Tetrahedral Chains Research Articles

Structural and Topological Evolution in SixSe1-x Glasses: Results from 1D and 2D 29Si and 77Se NMR Spectroscopy.

The coordination environments of Si and Se atoms and their connectivity in binary SixSe1-x glasses with 0.05 ≤ x ≤ 0.33 are investigated using a combination of one- and two-dimensional 29Si and 77Se nuclear magnetic resonance (NMR) and Raman spectroscopy. The high-resolution correlated isotropic and anisotropic 29Si and 77Se NMR spectra allow for the identification and quantitation of a variety of Si and Se environments. The results suggest that the structure of these glasses are characterized by a network with essentially perfect short-range chemical order, but with strong clustering at the intermediate range. Initial addition of Si to Se results in cross-linking of Se chain segments with nanoclusters of corner- and edge-shared SiSe4/2 tetrahedra. These clusters percolate via coalescence near x ≥ 0.2 to finally form a low-dimensional network with high molar volume, at the stoichiometric composition (x = 0.33) that is composed of chains of edge-sharing tetrahedra cross-linked by corner-shared tetrahedra. This structural evolution is shown to be consistent with the compositional variation of the glass transition temperature and the molar volume of these glasses.

Low-temperature structural transition in the quasi-one-dimensional spin-12compoundLi2Cu2O(SO4)2

A thorough structural exploration has been made on the quasi-one-dimensional S = 1/2 compound Li 2 Cu 2 O(SO 4) 2 by neutron and synchrotron x-ray diffraction. It reveals the occurrence of a structural transition at 125 K, characterized by a lowering of symmetry from P 4 2 /m to P ¯ 1, which is possibly driven by an exchange striction mechanism. This transition involves a dimerization of some Cu in the edge-sharing tetrahedral Cu chains. A symmetry mode analysis indicates that one representation, 3 + 4 + , dominates the structural transition. Interestingly, no intermediate structure with P 112/m symmetry is observed experimentally. Lastly, temperature dependent magnetic susceptibility measurements and neutron diffraction reveal that the magnetic ground state of this compound is a spin-singlet with a spin gap, characterized by the absence of long-range magnetic order down to 1.7 K.

Solar Cell Materials by Design: Hybrid Pyroxene Corner-Sharing VO4 Tetrahedral Chains.

Hybrid organic-inorganic frameworks provide numerous combinations of materials with a wide range of structural and electronic properties, which enable their use in various applications. In recent years, some of these hybrid materials-especially lead-based halide perovskites-have been successfully used for the development of highly efficient solar cells. The large variety of possible hybrid materials has inspired the search for other organic-inorganic frameworks that may exhibit enhanced performance over conventional lead halide perovskites. In this study, a new class of low-dimensional hybrid oxides for photovoltaic applications was developed by using electronic structure calculations in combination with analysis from existing materials databases, with a focus on vanadium oxide pyroxenes (tetrahedron-based frameworks), mainly due to their high stability and nontoxicity. Pyroxenes were screened with different cations [A] and detailed computational studies of their structural, electronic, optical and transport properties were performed. Low-dimensional hybrid vanadate pyroxenes [A]VO3 (with molecular cations [A] and corner-sharing VO4 tetrahedral chains) were found to satisfy all physical requirements needed to develop an efficient solar cell (a band gap of 1.0-1.7 eV, strong light absorption and good electron-transport properties).

Batisite, Na2BaTi2(Si4O12)O2, from Inagli massif, Aldan, Russia: crystal-structure refinement and high-temperature X-ray diffraction study

