AlO6 Octahedra Research Articles

Crystal structure stability and phase transition of celsian, BaAl2Si2O8, up to 1100 °C / 22 GPa

Celsian (BaAl2Si2O8) is an important component of aluminosilicate-based ceramics, which is widely used in high temperature/high pressure process. However, limited data available on its high pressure (HP)/high temperature (HT) behavior and HT studies were based on multi-component samples only. Here we present data on crystal structure stability and phase transformation processes of celsian studied under extreme conditions (up to ∼22 GPa/∼1110 °C) using in situ X-ray diffraction. Under HP conditions celsian undergoes three phase transitions: at the pressures below 10 GPa the main changes in the crystal structure of celsian associated with the changes in the coordination Ba-centered polyhedron (celsian-II and celsian-III), whereas at higher pressures AlO4 tetrahedra transforms into AlO6 octahedra (celsian-IV). Though the HP behavior of some isostructural compound has been studied before, the HP transformation of celsian is not similar to any previously studied mineral or synthetic compound. Celsian demonstrate stability and low thermal expansion coefficients (αV = 12 × 10−6 °C−1) in the studied temperature range.

Preparation, electronic structure and thermal properties of Zn-doped LaMgAl11O19 materials

LaMgAl11O19 (LMA) with magnetoplumbite structure is a promising thermal barrier material due to its excellent thermal properties. In this work, Zn-doped LMA materials were studied in order to improve the thermal performance, and LaZnxMg1-xAl11O19 (x = 0, 0.1, 0.3, 0.5, 0.7, 1.0) were prepared by sol-gel method. The as-synthesized powder materials possess pure LMA phase and show the morphology of hexagonal plates. X-ray photoelectron spectroscopy result shows that there are AlO4 tetrahedra and AlO6 octahedra units in the target materials, and Zn2+ ions replace Mg2+ ions to occupy the tetrahedral position of magnetoplumbite structure. The composition LaZn0.3Mg0.7Al11O19 material exhibits the lowest thermal conductivity of 0.38 W/(m·K) at 1000oC while maintaining a high coefficient of thermal expansion (10.86 × 10−6 K−1). Therefore, the thermal properties of LMA materials can be improved by adding an appropriate amount of Zn ions.

Na3[Al2B6P4O22(OH)3](H2O)6 and Na3[Al2BP2O11](H2O)0.5: Two Remarkable Complex Aluminum Borophosphates.

Two remarkable aluminum borophosphates (AlBPOs), namely, Na3[Al2B6P4O22(OH)3](H2O)6 (denoted as ABPO1) and Na3[Al2BP2O11](H2O)0.5 (denoted as ABPO2), have been designed and prepared by low-temperature flux syntheses. The exceptional open framework structure of ABPO1 is formed by a unique microanionic network [Al2B6P4O22(OH)3]n3-, which contains three types of 8-, 12-, and 16-membered ring (MR) tunnels. Interestingly, these tunnels are featured by a type of super-nanocage as large as ∼1.753 nm × 1.753 nm × 1.753 nm, which is the first example of AlBPOs containing extra-large cages. Importantly, it was found that Na+ can be partially exchanged by K+, Sr2+, Cd2+, and Ni2+, which means that it is a potential ionic exchanger for removing radionuclides and toxic cations. The structure of ABPO2 features a unique 2D anionic AlBPO layer composed of corner-sharing AlO6 octahedra and AlO4, BO4, and PO4 tetrahedra. To the best of our knowledge, this is the first example of both AlO6 octahedra and AlO4 tetrahedra being contained in the structure. 9-MRs can be observed along the b-axis. Herein, the syntheses and topological structures of ABPO1 and ABPO2 as well as elemental analysis, thermal stability, infrared spectroscopy, UV-vis diffuse reflectance, structural properties, and ionic exchange properties are also discussed.

