© Elsevier, Paris A new series of rare earth phosphates Ann. Chirn. Sci. Mat., 2001,26 (6), pp. 35-43 SYNTHESIS, CRYSTAL CHEMISTRY AND PHYSICAL STUDY OF A NEW SERIES OF RARE EARTH PHOSPHATES WITH SPECIFIC OPEN STRUCTURE MariaZAKI*, Ahmed AAMILP, Abderrahim SADEL', Mohammed ZAHIR °, Malika EL-GHOZZI b, Daniel AVIGNANT b "Laboratoire de Physico-chimie des Mat~riaux, Universit~ Choua~ Doukkali, Facult~ des Sciences, BP 20, 24000 EI-Jadida,Maroc. ~Laboratoire des Mat~riaux lnorganiques, UPRESA-6002 CNRS,Universit6 BlaisePascal,63177Anbi~re, France. Abstract - Hexagonal rare earth phosphates with the composition LnP04,xKOH (Ln = Eu - Lu, Y ; 0 < x < 1 ), were prepared by coprecipitation of rare earth nitrates and potassium phosphate in basic medium. These compounds were characterized by X-ray diffraction and chemical analysis. The best crystallinity was obtained for samples heat-treated at 660-750°C in air during 24 h, in which the K content was determined to be near 1% (wt). A Rietveld refinement of these hexagonal orthophosphates was performed in the same conditions as used for the hexagonal holmium orthophosphate reported earlier by us (rhabdophane-type, space group P6222 ). It indicated that a part of KOH is incorporated in the hexagonal form and that the other part remains in an amorphous form. Between 800 and 1200°C the two forms gradually transform into a zircon-type LnP04 phase for Ln = Tb-Lu, Y and a monazite-type LnP04 phase for Ln = Eu, Gd. The infi'ared spectra for all these compounds were obtained by the KBr disk method. R~sum~ - Synth~se, cristailochimie et ~tude physique d'une nouvelle gamme de phosphates de terre rare i structure ouverte. Des orthophosphates de terres rares hexagonaux de composition LnP04,xKOH (Ln = Eu - Lu, Y; 0 < x< 1 ) ont 6t6 pr6par6s par copr6cipitation de nitrates de terres rares et du phosphate de potassium en milieu basique. Ces compos6s ont 6t6 caracthris6s par diffraction des rayons X et par analyse chimique. La meilleure cristallisation a 6t6 obtenue quand les 6chantillom ont 6t6 calcin6s A 660-750°C ~t l'air pendant 24 h. Dans ces compos6s cristallis6s, la concentration de potassium est pmche de 1% (pds). Un affinement de structure par la m6thode de Rietveld a 6t6 r6alis6 sur ces orthophosphates hexagonaux dans les m~mes conditions que celles "utilis6es pour l'orthophosphate d'holmium hexagonal que nous avons 6tudi6 auparavant (rhabdophane, groupe d'espace P6222). I1 nous a montr6 qu'une pattie de KOH est incorpor6e dans la forrne hexagonale et le reste dans une phase amorphe. Vmisemblablement, la pr6sence de ces entit6s chimiques localis6es dans les tunnels pseudo-hexagonanx colin6aires/i l'axe cest ~ l'origine Renrints : M. Zaki, L.P.C.M., Univcrsit6 Chouia'l"b Doukkali, Facult6 des Sciences, B.P. 20, 24000 EL-JADIDA, Maroc.