Na3Sc(PO4)2: Thermal behaviors, distorted β-K2SO4-type structure and dielectric properties

Abstract

The Na3Sc(PO4)2 (NSPO) compound was obtained by solid-state synthesis method and has a new type of distorted β-K2SO4 (arcanite) crystal structure that was refined based on powder XRD data in the space group (SG) P21/c, with unit cell parameters: a = 11.5533(1) Å, b = 5.31759(5) Å, c = 16.7650(2) Å, β = 97.6439(6)°. The NSPO is a metastable phase with an optimal synthesis temperature range between 1173 and 1233 K. When a mixture of Na2CO3, NH4H2PO4 and Sc2O3 is annealed below this temperature range, the NaSICON-type compound and Na-polyphosphates is formed. However, annealing above T∼1245 K leads to the decomposition onset into the NaSICON-type compound (Na3Sc2(PO4)3), Na3PO4 and terminates at T∼1289 K. NSPO phase is dielectric with σbulk = 4.4 × 10−7 S/cm at 300 K. Increasing the temperature from 300 to 1000 K leads to an increase in σbulk by four orders (σbulk = 5.6 × 10−3 S/cm at 1000 K).