Showing papers in "Zeitschrift Fur Kristallographie in 2003"

Structural analysis of complex materials using the atomic pair distribution function — a practical guide

Abstract: Modern materials and their properties are of- ten characterized by varying degrees of disorder. Routine crystallographic structure solution only reveals the average structure. The study of Bragg and diffuse scattering yields the local atomic arrangements holding the key to under- standing increasingly complex materials. In this paper we review the pair distribution function technique used to un- ravel the local structure. We aim to give a practical over- view and make this method easily accessible to the wider scientific community.

Intermetallic compounds with ordered U3Si2 or Zr3Al2 type structure – crystal chemistry, chemical bonding and physical properties

Abstract: The crystal chemistry of binary and ternary intermetallic compounds with tetragonal U 3 Si 2 or Zr 3 Al 2 type structure is reviewed. Besides the preparation techniques and crystal chemistry, we especially focus on the structure-property relations and chemical bonding peculiarities.

Structural distortion of the α, β, and γ polymorphs of glycine on cooling

Abstract: The effect of cooling down to 150 K on the structures of α- (s.g. P2 1 /n), β- (s.g. P2 1 ), and γ- (s.g. P3 1 ) polymorphs of glycine was studied by X-ray single-crystal diffraction. No polymorphic transformations were detected. Relative volume changes and the anisotropy of structural distortion of the three polymorphs of glycine were compared. Maximum contraction of structures was measured in the b-direction for the α- and β-polymorphs, and in the α-direction for the γ-polymorph. Minimum contraction in the γ-polymorph was measured in c-direction. The structures of the α- and the β-polymorphs slightly expanded on cooling in the directions close to c. The directions of maximum and minimum lattice strain were related to the directions of weak and strong hydrogen bonds in the structures. Crystal structures for the three polymorphs were refined at 294 K and at 150 K. The effects of cooling on the intramolecular geometry of glycine molecules and on the intermolecular hydrogen bonds in all the three polymorphs were compared. Structural distortion on cooling was compared with that on increasing hydrostatic pressure.

A review of sample environments in neutron scattering

Abstract: Sample environment equipment, cryostats, furnaces, pressure cells etc. are an essential adjunct to most neutron scattering experiments to induce the sample being studied into a phase or state of particular interest. He we give a brief overview of the diverse range of Sample Environment equipment that is currently in use, a background to its development and also some recent trends.

A comparative study of pressure-induced lattice strain of α- and γ-polymorphs of glycine

Abstract: The effect of hydrostatic pressure up to 4 GPa on the two polymorphs of glycine (α - s,g. P21/n, and γ - s.g. P3 1 (P3 2 )) was studied by X-ray powder diffraction using a synchrotron radiation source (λ = 0.7 A, a MAR345 image plate detector). No polymorphic transformations were detected. Relative volume changes and the anisotropy of structural distortion of the two polymorphs were compared. The directions of maximum and minimum lattice strain were related to the directions of weak and strong hydrogen bonds in the structures.

Recrystallisation of HDA ice under pressure by in-situ neutron diffraction to 3.9 GPa

Abstract: We have studied by in-situ neutron diffraction the recrystallisation behaviour of HDA ice in the pressure range 0.3-3.9 GPa, i.e. the entire stability range of HDA. We report quantitative and detailed structural information on the various high pressure ice phases formed metastably at low temperatures. We find that between ∼0.4-0.7 GPa, HDA transforms at 175 K to mainly phases IV and V, and XII, and at 1-1.2 GPa to a mixture of ice VI and XII. On isothermal compression at 100 K, HDA recrystallises to an ice VII-like structure which is either partially ordered or mixed with ice VIII. Full structural data obtained by Rietveld refinements are reported for all these phases at low temperatures. Phases IV, V and XII are fully hydrogen disordered when obtained from HDA. The transformation behaviour in the 0.5-1.2 GPa range is in good agreement with the picture reported from quenched-recovered samples, although some differences persist in the recrystallisation to IV/XII mixtures. The recrystallisation behaviour of HDA over the entire pressure range appears to follow closely that of liquid water under pressure.

