Crystal structure of apatite type Ca2.49Nd7.51(SiO4)6O1.75

Le, T.H.; Brooks, N.R.; Binnemans, K.; Blanpain, B.; Guo, M.; Van Meervelt, L.

doi:10.1107/S205698901600089X

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Volume 72| Part 2| February 2016| Pages 209-211

doi:10.1107/S205698901600089X

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Crystal structure of apatite type Ca_2.49Nd_7.51(SiO₄)₆O_1.75

Thu Hoai Le,^a Neil R. Brooks,^b Koen Binnemans,^b Bart Blanpain,^a Muxing Guo ^a and Luc Van Meervelt ^b ^*

^aKU Leuven - University of Leuven, Department of Metallurgy and Materials Engineering, Kasteelpark Arenberg 44 - bus 2450, B-3001 Heverlee, Belgium, and ^bKU Leuven - University of Leuven, Department of Chemistry, Celestijnenlaan 200F - bus 2404, B-3001 Heverlee, Belgium
^*Correspondence e-mail: luc.vanmeervelt@chem.kuleuven.be

Edited by P. Roussel, ENSCL, France (Received 1 December 2015; accepted 15 January 2016; online 20 January 2016)

The title compound, Ca_2+xNd_8–x(SiO₄)₆O_2–0.5x (x = 0.49), was synthesized at 1873 K and rapidly quenched to room temperature. Its structure has been determined using single-crystal X-ray diffraction and compared with results reported using neutron and X-ray powder diffraction from samples prepared by slow cooling. The single-crystal structure from room temperature data was found to belong to the space group P6₃/m and has the composition Ca_2.49Nd_7.51(SiO₄)₆O_1.75 [dicalcium octaneodymium hexakis(orthosilicate) dioxide], being isotypic with natural apatite and the previously reported Ca₂Nd₈(SiO₄)₆O₂ and Ca_2.2Nd_7.8(SiO₄)₆O_1.9. The solubility limit of calcium in the equilibrium state at 1873 K was found to occur at a composition of Ca_2+xNd_8–x(SiO₄)₆O_2–0.5x, where x = 0.49.

Keywords: crystal structure; apatite structure type; calcium rare earth oxide silicate.

CCDC reference: 1447637

1. Chemical context

The study of calcium rare earth oxide silicates is important because they are usually observed in nuclear waste along with rare earth silicates. So far, the calcium rare earth oxide silicates of Nd (Fahey & Weber, 1982 ; Fahey et al., 1985 ), Sm (PDF 29–365; Smith, 1977 ), Eu (PDF 29–320; Smith, 1977), Gd (PDF 28–212; Smith, 1976 ), Tb (PDF 38–256; Lacout, 1986 ), and Ce (Skakle et al., 2000 ) have been studied. Fahey & Weber et al. (1982) and Fahey et al. (1985) published the structure and stoichiometry limits of the Ca_2+xNd_8–x(SiO₄)_2–0.5x system using X-ray and neutron powder diffraction. In that study, the samples were synthesized at 1523 or 1873 K and cooled at a rate of 250 K per hour. However, such a slow cooling process may lead to undesired modifications of the obtained specimens since the solubility of calcium does not remain constant but decreases with decreasing temperature. This problem is avoided in the present work by rapid quenching of the Ca_2+xNd_8–x(SiO₄)₆O_2–0.5x samples in their equilibrium state at 1873 K to room temperature within a few seconds. Consequently, compositions of the samples can be preserved better.

2. Structural commentary

The single crystal structure determined from room temperature data was found to belong to the space group P6₃/m and has the composition Ca_2.49Nd_7.51(SiO₄)₆O_1.75 and is isotypic with natural apatite and the previously reported Ca₂Nd₈(SiO₄)₆O₂ and Ca_2.2Nd_7.8(SiO₄)₆O_1.9 (Fahey & Weber, 1982; Fahey et al., 1985). The solubility limit of calcium in the equilibrium state at 1873 K was found to occur at a composition of Ca_2+xNd_8–x(SiO₄)₆O_2–0.5x, where x = 0.49.

There are two metal positions in the asymmetric unit of the structure (Fig. 1) and both contain disordered Nd and Ca ions: Nd1/Ca1 occupies the lower symmetry site 6h and Nd2/Ca2 the higher symmetry site 4f. The occupancies of these metal sites were refined resulting in 0.887 (5)/0.113 (5) for Nd1/Ca1 and 0.546 (4)/0.454 (4) for Nd2/Ca2. The majority (80%) of calcium is situated at the 4f site. In the structures of Ca₂Nd₈(SiO₄)₆O₂ and Ca_2.2Nd_7.8(SiO₄)₆O_1.9, these values are 89 and 73%, respectively (Fahey et al., 1985). The refined value of the amount of Nd in the structure gives a value of 0.49 for x in the equation Ca_2+xNd_8–x(SiO₄)₆O_2–0.5x. For charge-balance purposes, the occupancy of O²⁻ in the structure must be 2 − 0.5x or 1.755. Initially, the occupancy of the O²⁻ position O4 in the structure was allowed to refine freely and its value was close to what is required for charge balance; however, it was fixed at 0.146 as the refinement of heavy-atom positions is the most reliable and exact charge balance is required.

Figure 1
View of the coordination spheres of the Nd/Ca and Si atoms [displacement ellipsoids shown at the 50% probability level; symmetry codes: (i) x, y, −z + $[{1\over 2}]$ ; (ii) y, −x + y, −z; (iii) y, −x + y, z + $[{1\over 2}]$ ; (iv) −y + 1, x − y, z; (v) y − x, −x, −z + $[{1\over 2}]$ ; (vi) y − x, −x, z; (vii) y − x + 1, −x + 1, z; (viii) y, −x + y, z − $[{1\over 2}]$ ; (ix) −y + x + 1, x, z − $[{1\over 2}]$ ; (x) −x + 1, −y + 1, z − $[{1\over 2}]$ ].

