Pressure-dependent intermediate valence behavior in YbNiGa^sub 4^ and YbNiIn^sub 4

We report a comprehensive structural and valence study of the intermediate valent materials YbNiGa4 and YbNiIn4 under pressures up to 60 GPa. YbNiGa4 undergoes a smooth volume contraction and shows steady increase in Yb valence with pressure, though the Yb valence reaches saturation around 25 GPa. In YbNiIn4 , a change in pressure dependence of the volume and a peak in Yb valence suggest that a pressure-induced electronic topological transition occurs around 10–14 GPa. In the pressure region where YbNiIn4 and YbNiGa4 possess similar Yb-Yb spacings, the Yb valence reveals a precipitous drop. This drop is not captured by density functional theory calculations and implies that both the lattice degrees of freedom and the chemical environment play an important role in establishing the valence of Yb.