Disordered Cation Distribution Research Articles

Over-Stoichiometric Metastabilization of Cation-Disordered Rock Salts.

Cation-disordered rock salts (DRXs) are well known for their potential to realize the goal of achieving scalable Ni- and Co-free high-energy-density Li-ion batteries. Unlike in most cathode materials, the disordered cation distribution may lead to more factors that control the electrochemistry of DRXs. An important variable that is not emphasized by research community is regarding whether a DRX exists in a more thermodynamically stable form or a more metastable form. Moreover, within the scope of metastable DRXs, over-stoichiometric DRXs, which allow relaxation of the site balance constraint of a rock salt structure, are particularly underexplored. In this work, these findings are reported in locating a generally applicable approach to "metastabilize" thermodynamically stable Mn-based DRXs to metastable ones by introducing Li over-stoichiometry. The over-stoichiometric metastabilization greatly stimulates more redox activities, enables better reversibility of Li deintercalation/intercalation, and changes the energy storage mechanism. The metastabilized DRXs can be transformed back to the thermodynamically stable form, which also reverts the electrochemical properties, further contrasting the two categories of DRXs. This work enriches the structural and compositional space of DRX families and adds new pathways for rationally tuning the properties of DRX cathodes.

High-entropy silicide superconductors with W5Si3 -type structure

We report the synthesis, crystal structure and physical properties of two new high-entropy silicides (HESs), namely (Nb$_{0.1}$Mo$_{0.3}$W$_{0.3}$Re$_{0.2}$Ru$_{0.1}$)$_{5}$Si$_{3}$ and (Nb$_{0.2}$Mo$_{0.3}$W$_{0.3}$Re$_{0.1}$Ru$_{0.1}$)$_{5}$Si$_{3}$. Structural analysis indicates that both HESs consist of a (nearly) single tetragonal W$_{5}$Si$_{3}$-type phase (space group $I$4/$mcm$) with a disordered cation distribution. Electrical resistivity, magnetic susceptibility and specific heat measurements show that (Nb$_{0.1}$Mo$_{0.3}$W$_{0.3}$Re$_{0.2}$Ru$_{0.1}$)$_{5}$Si$_{3}$ and (Nb$_{0.2}$Mo$_{0.3}$W$_{0.3}$Re$_{0.1}$Ru$_{0.1}$)$_{5}$Si$_{3}$ are weakly coupled bulk superconductors, which represent the first superconducting high-entropy nonoxide ceramics. In particular, these HESs have higher $T_{\rm c}$ values (3.2-3.3 K) compared with those of the binary counterparts, and their $B_{\rm c2}$(0)/$T_{\rm c}$ ratios are the largest among superconductors of the same structural type.

Relation between cation distribution and chemical bonds in spinel NiFe2O4

In spinel oxides, the cation distribution plays an important role in determining their technologically important properties. Herein computational predictions based on the first principles calculations are presented to elucidate the relation between cation distribution and chemical bonds in spinel NiFe 2 O 4 . The variation of physical properties (such as system energy, lattice parameter, and atomic magnetic moment) with respect to the cation distribution (i.e., degree of inversion) in spinel NiFe 2 O 4 oxide is discussed. This study further reveals that the energy of spinel NiFe 2 O 4 oxide is linearly proportional to the number of different types of cation-oxygen-cation bonds in the spinel structure. The equilibrium degree of inversion in spinel NiFe 2 O 4 at elevated temperatures and the transformation from an ordered to disordered cation distribution in inverse spinel NiFe 2 O 4 are predicted and found to be driven by competition between bond enthalpy and entropy of cation site occupancy.

