 3 ) 2 , with the most Na-REE-rich areas in pyramidal (morphologically speaking) growth sectors representing a probable new mineral species. Although the Bear Lodge carbocernaite is consistently enriched in light REE relative to heavy REE and Y (chondrite-normalized La/Er = 500–4200), the pyramidal sectors exhibit a greater degree of fractionation between these two groups of elements relative to their associated prismatic sectors. A sample approaching the solid-solution midline [(Ca 0.57 Na 0.42 ) ∑0.99 (Sr 0.50 REE 0.47 Ba 0.01 ) ∑0.98 (CO 3 ) 2 ] was studied by single-crystal X-ray diffraction and shown to have a monoclinic symmetry [space group P 11 m , a = 6.434(4), b = 7.266(5), c = 5.220(3) A, γ = 89.979(17)°, Z = 2] as opposed to the orthorhombic symmetry (space group Pb 2 1 m ) proposed in earlier studies. The symmetry reduction is due to partial cation order in sevenfold-coordinated sites occupied predominantly by Ca and Na, and in tenfold-coordinated sites hosting Sr, REE, and Ba. The ordering also causes splitting of carbonate vibrational modes at 690–740 and 1080–1100 cm −1 in Raman spectra. Using Raman micro-spectroscopy, carbocernaite can be readily distinguished from burbankite- and ancylite-group carbonates characterized by similar energy-dispersive spectra.

The Configurational Space of Rocksalt‐Type Oxides for High‐Capacity Lithium Battery Electrodes

A unifying theory is presented to explain the lithium exchange capacity of rocksalt‐like structures with any degree of cation ordering, and how lithium percolation properties can be used as a guideline for the development of novel high‐capacity electrode materials is demonstrated. The lithium percolation properties of the three most common lithium metal oxide phases, the layered α‐NaFeO2 structure, the spinel‐like LT‐LiCoO2 structure, and the γ‐LiFeO2 structure, are demonstrated and a strong dependence of the percolation thresholds on the cation ordering and the lithium content is observed. The poor performance of γ‐LiFeO2‐type structures is explained by their lack of percolation of good Li migration channels. The spinel‐like structure exhibits excellent percolation properties that are robust with respect to off‐stoichiometry and some amount of cation disorder. The layered structure is unique, as it possesses two different types of lithium diffusion channels, one of which is, however, strongly dependent on the lattice parameters, and therefore very sensitive to disorder. In general it is found that a critical Li‐excess concentration exists at which Li percolation occurs, although the amount of Li excess needed depends on the partial cation ordering. In fully cation‐disordered materials, macroscopic lithium diffusion is enabled by ≈10% excess lithium.

Modular Construction of Oxide Structures—Compositional Control of Transition Metal Coordination Environments

The effects of reaction temperature and pO2 were investigated on a series of (Ba,Ca,Nd)FeO3-delta perovskite systems in order to isolate phases containing ordered arrangements of the distinct vacancy and cation ordering patterns identified in less compositionally complex iron oxide systems. Initial synthesis in air at high temperature yields cubic perovskite phases (I) with average iron oxidation states higher than 3; selected area electron diffraction together with diffuse features observed in the synchrotron X-ray diffraction (SXRD) patterns of these materials show evidence of small domains of short-range cation and vacancy order. Annealing these materials in nitrogen or in a sealed tube in the presence of an NiO/Ni buffer yielded the Fe(3+) phase Ca2Ba2Nd2Fe6O16 (II), closely related to Sr2LaFe3O8 but with partial cation order as well as anion order present the larger Ba cations are largely present in the 12-coordinate site between the octahedral iron layers, and Ca is largely present in 10-coordinate sites between octahedral and tetrahedral sites. Further reduction of Ca2Ba2Nd2Fe6O16 using a Zr getter yields the mixed-valence phase Ca2Ba2Nd2Fe6O15.6 (III). The structure of III was solved by maximum entropy analysis of XRD data coupled with analysis of high-temperature neutron diffraction data and refined against combined SXRD and high-Q ambient-temperature neutron data. This material crystallizes in a 20-fold perovskite super cell (Imma, a approximately square root(2 x a(p), b approximately 10 x a(p), c approximately square root(x 2a(p)) and can be visualized as an intergrowth between brownmillerite (Ca2Fe2O5) and the YBa2Fe3O8 structure. There are three distinct iron coordination environments, octahedral (O), square-pyramidal (Sp), and trigonal planar (Tp, formed by distorting the tetrahedral site in brownmillerite), which form a Sp-O-Tp-O-Sp repeat. Bond valence calculations indicate that Tp is an Fe(2+) site, while the O and Sp sites are Fe(3+). The A-site cations are also partially ordered over three distinct sites: 8-coordinate between the Sp layers, 10-coordinate between Tp and O layers, and 12-coordinate between Sp and O layers. Mossbauer spectroscopy, magnetometry, and variable-temperature neutron diffraction show that the material undergoes two magnetic transitions at approximately 700 and 255 K.

