Crystal Structures and Phase Evolutions of Potassium Decahydro-closo-Decaborate and Decahydro-1-Carba-closo-Decaborate: K2B10H10 and K-1-CB9H10.

The ordered crystal structures and phase evolutions of potassium decahydro-closo-decaborate (K2B10H10) and monocarba-closo-decaborate (K-1-CB9H10) were investigated by using synchrotron X-ray diffraction in combination with first-principles calculations and neutron vibrational spectroscopy. Upon cooling from room temperature to low temperature (100 K), both compounds exhibit order-to-order structure transitions with two ordered K2B10H10 and three ordered K-1-CB9H10 structures being identified and solved. For K2B10H10, a gradual monoclinic to monoclinic phase transition is observed. In contrast, KCB9H10 undergoes a more abrupt first-order tetragonal-to-orthorhombic phase transition. Our detailed structural analyses indicate that these transitions are driven by the reorientation of the quasi-ellipsoidal anions, optimizing the anion packing efficiency and cation-anion interactions. Notably, the reduced cation stoichiometry and weaker cation-anion interactions in KCB9H10 lead to significantly lower anion reorientational barriers compared with K2B10H10. Uncovering the orientational flexibility of these quasi-ellipsoidal anions at low temperatures is essential for understanding their dramatic transformation into superionic conductors at elevated temperatures. This work provides the structural foundation necessary for designing and developing next-generation polyborate-based solid-state electrolytes.