Growth, crystal structure, and properties of the Li3Ba2Ln3(MoO4)8 (Ln = Er, Tm) molybdates

The compounds Li3Ba2Ln3(MoO4)8 (Ln = Er, Tm) have been obtained by the high-temperature flux technique using the slow cooling method. Crystals grown spontaneously were used to characterize the materials and investigate their properties using X-ray diffraction (XRD), Raman spectroscopy, and magnetic measurements. Single crystal structures were solved and refined in the monoclinic space group C2/c. Lattice dimensions are a = 5.1935, b = 12.6364, c = 19.1022 Å, β = 91.399° for Er-based triple molybdate (pink crystals) and a = 5.1846, b = 12.5766, c = 19.1557 Å, β = 91.525° for Tm-molybdate (green crystals). The Ln cations, mainly at general position 8f, are coordinated by eight oxygen atoms and adopt a distorted antiprismatic geometry. The distribution of atoms over the three cationic sites is discussed comparatively to literature reports. Unpolarized Raman characterization supports both the disordered cation distribution, and the distortion of the MoO4 units. From the magnetic susceptibility measurements, it is found that the two triple molybdates follow a Curie-Weiss law in the 10–300 K domain, with a negative paramagnetic temperature θCW. The antiferromagnetic character is highlighted by the field-dependent magnetization curves.

Persistent Structure and Frustrated Magnetism in High Entropy Rare-Earth Zirconates.

The configurational complexity and distinct local atomic environments of high entropy oxides remain largely unexplored, leaving structure-property relationships and the hypothesis that the family offers rich tunability for applications ambiguous. This work investigates the influence of cation size and materials synthesis in determining the resulting structure and magnetic properties of a family of high entropy rare-earth zirconates (HEREZs, nominal composition RE2 Zr2 O7 with RE = rare-earth element combinations including Eu, Gd, Tb, Dy, Ho, La, or Sc). The structural characterization of the series is examined through synchrotron X-ray diffraction and pair distribution function analysis, and electron microscopy, demonstrating average defect-fluorite structures with considerable local disorder, in all samples. The surface morphology and particle sizes are found to vary significantly with preparation method, with irregular micron-sized particles formed by high temperature sintering routes, spherical nanoparticles resulting from chemical co-precipitation methods, and porous nanoparticle agglomerates resulting from polymer steric entrapment synthesis. In agreement with the disordered cation distribution found across all samples, magnetic measurements indicate that all synthesized HEREZs show frustrated magnetic behavior, as seen in a number of single-component RE2 Zr2 O7 pyrochlore oxides. These findings advance the understanding of the local structure of high entropy oxides and demonstrate strategies for designing nanostructured morphologies in the class.

High Rate Li-Ion Batteries with Cation-Disordered Cathodes

Summary Cation disordering in electrode materials for Li-ion batteries is deemed as an undesired structural feature for achieving high electrochemical activity owing to sluggish Li diffusion in the structure. Therefore, extensive efforts have been made to develop electrode materials having a well cation-ordered structure. We report a new class of cation-disordered material that can achieve reasonable Li diffusion by controlling the way of the cation distribution rather than the degree of cation disordering itself. The fully cation-disordered LiFeSO4F, activated by changes in the synthesis temperature and the Li/Fe ratio, delivers a substantially improved electrochemical performance. It exhibits a superior rate capability up to 100 C (36-s discharge) with ∼ 60 mAh/g and excellent capacity retention for 2,500 cycles at 5 C charge/20 C discharge rate. These results extend the scope of electrode materials to include cation-disordered materials and provide new opportunities for designing them for high-performance Li-ion batteries.

Full reoxidation of CuMn2O4 spinel catalyst triggered by epitaxial Mn3O4 surface nanocrystals

AbstractTransmission electron microscopy was performed on a model system of a novel CuMn2O4 spinel catalyst in order to study the effects of reduction by carbon monoxide and subsequent reoxidation by oxygen both processed at 600°C. A phase transition from CuMn2O4 spinel into separated MnO and Cu‐rich phases, which are CuO or Cu depending on the reduction period, was observed in the reduced samples. Furthermore, a surface coverage of the MnO grains by epitaxially grown, nanocrystalline Mn3O4 was found in all reduced samples. These Mn3O4 nanocrystals are assumed to act as seed crystals in the subsequent reoxidation step completely reversing the phase transitions and reconstituting single‐phase CuMn2O4 spinel. In addition, superlattice reflections due to cation ordering occurred as a typical feature in the as‐prepared spinel. In the reoxidized spinel, the superlattice reflections were absent, hence indicating a disordered cation distribution.

