Crystal structures of the NaLnF4 materials (Ln = La, Ce, Pr, Nd, Sm or Gd) were studied with synchrotron single-crystal and powder diffraction. The materials with Ln = La, Ce, Nd, Sm and Gd have the average β structure (P, Z = 1) with partial ordering of the cations. A new type of a superstructure due to ordering of the cations and vacancies was found in NaPrF4 (P3, Z = 6). It could be described using the group–subgroup relationships P ↔ P3. Our observations suggest that the β structure is unstable and that the ordering is a slow process at ambient conditions. Upon compression, β-NaNdF4, β-NaGdF4 and the superstructure NaPrF4 are stable to at least 8 GPa with no evidence for any pressure-induced disorder–order phenomena.
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