Mechanochemical synthesis of nonstoichiometric nanocrystals La1 − y Ca y F3 − y with a tysonite structure and nanoceramic materials from CaF2 and LaF3 crystals

Abstract

The nonstoichiometric phases La1 − y Ca y F3 − y (y = 0.15, 0.20) with a tysonite (LaF3) structure have been prepared for the first time by the mechanochemical synthesis from CaF2 and LaF3 crystals. The average size of coherent scattering regions is approximately equal to 10–30 nm. It has been shown that the compositions of the phases prepared by the mechanochemical synthesis are inconsistent with the phase diagram of the CaF2-LaF3 system. The “mechanohydrolysis” of the La1 − y Ca y F3 − y phase has been observed for the first time. Under these conditions, the La1 − y Ca y F3 − y phase partially transforms into lanthanum calcium oxyfluoride for a milling time of 180 min with intermediate sampling. The La1 − y Ca y F3 − y nanoceramic materials have been prepared from a powder of the mechanochemical synthesis product by pressing under a pressure of (2–6) × 108 Pa at room temperature. The electrical conductivity of the synthesized materials at a temperature of 200°C is equal to 4.9(6) × 10−4 S/cm, and the activation energy of electrical conduction is 0.46(2) eV. These data for the nanoceramic materials coincide with those obtained for migration of fluorine vacancies in single-crystal tysonite fluoride materials.