Selected Area Electron Diffraction Data Research Articles

The crystalline structure of ultrathin gold granular films

Researchers generally imagine a granular metal film as being constituted by a single type of stable spherical shaped particles with a mean diameter d . Indeed, even the most homogeneous films are made up by particles of various diameters d with different shapes and with stable or unstable crystalline structure. Recently, it has been shown that gold particles of diameter of 2 nm are in a “quasi-solid” state. In this paper the authors show the experimental results we obtained by selected area electron diffraction (SAED) from ultrathin ( d<3 nm ) gold films. The authors show that with gold particles of mean diameter less than 3 nm and electrons of 250keV, the dynamic effects can be neglected and the kinematic theory can be used in the data analysis. The authors conclude that, for films with d > 1.5 nm , SAED data can be well interpreted by means of a simple phenomenological model of the particles taking into account the effect on the diffraction patterns due to the particle size distribution. On the contrary, SAED data obtained from films of d<1.5 nm are in disagreement with the same model suggesting the occurrence in these films of “quasi-solid” particles.

Tin in a chondritic interplanetary dust particle

Abstract— Submicron platey Sn‐rich grains are present in chondritic porous interplanetary dust particle (IDP) W7029*A and it is the second occurrence of a tin mineral in a stratospheric micrometeorite. Selected Area Electron Diffraction data for the Snrich grains match with Sn2O3 and Sn3O4. The oxide(s) may have formed in the solar nebula when tin metal catalytically supported reduction of CO or during flash heating on atmospheric entry of the IDP. The presence of tin is consistent with enrichments for other volatile trace elements in chondritic IDPs and may signal an emerging trend towards non‐chondritic volatile element abundances in chondritic IDPs. The observation confirms small‐scale mineralogical heterogeneity in fine‐grained chondritic porous interplanetary dust.

Reply to R. Giovanoli's comment

In the reference book published by Dr. V. I. ]Viikheev (1957) he adduced the X-ray data for two samples of vernadite from Kusimovskoye deposit. The number of lines (up to 12) on X-ray powder patterns of these "vernadites" is quite unusual for the typical vernadife and their d-Ivalues are similar to d-I values of cryptomelane. Dr. Mikheev concluded that the vernadite structure is similar to that of pyrolusite, but is more complicated and with lesser symmetry. The data for "vernadites" published by Dr. iV!ikheev were included in ASTM (N 15604). Later on, Dr. P. F.Andrushchenko (1976), who studied manyvernadites from iron-manganese nodules collected in Pacific Ocean, ascertained that they are mostly amorphous to X-rays, but some give two lines: 2, 40 2, 45 and 1,40 1,45 A. The material studied by Dr. Andrushchenko corresponds to typical vernadite (compact, brittle, pitch black, with conchoidal fracture, powder dark brown, the lustre splendent). In the paper published in 1978 (Chukhrov et ai.,1978), after studying more than I00 samples of vernadite from different localities (including many samples of sea iron manganese nodules), we pointed out that vernadite often occurs in fine mixtures with cryptomelane. As examples we first named "vernadites" from Kusimvskoye manganese deposit (South Urals), which were studied by Mikheev, and so called copper pitch ore from Jeskasgan copper deposit (Kazakhstan) containing additionally to vernadite also cryptomelane, crysocolla and malakhite. The fine mixtures of vernadite and cryptomelane can macroscopically be very similar to vernadite and can be taken for vernadite. For vernadite often is X-ray amorphous; its fine mixtures with eryptomelane give on X-ray diffraction patterns only lines of cryptomelane. In our investigations we controlled in all cases the homogeneity of vernadite by means of selected area electron diffraction. So the data we obtained are the data for pure vernadite and not for cryptomelane or its mixture with vernadite. Our selected area electron diffraction data for pure vernadite correspond to the data for artificial and natural manganese oxide de-

Structural study of a new hexagonal form of tungsten trioxide

A new form of tungsten trioxide WO 3 has been obtained by dehydration of WO 3· 1 3 H 2O hydrate. The structural study was carried out from X-ray powder diffraction and selected area electron diffraction data. The crystallographic characteristics are: the hexagonal system; a = 7.298(2) Å, c = 7.798(3) Å; Z = 6. This hexagonal WO 3 is built up of slightly distorted (WO 6) octahedra sharing their corners arranged in six-membered rings in layers normal to the hexagonal axis; stacking of such layers leads to formation of large hexagonal tunnels. Some confirmations of this structure were made by high-resolution electron microscopy. Powder X-ray diffraction allowed us to determine an average structure. Absence of suitable single crystals has not permitted us to perform a complete structural determination. Although the existence of such a hexagonal structure for pure WO 3 had been considered as likely, it had not been hitherto observed.

Radial distribution studies of amorphous GexSe1−x alloy films

Radial distribution functions have been obtained from energy-filtered selected area electron diffraction data for four evaporated Ge x Se 1− x amorphous films ca. 300 Å thick. Peak positions in 4πr 2 p(r) are r 1 = 2.4, 2.3, 2.3, 2.4 A ̊ and r 2 = 4.0, 3.7, 3.5, 3.8 A ̊ for x = 0.00, 0.32, 0.56, 0.73 , respectively. First peak areas have been compared with those calculated for a random covalent model in which average coordinations of Ge and Se are four and two respectively. Areas for the alloys agree within estimated experimental error (±20%) with those predicted with this model.