Ca 8 MgR(PO 4 ) 7 (R = La, Pr, Nd, Sm−Lu, and Y) phosphates with a β-Ca 3 (PO 4 ) 2 related structure were prepared by a standard solid-state method in air. Second-harmonic generation, differential scanning calorimetry, and dielectric measurements led to the conclusion that all Ca 8 MgR(PO 4 ) 7 are centrosymmetric and go to another centrosymmetric phase in the course of a first-order antiferroelectric phase transition well above room temperature (RT). High-temperature electron diffraction showed that the symmetry changes from R3 ത c to R3 ത m during the phase transition.Structures of Ca 8 MgR(PO 4 ) 7 at RT were refined by the Rietveld method in centrosymmetric space group R3 ത c. Mg 2+ cations occupy the M5 site; the occupancy of the M1 site by R 3+ cations increases monotonically from 0.0389 for R = La to 0.1667 for R = Er-Lu, whereas the occupancy of the M3 site by R 3+ cations decreases monotonically from 0.1278 for R = La to 0 for R = Er-Lu.In the case of R = Er-Lu, the M3 site is occupied only by Ca 2+ cations. P1O 4 tetrahedra and cations at the M3 site are disordered in the R3 ത c structure of Ca 8 MgR(PO 4 ) 7 . Using synchrotron X-ray powder diffraction, we found that annealing conditions do not significantly affect the distribution of Ca 2+ and Eu 3+ cations between the structure positions of Ca 8 MgEu(PO 4 ) 7 .Luminescent properties of Ca 8 MgEu(PO 4 ) 7 powder samples were investigated under nearultraviolet (n-UV) light. Excitation spectra of Ca 8 MgEu(PO 4 ) 7 show the strongest absorption at about 395 nm that matches with commercially available n-UV-emitting GaN-based LED chips.Emission spectra show an intense red emission due to the 5 D 0 → 7 F 2 transition of Eu 3+ .

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 electricalconductivity 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, -Ca 9 Bi(VO 4 ) 7 , is stable below T c down to at least 160 K. The centrosymmetric ′-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+1.5x Bi 1-x (VO 4 ) 7 . The T ′ phase transition is reversible and of second order. Electrical conductivity of ′-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 R3c; 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 .

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