New calorimetric determinations of molar heat capacity C(p,m) of lithium tetrahydroborate (LiBH(4)) 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 LiBH(4) C(p,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 Delta(trs)S = 13.11 +/- 0.23J 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 C(p,m) of LiBH(4) "molten salt" is proposed.
Above room temperature heat capacity and phase transition of lithium tetrahydroborate
BARICCO, Marcello
2011-01-01
Abstract
New calorimetric determinations of molar heat capacity C(p,m) of lithium tetrahydroborate (LiBH(4)) 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 LiBH(4) C(p,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 Delta(trs)S = 13.11 +/- 0.23J 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 C(p,m) of LiBH(4) "molten salt" is proposed.
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