Crystal Field Stabilization Energy Research Articles

Mn3+ and the pink color of gem-quality euclase from northeast Brazil

Abstract Pink euclase of gem quality and centimeter size, presenting an unusual pink-orange coloration and a pink to orange pleochroism, was discovered near Livramento de Nossa Senhora, in Bahia State, Brazil. The origin of the pink coloration has been investigated using several spectroscopic techniques: UV-Vis-NIR spectroscopy, electron paramagnetic resonance (EPR), luminescence, and X-ray absorption near edge structure (XANES). The coloration is mainly due to the presence of Mn3+ substituted to octahedral Al3+ that causes an intense split band at about 18 500 and 21 000 cm−1. The crystal field splitting Dq, crystal field stabilization energy (CFSE), and Racah parameter B for Mn3+ are 2055.5 cm−1, 147 kJ/mol, and 886 cm−1, respectively. The Mn3+ molar extinction coefficient varies as a function of polarization, between 23 and 55 L.mol−1·cm−1. An additional absorption band, near 24 000 cm−1, together with the rising background toward the UV, tentatively assigned to the O → Fe3+ OMCT, contributes to the pink-orange hue. The in-situ UV-Vis-NIR spectra on heating up to 500 °C show a color change toward an intense, stable pink color. CIE colorimetric parameters demonstrate that the color of the investigated euclase remains in the pink domain before and after heat treatment. In the absence of Mn2+, shown by EPR and luminescence, the presence of Mn3+ evidences oxidative formation conditions due to contamination of the hydrothermal fluids by the surrounding host rock.

Structure and magnetic behavior of sol-gel grown spinel NixMn3-xO4 nanoparticles: Effect of Ni fraction and induction of superparamagnetism at room temperature

Spinel NixMn3-xO4 nanostructures have attracted a huge research interest due to their applications in catalysis, electrocatalysis, high-performance supercapacitors, lithium-ion batteries, among others. However, those applications strongly depend on the Ni:Mn ratio, valence states and spatial distribution of cations in the spinel lattice. Using a low-temperature sol-gel process, we synthesized NixMn3-xO4 nanoparticles of 14–26 nm average sizes with different Ni contents. Effects of Ni incorporation on the morphology, structure and magnetic properties of the nanostructures were studied by SEM, XRD, XPS, Raman spectroscopy and VSM. While a high Ni content in the spinel nanostructures causes lattice shrinkage, lattice expansion at lower Ni content occurs due to higher occupancy of Mn2+ ions in tetrahedral sites. Structural and magnetic behaviors of the nanostructures were explained by considering the crystal field stabilization energies of the cations. We demonstrate that superparamagnetic behavior can be induced in the NixMn3-xO4 nanostructures by incorporating Ni at x>1.0.

Effects of ferric-phosphate forms on phosphorus release and the performance of anaerobic fermentation of waste activated sludge

Five ferric-phosphate (Fe(III)Ps) with amorphous or crystalline structures were added to waste activated sludge (WAS) for anaerobic fermentation, aiming to investigate effects of Fe(III)Ps forms on phosphorus (P) release and the performance of WAS fermentation. The results revealed that the Fe(III) reduction rate of hexagonal-FePO4 was faster than that of monoclinic-FePO4·2H2O, thanks to its lower crystal field stabilization energy. FePO4·nH2O was reduced to vivianite and part of the phosphate was released as orthophosphate (PO4-P). Giniite (Fe5(PO4)4(OH)3·2H2O) as an iron hydroxyphosphate was transformed to βFe(III)Fe(II)(PO4)O-like compounds without PO4-P release. In addition, Fe(III)Ps had an adverse effect on the anaerobic fermentation of WAS. The specific hydrolysis rate constant and volatile fatty acids (VFAs) yield decreased by 38.4% and 41.9%, respectively, for the sludge sample with amorphous-FePO4·3H2O, which dropped the most. This study provides new insights into various forms of Fe(III)Ps performance during anaerobic fermentation and is beneficial to enhancing P recovery efficiency.

