Hydrothermal synthesis of pyrochlores and their characterization

Pyrochlores, microlites, and U-betafites of pyrochlore group minerals were obtained from mixing experiments of the corresponding oxides and fluorides by hydrothermal synthesis at T = 800 °C and P = 200 MPa in the solution of 1.0 M NaF. The presence of U⁴⁺ in pyrochlore does not affect the cell parameter, which for the phases of pyrochlore–microlite series is 10.42 ± 0.01 Å. In a system with an excess of UO₂, pyrochlores and microlites, containing uranium up to 0.2–0.3 atoms per formula unit (apfu), are formed. In the uranium-free system of betafites composition, perovskites and Ti-bearing pyrochlores are formed. U-pyrochlores of betafite series, containing 2Ti = Nb + Ta in moles, have cubic cell parameters of 10.26 ± 0.02 Å and U⁴⁺ isomorphic capacity of 0.4–0.5 apfu. In the pyrochlore structure, U⁴⁺ may substitute for Ca²⁺ and Na⁺ cations in the eightfold site. In pyrochlores of pyrochlore–microlite series, Ca²⁺ is replaced by U⁴⁺, while in pyrochlores of betafite series, U⁴⁺ replaces Na⁺. Phases with pyrochlore structure, containing U⁵⁺ and U⁶⁺ in the sixfold site, usually occupied by Nb⁵⁺, Ta⁵⁺, and Ti⁴⁺, are formed under oxidizing conditions (Cu–Cu₂O buffer). They are characterized by low content of Nb⁵⁺, Ta⁵⁺ (<0.1 apfu), and anomalous behavior of the crystal lattice (compression, instead of expansion). Under natural conditions, the formation of pyrochlores containing a significant amount of U⁵⁺ and U⁶⁺ is unlikely.