Phase transformations in ABO 4 type compounds under high pressure

For ABO ₄ type ternary oxides, high pressure phase transformations known up to the present are reviewed, and an attempt is made to explain and predict crystal structures of their high pressure phases. When ABO ₄ type compounds are plotted based on the two variables, k=r _A /r _B and t=(r _A +r _B )/2r _O, where r _A , r _B , and r _O are the ionic radii of A and B cations and divalent oxygen, they can be classified into the major structure types. It is found empirically that a compound basically transforms to the structure type isostructural with a compound lying in a classified area with the same k and larger t values in the diagram.     

Impact of amide–amide hydrogen bonding on the stability of two nicotinamide complexes of silver(I)

The nicotinamide (pyridine-3-carboxamide, nia) complexes of silver(I), [Ag(nia)₂(NO₃)]·H₂O (1), [Ag(nia)₂(NO₃)] (2), and {K[Ag(nia)₂](NO₃)₂} _n (3), were prepared and characterised by IR spectroscopy and TG/DTA thermal methods. The solid state structures of 2 and 3 were determined by single-crystal X-ray diffraction analysis. In both complexes two nicotinamide ligands are coordinated to silver(I) through the nitrogen atom of the pyridine ring in a near-linear fashion. In 2, additional coordination by two oxygen atoms of one nitrate group leads to the distorted tetrahedral coordination environment of silver(I). In 3, nitrate ions bridge potassium cations giving rise to a 2D coordination network which is further stabilised by cross-bridging of each two potassium atoms in [1 0 0] direction by complex cations, [Ag(nia)₂]⁺. Despite different aggregation of 2 and 3 in the solid state, both complexes demonstrate quite similar thermal stability. The amide self-complementary hydrogen bonds appear to be the main driving force for establishing the crystal structures of both 2 and 3.     

Deformation of shock-loaded andalusite studied with X-ray diffraction techniques

The structural deformation of an andalusite single crystal, shockloaded up to 400 kb with shock wave direction approximately parallel to c, was investigated by means of X-ray powder (Guinier) and single crystal techniques (Weissenberg, precession). Exposure to the dynamic pressure revealed a fracturing of the crystal into lattice blocks, with a mean size >1,000 Å. No change of the lattice constants could be observed after pressure release. From the streaks of X-ray reflection spots measured within the hk0, h0l, 0kl, and hhl planes the shock-induced lattice deformation is interpreted in terms of rotational gliding and/or microfracturing. The distortion mode is highly structure controlled. It follows preferrably two different structural motion systems: (1) Gliding parallel to (001) occurs, which produces lamellae parallel to (001), mainly arranged in two sublattices with common c-axis. The stacking sequence of lamellae along c is irregular. The lamellae-type structure may also result from an orientated transformation into a high pressure phase of lower symmetry and subsequent inversion into the original phase after pressure release. (2) Gliding parallel to (100) occurs. In this case the deformation mode is asymmetrical with respect to the undistorted crystal. The common direction b of the (001) and (100) deformation planes is probably the main direction of the shock-induced lattice deformation.     

Transmission electron microscope study of dislocations in orthopyroxene (Mg,Fe)2Si2O6

The orthopyroxene crystal structure can be viewed as the stacking of alternating tetrahedral and octahedral layers parallel to the (100) plane. Easy glide occurs in the (100) plane at the level of the octahedral layer to prevent breakage of the strong Si-O bonds. Dislocations with c and b Burgers vectors have been activated in (100) by room temperature indentation in an orthoenstatite gem quality single crystal. Investigations in transmission electron microscopy show that the b dislocations (b?9 Å) are not dissociated while the c's (c=5.24 Å) are dissociated into four partials. This result is interpreted by considering the oxygen sublattice as a distorted FCC one. The four c partials are thus Shockley partials bounding three stacking faults. For the two outer ones, synchroshear of the cations is necessary to keep unchanged their sixfold coordination; the oxygen sublattice is locally transformed into a HCP lattice. This accounts for the observed low splitting (?100 Å) of these faults as compared to the median one (?500 Å) which does not affect the oxygen sublattice and does not require cation synchroshear. In a Fe rich orthopyroxene (eulite), semi coherent exsolution lamellae have been studied. Either only c edge dislocations or both b and c edge dislocations occur in the phase boundaries depending upon the thickness of the lamellae. Only the c dislocations are dissociated. From the observed spacing between these mismatch dislocations a crude estimate of the exsolution temperature is proposed T _ex ? 700° C.     

