Trigonal Bipyramidal Sites Research Articles

The superstructure of domain-twinned η'-Cu6Sn5

The cell of the low-temperature modification of Cu 6 Sn 5 , the η-phase, has been determined by means of electron diffraction of single domains. The basic hexagonal NiAs (B8) type cell is pentupled, reflected in the reciprocal lattice by a fivefold superstructure running along [1121]. The extensive domain-twinning macroscopically gives rise to perfect hexagonal symmetry, which explains the previously proposed hexagonal cell [a=5a NiAs , c=5c NiAs ; Bernal (1928), Nature, 122, 54]. The structure was solved by occupying every fifth trigonal bipyramidal site of the NiAs-type structure and shifting the atoms surrounding these additional Cu atoms to form Edshammar 11 polyhedra with Cu-Sn and Cu-Cu distances in the range 2.60-2.74 A

Cation Distribution in the (Mg 1- xCu x) 3(PO 4) 2 Solid Solution, x = 0.46 and 0.79

Cation distributions in two related structures, (Mg 0.54Cu 0.46) 3(PO 4) 2(I) and (Mg 0.21Cu 0.79) 3(P0 4) 2(II) belonging to the system Mg 3(PO 4) 2-Cu 3(PO 4) 2 have been determined by single-crystal X-ray diffraction. (Mg 0.54Cu 0.46) 3(PO 4) 2I is isotypic with Mg(PO4)2; a = 7.608(3), b = 8.026(5), c = 5.116(3) Å, β = 92.73(4)°, monoclinic, P 2 1/ n, V = 312.0(3) Å3, Z = 2, D calc = 3.374 g cm -3, R = 5.8% for 549 observed reflections. (Mg 0.21Cu 0.79) 3(PO 4) 2II, is isotypic with Cu 3(PO 4) 2, a = 4.845(3), b = 5.265(3) c = 6.246(2) Å, α = 71.98(4), β = 93.04(4), γ = 111.42(4)°, triclinic, P1 bar, V = 140.7(1) Å 3, Z = 1, D calc = 4.198 g cm -3, R = 8.5% for 645 observed reflections. Both structures show mixed Mg/Cu trigonal bipyramidal sites (31% Mg 2+/69% Cu 2+). In I, additional positions of octahedral symmetry are occupied uniquely by Mg 2+ ions. In II, square planar sites contain only Cu 2+ ions.

Structure of the insertion compound Li 0.88U 3O 8 by powder neutron diffraction

The insertion compound Li 0.88U 3O 8, prepared from α-U 3O 8 at ambient temperature, was found to undergo an orthorhombic to hexagonal phase change on heating in vacuo. Powder neutron diffraction has been used to investigate the structures of the two modifications. Rietveld analysis of the data gave at, 573 K: space group P62 m, a = b = 6.849(3), c = 4.197(1), and at, 295 K: space group C2 mm, a = 6.9107(8), b = 11.7472(10), c = 4.1919(4). In the stable high-temperature modification, lithium atoms occupy 5-coordinate, trigonal-bipyramidal sites located in an ordered array between layers of the α′-U 3O 8 structure.

Calculation of the ferromagnetic resonance linewidth of barium ferrite

Two microscopic models have been proposed to explain the large intrinsic linewidth of barium ferrite. The first is a two-magnon scattering process due to an intrinsic disorder associated with the unique trigonal bipyramidal site in the crystal. [E. Tsantes and L. M. Silber, J. Appl. Phys. 63, 3350 (1988)]. The second is the Kasuya–LeCraw relaxation process [S. P. Marshall, J. B. Sokoloff, and C. Vittoria, IEEE Trans. Magn. MAG-25, 3491 (1989)]. Both of these processes involve modulation of the single-ion anisotropy. Analytical calculations using simplified models for the acoustic magnon and phonon spectra fail to account for the linewidths characteristic of barium ferrite. In order to understand both of these relaxation processes as they relate to barium ferrite, we have calculated the full phonon spectrum for this crystal based on the rigid-ion model. The calculated reflectivity compares well with the infrared reflectivity data that we have taken. Using phonon modes that we have calculated and the magnon modes calculated previously [S. P. Marshall and J. B. Sokoloff, J. Appl. Phys. 67, 2017 (1990)], we have calculated the ferromagnetic resonance linewidth on the basis of the Kasuya–LeCraw mechanism and two-magnon scattering by fluctuating 2b-site ions. We find that the trigonal-site disorder scattering mechanism is able to account for the observed magnitude of the linewidth.

