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

Structural, chemical, and physical properties of whitlockite-type Ca 9 Bi(VO 4 ) 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 Ca 9 Bi(VO 4 ) 7 with a transition temperature, T c, of 1053 ± 3 K. The polar phase, β -Ca 9 Bi(VO 4 ) 7, is stable below T c down to at least 160 K. The centrosymmetric β ‘ -phase is stable above T c up to 1273 ± 5 K. Above 1273 K, it decomposes to give BiVO 4 and whitlockite-type solid solutions of Ca 9+1.5 x Bi 1 - x (VO 4 ) 7 . The β ↔ β ‘ phase transition is reversible and of second order. Electrical conductivity of β ‘-Ca 9 Bi(VO 4 ) 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 Ca 9 Bi(VO 4 ) 7 are refined by the Rietveld method from XRD data measured at room temperature (space group R 3 c; Z = 6; a = 10.8992(1) Å, c = 38.1192(4) Å, and V = 3921.6(1) Å 3; R wp = 3.06% and R p = 2.36%). Bi 3+ ions together with Ca 2+ ions are statistically distributed among the M1, M2, M3, and M5 sites. Ca 9 Bi(VO 4 ) 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 Ca 9 Bi(VO 4 ) 7 is comparable with that for known nonlinear optical materials such as LiNbO 3 and LiTaO 3 .