Low-temperature thermal conductivity of the substrate material YAlO3 and its unconventional sister compound YbAlO3

TL;DR

This study addresses the problem of understanding lattice-dominated thermal transport in YAlO3 and its RE-substituted counterpart YbAlO3 at cryogenic temperatures, where phonon scattering and boundary effects compete. The authors perform steady-state κ measurements along two crystallographic directions from $2K$ to $300K$ and interpret the results using the Callaway model to separate boundary, defect, dislocation, normal, and Umklapp scattering channels, while comparing to elastic-constant and phonon-dispersion-based velocity estimates. They find that YAlO3 has a large κ with moderate anisotropy and long low-T phonon mean free paths, and that exchanging Y for Yb has only a modest effect on κ above $50K$, though YbAlO3 data below this temperature remain ambiguous due to sample size. The work highlights the limitations of Debye-based, isotropic-velocity descriptions for anisotropic, RE-containing perovskites and provides a practical baseline for using YAlO3 as a high-κ substrate or laser-host material at cryogenic temperatures.

Abstract

We present thermal conductivity data on single crystals of YAlO3 and YbAlO3 for temperatures between 2 K and 300 K and the heat current along b and c. Both materials are very good thermal conductors in the investigated temperature range. The thermal conductivity in these electrical insulators is due to phonons. The effect of Y-Yb exchange is found to be rather small despite the considerable difference in density and average atomic mass. For YAlO3 we find a moderate thermal conductivity anisotropy with weak temperature dependence and a ratio of c to b direction between at most 1 and 2.2. It is discussed with regard to the velocities of sound and relevant scattering processes. For YbAlO3 the small crystal size limits the precision of absolute thermal conductivity values and does not allow drawing conclusions on the anisotropy. Our results on YAlO3 confirm that the material is suitable for applications requiring a good thermal conductivity at temperatures down to liquid helium, such as lasers, substrates, and detectors.

Figures (5)

• Figure 1: Temperature dependence of the thermal conductivity $\kappa$ of YAlO$_3$ along the $b$-axis ($\kappa_b$) and the $c$-axis ($\kappa_c$) plotted in black and red, respectively. The inset presents the same data on a double-logarithmic scale in comparison to literature data from Aggarwal et al.Aggarwal-2005 and Zhang et al.Zhang-2023.
• Figure 2: Temperature dependence of the thermal conductivity $\kappa$ of YbAlO$_3$ along the $b$-axis ($\kappa_b$) and the $c$-axis ($\kappa_c$) plotted in black and red, respectively. Data for YAlO$_3$ are shown as dashed lines for comparison. The inset presents the YbAlO$_3$ data on a double-logarithmic scale.
• Figure 3: Temperature dependence of the thermal conductivity ratio $\kappa_c/\kappa_b$ of YAlO$_3$ and YbAlO$_3$ including literature data from Aggarwal et al.Aggarwal-2005 and Zhang et al.Zhang-2023 on YAlO$_3$. The error bars demonstrate maximum uncertainties from the upper and lower bounds for the geometry factors and measurement techniques. No respective estimate is provided by Zhang et al.
• Figure 4: (a) The upper panel shows the results of modeling the thermal conductivities $\kappa_b$ and $\kappa_c$ of YAlO$_3$ by the Callaway model. The calculated curves for $\kappa_b$ can be barely distinguished from the data and from each other, except for the range $T>200$ K. For $\kappa_c$, systematic deviations between model and data are seen at all $T$. Details of the calculation procedure are given in the main text. The corresponding parameters are summarized in Table \ref{['tab:YALO_fit']}. The dotted lines at low $T$ demonstrate the behaviors expected from the kinetic relation for constant mean-free paths $\lambda$. (b) The lower panel shows the deviations between model and data, emphasizing the much better agreement between for $\kappa_b$ than for $\kappa_c$.
• Figure 5: Influence of the different scattering contributions on the thermal conductivity of YAlO$_3$ for $j_Q \parallel b$ using $\Theta _\mathrm{D} = 760$ K. The dashed lines correspond to situations when only the specified relaxation times are taken into account.