The crystal structure of batisite, Na2BaTi2 (Si4O12)O2, from the Inagli massif (Aldan, Yakutia, Russia) was refined to R 1 = 0.032 for 1449 unique observed reflections. The mineral is orthorhombic, Imma, a = 8.0921(5), b = 10.4751(7), c = 13.9054(9) A, V = 1178.70(13) A3. The mineral is based upon three-dimensional titanosilicate framework consisting of chains of corner-sharing MO6 octahedra (M = Ti, Nb, Fe and Zr) and vierer chains of corner-sharing SiO4 tetrahedra. Both chains are parallel to the a axis and are linked by sharing peripheral O atoms. The octahedral chains display disorder of M atoms and bridging O sites related to the out-of-center distortion of octahedral geometry around Ti4+ cations. Electron microprobe analysis gives SiO2 39.46, TiO2 24.66, BaO 21.64, Na2O 7.56, K2O 4.38, Fe2O3 0.90, ZrO2 0.66, Nb2O5 0.36, (H2O)calc 0.58, sum 99.76 wt%. The seven strongest X-ray powder-diffraction lines [listed as d in A (I) hkl] are: 8.39 (94) 011, 3.386 (56) 031, 3.191 (36) 123, 2.910 (46) 222, 2.896 (100) 024, 2.175 (45) 035, 1.673 (57) 055. The thermal behaviour of batisite in the temperature range from 25 to 950 °C was studied using high-temperature powder X-ray diffraction. The thermal expansion coefficients along the principal crystallographic axes are: α a = 14.4 × 10−6, α b = 8.7 × 10−6, α c = 8.4 × 10−6, α V = 31.5 °C−1 for the temperature range 25–500 °C and α a = 19.6 × 10−6, α b = 9.1 × 10−6, α c = 8.8 × 10−6, α V = 37.6 °C−1 for the temperature range 500–900 °C. The direction of maximal thermal expansion is parallel to the chains of both MO6 octahedra and SiO4 tetrahedra, which can be explained by the stretching of silicate chains due to the increasing thermal vibrations of the Ba2+ cations. At 1000 °C, the titanosilicate framework in batisite collapses with the formation of fresnoite, Ba2TiSi2O7O.

Nolzeite, Na(Mn,□)2[Si3(B,Si)O9(OH)2]·2H2O, a new pyroxenoid mineral from Mont Saint-Hilaire, Québec, Canada

AbstractNolzeite, Na(Mn,□)2[Si3(B,Si)O9(OH)2]·2H2O, is a new mineral found in altered sodalite syenite at the Poudrette quarry, La Vallée-du-Richelieu, Montérégie (formerly Rouville County), Québec, Canada. Crystals are colourless to pale green and are acicular with average dimensions of 5 μm × 8 μm × 55 μm. They occur as radiating to loose, randomly oriented groupings within vugs associated with aegirine, nepheline, sodalite, eudialyte-group minerals, analcime, natron, pyrrhotite, catapleiite, steedeite and the unidentified mineral, UK80. Nolzeite is non-pleochroic, biaxial, withnmin= 1.616(2) andnmax= 1.636(2) and has a positive elongation. The average of six chemical analyses gave the empirical formula: Na1.04(Mn1.69□0.24Fe0.05Ca0.02)∑=2.00(Si2.96S0.04)∑=3.00(B0.70Si0.30)∑=1.00O9(OH)2·2H2O based on 13 anions. The Raman spectrum shows six distinct bands occurring at ∼3600–3300 cm–1and 1600–1500 cm–1(O–H and H–O–H bending), 1300–1200 cm–1(B–OH bending), 1030–800 cm–1(Si–O–Si stretching) as well as 700–500 cm–1and 400–50 cm–1(Mn–O and Na–O bonding, respectively). The FTIR spectrum for nolzeite shows bands at ∼2800 –3600 cm–1(O–H) stretching, a moderately sharp band at 1631 cm –1(H–O–H) bending, strong, sharp bands at ∼650 –700 cm–1, ∼800 –840 cm–1, and ∼900–1100 cm–1(Si–O and B –O) bonds. Nolzeite is triclinic, crystallizing in space groupPwitha= 6.894(1),b= 7.632(2),c= 11.017(2) Å, α= 108.39(3), β= 99.03, γ = 103.05(3)°,V= 519.27 Å3, andZ= 2. The crystal structure was refined toR= 12.37% and wR2= 31.07% for 1361 reflections (Fo> 4σFo). It is based on chains of tetrahedra with a periodicity of three (i.e. adreierchain) consisting of three symmetrically independent SiO4tetrahedra formingC-shaped clusters closed by BO2(OH)2tetrahedra, producing single loop-brancheddreierborosilicate chains. The chains are linked through shared corners to double chains of edge-sharing MnO5(OH) octahedra. Nolzeite is a chain silicate closely related to steedeite and members of the sérandite–pectolite series. Paragenetically, nolzeite is late-stage, probably forming under alkaline conditions and over a narrow range of low pressures and temperatures.