Polarized electronic absorption spectra of Cr2SiO4 single crystals

Polarized electronic absorption spectra, E∥a(∥X), E∥b(∥Y) and E∥c(∥Z), in the energy range 3000–5000?cm^–1 were obtained for the orthorhombic thenardite-type phase Cr₂SiO₄, unique in its Cr²⁺-allocation suggesting some metal-metal bonding in Cr²⁺Cr²⁺ pairs with Cr-Cr distance 2.75?Å along [001]. The spectra were scanned at 273 and 120?K on single crystal platelets ∥(100), containing optical Y and Z, and ∥(010), containing optical X and Z, with thicknesses 12.3 and 15.6?μm, respectively. Microscope-spectrometric techniques with a spatial resolution of 20?μm and 1?nm spectral resolution were used. The orientations were obtained by means of X-ray precession photographs. The xenomorphic, strongly pleochroic crystal fragments (X deeply greenish-blue, Y faint blue almost colourless, Z deeply purple almost opaque) were extracted from polycrystalline Cr₂SiO₄, synthesized at 35?kbar, above 1440?°C from high purity Cr₂O₃, Cr (10% excess) and SiO₂ in chromium capsules. The Cr₂SiO₄-phase was identified by X-ray diffraction (XRD). Four strongly polarized bands, at about 13500 (I), 15700 (II), 18700 (III) and 19700 (IV) cm^–1, in the absorption spectra of Cr₂SiO₄ single crystals show properties (temperature behaviour of linear and integral absorption coefficients, polarization behaviour, molar absorptivities) which are compatible with an assignment to localized spin-allowed transitions of Cr²⁺ in a distorted square planar coordination of point symmetry C₂. The crystal field parameter of Cr²⁺ is estimated to be 10?Dq?10700?cm^–1. A relatively intense, sharp band at 18400?cm^–1 and three other minor features can, from their small half widths, be assigned to spin-forbidden dd-transitions of Cr²⁺. The intensity of such bands strongly decreases on decreasing temperature. The large half widths, near 5000?cm^–1 of band III are indicative of some Cr-Cr interactions, i.e. δ-δ* transitions of Cr₂ ⁴⁺, whereas the latter alone would be in conflict with the strong polarization of bands I and II parallel [100]. Therefore, it is concluded that the spectra obtained can best be interpreted assuming both dd-transitions of localized d-electrons at Cr²⁺ as well as δ-δ* transitions of Cr₂ ⁴⁺ pairs with metal-metal interaction. To explain this, a dynamic exchange process 2 Cr_loc ²⁺?Cr_{2, cpl} ⁴⁺ is suggested wherein the half life times of the ground states of both exchanging species are significantly longer than those of the respective optically excited states, such that the spectra show both dd- and δ-δ*-transitions.     

Deformation mechanisms in experimentally deformed single crystals of pyrrhotite,Fe1−xS