Naalasite, NaAl(AsO3OH)2⋅H2O, the Al analogue of nafeasite from the Torrecillas mine, Iquique Province, Chile

AbstractThe new mineral naalasite (IMA2023–027), NaAl(AsO3OH)2⋅H2O, was found at the Torrecillas mine, Iquique Province, Chile, where it is a secondary alteration phase associated with anhydrite, juansilvaite, magnesiokoritnigite and a lavendulan-like phase. Naalasite occurs in tightly intergrown aggregates and druses of equant crystals. Crystals are light to medium pink and transparent, with vitreous lustre and white streak. The Mohs hardness is ~3½. The density is 3.19(2) g⋅cm–3. Optically, naalasite is uniaxial (+), with ω = 1.630(3) and ɛ = 1.660(3) (white light). The empirical formula (based on 9 O apfu) is Na0.92Al0.61Fe3+0.39As2O9H4.07. Naalasite is trigonal, space group R32, with cell parameters: a = 8.494(4), c = 26.430(13) Å, V = 1651.5(4) Å3 and Z = 9. The structure, refined to R1 = 3.78% for 641 I > 2σI reflections, is based on a loose 3D framework of alternating AsO3OH tetrahedra and AlO6 octahedra. The structure is topologically equivalent to that of nafeasite and can be regarded as its Al analogue, even though nafeasite is monoclinic with space group C2.

Temperature effect on surface structure of single crystal SrLaAlO4(001)

Development of next-generation electrochemical devices, such as solid oxide cells, requires control of the charge transfer processes across key interfaces. Structural strain at electrolyte:electrode interfaces could potentially alter the devices’ charge transport properties, therefore understanding the structural behavior of electrode surfaces under operating conditions is important. The functional oxide single crystal substrate SrLaAlO4 has been well-characterized with bulk structure studies; however, there are very few studies of SrLaAlO4 surface structures. Here, we present an investigation of the surface structure of SrLaAlO4 (001) substrates using surface x-ray diffraction under UHV conditions (10−10 Torr) with the substrate held at either room temperature or 650 °C. Best-fit models using a 1:1 ratio of Sr:La showed significant distortions to the surface AlO6 octahedra.

Petermegawite, Al6(Se4+O3)3[SiO3(OH)](OH)9⋅10H2O, a New AI-Bearing Selenite Mineral, from the El Dragón Mine, Potosí, Bolivia

Abstract A new mineral species, petermegawite, ideally Al6(Se4+O3)3[SiO3(OH)](OH)9⋅10H2O, was discovered at the El Dragón mine, Potosí Department, Bolivia. It occurs as aggregates of bladed or tabular crystals. Associated minerals are Co-bearing krut′aite–penroseite (matrix), chalcomenite, ‘clinochalcomenite’ (not IMA approved), molybdomenite, lepidocrocite, goethite, ahlfeldite, and calcite. Petermegawite is colorless in transmitted light and transparent with a white streak and vitreous luster. It is brittle and has a Mohs hardness of 2–2½. Cleavage is perfect on {001}. The measured and calculated densities are 2.27(5) and 2.32 g/cm3, respectively. Optically, petermegawite is biaxial (+), with α = 1.545(5), β = 1.554(5), γ = 1.567(5) (white light). An electron microprobe analysis yielded an empirical formula (based on 10 non-H cations pfu) of Al6.00[(Se0.89S0.11)Σ1.00O3]3[(Si0.90Al0.07)Σ0.97O2.81(OH)1.19](OH)9⋅10H2O, which can be simplified to Al6[(Se,S)O3]3[(Si,Al)O3(OH)](OH)9⋅10H2O. Petermegawite is orthorhombic, space group Cmc21 with unit-cell parameters a = 16.2392(2), b = 10.96370(10), c = 15.3367(2) Å, V = 2730.57(5) Å3, and Z = 4. Its crystal structure is characterized by clusters of six-membered rings of edge-sharing AlO6 octahedra with an SiO3(OH) tetrahedron situated at the center of each ring and three Se4+O3 triangular pyramids appended outside the ring. These clusters are joined together by hydrogen bonds to form heteropolyhedral layers parallel to (001). The H2O molecules that are not bonded to any non-H cations are sandwiched between the layers. The cluster formed by a six-membered AlO6 octahedral ring with an SiO3OH tetrahedron at the center of the ring in petermegawite has the composition [Al6(SiO3OH)O6(OH)9(H2O)6]6−. It is topologically identical to the clusters of composition [Al6(AsO4)O7(OH)9(H2O)5]8− that are found in bettertonite and penberthycroftite. As a new polyoxometalate building block, this type of cluster may be expressed with a general chemical formula [TM6X25], where T = tetrahedrally coordinated cation, M = octahedrally coordinated cation, and X = O, OH, H2O.

