Synthesis, characterization and thermodynamic properties of KNbO3

Abstract

The potassium niobate (KNbO3) sample was hydrothermally synthesized using Nb2O5 and KOH solution as precursors. X-ray diffraction (XRD) results show that the synthesized KNbO3 is a single-phase material with an orthorhombic crystal structure and the unit cell parameters are: a = 3.9743 (2) Å; b = 5.6954 (2) Å; c = 5.7229 (2) Å; α = β = γ = 90°, V = 129.54 (1) Å3. The temperature-dependent phase transitions and enthalpy changes of KNbO3 were investigated by Differential Scanning Calorimetry (DSC) method. The heat capacity was measured experimentally by relaxation time method via physical property measurement system (PPMS, 2–300 K) and DSC sapphire method (280–820 K), respectively. The heat capacity of KNbO3 (2–300 K) was piecewise fitted into polynomials as: Cp = 3.242 × 10−1 T + 1.540 × 10−3 T3 + 1.056 × 10−4 T5-1.098 × 10−6 T7 + 2.946 × 10−9 T9 (2–10 K), Cp = 3.275 + 0.005 T + 0.044 T2-6.816 × 10−4 T3 + 4.628 × 10−5 T4-1.343 × 10−6 T5 + 1.090 × 10−8 T6 (10–50 K), Cp = 88.211 + 69.280 × 10−3 T-0.580 × 105 T−2-8.685 × 105 T2 (50–300 K). Consequently, the changes of enthalpy, entropy, Gibbs free energy and equilibrium constant were calculated based the dependence of heat capacity and temperature (280–820 K).