Theoretical and experimental study on the topotactic transformation mechanism of Zn-Al layered double hydroxides

Layered double hydroxides (LDHs) are a class of minerals with nanoscale two-dimensional layered structure. The topotactic transformation of LDHs under calcination and the reconstruction behavior (memory effect) of the calcined LDHs in aqueous solution play a key role in the fabrication of high dispersed metal nanocatalysts. The thermal topotactic transformation mechanism of ZnAl-layered double hydroxides (LDHs) and memory effect of LDHs were investigated by density functional theory based molecular dynamics (MD) simulation, combined with thermogravimetric/differential thermal analysis (TG-DTA). TG-DTA results reveal that the LDH phase undergoes two key endothermic events at 273 and 800 °C. The results show that the anion in LDHs decomposes to CO2 and H2O via a monodentate intermediate at 273 °C, and H2O releases prior to CO2. After the decomposition of interlayer CO32− at 273 °C, the distribution of metal cations in LDH matrix changed heavily along both LDH(001) facet and the c -axis direction perpendicular to the (001) facet. The dehydrated products cannot reconstruct back to the hydroxide phase, showing no memory effect. At 800 °C, a complete collapse of layered structure occurs, resulting in a totally disordered cation distribution and plenty of holes in the final product. This work will be helpful for the design and preparation of highly dispersed nanocatalysts derived from LDHs precursors.

DFT-Based Simulation and Experimental Validation of the Topotactic Transformation of MgAl Layered Double Hydroxides.

The thermal topotactic transformation mechanism of MgAl layered double hydroxides (LDHs) is investigated by a combined theoretical and experimental study. Thermogravimetric differential thermal analysis (TG-DTA) results reveal that the LDH phase undergoes four key endothermic events at 230, 330, 450, and 800 °C. DFT calculations show that the LDH decomposes into CO2 and residual O atoms via a monodentate intermediate at 330 °C. At 450 °C, the metal cations almost maintain their original distribution within the LDH(001) facet during the thermal dehydration process, but migrate substantially along the c-axis direction perpendicular to the (001) facet; this indicates that the metal arrangement/dispersion in the LDH matrix is maintained two-dimensionally. A complete collapse of the layered structure occurs at 800 °C, which results in a totally disordered cation distribution and many holes in the final product. The structures of the simulated intermediates are highly consistent with the observed in situ powder XRD data for the MgAl LDH sample calcined at the corresponding temperatures. Understanding the structural topotactic transformation process of LDHs would provide helpful information for the design and preparation of metal/metal oxides functional materials derived from LDH precursors.

Analysis of conductivity and dielectric spectra of Mn0.5Zn0.5Fe2O4 with coupled Cole–Cole type anomalous relaxations

Most of the crystalline materials seldom show a well-defined dielectric loss peak due to domination of dc conductivity contribution, but effects of loss peaks are seen at high frequencies. Ac electrical data of nano-crystalline Mn0.5Zn0.5Fe2O4 synthesised by chemical co-precipitation method show such behaviour. Properly combined and formulated conduction and dielectric relaxation functions are required for such materials. Cole–Cole type relaxation function in the combined conduction and dielectric process is formulated for complex resistivity ρ⁎(ω), complex permittivity ε⁎(ω), complex conductivity σ⁎(ω) and complex electric modulus M⁎(ω). Conduction and dielectric relaxation are linked to Jonscher's idea of ‘pinned dipole’ and ‘free dipole’ to understand the relaxation dynamics. The physical parameters of ‘pinned dipole’ and ‘free dipole’ formalism are unique for all representations like ρ⁎(ω), ε⁎(ω), σ⁎(ω) and M⁎(ω). ‘Pinned dipole’ relaxation time τc related to conduction process and ‘free dipole’ relaxation time τd related to dielectric process show Arrhenius behaviour with the same activation energy. Correlation of dc conductivity σc with τc and τd indicates the coupled dynamics of ‘pinned dipole’ and ‘free dipole’. Time–temperature scaling of conduction and dielectric relaxation reveals that the mechanism of coupled dynamics of ‘pinned dipole’ and ‘free dipole’ is temperature independent. Hopping of charge carriers with dynamics of disordered cation distribution of host matrix generates a coupled conduction and dielectric relaxation in Mn0.5Zn0.5Fe2O4.