The Nd1/Ca1 site is seven coordinate and the Nd/Ca—O bond lengths vary between 2.3909 (19) and 2.721 (3) Å for oxygen atoms of the SiO₄²⁻ unit but the shortest bond length of 2.2681 (2) Å is to the O²⁻ ion, O4 (Fig. 1; Table 1). The Nd2/Ca2 site is nine coordinate and only bonds to SiO₄²⁻ units with six short distances [Nd—O = 2.4231 (17), 2.4715 (18) Å] and three long distances [Nd—O = 2.830 (2) Å] (Fig. 1; Table 1) are observed. The distances are similar to those reported by Fahey et al. (1985) for the structures of Ca₂Nd₈(SiO₄)₆O₂ and Ca_2.2Nd_7.8(SiO₄)₆O_1.9 determined by powder X-ray diffraction.

Table 1
Selected bond lengths (Å)

Nd1—O1	2.721 (3)	Nd2—O2^iv	2.4715 (18)
Nd1—O2ⁱ	2.463 (3)	Nd2—O3^v	2.830 (2)
Nd1—O3ⁱⁱ	2.3909 (19)	Si1—O1	1.621 (3)
Nd1—O3ⁱⁱⁱ	2.547 (2)	Si1—O2	1.623 (3)
Nd1—O4	2.2681 (2)	Si1—O3^vi	1.629 (2)
Nd2—O1ⁱ	2.4231 (17)
Symmetry codes: (i) -y+1, x-y, z; (ii) $[y, -x+y, z+{\script{1\over 2}}]$ ; (iii) $[-x+y, -x, -z+{\script{1\over 2}}]$ ; (iv) $[x-y+1, x, z-{\script{1\over 2}}]$ ; (v) -x+1, -y+1, -z; (vi) $[x, y, -z+{\script{1\over 2}}]$ .

The O4 atom (O²⁻ ion) is coordinated to three different Nd1/Ca1 ions whilst the SiO₄⁴⁻ group has eight contacts to different Nd/Ca positions. The O1 atom coordinates one Nd1/Ca1 position and two Nd2/Ca2 positions, the O2 atom coordinates one Nd1/Ca1 position and two Nd2/Ca2 positions and the O3 position coordinates one Nd1/Ca1 and one Nd2/Ca2 positions. These contacts generate the packing, which can be seen viewed down the c axis in Fig. 2.

Figure 2
View along the c axis of the packing arrangement.

3. Synthesis and crystallization

A mixture of appropriate amounts of fine powders of Nd₂O₃ (99.99%), CaO (99.9%) and SiO₂ (99.9%) was put into a sealed Pt-20%Rh tube and heated to 1873 K in an argon atmosphere and maintained at that temperature for 24 h. CaO was made by calcination of CaCO₃ at 1373 K for 12 h. The sample was then quenched in a cold-water bath to give a light-blue crystalline solid, from which a single crystal of the title compound was selected. The sample was further analyzed by EPMA–WDS, giving a composition of 20.2% SiO₂, 72.1% Nd₂O₃ and 7.7% CaO. The converted formula according to the EPMA–WDS result was Ca_2.45Nd_7.45Si₆O_25.775 (O was calculated).

4. Refinement details

Crystal data, data collection and structure refinement details are summarized in Table 2. There are two metal positions in the structure and the Nd and Ca ions are disordered on both of these sites. Nd/Ca occupancy on each of the two positions was refined and the occupancy of Nd was found to be 88.7 (5)% for one site and 54.6 (4)% for the other, giving a value of 0.49 for x in Ca_2+xNd_8–x(SiO₄)₆O_2–0.5x. The occupancy of the anionic O atom was fixed at 2 − 0.5x. Constraints were applied so that the Nd and Ca on the same site had identical positional and displacement parameters.

Table 2
Experimental details

Crystal data
Chemical formula	Ca_2.49Nd_7.51(SiO₄)₆O_1.75
M_r	1763.24
Crystal system, space group	Hexagonal, P6₃/m
Temperature (K)	298
a, c (Å)	9.5507 (3), 7.0513 (3)
V (Å³)	557.03 (3)
Z	1
Radiation type	Mo Kα
μ (mm⁻¹)	18.18
Crystal size (mm)	0.05 × 0.05 × 0.05

Data collection
Diffractometer	Agilent SuperNova (single source at offset, Eos detector)
Absorption correction	Multi-scan (CrysAlis PRO; Agilent, 2012 )
T_min, T_max	0.717, 1.000
No. of measured, independent and observed [I > 2σ(I)] reflections	2616, 878, 813
R_int	0.024
(sin θ/λ)_max (Å⁻¹)	0.821

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.019, 0.035, 1.11
No. of reflections	878
No. of parameters	42
Δρ_max, Δρ_min (e Å⁻³)	0.79, −0.86
Computer programs: CrysAlis PRO (Agilent, 2012), SHELXS97 (Sheldrick, 2008 ), SHELXL2014 (Sheldrick, 2015 ), OLEX2 (Dolomanov et al., 2009 ) and publCIF (Westrip, 2010 ).

Supporting information

CCDC reference: 1447637

Crystal structure: contains datablock I. DOI: 10.1107/S205698901600089X/ru2066sup1.cif

Structure factors: contains datablock I. DOI: 10.1107/S205698901600089X/ru2066Isup2.hkl

checkCIF report

Chemical context top

The study of calcium rare earth oxide silicates is important because they are usually observed in nuclear waste along with rare earth silicates. So far, the calcium rare earth oxide silicates of Nd (Fahey et al., 1982, 1985), Sm (PDF 29–365; Smith, 1977), Eu (PDF 29–320; Smith, 1977), Gd (PDF 28–212; Smith, 1976), Tb (PDF 38–256; Lacout, 1986), and Ce (Skakle et al., 2000) have been studied. Fahey et al. (1982, 1985) published the structure and stoichiometry limits of the Ca_{2 + x}Nd_{8 − x}(SiO₄)_2–0.5x system studied using X-ray and neutron powder diffraction. In that study, the samples were synthesized at 1523 or 16873 K and cooled at a rate of 250 K per hour. However, such a slow cooling process may lead to undesired modifications of the obtained specimens since the solubility of calcium does not remain constant but decreases with decreasing temperature. This problem is avoided in the present work by rapid quenching of the Ca_{2 + x}Nd_{8 − x}(SiO₄)₆O_2–0.5x samples in their equilibrium state at 1873 K to room temperature within a few s. Consequently, compositions of the samples can be preserved better.