Computer-simulation study of alkali-metal insertion into α-U3O8

Atomistic simulation techniques have been used to study the insertion of lithium and sodium into α-U3O8. Calculations for isolated guest ions predict that lithium will occupy five-co-ordinate trigonal bipyramidal sites whereas sodium will occupy nine-co-ordinate sites. The modelling of the stoichiometric phases MU3O8(M = Li, Na) reinforces these predictions. Calculations of ion migration are presented; lithium is demonstrated to be fairly mobile within the lattice whereas the diffusion of sodium is much more difficult.

An EXAFS study of the interaction of substrate with the ferric active site of protocatechuate 3,4-dioxygenase

X-ray crystallographic studies of the intradiol cleaving protocatechuate 3,4-dioxygenase from Pseudomonas aeruginosa have shown that the enzyme has a trigonal bipyramidal ferric active site with two histidines, two tyrosines, and a solvent molecule as ligands [Ohlendorf, D.H., Lipscomb, J.D., & Weber, P.C. (1988) Nature 336, 403-405]. Fe K-edge EXAFS studies of the spectroscopically similar protocatechuate 3,4-dioxygenase from Brevibacterium fuscum are consistent with a pentacoordinate geometry of the iron active site with 3 O/N ligands at 1.90 A and 2 O/N ligands at 2.08 A. The 2.08-A bonds are assigned to the two histidines, while the 1.90-A bonds are associated with the two tyrosines and the coordinated solvent. The short Fe-O distance for the solvent suggests that it coordinates as hydroxide rather than water. When the inhibitor terephthalate is bound to the enzyme, the XANES data indicate that the ferric site becomes 6-coordinate and the EXAFS data show a beat pattern which can only be simulated with an additional Fe-O/N interaction at 2.46 A. Together, the data suggest that the oxygens of the carboxylate group in terephthalate displace the hydroxide and chelate to the ferric site but in an asymmetric fashion. In contrast, protocatechuate 3,4-dioxygenase remains 5-coordinate upon the addition of the slow substrate homoprotocatechuic acid (HPCA). Previous EPR data have indicated that HPCA forms an iron chelate via the two hydroxyl functions.(ABSTRACT TRUNCATED AT 250 WORDS)

Study of the bipyramidal site in magnetoplumbite-like compounds, Sr M12O 19 ( M = Al, Fe, Ga)

Structures of magnetoplumbite-type crystals Sr M 12O 19 ( M = Al, Fe, Ga) have been refined by the single-crystal X-ray diffraction method. The space group is P6 3/ mmc with Z = 2. The cell dimensions are a = 5.5666(2) and c = 22.0018(8) Å for SrAl 12O 19, a = 5.8836(1) and c = 23.0376(9)Å for SrFe 12O 19, and a = 5.7929(1) and c = 22.8123(7) Å for SrGa 12O 19 at 22°C. Four structural models were tested with respect to the states of M atoms at trigonal bipyramidal sites: central atom and split atom models with and without anharmonic thermal vibrations. The simple split atom model gave lower R w values than the central atom model with anharmonic thermal vibrations for SrAl 12O 19 and SrGa 12O 19, whereas it gave an R w value approximately equal to that given by the central atom model with anharmonic vibration for SrFe 12O 19. The present study suggested that the potential around the M atom with trigonal bipyramidal coordination has double minima in all the magnetoplumbite-type crystals at lower temperatures.

Crystal structure of Nd 2Te 4O 11: An example of a rare earth tellurium oxide

The preparation and growth of single crystals and crystal structure determination of Nd 2Te 4O 11 are reported. It crystallizes in the monoclinic system, space group C/2 c,with parameters a = 12.635(6)Å, b = 5.204(1)Å, c = 16.277(3)Å, β = 106.02(8)°, and Z = 4.An agreement factor of R = 2.3%was obtained after refinement of positional and anisotropic thermal parameters. The structure is described as an interconnecting network of NdO 8 distorted square antiprisms, which link in two dimensions through edge sharing, with TeO 4 polyhedra which link in the same two dimensions through corner sharing. Sheets of (Nd 2O 10) ∞ join with those of (Te 8O 20) ∞ to form a three-dimensional network. The location and role of the lone pairs are discussed. The structure can also be described as a distorted hcp array of oxygen ions, neodymium ions, and lone pairs in which tellurium ions occupy selected trigonal bipyramidal sites.

