Cation-cation Repulsion Research Articles

Compression of cadmium orthosilicate, Cd2SiO4: a high-pressure single-crystal diffraction study

AbstractThe compression of cadmium orthosilicate, Cd2SiO4, was studied by high-pressure single-crystal diffraction carried out in a diamond-anvil cell. The equation of state at room temperature (third-order Birch-Murnaghan:V0= 695.71(3) Å3,K0.298= 119.2(5) GPa,K′ = = 6.17(4)) was determined from unit-cell volume data to 9.51 GPa. Compression of the orthorhombic structure (Fddd,Z= 8) was found to be anisotropic with relative compressibilities of 2.10:1.00:4.77 for thea,b, andcaxes. The variation of the lattice parameters with pressure is described by:a/a0= 0.99989(7) − 3.10(3) × × 10−GPa−1×P+ 8.2(3) × 10−5GPa−2×P2;b/b0= = 1.00007(8) − 8.8(9) × l0−2GPa−1×P;c/c0= 1.00001(5) − 4.16(3) × 10−3GPa−1×P+ 6.5(2) × 10−5GPa−2×P2. Crystal structures were determined at pressures of 3.01 GPa, 5.44 GPa, and 8.69 GPa. The anisotropy of the unit-cell compression is controlled by the distortion of the CdO6and SiO4polyhedra due to their unusual interconnection through shared edges. Cation-cation repulsion between Cd and Si atoms, which results in short and long Cd–O bonds, was found also to be responsible for unusual compressional behaviour related to the strength of the bond: the shorter the Cd–O bond the higher its compressibility. The large uniaxial distortion of the silicate tetrahedron is responsible for the relatively higher compressibility along thecaxis. The compressional differences between theaandbaxis result from the compressional anisotropy of the CdO6octahedron.

Refinement of the cookeite "r" structure

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Out-of-Center Distortions around Octahedrally Coordinated d0 Transition Metals

The bond valence approach is used to model the characteristic out-of-center electronic distortions around d 0 transition metal cations in octahedral coordination. The distortions are influenced not only by the electronic structure of the cation but also by the structure of the bond network, by lattice incommensurations, and by cation-cation repulsion. These latter effects often determine whether a distortion will occur and, if so, in what direction. Once the direction of an expected out-of-center distortion is known, its magnitude can be modeled using modified bond valence network equations, where certain bonds are weighted to take into account the intrinsic inequality of the bonds in such a distorted coordination. The arguments are illustrated by examples.

Li3Ni2TaO6: A novel rock salt superstructure phase with partial cation order

The new phase Li3Ni2TaO6 has a novel rock salt superstructure, a= 8.4259(3)A, b= 5.9073(3)A, c= 17.7329 A, Fddd. Within the cubic-close-packed oxide array, Ta occupies one set of octahedral sites giving isolated TaO6 octahedra which edge-share with Li/NiO6 octahedra Li and Ni are distributed non-randomly over three other sets of octahedral sites with Li : Ni occupancy ratios of 0.73 : 0.27, 0.59 : 0.41 and 0.55 : 0.45, respectively The higher Li : Ni ratio of the first set, 0.73 : 0.27 may be associated with cation-cation repulsion effects since this site is closest to Ta. Li3Ni2TaO6 is a very modest semiconductor with conductivity of ca. 4 × 10–6Ω–1 cm–1 at 300 °C and activation energy 0.77 eV, the disorder in the Li+ site occupancy is, therefore, static and does not yield significant levels of Li+ ion conductivity.