Mixed Zn–Ni spinel ferrites: Structure, magnetic hyperthermia and photocatalytic properties

Synthesis of high quality, narrow size distributed superparamagnetic nanoparticles with high saturation magnetization is imperative for biomedical and environmental remediation applications. This study reports the synthesis of narrow size distributed, small diameter, high-quality Zn1-xNixFe2O4 (x = 0.1, 0.3 and 0.5) nanoparticles with increased saturation mass magnetization for localized superparamagnetic hyperthermia and photocatalysis applications. Narrow size distributed samples with around 10 nm average diameter were synthesized, in the presence of surfactant, by solvothermal reflux method. All the samples show good colloidal stability through electrostatic stabilization with zeta potentials around - 60 mV. The heating capacity or specific heat generation rate (SHR) of synthesized superparamagnetic nanoparticles was determined through the calorimetric method. The x = 0.1, 0.3 and 0.5 samples show SHR = 372, 399 and 410 W g−1 of nanoparticles, respectively, under 35.28 kA/m and 316 kHz field parameters. These SHR values are higher than that of samples synthesized by other methods. Similarly, saturation mass magnetization value increases with Ni2+ concentration in the sample. The enhancement was attributed to preferential octahedral ligand sites occupation by Ni2+ ions in spinel lattice due to high crystal field stabilization energy. Photocatalytic activity of all nanoparticles was studied by methylene blue (MB) dye degradation under visible light. The data shows that degradation efficiency increases with Ni2+ concentration in the sample. All samples were characterized through X-ray powder diffraction (XRPD) profile, transmission electron microscopy (TEM), Fourier transformed infrared spectra (FT-IR), zeta potential distribution curve, thermogravimetric analysis (TGA) curve, vibrating sample magnetometer (VSM), transient temperature curves, and UV–visible–NIR photo-absorption spectra.

Predicting the Catalytic Activity of Surface Oxidation Reactions by Ionization Energies

Developing a descriptor to understand the reactivity of a catalyst is critical in achieving the rational design of heterogeneous catalysts. Ideally, the descriptor should be simple, predictive, as ...

Thermo spectral advances of CoO doped alkali bismuth phospho silicate glasses for thermo and photoluminescence applications

The glasses of composition (25-x) LiF–05 Bi2O3–15 P2O–55 SiO2: x CoO have prepared (where ‘x’ is ranging from 0 to 0.5 mol% in step size of 0.1). Prepared samples are characterized by both structural and optical techniques. The results drawn from structural reports suggest that the samples prepared were exhibiting covalently associated interlinked structure. Thermal stability findings from differential thermal analysis disclose that the samples prepared are glassy in nature. X-Ray diffraction reports strongly acclaim the differential thermal analysis conclusions back too. Thermoluminiscence measurements suggest that the sample of 0.4 mol% of CoO concentration exhibiting good shape symmetry factor and low activation energy (~ 0.65) under 15 min of UV - irradiation might be good thermoluminiscence asset. The optical evaluations such as Racah parameters, crystal field stabilization energy and nephelauxetic ratio are additionally projected. The variation in these evaluations with increase in CoO concentration further lead to decrease in optical band gap (~ 2.31 eV) evaluations, which are found to be least for the glass models of 0.4 mol% CoO concentration might be good optical asset to develop novel class of laser active mediums. The radiative evolutions of both emission cross section and transitions probability corresponding to the emissive peaks 2T1g(H) →4T1g(F) &4T1(4P) →4A2(4F) assessed by Photoluminescence technique. All the results drawn from radiative studies are due to octahedrally co-ordinated Co2+ ions and also found to be best for the sample of 0.4 mol% CoO concentration might be useful luminescent resource.

