Investigation of the anisotropic deuterium ordering inα-LuD single crystals by resistivity and heat-capacity measurements

The electrical resistivity and the heat capacity of single crystals of ${\mathrm{LuD}}_{0.183}$ were measured from ∼1 to 300 K. An anomaly due to ordering of deuterium pairs along the c axis is observed in both the resistivity and heat-capacity measurements. The binding energy of the ordered state is ∼15% larger in the c direction than in the a direction. Rapid cooling of the samples across the anomaly quenched in the single D atoms of the room-temperature α solid-solution phase giving rise to a resistivity increase Δ${\rho }_q$, which recovers in the anomaly region upon heating. No anisotropic effect was observed in Δ${\rho }_q$ as expected. The heat-capacity data show a small thermal effect at ∼200 K and an unusual flattening of the $C_p$ versus T curve between 70 and 190 K. This flattening is thought to be due to a stiffening of the lattice, which seems to peak at the anomaly temperature.