A refinement of the crystal structure of herderite, CaBePO₄OH Available to Purchase

The crystal structure of herderite [a = 9.789(2), b = 7.661(1), c = 4.804(1) Å, β = 90.02(1)°, P2₁/a] has been refined by least-squares methods to an unweighted R of 0.036. The PO₄ tetrahedra in the (BePO₄OH)_∞ sheet are more regular than the BeO₃OH tetrahedra, reflecting the more directional nature of the P-O bond and the absence of any shared O⋯O edges in the PO₄ tetrahedra. Plots of (O-T-O)₃, the average of the three tetrahedral angles common to a T-O bond, vs the length of the bonds in the two tetrahedra are consistent with extended Hückel molecular orbital theory. Variations in the P-O bond lengths can be rationalized in terms of Mulliken bond-overlap populations, the Baur (1970) extended electrostatic valence rule, and the Brown-Shannon (1973) relationship between bond length and bond strength.