Even the low-T garnet from the iconic Barrow’s zone is tetragonal

<p>Common (anhydrous) Fe-Mg-Ca-Mn garnet, the archetypal cubic mineral, has been recently discovered to be tetragonal in metapelites and metabasites from low-temperature regional metamorphic terranes (Cesare et al., 2018).</p><p>Despite the differences in bulk rock composition and pressure conditions, such low-T tetragonal garnets share common chemical features, namely high grossular (>25 mol%) and low pyrope (<7 mol%) contents. Similar compositions are documented in other contexts worldwide, both in blueschists-eclogites and in phyllites, including the metapelites from the garnet zone of the iconic Barrovian metamorphism of the Scottish highlands (Viete et al., 2011).</p><p>We have analysed a garnet crystal from a chlorite-biotite schist collected at the Barrow’s garnet zone in Glen Esk. The unit cell parameters were refined using diffraction reflections between 1.20 and 0.55 Å providing a tetragonal cell with a = 11.5731(5) Å and c = 11.5887(8) Å and volume V = 1552.15(15) Å3. Systematic absences analysis on complete intensity data collected up to 2theta = 80° indicated I41/acd space group confirming the cell parameters refinement.</p><p>Therefore, the garnet is tetragonal and not cubic, as suggested by its weak birefringence under crossed polarizers.</p><p>These results show that the tetragonal structure of common Fe-Mg-Ca-Mn garnet is verified whenever this mineral displays the Ca-rich, Mg-poor composition often observed in low-T metamorphic rocks. And support the hypothesis that the lowering of symmetry is composition-dependent.</p><p> </p><p>References</p><p>Cesare, B., et al. Garnet, the archetypal cubic mineral, grows tetragonal. Sci Rep <strong>9</strong>, 14672 (2019).</p><p>Viete, D.R., et al. The nature and origin of the Barrovian metamorphism, Scotland: Diffusion length scales in garnet and inferred thermal time scales. J. Geol. Soc. London <strong>168</strong>, 115–132 (2011).</p><p> </p>

Coherent crystal branches: the impact of tetragonal symmetry on the 2D confined polymer nanostructure

The symmetry of polymer crystals greatly affects the optical, thermal conductivity and mechanical properties of the materials. Past studies have shown that the two-dimensional (2D) confined crystallization of polymer nanorods could produce anisotropic structures. However, few researchers have focused on understanding confined nanostructures from the perspective of crystal symmetry. In this research, we demonstrate the molecular chain self-assembly of tetragonal crystals under cylindrical confinement. We specifically selected poly(4-methyl-1-pentene) (P4MP1) with a 41 or 72 helical conformation (usually crystallizing with a tetragonal lattice) as the model polymer. We found a coherent crystal branching of the tetragonal crystal in the P4MP1 nanorods. The unusual 45°- and 135°-{200} diffractions and the meridional 220 diffraction (from 45°-tilted crystals) have shown a uniform crystal branching between the a 1-axis crystals and the 45°-tilted crystals in the rod long axis, which originates from a structural defect associated with tetragonal symmetry. Surprisingly, this chain packing defect in the tetragonal cell can be controlled to develop along the rod long axis in 2D confinement.

Structural and Piezoelectric Characterization of Pr3+ Modified (1-x)Pb(Zr1-yTiy)O3 – xPb(Mn1/3Sb2/3)O3 Ceramic

Abstract0.88Pb(Zr0.52Ti0.48)O3 – 0.12Pb(Mn1/3Sb2/3)O3 – 0,02 at%E piezoelectric ceramics, with E = Pr3+ were synthesized by using a conventional method, namely a solid state reaction technique. X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM) were employed for the structural and microstructural investigations. Piezoelectric methods were used for the dielectric and piezoelectric properties determination. The results of XRD show a perovskite structure and XRD patterns, indexing on a tetragonal cell structure, was carried out for the most common phases. The SEM micrographs of the sintered compositions reveal a homogenous structure with a sharp or rounded grain boundary. The modified PZT ceramic presents still superior piezoelectric properties. Based on the results obtained, one can conclude that the analysed piezoelectric ceramics are useful for device applications.

Structural simplicity and complexity of compressed calcium: electronic origin

A simple cubic structure with one atom in the unit cell found in compressed calcium is counterintuitive to the traditional view of a tendency towards densely packed structures with an increase in pressure. To understand this unusual transformation it is necessary to assume electron transfer from the outer core band to the valence band, and an increase of valence electron number for calcium from 2 to ∼ 3.5. This assumption is supported by the Fermi sphere–Brillouin zone interaction model that increases under compression. The recently found structure of Ca-VII with a tetragonal cell containing 32 atoms (tI32) is similar to that in the intermetallic compound In5Bi3with 3.75 valence electrons per atom. Structural relations are analyzed in terms of electronic structure resemblance. Correlations of structure and physical properties of Ca are discussed.

Low-temperature Crystal Structure and 57Fe Mössbauer Spectroscopy of Sr3Sc2O5Fe2As2

The crystal structure of the layered iron arsenide Sr3Sc2O5Fe2As2 was determined between 300 and 10 K. The lattice parameters of the tetragonal cell decrease anisotropically according to Δc/c : Δa/a ≈ 4.2, which results in a slight flattening of the As-Fe-As bond angle within the FeAs layers. No indication of a structural instability could be detected. 57Fe Mössbauer spectroscopic data show only a single signal at 4.2, 77, and 298 K, subjected to quadrupole splitting. The isomer shift increases from 0.36(1) mms−1 at 298 K to 0.49(1) mms−1 at 4.2 K. No indication for magnetic ordering was found.

