FULL PAPER DOI: 10.1002/ejic.200900867 Mechanochemical Synthesis of Fe-Doped Apatite-Type Lanthanum Silicates Tamara Kharlamova,* [a] Svetlana Pavlova, [a] Vladislav Sadykov, [a] Marina Chaikina, [b] Tamara Krieger, [a] Arcady Ishchenko, [a] Yurii Pavlyukhin, [b] Sergei Petrov, [b] and Christos Argirusis [c] Keywords: Lanthanum / Silicates / Doping / Mechanical activation / Topochemistry Apatite-type lanthanum silicates (ATLS) doped by Fe were prepared by using mechanical activation, and the effect of the nature of the dopant raw materials on the formation of ATLS was considered. Genesis of the formation of apatite was studied by XRD, IR, HRTEM, and Mössbauer and UV/ Vis electron spectroscopy. The possibility of a partial substi- tution of Si in the apatite with a dopant and the mechanism of formation of ATLS in the course of mechanical activation depend on the dopant parent compound. The structural com- patibility of La 2 O 3 , La(OH) 3 , and apatite was shown to favor Introduction Apatite-type lanthanum silicates (ATLS) belong to a wide class of isostructural compounds. Recently, they have attracted considerable interest as a new type of solid electro- lyte possessing a high oxide ion conductivity at intermediate (500–800 °C) temperatures. [1] In contrast to the most tradi- tional oxide electrolytes based on fluorite- and perovskite- type systems, where the ion conductivity is caused by jump- ing of oxygen atoms into vacancies, the ion conductivity in the apatite system is suggested to be mediated by interstitial oxygen atoms. [2–4] This is caused by peculiarities of the apa- tite structure that tolerates different structural defects such as cation vacancies and interstitial oxygen atoms. In general, the hexagonal apatite structure can be pre- sented by a crystallographic formula M 10 (RO 4 ) 6 X 2 , where M = La 3+ , Mg 2+ , Ca 2+ , etc.; R = Si 4+ , Ge 4+ ,P 5+ etc.; X = O 2– , OH – ,F – , etc. It can be described as consisting of iso- lated tetrahedral RO 4 anions and M cations located in nine- coordinate 4f or seven-coordinate 6h sites (Figure 1). The seven-coordinate cations form channels along the c axis in [a] Boreskov Institute of Catalysis SB RAS, pr. Lavrentieva 5, 630090 Novosibirsk, Russian Federation Fax: +7-383-3308056 E-mail: kharlamova@catalysis.ru [b] Institute of Solid State Chemistry and Mechanochemistry of SB RAS, Kutatetadze Street 18, 630128 Novosibirsk, Russian Federation [c] Clausthal University of Technology, 38678 Clausthal-Zellerfeld, Germany Eur. J. Inorg. Chem. 2010, 589–601 © 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 589 the rapid formation of apatite-type silicate by a cluster-topo- tactic mechanism during mechanical activation, and the for- mation of an amorphous ferrosilicate intermediate in the case of the La(NO 3 ) 3 /SiO 2 precursor favors dopant incorporation into the apatite. However, dopant incorporation into the apa- tite structure was shown to be partially or fully hampered through milling when FeO(OH) and α-Fe 2 O 3 are used as rea- gents. Using crystalline hydrates as dopant precursors was shown to result in the formation of apatite through a dissoci- ative mechanism. which anions X are located. These anions, being oxide ions, are suggested to be responsible for the high anion conduc- tivity in oxyapatites such as rare-earth silicates. However, only systems possessing a defect structure due to the pres- ence of cation vacancies, for example La 9.33 Si 6 O 26 , and/or oxygen excess, for example La 9.67 Si 6 O 26.5 and La 9 SrSi 6 O 26.5 , show high ion conductivity, whereas stoi- chiometric systems such as La 8 Sr 2 Si 6 O 26 have rather low conductivity. [5] This seems to be caused by some displace- ment of the oxide ion in channels from the center into an interstitial position for systems with cation vacancies and/ or oxygen excess, which was shown by neutron diffraction studies. The latter along with atomistic modeling allows the interstitial mechanism of ion transport in apatite systems to be assumed. [2–4] Figure 1. The apatite structure.