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 group P212121 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(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 group P21/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 group P21/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 P212121 and can be obtained when the [21--] 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.

Optical properties of ferroic Fe2O(SeO3)2 and Fe2(SeO3)3·3H2O

“…Compounds of selenite have been extensively studied due to the stereochemically active lone pairs at the Se centers. 1–4 The combination of the inherently asymmetric (SeO 3 ) 2− , (HSeO 3 ) − and (Se 2 O 5 ) 2− oxoanion groups with various transition metals will result in a rich structural chemistry in transition metal selenite, 5–7 such as MnSeO 3 ·H 2 O, 8 ZnSe–Se 2 O 5 , 9 Nd(SeO 3 )(HSeO 3 ), 7 NiH 2 SeO 4 , 10 and Sc(HSeO 3 ) 3 . 11 Under the influence of lone electron pairs, some of them exhibit two-dimensional magnetoresistive frustration.…”