Phonons in YB$_6$ and LaB$_6$: Effects of temperature and pressure

Raman scattering by phonons in YB$_6$ and LaB$_6$ has been studied in wide temperature ($8$–$800$ K) and pressure (up to $10$ GPa) ranges. Acoustic phonons in both hexaborides exhibit anomalous softening with a decrease in the temperature. The positive isothermal Grüneisen coefficient of these modes in YB$_6$ appears to be an order of magnitude lower than the absolute value of the negative isobaric coefficient, which indicates the dominating role of temperature effects. The temperature shifts of the vibration frequencies of the B sublattice in YB$_6$ caused by phonon interactions have the opposite sign and exceed the respective shifts in LaB$_6$.