Structural commentary top

The single-crystal structure recorded at 298 K was found to belong to the space group P6₃/m and have the composition Ca_2.49Nd_7.51(SiO₄)₆O_1.75 and to be isomorphous with natural apatite and the previously reported Ca₂Nd₈(SiO₄)₆O₂ and Ca_2.2Nd_7.8(SiO₄)₆O_1.9 (Fahey et al., 1982, 1985). The solubility limit of calcium in the equilibrium state at 1873 K was found to occur at a composition of Ca_{2 + x}Nd_{8 − x}(SiO₄)₆O_2–0.5x, where x = 0.49.

There are two metal positions in the asymmetric unit of the structure (Fig. 1) and both contain disordered Nd and Ca ions: Nd1/Ca1 occupies the lower symmetry site 6h and Nd2/Ca2 the higher symmetry site 4f. The occupancies of these metal sites were refined resulting in 0.887 (5)/0.113 (5) for Nd1/Ca1 and 0.546 (4)/0.454 (4) for Nd2/Ca2. The majority (80%) of the Ca is situated at the 4f site. In the structures of Ca₂Nd₈(SiO₄)₆O₂ and Ca_2.2Nd_7.8(SiO₄)₆O_1.9, these values are 89 and 73%, respectively (Fahey et al., 1985). The refined value of the amount of Nd in the structure gives a value of 0.49 for x in the equation Ca_{2 + x}Nd_{8 − x}(SiO₄)₆O_2–0.5x. For charge balance purposes, the occupancy of O²⁻ in the structure must be 2 − 0.5x or 1.755. Initially, the occupancy of the O²⁻ position O4 in the structure was allowed to refine freely and its value was close to what is required for charge balance; however, it was fixed at 0.146 as the refinement of heavy-atom positions is the most reliable and exact charge balance is required.

The Nd1/Ca1 site is seven coordinate and the Nd/Ca—O bond lengths vary between 2.3909 (19) and 2.721 (3) Å for oxygen atoms of the SiO₄²⁻ unit but the shortest bond distance of 2.2681 (2) Å is to the O²⁻ ion, O4 (Fig. 1; Table 1). The Nd2/Ca2 site is nine coordinate and there only bonds to SiO₄²⁻ units with six short distances [Nd—O = 2.4231 (17), 2.4715 (18) Å] and three long distances [Nd—O = 2.830 (2) Å] (Fig. 1; Table 1) are observed. The distances are similar to those reported by Fahey et al. (1985) for the structures of Ca₂Nd₈(SiO₄)₆O₂ and Ca_2.2Nd_7.8(SiO₄)₆O_1.9 determined by powder X-ray diffraction.

The O4 atom (O²⁻ ion) is coordinated to three different Nd1/Ca1 ions whilst the SiO₄²⁻ group has eight contacts to different Nd/Ca positions. The O1 atom coordinates one Nd1/Ca1 position and two Nd2/Ca2 positions, the O2 atom coordinates one Nd1/Ca1 position and two Nd2/Ca2 positions and the O3 position coordinates one Nd1/Ca1 and one Nd2/Ca2 positions. These contacts generate the packing, which can be seen viewed down the crystallographic c axis in Fig. 2.

Synthesis and crystallization top

A mixture of appropriate amounts of fine powders of Nd₂O₃ (99.99%), CaO (99.9%) and SiO₂ (99.9%) was put into a sealed Pt-20%Rh tube and heated to 1873 K in an argon atmosphere and maintained at that temperature for 24 h. CaO was made by calcination of CaCO₃ at 1373 K for 12 h. The sample was then quenched in a cold-water bath to give a light-blue crystalline solid, from which a single-crystal of the title compound was selected. The sample was further analyzed by EPMA–WDS, giving a composition of 20.2% SiO₂, 72.1% Nd₂O₃ and 7.7% CaO. The converted formula according to the EPMA–WDS result was Nd_7.45Ca_2.45Si₆O_25.775 (O was calculated).

Refinement details top

Crystal data, data collection and structure refinement details are summarized in Table 2. There are two metal positions in the structure and the Nd and Ca ions are disordered on both of these sites. Nd/Ca occupancy on each of the two positions was refined and the occupancy of Nd was found to be 88.7 (5)% for one site and 54.6 (4)% for the other, giving a value of 0.49 for x in Ca_{2 + x}Nd_{8 − x}(SiO₄)₆O_2–0.5x. The occupancy of the anionic O atom was fixed at 2 − 0.5x. Constraints were applied so that the Nd and Ca on the same site had identical positional and thermal parameters.

Related literature top

For synthesis of Ca_{2 + x}Nd_{8 − x}(SiO₄)₂O_0.5x, see: Fahey, J. A., Weber, W. J. (1982) and Fahey, J. A., Weber, W. J., and Rotella, F. J. (1985). For structure refinement of Ca₂Nd₈(SiO₄)₂, see: Fahey, J. A., Weber, W. J. (1982). For solubility limits of Ca_{2 + x}Nd_{8 − x}(SiO4)_2–0.5x, see: Fahey, J. A., Weber, W. J., and Rotella, F. J. (1985).

Structure description top

The study of calcium rare earth oxide silicates is important because they are usually observed in nuclear waste along with rare earth silicates. So far, the calcium rare earth oxide silicates of Nd (Fahey et al., 1982, 1985), Sm (PDF 29–365; Smith, 1977), Eu (PDF 29–320; Smith, 1977), Gd (PDF 28–212; Smith, 1976), Tb (PDF 38–256; Lacout, 1986), and Ce (Skakle et al., 2000) have been studied. Fahey et al. (1982, 1985) published the structure and stoichiometry limits of the Ca_{2 + x}Nd_{8 − x}(SiO₄)_2–0.5x system studied using X-ray and neutron powder diffraction. In that study, the samples were synthesized at 1523 or 16873 K and cooled at a rate of 250 K per hour. However, such a slow cooling process may lead to undesired modifications of the obtained specimens since the solubility of calcium does not remain constant but decreases with decreasing temperature. This problem is avoided in the present work by rapid quenching of the Ca_{2 + x}Nd_{8 − x}(SiO₄)₆O_2–0.5x samples in their equilibrium state at 1873 K to room temperature within a few s. Consequently, compositions of the samples can be preserved better.