Frustrated Antiferromagnetic Spin Chains of Edge-Sharing Tetrahedra in Volcanic Minerals K3Cu3(Fe0.82Al0.18)O2(SO4)4 and K4Cu4O2(SO4)4MeCl

The calculation of the sign and strength of magnetic interactions in two noncentrosymmetric minerals (klyuchevskite, K3Cu3(Fe0.82Al0.18)O2(SO4)4 and piipite, K4Cu4O2(SO4)4Cu0.5Cl) has been performed based on the structural data. As seen from the calculation results, both minerals comprise quasi-one-dimensional frustrated antiferromagnets. They contain frustrated spin chains from edge-sharing Cu4 tetrahedra with strong antiferromagnetic couplings within chains and very weak ones between chains. Strong frustration of magnetic interactions is combined with the presence of the electric polarization in tetrahedra chains in piipite. The uniqueness of magnetic structures of these minerals caused by peculiarities of their crystal structures has been discussed. Keywords: Frustrated quasi-one-dimensional antiferromagnets, tetrahedral spin chains, klyuchevskite K3Cu3(Fe0.82Al0.18)O2(SO4)4, piypite K4Cu4O2(SO4)4MeCl

Ideal wollastonite and the structural relationship between the pyroxenoids and pyroxenes

A hypothetical ideal wollastonite with regular octahedra and T3 tetrahedron is presented and used to compare and contrast the pyroxenes and pyroxenoids. While clinopyroxenes have close-packed arrangements of oxygen anions, several lines of evidence demonstrate that pyroxenoids do not. One such line is the number of tetrahedra in a single tetrahedral chain per octahedra in a single associated octahedral chain (interior to the octahedral band), referred to as the “single-chain T:O ratio,” which is 1:1 in pyroxenes but 3:2 in wollastonite and always greater than 1:1 in other MSiO3 pyroxenoids. Because the Si-tetrahedron is extremely resistant to distortion, this forces marked distortion in at least one pyroxenoid octahedral site. The octahedral layers in pyroxenes and pyroxenoids are compared by placing them in the context of a fully occupied, closest-packed sheet of idealized octahedra, and it is shown that they are fundamentally different. The new mineral yangite is analyzed from the perspective developed in this study. It is structurally similar to the pyroxenoids, but the structure is a new type because it contains double tetrahedral chains and mixed polyhedral layers containing double chains of tetrahedra and bands of octahedra of width two. The tetrahedral chains are wollastonite-type chains and the wollastonite-type double chain is shown to have important differences from the amphibole double chain. A possible explanation for the existence of this crystal structure based on a hydrogen bond between Pb and O is presented.

Yangite, PbMnSi3O8·H2O, a new mineral species with double wollastonite silicate chains, from the Kombat mine, Namibia

A new chain-silicate mineral species, yangite, ideally PbMnSi 3 O 8 ·H 2 O, has been found on a specimen from the Kombat mine, Otavi Valley, Namibia. Associated minerals are melanotekite and rhodochrosite. Yangite is colorless to pale brown in transmitted light, transparent with white streak and vitreous luster. Broken pieces of yangite crystals are bladed or platy, and elongated along [010]. It is sectile with a Mohs hardness of ~5; cleavage is perfect on {101} and no twinning or parting was observed. The measured and calculated densities are 4.14(3) and 4.16 g/cm 3 , respectively. Optically, yangite is biaxial (−), with n α = 1.690(1), n β = 1.699(1), n γ = 1.705(1), Y = b, Z ^ c = 11°, and 2 V meas = 77(2)°. It is insoluble in water, acetone, and hydrochloric acid. An electron microprobe analysis demonstrated that the sample was relatively pure, yielding the empirical formula (with calculated H 2 O) Pb 1 . 00 Mn 1 . 00 2 + Si 3 . 00 O 8 ⋅ H 2 O . Yangite is triclinic and exhibits space group symmetry P 1 ¯ with unit-cell parameters a = 9.6015(9), b = 7.2712(7), c = 7.9833(8) A, α = 105.910(4), β = 118.229(4), γ = 109.935(5)°, and V = 392.69(7) A 3 . Its crystal structure is based on a skeleton of double wollastonite SiO 4 tetrahedral chains oriented parallel to [010] and interlinked with ribbons of Mn- and Pb-polyhedra. Yangite represents the first chain silicate with two-connected double chains and possesses all of the structural features of a hypothetical triclinic Ca 2 Si 3 O 8 ·2H 2 O phase proposed by Merlino and Bonaccorsi (2008) as a derivative of the okenite structure. The difference in the H 2 O component between the hypothetical phase and yangite likely is a consequence of the larger Pb 2+ with its lone-pair electrons in yangite replacing the smaller Ca 2+ in the hypothetical phase.

