MICROSTRUCTURE AND MECHANICAL PROPERTIES OF IN-SITU FORMATION FIBROUS POLYTYPE AlN COMPOSITE-MATERIAL

In-situ formation fibrous polytype AlN composite ceramic materials was prepared from AlN-Y_2O_3-SiO_2 system.In comparison with AlN ceramics,both bending strength and fracture toughness of the composite materials are much bettered.Microstructural observation revealed that a lot of epitaxial growth fibrous AlN polytype occurred in matrix.Y_2O_3 seems to act as a densifier for in-situ formation materials and as medium for growth of fibrous polytype.SiO_2 is the growth promoter for fibrous polytype.The occurrnce of fibrous polytype may increase the strength and toughness of AlN ceramic composite mateirals.

4H-POLYTYPE Sn-HGBOMITE

Ⅰ. INTRODUCTION H(?)gbomite, a titanium-bearing oxide mineral, is rare in nature, In structure, it is similar to spinel, especially to nigerite and taaffeite. A. S. Povarenneih includes nigerite, taaffeite and h(?)gbomite in the same group as an isostrueture. After it was first reported by Gavellin in 1917, h(?)gbomite has been found in Tanzania, Kamailong and Zam-

Two new long-period hexagonal silicon carbide polytypes 240H and 294H

Polytypism appears as one of the most striking features of structural chemistry and crystallography. A lot of close-packed or layered inorganic compounds, phyllosilicates and minerals, such as SiC, ZnS, CdI2, PbI2, chalcogenides, perovskites, clays and mica exhibit extensive polytypism and are able to admit many numbers of structural modifications or polytypes. The periodicity of the stacking of some polytypes may range up to thousands of Angstrm. The reason of the appearance of such a huge periodicity is one of the interesting questions generated by the polytypsim. Silicon carbide is a most typical substance of polytypism. Up to the present more than 157 polytypes of silicon carbide are discovered, in which we have discovered 84 new polytypes of SiC.

A Comparison Study of Synkinematic Illite Isolation,Quantitative X-ray Powder Diffraction,and K-Ar Dating for Direct Fault Gouge Analyses

K-Ar dating of synkinematic illite is increasingly recognized as a central method to constrain the timing of shallow crustal faulting.Methods of efficient sample preparation and quantitative identification of illite polytypes are critical to acquiring K-Ar isotope data for authigenic clays.In this respect,we compared the commonly used clay size separation method through centrifugation with vacuum filtration technology,showing that the former is prone to extract fractions with finer particle sizes under similar conditions,thus improving the error in the authigenic end-member age.Additionally,we demonstrated that the side-packed mounting method for X-ray diffraction analysis can significantly enhance the randomness in powder samples,thus improving the quantification accuracy compared with the front-packed and back-packed methods.The validity of our quantification method was confirmed by comparing Profex■modeling patterns with a suite of synthetic mixtures of known compositions,yielding an average analytical error of 3%.Dating results of these artificial mixtures and the reference materials indicated that a large range in percentages of detrital illite and a sufficient amount of age data will produce reliable results for ages of both extrapolated end-members.However,if the range is limited,the extrapolated age close to those of datasets is still reliable.

K-Ar Dating of Fault Gouges from the Red River Fault Zone of Vietnam

Constraining the timing of fault zone formation is fundamentally important in terms of geotectonics to understand structural evolution and brittle fault processes.This paper presents the first authigenic illite K-Ar age data from fault gouge samples collected from the Red River Shear Zone at Lao Cai province,Vietnam.The fault gouge samples were separated into three grain-size fractions（〈0.1 μm,0.1-0.4 μm and 0.4-1.0 μm）.The results show that the K-Ar age values decrease from coarser to finer grain fractions（24.1 to 19.2 Ma）,suggesting enrichment in finer fraction of morerecently grown authigenic illites.The timing of the fault movement are the lower intercept ages at 0%detrital illite（19.2 ± 0.92 Ma and 19.4 ± 0.49 Ma）.In combination with previous geochronological data,this result indicates that the metamorphism of the Day Nui Con Voi（DNCV） metamorphic complex took place before ca.26.8 Ma.At about 26.8 Ma-25 Ma,the fault strongly acted to cause the rapid exhumation of the rocks along the Red River-Ailoa Shan Fault Zone（RR-ASFZ）.During brittle deformation,the DNCV slowly uplifted,implying weak movement of the fault.This brittle deformation might have lasted for ca.5 Ma.

