MgAl 2 O 4 synthetic spinel: cation and vacancy distribution as a function of temperature, from in situ neutron powder diffraction A. Pavese * ,I,II , G. Artioli I and A. Hoser III I Universita Á di Milano, Dipartimento di Scienze della Terra, I-20133 Milano, Italy II National Research Council, Centro di Studio per la Geodinamica Alpina e Quaternaria, I-20133 Milano, Italy III BENSC-Berlin Neutron Scattering Centre, Hahn Meitner Institut, Glienicker Straûe 100, D-14109 Berlin-Wansee, Germany Received June 14, 1999; accepted February 14, 2000 Abstract. Neutron powder diffraction data collection has been carried out at BENSC Berlin, D), on synthetic Mg 0:94 Al 2:04 O 4 spinel, in the temperature range 300± 1700 K, to investigate the intracrystalline cation and va- cancy order-disorder OD) reaction, which begins to be detectable at 944 K. The Rietveld method has been used in combination with advanced minimization techniques to determine the occupancy coefficients of magnesium, alu- minum, and vacancies on the tetrahedral and octahedral sites. The temperature dependence of the cell parameter, of the T±±O and M±±O bond lengths, and of the oxygen u-coordinate are discussed in the light of the OD-reac- tions. The cation site partitioning has been interpreted as a function of temperature using the thermodynamic model of O'Neill and Navrotsky, whose parameters [a  27:41:4 and b  4:32:7 kJ/mol] are compared with earlier deter- minations from literature. Introduction Spinels are ternary oxides SG: Fd  3m; general chemical formula: AB 2 O 4 ) with unit cell containing 32 oxygen atoms in cubic close packing arrangement, 8 tetrahedrally T) and 16 octahedrally M) coordinated cations. In the ``normal structureº, the A- and B-atoms enter the T- and M-sites, respectively; in the ``inverse structureº, all the A- cations move to sixfold coordination, and half the B-atoms shift to the tetrahedral sites. Mixed configurations are of- ten observed, which are formally described by means of the ``inversion degreeº parameter x) corresponding to the tetrahedral content of the B-chemical species. Intracrystal- line order-disorder reactions involving cation diffusion be- tween the T- and M-sites take place in spinel by thermal activation and do not affect the space group symmetry non-convergent ordering: Carpenter, Powell, Salje, 1994). This phenomenology has been the subject of a number of investigations Yamanaka, Takeuchi, 1983; Peterson, La- ger, Hitterman, 1991; O'Neill, Dollase, Ross, 1991; O'Neill, 1992; O'Neill, Annersten, Virgo, 1992; see Della Giusta, Carbonin, Ottonello, 1996 for a survey), and it is still the focus of active research Lucchesi, Russo, Della Giusta, 1999; Pavese, Artioli, Russo, Hoser, 1999; Mene- gazzo, Carbonin, Della Giusta, 1997; Maekawa, Kato, Ka- wamura, Yokokawa, 1997). We here focus on synthetic MgAl-spinels that are well known to often bear vacancies due to an excess of triva- lent cations. Although vacancies may significantly affect the properties of this compound Viertel, Seifert, 1979), studies about their non-ambient behaviour are very scarce Kullerud, Donnay, Donnay, 1969; Lucchesi, Della Giusta, 1994), probably because the location of site vacancies and the measurement of their evolution versus intensive vari- ables temperature, pressure, etc) are particularly difficult. Even in the case of samples having a substantial amount of site vacancies, many authors have chosen to adopt an ideal composition during data analysis and interpretation for instance Peterson, Lager, Hitterman, 1991). In the present work we report and discuss the results from an in situ high temperature study 300±1700 K) of the cation ordering in a synthetic spinel sample [Mg 0:94 Al 2:04 O 4 ] by combining neutron powder diffraction methods with advanced minimization techniques, in order to get insight into the actual cation-vacancy partitioning versus T . The measured composition of our sample is such that 0.16 va- cancies per cell are present. This amount is sufficient to significantly affect the crystallographic features of MgAl- spinels, as shown by Lucchesi and Della Giusta 1994). Neutrons are the most appropriate radiation to study the site distribution of magnesium and aluminum, which are hardly distinguishable by X-ray. This work joins a recent issue by Redfern, Harrison, O'Neill and Wood 1999) de- voted to study quasi-stoichiometric synthetic MgAl 2 O 4 at high temperature. Experimental Sample Synthetic MgAl-spinel samples were obtained by heating for 24 hours a 1:1 stoichiometric mixture of analytical grade MgO and Al 2 O 3 oxides. The actual composition re- 406 Z. Kristallogr. 215 2000) 406±412 # by Oldenbourg Wissenschaftsverlag, Mu Ènchen * Correspondence author e-mail: pavese@p8000.terra.unimi.it) Brought to you by | University of California Authenticated Download Date | 6/4/15 5:48 PM