Single crystals of hexagonal and monoclinic pyrrhotite, Fe_1?xS, have been experimentally deformed by uniaxial compression at 300 MPa confining pressure, and at a strain rate of 1 × 10^?5 s^?1 in the temperature range from 200° C to 400° C. Very high anisotropy characterizes the mechanical behaviour of the crystal structure. During compression parallel to thec-axis, when no slip system may be activated, the maximum strength is observed. One or two degrees of non-parallelism between [c] and σ₁ results in slip on the basal plane, illustrating the very low resistance of the lattice against shear in this plane. At σ₁ Λ(0001)=45°, i.e. when maximum resolved shear stress is attained on the basal plane, the strength reaches a minimum. Thecritical resolved shear stress (CRSS) increases from less than 4.7 MPa at 400° C to 52 MPa at 200° C. A new slip system, \((10\overline 1 0)\parallel \left\langle {1\overline 2 10} \right\rangle \) prism slip, is described. It is activated only at high angles (>70°) between σ₁ and [c]. The CRSS of the prism slip ranges from 7 MPa (400° C) to 115 MPa (200° C). Twinning on \((10\overline 1 2)[(10\overline 1 2):(1\overline 2 10)]\) , earlier reported by several authors, has been produced only at the highest temperature either as secondary feature during pressure release (compression ‖[c]) or in heterogeneously strained areas (compression ⊥[c]). As twinning and prism slip attain their maximum values of the Schmidt factor under nearly equal stress conditions it is postulated that the former of the two deformation modes has the higher shear resistance.     

Electronic absorption spectra of Cr3+ ions in natural clinopyroxenes

The polarized (E‖a′, E‖b and E‖c) electronic absorption spectra of five natural chromium-containing clinopyroxenes with compositions close to chromdiopside, omphacite, ureyite-jadeite (12.8% Cr₂O₃), jadeite, and spodumene (hiddenite) were studied. The polarization dependence of the intensities of the Cr³⁺ bands in the clinopyroxene spectra cannot be explained by the selection rules for the point groups C ₂ or C _2v but can be accounted for satisfactorily with the help of the higher order pseudosymmetry model, i.e. with selection rules for the point symmetry group C _3v. The trigonal axis of the pseudosymmetry crystal field forms an angle of 20.5° with the crystallographic direction c in the (010) plane. D _q increases from diopside (1542 cm^?1) through omphacite (1552 cm^?1), jadeite (1574 cm^?1) to spodumene (1592 cm^?1). The parameter B which is a measure of covalency for Cr³⁺-O bonds at M1 sites in clinopyroxene depends on the Cr³⁺ concentration and the cations at M2 sites.     

Observations on the crystal structures of lueshite

Laboratory powder XRD patterns of the perovskite-group mineral lueshite from the type locality (Lueshe, Kivu, DRC) and pure NaNbO₃ demonstrate that lueshite does not adopt the same space group (Pbma; #57) as the synthetic compound. The crystal structures of lueshite (2 samples) from Lueshe, Mont Saint-Hilaire (Quebec, Canada) and Sallanlatvi (Kola, Russia) have been determined by single-crystal CCD X-ray diffraction. These room temperature X-ray data for all single-crystal samples can be satisfactorily refined in the orthorhombic space group Pbnm (#62). Cell dimensions, atomic coordinates of the atoms, bond lengths and octahedron tilt angles are given for four crystals. Conventional neutron diffraction patterns for Lueshe lueshite recorded over the temperature range 11–1,000 K confirm that lueshite does not adopt space group Pbma within these temperatures. Neutron diffraction indicates no phase changes on cooling from room temperature to 11 K. None of these neutron diffraction data give satisfactorily refinements but suggest that this is the space group Pbnm. Time-of-flight neutron diffraction patterns for Lueshe lueshite recorded from room temperature to 700 °C demonstrate phase transitions above 550 °C from Cmcm through P4/mbm to \(Pm\overline{3} m\) above 650 °C. Cell dimensions and atomic coordinates of the atoms are given for the three high-temperature phases. The room temperature to 400 °C structures cannot be satisfactorily resolved, and it is suggested that the lueshite at room temperature consists of domains of pinned metastable phases with orthorhombic and/or monoclinic structures. However, the sequence of high-temperature phase transitions observed is similar to those determined for synthetic NaTaO₃, suggesting that the equilibrated room temperature structure of lueshite is orthorhombic Pbnm.     

