JOURNAL OF MATERIALS SCIENCE: MATERIALS IN MEDICINE 5(1994) 263-268 Shape change and phase transition of needle-like non-stoichiometric apatite crystals L. YUBAO *~, C.P.A.T. KLEIN ~, J. DE WIJN*, S. VAN DE MEER ~, K. DE GROOT ~ $Department of Biomaterials, University of Leiden, Rijnsburgerweg 10, bid. 55, 2333 AA Leiden, The Netherlands * Institute of Materials Science and Technology, Sichuan University, Chengdu 610064, People's Republic of China Nanometre-size needle-like non-stoichiometric apatite crystals are made in our laboratory by hydrothermal treatment at 140°C at a pressure of 0.3 MPa for 2 h. The shape change and phase transition of these crystals are studied using transmission electron microscopy (TEM), X-ray diffractometer (XRD) and infrared spectroscopy (IR). The results show that water mole- cules are present in the crystal structure of non-stoichiometric apatite. The condensation of HP042- ions can happen over a wide temperature range more than 600°C and is followed closely by the reaction of P20 4- ions with OH- ions. The obvious TCP phase at 750°C is the outcome of the fusion and recrystallization of the needle-like crystals at 650 °C and 750 °C. Generally, stoichiometric apatite cannot contain water in its crystal structure while non-stoi- chiometric apatite can. Maybe this is the reason why bone contains non-stoichiometric apatite. 1. Introduction Hydroxyapatite (HA), Calo(PO4)6(OH)2 , and trical- cium phosphate (TCP), Ca3(PO4)2, are two main calcium phosphate salts which have been studied ex- tensively and used clinically [1-5]. It is known that the mineral constituent of human hard tissues gen- erally shows a porous biphasic (HA + TCP) structure after sintering at high temperature. Probably due to the analogy to bone mineral, artificially sintered por- ous biphasic or multiphasic (HA + c~- or/and 13-TCP) bioceramics are found to have better bone-bonding ability than pure HA in bony sites [6-8] and even show new bone formation within their porous struc- ture in non-osseous tissues (certain bone-inducing phenomenon) [9-12]. It seems that the more similar the phase composition between calcium phosphate materials and bone apatite, the better the bioperfor- mance of the materials in osseous environment. However, unsintered natural bone gives, in fact, a mineral phase of non-stoichiometric apatite whose presence in bone is in the form of thin needle-like crystals (5-20 nm in diameter by 60 nm in length) with a poorly crystallized apatite structure and a Ca/P molar ratio between 1.67 and 1.5 [-13 17]. Based on this, several methods have been used to prepare needle-like or fibrous non-stoichiometric apatite crys- tals which might be able to further improve the os- teointegration [18-20]. But, so far, although studies on non-stoichiometric apatite have been carried out for many years, both in material science and in bio- medical materials field [21-26], some properties are still unclear, such as whether water molecules are present in the crystal structure of non-stoichiometric apatite and whether the condensation of HPO 2- ions *To whom correspondence should be addressed. 0957-4530 © 1994 Chapman & Hall can occur over a wide temperature range more than 600 °C, as well as what is the relationship between the shape change and phase transition of non-stoichio- metric apatite crystals? The nanometre-size needle- like non-stoichiometric apatite crystals made in our laboratory by hydrothermal processing provide a means for analysing and answering those questions posed above. 2. Materials and methods Needle-like non-stoichiometric apatite crystals were prepared by hydrothermal treatment of fully washed as-prepared calcium phosphate precipitates at 140 °C under 0.3 MPa for 2 h. The precipitates were syn- thesized by dropping slowly an (NH4)zHPO4 aqueous solution into a stirred Ca(NO3) 2 aqueous solution according to a method similar to Jarcho et al. [27]. The pH value for both solutions was adjusted to 11 with ammonium solution and the reaction was set at room temperature. These crystals were fired at 650 °C, 750 °C, 850 °C, 920 °C and 1100 °C for 1 h in ambient atmosphere, separately. The Ca/P molar ratio was measured with an atomic absorption spectrometer for calcium and an ultraviolet spectrophotometer for phosphorus. The shape change was observed by transmission electron microscopy (TEM), and the phase transition was tested through X-ray diffractometer (XRD) and in- frared spectroscopy (IR). 3. Results 3.1. TEM photographs Fig. 1 shows the TEM photographs of the starting needle-like crystals and those fired at 650 °C, 750 °C, 263