Crystallization of LiAlSiO4 Glass in Hydrothermal Environments at Gigapascal Pressures-Dense Hydrous Aluminosilicates.

High-pressure hydrothermal environments can drastically reduce the kinetic constraints of phase transitions and afford high-pressure modifications of oxides at comparatively low temperatures. Under certain circumstances such environments allow access to kinetically favored phases, including hydrous ones with water incorporated as hydroxyl. We studied the crystallization of glass in the presence of a large excess of water in the pressure range of 0.25-10 GPa and at temperatures from 200 to 600 °C. The p and T quenched samples were analyzed by powder X-ray diffraction, scanning electron microscopy, and IR spectroscopy. At pressures of 0.25-2 GPa metastable zeolite Li-ABW and stable α-eucryptite are obtained at low and high temperatures, respectively, with crystal structures based on tetrahedrally coordinated Al and Si atoms. At 5 GPa a new, hydrous phase of LiAlSiO4, LiAlSiO3(OH)2 = LiAlSiO4·H2O, is produced. Its crystal structure was characterized from single-crystal X-ray diffraction data (space group P21/c, a = 9.547(3) Å, b = 14.461(5) Å, c = 5.062(2) Å, β = 104.36(1)°). The monoclinic structure resembles that of α-spodumene (LiAlSi2O6) and constitutes alternating layers of chains of corner-condensed SiO4 tetrahedra and chains of edge-sharing AlO6 octahedra. OH groups are part of the octahedral Al coordination and extend into channels provided within the SiO4 tetrahedron chain layers. At 10 GPa another hydrous phase of LiAlSiO4 with presently unknown structure is produced. The formation of hydrous forms of LiAlSiO4 shows the potential of hydrothermal environments at gigapascal pressures for creating truly new materials. In this particular case it indicates the possibility of generally accessing pyroxene-type aluminosilicates with crystallographic amounts of hydroxyl incorporated. This could also have implications to geosciences by representing a mechanism of water storage and transport in the depths of the Earth.

Enclathration of X@La4 Tetrahedra in Channels of Zn–P Frameworks in La3Zn4P6X (X = Cl, Br)

Two new quaternary lanthanum zinc phosphide-halides were synthesized via high-temperature solid-state reactions. Their complex crystal structures were determined by a combination of X-ray diffraction and advanced solid-state 31P NMR spectroscopy. La3Zn4P6Cl and La3Zn4P6.6Br0.8 share a common structural feature: a polyanionic Zn–P framework with large channels hosting complex one-dimensional cations. The cations are built from X@La4 tetrahedral chains with X = Cl (La3Zn4P6Cl) or Br0.8P0.2 (La3Zn4P6.6Br0.8). The X@La4 tetrahedra share two vertices forming one-dimensional chains. To accommodate larger bromine-containing cations the Zn–P framework is rearranged by breaking and forming several Zn–P and P–P bonds. This results in the formation of a unique [P3]3– cycle, which is isoelectronic to cyclopropane. Analysis of the electron localization and orbital overlaps confirmed the presence of different chemical bonding in the Zn–P networks in the Cl- and Br-containing compounds. La3Zn4P6Cl was predicted to be a ...

Raman modes inPbcaenstatite (Mg2Si2O6): an assignment by quantum mechanical calculation to interpret experimental results

Raman spectra of orthoenstatite have been computed from first principles, employing the hybrid Hamiltonian WC1LYP.[1] The calculated data show excellent agreement with the experimental data from literature with an absolute average difference of ~5 cm−1. The quantum mechanical simulation allowed the assignment of Raman features to specific vibrational modes. This enabled to assess quantitatively the contributions of internal (tetrahedral stretching) and external (tetrahedral chains and M1 and M2 cations) vibrations. Moreover, the mass substitution of 56Fe for 24Mg in the M1 and M2 sites and of 30Si and 18O for the 28Si and 16O sites, pointed out the relative contributions of the cations to each mode within different sites. The description of the Raman modes enabled to relate the major experimental peaks to specific structural vibrations, and to link the changes in crystal structure to those modes with pressure, temperature and composition. The results provide new clues to identify most suitable peaks for the investigation of the intracrystalline ordering of Fe and Mg in the M1 and M2 sites, and of Al in the tetrahedral and octahedral sites. Moreover we have been able to identify those peaks which are related to structural features, like tetrahedral bond distances. Copyright © 2016 John Wiley & Sons, Ltd.

