Er4F2[Si2O7][SiO4]:: The first rare-earth fluoride silicate with two different silicate anions

By the reaction of Er2O3 with ErF3 and SiO2 at 700 degreesC in sealed tantalum capsules using CsCl as flux (molar ratio 5:2:3:20), the compound Er4F2[Si2O7][SiO4] (triclinic, P (1) over bar; a=648.51(5), b=66034(5), c=1324.43(9)pm, alpha = 87.449(8), beta = 85.793(8), gamma = 60.816(7)degrees; V-m = 148.69(1) cm(3)/ mol, Z = 2) is obtained as pale pink: platelets or lath-shaped single crystals. It consists of disilicate anions [Si2O7](6-) in eclipsed conformation, ortho-silicate anions [SiO4](4-) and isolated [Er4F2](10+) units comprising two edge-shared [Er3F] triangles. Er3+ is surrounded by 7 +1 (Er1) or 7 (Er2-Er4) anionic neighbors, respectively of which two are F- in the case of Er1 and Er4 but only one for Er2 and Er3. The other ligands recruit from oxygen atoms of the different oxosilicate groups. The crystal structure can be described as simple and [Si2O7](6-)) along [001]. The necessity of a large excess ol rowing up of the three building groups ([SiO4](4-) [Er4F2](10+) fluoride for a successful synthesis of Er4F2[Si2O7][SiO4] will be discussed.