Infrared and Raman Spectroscopy of Mullite Ceramics Synthesized from Fly Ash and Kaolin

Infrared spectroscopy and Raman spectroscopy were used to characterize mullite ceramics prepared from fly ash and kaolin by annealing at 1000 °C, 1100 °C, 1200 °C, and 1300 °C. IR spectroscopy confirmed the presence of SiO4 tetrahedra and AlO6 octahedra in samples. The presence of mullite has been confirmed at all temperatures. The presence of quartz has been confirmed up to a temperature of 1100 °C, and the presence of an amorphous form of SiO2 has been confirmed at temperatures of 1200 °C and 1300 °C. The transformation of quartz into the amorphous form of SiO2 at temperatures above 1100 °C is assumed. Transformation was performed on the percentage intensity decrease of the bending vibration of Si-O-Si (at about 450 cm−1) and Al-O-Si (at about 550 cm−1). Raman spectroscopy confirmed the presence of mullite at different stages of structural ordering (a well-ordered structure at a temperature of 1100 °C and a disordered structure at a temperature of 1300 °C).

Al2O3 triggered in-situ crystallization of Gd3Al2Ga3O12 in glass and enhancement in magnetic and Faraday rotating properties

Nanocrystals doped transparent glasses or glass ceramics have shown promising tunable magnetic and magneto-optical performance. In this study, the Al2O3-induced in-situ crystallization of Gd3Al2Ga3O12 in tellurite glass was reported. 10–20 nm-cubic Gd3Al2Ga3O12 (Ia3d space group) nanocrystals formed in tellurite glass with Al2O3 content ≤ 0.75 mol% by 400 °C-heat treatment. When Al2O3 content in glass was higher than 0.75 mol%, excessive Al2O3 triggered the crystallization of orthorhombic GdAlO3 (Pnma) in which the Gd and Al ions existed as octahedral GdO8 and AlO6 units at a temperature higher than 398 °C. The in-situ crystallization influenced the glass network structure, broke the linkage of tetrahedral TeO4, and BO4, and formed trigonal pyramids TeO3 and BO3 instead. At the same time, nuclear magnetic resonance spectra revealed the conversion of AlO4→ AlO6, GaO4→ GaO6, and the changes from bridging oxygen to non-bridging oxygen as well. From energy-dispersive X-Ray analysis, Gd3+ clusters were observed, leading to the ferromagnetism of glass. Electron paramagnetic resonance spectra witnessed an enhancement of the Zeeman effect which is the reason for the improvement of Faraday rotation. Tellurite glass with 0.75 mol% Al2O3 after 400 °C-annealing (A75) showed a giant Verdet constant of 93 rad/T.m at 633 nm which is superior to most of the values from the literature.

Screening rare‐earth aluminates as promising thermal barrier coatings by high‐throughput first‐principles calculations

AbstractThermal barrier coatings (TBCs) play an important role in gas turbines to protect the turbine blades from the high‐temperature airflow damage. In this work, we use first‐principles calculations to investigate a specific class of rare‐earth (RE) aluminates, including cubic‐REAlO3(c‐REAlO3), orthorhombic‐REAlO3(o‐REAlO3),RE3Al5O12, andRE4Al2O9, to predict their structural stability, bonding characteristics, and mechanical and thermal properties. The polyhedron structures formed by the Al–O bonds are stronger and exhibit rigid characteristics, whereas the polyhedra formed by theRE–O bonds are relatively weak and soft. The alternating stacking of AlO4tetrahedra, AlO6octahedra, andRE–O polyhedra, as well as the selection ofREelements, shows intensive influences on the expected mechanical and thermal properties. TheB,G, andEof these four types of aluminates decrease in the order of c‐REAlO3 > o‐REAlO3 > RE3Al5O12 > RE4Al2O9.REAlO3andRE4Al2O9are brittle and quasi‐ductile ceramics, respectively, whereasRE3Al5O12is tailorable. The minimum thermal conductivity is in the range of 1.4–1.5 W m−1 K−1for c‐REAlO3, 1.3–1.4 W m−1 K−1for o‐REAlO3, 1.25–1.35 W m−1 K−1forRE3Al5O12, and 0.8–0.9 W m−1 K−1forRE4Al2O9.RE4Al2O9with low thermal conductivity and damage tolerance is predicted to be the potential candidates for next‐generation TBC materials.