Structural topology of potassium uranyl chromates: crystal structures of K8[(UO2)(CrO4)4](NO3)2, K5[(UO2)(CrO4)3](NO3)(H2O)3, K4[(UO2)3(CrO4)5](H2O)8 and K2[(UO2)2(CrO4)3(H2O)2](H2O)4

Abstract: Crystals of four potassium uranyl chromates, K 8 [(UO 2 )(CrO 4 ) 4 ](NO 3 ) 2 (1), K 5 [(UO 2 )(CrO 4 ) 3 ](NO 3 ) .(H 2 O ) 3 (2), K 4 [(UO 2 ) 3 (CrO 4 ) 5 ](H 2 O) 8 (3) and K 2 [(UO 2 ) 2 (CrO 4 ) 3 (H 2 O) 2 ](H 2 O) 4 (4), have been synthesized by evaporation of aqueous solutions of (UO 2 )(NO 3 ) 2 (H 2 O) 6 and K 2 CrO 4 . The structure of (1) (triclinic, P1, a = 7.0397(6), b = 9.7341(9), c = 9.7568(9) A, α = 105.846(2), β = 97.992(2), γ = 93.271(2)°, V = 633.8(1) A 3 , Z = 1) was solved by direct methods and refined to R1 = 0.044 (wR2 = 0.098). It is based upon clusters of composition [(UO 2 )(CrO 4 ) 4 ] that consist of UrO 4 square bipyramids (Ur: UO 2 2+ uranyl ion) that share comers with four different CrO 4 tetrahedra. The clusters are arranged in layers parallel to the (100) plane. K + cations and NO 3 - groups are located between the clusters. The crystal structure of (2) (orthorhombic, P2 1 2 1 2 1 , a = 6.1112(11), b = 12.136(2), c = 27.464(4) A, V = 2036.9(6) A 3 , Z = 4) was solved by direct methods and refined to R1 = 0.047 (wR2 = 0.071). it contains chains of composition [(UO 2 )(CrO 4 ) 3 ] that are parallel to the b axis. The chains consist of UrO 5 pentagonal bipyramids that share all equatorial comers with different CrO 4 tetrahedra. Planes containing the equatorial ligands of the UrO 5 pentagonal bipyramids are approximately parallel to the (100) plane. K + cations, NO 3 groups and H 2 O groups are between the chains. The crystal structure of (3) (monoclinic, P2 1 /c, a = 8.2336(7), b = 18.8042(17), c=21.2413(18) A, β = 89.979(2)°, V = 3288.7(5) A 3 , Z = 4) has been solved by direct methods from a crystal twinned on the (001) plane and refined to Rl = 0.060 (wR2 = 0.125). The structure is based upon topologically complex sheets of composition [(UO 2 ) 3 (CrO 4 ) 5 ] with UrO 5 pentagonal bipyramids that share their comers with CrO 4 tetrahedra. The sheets are parallel to the (010) plane. K + cations and H 2 O groups are located in the interlayer between the sheets. The crystal structure of (4) (monoclinic, P2 1 /c, a = 10.7417(5), b = 14.5290(7), c = 14.1387(6) A, β = 108.135(1)°, V = 2097.0(2) A 3 , Z = 4) was solved by direct methods and refined to R1 = 0.036 (wR2 = 0.067). It is based upon sheets of composition [(UO 2 ) 2 (CrO 4 ) 3 (H 2 O) 2 ] that are parallel to the (010) plane. K + cations and additional H 2 O groups are in the interlayer. Nodal representations of uranyl chromate structural units are developed, and a comparison with related structures is presented.

A neutron diffraction study of quartz-type FePO4: high-temperature behavior and α–β phase transition

Abstract: The structural evolution of quartz-type iron phosphate, FePO 4 , was studied by time-of-flight neutron powder diffraction from 294 K up to 1073 K. The refined structural parameters of the low-temperature α phase tend towards the values obtained for high-temperature β-quartz-type FePO 4 . In particular, the Fe-O-P bridging angles increase and the tetrahedral tilt angles decrease strongly as the α-β phase transition at 980 K is approached. Important discontinuities in these structural parameters are observed at this first-order transition. Thermal expansion in the α phase is strongly non-linear and dominated by these angular variations. There is essentially no thermal expansion in the β phase due to the absence of these mechanisms. The behavior in the α phase is distinct from those of other α-quartz homeotypes in that the angular variations are greater than for the other materials, including SiO 2 and AlPO 4 , and do not scale with the initial structural distortion. This may be an indication of a dynamic instability in α-quartz-type FePO 4 .