Powder X-ray diffraction and Rietveld analysis of Cd1−xCuxCr2O4(0.1≤x≤0.7)

Structural properties of Cd1−xCuxCr2O4(CCCO) have been investigated by means of X-ray powder diffraction and Rietveld analysis. A structural phase transformation from Fd3m to I42d at x=0.64 has been detected. The lattice constant a of the cubic unit cell decreases rapidly with increasing Cu content up to x=0.62. At x=0.64, the cubic unit cell is compressed into a tetragonal cell and CrO6 octahedrons are distorted. With continuing Cu content increases above 0.64, the distortion of the unit cell is released slightly according to the changes in c/a. Magnetic properties of Cd1−xCuxCr2O4(x=0.1,0.3,0.5,0.7) have also been measured and are discussed.

Compression of the perovskite-related mineral bernalite Fe(OH)3 to 9 GPa and a reappraisal of its structure

The octahedral-framework mineral bernalite, Fe(OH)3, provides a rare opportunity to examine directly the effects of a vacant A site upon the physical properties of perovskite-like structures. Here, we report the effect upon compressibility. Bernalite has been reported previously as having space group Immm (Birch et al., 1993), but numerous reflections violating I-centering were observed in the present study. A case is presented for bernalite having orthorhombic space group Pmmn. Lattice parameters were refined using the Le Bail method for a metrically tetragonal cell and their variation with pressure at room temperature was determined from 17 measurements at pressures from 10–4 to 9.3 GPa using synchrotron X-ray powder diffraction. No discontinuities in the compression curves of lattice parameters were observed. Fitting to a second-order Birch-Murnaghan equation-of-state (KT0' = 4) gives V0 = 438.51±0.06 Å3 and KT0 = 78.2±0.4 GPa. Second-order fits of (a/a0)3 and (c/c0)3 give elastic moduli KT0a = 82.0(6) GPa and KT0c = 71.6(4) GPa: the shorter cation–cation distance is the more compressible. These values are very close to those of stottite, FeGe(OH)6, which has tilt system a+a+c–. The difference in the elastic moduli KT0a and KT0c of bernalite and their close similarity to the stottite values support the revised Pmmn structure (tilt system a+b+c–) for bernalite proposed here. The compressional anisotropy observed in bernalite may reflect its highly anisotropic and directional H-bonding topology.

Space group and atomic structure determination of a nano-sized ordered phase derived from a f.c.c. structure in maraging steel 12Cr–9Ni–4Mo–2Cu using transmission electron microscopy

The unique properties of maraging steel Sandvik 1RK91 were attributed to unique precipitation: a nano-sized L phase in addition to the quasi-crystalline R′ phase, which differs from any precipitation system in conventional maraging steels. The L phase was observed after ageing at either 748 or 823 K. It has flake morphology with dimensions ∼100 × 500 × 500 Å. In the present study the structure of the L phase was examined using convergent-beam electron diffraction (CBED), energy-dispersive X-ray analysis (EDX) and high-resolution electron microscopy (HREM). The L phase could be described as Ti19Fe9Mo9Al8Cr5Ni50 or simply M 50Ni50 (M = Ti, Fe, Mo, Al and Cr). The L phase is isostructural to FeNi. Its crystal structure was determined to have the ordered structure of the uAu-I type (L10, P4/mmm, a = 3.52, c = 3.63 Å and Z = 2) with two Ni atoms at ½ 0 ½ and 0 ½ ½, and two M atoms at 0 0 0 and ½ ½ 0. The crystal structure of the L phase can also be described using a primitive tetragonal cell and lattice parameters: a = 2.49 and c = 3.63 Å, Z = 1. The volume of the primitive tetragonal unit cell is 22.5 Å3 and the density is ∼6.98 g cm−3. The present study has demonstrated the possibility of determining the structure of an extremely small crystal by utilizing the information from CBED, EDX analysis and HREM.

Occurrence of a monoclinic distortion in β-Fe2PO5

The structure of the mixed valence iron oxyphosphate β-Fe2PO5 has been reinvestigated using powder synchrotron X-ray data. A previous study based on the similarity with the powder diagrams of NiCrPO5 concluded that the structure existed in a tetragonal cell (I41/amd). In this space group both Fe atoms, Fe2+ and Fe3+, are sharing the same crystallographic site and, therefore, cannot be distinguished. Parts of the pattern have been measured at high resolution on the LURE D23 powder station. The instrumental resolution there clearly shows the splitting of some Bragg peaks. Among the subgroups of I4/amd, the monoclinic group I2/a permits a description of the spectra if the twofold b' of the new unit cell is chosen as the diagonal of the square base of the tetragonal cell. A Rietveld refinement was then performed against the full powder diffraction diagram. This reveals two iron sites with slightly different Fe—O distances. Finally, study as a function of temperature shows a monoclinic to tetragonal transition above 415 K, which may be related to a magnetocrystalline effect at TN = 408 K.