Phase Diagram of LiF-Li 3 PO 4 System: A New Mechanism of Heterovalent Anionic Isomorphism

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Phase Diagram of LiF-Li3PO4 System: A New Mechanism of Heterovalent Anionic Isomorphism G.V. Zimina1, M. Tsygankova1, M. Sadykova1, F.M. Spiridonov2, V.V. Fomichev1, P.P. Fedorov1,3 1

Moscow Technological University (Institute of Fine Chemical Technologies), Vernadskogo Avenue, 86, Moscow, 119571 2

Chemical Department, Lomonosov Moscow State University, Moscow 119991, Russia 3

Prochorov General Physics Institute, Russian Academy of Sciences, Moscow, 119991, Russia ABSTRACT

The phase diagram of LiF-Li3PO4 system is studied. The eutectic coordinates are 800±5°C, 8±1 mol% Li3PO4. A region of a solid solution based on lithium phosphate with a length of up to 11±2 mol% LiF was discovered. It is assumed that there is heterovalent isomorphism where anionic tetrahedron (PO4)3- is replaced by tetrahedron (LiF4)3-. INTRODUCTION At present, the rechargeable current sources, i.e., alkaline batteries intended for powering portable electronic devices, are of considerable interest. Alkaline batteries are mostly made using lithium and sodium compounds. The relevant issues are the increase in efficiency, explosion and fire safety. The task of chemical material science is to develop both materials with high cation conductivity and cathode materials, which must satisfy a number of specific requirements [1, 2]. Downloaded from https://www.cambridge.org/core. North Carolina State University, on 28 Dec 2017 at 10:53:29, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1557/adv.2017.622

A promising basis of materials for alkaline batteries shall be the compounds with tetrahedral anions, primarily phosphates [3] and fluorophosphates [4, 5]. During the search for fluorine-phosphate phases involving lithium, we paid attention to a particular system of lithium fluoride and lithium phosphate LiF-Li3PO4. The phase equilibria in this system have been partially studied by Bukhalova and Mardirosova [6]. A eutectic phase diagram was recorded (the eutectic coordinates are 800°C, 7 mol% of Li3PO4). While verifying these data, we did not find the existence of any intermediate phases; however, the parameters of the lattice of lithium phosphate synthesized in a mixture with fluoride were different from those for pure Li3PO4. This led to a thorough study of LiF-Li3PO4 system, which was the purpose of this research. EXPERIMENTAL DETAILS Fluoride and lithium orthophosphate were used as starting materials in the study of LiF-Li3PO4 system. Lithium orthophosphate was synthesized from Li2CO3 qualified “pure for analysis” and NH4H2PO4 qualified “chemically pure” at 800°C for 24 hours. Control of the purity of the samples was performed by X-ray diffraction. The resulting lithium orthophosphate is a mid-temperature modification, the γ-phase of Li3PO4. The X-ray diffraction patterns of the phosphate and lithium fluoride used in the study were in accordance with the literature data (JCPDS PDF card catalog). The studied samples were prepared at 10 mol%, if necessary at 5 or 2.5 mol%, annealed in