Synchrotron X-ray absorption spectroscopy study of disordered cation distribution of nanosized zinc ferrite powders

The zinc ferrite (ZnFe2O4) powders of various nanoparticle sizes were synthesised at different milling times (0 to 24 h) of the as-combusted powders. The non-equilibrium site occupancy of zinc (Zn2+) and ferric (Fe3+) ions was investigated through Zn and Fe K-edge X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra. The XRD and SEM strongly confirm the particle size of these ferrites decreasing with the increasing milling time. Compared with the bulk specimen of zinc ferrite, both XANES and EXAFS spectra of zinc ferrite powders clearly exhibit the large translocation of Zn2+ ions from the tetrahedral (A) sites to the octahedral (B) sites and the opposite translocation of some of Fe3+ ions without affecting the long-range structural order. Moreover, the curve-fitting analysis of Zn and Fe K-edge EXAFS spectra indicates that the degree of inversion increases as the particle size decreases resulting in significant differences in the magnetic behaviours.

Experimental determination of electrostatic properties of Na-X zeolite from high resolution X-ray diffraction.

High-resolution single crystal X-ray diffraction is used for the first time to obtain the charge density distribution in dehydrated Na-X zeolite. The electron density is extracted according to the Hansen & Coppens multipolar-model, from which Pval-κ-type atomic charges are derived. In order to compare the experimental electron density with theoretical calculations on zeolites and other minerals, a topological analysis is performed to derive AIM charges and electron density properties at bond critical points. The results are compared with that described in the literature. Finally, the electrostatic potential is evaluated in a periodic, mean field approach (disordered cation distribution in the Fd3[combining macron] space group) and for a given distribution of the cations (space group P1). The electrostatic energy is, then, derived in the neighbourhood of cation sites where the molecules are usually physisorbed.

Order/disorder phenomena in Zn1−xMnxGa2Se4 ordered vacancy compounds: high temperature neutron powder diffraction experiments

We present a study of order–disorder phenomena in the series of tetrahedral ordered vacancy compounds Zn1−xMnxGa2Se4 by means of time-of-flight neutron diffraction at high temperature together with dc magnetic susceptibility, Raman spectroscopy, differential thermal analysis and optical absorption experiments. Samples of nominal composition x = 0, 0.24, 0.5, 0.77 and 1 have been studied. An order–disorder phase transition has been detected, with Tc ranging from 472 to 610 ° C, which involves a structural change from a defect chalcopyrite phase, with space group (s.g.) and three different cation sites, to a partially disordered defect stannite, in which Zn, Mn and half of the Ga ions share the 4d site in s.g. Neither the vacancies nor the Ga ions occupying site 2a are involved in the phase transition. An additional ordering process is observed on approaching the phase transition from below, which is attributed to several factors: the activation of cation diffusion at ∼300 ° C, the partially disordered cation distribution exhibited by the as-grown single crystals and the preference of Mn atoms for the 2d crystallographic site in the structure. The reversibility of the phase transition is analysed with the aid of magnetic, optical and Raman experiments.

