Polytypism of Ln(SeO 3 )(HSeO 3 )·2H 2 O compounds: synthesis and crystal structure of the first monoclinic modification of Nd(SeO 3 )(HSeO 3 )·2H 2 O, DFT calculations and order/disorder description.

Compounds with the general formula Ln 3+ (SeO 3 )(HSeO 3 )·2H 2 O, where Ln = Sm 3+ , Tb 3+ , Nd 3+ and Lu 3+ , are characterized by orthorhombic symmetry with space group P2 1 2 1 2 1 and unit-cell parameters in the ranges a ∼ 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(SeO 3 )(HSeO 3 )·2H 2 O 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) Å 3 ; space group P2 1 /c. The crystal structures of Nd(SeO 3 )(HSeO 3 )·2H 2 O 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 group P2 1 /c can be obtained when the inversion centre is active in the L 2n -type layers, while the second MDO structure (MDO2-polytype) is orthorhombic with space group P2 1 2 1 2 1 and can be obtained when the [2 1 --] operation is active in the L 2n -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.