J.S. Anderson

OSTEOGENESIS IMPERFECTA IS LINKED TO BOTH TYPE I COLLAGEN STRUCTURAL GENES

The segregation of the two type I collagen structural gene loci COL1A1 and COL1A2 was analysed in eleven osteogenesis imperfecta pedigrees by means of restriction-site variants at, or close to, these loci. In each case, the OI gene was inherited with one or other collagen locus. As well as identifying the common OI loci the result of this analysis sets limits on the frequency of a third locus and lays the foundation for a widely available antenatal diagnostic test.

Electron microscopy of ferroelectric bismuth oxides containing perovskite layers

Ferroelectric bismuth oxides of the general formula (Bi2O2)2+(An-1BnO3n+1)2- (A = Ba, etc., B = Ti or other transition metal) have been examined by high-resolution lattice imaging electron microscopy. The lattice images show dark bands at the positions of the Bi20 2 layers, with n -1 lines between them due to the layers of the perovskite A cations or, in favourable circumstances, the fully resolved 0.4 nm square perovskite grid. Dislocations and domain boundaries have been imaged for the first time in ferroelectric crystals. The structure of the dislocations and domain walls is discussed in the light of the microstructural evidence.

Electron microscopy of the niobium oxides. I. Twinning and defects in HNb2O5

Two dimensional direct lattice imaging has been used to identify the detailed structure of faults in stoichiometric HNb2O5 crystals grown under different conditions. Material grown at 1000°C showed occasional twinning on (101); only one of two possible twin structures was identified. Material grown at lower temperatures frequently incorporated “wrong sized” blocks, equivalent to inserted lamellae of M- or NNb2O5. Heavily faulted twinned and intergrown structures are occasionally formed, and in such a regularly and recurrently twinned variant of the HNb2O5 structure was identified as a microdomain.

The Oxidation of Nb

The two forms-monoclinic and orthorhombic - of the mixed valence Nb (IV, V) oxide Nb12O29 oxidize at relatively low temperatures to two new modifications of Nb2O5. These, like the starting materials, are of the ‘block’ structure type, derived from the DO9 structure by crystallographic shear. Electron microscopy and electron diffraction show that the first distinct stage after complete oxidation is the formation of a super-lattice structure derived from the structure of the original Nb12O29; this undergoes internal reorganization leading, in the case of the monoclinic oxide, to the formation of a new modification of niobium pentoxide, Nb10O25, with a (3 x 3)1 block structure. By using high resolution lattice imaging methods, the course of the initial oxidation step, the formation of the superlattice and its subsequent rearrangement have been traced in considerable detail. This low temperature oxidation process, which takes place with measurable speed at temperatures as low as 110 °C, clearly turns upon the diffusion of oxygen through the open channels of the DO9 structure, and permits of a homogeneous solid state reaction involving only single unit jumps of atoms from original lattice sites into interstitial positions. At 440 °C and above, a second oxidation mechanism becomes competitive, involving reaction and rearrangement at the surface of the crystals. A reaction front then passes through the crystal by the co-operative diffusion processes that are involved in the migration of crystallographic shear planes.

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Abstract Reduction of the titanium-niobium oxides follows a common pattern. TiO2 is eliminated, to form a new phase richer in titanium than the original compound, and Nb(iv) replaces Ti(iv) in the original block structure, which is thereby enriched in niobium. With TiNb2O7, the second phase is a TiO2NbO2 solid solution, with the rutile structure, initially with a high titanium content, in equilibrium with a solid solution of composition Me3O7, isostructural with TiNb2O7. At log pO2 (atm) about −9.0 this reaches the limiting composition Ti0.72Nb2.28O7, in equilibrium with Ti0.56Nb0.44O2. The Me3O7 block structure then transforms into the Me12O29 block structure (Ti2Nb10O29Nb12O29 solid solution), which rapidly increases in niobium content as reduction continues. Reduction of Ti2Nb10O29 at oxygen fugacities above log pO2 (atm) = −9.0 forms the Me3O7 phase as the titanium-rich phase. At log pO2 = −9.0, and a composition about Ti1.6Nb10.4O29, the rutile solid solution takes over as second phase. The niobium/titanium ratio in both phases rises as reduction proceeds, and the last vestiges of the Me12O29 phase, in equilibrium with the final product, Ti0.17Nb0.67O2, are almost denuded of titanium.

O

Abstract Mixtures of MgF2·xNb2O5 with x = 7 and 14 have, on annealing at high temperatures yielded a wide variety of block structures in which MgF2 has replaced MeO2 (Me = Ti4+, Nb4+). With F− stabilizing the formation of large blocks, numerous defect structures have been characterized by direct lattice resolution electron microscopy. Features thus observed include planar faults, intergrowths, segregated domain structures, and disordered crystals. Chemical implications of these features are discussed.

