873 873 Journal of the Ceramic Society of Japan 115 [12] 873–876 (2007) Paper eóÇêçíÜÉêã~ä póåíÜÉëáë ~åÇ `êóëí~ääçÖê~éÜáÅ píìÇó çÑ pê–mÄ eóÇêçñó~é~íáíÉ pçäáÇ pçäìíáçåë Kongjun ZHU, Kazumichi YANAGISAWA,ಓ Rie SHIMANOUCHI,ಓಓ Ayumu ONDA,ಓ Koji KAJIYOSHIಓ and Jinhao QIU The Aeronautical Key Laboratory for Smart Materials and Structures, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, P. R. China ಓ Research Laboratory of Hydrothermal Chemistry, Faculty of Science, Kochi University, Kochi-shi 780–8520 Japan ಓಓ Department of Material Science, Faculty of Science, Kochi University, Kochi-shi 780–8520 Japan A continuous series of solid solutions in the system of strontium and lead hydroxyapatite, Pb x Sr 1x HAP x 0–1, was successfully synthesized by low-temperature mixing method LTMM at 160c C for 12 h under hydrothermal conditions. The samples were characterized by chemical analysis, electron microscopic observa- tion and X-ray powder-pattern fitting. The site of the metal ions in the solid solutions was analyzed by the Riet- veld method. The lattice constants of the prepared solid solutions varied linearly with Pb contents. It was found that Pb 2 ions in the solid solutions preferentially occupied the M2 site in the apatite structure. =Received August 28, 2007; Accepted October 18, 2007? Key-words : Hydroxyapatite, Solid solution, Hydrothermal synthesis, Rietveld method 1. Introduction Calcium hydroxyapatite =Ca 10 PO 4 6 OH 2 , designated as Ca HAp? is a primary constituent of vertebral animal's hard tissues and synthetic hydroxyapatite has attracted our attention due to its utility in the fields of bioceramics, catalyst, adsorbent, and so on. 1 Strontium hydroxyapatite =Sr 10 PO 4 6 OH 2 , designated as Sr HAp? has been also used as a catalyst for oxidative dehydrogenation of methane 2 and as an adsorbent for the protein. 3 In the oxidation of methane, the addition of a small quantity of lead to Sr HAp resulted in the conversion and the selectivity to ethylene. 4 In the apatite structure, the Ca ions occupy two types of nonequivalent sites: the M1 sites are at the fourfold sym- metry4f position and the M2 sites are at the sixfold sym- metry 6h position. 5 The apatite structure has a high flexibil- ity, so that many metal ions can incorporate in the structure. It is interesting to investigate the site of metal ions which incor- porate in the apatite structure. Badraoui et al. 6 prepared Sr–Pb HAp solid solutions from a double decomposition method, in aqueous medium. The precipitate was filtered, washed with hot distilled water, in order to eliminate traces of ammonium, nitrate and acetate ions, dried at 100c C for 12 h and calcined at 600c C for 4 h to improve its crystallinity. The results of the Rietveld analysis indicated a clear preference of lead for the M2 site of the apatitic structure, so that in the samples with low Pb content it is almost exclusively found in the M2 site. The HAp solid solutions have been prepared by the solid state reaction or aqueous reaction. 7–12 The former method gives higher crystallinity to the products, but the hydroxyl defect should be formed due to the high temperature treat- ments. On the other hand, the crystallinity of the products synthesized by the aqueous reaction is usually low even without hydroxyl defects in their structure. The low crystallinity and hydroxyl defect in the samples must cause inaccuracy in the structural investigation by the Rietveld anal- ysis. Hydrothermal synthesis method is a wet-chemical tech- nique for directly forming complex oxide powders with high crystallinity. 13 The technique has an advantage especially for preparation of HAp powders not to introduce the hydroxyl defect in the structure. Cd-Hap, 14 Ca–Cd Hap, 15,16 Ca–Sr Hap, 16,17 Ca–Pb Hap 16 were successfully synthesized by hydrothermal method in our former works. In this work, we have synthesized well-formed crystals of Sr–Pb HAp solid solution using LTMM under hydrothermal conditions at 160c C for 12 h. Powder X-ray diffraction ana- lyses were employed to determine the crystalline phases and investigate which site Pb ions preferentially occupy in the HAp structure by X-ray powder-pattern fitting. The obtained crystals were observed by SEM and TEM. The concentrations of Sr 2 , Pb 2 and PO 3 4 in the solid solutions were also meas- ured by inductively coupled plasma ICP spectrometry. 2. Experimental The starting materials were regent grade chemicals Wako Pure Chem., Ind., Co., Japan, NH 4 2 HPO 4 , SrNO 3 2 4H 2 O, and PbNO 3 2 4H 2 O. NH 4 2 HPO 4 was dissolved in deionized water to get a 0.200 mol L 1 solution. The pH of the solution was adjusted to 10 with ammonia. A mixed metal nitrate solution with 0.334 mol L 1 was prepared from SrNO 3 2 4H 2 O and PbNO 3 2 4H 2 O nitrates, varying the PbPbSr molar ratio from 0 to 1. We used a Teflon lined multi chamber autoclave which has two separate chambers. Each starting solution 8 cm 3 was separately poured into a different reaction chamber to get the HAp stoichiometric composition. These solutions were mixed at room temperature and then heated to desired temperature. Hydrothermal treat- ments were carried out at 160c C for 12 h. After washing with distilled water, the products were characterized by powder X-ray diffraction XRD; Model RTP-300RC, Rigaku Co., Japan with Cu Ka radiation 40 kV and 100 mA. The lattice constants were calculated by the least square method using Si as an internal standard. X-ray diffraction data used for the Rietveld analysis were collected by step scanning method under following conditions: 2u range 15c –130c ; step width 0.02c ; and counting time 6 s. Rietveld analysis was performed with the program RIETAN. 18 A scan- ning electron microscope SEM; Model S530, HITACHI Co., Japan and Transmission electron microscopic TEM; Model H–800, HITACHI Co., Japan were used to observe their morphologies. The composition of the solid solutions was determined by using inductively coupled plasma ICP spec-