Energetics and Crystal Chemical Systematics among Ilmenite, Lithium Niobate, and Perovskite Structures

Transitions from chain silicates to garnet, ilmenite, and perovskite structures are important in deep earth geophysics and solid-state chemistry. Titanates and other oxides also show polymorphism among ilmenite, lithium niobate (metastable), and perovskite structures. This review brings together the evidence from the recent mineralogical and materials science literature linking the crystal chemistry, thermodynamics, and occurrence of these polymorphs at high pressure and temperature. A-site ordered multicomponent titanate perovskites are also discussed. A common feature of the perovskite structure is its high vibrational entropy.