Large Low Temperature Specific Heat in Pyrochlore Bi₂Ti₂O₇

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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/T³ versus T. To understand the curious absence of long-range ordering of local distortions in the crystal structure of pyrochlore Bi₂Ti₂O₇, we have measured the specific heat of crystalline Bi₂Ti₂O₇ and related compounds. We find that the peak in C/T³ versus T in Bi₂Ti₂O₇ falls at a substantially lower temperature than other similar compounds, consistent with the presence of disorder. This thermodynamic evidence for disorder in crystalline Bi₂Ti₂O₇ is consistent with quenched configurational disorder among Bi lone pairs produced by geometrical frustration, which could represent a possible realization of "charge ice".