Vibrational spectra and optical properties ofFe1−xCrxVO4solid solutions: With a group theory analysis

The present manuscript reports vibrational spectra and optical studies of polycrystalline Fe1-xCrxVO4 solid solutions through FT-IR spectroscopy augmented with a group theory (G.T.) analysis and UV-Visible DRS spectroscopy. Full set of IR and Raman modes are determined by G. T. for various crystal symmetries in FeVO4-CrVO4 solid solutions where Triclinic, Monoclinic and Orthorhombic structures evolve with increasing Cr concentration. Experimentally obtained vibrational modes support the structural phase transitions and confirm formation of continuous solid solutions in Fe1-xCrxVO4. The Diffuse Reflectance Spectra (DRS) of Fe1-xCrxVO4 depicts the electronic structure and different optical transitions due to absorption of photon energy. The d-d transitions are manifested for all compounds in terms of crystal field stabilization energy (CFSE) caused by distorted lattice sites. The band gap energy of Fe1-xCrxVO4 is calculated using Tauc formula. It shows a red shift initially within triclinic structure then blue shift with the increase of Cr concentration. Urbach energy (Eu) tails in the spectra show the electronic structural disorder in Fe1-xCrxVO4 due to impurity energy levels of Cr ions within band gap region. It is observed that Eu decreases with the doping concentration due to the increase in crystal symmetry corresponding to the structural phase transitions in Fe1-xCrxVO4.

A rare flattened tetrahedral Mn(II) salen type complex: Synthesis, crystal structure, biomimetic catalysis and DFT study

A new flattened tetrahedral high spin Mn(II) complex (1) has been synthesized using N2O4 donor Schiff base ligand. Complex 1 was characterized by X-ray diffraction and various spectroscopic techniques. For further understanding of electronic structure of the complex, DFT calculations and electrochemical studies have been performed. This is a rare example of a flattened tetrahedral Mn(II) salen type Schiff base complex. High-spin d5 configuration of the metal center provides no crystal-field stabilization energy to the system and that is the main reason behind the significant deviation of this salen-type ligand from planarity. Notably, the propylenic linker in the ligand provides adequate flexibility so that such an uncommon binding mode of the salen type Schiff base ligand becomes possible. Complex 1 exhibits excellent catalytic property towards oxidation of o-aminophenols in aerobic condition. Detailed kinetic investigations together with the mass spectrometry studies reveal several important information relating to biomimetic catalytic activity of the present complex.

Tetrahedral coordination and low-spin configuration in a 5d oxide

Spin state plays a key role in a plethora of physical, biological, and geological phenomena. In octahedral coordination, spin state of $3d$ transition metals can be high spin or low spin, and can be tuned with external stimuli. In a tetrahedral coordination environment, with the smaller crystal field splitting energy, only high spin is considered possible for oxides. Herein we report the realization of low spin state in tetrahedral coordination in a novel oxide, $\mathrm{S}{\mathrm{r}}_{9}\mathrm{I}{\mathrm{r}}_{3}{\mathrm{O}}_{17}$. The low spin configuration was confirmed with density functional theory (DFT) calculations, including the effect of spin-orbit coupling (SOC), electron-electron repulsion (U), magnetic data, and crystal chemical consideration. This exotic electronic configuration was realized with the confluence of extended $5d$ orbitals, short bond lengths, and an electron count maximizing the crystal field stabilization energy. The discovery of low spin state for a tetrahedral environment provides a novel platform for spin state manipulation in solid state materials.

Apparent auxetic to non-auxetic crossover driven by Co2+ redistribution in CoFe2O4 thin films

Oxide spinels of general formula AB2O4 (A = Mg2+, Fe2+; B = Al3+, Cr3+, etc.) constitute one of the most abundant crystalline structures in mineralogy. In this structure, cations distribute among octahedral and tetrahedral sites, according to their size and the crystal-field stabilization energy. The cationic arrangement determines the mechanical, magnetic, and transport properties of the spinel and can be influenced by external parameters like temperature, pressure, or epitaxial stress in the case of thin films. Here, we report a progressive change in the sign of the Poisson ratio, ν, in thin films of CoFe2O4, defining a smooth crossover from auxetic (ν < 0) to non-auxetic (ν > 0) behavior in response to epitaxial stress and temperature. Microstructural and magnetization studies, as well as ab initio calculations, demonstrate that such unusual elastic response is actually due to a progressive redistribution of Co2+ among the octahedral and tetrahedral sites of the spinel structure. The results presented in this work clarify a long standing controversy about the magnetic and elastic properties of Co-ferrites and are of general applicability for understanding the stress-relaxation mechanism in complex crystalline structures.