There are two metal positions in the asymmetric unit of the structure (Fig. 1) and both contain disordered Nd and Ca ions: Nd1/Ca1 occupies the lower symmetry site 6h and Nd2/Ca2 the higher symmetry site 4f. The occupancies of these metal sites were refined resulting in 0.887 (5)/0.113 (5) for Nd1/Ca1 and 0.546 (4)/0.454 (4) for Nd2/Ca2. The majority (80%) of the Ca is situated at the 4f site. In the structures of Ca₂Nd₈(SiO₄)₆O₂ and Ca_2.2Nd_7.8(SiO₄)₆O_1.9, these values are 89 and 73%, respectively (Fahey et al., 1985). The refined value of the amount of Nd in the structure gives a value of 0.49 for x in the equation Ca_{2 + x}Nd_{8 − x}(SiO₄)₆O_2–0.5x. For charge balance purposes, the occupancy of O²⁻ in the structure must be 2 − 0.5x or 1.755. Initially, the occupancy of the O²⁻ position O4 in the structure was allowed to refine freely and its value was close to what is required for charge balance; however, it was fixed at 0.146 as the refinement of heavy-atom positions is the most reliable and exact charge balance is required.

Synthesis and crystallization top

A mixture of appropriate amounts of fine powders of Nd₂O₃ (99.99%), CaO (99.9%) and SiO₂ (99.9%) was put into a sealed Pt-20%Rh tube and heated to 1873 K in an argon atmosphere and maintained at that temperature for 24 h. CaO was made by calcination of CaCO₃ at 1373 K for 12 h. The sample was then quenched in a cold-water bath to give a light-blue crystalline solid, from which a single-crystal of the title compound was selected. The sample was further analyzed by EPMA–WDS, giving a composition of 20.2% SiO₂, 72.1% Nd₂O₃ and 7.7% CaO. The converted formula according to the EPMA–WDS result was Nd_7.45Ca_2.45Si₆O_25.775 (O was calculated).

Refinement details top

Crystal data, data collection and structure refinement details are summarized in Table 2. There are two metal positions in the structure and the Nd and Ca ions are disordered on both of these sites. Nd/Ca occupancy on each of the two positions was refined and the occupancy of Nd was found to be 88.7 (5)% for one site and 54.6 (4)% for the other, giving a value of 0.49 for x in Ca_{2 + x}Nd_{8 − x}(SiO₄)₆O_2–0.5x. The occupancy of the anionic O atom was fixed at 2 − 0.5x. Constraints were applied so that the Nd and Ca on the same site had identical positional and thermal parameters.

Computing details top

Data collection: CrysAlis PRO (Agilent, 2012); cell refinement: CrysAlis PRO (Agilent, 2012); data reduction: CrysAlis PRO (Agilent, 2012); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: OLEX2 (Dolomanov et al., 2009); software used to prepare material for publication: OLEX2 (Dolomanov et al., 2009) and publCIF (Westrip, 2010).

Figures top

Fig. 1. View of the coordination spheres of the Nd/Ca and Si centres (displacement ellipsoids shown at the 50% probability level; symmetry codes: (i) x, y, −z + 1/2; (ii) y, −x + y, −z; (iii) y, −x + y, z + 1/2; (iv) −y + 1, x − y, z; (v) y − x, −x, −z + 1/2; (vi) y − x, −x, z; (vii) y − x + 1, −x + 1, z; (viii) y, −x + y, z − 1/2; (ix) −y + x + 1, x, z − 1/2; (x) −x + 1, −y + 1, z − 1/2.

Fig. 2. View along the crystallographic c axis of the packing arrangement.

Dicalcium octaneodymium hexakis(orthosilicate) dioxide top

Crystal data top

Ca_2.49Nd_7.51(SiO₄)₆O_1.75	D_x = 5.256 Mg m⁻³
M_r = 1763.24	Mo Kα radiation, λ = 0.71073 Å
Hexagonal, P6₃/m	Cell parameters from 1649 reflections
Hall symbol: -P 6c	θ = 4.3–35.5°
a = 9.5507 (3) Å	µ = 18.18 mm⁻¹
c = 7.0513 (3) Å	T = 298 K
V = 557.03 (3) Å³	Block, light blue
Z = 1	0.05 × 0.05 × 0.05 mm
F(000) = 790

Data collection top

Agilent SuperNova (single source at offset, Eos detector) diffractometer	878 independent reflections
Radiation source: SuperNova (Mo) X-ray Source	813 reflections with I > 2σ(I)
Mirror monochromator	R_int = 0.024
Detector resolution: 15.9631 pixels mm^-1	θ_max = 35.7°, θ_min = 3.8°
ω scans	h = −15→15
Absorption correction: multi-scan (CrysAlis PRO; Agilent, 2012)	k = −15→15
T_min = 0.717, T_max = 1.000	l = −11→5
2616 measured reflections

Refinement top

Refinement on F²	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F² > 2σ(F²)] = 0.019	w = 1/[σ²(F_o²) + (0.0072P)² + 0.3232P] where P = (F_o² + 2F_c²)/3
wR(F²) = 0.035	(Δ/σ)_max = 0.001
S = 1.11	Δρ_max = 0.79 e Å⁻³
878 reflections	Δρ_min = −0.86 e Å⁻³
42 parameters	Extinction correction: SHELXL2014 (Sheldrick, 2015), Fc^*=kFc[1+0.001xFc²λ³/sin(2θ)]^-1/4
0 restraints	Extinction coefficient: 0.0062 (2)
4 constraints

Crystal data top

Ca_2.49Nd_7.51(SiO₄)₆O_1.75	Z = 1
M_r = 1763.24	Mo Kα radiation
Hexagonal, P6₃/m	µ = 18.18 mm⁻¹
a = 9.5507 (3) Å	T = 298 K
c = 7.0513 (3) Å	0.05 × 0.05 × 0.05 mm
V = 557.03 (3) Å³

Data collection top

Agilent SuperNova (single source at offset, Eos detector) diffractometer	878 independent reflections
Absorption correction: multi-scan (CrysAlis PRO; Agilent, 2012)	813 reflections with I > 2σ(I)
T_min = 0.717, T_max = 1.000	R_int = 0.024
2616 measured reflections

