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

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)   Å 3; 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 L2n-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 L2n-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.

Source: Acta Crystallographica Section B - March 9, 2023 Category: Chemistry Authors: Murtazoev, A.F. Berdonosov, P.S. Aksenov, S.M. Kuznetsov, A.E. Dolgikh, V.A. Nelyubina, Y.V. Merlino, S. Tags: order – disorder structures polytypism selenites rare-earth modularity research papers Source Type: research