Phase formation in molten system (Na/K)₂O‐TiO₂‐P₂O₅. Crystal structures of NASICON and langbeinite‐related phosphates (K/Na)_1+xTi₂(PO₄)₃ (x = 0 and 0.357)

Abstract

Crystallization of high temperature self-flux of system Na 2 O-K 2 O-TiO 2 -P 2 O 5 was investigated at different molar ratios (Na+K)/P = 0.9; 1.0 or 1.2 and Na/K = 1.0 or 2.0 over the temperature range 1000-650°C. The conditions of formation of complex phosphates K 0.10 Na 0.90 Ti 2 (PO 4 ) 3 (NASICONrelated) and K 0.877 Na 0.48 Ti ІІІ 0.357 Ti ІV 1.643 (PO 4 ) 3 (langbeiniterelated) have been established. The new obtained compounds were investigated using FTIR-spectroscopy, powder and single crystal X-ray diffraction and optical microscopy methods. The influence of alkaline metal nature on the structure formation of complex phosphates in the high temperature self-fluxes is discussed.

Partial Substitution of Potassium with Sodium in the K2Ti2(PO4)3 Langbeinite‐Type Framework: Synthesis and Crystalline Structure of K1.75Na0.25Ti2(PO4)3

“…Analysis of previously reported results for the interaction of TiO 2 with mixed phosphate melts such as Na 2 O–K 2 O–P 2 O 5 showed that the formation of langbeinite‐related phosphate took place at molar ratios of (Na+K)/P=0.9 and Na/K=1.0 . Structure determination of K 0.877 Na 0.48 Ti III 0.357 Ti IV 1.643 (PO 4 ) 3 showed partial substitution of K + by Na + and the formation of vacancies in the crystallographic positions of the alkaline metals . This latter fact significantly complicates estimation of the possibility to substitute potassium by sodium in the langbeinite‐related structure.…”

“…On the basis of previous results, the interactions of TiN with Na 2 O–K 2 O–P 2 O 5 melts were studied in this work at (Na+K)/P molar ratios ranging from 0.9 to 1.2 to identify a route to fabricate L‐KNTP solid solutions. To achieve competitive conditions for sodium and potassium in the self‐flux, two values of the initial Na/K ratio (1.0 and 2.0) and a fixed value of added TiN (3.0 wt %) were selected.…”

“…The langbeinite‐type K 0.877 Na 0.48 Ti 2 (PO 4 ) 3 solid solution was obtained earlier, for which the degree of substitution of potassium by sodium was greater than that in K 1.75 Na 0.25 Ti 2 (PO 4 ) 3 . We compared the results of the structural studies for these two compounds.…”

“…Another way to search for novel langbeinite‐type phosphates with sufficient ionic conductivity is to obtain compounds containing both big K + cations and small Na + cations. Recently, the synthesis of a langbeinite‐type solid solution with the chemical formula K 0.877 Na 0.48 Ti 2 (PO 4 ) 3 was reported . Reliable information about the structure, stability, and formation principles is crucially important for further study of such kinds of materials.…”

“…Recently,t he synthesis of al angbeinite-type solid solution with the chemical formula K 0.877 Na 0.48 Ti 2 (PO 4 ) 3 was reported. [27] Reliable information about the structure, stability, and formation principles is crucially important for furthers tudy of such kinds of materials. Motivated by this need, we directed our efforts to the synthesiso fK -Na-containing complex phosphates of the langbeinite type to obtain single crystals for X-ray diffraction analysist oe nable structural analysisa nd furthert heoretical calculation of the electronics tructure and related properties.…”