Our interest for lanthanide hexaaluminates began about twenty years ago. At this time, we were investigating the alumina-lanthanide oxide phase diagrams.New compouns with approximate composition "LnAl11O18", Ln = La-Sm, and X-ray powder patterns very similar to the β-alumina one were found . β-alumina - in fact a sodium aluminate with theoretical formula NaAl11017 - has also been subjected to detailed studies in this laboratory because of its high ionic conductivity . At the end of the seventies, a serie of papers by Verstegen and Stevels from Philips laboratory have appeared [3-5]. They reported the highly efficient luminescent properties of rare-earth and transition-metal activated n-alumina and magnetoplumbite. The magnetoplumbite family, to whom belongs for instance CaA112O19 is structurally related to β-alumina. The Philips'papers decided us to come back to the "LnAl1 1O18" phases. Their thermal stability were very poor, but we remarked that β-alumina structure has an oxygen sublattice in O17 while magnetoplumbite has an O19 one. Therefore, we tried to stabilize the "LnAl11O18" phases by incorporating one MO unit in it, to reach the O19 lattice of MP compounds. With M = Mg, the phase turned to have the formula LaMgAl11O19, latter referred to as LMA.