Beraunite: Refinement, comparative crystal chemistry, and selected bond valences

Deep green beraunite from Campanian age marls of Mullica Hill, N. J. (USA) was investigated owing to its unique crytal chemistry (two previous refinements involved orange crystals). Our crystal gave nearly 4[Fe +Fe~+(OH)s(H20)4(P04)4 · 2H20], a= 20.953(5), b = 5.171(1), c = 19.266(4) A, f3 = 93.34(2t, C2Jc, R = 0.043 (isotropic) for 2065 unique Fo and 91 variable parameters. Common to several known ferrosoferric phosphate structures, the underlying principle is a linear face-sharing oxygen octahedral trimer whose cations are M(4)-M(1)-M(4). For our beraunite, ca. 83% Fe2+ resides in the central M(l) site with mean M(1)-0 = 2.11 A. The other octahedral cation sites M(2), M(3), and M(4) (mean M -0 distances 2.01 to 2.02 A) are Fe3 +. The orange crystals of the other studies have predominant Fe3 + at all M(1) to M(4) octahedral sites (mean M -0 distances 1.97 to 2.04 A). Distortion of the octahedral face-trimer is pronounced, with M(1)M(4) distance lengthening of 19% compared with the perfect octahedral model. This results from cation-cation repulsion, which distorts the entire immediate environment from the perfect model. The OH(1) site on the trimer is coordinated by M(1), M(3), M{4), and H(1). Bond valence computations for the trimer suggest that Fe2+Fe3 + (green) --+ Fe3+Fe3 + (orange) may involve (1) 0-H(t) distance response to bond valence deviation or (2) an averaged model 20H---..2(0Ht120 112)L 5 -, but independent spectroscopic study is required to unravel the mechanism. * Present address: Natural History Museum of Los Angeles County, 900 Exposition Blvd., Los Angeles, California 90007 USA. 264 Paul B. Moore and Anthony R. Kampf The aristotype for this family of structures is believed to be ideal lipscombite, Fe~+(OH)4(P04)4, I4tfamd. Included are the several5 x 14 A2 derivatives; scheelite, Ca W04 , which shares partial antitypy; and MHSH = (Mgt6;3 D 813)(0HMS04MH413), l4tfamd with similar cell metric.

Beraunite: Refinement, comparative crystal chemistry, and selected bond valences

Abstract Deep green beraunite from Campanian age marls of Mullica Hill, N. J. (USA) was investigated owing to its unique crytal chemistry (two previous refinements involved orange crystals). Our crystal gave nearly 4[Fe2+Fe5 3+(OH)5(H2O)4(PO4)4 · 2H2O], a = 20.953(5), b = 5.171(1), c = 19.266(4) Å, β = 93.34(2)°, C2/c, R = 0.043 (isotropic) for 2065 unique F o and 91 variable parameters. Common to several known “ferrosoferric” phosphate structures, the underlying principle is a linear face-sharing oxygen octahedral trimer whose cations are M(4) – M(1) – M(4). For our beraunite, ca. 83% Fe2+ resides in the central M(1) site with mean M(1) – O = 2.11 Å. The other octahedral cation sites M(2), M(3), and M(4) (mean M – O distances 2.01 to 2.02 Å) are Fe3+. The orange crystals of the other studies have predominant Fe3+ at all M(1) to M(4) octahedral sites (mean M – O distances 1.97 to 2.04 Å). Distortion of the octahedral face-trimer is pronounced, with M(1) – M(4) distance lengthening of 19% compared with the perfect octahedral model. This results from cation-cation repulsion, which distorts the entire immediate environment from the perfect model. The OH(1) site on the trimer is coordinated by M(1), M(3), M(4), and H(1). Bond valence computations for the trimer suggest that Fe2+ Fe3+ (green) → Fe3+ Fe3+ (orange) may involve (1) O – H(1) distance response to bond valence deviation or (2) an averaged model 2OH−→2(OH½O½)1.5−, but independent spectroscopic study is required to unravel the mechanism.

Thermodynamic Quantities for Reactions of Alkali Metaborate Monomer and Dimer Vapors.

Alkali metaborate monomer and dimer vapors have been studied thermodynamically, and these results have been compared with those for alkali halides. The ratios of mean dissociation energies for one M-BO2 bond in the M2(BO2)2(g) dimers to dissociation energies for one M-BO2 bond in the MBO2(g) monomers are almost constant for all the alkali metals (M) and an average value is 0.63±0.01. This value is just equal to the corresponding ratios for halides. For the dimerization reactions of MBO2(g), both free energies and enthalpies decrease in going from M=Li to M=Cs, while entropies are almost constant. A similar tendency can be seen in chlorides. For the alkali-alkali exchange reactions in the dimers, free energies, enthalpies, and entropies are very small for both metaborates and chlorides. Excess stabilities in the alkali-alkali mixed-dimer formations are explained from the correlation with the increases in the alkali cation-cation repulsion energies.

LiNaSe, the first inter alkali metal selenide: Synthesis and structure

LiNaSe was synthesized from lithium carbonate and sodium carbonate at 750 °C in a stream of hydrogen loaded with selenium vapour. The crystal structure was determined from single-crystal diffractometer data. The atomic arrangement corresponds to the lead chloride-type structure. The selenium ions are slightly distorted hexagonal close packed. Lithium and sodium ions occupy tetrahedral and octahedral positions respectively. The cations' displacement from the centre of the surrounding chalcogene polyhedra is believed to be caused by cation-cation repulsion.