Growth defects in metamorphic biotite

Metamorphic biotites examined by transmission electron microscopy contain planar defects on the (001) plane, superlattices, twins and a microstructure causing streaking of k≠3n rows. Analysis of the fringe contrast shows that the fault vectors associated with the planar defects are either R ₁=±1/3 [010], R ₂=±1/6 [310] or R ₃=±1/6 [3 \(\bar 1\) 0]. Structural considerations indicate that a stacking fault R ₁, R ₂ or R ₃ is most likely to exist in the octahedral layer rather than the potassium layer. The result of such a fault on a unit layer of mica is effectively to rotate it through ±120° about c* (equivalent to the common mica twin law). These stacking faults can provide the mechanism for producing the ±120° rotations associated with the common mica polytypes. Furthermore, many of the observed microstructures can be generated by these stacking faults.     

A crystal chemical study of protoanthophyllite: orthoamphiboles with the protoamphibole structure

Two new protoamphibole-type amphiboles with space group type Pnmn, have been found in nature: protoferro-anthophyllite (Fe_0.80Mn_0.20)₂ (Fe_0.98Mg_0.02)₅ (Si₄O₁₁)₂(OH)₂, and protomangano-ferro-anthophyllite, (Mn_0.70Fe_0.30)₂ (Fe_0.82Mg_0.18)₅ (Si₄O₁₁)₂(OH)₂. Protoferro-anthophyllite (PFA) occurs in pegmatites at both Gifu Prefecture, Japan and at Cheyenne Mountain, El Paso County, Colorado, USA. Protomangano-ferro-anthophyllite, (PMFA) occurs in pegmatites at Fukushima Prefecture and in a Mn mine at Tochigi Prefecture, Japan. Structure determinations of the two amphiboles show that both are isostructural with the synthetic fluorian-amphibole, protoamphibole (= protofluorian-lithian-anthophyllite). A calculation of the procrystal electron density distributions, the bond paths and the bond critical point properties of PFA, PMFA, grunerite and protoamphibole indicates that the M4 cation in these amphiboles is 4-coordinated. A calculation of the electron density distributions at the Becke3LYP/6-311G(2d,p) level for model silicate tetrahedra for these amphiboles and anthophyllite reveals that the value of the electron density at the bond critical points, ρ(r _c ), for the SiO(nbr) bonds is larger, on average (0.93 e/ų), than that for the SiO(br) bonds (0.90 e/ų). The observed SiO bond lengths decrease linearly with increasing ρ(r _c ) while the magnitudes of the curvatures of ρ(r _c ) both perpendicular and parallel to the bonds and the Laplacian of ρ(r _c ) each increases. These trends are associated with an increase in the electronegativity of the Si cation, a possible increase in the covalent character of the SiO bond and a tendency for SiO(nbr) bonds to be involved in wider OSiO angles than SiO(br) bonds. It is possible, if not likely, that protoanthophyllite has often been misidentified as anthophyllite.     

Magnetic properties of the clinopyroxenes aegirine and hedenbergite: a magnetic susceptibility study on single crystals

Two natural clinopyroxene single crystals were investigated, an aegirine-augite (AEG) and a magnesian hedenbergite (HED). Both samples were carefully characterized by electron microprobe, X-ray diffraction, and Mössbauer spectroscopy. Magnetic susceptibility measurements of powdered samples reveal low temperature antiferromagnetic coupling and Curie-Weiss behaviour with T _N =7.5(5)?K, Θ _P =?19(1)?K for AEG, and T _N =31(1)?K, Θ _P =+21(1)?K for HED, respectively. Low temperature Mössbauer spectra exhibit relaxation phenomena. Magnetic susceptibility measurements of the single crystals show the direction of the magnetic moments to be lying within the a/c plane for both samples: 50(±2)° from a and 57(±2)° from c in AEG, and 45(±2)° from a and 60(±2)° from c in HED, respectively. The antiferromagnetic interchain interaction competes with the ferromagnetic intrachain interaction in both pyroxenes. In the magnesian hedenbergite a field induced magnetic transition is found. Its dependence on temperature, magnetic field and crystallographic direction is investigated and described.     

