ИСТИНА

In the 1970s, the Ln2O3- Mo(W)O3 (Ln = La-Tb) systems were intensively studied with the aim of finding double molybdates and tungstates in them. It is known that in a number of Ln2O3-MoO3 systems there are compounds with the composition 5:6 (Ln5Mo3O16) with the fluorite-like structure [1]. Fluorite-like Ln5Mo3O16+δ (0 ≤ δ ≤ 0.5) compounds have mixed oxygen-electronic conductivity, with a magnitude of the order of 10-2 S/cm at 800 °C [2, 3]. A study by Faurie [4] showed that when one rare-earth element is replaced by an alkaline element and one oxygen by fluorine, MeLn4Mo3O15F compounds (Me = Li, K, Na) are formed with the safekeeping of the fluorite structure. However, the properties of these compounds changed dramatically. A phase transition occurred in the region of which the conductivity increased abruptly [5]. The reason for this phenomenon has not yet been explained. In this work, we synthesized the solid solutions in Ln5Mo3O16.5 - LiLn4Mo3O15F system in air, where Ln = La, Pr, Nd. We study the conditions of solid state synthesis, polymorphism and the properties of LixLn5–xMo3O16.5–yFx (x = 0–1.6) compounds using X-ray, DSC and measuring the electrophysical properties, in particular, their piezoelectric response was investigated. Wide region of LixLn5–xMo3O16.5–yFx solid solutions (x = 0 - 1.6) with a cubic fluorite-like structure have been found in the above system. It should be noted that successful synthesis of these compounds significantly depends on the temperature and time of synthesis. The upper limit of time and temperature is strongly limited by the volatility of fluorine. According to thermogravimetry fluorine evaporates from the compounds above 750–800 °C, and then the LixLn5–xMo3O16.5–yFx compounds decompose. The optimum synthesis temperature was in the region of 600-700 °C. Polymorphism and properties of LixLn5–xMo3O16.5–yFx samples with high fluorine content are of particular interest. At a temperature of about 580 °C, a reversible phase transition is observed for these samples, which is accompanied by DSC anomalies, a peak of the dielectric constant and an intense jump in conductivity by two orders of magnitude. These samples also demonstrate significant piezoelectric response, which may indicate the ferroelectric nature of the phase transition. XRD study did not reveal any difference between XRD diffraction patterns of pure Ln5Mo3O16 and LixLn5–xMo3O16.5–yFx. However, it can be assumed, taking into account the above mentioned changes in physical properties, that upon doping, the cubic structure is distorted, although only slightly. [1] Hubert P., Michel P., Thozet A. Cristallochimie structure du molibdite de nèodyme Nd5Mo3O16. C. R. Seances Acad. Sci., 1973, C276, 1779-1781. [2] Tsai M., Greenblatt M., McCarroll W. H. Oxide ion conductivity in Ln5Mo3O16+x (Ln= La, Pr, Nd, Sm, Gd; x~0.5) with a fluorite-related structure. Chem. Mater., 1989, 1, 253-259. [3] Voronkova V.I., Leonidov I.A., Kharitonova E.P, et al. Oxygen ion and electron conductivity in fluorite-like molybdates Nd5Mo3O16 and Pr5Mo3O16. J.Alloys. Compd., 2014, 615, 395-400. [4] Faurie J. Preparation de nouvelles phases MLn4Mo3O16, MLn6Mo4O22 de structure derivee du type fluorine. Bull.Soc.Chim. Fr., 1971,11, 3865-3870. [5] Voronkova V., Kharitonova E., Orlova E., Kezionis A., Petrulionis D. Effect of sodium and fluorine co-doping on the properties fluorite-like, rare-earth molybdates of Nd5Mo3O16 type. Eur. J. Inorg. Chem. 2019. in press.