Phase Transition in Lanthanum Tris(dipivaloylmethanate) near 455 K: Thermal and Structural Analysis
摘要
Thermogravimetric analysis with simultaneous DTA signal recording is performed for lanthanum tris-dipivaloylmethanate, [La(dpm)3]2. An endothermic effect associated with an irreversible phase transition is revealed in the 455-474 K range. The phase transition temperature of 455.2 ± 0.5 K is established by differential scanning calorimetry. Crystals of the high-temperature phase are obtained from the gas phase by sublimation transport in an evacuated sealed ampoule under a temperature gradient from 470 K to 480 K. It is shown by single-crystal XRD that the observed phase transition is caused by a change in the [La(dpm)3]2 molecular structure. The high‑temperature isomer (triclinic, a = 11.43 Å, b = 12.98 Å, c = 15.14 Å, α = 101.5°, β = 101.0°, γ = 113.6°, V = 1929 Å3, Z = 1) is also formed by dimeric molecules, but the coordination environment of the lanthanum atom changes from a distorted trigonal monocapped prism (CN = 7) to a distorted square antiprism (CN = 8), and the number of μ2-oxygen atoms increases from 2 to 4.