New insight in the structure–luminescence relationships of Ca9Eu(PO4)7

The double phosphate Ca9Eu(PO4)7, obtained by solid state reaction, was found to be isotypic with Ca3(PO4)2, with space group R3c and unit cell parameters a=10.4546(1) Å, c=37.4050(3) Å, V=3540.67(9) Å3, Z=6. The structure parameters refined using the Rietveld method showed that europium shares positions M1, M2 and M3 with calcium, contradicting previously published Mössbauer results. Low temperature luminescence under selective excitation of Eu3+ in Ca9Y1−xEux(PO4)7 and in Ca9Eu(PO4)7 samples was studied, confirming the Eu3+ distribution into these sites. At 10 K, 5D0→7F0 emission lines of Eu3+ were observed at 578.5, 579.5, 580.1 nm for the M3, M1 and M2 sites, respectively. High temperature X-ray powder diffraction evidenced a second-order phase transition around 573 °C.