Natural and synthetic compounds with kröhnkite-type chains. An update

Abstract: The crystal structures of five new triclinic double salt dihydrates have been determined from single-crystal X-ray diffraction data. The following four compounds all contain krohnkite-[Na 2 Cu(SO 4 ) 2 . 2 H 2 O]-type tetrahedral-octahedral chains: the three isotypic synthetic compounds K 2 Mg(CrO 4 ) 2 . 2 H 2 O (a = 5.674(1), b = 6.462(1), c = 7.517(2) A, α = 110.38(3), β = 95.24(3), γ = 109.86(3)°, V = 236.0(1) A 3 , R(F) = 0.036, space group P1, no. 2, Z = 1), K 2 Mg(MoO 4 ) 2 . 2 H 2 O (a = 5.884(1), b = 6.491(1), c = 7.700(1) A, α = 111.67(2), β = 96.59(2), γ =108.62(2)°, V = 249.8(1) A 3 , R(F) = 0.019, space group P1, no. 2, Z = 1) and K 2 Mn(SeO 4 ) 2 . 2 H 2 O (a = 5.674(1), b = 6.608(1), c = 7.523(2) A, α = 110.31(3), β = 95.69(3), γ = 108.3513)°, V = 244.10(9) A 3 , R(F) = 0.024, space group P1, no. 2, Z = 1), and the mineral messelite Ca 2 (Fe 2+ ,Mn 2+ ,Mg)(PO 4 ) 2 . 2 H 2 O (a = 5.480(1), b = 5.759(1), c = 6.569(1) A, α = 90.18(3), β = 102.62(3), γ = 108.45(3)°, V = 191.3(1) A 3 , R(F) = 0.022, space group P1, no. 2, Z 1). The fifth compound, the synthetic dichromate K 2 Zn(Cr 2 O 7 ) 2 . 2 H 2 O (a = 6.794(1), b = 7.735(2), c = 7.834(2) A, α = 88.97(3), β = 80.90(3), γ = 64.57(3)°, V = 366.5(2) A 3 , R(F) = 0.036, space group P1, no. 2, Z = 1) is closely related to the krohnkite-group; it contains Cr 2 O 7 groups assuming the same bridging role that XO 4 groups have in krohnkite-type oxysalts. The atomic arrangements of the title compounds are described and structural relations with other krohnkite-type and -like compounds are discussed. In addition, related yavapaiite-type sheet structures of AM'(XO 4 ) 2 compounds are briefly reviewed, including the rod group symmetry of the underlying krohnkite-chain building unit. Twelve different space groups have been found for these sheet structures. The previously reported space group symmetries for α-NH 4 Fe(CrO 4 ) 2 (P2 1 ) and CsTa(PO 4 ) 2 (P1) are obviously incorrect and should be revised to P2 1 /n and C2/m, respectively.

Geometrical isomerism in uranyl chromates I. Crystal structures of (UO2)(CrO4)(H2O)2, [(UO2)(CrO4)(H2O)2](H2O) and [(UO2)(CrO4)(H2O)2]4(H2O)9

Abstract: Three uranyl chromate hydrates, (UO 2 )(CrO 4 )(H 2 O) 2 , [(UO 2 )(CrO 4 )(H 2 O) 2 ](H 2 O) and [(UO 2 )(Cr0 4 )(H 2 O) 2 ]4(H 2 O) 9 , have been prepared by evaporation from aqueous solutions. The crystal structure of (UO 2 )(CrO 4 )(H 2 O) 2 (monoclinic, C2/m, Z = 16, a = 16.786(2), b = 22.731(2), c = 6.9969(7) A, β = 90.051(3)°, V = 2669.8(5) A 3 ) has been solved by direct methods and refined to R1 = 0.065, calculated for 6702 unique observed reflections (|F o | ≥ 4σ F ). The crystal structure of [(UO 2 )(CrO 4 )(H 2 O) 2 ](H 2 O) (monoclinic, P2 1 , Z = 4, a = 9.7206(14), b = 7.1617(10), c = 11.0909(16) A, β = 92.388(3)°, V = 771.43(19) A 3 ) has been solved by direct methods and refined to R1 = 0.052, calculated for 4879 unique observed reflections (|F o | > 4σ F ). The crystal structure of [(UO 2 )(CrO 4 )(H 2 O) 2 ] 4 (H 2 O) 9 (monoclinic, P2 1 /c, Z = 4, a = 31.397(1), b = 7.1701(3), c = 16.2480(7) A, β = 97.515(1)°, V= 3626.3(3) A 3 ) has been solved by direct methods and refined to R1 = 0.051, calculated for 8164 unique observed reflections (|F o | ≥ 4σ F ). Each structure contains chains of composition [(UO 2 )(CrO 4 )(H 2 O) 2 ]. Within the chains, (UO 2 )O 3 (H 2 O) 2 pentagonal bipyramids share three equatorial O atoms with CrO 4 tetrahedra. The CrO 4 tetrahedra in the [(UO 2 )(CrO 4 )(H 2 O) 2 ] chains each share three corners with uranyl polyhedra, and contain one non-shared (terminal) ligand pointing either up or down relative to the plane of the chain. This results in a geometrical isomerism of the uranyl chromate chains that can be described as a sequence of tetrahedra orientations along the chain extension, e.g ... up-down-up-down... or... ududud... [or (ud) ] . The structures of (UO 2 )(CrO 4 )(H 2 O) 2 and [(UO 2 )(CrO 4 )(H 2 O) 2 ](H 2 O) are based upon the chains of the (ud) type, whereas [(UO 2 )(CrO 4 )(H 2 O) 2 ] 4 (H 2 O) 9 contains two types of chains: (ud) and (u) . The [(UO 2 )(CrO 4 )(H 2 O) 2 ] chains extend in the same direction in each structure and are linked via hydrogen bonds involving H 2 O molecules.

