Breakdown of Arrhenius law of temperature-dependent vacancy concentration in fcc lanthanum

摘要: We measured the temperature dependent equilibrium vacancy concentration using in-situ positron annihilation spectroscopy in order to determine enthalpy $H_\text{f}$ and entropy $S_\text{f}$ of formation elementary fcc-La. The Arrhenius law applied for data analysis, however, is shown fail explaining unexpected high values both $H_\text{f}$: particular $S_\text{f}=17(2)~k_\text{B}$ one magnitude larger compared other elemental metals, experimental value found be more than three standard deviations off theoretical $H_\text{f}=1.46~\text{eV}$ (our \acs{dft} calculation La at $T=0~\text{K}$). A consistent explanation given beyond classical approach terms a dependence with $S_\text{f}^\prime=-0.0120(14)~k_\text{B}/\text{K}$ accounting anharmonic potential introduced by vacancies.