Synthesis and electrochemistry of layered 0.6LiNi 0.5Mn 0.5O 2· xLi 2MnO 3· yLiCoO 2 ( x+ y=0.4) cathode materials

Layered cathode materials with the nominal compositions of 0.6LiNi 0.5Mn 0.5O 2· xLi 2MnO 3· yLiCoO 2 ( x+ y=0.4) were synthesized by conventional solid-state reaction. Their XRD patterns indicated the formation of solid solutions. The content of Li 2MnO 3 in the solid solutions presented important effects on structure and electrochemistry. The end member, 0.6LiNi 0.5Mn 0.5O 2·0.4LiCoO 2, was isostructural to LiCoO 2, but increasing Li 2MnO 3 content in solid solution materials brought about partial cation ordering. Electrochemical results demonstrated that the increasing of Li 2MnO 3 content in materials improved cycleability. The samples with 0.2≤ x≤0.4 delivered the initial discharge capacity of around 190 mA h/g with good cycling performance and are the promising cathode materials in view of capacity and cycleability.

Mechanically-Activated Synthesis and Mixed Conductivity of TbMO4-δ(M = Zr, Hf) Ceramics

Terbium hafnate and zirconate ceramics with submicron grain sizes were prepared via mechanically-activated synthesis. X-ray and electron diffraction and infrared (IR) absorption spectroscopy showed that TbZrO4−δ has a disordered fluorite-type structure, while TbHfO4−δ is partially ordered, containing pyrochlore microdomains. The oxygen ion transference numbers determined by the modified e.m.f. technique under oxygen/air gradient, vary in the range 0.08–0.26 at 873–1123 K, increasing with temperature. The activation energies for ionic and p-type electronic transport are 82–83 and 29–40 kJ/mol, respectively. The ionic conduction becomes dominant in reducing atmospheres, but tends to decrease at low p(O2). Oxygen partial pressure dependencies of Seebeck coefficient can be described by a model common for oxide phases with mixed ionic and electron-hole conductivity. Due to partial cation ordering, terbium hafnate exhibits lower ionic and hole transport as compared to TbZrO4−δ. The average thermal expansion coefficients of TbMO4−δ (M = Zr, Hf) ceramics in air, calculated from dilatometric data, are (11.5–12.4) × 10−6 K−1 at 600–1200 K and (18.4–20.3) × 10−6 K−1 at 1200–1420 K.

The mechanism of self-reversal of thermoremanence in natural hemoilmenite crystals: new experimental data and model

New magnetic and mineralogical findings on self-reversing hemoilmenite (Fe 2− y Ti y O 3) grains from Pinatubo lavas (1991 eruption) provide important clues regarding the acquisition process of reverse thermoremanent magnetization (rTRM) in this solid solution series. Magnetic force microscopy indicates the presence of multidomain magnetic structures in coexisting strongly and weakly magnetic crystallographic regions having compositions of y≅0.54 and 0.53, respectively. Continuous thermal demagnetization of natural and laboratory TRM carried out on both whole rock samples and single hemoilmenite crystals shows that the magnitude of a normal TRM (nTRM) component, observed at temperatures above the Curie point of the self-reversing phase, is much too large to be carried by a phase that is entirely cation-disordered. Consistent with this observation are findings using transmission electron microscopy (TEM) which, in contrast to that what is commonly assumed, reveals the weakly magnetic regions to be magnetically heterogeneous. Specifically, these regions are found to contain tiny (20–40 nm) domains that are cation-ordered and evidently ferrimagnetic dispersed within the cation-disordered, presumably spin-canted antiferromagnetic matrix. Given these findings, we argue that the so-called nTRM-carrying x-phase is itself partially cation-ordered, and, thus, ferrimagnetic, as postulated first by Ishikawa and Syono (J. Phys. Soc., Jpn. 17 (1962) 714). We propose a “nanophase” self-reversal model for the ilmenite–hematite solid solution series in which the rTRM and nTRM components are carried by the cores and margins, respectively, of the tiny, partially cation-ordered nano-sized domains observed by TEM. Due to the partial cation order, both the core and the margin of each domain are expected to behave in a ferrimagnetic fashion at temperatures below their respective Curie points. However, given the kinetics of the ordering process, their cation distributions need be antiphase, which causes their magnetic moments to be oppositely aligned. Since it is most reasonable to consider each margin to be slightly more Fe-rich than the inside core, upon cooling, the margins acquire a magnetic remanence first (a nTRM). Then, upon further cooling, given that the intralayer and interlayer nearest-neighbor superexchange interactions are ferromagnetic and antiferromagnetic, respectively, the net magnetic moment of the core material need be oppositely aligned (producing a rTRM). The nano-sized regions would indeed behave in a superparamagnetic (SP) fashion if magnetically uncoupled to adjacent material; however, the spins in the margins (the x-phase) must be locked through superexchange to those of the surrounding disordered matrix, which we also claim to be locally enriched in iron. If so, then the magnetization of the x-phase can be both highly-coercive and thermally stable, as observed experimentally. Upon stepwise thermal demagnetization, the self-reversed remanence measured at room temperature is not destroyed until the unblocking temperature of the disordered Fe-enriched aureole (approximately 410°C) is reached. Mineralogical considerations and magnetic evidence from previous works suggest that this model is generally valid for self-reversed dacitic pumice, in particular the Mt. Haruna dacite and the 1985 Nevado del Ruiz dacitic andesite.