Influence of TiO2 on structural, luminescent and conductivity investigations of CaF2−CaO−Y2O3−B2O3−P2O5 glasses

Glasses with different molar ratios of (20-x)CaF2―10CaO―5Y2O3―10B2O3―55P2O5: x TiO2 (0 ≤ x ≤ 1 mol%) were synthesized. Structural, optical, luminescent and ionic conductivity studies of the prepared glasses were studied. The investigations on these glass materials had indicated that the titanium ions exhibit two different oxidation states visually Ti3+ and Ti4+. Different physical parameters were calculated on the prepared glasses and reported. The phase transition points such as Tg and Tc of these glasses were analyzed by DTA analysis. The shift in the wavenumbers of different asymmetrical and symmetrical band positions of these glass materials were identified by FT-IR and Raman studies. By increasing the TiO2 concentration in the glass matrix, the decrement in the theoretical and optical basicity values had been observed. This may be due to the covalent bond formation between the dopant and the glass matrix. The crystal field stabilization energy, ligand field parameters such as B and C were calculated. The luminescent properties and ionic conductivity of these glass materials were also studied.

Transition metals in the transition zone: partitioning of Ni, Co, and Zn between olivine, wadsleyite, ringwoodite, and clinoenstatite

Ni, Co, and Zn are widely distributed in the Earth’s mantle as significant minor elements that may offer insights into the chemistry of melting in the mantle. To better understand the distribution of Ni2+, Co2+, and Zn2+ in the most abundant silicate phases in the transition zone and the upper mantle, we have analyzed the crystal chemistry of wadsleyite (Mg2SiO4), ringwoodite (Mg2SiO4), forsterite (Mg2SiO4), and clinoenstatite (Mg2Si2O6) synthesized at 12–20 GPa and 1200–1400 °C with 1.5–3 wt% of either NiO, CoO, or ZnO in starting materials. Single-crystal X-ray diffraction analyses demonstrate that significant amounts of Ni, Co, and Zn are incorporated in octahedral sites in wadsleyite (up to 7.1 at%), ringwoodite (up to 11.3 at%), olivine (up to 2.0 at%), and clinoenstatite (up to 3.2 at%). Crystal structure refinements indicate that crystal field stabilization energy (CFSE) controls both cation ordering and transition metal partitioning in coexisting minerals. According to electron microprobe analyses, Ni and Co partition preferentially into forsterite and wadsleyite relative to coexisting clinoenstatite. Ni strongly prefers ringwoodite over coexisting wadsleyite with $${D}_{\text{Ni}}^{\text{Rw}/\text{Wd}}$$ = 4.13. Due to decreasing metal–oxygen distances with rising pressure, crystal field effect on distribution of divalent metal ions in magnesium silicates is more critical in the transition zone relative to the upper mantle. Analyses of Ni partitioning between the major upper-mantle phases implies that Ni-rich olivine in ultramafic rocks can be indicative of near-primary magmas.

Atomistic and ab initio DFT modelling of the defect structures in Al3+/Cr3+-doped and co-doped Y3Fe5O12

