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) ų; 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.

Computing details

(I)

Crystal data

H5NdO8Se2	F(000) = 788
Mr = 435.2	Dx = 3.704 Mg m^−3
Monoclinic, P21/c	Mo Kα radiation, λ = 0.71075 Å
Hall symbol: -P 2ycb	Cell parameters from 10283 reflections
a = 7.0815 (2) Å	θ = 2.5–30.6°
b = 6.6996 (2) Å	µ = 15.98 mm^−1
c = 16.7734 (5) Å	T = 293 K
β = 101.2555 (13)°	Thin plate crystal
V = 780.48 (6) Å^3	0.11 × 0.08 × 0.06 mm
Z = 4

Data collection

Bruker Apex II diffractometer	Rint = 0.049
Radiation source: X-ray tube	θmax = 30.6°, θmin = 2.5°
10283 measured reflections	h = −10→9
2294 independent reflections	k = −9→6
2229 reflections with I > 3σ(I)	l = −24→23

Refinement

Refinement on F	H atoms treated by a mixture of independent and constrained refinement
R[F^2 > 2σ(F^2)] = 0.038	Weighting scheme based on measured s.u.'s w = 1/(σ^2(F) + 0.0009F^2)
wR(F^2) = 0.061	(Δ/σ)max = 0.037
S = 1.55	Δρmax = 1.59 e Å^−3
2294 reflections	Δρmin = −1.50 e Å^−3
108 parameters	Extinction correction: B-C type 1 Gaussian isotropic (Becker & Coppens, 1974)
3 restraints	Extinction coefficient: 2160 (190)
14 constraints

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Nd1	0.26719 (4)	0.12607 (4)	0.037072 (17)	0.01009 (12)
Se1	0.23693 (8)	0.65639 (9)	−0.02447 (4)	0.01272 (17)
Se2	−0.02253 (10)	0.15016 (11)	−0.17012 (4)	0.0216 (2)
O1	−0.0773 (6)	0.1942 (6)	−0.0098 (3)	0.0149 (11)
O2	0.2731 (7)	0.4770 (7)	0.0463 (3)	0.0204 (13)
O3	0.1141 (8)	0.0759 (8)	0.1540 (3)	0.0257 (14)
O4	0.5737 (6)	0.1904 (6)	−0.0056 (3)	0.0163 (12)
O5	0.1959 (8)	0.1527 (8)	−0.1123 (3)	0.0266 (15)
H2o1	0.5207 (9)	0.1168 (12)	0.15884 (9)	0.045 (2)
Oh	0.0389 (10)	0.0946 (10)	−0.2631 (3)	0.0358 (19)
H2o2	0.5285 (14)	0.6197 (14)	0.1874 (5)	0.062 (3)
H1_h2o1	0.652637	0.081326	0.154529	0.0535*
H2_h2o1	0.490554	0.148466	0.212007	0.0535*
H1_oh	0.07352	0.063235	−0.315514	0.0429*
H1_h2o2	0.624 (13)	0.633 (19)	0.153 (6)	0.0745*
H2_h2o2	0.421 (12)	0.54 (2)	0.157 (6)	0.0745*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Nd1	0.0088 (2)	0.00648 (19)	0.0150 (2)	−0.00034 (8)	0.00232 (12)	0.00053 (8)
Se1	0.0129 (3)	0.0083 (3)	0.0165 (3)	0.00000 (17)	0.0020 (2)	−0.00249 (18)
Se2	0.0207 (4)	0.0254 (4)	0.0182 (3)	−0.0001 (2)	0.0030 (2)	0.0043 (2)
O1	0.0062 (17)	0.0122 (18)	0.0242 (19)	0.0002 (14)	−0.0017 (14)	−0.0021 (15)
O2	0.026 (2)	0.0066 (18)	0.028 (2)	0.0008 (15)	0.0034 (17)	0.0031 (15)
O3	0.026 (2)	0.033 (3)	0.020 (2)	−0.008 (2)	0.0093 (18)	−0.0028 (19)
O4	0.0138 (19)	0.0077 (17)	0.028 (2)	−0.0015 (14)	0.0062 (16)	−0.0026 (15)
O5	0.022 (3)	0.033 (3)	0.023 (2)	−0.007 (2)	0.0010 (19)	0.0001 (19)
H2o1	0.024 (3)	0.081 (6)	0.025 (3)	0.022 (3)	−0.006 (2)	0.005 (3)
Oh	0.043 (3)	0.050 (4)	0.017 (2)	−0.004 (3)	0.013 (2)	0.005 (2)
H2o2	0.074 (6)	0.084 (6)	0.026 (3)	−0.048 (5)	0.005 (3)	−0.008 (3)

Geometric parameters (Å, º)

Nd1—O1	2.456 (4)	Se1—O1^i	1.692 (4)
Nd1—O2	2.356 (5)	Se1—O2	1.673 (5)
Nd1—O3	2.443 (5)	Se1—O4^ii	1.686 (4)
Nd1—O4	2.452 (5)	Se2—O3^iii	1.690 (6)
Nd1—O5	2.463 (5)	Se2—O5	1.658 (5)
Nd1—H2o1	2.443 (4)	Se2—Oh	1.740 (6)
O1—Nd1—O2	80.74 (15)	O4—Nd1—H2o1	72.56 (16)
O1—Nd1—O3	73.37 (15)	O5—Nd1—H2o1	145.43 (18)
O1—Nd1—O4	138.99 (14)	O1^i—Se1—O2	101.8 (2)
O1—Nd1—O5	70.57 (17)	O1^i—Se1—O4^ii	94.8 (2)
O1—Nd1—H2o1	142.61 (16)	O2—Se1—O4^ii	102.9 (2)
O2—Nd1—O3	95.16 (18)	O3^iii—Se2—O5	105.0 (3)
O2—Nd1—O4	80.69 (15)	O3^iii—Se2—Oh	97.0 (3)
O2—Nd1—O5	89.52 (17)	O5—Se2—Oh	99.1 (3)
O2—Nd1—H2o1	88.3 (2)	Nd1—O1—Se1^i	133.5 (2)
O3—Nd1—O4	144.60 (15)	Nd1—O2—Se1	132.2 (2)
O3—Nd1—O5	142.34 (17)	Nd1—O3—Se2^iii	119.7 (3)
O3—Nd1—H2o1	72.18 (17)	Nd1—O4—Se1^ii	135.9 (2)
O4—Nd1—O5	73.03 (17)	Nd1—O5—Se2	125.2 (3)

Symmetry codes: (i) −x, −y+1, −z; (ii) −x+1, −y+1, −z; (iii) −x, −y, −z.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
H2o1—H1_h2o1···O5^iv	0.98	2.10	2.917 (9)	139.72
H2o1—H2_h2o1···H2o2^v	0.98	1.73	2.669 (9)	159.38
Oh—H1_oh···O3^vi	0.98	2.50	2.717 (9)	92.01
H2o2—H1_h2o2···O5^ii	0.98 (11)	2.12 (12)	2.947 (12)	141 (10)
H2o2—H2_h2o2···O2	0.98 (10)	1.99 (9)	2.849 (9)	145 (9)

Symmetry codes: (ii) −x+1, −y+1, −z; (iv) −x+1, −y, −z; (v) −x+1, y−1/2, −z+1/2; (vi) x, −y+1/2, z−1/2.