Structure, phase transition and negative thermal expansion in ammoniated ZrW₂O₈

In the present work, we have developed an effective way to control the thermal expansion of ZrW₂O₈ by the insertion of NH₃ into the void of the framework structure. Neutron powder diffraction (NPD) and XPS studies reveal that N is bonding to W in the ammoniated ZrW₂O₈ and retains the original structure with the space group P2₁3. This kind of ammoniation improves the thermal stability, raised the phase transition temperature about 50 K, and weakens the negative thermal expansion (NTE) from −7.8 × 10⁻⁶ K⁻¹ to −2.1 × 10⁻⁶ K⁻¹. These behaviors account for bonding of NH₃ to the neighboring atoms of ZrW₂O₈, and hinder the rocking motion of the corner shared polyhedral structure.