Nanostructured crystals of fluorite phases Sr1 − xRxF2 + x (R Are Rare Earth Elements) and their ordering: 10. Ordering under spontaneous crystallization and annealing of Sr1 − xRxF2 + x Alloys (R = Tb-Lu, Y) with 23.8–36.1 mol % RF3

The products of spontaneous crystallization (at a cooling rate of ∼200 K/min) of Sr1 − x R x F2 + x melts in the homogeneity range of the fluorite phase have been investigated. Thirty-two irrational compositions with 23.8–36.1 mol % RF3 and eight rational Sr2 RF7 compositions are obtained. With respect to the RF3 content, these compositions form five groups: (1) Sr0.762 R 0.238F2.238 (23.8% RF3), (2) Sr0.744 R 0.256F2.256 (25.6%), (3) Sr0.718 R 0.282F2.282 (28.2%), (4) Sr2 RF7 (33.3%), and (5) Sr0.639 R 0.361F2.361 (36.1%). R = Tb-Lu, Y for all groups. Quenching melts of group 5 with R = Tb, Dy, and Ho leads to the formation of ordered phases with the trigonal distortion of the rhβ-Na7Zr6F31 type, while for melts of group 5 with R = Lu, quenching yields a phase of the trigonal rhα′-Sr4Lu3F17 type. In group 5 with R = Y, Er, Tm, or Yb and in groups 1–4 with all REEs, fluorite phases are formed. Annealing at 900 ± 20°C for 96 h with subsequent cooling at a rate of ∼200 K/min expands the variety of ordered phases: a phase with a new r type of orthorhombic distortion is formed in group 1 with R = Lu, in group 2 with R = Tm or Lu, and in group 3 with R = Ho-Lu, Y; a t-Sr2 RF7 phase with tetragonal distortion is formed in group 4 with R = Tb-Er, Y; and a phase of trigonal rhα′ type is formed in group 5 with R = Y, Yb, or Lu. A fluorite phase arises in group 1 with R = Tb-Lu, Y as a result of quenching and annealing. The tendency to ordering becomes more pronounced with an increase in the RF3 content and REE atomic number. The annealing conditions do not provide equilibrium or the completely ordered state of all alloys.