(Na,Li)3(Cl,OH)[Cu3OAl(PO4)3]: a first salt-inclusion aluminophosphate oxocuprate with a new type of crystal structure.

The synthesis and characterization of a first salt-inclusion aluminophosphate oxocuprate, (Na,Li)3(Cl,OH)[Cu3OAl(PO4)3], obtained as single crystals, is reported. A novel phase, with a strongly pseudo-orthorhombic structure, is described as a monoclinic crystal structure established by the study of a pseudomerohedric microtwin. It was investigated using scanning electron microscopy, microprobe analysis and low-temperature X-ray diffraction. The composite crystal structure represents an original framework assembled from Cu-centered polyhedra, AlO6 octahedra and PO4 tetrahedra with channels, which incorporate the Na/Li salt component [(Na,Li)3(Cl,OH)]2+ that ensures electroneutrality of the compound. Layers of strongly corrugated chains of Cu-centered octahedra with shared edges and linked by PO4 tetrahedra are shown to be topologically identical with the layers also built from Cu-centered polyhedra and AsO4/VO4 tetrahedra forming the crystal structure of a fumarolic mineral aleutite, (M0.5Cl)[Cu5O2(AsO4)(VO4)] [Siidra et al. (2019). MinMag, 83, 847-853]. `Sawtooth chains' and pairs of Cu-centered octahedra inherent in the title structure may be of interest in solid-state physics, engaging studies in the field of low-dimensional and frustrated magnetism.

Lithium-Ion Conduction in a Class of Aluminoborates LinMAlB12O24 (M = Ba, Sr, Ca, or La; n = 7 or 6)

With diverse structures and high ambient stability, Li-containing borate materials are promising candidates as Li solid-state conductors. Here, we report a group of aluminoborate compounds, LinMAlB12O24 (n = 6 for M = La; n = 7 for M = Ba, Sr, Ca), which were synthesized by a conventional solid-state method. Their crystal structures feature a common three-dimensional framework consisting of B6O14 clusters and AlO6 octahedra and the Li ions are positioned around the chained B6O14 clusters. The Li content n can be modified, i.e. either 6 or 7, depending on the chosen metal species M to keep the overall charge balance. The Li-ion conduction properties of the series were studied by bond valence site energy calculation and electrochemical impedance spectroscopy. We observed much higher ionic conductivity and a lower migration barrier in Li7BaAlB12O24 than in Li6LaAlB12O24, suggesting that the Li-ion site configuration plays an important role in determining ion transport.

The effects of pressure on the lattice of the rare-earth-based perovskite-type oxides SmAlO3 and NdAlO3

This paper studies the behavior of SmAlO3 and NdAlO3 when they are subject to high pressures. This work is undertaken using angle-dispersive synchrotron x-ray powder diffraction and Raman spectroscopy at pressures up to 24.2 and 39.0 GPa, respectively. It is found that SmAlO3 undergoes an orthorhombic (Pnma) to rhombohedral (R-3c) structure transition at around 10 GPa; this transition is induced by the rotation of the AlO6 octahedra toward that of the ideal perovskite structure when the material is subject to high pressures. The tilting of the AlO6 octahedra also decreases at high pressures in NdAlO3. It is found that NdAlO3 maintains its original rhombohedral structure for pressures of up to 39.0 GPa. The structural changes observed in these compounds help establish the electrical and magnetic properties of RAlO3 (R = Sm or Nd) at high pressures.

EPR, NMR, DFT and XPS study on LaAlO3 phase transition in glass and influence on glass structure and properties

Thermal treatment induced phase transition of LaAlO3 in glass and its influence on glass structure and dielectric, magnetic, and Faraday rotation properties were characterized through various techniques such as nuclear magnetic resonance, X-ray neutron diffraction, transmission electron microscopy, X-ray photoelectron microscopy, vibrating-sample magnetometer, and electron paramagnetic resonance. Cubic LaAlO3 nanocrystals were synthesized and incorporated into lead-tellurite glasses, after crystallization treatment at 380 °C for 2 h, 14-17 nm-cubic LaAlO3 nanocrystals were changed to 14-27 nm-rhombohedral LaAlO3 perovskite due to the temperature-induced viscosity and strain in the melt. The phase transition of LaAlO3 nanocrystals in glass matrix was verified through versatile techniques in terms of the octahedral AlO6 distortion, SAED patterns, changes of lattice parameters calculated by Rietveld refinement, and coordination number changes of AlO6 by NMR and XPS techniques. The modification to the glass structure was yielded by phase transition through converting TeO4→TeO3, AlO6→AlO4, BO4→BO3, and producing non-bridging oxygen. In addition, the energy band gap and density of states of glass after phase transition was calculated through Density functional theory (DFT) simulation. Glass doped with 10% LaAlO3 exhibited a promising dielectric constant and good thermal stability due to the phase transition induced polarizability, and improved optical and magnetic performance which is promising for photonics device applications.