New Data on Chemical Delithiation of Li xNi 2- xO 2 (0.6 < x < 1)

New data on acid delithiation of lithium-nickel oxides, Li x Ni 2- x O 2 (0.6 < x < 1), are obtained using EPR of low-spin Ni 3+ ions, magnetic susceptibility measurements, and thermal analysis. It is shown that short-range and long-range cation order in the solid solutions Li x Ni 2- x O 2 specify their behavior toward acids. For Li x Ni 2- x O 2 oxides (0.6 < x < 0.9), where partial cation order occurs, acids remove lithium and impurity nickel ions from the LiO 2-layers, but do not attack Ni 2+ and Ni 3+ ions, which are segregated in the neighboring (111) cubic planes. For Li x Ni 2- x O 2 oxides ( x > 0.9), where long-range cation order is developed, the extraction of lithium and impurity nickel ions proceeds concomitantly with partial exchange between lithium from the LiO 2-layers and protons from the acid solution. The different chemical compositions of acid-treated oxides Li x Ni 2- x O 2 (0.6 < x < 0.9) and Li x Ni 2- x O 2 (0.9 < x < 1) are manifested by their thermochemical properties up to 200°C: for the partially ordered oxides (0.6 < x < 0.9), reduction of highly oxidized nickel ions and cation redistribution occur within the initial structural framework, whereas for the layered oxides ( x > 0.9), dehydration processes also take place.

Li3Ni2TaO6: A novel rock salt superstructure phase with partial cation order

The new phase Li3Ni2TaO6 has a novel rock salt superstructure, a= 8.4259(3)A, b= 5.9073(3)A, c= 17.7329 A, Fddd. Within the cubic-close-packed oxide array, Ta occupies one set of octahedral sites giving isolated TaO6 octahedra which edge-share with Li/NiO6 octahedra Li and Ni are distributed non-randomly over three other sets of octahedral sites with Li : Ni occupancy ratios of 0.73 : 0.27, 0.59 : 0.41 and 0.55 : 0.45, respectively The higher Li : Ni ratio of the first set, 0.73 : 0.27 may be associated with cation-cation repulsion effects since this site is closest to Ta. Li3Ni2TaO6 is a very modest semiconductor with conductivity of ca. 4 × 10–6Ω–1 cm–1 at 300 °C and activation energy 0.77 eV, the disorder in the Li+ site occupancy is, therefore, static and does not yield significant levels of Li+ ion conductivity.

Thermoanalytic investigations of the semiconductor TlSbSe 2

Differential thermal analyses (DTA), dilatometric, and X-ray powder diffraction studies of the semiconductor TISbSe 2 show the existence of three phases. Between the high temperature (HT) phase with statistically distributed cations and the room temperature (RT) phase, an intermediate phase with partial cation order (PO phase) was found. The phase transition from HT to PO phase is of first order, where both displacement and atomic exchange processes occur. The phase transition from PO to RT is of higher order, accompanied by diffusion processes, which increase the order of the cations. The completely ordered RT phase can be obtained by long time annealing (e.g. four weeks at 350 °C). Another method to obtain the well ordered RT phase is to pass repeatedly the phase transitions without melting the material. The coefficients of thermal expansion have been measured for the temperature range from 25 to 300 °C.

Bismuth valence order-disorder study in BaBiO3 by powder neutron diffraction

Two structural arrangements have been shown to exist for the Perovskite-like compound BaBi0.53+ Bi0.55+ O3 by powder neutron diffraction data. The first is characterized by a partial (75%) order of the Bi3+ and Bi5+ cations on the two crystallographically independent sites, while in the second arrangement the two cations are almost 100% disordered. The structure of eleven differently-prepared BaBiO3 samples showed that the deciding factor for obtaining one or the other arrangement is the temperature at which the sample is prepared or subsequently heat-treated. On the contrary, the cooling rate and the atmosphere (air or oxygen) do not seem to be important parameters. DTA measurements indicated that BaBiO3 undergoes an additional phase transition at 860°C on heating and 801°C on cooling. This transition probably corresponds to the disproportionation of the Bi into Bi3+ and Bi5+ cations. This conjecture explains why the heat-treatment at 800°C yields compounds containing partial cation ordering. Since the symmetry is cubic above and below the 801°C transition, the 75% ordered BaBiO3 stable between 801°C and 480°C would contain a trigonal dynamical distortion which would be the precursor of the static trigonal distortion which takes place at 480°C. In every compound the partial ordering is always 75%. This could be interpreted as an indication that an additional ordering of Au3Cu type exists on the two Bi sublattices.