In 1.2Ga 0.8MgO 4: Powder neutron refinement and crystal chemistry

The crystal structure of YbFe 2O 4-type In 1.2Ga 0.8MgO 4 has been refined using neutron powder data. The hexagonal parameters of its rhombohedral unit cell are a = 3.3243(4), c = 25.954(3) Å, and Z = 3. The structure contains In atoms in octahedral coordination and In + Ga + Mg atoms disordered on trigonal bipyramidal sites. It is shown to be simply related to the YAlO 3-type InGaO 3(II) structure via a crystallographic shear operation, both structures belonging to the n InGaO 3 · MgO homologous series.

Structural and spectroscopic investigations of LaFeAl 11O 19 compounds

Single crystals of nonstoichiometric LaFeAl 11O 19, isotypic with magnetoplumbite, have been prepared. Structural refinement and investigations by ESR and optical and Mössbauer spectroscopies have been performed. It is deduced that iron is in the 2+ state, and that Fe 2+ ions are localized in the tetrahedral sites of the spinel blocks. By air annealing, Fe 2+ is partially converted into Fe 3+. A significant amount of Fe 3+ ions appears in trigonal bipyramidal sites.

Neutron diffraction study of the cation distributions in the systems Fe7 –xMx(PO4)6(M = Mn or Co)

The cation distributions in the mixed-metal phosphate systems Fe7 –xMx(PO4)6[(1) M = Mn, x= 2; (2) M = Co, x= 3] have been studied by Rietveld analysis of powder neutron diffraction data. Both systems are synthesised hydrothermally, with subsequent low- temperature (500 °C) annealing. They adopt the Fe7(PO4)5 structure type, which provides four crystallographically distinct metal sites, three of which are octahedral and one five-co-ordinated. Iron is found to be preferentially oxidised over the dopant cation in both cases, giving idealised compositions ‘FeIIFeIII4MnII2(PO4)6’ and ‘FeIII4CoII3(PO4)6’. The cation sites are found to be strongly ordered, with Fe3+ preferentially occupying the two most distorted octahedral sites, rather than the more regular octahedral site and the five-co-ordinated (trigonal bipyramidal) site, in both cases. These observations may be rationalised in terms of a balance of coulombic, ligand-field, and other considerations.

Laser and phosphor host La1−xMgAl11+xO19 (x=0.050): Crystal structure at 295 K

The laser and phosphor host material with nominal composition LaMgAl11O19 is shown to be nonstoichiometric. Czochralski grown crystals are hexagonal, with space group P63/mmc, and a=5.589 82(5), c=21.9738(12) Å at 295 K. The thermal expansivity has been determined over the range 295 to 860 K: at 295 K, α1=4.97(2)×10−6 K−1 and α3=8.14(3)×10−6 K−1and at 775 K, α1=7.01(1)×10−6 K−1 and α3=10.1(2)×10−6 K−1. The integrated intensities of 20 487 reflections within a reciprocal lattice sphere of radius (sin θ)/λ=1.0 Å−1 were measured on a CAD-4 diffractometer with MoKα radiation, resulting in 18 232 reflections above background. Correction and averaging gave 864 independent structure factors with an internal agreement factor of 0.0145. The crystal structure was solved from the Patterson function and Fourier series and refined by the method of least squares. The final agreement factor R=0.013 98. The composition corresponds to La1−xMgAl11+xO19, with x=0.0502(8): the La deficiency is exactly compensated by excess Al. The principal La site is not fully occupied, but contains 4.76 at. % Al and 5.2 at. % Schottky defects. The remaining 4.94 at. % La and 0.26 at. % Al are distributed over two threefold satellite sites 0.583(3) Å distant from the principal La sites. The Mg atoms share a tetrahedral site equally with Al, while the Al atoms in the trigonal bipyramidal site undergo anharmonic motion. The remaining Al atoms occupy distorted octahedral or trigonal antiprismatic sites, with the latter sharing faces to give an Al–Al distance of 2.6541(9) Å. Other polyhedra share only oxygen atom corners or edges. The superior lasing characteristics are due at least partially to the wide separation of 5.5898 Å between principal La sites. The small average atomic radial vibrational amplitude of 0.071 Å corresponds to a Debye characteristic temperature of 631 K.

