Fluorite‐like Li_xLn_5–xMo₃O_16.5–1.5xF_x (Ln = La, Pr, Nd) compounds isostructural with Nd₅Mo₃O₁₆

Abstract

LixLn5–xMo3O16.5–1.5xFx (0.8 ≤ x ≤ 1.4 (Ln = La), 0.4 ≤ x ≤ 1 (Ln = Pr), 0.6 ≤ x ≤ 1 (Ln = Nd)) solid solutions on the Ln5Mo3O16–LiLn4Mo3O15F joins of the Ln2O3–MoO3–LiF ternary systems, similar to the fluorite‐related cubic compound Nd5Mo3O16, have been prepared by solid‐state reactions. Substitutions of lithium on the rare‐earth site and fluorine on the oxygen site have been shown to lead to a reversible phase transition accompanied by an anomaly in permittivity and a jump in conductivity by one and half orders of magnitude. In a wet atmosphere, the LixLn5–xMo3O16.5–1.5xFx compounds may have proton conductivity.

The fluorite-like Ln5Mo3O16+δ family extension: LiLn4Mo3O15F (Ln = Eu, Gd, or Dy) ceramics

“…8). A similar reversible first-order phase transition was also observed in previous works 24,25 for F-containing LiLn 4 Mo 3 O 15 F phases (Ln = La, Pr, Nd, or Sm). It is important to note that the transition is not observed for Ln 5 Mo 3 O 16 compounds.…”

“…As mentioned above, it has been proven 24,28 that MLn 4 Mo 3 O 15 F (M = Li and Na) phases with larger rare earth cations, such as lanthanum, praseodymium, and neodymium, have the ability to dissociate water capture from the vapor phase and, consequently, facilitate proton transport. The LiSm 4 Mo 3 O 15 F compound is much less hydroscopic.…”

“…Heating above 800 °C for fluorine-containing ceramics would lead to the sublimation of fluorine from the structure, as reported in previous works. 23–25…”

“…At higher temperatures, above 700 °C, the LiEu 4 Mo 3 O 15 F sample loses weight while fluorine sublimates from the ceramics, which was also observed in previous works. 23–25…”

“…It can be concluded that the loss of the ability to absorb water in LiLn 4 Mo 3 O 15 F phases containing small rare earth cations such as europium is associated with the crystal lattice shrinking and, as a result, a decrease in the size of octahedral cavities, in the structure, which can be filled with groups OH − , enabling the possibility of proton transfer. It should be noted that the unit cell parameter of the LiLn 4 Mo 3 O 15 F compounds decreases as follows with decreasing radius of the rare earth cation: a = 11 304 Å (Ln = La); 24 11 128 Å (Ln = Pr); 24 11 033 Å (Ln = Nd); 24 10.909(1) Å (Ln = Sm); 25 1and 0.873(2) Å (Ln = Eu) (present study).…”