Structures, Compositions, and X-Ray Diffraction Identification of Dioctahedral Chlorites

AbstractAl-rich di, trioctahedral chlorite exists as the species cookeite and sudoite. Di,dioctahedral chlorite exists as the species donbassite. Cookeite has essential Li in its structure, sudoite has essential Mg, and donbassite has only small amounts of either element. To date, sudoite has been reported to have only IIb structural units and donbassite to have only Ia structural units. Cookeite is based primarily on Ia structural units, but IIb units are present in specimens from two localities. Most Al-rich chlorite species have regular-stacking “r” or “s” 2-layer stacking sequences, but 1-layer Ia-2 and Ia-6 polytypes also are known. The structural units (Ia or IIb) and the specific stacking sequences can be explained by a combination of local charge balance and minimization of cation-cation repulsion involving the interlayer and tetrahedral cations. X-ray powder diffraction data are adequate to differentiate Al-rich chlorite from trioctahedral chlorite and to identify the type of structural unit present, but single crystal study is necessary to identify the 2-layer and 1-layer sequences with certainty.

Refinement of the Crystal Structure of A Monoclinic Ferroan Clinochlore

AbstractA monoclinic IIb-2 clinochlore from Washington, D.C., contrary to previous studies, is primarily a ferroan rather than a ferrian chlorite. Disorder of tetrahedral Si,Al cations is indicated because of unsuccessful structural refinements in subgroup symmetries. The true space group is C2/m. Slight ordering of Mg, Fe2+, and Fe3+ over octahedra M(l) and M(2) within the 2:1 layer (mean M-O,OH = 2.092 and 2.084 Å, respectively), complete ordering of trivalent Al into the centrosymmetric octahedron M(4) of the interlayer sheet (M-OH = 1.929 Å), and ordering of primarily divalent cations (Mg and Fe) into the two interlayer M(3) octahedra (M-OH = 2.117 Å) exist. The excess of negative charge above unity due to tetrahedral substitution of Al for Si (1.378 atoms) is compensated entirely within the octahedral sheet of the 2:1 layer.Ordering of a trivalent cation into one octahedron in the interlayer should be universal for all stable trioctahedral chlorites. In this specimen the ordering is due to (1) minimization of cation-cation repulsion by layer offsets which provide more space around the trivalent element, and (2) energy minimization by localization of the source of positive charge on the interlayer sheet in one octahedron rather than two. In other structures or for different compositions additional factors can be important also. Most chlorites of the IIb and Ib (β = 90°) types are expected to show disorder of the tetrahedral cations. The b positioning of interlayer and layer provides no preferential driving force for concentration of Si and A1 in any tetrahedron as a consequence of the expected ordering of the interlayer cations. The monoclinic IIb-2 polytype is less abundant in nature than the triclinic IIb-4 and IIb-6 structures, because only half as many possible superpositions of layers exist that will produce monoclinic symmetry. Crystallization factors must also be important, because the IIb-2 chlorite is much less abundant than predicted by this purely geometrical argument.

Coordination of Fe, Ga and Ge in high pressure glasses by Mössbauer, Raman and X-ray absorption spectroscopy, and geological implications