Crystal Chemistry Research of Minerals of Nigerite Group and Its Significance

There is a type of complex oxide mineral, composed of many elements such as Sn, Mg, Fe, Zn, Ti, Mn, Al, etc., in the areas of Anhua, Linwu and Shizhuyuan of Hunan Province. These minerals belong to nigerite, Mg-nigerite (pengzhizhongite), etc.. According to the principle of closest packing, the crystal chemical properties of nigerite (brown), pengzhizhongite (buff), Zn-nigerite (fawn), Fe-nigerite, Mn-nigerite (greenish-black) etc. have been analyzed. Their crystal structures may be characterized by O (the layers of cation octahedral coordinations), T_1 (the mixed layers of cation octahedral coordinations and cation tetrahedral coordinations in same directions), T_2 (the mixed layers of cation octahedral coordinations and cation tetrahedral coordination in different directions). The position of layer-O and layer-T is alternate permutation. The crystal structure of pengzhizhongite (6 H ) may be expressed by …OT_2OT_1OT_1…, taaffeite (8 H ), …OT_2OT_1OT_2OT_1…, and nigerite (24 R ), … OT_1OT_2 OT_2OT_1 …×3 etc.. In their structure, there are not only the crystal structure units of spinel … OT_2 OT_2 … but also the units of nolanite … OT_1 OT_1 …. The research of these minerals has important theoretic and practical significance in the fields of minerals, gemology, material science etc..

Optical Characterization of 4H-,6H-and 15R-SiC Crystals

The optical absorption of 4H-, 6H- and 15R-SiC single crystals has been measured at room temperature. The band gaps were calculated, and the reasons for band gap shrinking were discussed. Influence of free carder concentration was considered. The fast- and second-order Raman spectra of 4H-, 6H- and 15R-SiC samples were analyzed. Raman spectra of disorder structure in 6H-SiC grown by Lely method were given and simulated. The low wave-number Raman spectrum is a reliable method to distinguish the SiC polytypes. We analyzed the similarity of the second-order Raman spectra of all polytypes.

Investigation of the 6H-SiC (0001) surface by AFM

Micropipe and step structures on 6H-SiC （0001） surface were investigated by an atomic force microscopy （AFM）. On the facet,all micropipes examined are the origins of spiral steps,indicating that dislocations intersect the surface at these points. Micropipes are empty-core super-dislocations as originally described by Frank. The micropipe radius increases with the square of the dislocation Burgers vector. From the center to the periphery,step structures change with different surface inclinations. Regular step is observed within the central faceted area. Step bunching and atomically rough surfaces are observed within the peripheral convex area. If the inclination with respect to the （0001） plane is large enough,step bunching of 15R-SiC can be observed.

Domain Structure in Calcium Cerium Fluorocarbonate Minerals, Mianning County, Sichuan Province, China

The domain structures in calcium-cerium fluorocarbonate mineral series from a rare earth mineral deposit within an aegirine alkali granite massif in Mianning County, Sichuan Province, China, were studied by means of transmission electron microscopy. Many categories of domain structures in this mineral series were observed and investigated, including the microtwin domains of parisite-6R(2) and B2S-6R, the antiphase domains of B2S-2H. The results show that the antiphase domains which are faults along crystal plane (0001) are formed by the displacement of crystal lattice in c * direction and the displacement is 0.471 nm. A new polytype (B4S2-3R) of regular mixed-layer structure with B4S2 type is found in the domain region of mixed-layer structure.

Raman Scattering Detection of Stacking Faults in Free-Standing Cubic-SiC Epilayer

We report on stacking fault （SF） detection in free-standing cubic-SiC epilayer by the Raman measurements. The epilayer with enhanced SFs is heteroepitaxially grown by low pressure chemical vapour deposition on a Si（100） substrate and is released in KOH solution by micromechanical manufacture, on which the Raman measurements are performed in a back scattering geometry. The TO line of the Raman spectra is considerably broadened and distorted. We discuss the influence of SFs on the intensity profiles of TO mode by comparing our experimental data with the simulated results based on the Raman bond polarizability （BP） model in the framework of linearchain concept. Cood agreement with respect to the linewidth and disorder-induced peak shift is found by assuming the mean distance of the SFs to be 11 A in the BP model.