Refinement top

R[F² > 2σ(F²)] = 0.019	42 parameters
wR(F²) = 0.035	0 restraints
S = 1.11	Δρ_max = 0.79 e Å⁻³
878 reflections	Δρ_min = −0.86 e Å⁻³

Special details top

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å²) top

	x	y	z	U_iso*/U_eq	Occ. (<1)
Nd1	0.24279 (2)	0.01102 (2)	0.2500	0.00756 (7)	0.887 (5)
Ca1	0.24279 (2)	0.01102 (2)	0.2500	0.00756 (7)	0.113 (5)
Nd2	0.6667	0.3333	−0.00110 (5)	0.00906 (10)	0.546 (4)
Ca2	0.6667	0.3333	−0.00110 (5)	0.00906 (10)	0.454 (4)
Si1	0.37185 (11)	0.40114 (11)	0.2500	0.0077 (2)
O1	0.4886 (3)	0.3232 (3)	0.2500	0.0127 (5)
O2	0.4707 (3)	0.5974 (3)	0.2500	0.0144 (5)
O3	0.2528 (2)	0.3424 (3)	0.0659 (3)	0.0209 (5)
O4	0.0000	0.0000	0.2500	0.0141 (10)	0.88

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
Nd1	0.00740 (10)	0.00713 (10)	0.00733 (9)	0.00301 (7)	0.000	0.000
Ca1	0.00740 (10)	0.00713 (10)	0.00733 (9)	0.00301 (7)	0.000	0.000
Nd2	0.00913 (12)	0.00913 (12)	0.00892 (15)	0.00456 (6)	0.000	0.000
Ca2	0.00913 (12)	0.00913 (12)	0.00892 (15)	0.00456 (6)	0.000	0.000
Si1	0.0069 (4)	0.0079 (4)	0.0085 (4)	0.0039 (3)	0.000	0.000
O1	0.0116 (12)	0.0187 (13)	0.0123 (12)	0.0108 (10)	0.000	0.000
O2	0.0120 (12)	0.0085 (11)	0.0222 (14)	0.0049 (9)	0.000	0.000
O3	0.0158 (9)	0.0382 (13)	0.0131 (9)	0.0166 (9)	−0.0042 (7)	−0.0098 (8)
O4	0.0052 (12)	0.0052 (12)	0.032 (3)	0.0026 (6)	0.000	0.000

Geometric parameters (Å, º) top

Nd1—Nd1ⁱ	3.9284 (3)	Si1—Nd2^viii	3.2527 (8)
Nd1—Nd1ⁱⁱ	3.9284 (3)	Si1—Nd2^xi	3.2527 (8)
Nd1—Nd2ⁱⁱⁱ	4.0666 (3)	Si1—Ca2^xi	3.2527 (8)
Nd1—Si1	3.2877 (9)	Si1—Ca2^viii	3.2527 (8)
Nd1—Si1ⁱ	3.1738 (9)	Si1—O1	1.621 (3)
Nd1—O1	2.721 (3)	Si1—O2	1.623 (3)
Nd1—O2^iv	2.463 (3)	Si1—O3^xii	1.629 (2)
Nd1—O3^v	2.3909 (19)	Si1—O3	1.629 (2)
Nd1—O3^vi	2.3909 (19)	O1—Nd2ⁱⁱⁱ	2.4230 (17)
Nd1—O3ⁱ	2.547 (2)	O1—Ca2ⁱⁱⁱ	2.4230 (17)
Nd1—O3^vii	2.547 (2)	O2—Nd1ⁱⁱⁱ	2.463 (3)
Nd1—O4	2.2681 (2)	O2—Ca1ⁱⁱⁱ	2.463 (3)
Nd2—Si1^vi	3.2527 (8)	O2—Nd2^viii	2.4715 (18)
Nd2—Si1^viii	3.2527 (8)	O2—Nd2^xi	2.4715 (18)
Nd2—Si1^ix	3.2527 (8)	O2—Ca2^xi	2.4715 (18)
Nd2—O1ⁱⁱⁱ	2.4231 (17)	O2—Ca2^viii	2.4715 (18)
Nd2—O1^iv	2.4231 (17)	O3—Nd1ⁱⁱ	2.547 (2)
Nd2—O1	2.4230 (17)	O3—Nd1^xiii	2.3909 (19)
Nd2—O2^viii	2.4715 (18)	O3—Ca1^xiii	2.3909 (19)
Nd2—O2^ix	2.4715 (18)	O3—Ca1ⁱⁱ	2.547 (2)
Nd2—O2^vi	2.4715 (18)	O3—Nd2^viii	2.830 (2)
Nd2—O3^x	2.830 (2)	O3—Ca2^viii	2.830 (2)
Nd2—O3^viii	2.830 (2)	O4—Nd1ⁱⁱ	2.2681 (2)
Nd2—O3^vi	2.830 (2)	O4—Nd1ⁱ	2.2681 (2)
Si1—Nd1ⁱⁱ	3.1738 (9)	O4—Ca1ⁱⁱ	2.2681 (2)
Si1—Ca1ⁱⁱ	3.1738 (9)	O4—Ca1ⁱ	2.2681 (2)