3T-phlogopite from Kasenyi kamafugite (SW Uganda): EPMA, XPS, FTIR, and SCXRD study

A 3 T mica polytype from Kasenyi (southwest Uganda), kamafugite, was studied by electron probe microanalysis (EPMA), single-crystal X-ray diffraction (SCXRD), micro-Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS) to characterize its crystal chemistry and the relationships with phlogopites from the same rock but showing different stacking sequence and to get insights into factors affecting polytypism in Ugandan phlogopites. EPMA data gave: SiO 2 = 38.7(2), Al 2 O 3 = 13.08(9), MgO = 20.4(2), TiO 2 = 4.8(1), MnO = 0.03(3), FeO tot = 5.51(9), Cr 2 O 3 = 0.90(7), NiO = 0.11(5), SrO = 0.03(3), ZnO = 0.04(3), ZrO 2 = 0.01(2), K 2 O = 9.64(5), Na 2 O = 0.29(1), BaO = 0.15(5), F = 0.13(5), and Cl = 0.01(1) wt%. The analyzed sample may be classified as a Ti-rich phlogopite. X-ray photoelectron spectroscopy provided Fe 3+ /Fe 2+ and O 2− /OH equal to ~0.75 and 7.14, respectively, which are in agreement with the results of previous Mossbauer investigation on the BU1 phlogopites from the same rock and with the structural formula of the studied crystal. Infrared spectra showed a shoulder at ~3660 cm −1 in the OH − stretching region (~3740–3600 cm −1 ), which is assigned to MgMgFe 3+ -OH − -K-O 2− local configurations. No evidences of vacancy substitutions were observed. Structure refinement based on single-crystal X-ray diffraction data was performed in space group P 3 1 12 using anisotropic displacement parameters and converged to R 1 = 4.34 and wR 2 = 3.33%. Unit-cell parameters are: a = b = 5.3235(3) and c = 30.188(2) A. Geometrical and chemical considerations point to a disordered cation distribution over T1 and T2 tetrahedral sites, whereas partial cation ordering characterizes the octahedral sites with high-charge cations preferentially located as expected on M2 and M3. Tetrahedral bond length distortion and angular variance parameters describe more distorted polyhedra in 3 T polytype than those found in coexisting 1 M and 2 M 1 polytypes. Finally, the overall crystal-chemical features indicate the occurrence of the following substitution mechanisms in the studied sample: Ti-oxy [ VI M 2+ +2(OH) − ↔ VI Ti 4+ +2(O 2− )+H 2 ↑] and Al, Fe 3+ , Cr-oxy [ VI M 2+ +(OH) − ↔ VI M 3+ +O 2− +½(H 2 )↑]; Al, Fe 3+ -Tschermak [ VI M 2+ + IV Si 4+ ↔ VI (Al 3+ , Fe 3+ )+ IV Al 3+ ]; XII K + + IV Al 3+ ↔ IV Si 4+ + XII □ tetraferriphlogopite [ IV Fe 3+ ↔ IV Al].

Short-Range Layered A-Site Ordering in Double Perovskites NaLaBB′O6 (B = Mn, Fe; B′ = Nb, Ta)

The new compounds NaLaFeTaO6, NaLaFeNbO6, NaLaMnTaO6, and NaLaMnNbO6 have been synthesized and characterized with a combination of transmission electron microscopy, X-ray powder diffraction (XRPD), neutron powder diffraction (NPD), and magnetization measurements. Through electron microscopy study, a local layered order of the A-cations has been detected without the typical occurrence of rock salt order at the B-cation site. Satellite reflections in the electron diffraction related to the local layered order are not visible on the XRPD or NPD patterns. The occurrence of local layered order is supported by pair distribution function analysis, which also reveals the presence of uncorrelated displacements of the Nb and Ta cations. The octahedra are tilted according to the system a−b+a−, and the coordinates were refined from XRPD and NPD with a disordered cation distribution in the space group Pnma. The magnetic exchange interactions in NaLaFeTaO6 and NaLaFeNbO6 are antiferromagnetic, while they are ferromagne...

