Thermal properties of ${\text{Cd}}_2{\text{Re}}_2{\text{O}}_7$ and ${\text{Cd}}_2{\text{Nb}}_2{\text{O}}_7$ at the structural phase transitions

The structural phase transitions in ${\text{Cd}}_2{\text{Re}}_2{\text{O}}_7$ and ${\text{Cd}}_2{\text{Nb}}_2{\text{O}}_7$ pyrochlores have been studied through capacitive thermal expansion, heat capacity, and thermal conductivity. The superconductor ${\text{Cd}}_2{\text{Re}}_2{\text{O}}_7$ $\left(T_c=1\text{K}\right)$ shows well-defined anomalies at 200 and 120 K due to structural transitions. The pressure dependence of the transition temperatures is calculated from the thermal expansion and heat capacity, which shows good agreement with the results of direct measurements. The ferroelectric ${\text{Cd}}_2{\text{Nb}}_2{\text{O}}_7$ shows distinct anomalies due to structural transitions at 204, 85, and 46 K, in addition to a broad feature around 190 K associated with the relaxorlike behavior. For both compounds, the lattice thermal conductivity shows a glasslike temperature dependence in the high-temperature cubic phase.

Figure 1

Resistivity $\rho$ and magnetic susceptibility $\chi$ of ${\text{Cd}}_2{\text{Re}}_2{\text{O}}_7$. $\chi$ was measured under a magnetic field of 10 kOe. The inset shows $\chi$ in the vicinity of the 120 K transition.

Figure 2

The temperature dependence of the linear thermal expansivity $\Delta L/L_0$ of ${\text{Cd}}_2{\text{Re}}_2{\text{O}}_7$ and ${\text{Cd}}_2{\text{Nb}}_2{\text{O}}_7$ along the cubic $⟨111⟩$ direction.

Figure 3

(Color online) Heat capacity $C_p$ and thermal-expansion coefficient $\alpha$ of ${\text{Cd}}_2{\text{Re}}_2{\text{O}}_7$. The lines are extrapolations from the high-temperature cubic phase, which are used to obtain the jumps in $C_p$ and $\alpha$ at $T_{\text{R}1}=200\text{K}$ (dashed lines). The inset shows $C_p$ in the vicinity of the superconducting transition at 1 K.

Figure 4

Heat capacity divided by temperature, $C_p/T$, and thermal expansion coefficient $\alpha$ of ${\text{Cd}}_2{\text{Re}}_2{\text{O}}_7$ in the vicinity of $T_{\text{R}1}=120\text{K}$. The lines are the baselines used to obtain the excess contributions in $C_p/T$ and $\alpha$.

Figure 5

(a) The real part of dielectric susceptibility ${ϵ}^{\prime }$ at measurement frequencies of 1, 3, 10, 30, 100, 300, and 1000 kHz (left to right), (b) the thermal-expansion coefficient $\alpha$, and (c) the excess contributions to the heat capacity $\Delta C_p$, for ${\text{Cd}}_2{\text{Nb}}_2{\text{O}}_7$. The data shown in (a) and (c) are reproduced from Ref. 15. Note the change in scale for $\Delta C_p$ at 100 K in (c). Dashed lines are drawn at 46, 84.5, and 204 K to locate the anomalies associated with each transition.

Figure 6

Thermal conductivity $\kappa$ of ${\text{Cd}}_2{\text{Re}}_2{\text{O}}_7$ and ${\text{Cd}}_2{\text{Nb}}_2{\text{O}}_7$ measured on heating direction. For ${\text{Cd}}_2{\text{Re}}_2{\text{O}}_7$, the closed circles represent the total $\kappa$, whereas the solid line corresponds to the phonon contribution ${\kappa }_{ph}$ obtained by subtracting the estimated electronic contribution from the total $\kappa$. The $\kappa$ of amorphous silica ${\text{a-SiO}}_2$ (Ref. 49) is shown as a dashed line.