Geometrical isomerism in uranyl chromates II. Crystal structures of Mg2[(UO2)3(CrO4)5](H2O)17 and Ca2[(UO2)3(CrO4)5](H2O)19

Abstract: Geometrical isomerism of uranyl chromate sheets occurs in the structures of Mg 2 [(UO 2 ) 3 (CrO 4 ) 5 ](H 2 O) 17 and Ca 2 [(UO 2 ) 3 (CrO 4 ) 5 ](H 2 O) 19 . Both compounds were prepared by evaporation from aqueous solutions. Compound 1 is orthorhombic, Pbca, a = 19.9206(10), b = 21.0526(10), c = 18.4966(9) A, V= 7757.1(7) A 3 ; R 1 = 0.050 for 6332 unique reflections with |F o | ≥ 4σ F . Compound 2 is monoclinic, P2 1 /m, a = 11.0359(5), b = 17.5364(7), c = 11.5056(5) A, β = 118.178(1)°, V = 1962.8(2) A 3 ; R 1 = 0.037 for 6209 unique reflections with |F o | = 4σ F . Both structures contain sheets of composition [(UO 2 ) 3 (CrO 4 ) 5 ] involving comer-sharing UO 7 pentagonal bipyramds and CrO 4 tetrahedra. The sheets are parallel to (100) in both structures. Mg 2+ and Ca 2+ cations are coordinated by H 2 O molecules and are located in the interlayer. The topology of the [(UO 2 ) 3 (CrO 4 ) 5 ] sheets in both structures is the same, whereas their geometrical structures are different, and they are geometrical isomers. The sheets differ in the orientations of non-shared corners of chromate tetrahedra relative to the plane of the sheet. In order to describe the geometrical structure of the isomers, the concept of orientation matrix of tetrahedra is introduced. It has been shown that there are at least three geometrical isomers of the [(UO 2 ) 3 (CrO 4 ) 5 ] sheet in uranyl chromates that differ in their orientation matrices of the tetrahedra.

Compositional and structural variations in the ternary system Li - Al - Si

Abstract: The Li-Al-Si ternary system has been investigated and four ternary phases characterized using both powder and single crystal X-ray diffraction data. LiAlSi and Li 7 Al 3 Si 4 crystallize in the F43m, cubic system, Li 18 Al 2 Si 6 in tetragonal I4 1 /amd and Li 15 Al 3 Si 6 in hexagonal P6 3 /m space group. The structure of LiAlSi (a = 5.94 A), previously reported from powder data, has been confirmed. The new compound Li 7 Al 3 Si 4 has been identified and, due to a higher lithium content, its cell parameter is slightly larger (a = 6.115 A). The tetragonal cell (a = 6.179(1) A, c = 12.199(4) A) of Li 18 Al 2 Si 6 may be considered as a (1 x 1 x 2) supercell of the cubic one. Structural variations are strongly correlated to compositional fluctuations and atomic substitutions, in every case the Si-Al network displays some covalent character. In the Li 15 Al 3 Si 6 hexagonal cell, bonding is more covalent leading to a heterographite-like Si-Al network.

Theoretical analysis and empirical manifestation of the distortion theorem

Abstract: The distortion theorem is analyzed by the use of the Bond Valence Model for the case of an isolated coordination polyhedron with bonds of different lengths. It is shown that a correlation should exist between the average bond length increase and the mean-square deviation of individual bond lengths from the average one, at least for nonstrained crystal structures. It is proved that the correlation parameters are strongly dependent on a mode of distortion (symmetry of coordination environment). These conclusions are in general and sometimes in quantitative agreement with available experimental data, although in most cases the distortion of coordination polyhedra results from a variety of factors of chemical, geometric and electronic origin. It is assumed that an observed correlation between the average bond distance and the distortion degree may be taken as a measure of relative role of intrinsic bond strengths and influence of crystalline environment.

Stroboscopic neutron diffraction from spatially modulated systems

Abstract: The combination of stroboscopic techniques and neutron diffraction can be used to study the kinetics of structural changes in condensed matter on a microscopic level. Transient states may be identified and characterized on time-scales down to the microsecond regime. Hence, valuable information about the underlying mechanisms can be obtained from time-resolved experiments. Particularly interesting subjects for this type of investigation are spatially modulated systems which undergo phase transitions or phase separation. The potential of stroboscopic neutron diffraction is demonstrated using three different examples in which samples are periodically perturbed by the variation of temperature, mechanical stress or electric field and their structural response is characterized by time-resolved diffraction. Spinodal decomposition in ionic crystals of the silver-alkali halide type is shown to be dominated by two different processes on different time-scales. The stress-induced phase transition into the incommensurate phase of quartz involves relaxation processes which are reflected by different kinetic behaviours of Bragg peaks and satellite reflections, respectively. Finally, metastable transient states are observed during the field induced lock-in transition in ferroelectric Rb 2 ZnCl 4 which are most probably due to strain fields and pinning effects.

