Above room temperature heat capacity and phase transition of lithium tetrahydroborate

New calorimetric determinations of molar heat capacity Cp,m of lithium tetrahydroborate (LiBH4) were performed in order to analyze the origin of the previously observed “anomaly” before the polymorphous transition at 386 K. The above room temperature dependence of LiBH4Cp,m was measured until approaching the melting point of the compound at 553 K and the abnormal behaviour was attributed to lattice defects independent of the main crystalline phase transition. As a result, lower entropy of the transition ΔtrsS = 13.11 ± 0.23 J K−1 mol−1 than that proposed in literature was obtained, which is in agreement with the observed anisotropy and crystal density decrease before the transition from recent X-ray diffraction indexing. An estimate of the liquid Cp,m of LiBH4 “molten salt” is proposed.

► LiBH4 as hydrogen storage material.
► Thermodynamic properties of this compound are lacking in literature. Molar heat capacity, Cp,m, of the different phases (ortho-phase, hexa-phase and liquid phase) needs to be determined.
► Cp,m “anomaly” is observed to be independent of the main phase transition.