Discovery of five- and six-fold coordinated beryllium

For a long time it has been believed that elements of the second period of the Periodic Table are not able to possess coordination higher that four due to the absence of 3d atomic orbitals. Our team has discovered high pressure phases featuring Be in five- and six-fold coordination by oxygen atoms.

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Due to the broad technological applications of beryllium oxocompounds, their structure and chemical bonding became a focus of a number of recent experimental and theoretical studies. At ambient conditions BeO crystallizes in the hexagonal wurtzite structure, featuring tetrahedrally coordinated Be2+ and O2− ions. Bonding of beryllium to four oxygen atoms with the formation of BeO4 tetrahedra is also exclusive for its natural occurrence. Cases with Be in coordination higher than four have not been observed experimentally for inorganic compounds, though recent ab initio calculation studies have predicted the formation of BeO6 octahedra in BeO2 and BeO. 

In situ high-pressure X-ray diffraction a powerful tool in simultaneous synthesis and structural characterization of new compounds. Pressure-induced densification of matter is accompanied by the rearrangement of atomic bonds and structural units in order to fill the available space as effective as possible, which usually results in the increase of the atomic coordination numbers. Thus, in a recent high-pressure study on CaB2Si2O8 we have found rare five-fold coordination state of silicon with a formation of SiO5 trigonal bypiramids (Pakhomova et al., 2017). This case has inspired us to probe high-pressure behavior of structurally similar compound CaBe2P2O8. The question whether beryllium could experience the same increase in coordination number is of general chemical interest as well as of particular importance for the understanding the nature of Be–O bonding. 

In our present high-pressure experiments, we pressurized a single crystal of CaBe2P2O8 in a diamond anvil cell and tracked structural changes using single-crystal X-ray diffraction. To our great astonishment, we discovered two two high pressure phases those crystal structures feature beryllium being coordinated by five and six oxygens with formation of BeO5 trigonal bipyramids and BeO6 octahedra. In addition, performed ab initio simulations were found to be in excellent agreement between measured and calculated high-pressure behavior. 

The crystal structures of high-pressure modifications of CaBe2P2O8: at 83.2 GPa (left)  and at 89.5 GPa (right). POn polyhedra are given in yellow, BeOn polyhedra are given in light blue. Ca and O atoms are presented as blue and red spheres. 

Both experimental observations of the BeO5/BeO6 configurations and ab initio calculations are in line with previous quantum chemical calculations and demonstrate that the involvement of d orbitals is not mandatory for the formation of species with trigonal–bipyramidal and octahedral geometries. Instead, an electron-deficient multicenter bonding can be proposed as a mechanism of formation of such exotic configurations and, generally, as a densification mechanism for the CaP2Be2O8 crystal structure adopting to high-pressure conditions.

The present study further proves the powerful capabilities of high pressure as a tool for tuning chemical properties of matter. Growing interest of the chemical community in high-pressure SCXRD techniques using diamond anvil cells (DACs) ensures that the upcoming studies will bring further examples of unique phases as well as provide a solid experimental basis for the future development of novel high-pressure crystal chemistry.

Learn more about our experiments and results in: 

Penta- and hexa-coordinated beryllium and phosphorus in high-pressure modifications of CaBe2P2O8; A.Pakhomova, G. Aprilis, M. Bykov, L.Gorelova, S. Krivovichev, M. Belov, I. Abrikosov, L.Dubrovinsky; Nature Communications, 2019; DOI: 10.1038/s41467-019-10589-z

Post and Image are created by Anna Pakhomova and Leonid Dubrovinsky. 


Pakhomova, A. et al. A closer look into close packing: pentacoordinated silicon in a high-pressure polymorph of danburite. IUCr. 4, 671-677 (2017)

Anna Pakhomova

beamline scientist & postdoctoral fellow, Deutsches Elektronen-Synchrotron