Oriented graphite single-crystal inclusions in diamond

Abstract: A crystallographic study of 13 specimens of diamonds with euhedral single-crystalline graphite inclusions in their centres is presented. All inclusions belong to the hexagonal graphite modification (space group P6 3 /mmc; a 0 ≃ 2.46 A, c 0 ≃ 6.70 A) and are up to 300 μm in diameter. Comparison of the measured c-lattice parameters of the graphite crystals with lattice parameters of graphite at elevated isostatic pressures indicates remnant pressures of up to 2.6 GPa acting on the inclusions. All samples exhibit distinct orientation relations between graphite and diamond. In 12 samples the direction [001] of graphite (G[001]) approximately parallels one of the directions (111) of cubic diamond (D ). The largest deviation is about 4°. A further division of these 12 samples is: (a) The three G(100) directions have angles of about 4° with three D(110) directions. This orientation relation is observed in five samples. (b) The angle between the same directions is about 34° in six samples. (c) In one sample this angle is about 16°. In a 13th specimen G[001] approximately parallels one of the three D(100). The deviation is of about 6°, and one of the G(100) directions nearly parallels one of the D . These orientation relations are analysed with a simplified application of the coincidence site lattice (CSL) concept. A 14th specimen is different to all others, as it exhibits a large (about 300 μm edge length) region with a sharp hexagonal borderline filled with a patchwork of tiny graphite islands. Several parallel lamellae of up to 5 μm thickness result in a hexagonal pyramidal form. This graphite inclusion is suggested to be protogenetic with respect to the diamond.

Hydrogen in novel solid-state metal hydrides

Abstract: Solid-state metal hydrides display hydrogen densities close to that of liquid hydrogen and thus provide a safe and efficient way of storing hydrogen. As a result of recent neutron and synchrotron diffraction work some novel metal hydrides have been characterized that shed new light on the nature of metal-hydrogen interactions. While hydrogen appears as an anion surrounded by a large inventory of cation configurations in ionic hydrides such as Ca 4 Mg 3 H 14 , Ca 19 Mg 8 H 54 , Eu 2 MgD 6 , Eu 6 Mg 7 D 26 and Eu 2 Mg 3 D 10 , it acts as a terminal ligand in covalently bonded hydride complexes based on p-elements such as [BH 4 ] - and d-elements such as [IrH 5 ] 4- and [IrH 4 ] 5- in the complex hydrides LiBH 4 and Mg 6 Ir 2 H 11 , respectively. Surprisingly, hydride complexes and hydride anions can also be discerned in typically metallic (interstitial) hydrides such as NdMgNi 4 H 4 (= Nd 3+ Mg +2 . [Ni 4 H 4 ] 5- ) and LaMg 2 NiD 7 (= La 3+ Mg +2 2 . [NiH 4 ] 4- . 3 H - ). Some hydrides also reveal other interesting features such as a hydrogenation induced Ce 4+ → Ce 3+ valence change in CeMn 18 Al 0.2 H 4.4 at room temperature that is accompanied by a Mn/Al metal atom exchange over distances of ∼2.6 A, and a hydrogen induced metal-to-nonmetal transition near ambient conditions that leads from the metallic compound Mg 3 Ir to the red colored hydride Mg 6 Ir 2 H 11 . In this article recent work and some methodological aspects are highlighted.

Rare earth disilicates R2Si2O7 (R = Gd, Tb, Dy, Ho): type B

Abstract: The type B structure of the single rare earth element disilicates has been revised using room temperature and pressure single-crystal X-ray diffraction measurements on Gd 2 Si 2 O 7 , Tb 2 Si 2 O 7 and Ho 2 Si 2 O 7 synthesized at 2-2.5 GPa, 1400-1500 °C in a piston-cylinder apparatus and Dy 2 Si 2 O 7 synthesized hydrothermally at 0.1 GPa. Crystal data are: triclinic, space group P1, Z = 4; Gd 2 Si 2 O 7 - a = 6.6609(2), b = 6.7081(2), c = 12.1390(4) A, α = 94.277(1), β = 90.577(1), γ = 91.441(1)°, R = 0.033, and D x = 5.929 g/cm 3 ; Th 2 Si 2 O 7 - a = 6.6331(3), b = 6.6799(2), c = 12.0967(2) A, α = 94.128(1), β = 90.609(2), γ = 91.541(1)°, R = 0.038, and D x = 6.041 g/ cm 3 ; Dy 2 Si 2 O 7 - a = 6.6158(2), b = 6.6604(2), c = 12.0551(4) A, α = 94.373(2), β = 90.836(2), γ = 91.512(2)°, R = 0.031, and D x = 6.156 g/cm 3 ; and Ho 2 Si 2 O 7 - a = 6.5960(2), b = 6.6328(3), c = 12.0214(6) A, α = 94.479(1), β = 90.856(1), γ = 91.749(2)°, R = 0.034, and D x = 6.313 g/cm 3 . Substitution of Gd 3+ , Tb 3+ , Dy 3+ , and Ho 3+ cations into the type B structure results in proportional linear decrease in size of the REEO n polyhedra and unit-cell parameters, and leaves the silicate stereochemistry essentially unchanged.