Antigorite equation of state and anomalous softening at 6 GPa: an in situ single-crystal X-ray diffraction study

The compressibility of antigorite has been determined up to 8.826(8) GPa, for the first time by single crystal X-ray diffraction in a diamond anvil cell, on a specimen from Cerro del Almirez. Fifteen pressure–volume data, up to 5.910(6) GPa, have been fit by a third-order Birch–Murnaghan equation of state, yielding V ₀ = 2,914.07(23) Å³, K _T0 = 62.9(4) GPa, with K′ = 6.1(2). The compression of antigorite is very anisotropic with axial compressibilities in the ratio 1.11:1.00:3.22 along a, b and c, respectively. The new equation of state leads to an estimation of the upper stability limit of antigorite that is intermediate with respect to existing values, and in better agreement with experiments. At pressures in excess of 6 GPa antigorite displays a significant volume softening that may be relevant for very cold subducting slabs.     

Transmission electron microscopy of experimentally deformed chalcopyrite single crystals

Two crystals of natural chalcopyrite, CuFeS₂, experimentally deformed at 200° C have been studied by means of transmission electron microscopy (TEM). The activated glide planes are (001) and {112}. The dislocations in (001) have the Burgers vector [110] and a predominating edge character. They are split into two colinear partials b=1/2[110] and can cross split into {112}. The dislocations in {112} consist of straight segments along low index lattice lines. They are often arranged in dipoles generating trails of loops. Few dislocations with b=1/2[ \(\overline {11} \) 1] and [1 \(\bar 1\) 0] are present and dislocations with b=[0 \(\bar 2\) 1] occur in low angle subgrain boundaries. From weak beam contrasts it is presumed that most of the dislocations gliding in {112} have b=1/2〈3 \(\overline {11} \) 〉. They are dissociated into up to four partials. Microtwins and different types of stacking faults in {112} also occur. Models of the dissociation of dislocations are discussed.     

Exploration of structure and bonding in stishovite with fourier and pseudoatom refinement methods using single crystal and powder X-ray diffraction data

The structure and bonding in stishovite, SiO₂, is explored with Fourier summation and pseudoatom refinement of merged x-ray single crystal and powder diffraction data. Replacement of the 25 lowest-angle, highly extinction-affected, single crystal reflections with structure factors obtained from low-extinction powder diffraction data has resulted in a significant improvement in the analysis compared with earlier studies. The deformation electron density, total electrostatic potential and total and valence electron densities are mapped. Accumulations of electron density are observed in both SiO bonds, together with non-bonding features displayed about the oxygen on both sides of a plane formed by three bonds with Si. Deficits of electron density between O atoms across the shared-edges are rationalized in terms of the Pauli exclusion principle. There is no evidence for strong repulsion of Si atoms across the same ring. The total electrostatic potential has a continuous low value for the vacant channels in the structure along c with localized minima between O atoms on opposite sides of the channel. The sizes of Si and O are related to the electron density and to the electrostatic potential.     

Numerical estimates of mechanical energy transfer from the atmosphere to the Indian ocean

The field of mechanical energy transfer from the atmosphere to the ocean is computed for the first time. The numerical simulation of waves within the Indian Ocean (IO) water area for the period of 1998?C2009 is used. Mechanical energy transfer is described by two integrated parameters calculated per area unit: the speed of complete energy flux from wind to waves, I _E (x, t), and the speed of complete losses in the energy of wind waves, D _E (x, t). In order to solve this problem, the wind field W(x, t) (the NCEP/NOAA data) is used; the I _E (x, t) and D _E (x, t) fields are calculated on the basis of the WAM numerical model containing a modified source function. The results obtained allow us, first, to assess the characteristic spatial distribution of zones ??pumped?? by the wind with mechanical energy for both the wave field and the upper layer of the ocean by seasons, years, and the whole period discussed, second, to determine the extreme and average zonal values of I _E (x, t) and D _E (x, t), the degree of their shift spacing and balance B _E = (I _E + D _E ); and third, to define the characteristic time scales of variations in the wind field and wave field energies, caused by energy transfer from the wind to waves in the zones and within the Indian ocean as a whole. These results significantly specify the climatic estimates obtained earlier.     

