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

• Published in: Acta crystallographica Section B, Structural science, crystal engineering and materials Vol. 79; no. Pt 2; pp. 176 - 183
• Main Authors: Murtazoev, Alisher F, Berdonosov, Peter S, Aksenov, Sergey M, Kuznetsov, Alexey N, Dolgikh, Valery A, Nelyubina, Yulia V, Merlino, Stefano
• Format: Journal Article
• Language: English
• Published: England 01-04-2023
• Subjects: selenites; polytypism; modularity; rare-earth; order–disorder structures
• ISSN: 2052-5206; 2052-5206
• DOI: 10.1107/S2052520622012227

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