MIREA — Russian Technological University (Lomonosov Institute of Fine Chemical Technologies), Moscow, Russia:

V. I. Kupenko, Master, Bolshakov Department of Chemistry and Technology of Rare and Trace Elements, Nanosized and Composite Materials
M. M. Sadykova, postgraduate student, Bolshakov Department of Chemistry and Technology of Rare and Trace Elements, Nanosized and Composite Materials, e-mail: sadykova.mm@mail.ru
G. V. Zimina, Lead Researcher, Bolshakov Department of Chemistry and Technology of Rare and Trace Elements, Nanosized and Composite Materials, Candidate of Chemical Sciences
V. V. Fomichev, Professor, Bolshakov Department of Chemistry and Technology of Rare and Trace Elements, Nanosized and Composite Materials

Compound phosphates of lithium (sodium) and triply charged metals represent a large family of chemical compounds. Many of them have unique conductive properties and can be used as cathode or anode materials for chemical sources of current. Thus, a keen interest is expressed to crystallizing NASICON-like phases in the three-component system YbPO₄ – Na₃PO₄ – Li₃PO₄, where both alkali cations could participate in conductivity. We studied stable cross sections in the three-component system YbPO₄ – Na₃PO₄ – Li₃PO₄ applying solid-phase synthesis and X-ray analysis methods. Original substances under study are lithium carbonate, sodium monophosphate and ytterbium oxide. Samples of the original components, thoroughly dispersed in ethanol, were pressed into tablets, 9–10 mm in diameter and 1 mm in thickness, annealed at 950 ^oС within 100–200 h. X-ray diffraction patterns of phases were obtained using Shimadzu XRD 6000 (Cu K). The crystal lattice parameters were calculated using FullProf software, indexing was performed with RTP 32 software. Cation conductivity of the samples was studied using Agilent 4284 with the Netzsch STA 449 С Jupiter thermal analyzing system within a temperature range of 20–300 ^oС and a frequency range of 1 Hz – 1 MHz, applying impedance spectroscopy. We made three triangulated sections of the three-component system: YbPO₄ – LiNa₅(PO₄)₂, Na₆Yb₃(PO₄)₅ – Li₃PO₄ and YbPO₄ – LiNa₅PO₄, revealing three compound phosphates LiNa₅Yb₄(PO₄)₆, LiNa₅Yb₃(PO₄)₅, LiNa₅Yb(PO₄)₃ and the ytterbium stabilized high-temperature modification of lithium – sodium phosphate resulting from introducing lithium into the crystal lattice of relevant double phosphates of sodium – ytterbium. Na₆Yb₃(PO₄)₅-based solid solutions were found and the а parameter was considerably increased, when increasing lithium in the compound, indicating that the interstitial solid solution was formed. We measured cation conductivity (Li⁺, Na⁺) of the samples with 40 and 60 % (mol.) Li₃PO₄ and found that lithium introduced into the matrix of compound phosphate Na₆Yb₃(PO₄)₅ negatively influenced the conductivity value. This is likely to be a strong polarization effect of cation Li⁺.

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