Molybdenite as a Rhenium Carrier:First Results of a Spectroscopic Approach Using Synchrotron Radiation

The chemical and physical properties of rhenium render it a highly demanded metal for advanced applications in important industrial fields. This very scarce element occurs mainly in ores of porphyry copper-molybdenum deposits associated with the mineral molybdenite, MoS2, but it has also been found in granite pegmatites and quartz veins as well as in volcanic gases. Molybdenite is a typical polytype mineral which crystal structure is based on the stacking of [S-Mo-S] with molybdenum in prismatic coordination by sulphide anions;however, it is not yet clearly established if rhenium ions replace Mo4+ cations in a disordered way or else, if such replacement gives rise to dispersed nanodomains of a rhenium-rich phase. As a contribution to clarify this question, an X-ray absorption spectroscopy (XANES) study using synchrotron radiation was performed at the Re L3-edge of rhenium-containing molybdenite samples. Obtained results are described and discussed supporting the generally accepted structural perspective that rhenium is mainly carried by molybdenite through the isomorphous replacement of Mo, rather than by the formation of dispersed Re-specific nanophase(s).

Anchimetamorphism of the Neoproterozoic and the Lower Paleozoic along the Profile of Yuanguping in Western Hunan Province, China

The Neoproterozoic and Lower Paleozoic along the profile of Yuanguping in western Hunan Province, China underwent anchimetamorphism. The illite crystallinity (IC) of the <2 μm fractions ranges from 0.23-0.34°△2θfor the Neoproterozoic to 0.23-0.35°△A2θ for the Lower Paleozoic (calibrated with the Kisch IC set, Kisch, 1991). This indicates that the metamorphic grade of the Neoproterozoic and Lower Paleozoic is the anchizone. The peak metamorphic temperature is estimated to be 290-210℃. This result does not agree with the greenschist or subgreenschist facies of the Banxi Group, nor with the lower-greenschist facies or sedimentary cover of the Sinian to Lower Paleozoic, as most previous researchers thought. The illite (K-mica) b0 values range from 0.9074 to 0.8963 (nm) for the Neoproterozoic and the Lower Paleozoic. Based on cumulative frequency curves of the illite (K-mica) b0, the peak metamorphic pressure of the Banxi Group was derived to be of a type that is slightly higher than that of the N. New Hampshire low-intermediate pressure type. Most illites occur as the 2M1 polytype and some in the Neoproterozoic as a mixture of the 2M1+1M types. The distributions of coherent scanning domains (CSDs) of illites along the profile, measured with the XRD method, display a lognormal model and spread out with decreasing Kubler Index of IC. It indicates that illites underwent an Ostwald ripening. The post-anchimetamorphic structural movement not only results in a series of faults but also induces the lattice strain in minerals along the faults and hence impacts the illite crystallinity and causes diagenetic samples to occur within the anchizone. Compared with the cases in eastern and central Hunan Province, the Neoproterozoic and Lower Paleozoic rocks in the west underwent a lower-temperature anchimetamorphism with a pressure lower than that in the east and higher than that in the center of Hunan Province.

Lead Iodide Crystals as Input Material for Radiation Detectors

Lead iodide is an important inorganic solid for fundamental research and possible technological applications and is considered to be a potential room temperature nuclear radiation detector. In lead iodide the phenomenon of polytypism is posing an interesting problem of phase transformations amongst its various polytypic modifications. The transformations have also been observed even when the crystals are stored for few months. It causes deterioration in functioning of PbI2 devices. Taking into account the known structures of PbI2 and the data available on the mode of growth and storage of crystals, it has been concluded that purified melt grown crystals of PbI2 are the best suited for nuclear radiation detectors.