Structural reinvestigation of Li3FeN2: Evidence of cationic disorder through XRD, solid-state NMR and Mössbauer spectroscopy

A significant cationic disorder is evidenced on Li3FeN2 prepared through solid-state reaction under controlled atmosphere. This derivative anti fluorite type structure (orthorhombic, space group Ibam, a=4.870(1)Å, b=9.652(1)Å and c=4.789(1)Å), solved first through single crystal X-ray diffraction [7], is usually described by Li+ and Fe+3 ordered distribution in tetrahedral sites formed by the nitrogen network, leading to [FeN4/2]3− edge-sharing tetrahedral chains. From 7Li/6Li Nuclear Magnetic Resonance spectroscopy, 57Fe Mössbauer spectroscopy and powder X-ray and neutron diffraction, we demonstrate that about 4% of lithium sites are filled by iron and about 11% of iron sites are occupied by Li, which can explain the discrepancy within the Gudat's model observed on larger scale solid-state synthesis samples.

Single-crystal X-ray diffraction study of Cs2Er[Si6O14]F and Cs2Er[Si4O10

Abstract Single-crystal growth experiments in the system CsF-Er2O3-SiO2 resulted in the simultaneous crystallization of two chemically related compounds within the same run: Cs2Er[Si6O14]F (phase I) and Cs2Er[Si4O10]F (phase II). They represent the first examples for cesium erbium silicates containing fluorine. Basic crystallographic data are – phase I: space group Cmca, a=17.2556(6) Å, b=24.6565(7) Å, c=14.4735(5) Å, V=6157.9(3) Å3, Z=16; phase II: space group Pnma, a=22.3748(7) Å, b=8.8390(2) Å, c=11.9710(4) Å, V=2367.5(1) Å3, Z=8. The structures were determined by direct methods and refined to residuals of R(|F|)=0.0229 for 2920 (phase I) and 0.0231 for 2314 (phase II) independent observed reflections with I>2σ(I). The structure of phase I represents a previously unknown structure type with a three dimensional tetrahedral framework consisting of Q3 and Q4 groups in the ratio 2:1. Basic building units of the network are unbranched sechser single-chains running parallel to [001]. The network can be conveniently built up from the condensation of tetrahedral layers parallel to (010) or (100), respectively. The crystal structure of phase II can be classified as a tubular or columnar chain silicate indicating that the backbones of the structure are multiple chains of silicate tetrahedra. This structure is isotypic to a Cs2Y[Si4O10]F, a compound that has been characterized previously. Alternatively, both compounds can be described as mixed octahedral-tetrahedral frameworks, which can be classified according to their polyhedral microensembles. A topological analysis of both nets is presented.

Structural systematics of Ge substitution in primitive pyroxenes

Pyroxenes of general stoichiometry Mg(GexSi1−x)O3 were encountered in attempts to synthesise Ge-substituted talcs at 0.2 GPa, 650–700 °C. Orthopyroxenes (Pbca) of compositions x = 0.21, 0.30, and 0.34 were identified, and also a P21/c clinopyroxene of composition x = 0.63, and C2/c clinopyroxenes of compositions x = 0.91 and 1. End-member clinoenstatite MgSiO3-P21/c synthesised at 16 GPa, 1300 °C and transformed from C2/c was also included in the study. Crystal structure refinements using single-crystal XRD data showed that unit-cell parameters vary linearly with Si–Ge for the Pbca and P21/c pyroxenes, both of which have two symmetrically non-equivalent tetrahedral chains. Refinement of Si–Ge occupancies at tetrahedral sites showed that the two chains of all primitive pyroxenes have very different compositions, with XGe(TB) ≫ XGe(TA). This difference arises from the greater flexibility of the B-chain to rotate in response to tetrahedral expansion due to increasing Ge content. The TA-M2 shared polyhedral edge imposes significant constraints on the flexibility of the A-chain, which can accommodate much less Ge than the B-chain. Linear trends of cell parameters, site occupancies, and structural parameters for the primitive pyroxenes, when extrapolated to published data for MgGeO3–Pbca, extend across the entire Si–Ge join.

