Glasslike thermal conductivity and narrow insulating gap of EuTiO₃

Crystals and glasses differ by the amplitude and the temperature dependence of their thermal conductivity. However, there are crystals known to display glasslike thermal conductivity. Here, we show that EuTiO3, a quantum paraelectric known to order antiferromagnetically at 5.5 K, is one such system. The temperature dependence of resistivity and Seebeck coefficient yield an insulating band gap of similar to 0.22 eV. Thermal conductivity is drastically reduced. Its amplitude and temperature dependence are akin to what is seen in amorphous silica. Comparison with nonmagnetic perovskite solids, SrTiO3, KTaO3, and EuCoO3, shows that what impedes heat transport are 4 f spins at Eu2+ sites, which couple to phonons well above the ordering temperature. Thus, in this case, superexchange and valence fluctuations, not magnetic frustration, are the drivers of the glasslike thermal conductivity.