A Mössbauer investigation of natural troilite from the Agpalilik meteorite

Troilite close to FeS, with 0.17 weight percent Cr as main impurity, was obtained from the Agpalilik meteorite. Powder Mössbauer spectroscopy was made in the temperature range 77–645 K. The full Hamiltonian was applied in the fittings. Assuming the asymmetry parameter η to be constant on passing from the high-temperature NiAs-type structure to the medium-temperature MnP-type structure yields a quadrupole splitting (dq=0.5e² qQ(1+(η²)/3)^1/2) value of ?0.25(2) mm/s for these phases. In low-temperature troilite |dq|=0.85 mm/s at room temperature. The combinations of (η, θ, φ) in troilite giving identical spectra range from (0, 49°, -) to (1, 45°, 50°) for negative V _zz or from (0.3, 57°, 78°) to (1, 58°, 54°) for positive V _zz . Assuming a negative V _zz and B‖c gives a θ value in agreement with the shortest Fe-S join being the V _zz orientation. The magnetic spin flip of 90° is proposed to occur in the MnP-phase only. The MnP phase-troilite transition occurs at lower temperatures and is more sluggish than in pure FeS.     

Biradical states of oxygen-vacancy defects in ��-quartz: centers E_{2}^{\prime \prime } and E_{4}^{\prime \prime }

Several new radiation defects with total electron spin S?=?1 occurring in electron-irradiated, synthetic ??-quartz have been observed by using electron paramagnetic resonance spectroscopy. These defects are considered to be biradicals, i.e., pairs of S?=?1/2 species. The concentration of these centers depends on the condition of the fast-electron irradiation. They have different decay behaviors that allow measurements of any individual species especially when it predominates over the others. The primary spin Hamiltonian parameter matrices g ₁, g ₂, D have now been determined for two similar defects, which herein are labeled $ E_{2}^{\prime \prime } $ and $ E_{4}^{\prime \prime } $ . Inter-electron distances estimated by using the magnetic dipole model, suggest that the structures of centers $ E_{2}^{\prime \prime } $ and $ E_{4}^{\prime \prime } $ both involve the unpaired electrons each located in orbitals of two silicon atoms next to a common oxygen vacancy but which have slightly different Si?CSi distances at 0.90 and 0.79?nm, respectively. This model is consistent with previous DFT calculations of the triplet configurations with local energetic minima. Observed decay behaviors suggest a transformation of centers $ E_{2,4}^{\prime \prime } $ to the analogous $ E_{1}^{\prime \prime } $ center. These triplet centers in quartz provide new insights into the structures of analogous defects in amorphous silica.     

Magnetic structure and cation distribution in (Fe,Mn)2SiO4 (olivine) by neutron diffraction

The cation distribution in the synthetic samples of olivine-type structure with composition (Fe _x Mn_1?x )₂SiO₄ was determined at room temperature and confirms previous Mössbauer results. At low temperature an antiferromagnetic ordering is observed. The magnetic structures can be described in the crystallographic cell (i.e. k=0). They are interpreted on the basis of the irreducible representations (modes) of the symmetry groups which are compatible with Pnma. The dominant modes observed for all compounds, including Fe₂SiO₄ and Mn₂SiO₄, only differ in their direction. The main direction of magnetization is dominated by the Fe²⁺ single-ion anisotropy. At 4.2K, for x=0.29, it is parallel to the c-axis, whereas for x=0.76 the direction is parallel to the b-axis. The anisotropy of the M1-sites dominates in the first case, whereas M2-anisotropy dominates in the second case. The influence of temperature is demonstrated for x=0.50 where c is the main direction at 4.2K, when it is b at 38K.