Nd1ⁱⁱ—Nd1—Nd1ⁱ	60.0	O3^viii—Nd2—Si1^ix	92.88 (4)
Nd1ⁱⁱ—Nd1—Nd2ⁱⁱⁱ	103.981 (6)	O3^vi—Nd2—Si1^viii	92.88 (4)
Nd1ⁱ—Nd1—Nd2ⁱⁱⁱ	150.673 (5)	O3^vi—Nd2—Si1^vi	30.06 (4)
Si1—Nd1—Nd1ⁱⁱ	51.249 (17)	O3^x—Nd2—Si1^viii	123.63 (4)
Si1ⁱ—Nd1—Nd1ⁱⁱ	113.889 (17)	O3^vi—Nd2—O3^viii	117.44 (2)
Si1ⁱ—Nd1—Nd1ⁱ	53.889 (17)	O3^x—Nd2—O3^vi	117.45 (2)
Si1—Nd1—Nd1ⁱ	111.249 (17)	O3^x—Nd2—O3^viii	117.45 (2)
Si1—Nd1—Nd2ⁱⁱⁱ	58.326 (14)	Nd1ⁱⁱ—Si1—Nd1	74.86 (2)
Si1ⁱ—Nd1—Nd2ⁱⁱⁱ	134.034 (14)	Nd1ⁱⁱ—Si1—Nd2^viii	81.18 (2)
Si1ⁱ—Nd1—Si1	165.14 (2)	Nd1ⁱⁱ—Si1—Nd2^xi	81.18 (2)
O1—Nd1—Nd1ⁱⁱ	80.66 (5)	Nd1ⁱⁱ—Si1—Ca2^xi	81.18 (2)
O1—Nd1—Nd1ⁱ	140.66 (5)	Nd1ⁱⁱ—Si1—Ca2^viii	81.18 (2)
O1—Nd1—Nd2ⁱⁱⁱ	35.26 (3)	Ca1ⁱⁱ—Si1—Nd1	74.86 (2)
O1—Nd1—Si1ⁱ	165.45 (5)	Ca1ⁱⁱ—Si1—Nd1ⁱⁱ	0.000 (8)
O1—Nd1—Si1	29.41 (5)	Ca1ⁱⁱ—Si1—Nd2^xi	81.18 (2)
O2^iv—Nd1—Nd1ⁱ	120.15 (6)	Ca1ⁱⁱ—Si1—Nd2^viii	81.18 (2)
O2^iv—Nd1—Nd1ⁱⁱ	179.85 (6)	Ca1ⁱⁱ—Si1—Ca2^xi	81.18 (2)
O2^iv—Nd1—Nd2ⁱⁱⁱ	75.88 (5)	Ca1ⁱⁱ—Si1—Ca2^viii	81.18 (2)
O2^iv—Nd1—Si1	128.60 (6)	Nd2^xi—Si1—Nd1	139.386 (19)
O2^iv—Nd1—Si1ⁱ	66.27 (6)	Nd2^viii—Si1—Nd1	139.386 (19)
O2^iv—Nd1—O1	99.18 (8)	Nd2^xi—Si1—Nd2^viii	65.31 (2)
O2^iv—Nd1—O3ⁱ	71.01 (7)	Nd2^xi—Si1—Ca2^viii	65.31 (2)
O2^iv—Nd1—O3^vii	71.01 (7)	Ca2^xi—Si1—Nd1	139.386 (19)
O3^v—Nd1—Nd1ⁱ	110.42 (6)	Ca2^viii—Si1—Nd1	139.386 (19)
O3ⁱ—Nd1—Nd1ⁱ	58.32 (5)	Ca2^xi—Si1—Nd2^xi	0.000 (10)
O3^v—Nd1—Nd1ⁱⁱ	94.99 (5)	Ca2^viii—Si1—Nd2^viii	0.000 (10)
O3^vi—Nd1—Nd1ⁱ	110.42 (6)	Ca2^xi—Si1—Nd2^viii	65.31 (2)
O3ⁱ—Nd1—Nd1ⁱⁱ	109.12 (5)	Ca2^xi—Si1—Ca2^viii	65.31 (2)
O3^vii—Nd1—Nd1ⁱⁱ	109.12 (5)	O1—Si1—Nd1	55.53 (10)
O3^vii—Nd1—Nd1ⁱ	58.32 (5)	O1—Si1—Nd1ⁱⁱ	130.39 (10)
O3^vi—Nd1—Nd1ⁱⁱ	94.99 (5)	O1—Si1—Ca1ⁱⁱ	130.39 (10)
O3^vi—Nd1—Nd2ⁱⁱⁱ	94.52 (6)	O1—Si1—Nd2^xi	136.87 (6)
O3ⁱ—Nd1—Nd2ⁱⁱⁱ	112.89 (5)	O1—Si1—Nd2^viii	136.87 (6)
O3^vii—Nd1—Nd2ⁱⁱⁱ	146.38 (5)	O1—Si1—Ca2^viii	136.87 (6)
O3^v—Nd1—Nd2ⁱⁱⁱ	42.90 (6)	O1—Si1—Ca2^xi	136.87 (6)
O3^v—Nd1—Si1	77.25 (5)	O1—Si1—O2	113.16 (14)
O3^vi—Nd1—Si1ⁱ	106.70 (5)	O1—Si1—O3	111.34 (9)
O3ⁱ—Nd1—Si1ⁱ	30.67 (4)	O1—Si1—O3^xii	111.34 (9)
O3ⁱ—Nd1—Si1	145.64 (5)	O2—Si1—Nd1ⁱⁱ	116.45 (10)
O3^vii—Nd1—Si1ⁱ	30.67 (4)	O2—Si1—Nd1	168.69 (10)
O3^v—Nd1—Si1ⁱ	106.70 (5)	O2—Si1—Ca1ⁱⁱ	116.45 (10)
O3^vii—Nd1—Si1	145.64 (5)	O2—Si1—Nd2^viii	47.71 (6)
O3^vi—Nd1—Si1	77.25 (5)	O2—Si1—Nd2^xi	47.70 (6)
O3^v—Nd1—O1	70.49 (5)	O2—Si1—Ca2^xi	47.70 (6)
O3ⁱ—Nd1—O1	146.95 (5)	O2—Si1—Ca2^viii	47.71 (6)
O3^vi—Nd1—O1	70.49 (5)	O2—Si1—O3^xii	107.49 (10)
O3^vii—Nd1—O1	146.95 (5)	O2—Si1—O3	107.49 (10)
O3^vi—Nd1—O2^iv	84.96 (5)	O3^xii—Si1—Nd1ⁱⁱ	52.89 (7)
O3^v—Nd1—O2^iv	84.96 (5)	O3^xii—Si1—Nd1	78.94 (9)
O3^vii—Nd1—O3ⁱ	61.28 (9)	O3—Si1—Nd1ⁱⁱ	52.89 (7)
O3^vi—Nd1—O3^vii	77.13 (4)	O3—Si1—Nd1	78.94 (9)
O3^vi—Nd1—O3ⁱ	136.63 (7)	O3—Si1—Ca1ⁱⁱ	52.89 (7)
O3^v—Nd1—O3^vii	136.63 (7)	O3^xii—Si1—Ca1ⁱⁱ	52.89 (7)
O3^v—Nd1—O3ⁱ	77.13 (4)	O3^xii—Si1—Nd2^xi	60.46 (9)
O3^v—Nd1—O3^vi	137.40 (11)	O3—Si1—Nd2^xi	111.52 (8)
O4—Nd1—Nd1ⁱ	30.0	O3^xii—Si1—Nd2^viii	111.52 (8)
O4—Nd1—Nd1ⁱⁱ	30.0	O3—Si1—Nd2^viii	60.46 (9)
O4—Nd1—Nd2ⁱⁱⁱ	130.005 (6)	O3^xii—Si1—Ca2^xi	60.46 (9)
O4—Nd1—Si1	81.249 (17)	O3^xii—Si1—Ca2^viii	111.52 (8)
O4—Nd1—Si1ⁱ	83.888 (17)	O3—Si1—Ca2^xi	111.52 (8)
O4—Nd1—O1	110.66 (5)	O3—Si1—Ca2^viii	60.46 (9)
O4—Nd1—O2^iv	150.15 (6)	O3^xii—Si1—O3	105.63 (15)
O4—Nd1—O3^vi	104.58 (5)	Nd2ⁱⁱⁱ—O1—Nd1	104.33 (7)
