Redetermination of Mg2B25 based on single-crystal X-ray data

The crystal structure of Mg2B25, dimagnesium pentaeicosaboride, was reexamined from single-crystal X-ray diffraction data. The structural model previously reported on the basis of powder X-ray diffraction data [Giunchi et al. (2006 ▶). Solid State Sci. 8, 1202–1208] has been confirmed, although a much higher precision refinement was achieved, leading to much smaller standard uncertainties on bond lengths and refined occupancy factors. Moreover, all atoms were refined with anisotropic displacement parameters. Mg2B25 crystallizes in the β-boron structure type and is isostructural with other rhombohedral compounds of the boron-rich metal boride family. Magnesium atoms are found in interstitial sites on special positions (two with site symmetry .m, one with .2 and one with 3m), all with partial occupancies.

The crystal structure of Mg 2 B 25 , dimagnesium pentaeicosaboride, was reexamined from single-crystal X-ray diffraction data. The structural model previously reported on the basis of powder X-ray diffraction data [Giunchi et al. (2006). Solid State Sci. 8, 1202-1208] has been confirmed, although a much higher precision refinement was achieved, leading to much smaller standard uncertainties on bond lengths and refined occupancy factors. Moreover, all atoms were refined with anisotropic displacement parameters. Mg 2 B 25 crystallizes in the -boron structure type and is isostructural with other rhombohedral compounds of the boron-rich metal boride family. Magnesium atoms are found in interstitial sites on special positions (two with site symmetry .m, one with .2 and one with 3m), all with partial occupancies. MgB 20 (Brutti et al., 2002), Mg 2 B 25 exhibits very small differences in the unit-cell parameters, but significant differences in the occupation of the interstitial magnesium sites, also with the identification of a new structural site never observed in the related metal borides. This site was labelled as Mg(N) by Giunchi et al. (2006). However, since the presence of partially occupied structural sites in the model obtained from XRPD data, some scepticism persisted in the literature about the true nature of the structure of Mg 2 B 25 . Here we present a re-examination of the crystal structure of Mg 2 B 25 based on single-crystal X-ray diffraction data. The structural model previously reported by Giunchi et al. (2006) has been confirmed, although a much higher precision refinement was achieved, accompanied with the refinement of all atoms with anisotropic displacement parameters.

Related literature
Mg 2 B 25 adopts the β-boron structure and crystallizes isostructurally with other rhombohedral compounds of the boronrich metal boride family (Fig. 1). Magnesium atoms occupy interstitial sites with partial occupancies that can be interpreted with the aid of the analysis of impossible interatomic Mg···Mg contacts.

Experimental
A small crystal fragment was cut out from a polished thin section of a MgB 2 bulk sample. This sample was obtained by the Mg-RLI process (Giunchi, 2003) under particular mild conditions (low temperature and short heat treatment) in order to have a consistent Mg 2 B 25 fraction in the final sample. Thus, this small crystal does not have to be considered as a representative product of the Mg-RLI process, but just a sample synthesized ad hoc. The extracted crystal fragment was used for the X-ray single-crystal diffraction study.

Refinement
The crystal structure refinement was performed starting from the atomic coordinates reported by Giunchi et al. (2006).
Convergence was rapidly obtained for an anisotropic model of the structure. The site-occupancy factor of the Mg positions and of some of the B positions (i.e., B4 and B13) was left free to vary.

Figure 1
The  where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 0.14 e Å −3 Δρ min = −0.31 e Å −3 Special details 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 F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.