Topological Features of A+B2+[B5O9] Layered Pentaborates: Structural Changes in NaSr[B5O9] at High Temperatures or Why KCa[B5O9] Is Unstable?

Temperature-dependent structural investigations of layered pentaborate NaSr[B5O9] belonging to a family of compounds with the general formula A(+)B(2+)[B5O9] (A = Na, K; B = Ca, Sr, Ba, Pb) have been performed. The thermal expansion shows slightly anisotropic behavior (alpha(11) = 7, alpha(22) = 10, alpha(33) = 22 x 10(-6) degrees C1- at 100 degrees C) which is caused by preferential orientation of the pentaborate [B5O11] rigid groups (fundamental building units, FBUs) of the layer. The monoclinic beta angle increases above 600 degrees C due to a heterovalent Sr2+ <-> Na+ cation substitution. Members of the A(+)B(2+)[B5O9] family are represented by two structural types which are characterized by the different orientations of the pentaborate FBUs within the layers. A topological analysis of the layers has been made, and corresponding classifications of the cationic net are given. It was found that the type of the structure strongly depends on the difference in the ionic radii of alkaline A(+) and alkaline-earth B2+ cations. Our ab initio calculations also indicate the KCa[B5O9] compound unstability. Based on the density functional theory (DFT) calculations, it was also shown that the compound KCa[B5O9] is unstable under ambient conditions. According to the geometry analysis, the reason for the instability is the K+ and Ca2+ cations are size mismatched.

Automated summary

Temperature-dependent structural investigations were conducted on layered pentaborate NaSr[B5O9], showing slightly anisotropic thermal expansion behavior and an increase in monoclinic beta angle above 600 degrees C. The structure type strongly depends on the ionic radii difference between alkaline A(+) and alkaline-earth B2+ cations. Furthermore, ab initio calculations revealed the instability of the compound KCa[B5O9] due to size mismatch of the K+ and Ca2+ cations.