Structural Characteristic of AREI_3 Compounds

The structure of AREI_3 is reported in this paper.Their crystal structures,which include AREI_3(A=K,Rb,Cs;RE=Sm,Eu,Yb,Tm) were determined by X-ray powder diffraction method,Gui- nier technique was used.CsEuI_3 was investigated by electron microscopy to confirm the polytype of AREI_3(SrCeO_3-type,Pmmm) observed in the X-ray diffraction experiments. The values as a function of structure type show that the structure of AREI_3 are different from that of other halides of AREX_3(X=F,Cl,Br)

Very low-grade metamorphism of the Meso-Neoproterozoic and the Lower Paleozoic along the profile from Huangtudian to Xianxi in the central-northern part of Hunan Province,China

The Lengjiaxi and Banxi groups of the Meso-Neoproterozoic and the Lower Paleozoic along the profile at Huangtudian-Xianxi of the central-northern Hunan Province, China underwent very low-grade metamorphism. Illite crystallinity (IC) data of the <2 mm fractions range from 0.18—0.21癉2q for the Lengjiaxi Group to 0.19—0.23癉2q for the Banxi Group to 0.20—0.29癉2q for the Sinian to the Lower Paleozoic (K黚ler index, corrected with Kisch IC set, 1991). This indicates that the metamorphic grade of the Lengjiaxi Group is in the epizone, the Banxi Group in the epizone-high anchizone, and the Sinian to the Lower Paleozoic mostly in the high anchizone along the profile investigated. The peak metamorphic temperature evaluated is roughly 340—240℃. This result does not agree with the greenschist or subgreenschist facies of the Lengjiaxi and Banxi groups and neither with the lower-greenschist facies or diagenitic zone of the Sinian to the Lower Paleozoic, of which most previous researchers thought. The illites (K-mica) b0 values range from 0.8989—0.9050 nm for the Lengjiaxi Group to 0.8984—0.9037 nm for the Banxi Group to the Lower Paleozoic. Based on the cumulative frequency curves of illite (K-mica) b0, the peak metamorphic pressure of the Banxi Group to the Lower Paleozoic along the profile was derived in a low-intermediate pressure type. Illites occur as 2M1 polytype in the Lengjiaxi and Banxi groups and as 2M1+1M types in some samples of the Lower Paleozoic. This indicates that the transformation of illite polytype from 1M to 2M1 was fulfilled within the high anchizone.

Kinetically-Induced Hexagonality in Chemically Grown Silicon Nanowires

Various silicon crystal structures with different atomic arrangements from that of diamond have been observed in chemically synthesized nanowires.The structures are typified by mixed stacking mismatches of closely packed Si dimers.Instead of viewing them as defects,we define the concept of hexagonality and describe these structures as Si polymorphs.The small transverse dimensions of a nanowire make this approach meaningful.Unique among the polymorphs are cubic symmetry diamond and hexagonal symmetry wurtzite structures.Electron diffraction studies conducted with Au as an internal reference unambiguously confirm the existence of the hexagonal symmetry Si nanowires.Cohesive energy calculations suggest that the wurtzite polymorph is the least stable and the diamond polymorph is the most stable.Cohesive energies of intermediate polymorphs follow a linear trend with respect to their structural hexagonality.We identify the driving force in the polymorph formations as the growth kinetics.Fast longitudinal elongation during the growth freezes stacking mismatches and thus leads to a variety of Si polymorphs.The results are expected to shed new light on the importance of growth kinetics in nanomaterial syntheses and may open up ways to produce structures that are uncommon in bulk materials.

Origin of clay minerals on section of Luochuan loesspalaeosol in Shaanxi Province,northwest China