The defect structures when Y3Fe5O12 is doped with either Al3+ or Cr3+, and evenly co-doped with both, which have been a matter of controversy in the literature, are modeled using atomistic and ab initio DFT methods. When Y3Fe5O12 is doped with Al3+, the defect reaction energy obtained marginally favors the preferential substitution of Al3+ for Fe3+ at the tetrahedral sites as opposed to octahedral ones. This is indicative that for Al3+-doped samples processed at elevated temperatures, or containing undetected impurities, the substitution of Al3+ for octahedral Fe3+ is likely. To model the defect structure of the Cr3+ -doped Y3Fe5O12, it was essential that the Cr3+ ions crystal field stabilization energy (CFSE) and the Fe3+-O2-- Cr3+ spin-spin coupling derived from the ab initio DFT calculations ,be taken into account. The results show the substitution of the Cr3+ ion for an octahedral Fe3+ ion to be energetically favorable relative to its substitution for a tetrahedral Fe3+ one. It is also shown that the antisite defect, where the Cr3+ ion substitutes for Y3+ at a dodecahedral site with the expelled Y3+ ion substituting for an octahedral Fe3+ ion, is possible under certain processing conditions. For the Al3+ /Cr3+ co-doped Y3Fe5O12, the Al3+ and Cr3+ ions were found to, respectively, substitute for the tetrahedral and octahedral Fe3+ ions. The energy values obtained suggest this defect structure to be insensitive to the processing conditions and/or the presence of undetected impurities. The structural and magnetic implications of these defect structures are discussed.

A novel bis-1,2,4-benzothiadiazine pincer ligand: synthesis, characterization and first row transition metal complexes.

Reaction of 2,6-dicyanopyridine with 2 equiv. of 2-(propylthio)benzenamine in the presence of lithium bis(trimethylsilyl)amide, followed by ring-closing oxidation with N-chlorosuccinimide affords the novel tridentate ligand, 2,6-bis-(1',2',4'-benzothiadiazinyl)pyridine (LH2). Electrochemical studies on the free ligand LH2 reveal a single well-defined 2e- oxidation process with E1/2 = +0.90 V. EPR studies of the in situ chemical oxidation of LH2 reveal the generation of a benzotriazinyl radical. Reactions of ligand LH2 with a range of divalent transition metal salts in either MeOH or MeCN in a 2 : 1 ratio at ambient temperatures afforded mononuclear complexes with general formula [M(LH2)2][X]2 (M = Mn, X = CF3SO3 (1); Fe, X = CF3SO3 (2); Fe, X = BF4 (3); Co, X = Cl (4); Ni, X = Cl (5); Zn, X = CF3SO3 (6)) and the 1 : 1 complex [Cu(LH2)(NO3)2] (7). In all cases the LH2 ligand binds in a tridentate N,N',N'' chelate fashion via benzothiadiazinyl NBTDA and pyridyl Npy atoms. The low spin FeII complexes (2 and 3) were implemented for NMR and UV-Vis solution studies of ligand reactivity as well as cyclic voltammetry which reveal two 1e-oxidation waves. The metal complexes 1-6 are discussed and reveal a range of geometries between octahedral and trigonal prismatic with the greatest deviation from octahedral symmetry apparent for ions with no crystal field stabilisation energy, i.e. d10 ZnII and high spin d5 MnII ions.

The absorption- and luminescence spectra of Mn3+ in beryl and vesuvianite

The electron absorption-, photoluminescence- and electron paramagnetic-resonance spectra of Mn3+ in red beryl from Wah Wah Mountains (Utah USA) and of pink- and purple vesuvianite from Jeffrey Mine (Asbestos, Canada) were measured at room- and low temperatures. The crystal field stabilization energies are equal to 130.9 kJ/mol for the red beryl, and 151.5–158.0 and 168.0 kJ/mol for for the pink- and the purple vesuvianite, respectively. The red photoluminescence of Mn3+ was not intensive either at room- or at low temperatures. The high Mn content in the crystals caused the emergence of an additional emission band and short photoluminescence-decay lifetimes. The latter are only 183 μs for beryl and 17 μs for vesuvianite.

Effect of doping on the properties and photocatalytic performance of titania thin films on glass substrates: Single-ion doping with Cobalt or Molybdenum

