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

Compounds with the general formula Ln(3+)(SeO3)(HSeO3)center dot 2H(2)O, where Ln = Sm3+, Tb3+, Nd3+ and Lu3+, are characterized by orthorhombic symmetry with space group P2(1)2(1)2(1) and unit-cell parameters in the ranges alpha similar to 6.473-6.999, b similar to 6.845-7.101, c similar to 16.242-16.426 angstrom. Light-purple irregularly shaped crystals of a new monoclinic polytype of neodymium selenite Nd(SeO3)(HSeO3)similar to 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) angstrom, ss = 101.256 (1)degrees, V = 780.48 (6) angstrom (3); space group P2(1)/c. The crystal structures of Nd(SeO3)(HSeO3)center dot 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.