Crystal structures of 2,2′-bipyridine adducts of two cadmium O-alkyl dithiocarbonates: rationalisation of disparate coordination geometries based on different crystal packing environments

Abstract: Two distinct coordination geometries are found in the structures of Cd(S 2 COR) 2 (2,2'-bipyridine). For R = CH 2 CH 2 OCH 3 (1) a N 2 S 2 donor set defines a distorted tetrahedral geometry for cadmium as the xanthate ligands adopt a monodentate coordination mode. By contrast, a N 2 S 3 donor set is found in the structure where R is butyl (2) as one of the xanthate ligands is chelating. An analysis of the crystal structures shows that both lattices feature extensive C-H...π interactions and that in (1) there are C-H...O interactions that are not present in (2). In (2) there are π...π interactions that are absent in (1). A qualitative argument based on crystal packing considerations is proposed to explain the differences in molecular structures.

On the discovery of new low temperature monoclinic phases with Cm and Cc space groups in Pb(Zr0.52Ti0.48)O3: an overview

Abstract: The decades old phase diagram of the technologically important Pb(Zr x Ti 1-x )O 3 (PZT) ceramics in the vicinity of the morphotropic phase boundary has been under review during the last few years This article summarises the results of the discoveries of two new monoclinic phases (FM HT ) and (FM LT ) with Cm and Cc space group by the Brookhaven group and our group, respectively, in the PZT ceramics

Crystal structure of new hexahydrate dicobalt pyrophosphate Co2P2O7·6H2O: comparison with Co2P2O72H2O, α-, β- and γ-Co2P2O7

Abstract: A new dicobalt pyrophosphate, Co 2 P 2 O 7 .6 H 2 O, has been synthesized and characterized by single crystal X-ray diffraction. The compound belongs to the monoclinic space group P2 1 /n, and its unit cell dimensions are: a = 7.2077(2), b = 18.3373(5), c = 7.6762(2) A, β = 92.4356(15)°, V = 1013.64(5) A 3 , Z = 4. The structural parameters were refined to an R index of 0.0570 for 1937 reflections with I > 2σ(I o ). The framework of this pyrophosphate is made by the packing of layers of discrete CoO 6 octahedra connected by double tetrahedra P 2 O 7 groups. The (P 2 O 7 ) 4- anion group shows bent eclipsed conformation. A comparative study among anhydrous (α-, β- and γ-Co 2 P 2 O 7 ) and hydrated forms (Co 2 P 2 O 7 . 2 H 2 O) of dicobalt pyrophosphates known in literature is presented.

The crystal structure of CaSiO3-walstromite, a special isomorph of wollastonite-II

Abstract: The crystal structure of CaSiO 3 -walstromite has been solved and refined from a data set collected in situ from an inclusion in a diamond single crystal (1135 observed reflections, MoK α , Nonius CAD4, R = 3.46%). The crystal structure is almost identical to that of walstromite (Ca 2 BaSi 3 O 9 ). The replacement of Ba 2+ by Ca 2+ causes no significant structural changes (ring silicate; according to F. Liebau: dreier single rings). The structural differences to wollastonite-II (Ca 3 Si 3 O 9 ) are also relatively small: The Ca positions are almost the same, but the axial Si 3 O 9 -ring is isomorphously replaced by the same ring with the inverse axiality. This constitutes a new principle of isomorphous replacement. Based on density functional theory the equation of state has been predicted. This allows us to estimate the remaining pressure of the investigated CaSiO 3 inclusion in the diamond to be 3 GPa.