O4—Nd1—O3^vii	83.45 (5)	Nd2—O1—Nd1	104.33 (7)
O4—Nd1—O3ⁱ	83.45 (5)	Nd2ⁱⁱⁱ—O1—Nd2	93.89 (9)
O4—Nd1—O3^v	104.58 (5)	Ca2ⁱⁱⁱ—O1—Nd1	104.33 (7)
Si1^vi—Nd2—Si1^viii	93.628 (14)	Ca2ⁱⁱⁱ—O1—Nd2	93.89 (9)
Si1^ix—Nd2—Si1^viii	93.628 (14)	Ca2ⁱⁱⁱ—O1—Nd2ⁱⁱⁱ	0.000 (11)
Si1^ix—Nd2—Si1^vi	93.628 (14)	Si1—O1—Nd1	95.06 (11)
O1^iv—Nd2—Si1^ix	98.07 (6)	Si1—O1—Nd2	127.73 (7)
O1ⁱⁱⁱ—Nd2—Si1^vi	165.42 (5)	Si1—O1—Nd2ⁱⁱⁱ	127.73 (7)
O1^iv—Nd2—Si1^vi	94.28 (5)	Si1—O1—Ca2ⁱⁱⁱ	127.73 (7)
O1—Nd2—Si1^vi	98.07 (6)	Nd1ⁱⁱⁱ—O2—Nd2^viii	115.89 (7)
O1ⁱⁱⁱ—Nd2—Si1^viii	98.07 (6)	Nd1ⁱⁱⁱ—O2—Nd2^xi	115.89 (7)
O1^iv—Nd2—Si1^viii	165.42 (5)	Nd1ⁱⁱⁱ—O2—Ca2^viii	115.89 (7)
O1ⁱⁱⁱ—Nd2—Si1^ix	94.28 (5)	Nd1ⁱⁱⁱ—O2—Ca2^xi	115.89 (7)
O1—Nd2—Si1^ix	165.42 (5)	Ca1ⁱⁱⁱ—O2—Nd1ⁱⁱⁱ	0.000 (9)
O1—Nd2—Si1^viii	94.28 (5)	Ca1ⁱⁱⁱ—O2—Nd2^xi	115.89 (7)
O1ⁱⁱⁱ—Nd2—O1	72.49 (7)	Ca1ⁱⁱⁱ—O2—Nd2^viii	115.89 (7)
O1^iv—Nd2—O1ⁱⁱⁱ	72.49 (7)	Ca1ⁱⁱⁱ—O2—Ca2^xi	115.89 (7)
O1^iv—Nd2—O1	72.49 (7)	Ca1ⁱⁱⁱ—O2—Ca2^viii	115.89 (7)
O1—Nd2—O2^vi	125.79 (8)	Nd2^xi—O2—Nd2^viii	90.49 (8)
O1^iv—Nd2—O2^vi	94.22 (6)	Nd2^xi—O2—Ca2^viii	90.49 (8)
O1—Nd2—O2^ix	153.93 (8)	Nd2^viii—O2—Ca2^viii	0.0
O1^iv—Nd2—O2^viii	153.93 (8)	Ca2^xi—O2—Nd2^viii	90.49 (8)
O1ⁱⁱⁱ—Nd2—O2^ix	94.22 (6)	Ca2^xi—O2—Nd2^xi	0.0
O1ⁱⁱⁱ—Nd2—O2^vi	153.93 (8)	Ca2^xi—O2—Ca2^viii	90.49 (8)
O1^iv—Nd2—O2^ix	125.79 (8)	Si1—O2—Nd1ⁱⁱⁱ	122.72 (13)
O1—Nd2—O2^viii	94.22 (6)	Si1—O2—Ca1ⁱⁱⁱ	122.72 (13)
O1ⁱⁱⁱ—Nd2—O2^viii	125.79 (8)	Si1—O2—Nd2^xi	103.23 (9)
O1ⁱⁱⁱ—Nd2—O3^vi	139.76 (6)	Si1—O2—Nd2^viii	103.23 (9)
O1—Nd2—O3^viii	87.87 (7)	Si1—O2—Ca2^xi	103.23 (9)
O1^iv—Nd2—O3^x	68.15 (7)	Si1—O2—Ca2^viii	103.23 (9)
O1—Nd2—O3^vi	68.15 (7)	Nd1^xiii—O3—Nd1ⁱⁱ	116.16 (8)
O1ⁱⁱⁱ—Nd2—O3^x	87.87 (7)	Nd1^xiii—O3—Ca1ⁱⁱ	116.16 (8)
O1^iv—Nd2—O3^viii	139.76 (6)	Nd1ⁱⁱ—O3—Nd2^viii	101.98 (7)
O1^iv—Nd2—O3^vi	87.87 (7)	Nd1^xiii—O3—Nd2^viii	101.99 (8)
O1ⁱⁱⁱ—Nd2—O3^viii	68.15 (7)	Nd1ⁱⁱ—O3—Ca2^viii	101.98 (7)
O1—Nd2—O3^x	139.76 (6)	Nd1^xiii—O3—Ca2^viii	101.99 (8)
O2^vi—Nd2—Si1^ix	64.81 (6)	Ca1ⁱⁱ—O3—Nd1ⁱⁱ	0.000 (14)
O2^ix—Nd2—Si1^viii	64.81 (6)	Ca1^xiii—O3—Nd1^xiii	0.0
O2^ix—Nd2—Si1^vi	98.63 (5)	Ca1^xiii—O3—Nd1ⁱⁱ	116.16 (8)
O2^ix—Nd2—Si1^ix	29.07 (6)	Ca1^xiii—O3—Ca1ⁱⁱ	116.16 (8)
O2^viii—Nd2—Si1^viii	29.07 (6)	Ca1ⁱⁱ—O3—Nd2^viii	101.98 (7)
O2^viii—Nd2—Si1^ix	98.63 (5)	Ca1^xiii—O3—Nd2^viii	101.99 (8)
O2^viii—Nd2—Si1^vi	64.81 (6)	Ca1ⁱⁱ—O3—Ca2^viii	101.98 (7)
O2^vi—Nd2—Si1^vi	29.07 (6)	Ca1^xiii—O3—Ca2^viii	101.99 (8)
O2^vi—Nd2—Si1^viii	98.63 (5)	Ca2^viii—O3—Nd2^viii	0.000 (14)
O2^vi—Nd2—O2^viii	75.14 (6)	Si1—O3—Nd1^xiii	141.68 (11)
O2^vi—Nd2—O2^ix	75.14 (6)	Si1—O3—Nd1ⁱⁱ	96.43 (9)
O2^ix—Nd2—O2^viii	75.14 (6)	Si1—O3—Ca1^xiii	141.68 (11)
O2^vi—Nd2—O3^viii	125.24 (6)	Si1—O3—Ca1ⁱⁱ	96.43 (9)
O2^vi—Nd2—O3^x	66.19 (7)	Si1—O3—Nd2^viii	89.48 (10)
O2^viii—Nd2—O3^viii	58.84 (7)	Si1—O3—Ca2^viii	89.48 (10)
O2^ix—Nd2—O3^x	58.84 (7)	Nd1ⁱ—O4—Nd1ⁱⁱ	120.0
O2^vi—Nd2—O3^vi	58.85 (7)	Nd1—O4—Nd1ⁱⁱ	120.0
O2^viii—Nd2—O3^x	125.24 (6)	Nd1—O4—Nd1ⁱ	120.0
O2^ix—Nd2—O3^viii	66.19 (7)	Nd1—O4—Ca1ⁱⁱ	120.0
O2^ix—Nd2—O3^vi	125.24 (6)	Nd1ⁱ—O4—Ca1ⁱⁱ	120.0
O2^viii—Nd2—O3^vi	66.19 (7)	Nd1—O4—Ca1ⁱ	120.0
O3^vi—Nd2—Si1^ix	123.63 (4)	Ca1ⁱ—O4—Nd1ⁱ	0.000 (16)
O3^viii—Nd2—Si1^viii	30.06 (4)	Ca1ⁱⁱ—O4—Nd1ⁱⁱ	0.000 (9)
O3^x—Nd2—Si1^ix	30.06 (4)	Ca1ⁱ—O4—Nd1ⁱⁱ	120.0
O3^x—Nd2—Si1^vi	92.88 (4)	Ca1ⁱ—O4—Ca1ⁱⁱ	120.0
O3^viii—Nd2—Si1^vi	123.63 (4)

