Thermochemistry of Li1+xMn2−xO4 (0⩽x⩽1/3) spinel

✍ Scribed by Miaojun Wang; Alexandra Navrotsky

• Publisher: Elsevier Science
• Year: 2005
• Tongue: English
• Weight: 237 KB
• Volume: 178
• Category: Article
• ISSN: 0022-4596
• DOI: 10.1016/j.jssc.2004.12.007

✦ Synopsis

Lithium substituted Li 1+x Mn 2Àx O 4 spinel samples in the entire solid solution range (0pxp1=3) were synthesized by solid-state reaction. The samples with xo0:25 are stoichiometric and those with xX0:25 are oxygen deficient. High-temperature oxide melt solution calorimetry in molten 3Na 2 O Á 4MoO 3 at 974 K was performed to determine their enthalpies of formation from constituent binary oxides at 298 K. The cubic lattice parameter was determined from least-squares fitting of powder XRD data. The variations of the enthalpy of formation from oxides and the lattice parameter with x follow similar trends. The enthalpy of formation from oxides becomes more exothermic with x for stoichiometric compounds (xo0:25) and deviates endothermically from this trend for oxygen-deficient samples (xX0:25). This energetic trend is related to two competing substitution mechanisms of lithium for manganese (oxidation of Mn 3+ to Mn 4+ versus formation of oxygen vacancies). For stoichiometric spinels, the oxidation of Mn 3+ to Mn 4+ is dominant, whereas for oxygen-deficient compounds both mechanisms are operative. The endothermic deviation is ascribed to the large endothermic enthalpy of reduction.