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Lattice images of the niobium oxides, structures based on the linkage of octahedral groups in continuous networks, occasionally contain features recognizable as dislocations. Since lattice imaging enables the micro-structure to be resolved in great detail, at the level of local structural organization, it is possible to determine the configuration, and also to infer the chemical composition, of dislocated areas. By treating the niobium oxide ‘block’ structures as superstructures of the ReO3 (DO9) type, the topology of dislocations can be expressed by relations between the insertion (or deletion) of one or more half-planes of cations, or of oxygen atoms only, changes in the number of crystallographic shear plane interfaces between blocks or columns, changes in (idealized) dimensions and any requisite distortion in the third dimension. Mapping the structure around a dislocation, from the lattice image, is directly equivalent to plotting the Burgers’ circuit. In this way, the precise nature of a dislocating perturbation and its implications for the local chemical composition of the crystal can be directly identified. The method is exemplified by analysis of dislocations and of related extended defects of several types, associated with twinning phenomena, semicoherent intergrowth between different ReO3-type superstructures and arrays building up a low angle boundary. The essential features of the analysis are not restricted to structures of the niobium oxide type, but can be extended to other types of polyhedron networks.

: Electron Microscopy of a Solid-State Reaction

Abstract Lattice imaging electron microscopy has been used to study the mechanism of solid state reactions of the type: A s → B s + C s , in which the product B is able to intergrow coherently with the starting material A , but the product C cannot do so. C must be formed by a fully reconstructive, heterogeneous process; formation of B is only partially reconstructive, and essentially homogeneous. Reactions were the reversible phase reactions in the system Nb 2 O 5 WO 3 : disproportionation of the (5 × 4) 1 block structures of 8Nb 2 O 5· 5WO 3 , to form (4 × 4) 1 blocks of 7Nb 2 O 5· 3WO 3 as coherent product, and that of 9Nb 2 O 5· 8WO 3 (with (5 × 5) 1 blocks), forming (5 × 4) 1 blocks of 8Nb 2 O 5· 5WO 3 as coherent product. The coherent product structure is formed in isolated rows of blocks, or small packets of such rows, running across each crystal. The reaction does not work in progressively from some surface initiating step, with an interface between unchanged and converted material, but represents a block-by-block conversion, linearly propagated. Nb 2 O 5 and WO 3 must be abstracted, in appropriate stoichiometric ratio, from each block but must ultimately reach and react at the surface, to form the incoherent product (a pentagonal tunnel network structure, in both cases). Some homogeneous transport process involving lattice diffusion must be invoked. Domains of highly anomalous structure, regarded as relicts of transient conditions, are occasionally observed. From reactions at relatively low temperatures, these have structures that can be regarded as partially ordered nonstoichiometric solid solutions; after prolonged heating, and at higher temperatures they form well ordered strips of metastable block structures. Both types represent strong, spontaneous fluctuations of composition, which impose a corresponding structure locally. These fluctuations may be associated with the transport of WO 3 and Nb 2 O 5 away from the locus of reaction. Evidence about the mechanism of the reactions, the role of dislocations and the nature of cooperative processes is considered.

Reduction of the titanium niobium oxides. I. TiNb2O7 and Ti2Nb10O29

Abstract Interpretation of the reduction path of TiNb 24 O 62 is complicated by uncertainty about both the stoichiometric ranges of the possible block structures and the formation of TiNb solid solutions. Reduction forms the Me 12 O 29 phase, probably from the outset, with an initial composition close to Ti 2 Nb 10 O 29 , thereby rapidly depleting the Me 25 O 62 phase of titanium. When log p O 2 (atm) has dropped to −9.62, a phase approximately Ti 0.95 Nb 11.05 O 29 is in equilibrium with titanium-free Nb 25 O 62 at its lower composition limit (NbO 2.471 ). Nb 25 O 62 is then reduced to Nb 47 O 116 without change in the Me 12 O 29 . At −9.62>log p O 2 (atm) > −10.0, niobium is transferred to the Me 12 O 29 phase and Nb 47 O 116 is consumed. A second univariant equilibrium is set up as Nb 47 O 116 is reduced to Nb 22 O 54 . This is consumed in turn, to increase the niobium content of the Me 12 O 29 until, at log p O 2 close to −10.8, monophasic Ti 0.48 Nb 11.52 O 29 is formed. The (Ti,Nb)O 2 solid solution then appears and the final product is Ti 0.04 Nb 0.96 O 2 , with the rutile superstructure cell reported for NbO 2 .

Multiple phases in the system MgF2Nb2O5 an electron microscope study of intergrowths, defects, and disordered crystals

The high-resolution electron microscopic technique of lattice imaging shows 15 and 3 AA repeat sequences in 6H SiC. The 15 AA sequence is further resolved into 2.5 AA layers corresponding to the interlayer separation.