The red long-lasting luminescence properties of the ZnGa2O4:Cr3+ spinel material are shown to be much improved when germanium or tin is substituted to the nominal composition. The resulting Zn1+x Ga2–2x (Ge/Sn) x O4 (0 ≤ x ≤ 0.5) spinel solid solutions synthesized here by a classic solid state method have been structurally characterized by X-ray and neutron powder diffraction refinements coupled to 71Ga solid state NMR studies. In contrast to ZnGa2O4:Cr3+ for which long lasting luminescence properties have been reported to arise from tetrahedral positively charged defects resulting from the spinel inversion, our results show that a different mechanism occurs complementary for Zn1+x Ga2–2x (Ge/Sn) x O4. Here, the great enhancement of the brightness and decay time of the long lasting luminescence properties is directly driven by the substitution mechanism which creates distorted octahedral sites surrounded by octahedral Ge and Sn positive substitutional defects which likely act as new efficient traps.

Bi2ZnTiO6, a lead-free analogue of PbTiO3, has been prepared by high-pressure solid-state synthesis methods. Structural analysis reveals that the tetragonal distortion (c/a ratio = 1.21) of Bi2ZnTiO6 is the largest reported for any d0 B site Pb or Bi based perovskite. Significant cation displacements result in a point-charge calculated polarization of 150 μC cm-2.

Tetrahedral units can transport oxide anions via interstitial or vacancy defects owing to their great deformation and rotation flexibility. Compared with interstitial defects, vacancy-mediated oxide-ion conduction in tetrahedra-based structures is more difficult and occurs rarely. The isolated tetrahedral anion Scheelite structure has showed the advantage of conducting oxygen interstitials but oxygen vacancies can hardly be introduced into Scheelite to promote the oxide ion migration. Here we demonstrate that oxygen vacancies can be stabilized in the BiVO4 Scheelite structure through Sr2+ for Bi3+ substitution, leading to corner-sharing V2O7 tetrahedral dimers, and migrate via a cooperative mechanism involving V2O7-dimer breaking and reforming assisted by synergic rotation and deformation of neighboring VO4 tetrahedra. This finding reveals the ability of Scheelite structure to transport oxide ion through vacancies or interstitials, emphasizing the possibility to develop oxide-ion conductors with parallel vacancy and interstitial doping strategies within the same tetrahedra-based structure type.