Two-Dimensional Cationic Aluminoborate as a New Paradigm for Highly Selective and Efficient Cr(VI) Capture from Aqueous Solution.

Water pollutants existing in their oxyanion forms have high solubility and environmental mobility. To capture these anionic pollutants, cost-effective inorganic materials with cationic frameworks and outstanding removal performance are ideal adsorbents. Herein, we report that two-dimensional (2D) cationic aluminoborate BAC(10) sets a new paradigm for highly selective and efficient capture of Cr(VI) and other oxyanions from aqueous solution. The structure of Cr(VI)-exchanged BAC(10) sample (Cr(VI)@BAC(10), H0.22·Al2BO4.3·(HCrO4)0.22·2.64H2O) has been successfully solved by continuous rotation electron diffraction. The crystallographic data show that the 2D cationic layer of BAC(10) is built by AlO6 octahedra, BO4 tetrahedra, and BO3 triangles. Partial chromate ions exchanged with Cl– ions are located within the interlayer region, which are chemically bonded to the aluminoborate layer. BAC(10) shows faster adsorption kinetics compared to the commercial anion exchange resin (AER) and layered double hydroxides (LDHs), a higher maximum adsorption capacity of 139.1 mg/g than that of AER (62.77 mg/g), LDHs (81.43 mg/g), and a vast majority of cationic MOFs, and a much broader working pH range (2–10.5) than LDHs. Moreover, BAC(10) also shows excellent Cr(VI) oxyanion removal performance for a solution with a low concentration (1–10 mg/L), and the residual concentration can be reduced to below 0.05 mg/L of the WHO drinking water criterion. These superior properties indicate that BAC(10) is a promising material for remediation of Cr(VI) and other harmful oxyanions from wastewater.

Probing the effect of R-cation radii on structural, vibrational, optical, and dielectric properties of rare earth (R=La, Pr, Nd) aluminates

The present study correlates the effect of R-cation radii on structural, vibrational, optical, and dielectric properties of rhombohedral rare earth aluminates RAlO3 (R = La, Pr, Nd). The polycrystalline samples of RAlO3 have been synthesized using sol-gel synthesis technique. Pure rhombohedral phase of RAlO3 samples has been confirmed with X-ray diffraction. Systematic decrements in the lattice parameter, bond length, and bond angle have been observed, giving rise to structural distortion due to decrease in ionic radii of R-cation. The phononic properties of RAlO3 have been investigated through Raman spectroscopy, where the degree of distortion of AlO6 octahedra can be analyzed with the peak position of Eg and A1g modes. An increase in the energy bandgap with decreasing R-cation radii shows an interconnection with the decrease in Al–O bond length. Interestingly, the decreasing dielectric constant with decreasing ionic radii of R-cation has been correlated with the difference in electronegativity of cation(R3+)-anion(O2−) pair. Also, a positive linear relationship between dielectric constant and energy bandgap has been investigated using Penn model.

Effect of SnO–P2O5–MgO glass addition on the ionic conductivity of Li1.3Al0.3Ti1.7(PO4)3 solid electrolyte

