CRYSTAL-STRUCTURES AND LUMINESCENCE PROPERTIES OF LA3ZR4F25 AND ALPHA-LAZR3F15

During a study of the LaF3-ZrF4 system, both La3Zr4F25 and alpha-LaZr3 F15 compounds have been evidenced. Their crystal structures have been determined from single-crystal X-ray diffraction data. La3Zr4F25 cryst allises in the cubic system with a= 12.384 Angstrom and I (4) over bar 3d space group (no. 220). Its crystal structure is built up of (ZrF6) (2-) octahedra and (LaF8)(5-) dodecahedra sharing corners. The low tem perature form, alpha, of LaZr3F15 is orthorhombic (space group Pmmn, n o. 59) with a = 15.721 Angstrom, b = 16.299 Angstrom, c= 8.438 Angstro m. Its structure is built of corner-sharing tricaped trigonal prisms s urrounding the La3+ ions and both octahedra and monocapped trigonal pr isms encompassing the Zr4+ ions. This structure is characterised by dy namically disordered (ZrF6)(2-) complex anions. The Eu3+ luminescence properties of these phases have been investigated and are discussed in relationship with their crystal structures.