Low-temperature thermodynamic properties of dimeric tris(2,2,6,6-tetramethyl-3,5-heptanedionato) europium

A sample of dimeric tris(2,2,6,6-tetramethyl-3,5-heptanedionato) europium ([Eu(C₁₁H₁₉O₂)₃]₂ or dipivaloylmethanate europium) was synthesized. Chemical analysis, elemental analysis and X-ray crystallography were used to characterize composition and structure of the complex. Low-temperature heat capacity of the sample was measured by adiabatic method in a temperature range from 8.63 K to 312.90 K. The Debye temperature at 0 K was calculated. Thermodynamic functions (entropy, enthalpy, reduced Gibbs energy) in a range (0–312) K have been calculated using the obtained experimental heat capacity data. An anomaly in the functional behaviour of heat capacity was detected in a temperature range from 250 K to 270 K, with a maximum at T_c = (262.36 ± 0.01) K. A reversible colour change (thermochromism) of europium dipivaloylmethanate crystals upon cooling below 245 K was detected. Anomalous contributions to entropy and enthalpy have been revealed. The nature of the effects detected is discussed.