NASICON-type structured compounds Li 1+ x M x Ti 2- x (PO 4 ) 3 (M = Al, Fe, Y, etc.) have captured much attention due to their air stability, wide electrochemical window and high lithium ion conductivity. Especially, Li 1.3 Al 0.3 Ti 1.7 (PO 4 ) 3 (LATP) is a potential solid electrolyte due to its high ionic conductivity. However, its actual density usually has a certain gap with the theoretical density, leading the poor ionic conductivity of LATP. Herein, LATP solid electrolyte with series of SnO–P 2 O 5 –MgO (SPM, 0.4 wt%, 0.7 wt%, 1.0 wt%, 1.3 wt%) glass addition was successfully synthesized to improve the density and ionic conductivity. The SPM addition change Al/Ti–O bond and P–O bond distances, leading to gradual shrinkage of octahedral AlO 6 and tetrahedral PO 4 . The bulk conductivity of the samples increases gradually with SPM glass addition from 0.4 wt% to 1.3 wt%. Both SPM and the second-phase LiTiPO 5 , caused by glass addition, are conducive to the improvement of compactness. The relative density of LATP samples increases first from 0 wt% to 0.7 wt%, and then decreases from 0.7 wt% to 1.3 wt% with SPM glass addition. The grain boundary conductivity also changes accordingly. Especially, the highest ionic conductivity of 2.45 × 10 −4 S cm −1 , and a relative density of 96.72% with a low activation energy of 0.34 eV is obtained in LATP with 0.7 wt% SPM. Increasing the density of LATP solid electrolyte is crucial to improve the ionic conductivity of electrolytes and SPM glass addition can promote the development of dense oxide ceramic electrolytes.

Dendoraite-(NH4), a new phosphate–oxalate mineral related to thebaite-(NH4) from the Rowley mine, Arizona, USA

AbstractDendoraite-(NH4), (NH4)2NaAl(C2O4)(PO3OH)2(H2O)2, is a new mineral species from the Rowley mine, Maricopa County, Arizona, USA. It occurs in an unusual bat-guano-related, post-mining assemblage of phases that include a variety of vanadates, phosphates, oxalates and chlorides, some containing NH4+. Other secondary minerals found in association with dendoraite-(NH4) are antipinite, fluorite, mimetite, mottramite, relianceite-(K), rowleyite, salammoniac, struvite, vanadinite, willemite, wulfenite and at least one other new mineral. Crystals of dendoraite-(NH4) are colourless blades up to ~0.1 mm in length. The streak is white and lustre is vitreous, Mohs hardness is 2½, tenacity is brittle and fracture is splintery. The calculated density is 2.122 g⋅cm–3. Dendoraite-(NH4) is optically biaxial (–) with α = 1.490(5), β = 1.540(5) and γ = 1.541(5) (white light); 2Vcalc = 15.7°; and orientation X = b. Electron microprobe analysis gave the empirical formula [(NH4)1.48K0.52]Σ2.00Na0.96(Al0.96Fe3+0.03)Σ0.99(C2O4)[PO2.97(OH)1.03]2(H2O)2, with the C, N and H contents constrained by the crystal structure. Dendoraite-(NH4) is monoclinic, P21/n, with a = 10.695(6), b = 6.285(4), c = 19.227(12) Å, β = 90.933(10)°, V = 1292(2) Å3, and Z = 4. The structural unit in the crystal structure of dendoraite-(NH4) (R1 = 0.0467 for 1322 Io > 2σI reflections) is a double-strand chain of corner-sharing AlO6 octahedra and PO3OH tetrahedra decorated by additional PO3OH tetrahedra and C2O4 groups. Topologically, this is the same chain found in the structure of thebaite-(NH4). The decorated chains connect to one another through links to NaO7(H2O) polyhedra to form a [Na(H2O)Al(C2O4)(PO3OH)2]2– sheet, which connect to one another through bonds to (NH4)/K and through hydrogen bonds.

Kahlenbergite KAl<sub>11</sub>O<sub>17</sub>, a new <i>β</i>-alumina mineral and Fe-rich hibonite from the Hatrurim Basin, the Negev desert, Israel

Abstract. Kahlenbergite, ideally KAl11O17, and Fe-rich hibonite, CaAl10Fe2O19, are high-temperature minerals found in “olive” subunits of pyrometamorphic rocks, in the Hatrurim Basin, the Negev desert, Israel. The crystal structures of both minerals are refined using synchrotron radiation single-crystal diffraction data. The structure of kahlenbergite (P63/mmc; a=5.6486(1) Å; b=22.8970(3) Å; Z=2) exhibits triple spinel blocks and so-called R blocks. The spinel blocks show mixed layers with AlO6 octahedra and (Al0.56Fe0.44)O4 tetrahedra and kagome layers with (Al0.92Fe0.08)O6 octahedra. One-dimensional diffuse scattering observed parallel to c* implies stacking faults in the structure. Also, in one of the investigated kahlenbergite crystals additional reflections can be identified, which obviously belong to a second phase with a smaller lattice parameter c: Fe3+-rich hibonite. The structure of hibonite contains the same spinel blocks as kahlenbergite. The R blocks in hibonite contain Ca atoms, AlO5 bipyramids, and AlO6 octahedra, whereas the R blocks in kahlenbergite contain potassium atoms and AlO4 tetrahedra.