The crystal chemistry of (Mn3 +, Fe3+)-substituted andalusites (viridines and kanonaite), (Al1−x−yMn3x+Fe3y+)2(OǀSiO4): crystal structure refinements, Mössbauer, and polarized optical absorption spectra

AbstractThe crystal chemistry of viridines and kanonaite, (Al1-x-yMn3x+Fe3y+)2(OǀSiO4), has been evaluated with special reference to the behavior of Mn3+in this andalusite type structure. Five natural samples (from Ultevis:x= 0.012,y= 0.028; from Yakutia:x= 0.076,y= 0.046; from Tanzania:x= 0.091,y= 0.031; from Darmstadt:x= 0.171,y= 0.048; from Kanona:x= 0.340,y= 0.009) and two synthetic viridines (P135:x= 0.173,y= 0.005;P150:x= 0.22,y= 0) have been studied by means of X-ray powder diffraction, single crystal-structure refinements,57Fey-resonance spectroscopy, and optical absorption microspectroscopy.The structure refinements reveal that the transition metal ions substitute for Al almost exclusively in the distorted octahedral Al(l) site of the andalusite structure type within the entire mixed crystal series. This is independently proven by the Mössbauer results for57Fe, which show that only 10 to 15 % of total iron is present in the Al(2) trigonal-bipyramidal site. With increasing substitution, the octahedral (c/a)octratio increases. This result is corroborated by the increasing energy of the Mn3+5B1g→5A1gtransition as determined from the optical spectra. The increasing octahedral elongation leads to a tilting of both the Al(2)O5trigonal bipyramids and SiO4tetrahedra, and to slight changes of several of the Al(2)– O bond distances. These changes, although similar to those observed at high temperatures (Winter and Ghose, 1979), are considerably stronger than those caused by high temperature (e.g.,Δ(c/a)oct/(c/a)octto be extrapolated forx= 0.5 is approximately 0.1 while at 1000°C this relative change is only 0.035]. In the optical spectra, spin-allowed and spin-forbidden transitions of Mn3 +are identified near 15000 cm-1[5B1g→5A1g(D)], 18000 cm-1{5B1g→[3T1g(H)]}, 19700cm-1{5B1g→[3T1g(H)]}, 21800cm-1[5B1g→5B2g(D)], 23300cm-1[5B1g→5Eg(D)] and spin forbidden Fe3+transitions near 19700cm-1{6A1g→[4T2g(G)]}, 20800 cm-1{6A1g→[4T2g(G)]}, 22300 cm-1{6A1g→ [4A1g,4Eg](G)}, 23300cm-1{6A1g→[4A1g,4Eg](G)}. The crystal-field parameter 10Dqfor Mn3+decreases in the whole series by approximately 10%. However, this effect is compensated by increasing groundstate splitting such that the crystal field stabilazation energy of Mn3 +is nearly constant, 198 ± 2 kJ/g-atom Mn3 +, in the whole range of solid solutions, 0.0≤ x ≤ 0.4, which were studied.

Stereochemical role of lone pairs in main-group elements. Part 2. Structure and bonding in trichloro(tetramethylthiourea)antimony(III) studied by means of X-ray crystallography and antimony-121 Mössbauer spectroscopy

The crystal structure of the title compound has been determined from X-ray diffractometer data by Patterson and Fourier methods. Crystals are monoclinic, space group Cc, with Z= 4 in a unit cell of dimensions a= 10.180(2), b= 15.306(2), c= 7.699(1)A, and β= 105.48(1)°. The structure was refined to R 0.019 for 1 426 observed reflections. The antimony(III) atoms are in distorted trigonal-bipyramidal sites surrounded by three chlorine atoms at 2.428(2), 2.527(3), and 2.687(2)A and by a tetramethylthiourea sulphur atom at 2.505(2)A. The stereochemistry and distortion at the antimony(III) site is consistent with the lone pair occupying an equatorial position. The 121Sb Mossbauer parameters are reported and compared with literature values for related complexes. The increased negative shifts on going from SbCl3 to the complexes, together with the signs and magnitudes of the quadrupole interactions, are interpreted within a covalent framework in terms of the use of antimony 5s and 5p valence orbitals and the desire of these systems to lower their total energies by distortions which maximise the populations of the lower-energy 5s orbital.