The structure of a super-aluminous version of the dense hydrous-magnesium silicate phase D

The dense hydrous-magnesium silicate phase D, which has the ideal formula MgSi2H2O6, may be an important link in a chain of hydrous phases that carry H2O in the ultramafic portions of subducting lithosphere, into the Earth’s lower mantle. We have synthesized a new Al-rich form of phase D, containing up to 50 wt% Al2O3, using a multi-anvil device at ~1300 °C and 25 GPa. The phase, with the formula Mg0.2Fe0.15Al1.8H1.8SiO6, was initially produced in a bulk composition designed to synthesize Al- and Fe-rich magnesium silicate perovskite with a composition similar to that produced in experiments on mid-ocean ridge basalt bulk compositions at lower mantle conditions. Further experiments using a starting mixture based on the composition of this Al-rich phase resulted in the synthesis of 60–70 μm long single crystals at similar conditions. The recovered crystals were slightly richer in H2O (Mg0.2Fe0.12Al1.5Si0.92H3.1O6) and their unit-cell parameters were similar to those of MgSi2H2O6 phase D. A refinement of the crystal structure was carried out in the P 31 m space group and revealed a more disordered cation distribution than magnesium silicate phase D. All cation-oxygen distances are similar, suggesting a high degree of Si/Al disorder. Although the stability field of this new variant of phase D is yet to be determined, this phase may be an important host for H2O within portions of subducted oceanic crust in the lower mantle.

First-principles XANES simulations of spinel zinc ferrite with a disordered cation distribution

Theoretical calculations of Zn $K$ and Fe $K$ x-ray absorption near-edge structures (XANES) using a first-principles method have been performed to evaluate the degree of cation disordering in spinel zinc ferrite $(\mathrm{Zn}{\mathrm{Fe}}_{2}{\mathrm{O}}_{4})$ thin film prepared by a sputtering method, $\mathrm{Zn}{\mathrm{Fe}}_{2}{\mathrm{O}}_{4}$ thin films annealed at elevated temperatures, and $\mathrm{Zn}{\mathrm{Fe}}_{2}{\mathrm{O}}_{4}$ bulk specimen prepared by a solid-state reaction. Using the full-potential linearized augmented plane-wave + local orbitals method, a theoretical spectrum is generated for the tetrahedral and octahedral environments for each of the two cations. The experimental XANES spectrum of the thin film annealed at $800\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ as well as that of bulk specimen is successfully reproduced by using either the theoretical spectrum for ${\mathrm{Zn}}^{2+}$ on the tetrahedral site ($A$ site) or that for ${\mathrm{Fe}}^{3+}$ on the octahedral site ($B$ site), which is indicative of the normal spinel structure. For the as-deposited film, on the other hand, excellent agreement between theoretical and experimental spectra is obtained by considering the presence of either ion in both the $A$ and $B$ sites. The degree of cation disordering, $x$, defined as ${[\mathrm{Zn}_{1\ensuremath{-}x}{}^{2+}\mathrm{Fe}_{x}{}^{3+}]}_{A}{[\mathrm{Zn}_{x}{}^{2+}\mathrm{Fe}_{2\ensuremath{-}x}{}^{3+}]}_{B}{\mathrm{O}}_{4}$, is estimated to be approximately 0.6 in the as-deposited film, which is consistent with the analysis of the extended x-ray absorption fine structure on the Zn $K$ edge. Curious magnetic properties as we previously observed for the as-deposited thin film---i.e., ferrimagnetic behaviors accompanied by large magnetization at room temperature and cluster spin-glass-like behavior---are discussed in connection with disordering of ${\mathrm{Zn}}^{2+}$ and ${\mathrm{Fe}}^{3+}$ ions in the spinel-type structure.