Mössbauer spectra of glasses of NaFeSi 3O 8 and 3NaAlSi 2O 6 · NaFeSiO 4 starting compositions consist of a dominant Fe 3+ and subordinate Fe 2+ quadrupole-split doublet, in agreement with previous work. Fe 3+ is assigned to tetrahedral coordination. Pressure-induced coordination changes are not observed in the pressure range 1 bar to 30 kbar. A gradual increase in isomer shift of the Fe 3+ doublet with increase in pressure is attributed to steric effects. Raman spectra of GeO 2, NaGaSi 3O 8 and NaGaSiO 4 glasses are dominated by network structure vibrations. There is no detectable change in the nearest-neighbor coordination of Ge 4+ in GeO 2 from 1 bar to 14 kbar, of Ga 3+ in NaGaSi 3O 8 from 1 bar to 28 kbar and of Ga 3+ in NaGaSiO 4 from 1 bar to 25 kbar. However, some structural reorganization outside of the first coordination sphere occurs in the high pressure glasses. XANES and EXAFS spectra on powdered samples of 1 bar and 25 kbar NaGaSiO 4 glasses and crystalline NaGaSiO 4 were obtained from K edge absorption spectra at the Stanford Synchrotron Radiation Laboratory using a double crystal monochromator equipped with Si(220) crystals. The XANES spectra indicate that Ga 3+ has a similar extended coordination geometry in both glasses. The EXAFS spectra reveal that Ga 3+ is four-coordinated with oxygen in all three samples with a Ga 3+-O distance of about 1.83 Å. The radial distribution functions of the two glasses are virtually identical. However, both XANES and EXAFS spectra indicate significant structural differences between crystalline NaGaSiO 4 (nepheline-type structure) and vitreous NaGaSiO 4 beyond the first coordination shell of Ga 3+. Thus, X-ray absorption spectroscopy independently confirms the Raman results on the unchanged coordination of Ga 3+ in NaGaSiO 4 glasses with pressures up to 25 kbar. Glass compositions were selected in anticipation that larger and/or lower charged cations would exhibit pressure-induced coordination changes at lower pressures than Al 3+ and Si 4+. The present null result suggests that the stabilizing features of open network structures in the liquid state (large entropy and minimized cation-cation repulsion) more than compensate for large molar volume in the pressure range accessible to experimentation. It appears that network structures in natural magmas should remain stable throughout the upper mantle. Consequently, the densities of magmas at high pressures which are calculated from compressibility data and the appropriate equation of state will be only slightly underestimated, due to the effect of minor structural changes beyond the first coordination sphere.

Crystal structural features of the olivine ? spinel transition

Comparisons of structural features of olivine (α phase), spinel (γ phase), and the modified spinel (β phase) lead to predictions of possible mechanisms for the olivine → spinel transitions. In the olivine structure, rigid tetrahedral edges and shared octahedral edges form columns of corner-sharing trigonal dipyramids parallel to the a axis. These rigid columns are separated by weaker, unshared octahedral edges which may be stretched to reduce cation-cation repulsion. As a result, olivine has a relatively loose structure and is stable at low pressure. At elevated pressure, olivine transforms to the more compact spinel structure, in which the rigid tetrahedral edges and shared octahedral edges form a three dimensional network instead of aligned columns. These structural differences explain how compressibility and thermal expansion may be taken up mainly by octahedral sites in olivine, but are evenly distributed over both octahedral and tetrahedral sites in spinel. Because the closest packings of oxygens and interstitial cation distributions differ between olivine (h.c.p.) and spinel (c.c.p.), the olivine structure may have to disintegrate during its transformation to spinel, so that the olivine → spinel transition involves processes of nucleation and growth. The migration of atoms across the olivine-spinel interface is thus a complicated process of random walk without a definite path. In the β phase → spinel transition, however, the diffusion of cations may follow a definite path in restricted regions because oxygen closest packings and cation distributions are similar in the two structures. If the oxygen packing remains intact during the β → γ transition, the transformation will be an intracrystalline process leading to domain structure in the spinel product.

Synthesis of new high-pressure columbite phases containing pentavalent vanadium

A series of MV 2O 6 compounds with M = Ni, Mg, Co, Zn, Mn, and Cd having the orthorhombic columbite-type structure has been prepared at pressures of 50–80 kbar and temperatures of 800–1200°C. CuV 2O 6II was found to have a monoclinic distortion with probable space group P2 1/ c and a = 4.824(1), b = 13.483(2), c = 5.652(1)A˚and β = 91.02(2). The transition from the brannerite to columbite-type structures involves an increase of V 5+ coordination from 5 + 1 → 6, a change in oxygen packing from cubic close-packed to hexagonal close-packed, a retention of part of the structure of the VO 6 sheets and a collapse of the MO 6 rutile-like chanins into αPbO 2 chains. The relative stabilities of the trirutile, columbite, and PbSb 2O 6 structures are discussed in terms of electrostatic repulsion forces, polarization, and covalency. It is concluded that minimization of cation-cation repulsion for d 10 ions cannot be responsible for the stability of the trirutile structure, since the d 10 ions As 5+ and Sb 5+ are frequently found in the PbSb 2O 6 structure in which each Sb 5+ or As 5+ ion has 3 Sb 5+ or As 5+ neighbors. In plots of r M 3 vs unit cell volumes for MV 2O 6 and MNb 2O 6 compounds, the volume of the Mg-containing compound always lies above the line connecting the other compositions. This deviation is attributed to the relatively greater covalence and consequent shortening of M O bonds where M = Ni, Co, Zn, Mn, and Cd.