High-temperature second-harmonic-generation, x-ray powder diffraction, dielectric, differential scanning calorimetry, and structural data are presented to show that ${\mathrm{Sr}}_9\mathrm{In}{\left(\mathrm{P}{\mathrm{O}}_4\right)}_7$ exhibits an antiferroelectric (AFE) phase transition of the second order at $773\pm 3\mathrm{K}$. ${\mathrm{Sr}}_9\mathrm{In}{\left(\mathrm{P}{\mathrm{O}}_4\right)}_7$ complements a group of ${\mathrm{Ca}}_3{\left(\mathrm{V}{\mathrm{O}}_4\right)}_2$-type ferroelectric phosphates and vanadates and is the first example of an AFE material in this family. Structural data obtained for the AFE and the paraelectric phase of ${\mathrm{Sr}}_9\mathrm{In}{\left(\mathrm{P}{\mathrm{O}}_4\right)}_7$ show evidence that both displacive and order-disorder contributions are responsible for the physical properties of the ${\mathrm{Ca}}_3{\left(\mathrm{V}{\mathrm{O}}_4\right)}_2$-type materials.

• Received 1 September 2004

DOI:https://doi.org/10.1103/PhysRevB.70.172103