Large Low Temperature Specific Heat in Pyrochlore Bi$_2$Ti$_2$O$_7$

TL;DR

This study investigates the low temperature specific heat of crystalline Bi$_2$Ti$_2$O$_7$, revealing disorder-related features that suggest a 'charge ice' state due to geometrical frustration among Bi lone pairs.

Contribution

It provides thermodynamic evidence of disorder in crystalline Bi$_2$Ti$_2$O$_7$, linking low temperature specific heat anomalies to geometrical frustration and quenched disorder.

Findings

Peak in $C/T^3$ occurs at lower temperature than in similar compounds

Evidence of disorder consistent with quenched configurational disorder

Supports the concept of 'charge ice' in this material

Abstract

Both amorphous and crystalline materials frequently exhibit low temperature specific heats in excess of what is predicted using the Debye model. The signature of this excess specific heat is a peak observed in $C/T^3$ \textit{versus} $T$. To understand the curious absence of long-range ordering of local distortions in the crystal structure of pyrochlore Bi$_2$Ti$_2$O$_7$, we have measured the specific heat of crystalline Bi$_2$Ti$_2$O$_7$ and related compounds. We find that the peak in $C/T^3$ versus $T$ in Bi$_2$Ti$_2$O$_7$ falls at a substantially lower temperature than other similar compounds, consistent with the presence of disorder. This thermodynamic evidence for disorder in crystalline Bi$_2$Ti$_2$O$_7$ is consistent with quenched configurational disorder among Bi lone pairs produced by geometrical frustration, which could represent a possible realization of "charge ice".