TiO2 thin films of varying Co and Mo individual doping levels (0.00–1.00 mol% metal basis) were fabricated on soda-lime-silica glass substrates by spin coating, followed by annealing at 450 °C for 2 h. Mineralogical analyses showed that all of the films were comprised only of anatase and suggested that both Co and Mo formed interstitial solid solutions. For the Co-doped films, the variation in anatase XRD peak intensities indicated three discrete stages of structural and nanostructural development: recrystallization (≤0.05 mol% Co), lattice distortion (0.05–0.60 mol% Co), and precipitation (≥0.60 mol% Co). For the Mo-doped films, Mo consistently enhanced nucleation, recrystallization, and lattice distortion within the solubility limit, which was not reached at 1.00 mol% Mo. Chemical analyses showed that Ti4+/Ti3+, Co3+/Co2+, and Mo5+/Mo4+ (and possibly Mo6+) were present in the films. The formation of non-equilibrium valences was considered in terms of the effects of crystal field stabilization energy (CFSE), intervalence charge transfer (IVCT), and electronegativity. Nanostructural analyses of the Co-doped films showed that, with increasing doping level, the grain size increased at the lowest level, followed by a decrease owing to changes in the structure (lattice destabilization) and nanostructure (localized liquid formation). In contrast, the grain size of the Mo-doped films increased with increasing doping level due to the maintenance of solid solubility, defect formation (and associated enhanced nucleation and recrystallization), and grain growth. Defect formation and charge compensation resulted in lattice contraction, which was confirmed by laser Raman microspectroscopy and resultant destabilization. Optical analyses showed that all of the thin films were flat, smooth, and nanostructurally homogeneous. The band gap data suggested that the governing effect derived from the lattice distortion associated with the lattice incorporation of both dopants. Photocatalytic testing showed that the lowest level of doping for both dopants (0.01 mol%) yielded maximal photocatalytic activity and that the performance decreased with increasing doping levels. This was attributed to the effects of nucleation and recrystallization, semiconducting effect, dopant size effect, and minimal lattice distortion.

Site selective Cu2+ substitution in single crystal Fe3O4 biocompatible nanospheres by solvothermal reflux method

Transition metal cations distribution among tetrahedral and octahedral sites in Fe3O4 has profound influence on it properties. The cations distribution among these sites were influenced by crystal field stabilization energy (CFSE) of a substituent ion. It was reported that Cu2+ substitutes both Fe3+ in tetrahedral and Fe2+ in octahedral sites of magnetite though it has high CFSE for octahedral sites. Thus it promotes mixed (normal & inverse) spinel structure and hence the properties deviate from the prediction. Here we show that selective octahedral site substitution of Fe2+ by Cu2+ in single crystal Fe3O4 nanoparticles was possible by solvothermal reflux method at moderate temperatures (≈300°C). It was attributed to reduced energy barrier for crystallization and high diffusion coefficient of the metal cations at moderate temperature in low viscous organic solvents mixture, at ambient pressure, compared to coprecipitation and solid state reaction protocols. In addition, effective removal of exothermic crystal lattice energy, released during nucleation and growth process, by natural gas bubbles formed in the reflux organic solvent mixture. This promotes effective diffusion of cations during the growth process and easy octahedral occupation by Cu2+ in Fe1−xCuxFe2O4 (x=0, 0.1, 0.2, 0.3, 0.4, 0.5 & 0.6) single crystal nanospheres. The octahedral substitution was confirmed by reduction of crystal lattice parameter as well as ferromagnetic strength of octahedral sublattice with enhanced Cu2+ concentration. In addition, morphology, lattice vibrational frequency and electron excitation spectra of the nanospheres were studied. The Langevin function fit analysis reveals that superparamagnetic domain diameter were slightly less than the particle diameter obtained from TEM micrograph.

Electronic, magnetic and spectroscopic properties of doped Mn(1−x)AxWO4 (A = Co, Cu, Ni and Fe) multiferroic: an experimental and DFT study

