Structural geometry of anion-centered tetrahedra [XA₄] (X = O, N; A = Pb, REE, etc.) is controlled by both A ⋯ A interactions and X-A bonds. If the A₄ cage has an interstition smaller than the X atom, the encapsulation of X into A₄ results in a bond strain caused by stretching of A ⋯ A contacts and compressing of X-A ones. As a consequence, A-X bond-valence parameters for encapsulated X atoms are decreased in comparison with standard ones.