Specimen preparation of low-melting-point solidified gases for low-temperature powder diffractometry

Abstract

The quality of powder specimens in diffraction experiments is very important. Any preferred orientation in the specimen under study affects the observed integrated intensities of the powder peaks and makes them unreliable; thus, results obtained from these intensities hardly appear to be trustworthy. The existing methods of powder preparation were considered to be unsuitable for samples such as isotopic methanes, nitrogen, oxygen, hydrogen, etc. which are gaseous at room temperature and have relatively low melting points. Therefore, a new method was developed for producing fine-grained powder specimens of solidified gases without preferred orientation directly from the gas phase using a specially designed liquid-helium cryostat which allowed visual inspection of the formation of powder in the specimen container. Subsequent neutron diffraction tests proved that the powder specimens prepared by this method were free of preferred orientations.

A variable-temperature sample container for low-temperature neutron powder diffraction

“…Apart from being a compact, effective and economical device for varying sample temperatures in the 4-2 to 77.4 K temperature range, the VTSC has two further properties which make it eminently suitable for producing fine-grained powder samples directly from the gas phase [a method for preparing powder specimens of low-melting-point solidified gases is described elsewhere, see Arzi (1975Arzi ( , 1982.] One is the low heat conductivity of the fused silica tubes which allows setting up and maintaining large vertical temperature gradients along them and the other is their transparency which allows visual observation of the powder formation.…”