Single-point ground states and residual entropies in the antiferromagnetic Ising model with multisite interaction on the tetrahedral chain: exact results

.The antiferromagnetic spin-1/2 Ising model with the presence of the multisite interaction and in the external magnetic field on the infinite tetrahedral chain is solved exactly by using the transfer matrix method. Using the exact expression for the free energy per site of the model the thermodynamic properties of the model are investigated in detail. It is shown that the total magnetisation per site forms the magnetisation plateaus at low temperatures, the properties of which depend on the strength of the multisite interaction. The set of all possible ground states is found and discussed. The existence of the so-called single-point ground states is shown and their properties are studied in detail. The entropy per site of the model is investigated and the exact expressions for the residual entropies of all ground states are found and their macroscopic degeneracies are discussed. The existence of the ground states with the same value of magnetisation but with different entropies is observed. In addition, the behaviour of the susceptibility and the specific heat is also studied.

Synthesis, characterization and chemical stability of silicon dichalcogenides, Si(SexS1−x)2

Silicon dichalcogenides have an intriguing crystal structure consisting of long tetrahedral chains held together by van der Waals forces but the electronic and optical properties have been less explored. In the present work, bulk SiSe2, SiS2, and Si(SexS1−x)2 were synthesized by the congruent melt growth method and characterized by Raman spectroscopy, X-ray Diffraction and UV/visible/IR transmission measurements supported by first-principles calculations. First-principles calculations reveal a nearly linear decrease of band gap energy in Si(SexS1−x)2 with increasing Se content, i.e., from SiS2 to SiSe2 which corresponds with a blue-shift in the transmission spectra from bulk SiSe2 to Si(Se0.6S0.4)2, and to SiS2. Air stability tests demonstrate the formation of toxic H2Se/H2S gas from sample oxidation at room temperature upon exposure to ambient air, and great care should be paid when handling these materials.

Thermal expansion and structural complexity of Ba silicates with tetrahedrally coordinated Si atoms

Thermal expansion of Ba silicates with tetrahedrally coordinated Si atoms in the temperature range of 25–1100°C had been studied by high-temperature X-ray powder diffraction. The volume thermal expansion coefficients (TECs) are in the range 41–50×10−6°C−1 with an average value of 〈αV〉=45×10−6°C−1. In the structures with chain and layered silicate anions, thermal expansion is anisotropic: the direction of maximal TEC is parallel to the extension of the zweier chains of silicate tetrahedra, which are strained owing to the interactions with Ba2+. The strain is released during thermal expansion due to the increasing effective size of Ba2+ induced by thermal vibrations. Information-theoretic analysis of the structural and topological complexities of Ba silicates indicates that their structural complexity is a function of the topological complexity of their silicate anions. The latter displays a non-linear behaviour with increasing SiO2 content (=the increasing degree of polymerization and increasing dimensionality): it starts from simple topologies, reaches a maximum at topologies of intermediate complexity, and ends up at simple topologies again. The specificity of the interactions of Ba2+ with the silicate anions results in higher complexity of high-temperature α-BaSi2O5 compared to that of low-temperature β-BaSi2O5. This uncommon behaviour may be explained by the vibrational advantages provided by flatter and more complex silicate layers in the α-phase, which overcome negative differences in configurational entropies of the two modifications apparent in the differences of their structural Shannon information.

Combined dilatometric and calorimetric study of kinetic processes occurring in Ge20Te76Se4 infrared bulk glass

Kinetic processes (cold crystallization and structural relaxation) occurring in the Ge20Te76Se4 infrared bulk glass were studied by means of differential scanning calorimetry (DSC) and thermomechanical analysis (TMA). The crystallization behaviour was described in terms of the Autocatalytic and Johnson–Mehl–Avrami kinetic models. The crystallization activation energies evaluated by the two techniques were: 164±7kJmol−1 (DSC) and 153±8kJmol−1 (TMA). The agreement between the two values was explained in terms of the marked glass stability and glass-forming ability of the Ge20Te76Se4 glass, where the crystal growth proceeds at rather low viscosities. Regarding the process complexity, increasing heating rate led to a decrease of the dimensionality of the formed crystallites (3D→2D). Structural relaxation kinetics was described in terms of the Tool–Narayanaswamy–Moynihan model. Values of the parameters of the TNM model were interpreted with respect to the molecular structures determined by Raman spectroscopy. The Se↔Te substitution appears to lead to larger interconnection between the GeTe4 tetrahedral chains rather than to significant changes in the intra-chain bonding. Good agreement between the activation energies of the relaxation process determined by the two experimental techniques was again obtained: 501±10kJmol−1 (DSC) and 490±34kJmol−1 (TMA).

