Structure cristalline de Eu2(II)SiO4β

Stabilization of Pr4+ in Silicates─High-Pressure Synthesis of PrSi3O8 and Pr2Si7O18

The reaction between PrO2 and SiO2 was investigated at various pressure points up to 29 GPa in a diamond anvil cell using laser heating and in situ single-crystal structure analysis. The pressure points at 5 and 10 GPa produced Pr2III(Si2O7), whereas Pr4IIISi3O12 and Pr2IV(O2)O3 were obtained at 15 GPa. Pr4IIISi3O12 can be interpreted as a high-pressure modification of the still unknown orthosilicate Pr4III(SiO4)3. PrIVSi3O8 and Pr2IVSi7O18 that contain praseodymium in its rare + IV oxidation state were identified at 29 GPa. After the pressure was released from the reaction chamber, the Pr(IV) silicates could be recovered, indicating that they are metastable at ambient pressure. Density functional theory calculations of the electronic structure corroborate the oxidation state of praseodymium in both PrIVSi3O8 and Pr2IVSi7O18. Both silicates are the first structurally characterized representatives of Pr4+-containing salts with oxoanions. All three silicates contain condensed networks of [SiO6] octahedra which is unprecedented in the rich chemistry of lanthanoid silicates.

Red luminescence and ferromagnetism in europium oxynitridosilicates with a β-K2SO4 structure

LaSrSiO₃N and LaBaSiO₃N are the first examples of nitride-based alkaline earth orthosilicates with β-K₂SO₄ structure, and they show red-orange luminescence after activation with Eu²⁺. The analogous compound LaEuSiO₃N is, in addition to a red phosphor material, a soft ferromagnet with a Curie temperature of 3 K.

Structural chemistry of A 2 MX 4 compounds (X = O, F) with isolated tetrahedral anions: search for the densest structure types

The packing density of various structures is important not only for understanding and the prediction of high-pressure phase transitions, but also because of its reported correlation with thermodynamic stability. Plotting the cube root of formula volume against the cation radii (R) for nine morphotropic series with isolated tetrahedral anions, A 2 MO4 (M = Si, Ge, S, Se, Cr, Mn, Mo, W) and A 2BeF4, permits the comparison of packing densities for 13 structure types (about 80 individual compounds and several solid solutions) stable at (or near) ambient temperature. The spinel type is the densest. The next densest types are those of K2MoO4, Tl2CrO4, β-Ca2SiO4, β-K2SO4, Ag2CrO4 and Sr2GeO4. In three series (M = Ge, Mo, W) the densest type comes with somewhat intermediate values of R, and not the largest, in contrast to the classical homology rule. Another contradiction with traditional views is that some of the densest phases have abnormally low overall binding energies. The correlation between packing density and coordination number (CN) is better when CN of A counts entire MX 4 groups rather than individual X atoms; many, but not all, A 2 MX 4 structures have binary A 2 M analogues (of course, A and M are not necessarily the same in these structure types). The most frequent arrangement of A around M is of the Ni2In type: a (distorted) pentacapped trigonal prism.