2016. 57, 3 – UDC 548.73:546.18:546.21 NEW POLYMORPH OF CaHPO4 (MONETITE): SYNTHESIS AND CRYSTAL STRUCTURE N. Ouerfelli1, 2, M.F. Zid 1 1Laboratoire de Matйriaux et Cristallochimie, Facultй des Sciences de Tunis, Universitй de Tunis El Manar, El Manar II, Tunis, Tunisia E-mail: najouerf@yahoo.fr 2Universitй de Tunis Carthage, Institue Prйparatoire aux Etudes d'Ingйnieur de Bizerte BP 64-7021 - Zarzouna (Bizerte), Tunisia Received February, 25, 2015 A novel form of anhydrous dicalcium phosphate CaHPO4 (monetite) is synthesized hydrothermally and characterized by single crystal X-ray diffraction. <...> A threedimensional crystal structure can be described by {HPO4}n infinite zigzag chains linked by Ca—O bonds. <...> The chemistry and crystal structure of calcium phosphates is an interesting field of study for a long time. <...> The synthesis and structural characterization of anhydrous calcium hydrogen phosphates have been well known for a long time [5 ]. <...> In this work we focus on the crystal structure of new polymorph monetite. <...> Colourless transparent single crystals of CaHPO4 were prepared hydrothermally from a mixture of CaCO3, Fe(NO3)9H2O, and H3PO4 in the molar ratio Ca:Fe:P = 1:1:4 in an aqueous solution. <...> Colourless and light pink single crystals were obtained. <...> They were identified using single crystal diffraction analyses as and orthorhombic polymorph CaHPO4 and Ca0.5Fe(HP3O10) respectively. <...> The structures were solved by direct methods and subsequently refined by Fourier difference techniques with full-matrix least squares based on F2 using SHELX [ 8 ] under the WinGX software package [ 10 ]. <...> It was found that the new compound crystallized in the orthorhombic system, space group Ccm21. <...> Anisotropic thermal parameters were assigned to all non-hydrogen atoms. <...> Even though the H atom occupies a general position and is only 50 % occupied, it was located in a difference Fourier map and found to be attached to a P—O vertex. <...> Restraints were required to ensure a stable refinement of this H atom species. <...> The refinement of the atomic coordinates led to the reliability factors R(F) = 0.025 and wR(F2) = = 0.070 considering <...>