Chukochenite, (Li0.5Al0.5)Al2O4, a new lithium oxyspinel mineral from the Xianghualing skarn, Hunan Province, China

Abstract Chukochenite, (Li0.5Al0.5)Al2O4, is a new mineral species from the Xianghualing skarn, Hunan Province, southern China. It occurs as subhedral to euhedral crystals up to 200 μm across in the green rock of Xianghualing skarn, closely associated with fluorite, phlogopite, chrysoberyl, margarite, chlorite, ferronigerite-2N1S, and zinconigerite-2N1S. The crystals are colorless and transparent with a vitreous luster. Chukochenite is brittle with irregular fracture, has a Mohs hardness of 8, and shows light red fluorescence under 253.7 nm UV radiation and light green fluorescence under 365 nm UV radiation. The calculated density is 3.771 g/cm3. Chukochenite is optically biaxial (–) with α = 1.79(2), β = 1.82(2), and γ = 1.83(2) (589 nm). The calculated 2V is 60°, with the optical orientations X, Y, and Z parallel to the crystallographic a, b, and c, respectively. Electron microprobe analysis (Li by LA-ICP-MS) yielded in wt% Al2O3 80.70, Fe2O3 8.16, Li2O 3.68, ZnO 3.25, MnO 2.49, MgO 1.70, Na2O 0.11, CaO 0.08, TiO2 0.02, K2O 0.01, and Cr2O3 0.01 (total 100.24 wt%), giving an empirical formula [(Li0.355Al0.138Na0.005Ca0.002)Σ0.5(Al0.145Fe0.147+3Mg0.061Zn0.058Mn0.051Si0.001)Σ0.463]Al2O4 on a basis of 4 O atoms per formula unit. Chukochenite is orthorhombic, Imma, a = 5.659 (1), b = 16.898 (1), c = 7.994 (1) Å, V = 764.46 (8) Å3, and Z = 12. The nine strongest lines of powder XRD [d in Å (I) (hkl)] are: 2.405 (53) (231); 1.996 (29) (260); 1.535 (77) (303); 1.413 (100) (264); 1.260 (52) (2 12 0); 1.068 (36) (1 13 4); 1.039 (61) (503); 0.999 (59) (008); and 0.942 (35) (3 13 4). Chukochenite has a framework structure of spinel with low symmetry (orthorhombic Imma) due to the ordering of Li cations over octahedrally coordinated sites, which has not been previously reported for synthetic (Li0.5Al0.5) Al2O4. This structure type is based on a framework of AlO4 tetrahedra, AlO6, and LiO6 octahedra. AlO6 edge-sharing octahedra form chains along the a axis. AlO6 octahedra and LiO6 octahedra in a 2:1 ratio share edges, forming octahedral chains along b. These octahedral chains are connected by AlO4 tetrahedra and each corner of an AlO4 tetrahedron shares with three AlO6 octahedra or two AlO6 + one LiO6 octahedra. The discovery of chukochenite adds a new perspective on the cation ordering and the mechanism of luminescence and magnetism in (Li0.5Al0.5)Al2O4.

The crystal structure of a new calcium aluminate phase containing formate

A new calcium aluminate phase containing formate ions was synthesized and its crystal structure determined. This new phase is indicated as M-phase and was firstly observed in Portland cement pastes hydrated in presence of Ca-formate and in excess of water. The crystal structure of the M-phase was successfully solved in the R-3 space group of the trigonal system on the basis of synchrotron X-ray single crystal diffraction data. The structural model was confirmed by Rietveld refinement of the powder diffraction data acquired on the synthesized pure sample. The crystal structure of the M-phase is similar to that of ettringite, being characterized by columns of AlO6 octahedra alternating with groups of three edge-sharing CaO7 polyhedra. The formate ions (HCOO)− share two oxygens with Ca polyhedra and are located in the interspace between the columns. The crystal structure of the M-phase testifies the strong interaction occurring between small organic molecules as formate and the calcium aluminate components of Portland cement.