Ferroelectric and Ionic-Conductive Properties of Nonlinear-Optical Vanadate, Ca9Bi(VO4)7

Structural, chemical, and physical properties of whitlockite-type Ca9Bi(VO4)7 were studied by X-ray powder diffraction (XRD), electron diffraction (ED), second-harmonic generation (SHG), thermogravimetry, differential scanning calorimetry, dielectric, and electrical-conductivity measurements. A new phase-transition of the ferroelectric type was found in Ca9Bi(VO4)7 with a transition temperature, Tc, of 1053 ± 3 K. The polar phase, β-Ca9Bi(VO4)7, is stable below Tc down to at least 160 K. The centrosymmetric β'-phase is stable above Tc up to 1273 ± 5 K. Above 1273 K, it decomposes to give BiVO4 and whitlockite-type solid solutions of Ca9+1.5xBi1-x(VO4)7. The β ↔ β' phase transition is reversible and of second order. Electrical conductivity of β'-Ca9Bi(VO4)7 is rather high (σ = 0.6 × 10-3 S/cm at 1200 K) and obeys the Arrhenius law with an activation energy of 1.0 eV. Structure parameters of Ca9Bi(VO4)7 are refined by the Rietveld method from XRD data measured at room temperature (space group R3c; Z = 6; a = 10.8992(1) Å, c = 38.1192(4) Å, and V = 3921.6(1) Å3; Rwp = 3.06% and Rp = 2.36%). Bi3+ ions together with Ca2+ ions are statistically distributed among the M1, M2, M3, and M5 sites. Ca9Bi(VO4)7 has a SHG efficiency of about 140 times that of quartz. Through the powder SHG measurements, we estimated the nonlinear optical susceptibility, 〈d〉, at about 6.1−7.2 pm/V. This value for Ca9Bi(VO4)7 is comparable with that for known nonlinear optical materials such as LiNbO3 and LiTaO3.