Crystal structure and electronic properties of bulk and thin film brownmillerite oxides

The equilibrium structure and functional properties exhibited by brownmillerite oxides, a family of perovskite-derived structures with alternating layers of $B$O$_6$ octahedra and $B$O$_4$ tetrahedra, viz., ordered arrangements of oxygen vacancies, is dependent on a variety of competing crystal-chemistry factors. We use electronic structure calculations to disentangle the complex interactions in two ferrates, Sr$_2$Fe$_2$O$_5$ and Ca$_2$Fe$_2$O$_5$, relating the stability of the equilibrium (strain-free) and thin film structures to both previously identified and newly herein proposed descriptors. We show that cation size and intralayer separation of the tetrahedral chains provide key contributions to the preferred ground state. We show the bulk ground state structure is retained in the ferrates over a range of strain values; however, a change in the orientation of the tetrahedral chains, i.e., a perpendicular orientation of the vacancies relative to the substrate, is stabilized in the compressive region. The structure stability under strain is largely governed by maximizing the intraplane separation of the `dipoles' generated from rotations of the FeO$_4$ tetrahedra. Lastly, we find that the electronic band gap is strongly influenced by strain, manifesting as an unanticipated asymmetric-vacancy alignment dependent response. This atomistic understanding establishes a practical route for the design of novel functional electronic materials in thin film geometries.

Investigations on alunogen under Mars-relevant temperature conditions: An example for a single-crystal-to-single-crystal phase transition

The low-temperature (LT) dependent behavior of a synthetic alunogen sample with composition Al 2 (SO 4 ) 3 ·16.61H 2 O has been studied in the overall temperature range from −100 to 23 °C by DSC measurements, in situ powder and single-crystal X-ray diffraction as well as Raman spectroscopy. Cooling/heating experiments using the different techniques prove that alunogen undergoes a reversible, sluggish phase transition somewhere between −30 and −50 °C from the triclinic room-temperature (RT) form to a previously unknown LT-polymorph. A significant hysteresis for the transition was observed with all three methods and the transition temperatures were found to depend on the employed cooling/heating rates. The crystal structure of the LT-modification has been studied at −100 °C using single crystals, which have been grown from an aqueous solution. Basic crystallographic data are as follows: monoclinic symmetry, space group type P 2 1 , a = 7.4125(3), b = 26.8337(16), c = 6.0775(3) A, β = 97.312(4)°, V = 1199.01(10) A 3 , and Z = 2. Structure analysis revealed that LT-alunogen corresponds to a non-stoichiometric hydrate with 16.61 water moieties pfu. Notably, the first-order transition results in a single-crystal-to-single-crystal transformation. In the asymmetric unit there are 2 Al-atoms, 3 [SO 4 ]-tetrahedra, and 17 crystallographically independent sites for water molecules, whose hydrogen positions could be all located by difference-Fourier calculations. According to site-population refinements only one water position (Ow5) shows a partial occupancy. A comfortable way to rationalize the crystal structure of the LT-modification of alunogen is based on a subdivision of the whole structure into two different slabs parallel to (010). The first type of slab (type A) is about 9 A thick and located at y ≈ 0 and y ≈ ½, respectively. It contains the Al(H 2 O) 6 -octahedra as well as the sulfate groups centered by S1 and S2. Type B at y ≈ ¼ and y ≈ ¾ comprises the remaining tetrahedra about S3 and a total of five additional “zeolitic” water sites (Ow1–Ow5), which are not a part of a coordination polyhedron. Within slab-type A alternating chains of (unconnected) octahedra and tetrahedra can be identified, which are running parallel to [100]. In addition to electrostatic interactions between the Al(H 2 O) 6 3+ - and the (SO 4 ) 2− -units, hydrogen bonds are also essential for the stability of these slabs. A detailed comparison between both modifications including a derivation from a hypothetical aristotype based on group-theoretical concepts is presented. Since alunogen has been postulated to occur in martian soils the new findings may help in the identification of the LT-form by X-ray diffraction using the Curiosity Rover’s ChemMin instrument or by Raman spectroscopy.