Phosphoranes. 6. Determination of ground state structures of trifluoromethylphosphoranes using nuclear magnetic resonance spectroscopy. Correlation of 1JPC with the trigonal-bipyramidal site and the relation between 1JPC and 2JPF of trifluoromethyl substituents in phosphoranes

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTPhosphoranes. 6. Determination of ground state structures of trifluoromethylphosphoranes using nuclear magnetic resonance spectroscopy. Correlation of 1JPC with the trigonal-bipyramidal site and the relation between 1JPC and 2JPF of trifluoromethyl substituents in phosphoranesRonald G. Cavell, J. Andrew Gibson, and Kwat I. TheCite this: J. Am. Chem. Soc. 1977, 99, 24, 7841–7847Publication Date (Print):November 1, 1977Publication History Published online1 May 2002Published inissue 1 November 1977https://doi.org/10.1021/ja00466a015RIGHTS & PERMISSIONSArticle Views87Altmetric-Citations33LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (888 KB) Get e-Alerts Get e-Alerts

Mössbauer spectra of ternary tin(IV) sulphides in the systems Na2S–SnS2, BaS–SnS2, and PbS–SnS2

119 Sn Mössbauer spectra have been recorded for a number of ternary sulphides isolated from the systems Na2S–SnS2, BaS–SnS2, and PbS–SnS2. The series includes compounds of known crystal structure such as Na4SnS4, Na6Sn2S7, the high- and low-temperature forms (HT and LT) of Ba2SnS4, and Ba3Sn2S7, all of which contain tin tetrahedrally co-ordinated by sulphur atoms; Na2SnS3(LT) and PbSnS3, in which the tin is octahedrally co-ordinated; and Na4Sn3S8 which contains tin in both tetrahedral and trigonal bipyramidal sites. The measurements establish that the chemical isomer shift is sensitive to the co-ordination number of the central tin atom, and decreases by ca. 0.2 mm s–1 in going from tetrahedral, through trigonal bipyramidal, to octahedral co-ordination. The data for the compounds Na2SnS3(HT) and BaSnS3, Whose crystal structures are not known, are consistent with the presence of tin in distorted tetrahedral environments.

Stability and structure of noncentrosymmetric hexagonal LnInO 3 (Ln = Eu, Gd, Tb, Dy, Ho, Y)

LnInO 3 (Ln = Eu-Ho, Y) can be prepared at atmospheric pressure. These phases are hexagonal, a ∼ 6.3 A ̊ , c ∼ 12.3 A ̊ , Z = 6 , and possess the noncentrosymmetric space group P6 3 cm and the LuMnO 3 structure. The In 3+ ions are located in fivefold-coordinated trigonal-bipyramidal sites, and the Ln 3+ ions are sevenfold coordinated. HoInO 3 and YInO 3 transform to cubic solid solutions with the C-type rare-earth oxide structure above ∼ 1000°C. TbInO 3, GdInO 3 and EuInO 3 transform to ∼9% denser orthorhombic perovskite at elevated pressure, but DyInO 3, HoInO 3 and YInO 3 first transform to novel ∼6% denser pseudohexagonal orthorhombic phases at ∼20 kbar, 1000°C. No hexagonal form of ErInO 3 could be prepared at 950–1050°C, 0–34 kbar.

Crystal structure of anhydrous potassium carbonate

Crystals of the title compound are monoclinic, space group P21/c with unit cell dimensions: a= 5·64, b= 9·80, c= 6·88 A, β= 98·8°. The structure, which was determined from photographic X-ray data by Patterson and Fourier methods, consists of a slightly distorted hexagonal close packed array of carbonate ions in which potassium ions occupy both octahedral and trigonal bipyramidal sites. The carbonate ions have non-crystallographic trigonal planar geometry to within the accuracy of this determination. The structure was refined by least-squares methods to R 0·129 for 316 observed reflections.

A Mössbauer and ESR study of CaO·6Al 2O 3CaO·6Fe 2O 3 solid solutions

Mössbauer spectra of CaAl 12− x Fe x O 19 solid solutions with x ⩽ 4.8 show three resolvable quadrupole doublets arising from ferric ions in tetrahedral, trigonal bipyramidal and octahedral sites. Analysis of the relative area ratios shows a marked preference for iron occupation of the tetrahedral sites. Magnetic ordering is observed at 77°K for x = 4.8 and 6. An ESR spectrum of x = 1.2 is dominated by a g′ = 2 super-exchange resonance; that of x = 0.1 shows a complex pattern attributable mainly from ferric ions in an axially distorted site with zero rhombic distortion.

Band model for transition-metal chalcogenides having layer structures with occupied trigonal-bipyramidal sites

From symmetry arguments, the occupied d bands of superconducting NbSe 2 and semiconducting WSe 2 are identified as | ± 2, ∓ 1 2 〉 , which is split from the next higher d band | ± 2, ± 1 2 〉 by the energy gap E g = 2k c λ − W b. W b is the mean bandwidth and k cλ the effective spinorbit coupling parameter.