Structural studies of the high temperature phases of AgTaO₃

Highlights: • Crystal Structure of AgTaO₃ studied using a combination of synchrotron X-ray and Neutron Diffraction. • High temperature phase transitions studied using synchrotron X-ray diffraction. • Sequence of structures established. - Abstract: The temperature dependence of the structure of a polycrystalline sample of AgTaO₃ has been determined using in-situ Synchrotron X-ray powder diffraction methods. This work finds no evidence for the presence of a monoclinic phase, rather three phase transitions have been identified, namely $R3c\stackrel{390°C}{\leftrightarrow }\mathrm{Cmcm}\stackrel{465°C}{\leftrightarrow }P4/\mathrm{mbm}\stackrel{580°C}{\leftrightarrow }\mathrm{Pm}\bar{3}m$. The rhombohedral phase was further studied at room temperature by neutron powder diffraction. The co-existence of the rhombohedral and orthorhombic phases around 380–400 $°$ C indicates that the transition between these is first order, and gives rise to unusual peak shapes in the diffraction patterns.