Crystal structures and crystal chemical relationships of kröhnkite- and collinsite-type compounds Na2 Me 2+(XO4)2 · 2 H2O (X = S, Me = Mn, Cd; and X = Se, Me = Mn, Co, Ni, Zn, Cd) and K2Co(SeO4)2 · 2 H2O

Abstract: The crystal structures of eight compounds with compositions A 2 Me 2+ (XO 4 ) 2 . 2H 2 O (A = Na, X = S, Me 2+ = Mn, Cd; A = Na, X = Se, Me 2+ = Mn, Co, Ni, Zn, Cd; and A = K, X= Se, Me 2+ = Co) were investigated using single crystal X-ray CCD diffraction data. The Mn- and Cd-compounds crystallize in the monoclinic krohnkite structure type [Na 2 Cu(SO 4 ) 2 .2H 2 O, space group P2 1 /c], the Co-, Ni-, and Zn-selenates in the closely related triclinic structure type of collinsite [Ca 2 Mg(PO 4 ) 2 .2H 2 O, space group P1]. Both types are built up from infinite Me(XO 4 ) 2 (H 2 O) 2 chains, composed of MeO 4 (H 2 O) 2 octahedra alternating with each two XO 4 tetrahedra by sharing comers. A-O bonds and hydrogen bonds link the chains to layers and further on to the three-dimensional structures. The influence of the ionic radii of the various A, Me, and X cations on the distances between these structural units is discussed. Special attention is paid to polyhedral distortions as well as to individual peculiarities of the alkaline coordinations and the hydrogen bonding systems. Thus, the crystal chemical relationships between the two structure types as well as to isotypic copper compounds, which are characterized by the strong octahedral Jahn-Teller distortions, are elucidated.

High resolution neutron protein crystallography. Hydrogen and hydration in proteins

Abstract: Neutron diffraction provides an experimental method of directlylocating hy drogen atoms in proteins, and the development of the neutron imaging plate (NIP) became a breakthrough event in neutron protein crystallo- graphy. The general features of the NIP are reviewed. A high resolution neutron diffractometer dedicated to biolo- gical macromolecules (BIX-3) with the NIP has been con- structed at Japan Atomic EnergyResearch Institute and this has enabled 1.5 � resolution structural analyses of several proteins to be carried out. The specifications of BIX-3 and LADI (a quasi-Laue type diffractometer in- stalled in the Institut Laue-Langevin) are compared. The crystal structures of myoglobin, wild type rubredoxin and a mutant of rubredoxin have been carried out using BIX-3. From these studies, several topics, such as the location of hydrogen bonds and certain acidic hydrogen atoms, the identification of methyl hydrogen atoms, details of H/D exchange and dynamical behavior of hydration structures have been investigated, and important information has been extracted from the structural results. Finally, a sys- tematic procedure to grow large single crystals of proteins or nucleic acids is described.

The determination of anharmonic probability densities from static and dynamic disorder by neutron powder diffraction

Abstract: Atomic probability densities as derived from a careful analysis of the Debye-Waller factors contain important information about the average static and dynamic disorder in crystals. Although the inclusion of anharmonic terms often requires single crystal data, in favourable cases, in particular at high temperatures, it is possible or even necessary to rely on neutron powder experiments too. This is illustrated by several examples including the anharmonic motion of the atoms in ZnS, the anionic diffusion and the static disorder in zirconia based super-ionic conductors, and the critical behaviour at the structural phase transition in PbTiO 3 . Anharmonic and split models are compared on the example of Zr-Mg-O-N.

Crystal chemistry of lead oxide phosphates: crystal structures of Pb4O(PO4)2, Pb8O5(PO4)2 and Pb10(PO4)6O

Abstract: Crystals of three Pb oxide phosphates, Pb 4 O(PO 4 ) 2 , Pb 8 O 5 (PO 4 ) 2 and Pb 10 (PO 4 ) 6 O, were synthesized from mixtures of PbO and NH 4 H 2 PO 4 heated to 950 °C. The crystal structure of Pb 4 O(PO 4 ) 2 (monoclinic, P2 1 /c, a = 9.4894(8), b = 7.1402(6), c = 14.4077(13) A, β = 104.549(2)°, V = 944.91(14) A 3 ) was solved by direct methods and refined to R1 = 0.044 (wR = 0.103). It contains isolated OPb 4 tetrahedra that are linked through Pb-O bonds to O atoms of PO 4 tetrahedra. The crystal structure of Pb 8 O 5 (PO 4 ) 2 (monoclinic, C2/m, a = 10.6427(4), b = 10.2078(4), c = 14.3530(6) A, β = 98.325(1)°, V = 1542.86(11)A 3 ) was solved by direct methods and refined to R1 = 0.032 (wR = 0.068). It is based upon a complex sheet with composition [O 5 Pb 8 ]. The basic element of the sheet is a cluster of composition [O 5 Pb 10 ] that consists of four OPb 4 tetrahedra that share edges. The adjacent [O 5 Pb 10 ] clusters are linked by sharing Pb atoms, resulting in sheets that are parallel to (001). The PO 4 tetrahedra are located in the interlayers between the sheets. The crystal structure of Pb 10 (PO 4 ) 6 O (hexagonal, P6 3 /m, a = 9.8650(3), c = 7.4306(3) A, V = 626.25(4) A 3 ) has been solved by direct methods and refined to R1 = 0.040 (wR = 0.106). Pb 10 (PO 4 ) 6 O crystallizes with an apatite-type structure. The structure contains a single O atom that is not part of a PO 4 tetrahedron; it has a site occupancy factor of 0.25 and is located on the 6 3 axis.

