Crystal Structures of the Europium and Yttrium Hydroxychromates Eu(OH)(CrO4) and Y(OH)(CrO4); Structural Evolution as a Function of the Ln3+ Ionic Radius

The compounds Eu(OH)(CrO4) and Y(OH)(CrO4) were obtained under hydrothermal conditions and characterized by single-crystal X-ray diffraction analysis. They are isostructural and crystallize in the monoclinic system, space group P2(1)/n (no. 14) with lattice parameters a = 8.278(1) angstrom, b = 11.400(2) angstrom, c = 8.393(1) angstrom, beta = 93.76(2)degrees, V = 790.3(2) angstrom(3), Z = 4, d = 4.79 g.cm(-3) for Eu(OH)(CrO4) and a = 8.151(1) angstrom, b = 11.362(2) angstrom, c = 8.285(1) angstrom, beta = 94.23(1)degrees, V = 765.2(2) angstrom(3), Z = 4, d = 3.85 g.cm(-3) for Y(OH)(CrO4). The [EuO8] polyhedra form infinite double chains along the a direction, which are connected by common edges and corners. These double chains are related together in the two other directions by the [CrO4](2-) tetrahedra to form a three-dimensional network in which channels appear parallel to the [100] direction. We examine the structural evolution, as a function of the Ln(3+) ionic radius, in the series Ln(OH)(CrO4) compounds (with Ln = Nd, Eu, Gd, Tb, Er, Yb) and Y(OH)(CrO4). To determine the best coordination number of each lanthanide and yttrium ions, different calculations of bond valence sum were realized.