Symmetry codes: (i) −x+y, −x, −z+1/2; (ii) −y, x−y, z; (iii) −x+y+1, −x+1, −z+1/2; (iv) −y+1, x−y, z; (v) y, −x+y, z+1/2; (vi) y, −x+y, −z; (vii) −x+y, −x, z; (viii) −x+1, −y+1, −z; (ix) x−y+1, x, z−1/2; (x) x−y+1, x, −z; (xi) x−y, x, z+1/2; (xii) x, y, −z+1/2; (xiii) x−y, x, z−1/2.

Selected bond lengths (Å) top

Nd1—O1	2.721 (3)	Nd2—O2^iv	2.4715 (18)
Nd1—O2ⁱ	2.463 (3)	Nd2—O3^v	2.830 (2)
Nd1—O3ⁱⁱ	2.3909 (19)	Si1—O1	1.621 (3)
Nd1—O3ⁱⁱⁱ	2.547 (2)	Si1—O2	1.623 (3)
Nd1—O4	2.2681 (2)	Si1—O3^vi	1.629 (2)
Nd2—O1ⁱ	2.4231 (17)

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

Experimental details

Crystal data
Chemical formula	Ca_2.49Nd_7.51(SiO₄)₆O_1.75
M_r	1763.24
Crystal system, space group	Hexagonal, P6₃/m
Temperature (K)	298
a, c (Å)	9.5507 (3), 7.0513 (3)
V (Å³)	557.03 (3)
Z	1
Radiation type	Mo Kα
µ (mm⁻¹)	18.18
Crystal size (mm)	0.05 × 0.05 × 0.05

Data collection
Diffractometer	Agilent SuperNova (single source at offset, Eos detector)
Absorption correction	Multi-scan (CrysAlis PRO; Agilent, 2012)
T_min, T_max	0.717, 1.000
No. of measured, independent and observed [I > 2σ(I)] reflections	2616, 878, 813
R_int	0.024
(sin θ/λ)_max (Å⁻¹)	0.821

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.019, 0.035, 1.11
No. of reflections	878
No. of parameters	42
Δρ_max, Δρ_min (e Å⁻³)	0.79, −0.86

Computer programs: CrysAlis PRO (Agilent, 2012), SHELXS97 (Sheldrick, 2008), SHELXL2014 (Sheldrick, 2015), OLEX2 (Dolomanov et al., 2009) and publCIF (Westrip, 2010).

Acknowledgements

The authors thank the Hercules Foundation for supporting the purchase of the diffractometer through project AKUL/09/0035.

References

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