Crystallinity,polytype,and morphology of clay minerals in the Luochuan loess-palaeosols in Shaanxi Province,northwest China were studied in order to have an insight into their origin using X-ray diffraction(XRD)analysis and scanning electron microscopy(SEM)methods.The SEM observations showed that the morphology of some illites seems to be lath-shaped crystals.An analysis of illite crystallinity(IC)on the Luochuan section indicated that the origin of illite was related to the rocks of an anchizone.Most illite in the Luochuan loess-palaeosol section were of 2M1 polytype,but some were of 1M polytype formed by weathering of feldspar in the process of pedogenesis.Illite in the Luochuan section has undergone both physical and chemical weathering.These results revealed that most illite were of detrital origin related to the source area of an anchizone,but parts of the illite were of an authigenic origin formed during pedogenesis after sedimentation.Chlorites in the samples of the Luochuan section were mainly composed of irregular flaky grains and their crystallinity was good.These showed that chlorite had the detrital origin formed by physical weathering.Kaolinite crystallinity was relatively good.The value of CIA on the Luochuan section ranges from 61.9 to 69,and therefore kaolinite could not be formed during weathering and pedogenesis.These results indicated that the kaolinite had a detrital origin.Morphology of smectite seems to be capillaceous.The XRD patterns of all samples contained diffraction peaks at 1.5218Å(nontronite)and 1.5052Å(montmorillonite),thus indicating an intermediate composition between trioctahedral and dioctahedral smectite.The smectite crystallinity was very poor.These results revealed that smectite in the study area was authigenic rather than detrital origin.

Anomalous Resistivity in Vanadium-Doped Semi-Insulating 4H-SiC Wafers

The resistivities of vanadium-doped semi-insulating 4H-SiC wafers were measured by a contactless resistivity measurement system. Anomalous resistivity was found in semi-insulating 4H-SiC wafer. Raman spectra of semi-insulating4H-SiC wafer indicated that the anomalous resistivity was caused by polytype inclusion. Based on the activation energies of different SiC polytypes calculated from resistivity versus temperature data measured by COREMA-VT, the resistivities in the vanadium-doped semi-insulating 4H-SiC wafer with 6H polytype inclusion were calculated. The calculated resistivities are quite consistent with the measured resistivities. Furthermore, the compensation mechanism for the formation of anomalous resistivity was proposed.

Engineering crystal phase of polytypic CuInS2 nanosheets for enhanced photocatalytic and photoelectrochemical performance

Crystal phase engineering on CulnS2(CIS)nanocrystals,especially polytypic structure,has become one of the research hotspots to design the advanced materials and devices for energy conversion and environment treatment.Here,the polytypic CIS nanosheets(NSs)including zincblende/wutzite and chalcopyrite/wurtzite types were first time achieved in a hot-injection system using oleic acid and liquid paraffin as the reaction media.As-obtained polytypic CIS NSs exhibit significantly enhanced light-absorption abillty and visible-light-driven photocatalytic performance originating from the rational hetero-crystalline interfaces and surface defect states,which efficiently inhibit the recombination of photo-generated carriers.Meanwhile,the polytypic CIS NSs were spin-coated onto the surface of fluorinated-tin oxide glass substrate and used as the photoelectrode,which shows an excellent photoelectrochemical(PEC)activity in aqueous solution.The present work not only provides a facile,rapid,low-cost,and environmental-friendly synthesis strategy to design the crystal phase and defect structure of ternary chalcogenides,but also demonstrates the relationships between the polytypic structure and photocatalytic/photoelectrochemical properties.

Induced structural modifications in ZnS nanowires via physical state of catalyst:Highlights of 15R crystal phase

Peculiar and unique growth mechanisms involved in semiconductor nanowires(NWs)pave the way to the achievement of new crystallographic phases and remarkable material properties,and hence,studying polytypism in semiconductor NWs arouses a strong interest for the next generation of electronic and photonic applications.In this context,the growth of ZnS nanowires has been investigated,as bulk ZnS compound exhibits numerous unstable polytypes at high temperatures,but their stable occurrence is highly anticipated in a nanowire due to its special quasi-dimensional shape and growth modes.In this work,the idea is to provide a change in the growth mechanism via the physical state of catalyst droplet(liquid or solid)and hence,study the induced structural modifications in ZnS nanowires.The HRTEM images of VLS(via liquid alloyed catalyst)grown ZnS NWs show periodic stacking faults,which is precisely identified as a stacking sequence of cubic or hexagonal individual planes leading to an astonishing 15R crystal polymorph.This crystallographic phase is observed for the first time in nanowires.Contrastingly,NWs grown with VSS(via solid catalyst)show crystal polytypes of zinc blende and wurtzite.We calculate and discuss the role of cohesive energies in the formation of such ZnS polytypes.Further,we present the selection rules for the crystallization of such 15R structure in NWs and discuss the involved VLS and VSS growth mechanisms leading to the formation of different crystal phases.