No abstract

A study has been made of the influence of variations in cooling rate (supersaturation) and impurity content of the changes in morphology (habit and surface features) of sodium chlorate crystals grown from aqueous solution. The habit was found to vary from pure cubic at high cooling rates to cuboid with relatively small (1 10) and ( i l I), (Til), (1 Ti), and (1 11) faces at low cooling rates. Optical examination of the as grown surfaces of the resulting crystals revealed the presence of growth hillocks and surface features characteristic of dislocationassociated growth mechanisms. This speculation was supported by the distribution of dislocations in the various growth sectors, identified by Lang topography. These observations were supplemented by growth rate studies which confirmed that the morphological changes resulted from changes in supersaturation during growth. Controlled addition of sodium dithionate to the solution during growth had no influence on the growth mechanism of the natural habit faces, as evidenced by the surface features, but led to the rapid development of new tetrahedral ( l i l ) , (1 IT), (11 l), and (Til) faces. It is shown that these faces play the major role in the habit modification as a consequence of the structural similarity of the dithionate ion to the chlorate ions which compose these faces. The ready substitution of the so3 moiety for the C103-ion obstructs the normal development of the face in the manner of a "tailor-made" habit modifier.The habit of a crystal is determined by the relative growth rates of the various faces bounding the crystal. It is dependent on several internal and external factors. Internal factors are those which are associated with the crystal itself, such as crystal structure, dislocations, and other defects. External factors are those imposed on the crystal by the crystallization conditions. These conditions are defined either by external growth parameters, such as temperature, supersaturation, and hydrodynamics, or by the introduction of impurity into the solution.Sodium chlorate (NaClO3) is comparatively easy to crystallize, and its habit is known to be easily modified. It crystallizes in the cubic structure (space group P213) and normally exhibits a morphology dominated by { 100) faces with occasionally formed (1 lo), (1201, and some { 11 1) facets. The relative simplicity of its structure and behavior makes it a useful material for fundamental studies of habit modification caused by both internal and external factors.The first studies of habit modification of sodium chlorate were carried out by Burin,' who concluded that in well-stirred highly supersaturated solutions of pure material at constant temperature the morphology of the growing crystal is determined by the crystallographic structure and is cubic. Later, Kern2 proposed that the habit of NaC103 is purely cubic only at low supersaturations and that tetrahedral faces tend to appear at higher supersaturations. Hemade no mention, however, about theeffect of growth rates on morphology.Kern's propos...

Structural properties of the AgSbTe 2 -AgSbSe 2 pseudobinary system were examined using thermal analysis, scanning electron microscopy, and x-ray powder diffractometry. It was found that partial substitution of Te by Se atoms leads to stabilization of the cubic crystal structure of alloys. The electronic-transport properties of materials were measured in order to investigate carrier conduction, band-gap features, and thermoelectric properties. The undoped homogeneous solid solution exhibits extremely low thermal conductivity of 0.5 W m −1 K −1 , a very large positive Seebeck coefficient of about 400-600 V K −1 at room temperature, low carrier densities of 10 16 -10 18 cm −3 , and thermally activated conduction. The influence of alloying on thermal-conductivity mechanisms and electron properties was discussed. The highest experimental dimensionless figure of merit ZT of the undoped AgSbSe 0.25 Te 1.75 sample is about 0.65 at a temperature of 520 K. The influence of doping on enhancement of thermoelectric properties of these materials was analyzed and optimal values of transport parameters were estimated.

a b s t r a c tSuperconducting FeSe has been investigated by measurements of the magnetic susceptibility versus temperature and Mössbauer spectroscopy at various temperatures including strong external magnetic fields applied to the absorber. It was found that isomer shift exhibits sharply defined increase at about 105 K leading to the lowering of the electron density on iron nucleus by 0.02 electron a.u. −3 . Above jump in the electron density is correlated with the transition from the P4/nmm to the Cmma structure, while decreasing temperature. Mössbauer measurements in the external magnetic field and for temperatures below transition to the superconducting state revealed null magnetic moment on iron atoms. Hence, the compound exhibits either Pauli paramagnetism or diamagnetic behavior. The principal component of the electric field gradient on the iron nucleus was found as negative on the iron site.

The thermal conductivity of Cu2Te is around 4 W m−1 K−1 at 300 K with low Cp values.