The influence of dopants (Co, Cu, Fe and Ni) on the optical, electronic and magnetic properties of multiferroic MnWO4 was studied using Raman spectroscopy, ultraviolet–visible spectroscopy (UV–Vis), magnetization measurements and density functional theory (DFT) calculations. The evolution of Raman spectra with different elemental substitutions at the Mn site was also studied, where the peak width increased with doping of higher mass elements (Co, Cu, Fe and Ni). UV–Vis diffuse reflectance spectroscopy on polycrystalline Mn(1−x)AxWO4 (A = Co, Cu, Fe and Ni) (0 ⩽ ⩽ 0. was performed. The evaluated electronic band gap decreasing with successive Co, Cu and Fe doping reflected the lower ionic radius of the substituted element, and for Ni-doped MnWO4 the band gap increased slightly compared to the parent MnWO4. Bader charge transfer and a partial density of states (PDOS) analysis from DFT simulations predict the appearance of impurity states in the band gap region (of pure MnWO4) from the d orbital of the dopant (Co, Cu and Fe) hybridized with the p orbital of the bonded O atoms due to charge transfer from O to the dopant, and reduced the band gap of Co, Cu and Fe-doped MnWO4. On the other hand, for Ni-doped MnWO4 strong W–O hybridization occurring due to large charge transfer from oxygen to tungsten leads to an increase in the band gap. The band gap, computed using the GGA + U method, is close to the experimental value. The signature of the d–d transition observed in the UV spectra is explained in terms of the crystal field stabilization energy caused by the octahedral distortion present in the lattice. Three different antiferromagnetic phases (AF1, AF2 and AF3) are identified in MnWO4 and also for the Co (18.75%)-doped sample. For Cu-doped samples, suppression of the AF1 phase and stabilization of the AF2 phase is observed up to 2 K. Successive doping of Cu leads to the diminution of magnetic frustration. A new magnetic order is identified for Ni-doped MnWO4 in the temperature range 13.7–20 K.

Transition metals in the transition zone: Crystal chemistry of minor element substitution in wadsleyite

As the most abundant solid phase at depths of 410–525 km, wadsleyite constitutes a large geochemical reservoir in the Earth. To better understand the implications of minor element substitution and cation ordering in wadsleyite, we have synthesized wadsleyites coexisting with pyroxenes with 2–3 wt% of either TiO 2 , Cr 2 O 3 , V 2 O 3 , CoO, NiO, or ZnO under hydrous conditions in separate experiments at 1300 °C and 15 GPa. We have refined the crystal structures of these wadsleyites by single-crystal X-ray diffraction, analyzed the compositions by electron microprobe, and estimated M3 vacancy concentration from b / a cell-parameter ratios. According to the crystal structure refinements, Cr and V show strong preferences for M3 over M1 and M2 sites and significant substitution up to 2.9 at% at the tetrahedral site (T site). Ni, Co, and Zn show site preferences similar to those of Fe with M1≈ M3 > M2 > T. The avoidance of Ni, Co, and Fe for the M2 site in both wadsleyite and olivine appears to be partially controlled by crystal field stabilization energy (CFSE). The estimated CFSE values of Ni 2+ , Co 2+ , and Zn 2+ at three distinct octahedral sites show a positive correlation with octahedral occupancy ratios [M2/(M1+M3)]. Ti substitutes primarily into the M3 octahedron, rather than M1, M2, or T sites. Ti, Cr, and V each have greater solubility in wadsleyite than in olivine. Therefore these transition metal cations may be enriched in a melt or an accessory phase if hydrous melting occurs on upward convection across the wadsleyite-olivine boundary and may be useful as indicators of high-pressure origin.

Tuning the Phase Stability of Sodium Metal Pyrophosphates for Synthesis of High Voltage Cathode Materials

Properties of the electrode materials are strongly influenced by their crystal structures, yet there is still a lack of design principles to control the polymorphism, showing multiple structures for a given composition with varying battery performance. Here, the underlying mechanism that governs the phase stability of Na2CoP2O7, which has two polymorphs with different electrochemical properties, and a strategy to control it via transition metal substitution are investigated. It is found that the relative stability between the triclinic and orthorhombic polymorphs of Na2MP2O7 (M = transition metals) is determined by two factors, the ionic size and crystal field stabilization energy. On the basis of this understanding, a computational strategy is devised for selecting the optimal substituents to produce a desired polymorph, from which the introduction of Ca, Ni, or Mn into Na2CoP2O7 is identified to stabilize the preferred triclinic phase that has a higher voltage than the orthorhombic counterpart. This pre...