METAL SELECTIVE REDOX PROCESSES IN THE TERNARY COMPOUND ZRSITE

The thermochemical reactivity of the ternary compound ZrSiTe has been studied. The material is stable in inert or dry oxidizing atmosphere b ut is highly sensitive to moisture, i.e. it shows a spontaneous redox reaction with O-2/H2O. During the reaction the volume of the sample in creases drastically by a factor of about 20. The volume increase is no t caused by an intercalation reaction but is rather due to the formati on of very finely grained particles. This finding is supported by in s itu X-ray powder diffractometry investigations which demonstrate that the interlayer ordering is destroyed in the first steps of the reactio n. The loss of intralayer ordering takes a longer reaction time and in the final product the 'crystallites' exhibit no long-range order. The reaction product consists of a mixture of amorphous elemental telluri um, ZrO2 and SiO2. Subsequent heating in an N-2 atmosphere results fir st in the re-crystallization of elemental Te at 100 degrees C followed by a re-amorphization at about 300 degrees C. At even higher temperat ures the reaction between the different components leads to the format ion of the complex oxide ZrTe3O8. No reaction takes place between ZrSi Te and dry oxygen at ambient temperatures. Up to about 500 degrees C a continuous weight increase of only 3.5% is observed. Above this tempe rature a strong exothermic reaction starts at about 530 degrees C whic h is accompanied by a significant weight increase. A second exothermic reaction is observed at about 720 degrees C which is again accompanie d by a weight increase. The composition after the last step is ZrSiTeO 5.8. X-ray powder patterns reveal that until 600 degrees C the main ph ase is ZrSiTe. As intermediate products the formation of elemental Te, ZrTe3O8 and ZrTe3 is observed. Above 900 degrees C the final product consists of a mixture of microcrystalline SiO2, ZrO2 and crystalline c ubic ZrTe3O8. The results of combined thermogravimetry and differentia l thermal analysis suggest that the final product is formed by a react ion between the binary oxides. All attempts to intercalate donor atoms or donor molecules were unsuccessful. In all cases the host material shows either no reaction or was irreversibly destroyed.