Superlinear Temperature-Dependent Resistivity and Structural Phase Transition in BaNi$_2$P$_4$

Abstract

The mechanism of anomalous superlinear temperature-dependent resistivity, $\rho (T)$, in the metallic unconventional clathrate BaNi$_2$P$_4$ was studied by examining its evolution with artificial disorder induced by low-temperature ($\sim$ 20 K) 2.5 MeV electron irradiation. We find a dominant effect of the tetragonal-orthorhombic transition at $T_s$ ($ \sim$373 to 378 K, depending on heat cycle rate and direction) on $\rho (T)$, with standard metallic $T-$linear resistivity above the transition and anomalous behavior in the orthorhombic phase below. The transition is accompanied by the formation of structural domains and a notable (about 4~K) hysteresis in the magnetization and resistivity measurements, clearly showing its first order character. Matthiessen rule is obeyed both above and below the transition, suggesting negligible changes in the electronic structure. This conclusion is supported by the smooth evolution of the Hall effect through the transition. The Hall number is in good agreement with band structure calculations both above and below the transition. The transition temperature is notably suppressed with electron irradiation. Raman scattering at temperatures above room temperature find softening of local Ba vibration mode in the orthorhombic phase on approaching the transition. $^{31}P$ NMR line splits in the orthorhombic phase, suggesting a partial shift of the Ba atom from the central position in the cage. We suggest that local Ba rattling leads to enhanced residual contribution to resistivity in the high temperature tetragonal phase, the decay of which is responsible for the anomalous temperature-dependent resistivity in the orthorhombic phase.