Crystal Structures and Phase Evolutions of Potassium Decahydro- closo -Decaborate and Decahydro-1-Carba- closo -Decaborate: K ₂ B ₁₀ H ₁₀ and K-1-CB ₉ H ₁₀

High Resolution Image Download MS PowerPoint Slide The ordered crystal structures and phase evolutions of potassium decahydro- closo -decaborate (K 2 B 10 H 10 ) and monocarba- closo -decaborate (K-1-CB 9 H 10 ) 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 K 2 B 10 H 10 and three ordered K-1-CB 9 H 10 structures being identified and solved. For K 2 B 10 H 10, a gradual monoclinic to monoclinic phase transition is observed. In contrast, KCB 9 H 10 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 KCB 9 H 10 lead to significantly lower anion reorientational barriers compared with K 2 B 10 H 10 . 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.