To whom correspondence should be addressed. Journal of Solid State Chemistry 153, 231 } 236 (2000) doi:10.1006/jssc.2000.8746, available online at http://www.idealibrary.com on Synthesis, Stoichiometry, and Electrical Transport Properties of (Cd 1x Mn x )Mn 2 O 4 Paolo Ghigna, Giovanni Battista Barbi, Gaetano Chiodelli, Giorgio Spinolo, Lorenzo Malavasi, and Giorgio Flor INSTM, C.S.T.E./CNR, and Dipartimento di Chimica Fisica, Universita ` di Pavia, Viale Taramelli 16, I 27100-Pavia, Italy Received December 15, 1999; in revised form April 12, 2000; accepted April 20, 2000; published online July 17, 2000 This paper describes the synthesis and discusses the thermal stability, oxygen and cation stoichiometry, and electrical trans- port properties of Cd/ Mn spinel materials. It has been found that an e4ective synthesis can be obtained by reacting in the solid state CdO and Mn 2 O 3 in a closed quartz ampoule at 8003C. By following this synthetic procedure it was possible to prepare materials with di4erent Cd/ Mn ratios. The as-prepared spinels are slightly understoichiometric in the oxygen content, but at appropriate T and P(O 2 ) they became overstoichiometric. The electrical transport takes place by motion of electron holes that are created by oxygen overstoichiometry at low T. At higher temperatures a di4erent conduction mechanism is active. This mechanism is likely related to the thermal promotion of elec- trons from the valence into the conduction band.  2000 Academic Press 1. INTRODUCTION CdMn  O  is a promising material for electrochemical (potentiometric) sensors of oxygenic gases (NO  ) in high- temperature detection of combustion exhausts (1} 3). The working principle of these sensors (1}3) may be thought of as being based on two reactions, 2NO  #4eP2NO#2O [1] and O  #4eP2O, [2] taking place simultaneously (opposite to each other) at the sensing electrode, thus determining its mixed potential. The sensing electrode is typically connected to an O conduct- ing solid electrolyte (usually a stabilized zirconia layer), which acts also as a reference electrode. In a recent investigation we have shown that Cd/Mn spinels can be regarded as solid solutions between CdMn  O  and Mn  O  . A material with a Cd stoichiomet- ric coe$cient lower than 2 can easily be prepared: in this case the Mn(II) enters the tetrahedral sites, thus producing a range of solid solutions that ideally require no vacancies in the tetrahedral sites nor oxygen vacancies (4). It is the aim of the present work to investigate the synthesis, structure, defect equilibria, and electrical properties of CdMn  O  and related materials, and to gain the basic knowledge needed to provide a rational understanding of their promis- ing applications. 2. EXPERIMENTAL 2.1. Materials Several samples of cadmium manganese spinel with dif- ferent cadmium contents were prepared as reported in the results section, following di!erent synthetic paths. In particular Cd(NO  )  ) 4H  O (Aldrich, 98%) and Mn(NO  )  ) nH  O (Aldrich, 98%) were used for the sol}gel trials, while CdO (Aldrich, 99.5%) and Mn  O  (Aldrich, 99%) were used for the solid state syntheses. 2.2. Instruments and Methods X-ray powder di!raction patterns were acquired on a Philips 1710 di!ractometer operated at 40 keV and 35 mA and equipped with a Cu radiation tube, adjustable diver- gence slit, graphite monochromator on the di!racted beam, and proportional detector. The lattice constants were deter- mined by minimizing the weighted square di!erence be- tween calculated and experimental Q  values, where Q  "4 sin   /   ; weight"sin(2  ). Instrumental aberra- tions were considered by inserting additional terms in the linear least-squares model (5). SEM and EMPA inspections were performed with a JEOL JXA-840 A scanning electron microscope. A Zeiss Axioplan was used for optical micro- scopic inspections. Thermogravimetric and isothermal SHANTHA BALA GNS JSSC 8746 DATE.............................. DISC USED YES ! NO ! 231 0022-4596/00 $35.00 Copyright  2000 by Academic Press All rights of reproduction in any form reserved.