Thermal annealing effect on magnetism and cation distribution in disordered ZnFe 2O 4 thin films deposited on glass substrates

The effect of thermal annealing on magnetic properties has been investigated for disordered zinc ferrite (ZnFe 2O 4) thin films deposited on silica glass substrates using a sputtering method. The magnetization at room temperature is enhanced by annealing at relatively low temperature such as 300 °C and decreased by annealing at higher temperatures. X-ray absorption near edge structures indicate that ZnFe 2O 4 thin film annealed at 300 °C possesses disordered cation distribution similar to that for as-deposited thin film. It is presumed that the enhancement of magnetization by annealing at 300 °C is due to precipitation of disordered ZnFe 2O 4 crystal from an amorphous matrix formed in the as-deposited thin film.

Crystal chemistry of the schorl-dravite series

Nineteen tourmaline samples of various provenances and geological settings were studied by EMPA, SREF and MS to represent the schorl-dravite compositional field. All samples belong to the Alkali group (except one with an X -site vacancy content of 0.53 apfu) and to the Oxy- and Hydroxy-subgroups. Among divalent cations, the main substitution involves Y Mg for Y Fe 2+ , to produce the two end-members dravite and schorl. Site populations were determined by a new minimization procedure that simultaneously accounts for both structural and chemical data. Results show that the crystals are characterized by disordered cation distribution between Y and Z sites: Al populates both sites, with a marked preference for the smaller Z octahedron; Mg is often equally distributed between Y and Z . Both Fe 2+ and Fe 3+ populate both Y and Z sites, but show a strong preference for Y . Specific mean bond distances (A) optimised for major elements are: Y Al-O = 1.908, Y Mg-O = 2.084, Y Fe 2+ -O = 2.139, z Al-O = 1.900, z Mg-O = 2.077 and z Fe 2+ -O = 2.131. In the schorl-dravite solid solution, structural variations appear to be primarily due to Y and Z interactions. These effects are conspicuous over the entire structure, as Y dimensions directly affect the a cell parameter, while Z is similarly correlated with c. The dimensions of Y and Z octahedra are determined by Al contents. Dimensional variations of Z are well described by its bond-distance variations, except for Z -07D. Both octahedra reciprocally interact, influencing their distortions: inverse correlations exist between Y dimension vs. Z quadratic elongation and Z dimension vs. Y quadratic elongation. As a common feature, the effects of the octahedral second coordination sphere are only confined to polyhedral distortions instead of dimensional variations, which only depend on site populations.

Crystal chemistry of the dravite-chromdravite series

Five Cr-dravite-chromdravite samples, representative of the high-chromium part of the compositional field observed in tourmalines occurring in rocks surrounding Lake Baikal, were selected and studied by electron microprobe analysis and structural refinement. All examined tourmalines belong to the Alkali group, Oxy-subgroup. Their most striking feature is the exceptionally high Cr content (from 3.2 to 4.41 apfu), which substitutes for Al. The main divalent cation is Mg. Site populations were determined by a minimization procedure which simultaneously takes into account both structural and chemical data. Empirical bond distances for Y Cr 3+ -O (1.978 A) and z Cr 3+ -O (1.970 A) were optimised within the minimization procedure. Results indicate that the crystals are characterized by a quite disordered cation distribution between Y and Z octahedral sites, the various cations showing different degrees of preference. Cr 3+ and Mg populate both sites but show opposite behaviour: Mg has a marked preference for the Z octahedron and Cr 3+ for Y. Al almost exclusively populates Z. Most structural features are related to variations in the Z octahedron, whereas the dimensions of other polyhedra remain almost constant. Z dimensions depend on site populations, and are in particular determined by the z Cr 3+ ⟷ Z Al substitution. In the dravite-chromdravite series, both current and literature data show that the unit cell parameter c is strongly and positively correlated with Z dimensions, while correlation with a is evident only within the Cr-dravite-chromdravite subseries. Y does not actively participate in structural variations and, in particular, no correlation was ever observed between Y dimensions and unit cell parameters. Most structural variations are thus due to Z , while the effects of Y are negligible. This behaviour stands out when examining the a vs. c plot, for the whole series: a strong correlation between the two unit cell parameters was observed in the Cr-dravite-chromdravite subseries, but none within the dravite-Cr-dravite subseries or schorl-dravite series.