Polytypism of Ln(SeO3)(HSeO3)·2H2O compounds: synthesis and crystal structure of the first monoclinic modification of Nd(SeO3)(HSeO3)·2H2O, DFT calculations and order/disorder description

Abstract

Compounds with the general formula Ln3+(SeO3)(HSeO3)·2H2O, where Ln = Sm3+, Tb3+, Nd3+and Lu3+, are characterized by orthorhombic symmetry with space groupP212121and unit-cell parameters in the rangesa∼ 6.473–6.999,b∼ 6.845–7.101,c∼ 16.242–16.426 Å. Light-purple irregularly shaped crystals of a new monoclinic polytype of neodymium selenite Nd(SeO3)(HSeO3)·2H2O have been obtained during a mild-condition hydrothermal synthesis. The monoclinic unit-cell parameters are:a= 7.0815 (2),b= 6.6996 (2),c= 16.7734 (5) Å, β = 101.256 (1)°,V= 780.48 (6) Å3; space groupP21/c. The crystal structures of Nd(SeO3)(HSeO3)·2H2O polymorphs show order–disorder (OD) character and can be described using the same OD groupoid family, more precisely a family of OD structures built up from two kinds of non-polar layers (category IV). The first monoclinic maximum degree order (MDO) structure (MDO1-polytype) with space groupP21/ccan be obtained when the inversion centre is active in theL2n-type layers, while the second MDO structure (MDO2-polytype) is orthorhombic with space groupP212121and can be obtained when the [21--] operation is active in theL2n-type layers. The structural complexity parameters and DFT calculations of both polytypes show that the polytype structures are extremely close energy-wise and almost equally viable from the point of total energy of the structure.