Please cite this article in press as: L. Carbajal, et al., Phase and melting relationships of ,  and  ′ -Ca 3 (PO 4 ) 2 polymorphs in the Ca 3 (PO 4 ) 2 -Zn 3 (PO 4 ) 2 system, J Eur Ceram Soc (2016), http://dx.doi.org/10.1016/j.jeurceramsoc.2016.12.022 ARTICLE IN PRESS G Model JECS-10974; No. of Pages 7 Journal of the European Ceramic Society xxx (2016) xxx–xxx Contents lists available at www.sciencedirect.com Journal of the European Ceramic Society journal homepage: www.elsevier.com/locate/jeurceramsoc Feature article Phase and melting relationships of ,  and  ′ -Ca 3 (PO 4 ) 2 polymorphs in the Ca 3 (PO 4 ) 2 -Zn 3 (PO 4 ) 2 system Leticia Carbajal, Sara Serena ∗ , Maria Antonia Sainz, Angel Caballero Ceramic Department, Institute of Ceramic and Glass, ICV-CSIC, 28045, Madrid, Spain a r t i c l e i n f o Article history: Received 24 October 2016 Accepted 10 December 2016 Available online xxx Keywords: Phase diagram Tricalcium phosphate Zinc Biomaterials a b s t r a c t In order to provide an exact knowledge of the phase transitions and melting relationships of Ca 3 (PO 4 ) 2 (TCP) in the presence of zinc, a revisited version of the rich-Ca 3 (PO 4 ) 2 region of the phase diagram of the system Ca 3 (PO 4 ) 2 -Zn 3 (PO 4 ) 2 has been established in the present work. Experimental determination of this diagram was carried out by solid-state reactions of samples prepared from pure NH 4 H 2 PO 4 , CaCO 3 and ZnO raw materials. X-ray Diffraction, Differential Thermal Analyses and Field Emission Scanning Electron Microscopy studies allowed to revise the , ,  + -TCP phase stability fields, delimitating for the first time the biphasic  +  ′ -TCP field and the melting relationships in the high temperature region of the system. The results allowed to determine two peritectic invariant points, at ≈1400 ◦ C for 95 mol% Ca 3 (PO 4 ) 2 and at ≈1490 ◦ C for ≈99.5 mol% Ca 3 (PO 4 ) 2 . © 2016 Elsevier Ltd. All rights reserved. 1. Introduction Tricalcium phosphate (Ca 3 (PO 4 ) 2 -TCP) based biomaterials are excellent candidates in hard tissue engineering due to its similar- ity to the natural bone composition and outstanding properties [1–6]. The presence of additives such as Zn 2+ , Mg 2+ , F − , CO 3 2− and/or SiO 4 2− among others in solid solution in the structure of TCP affect the stability of its different polymorphs and therefore the properties of TCP based biomaterials. It is well known that the incorporation of zinc in TCP structure within the non-toxic level stimulates bone growth and its mineralization, hence its interest [7–9]. From the late–90s zinc substituted TCP based biomaterials have received a considerable attention from worldwide researchers, as regards the publications on the synthesis, obtaining, solubil- ity, bioactivity and biological performance “in vitro” and “in vivo” of this family of materials [10–17]. Nevertheless none of these studies related their results with the information on the phases assemblage and microstructure available in the corresponding equilibrium phase diagrams. Recently our group has published results relating the effect of phase assemblage in Zn-TCP materials and physico-chemical properties and bioactive behaviour of Zn- TCP biomaterials [18–20]. These studies evidenced that consistent and detailed descriptions on the rich tricalcium phosphate region ∗ Corresponding author. E-mail address: serena@icv.csic.es (S. Serena). of Ca 3 (PO 4 ) 2 -ZnO phase diagrams are necessary to obtain, design and develop “smart” materials with tailored physic-chemical char- acteristics and biological performance “in vivo”, able to replace and regenerate the mineral component of the bone. TCP has three polymorphs ,  and  ′ in order of increasing tem- perature. The low temperature phase -TCP (Rhombohedral, R3CH) [21] is stable from room temperature up to 1125 ◦ C and trans- forms reconstructively to -TCP (Monoclinic, P121/A1) [22] phase. -TCP phase is stable in the temperature range 1150–1470 ◦ C. Moreover, -TCP phase transforms rapid and reversible to  ′ -TCP (Hexagonal, P63/mmc) [23] at temperatures above 1470 ◦ C.  ′ -TCP polymorph spontaneously transform to -TCP on cooling and can- not be retained at room temperature even by quenching. The presence of metal ions as solid solutions in the structure of TCP has significant consequences in the relative stability of these polymorphic forms, especially in the stabilization of -TCP. As an example, the presence of Mg +2 increase the temperature of poly- morphic transformation -TCP → -TCP in more than 300 ◦ C as a function of magnesium content [24]. The dissolutions of different metals in the crystalline structure of -TCP has been studied in var- ious works [19,25–27] and solubility limit of divalent ions has been assessed ∼13.6 mol% [25]. The most detailed version of the Ca 3 (PO 4 ) 2 -Zn 3 (PO 4 ) 2 multi- component system was published by Kreidler et al. [28], back 1967. These authors, based on previous data [29–32] coupled with their own studies, published the first and most complete versions of the systems CaO-P 2 O 5 [33], ZnO-P 2 O 5 [34], and Ca 3 (PO 4 ) 2 -Zn 3 (PO 4 ) 2 [29], (Fig. 1). Kreidler et al. [28] determined the phase relations http://dx.doi.org/10.1016/j.jeurceramsoc.2016.12.022 0955-2219/© 2016 Elsevier Ltd. All rights reserved.