The thermal conductivities of pyrochlore oxide solid solutions of general formula (Er 1−x Y x) 2 Ti 2 O 7 with 0 x 1 have been determined for high-quality single crystals aligned along the [110] direction, over the temperature range from 3 to 300 K. Er 2 Ti 2 O 7 and Y 2 Ti 2 O 7 are isostructural and Er 3+ and Y 3+ are within 1% in size, but differ by a factor of about 2 in mass. Therefore, this system allows a clear test of the influence of mass of dopant on thermal conductivity, while controlling for other factors such as dopant size and sample purity. Although Y 2 Ti 2 O 7 has a higher thermal conductivity than Er 2 Ti 2 O 7 at T = 300 K, from 3 to 200 K their relative thermal conductivities reverse. Furthermore, we observe significant decrease in thermal conductivity upon doping Er 2 Ti 2 O 7 with Y 3+ ions, showing definitively that, in the temperature range from about 3 to 300 K, the impurity scattering effect of the lighter Y 3+ ions is the predominant limiter of the thermal conductivity. This conclusion is supported by the finding that the phonon mean free path of the doped compounds decreases with increased dopant concentrations, increasing again as pure Y 2 Ti 2 O 7 is approached.

We describe a laboratory experiment that serves as an introduction to solid-state and materials science, a topic that requires additional attention in the undergraduate chemistry laboratory curriculum. The experiment illustrates the longrange translational order, crystal growth, and the macroscopic manifestations of that order. This is demonstrated through the preparation and characterization of large, well-formed bismuth crystals, an aesthetically pleasing product. The characterization of the grown bismuth crystals involves determination of melting point and enthalpy of fusion via differential scanning calorimetry. The temperature dependence of the electrical resistance of grown bismuth crystals is also measured. Students are encouraged to consider the effect of metallic bonding interactions on the melting of the crystal samples and on their ability to conduct electricity. Students also analyze how the impurities influence the melting point and the electrical properties. The experiment is suitable for use in the third-or fourth-year undergraduate laboratory and is performed by students in one four-hour session. The experiment could be adapted to two laboratory sessions, with the first two-hour session covering crystal growth, and the second two-hour session focused on thermal and electrical characterization.