Polymorphism and Temperature-Induced Phase Transitions of Na₂CoP₂O₇

Polymorphism and temperature-induced phase transitions of Na₂CoP₂O₇ were studied by in situ neutron powder diffraction and complemented by ab initio calculations to reconcile previous reports of its three polymorphs. We show that the "blue" form prepared at 873 K exists at room temperature in the orthorhombic Pna2₁ (= P2₁cn) phase, which transforms via a first-order transition to the tetragonal form at the temperature close to room temperature (∼335 K). Just above the transition, the tetragonal form is likely incommensurately modulated with the modulation vanishing at ∼423 K. Above that temperature the phase remains in the unmodulated tetragonal state (P4₂/mnm) until melting at ∼900 K. Upon cooling after melting, Na₂CoP₂O₇ crystallizes into the "rose" triclinic P1 form which persists while it cools to room temperature, apparently stabilized by the barrier of the reconstructive "rose"-"blue" transition. We also discuss the relationship between the tetragonal and orthorhombic structures, the driving forces of the orthorhombic distortion, and similarity to Na₂ZnP₂O₇ and the melilite-type structural family.