Visual computing in electron crystallography

Abstract: A simple image contrast theory in high-resolution electron microscopy (HREM) is introduced to demonstrate that below a certain critical crystal thickness the intensity of the Scherzer focus image is linear to the projected potential of an artificial crystal that is isomorphic to the examined one. It has become the theoretical base of electron crystallographic image processing techniques relying on the weak-phase-object approximation and kinematical diffraction. Two techniques of image processing are introduced. One of them aims at determining crystal structures by combining electron diffraction data and applying diffraction analysis methods. To reduce various kinds of electron diffraction intensity distortion the diffraction data are corrected by means of an empirical method set up by referring to the heavy atom method and Wilson statistic. The other one aims at revealing crystal defects at atomic resolution from the image taken with a medium-voltage field-emission high-resolution electron microscope. The dynamical effect is corrected by forcing the integral amplitudes of reflections in the diffractogram, of image equal to the amplitudes of corresponding structure factors for the perfect crystal. The principle of the two techniques is briefly introduced, and examples of applications to crystal structure and defect determination are given.

Geminography - The science of twinning applied to the early-stage derivation of non-merohedric twin laws

Abstract: The recognition of twinning and the determi- nation of the twin laws are nowadays often entrusted to "black-box" software packages. This approach may be bothrisky and uneconomical. It is risky because never a procedure should be blindly applied. It is uneconomical if procrastinates the analysis of twinning at the very late stage of structure solution and refinement, whereas in many cases this analysis can and should be applied much earlier. In this paper we present a brief survey of the working strategies to be applied to the structural study of twinned crystals, emphasizing the role of the crystallogra- pher, rather than that of the machine.

Electron crystallography of zeolites. 1. Projected crystal structures of ZSM-5 and ZSM-11

Abstract: The prospect of carrying out quantitative crystal structure analyses by direct methods with electron diffraction intensity data from zeolites is evaluated for two related materials: ZSM-5 and ZSM-11. The stacked platelike arrays of ZSM-5 induce intensity perturbations from secondary electron scattering; nevertheless, the T-site positions can be found by direct methods. Intensity data from smaller ZSM-11 microcrystals are more favorable for ab initio analysis so that some oxygen positions are found during Fourier refinement. For intergrowths of one zeolite in the other, the nature of the inclusions has been found to depend on the dominant species. If ZSM-5 is dominant, the apparent ZSM-11 inclusions (assuming a preferred platy orientation) could be rather large, whereas if ZSM-11 is dominant, the ZSM-5 inclusions extend over only 3 or 4 unit cells.

Anomalous thermal expansion behaviour of Na8[AlSiO4]6(NO3)2-sodalite: P4̅3n to Pm3̅n phase transition by untilting and contraction of TO4 units

Abstract: X-ray single crystal, X-ray powder and specific heat investigations were carried out on alumosilicate NaNO 3 -sodalite between 290 K and 1100 K. Single crystal structure determinations at 295 K, 505 K and 720 K reveal the sodalite structure in its partially collapsed form in space group P43n whereas at 935 K the z coordinate of the framework oxygen is shifted to z = 0.5 according to an untilted sodalite framework in Pm3n. The sodium ions are located on 8e (x, x, x) position in P43n and become statistically distributed on 16i (x, x, x) in Pm3n. The displacements of the nitrate atoms are extraordinarily enlarged at 935 K compared to the values at the lower temperatures. Analysis of the X-ray powder data show a discontinuity in the thermal expansion behaviour at T c = 930 K with the temperature dependence of the lattice parameter described by a calc = a 1 - A(T c - T) 0.5 , a 1 = BT + a 0 (A = 0.57 pm K -0.5 , B = 5.84 . 10 3 pm K -0.5 , a 0 = 909.9 pm, T in K) which is related to a square root decrease of the volume strain. The anomalous thermal expansion compromises a gradual contraction of TO 4 units and the disappearence of structural tilt with increasing temperature. The refined data reveal a change of z coordinate of O1 proportional to (T c - T) 1/4 in P43n which is considered as an effective order parameter in a Landau tricritical description of the phase transition triggered by the displacement of the cage filling ions.

Analytic expressions for the vertex coordinates of quasiperiodic lattices

Abstract: Using the generalized dual method, closed analytical expressions for the coordinates of quasiperiodic lattices, derived from periodic or quasiperiodic grids, are given. The obtained formulae constitute a useful and prac- tical tool to generate and perform calculations in quasi- periodic structures.