Nanostructured crystals of fluorite phases Sr1 − x R x F2 + x and their ordering: 9. The defect crystal and real structure of quenched fluorite phases Sr1 − x Ce x F2 + x (x = 0–0.5)
Tóm tắt
A Sr0.7Ce0.3F2.3 crystal (CaF2 type, sp. gr.
$$Fm\bar 3m$$
), obtained by quenching from melt, has been studied for the first time by X-ray diffraction. Fluorine vacancies and interstitial anions are found in the 8c and 32f sites, respectively. The defect ratio in the Sr0.7Ce0.3F2.3 structure corresponds to the tetrahedral cluster configuration of defects {Sr4 − n
Ce
n
F26}. The defect structure of quenched (at a rate of ∼25 K/min) crystal differs from that of a crystal grown from melt (cooling at a rate of ∼3 K/min) by the displacement of some cations (presumably Ce3+) along the threefold axis to the 32f site and the anisotropy of thermal vibrations of ions in the cluster core (F
int(32f)3). The concentration dependence of the lattice parameters of quenched Sr1 − x
Ce
x
F2 + x
phases (x = 0–0.5) is described by a third-order polynomial: a = 5.80009 + 1.166518 × 10−3
x − 1.124969 × 10−5
x
2 + 8.258155 × 10−8
x
3. The compositional dependence of microdistortions is also nonlinear; maximum microdistortions are observed in the SrF2 crystal. They decrease with an increase in the cerium concentration x to ∼ 0.35. The minimum in the range x = 0.30–0.35 correlates with a composition corresponding to the peak (at x ∼ 0.29) in the melting curves of the fluorite phase estimated from the phase diagram of the SrF2-CeF3 system (the method of thermal analysis).
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