Anomalous lattice specific heat and rattling phonon modes in quadruple perovskites

TL;DR

The paper addresses anomalous low-temperature lattice contributions to the specific heat in quadruple perovskites ACu$_3$Fe$_2$Re$_2$O$_{12}$ and tests the presence of rattling phonon modes. It combines detailed Cp measurements with Debye–Einstein fits and first-principles DFT+U calculations, demonstrating clear rattling behavior in Mn- and Cu-containing compounds and outlining a pseudo-Jahn-Teller mechanism as a probable origin. The results reveal low-energy Einstein-like modes and a complex energy landscape for A-site displacements, linking structural anharmonicity to magnetic and electronic properties in these half-metallic ferrimagnets. The work suggests potential for reduced thermal conductivity and thermoelectric relevance, and lays groundwork for deeper microscopic modeling of rattling in complex oxides.

Abstract

Experimental data on the specific heat $C_p$ of quadruple perovskites ACu$_3$Fe$_2$Re$_2$O$_{12}$ (A = Mn, Cu, La, Ce, Dy) are presented, demonstrating an anomalous concave-down $C_p/T$ vs. $T^2$ curve and a bell-shaped feature in $β(T) = (C_p - γT)/T^3$ plotted against $T$ on a logarithmic scale. This feature is most pronounced for A = Cu and Mn. These findings can be explained by the rattling phenomenon, previously identified in other systems such as filled skutterudites and $β$-pyrochlores. Using first-principles DFT+U calculations, the presence of a rattling mode in A = Mn system is directly confirmed. A qualitative interpretation of the rattling mechanism in terms of a pseudo-Jahn-Teller effect is proposed.

Figures (8)

• Figure 1: The crystal structure of ACu$_3$Fe$_2$Re$_2$O$_{12}$ quadruple perovskites. $A$ ions sits in the distorted icosahedral cage and can rattle.
• Figure 2: $C_p/T$ vs $T^2$ of quadruple perovskites $A$Cu$_3$Fe$_2$Re$_2$O$_{12}$, (a) $A$ = Cu, (b) $A$ = Ca, (c) $A$ = Mn, (d) $A$ = La, (e) $A$ = Dy, $A$ = Ce. The solid lines show the fitting results basing on function (\ref{['C_p_fitting']}) with one $r$ parameter for $A$Cu$_3$Fe$_2$Re$_2$O$_{12}$ ($A$=Cu, Ca, La, Ce and Dy) and two parameters $r_1$ and $r_2$ in the sum for MnCu$_3$Fe$_2$Re$_2$O$_{12}$. The Sommerfeld (electronic), Debye and Einstein contributions are shown by different lines, notations being given in (a).
• Figure 3: Temperature-dependent $\beta(T)=(C_p-\gamma T)/T^3$ of quadruple perovskites $A$Cu$_3$Fe$_2$Re$_2$O$_{12}$. (a) $A$ = Cu, (b) $A$ = Ca, (c) $A$ = Mn, (d) $A$ = La, (e) $A$ = Dy and (f) $A$ = Ce.
• Figure 4: (a) Temperature dependence of specific heat of DyCu$_3$Fe$_2$Re$_2$O$_{12}$ between 2 and 201 K. (b) Temperature dependence of $C_p/T$ measured at some selected magnetic fields.
• Figure 5: Total and partial density of states calculated in DFT+U for MnCu$_3$Fe$_2$Re$_2$O$_{12}$. Positive (negative) DOS corresponds to spin up (down). The Fermi energy is set to zero.