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Recent results with solid-state semiconductor gas sensors based on organic sensor elements are reviewed. Devices based on metal phthalocyanines show useful responses to NO,. Lead phthalocyanine combines the highest conductivity with the maximum sensitivity to NO,. A thin-film lead phthalocyanine sensor has successfully been used to monitor NO, produced by shot-firing in coal mines. To obtain reasonable conductance and speed of response and recovery, phthalocyanine sensors have been operated at 170 "C. Conducting polymer materials, and particularly chemically doped polypyrrole, show responses to toxic gases at ambient temperature. Initial work, using polypyrrole black impregnated filter paper, showed a response to ammonia. More recently, using polypyrrole films electrochemically deposited over electrode arrays, responses to nitrogen dioxide and hydrogen sulphide have also been obtained. Organic-semiconductor gas sensors may have advantages compared to metal-oxide devices in their sensitivity to toxic gases and in their ability to operate at or near room temperature. However, the mechanisms of device function are not yet well understood.

The nuclear and magnetic structure of Sr2Fe205 has been determined by profile analysis of powder diffraction data. The results indicate disorder in the displacements of the iron atoms in the approximately tetrahedral sites in space group Icmm. The relationship of this structure to those of Ca2Fe206 (Pcmn) and Ca2(FeA1)20~ (Ibm2) is discussed. The values of the octahedral and tetrahedral site magnetic moments are compared with results for other Fem compounds.

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