High-temperature phase transformations in LiH2PO4 and possible solid-state polymerization

The dielectric constant and electrical conductivity were measured on heating polycrystalline LiH2PO4 from room temperature to 220 ∘C. Two dielectric anomalies appeared at 178 ∘C  ( T p 1 ) and 196 ∘C  ( T p 2 ). The electrical conductivity showed the same anomalies near the corresponding temperatures and reached the magnitude of the superprotonic phases: 3 × 1 0 − 2 Ω − 1 cm − 1 at 178 ∘C ( T p 1 ) and 10 Ω − 1 cm − 1 at 196 ∘C ( T p 2 ). Whether the superprotonic phase transformations are due to polymorphic transitions in the bulk, surface transitions, or chemical reactions (thermal decomposition) at the surface is uncertain. Considering several previous thermal studies, our experimental results seem to be related to the last case: chemical reactions (thermal decomposition) at the surface with the progressive solid-state polymerization. The anomalies at 178 ∘C ( T p 1 ) and 196 ∘C ( T p 2 ) are suggested to be the onset of dimerization and oligomerization, respectively.