Pressure-dependent intermediate valence behavior in YbNiGa4 and YbNiIn4

Author(s): Brubaker, ZE; Stillwell, RL; Chow, P; Xiao, Y; Kenney-Benson, C; Ferry, R; Popov, D; Donald, SB; Soderlind, P; Campbell, DJ; Paglione, J; Huang, K; Baumbach, RE; Zieve, RJ; Jeffries, JR | Abstract: We report a comprehensive structural and valence study of the intermediate valent materials YbNiGa$_{4}$ and YbNiIn$_{4}$ under pressures up to 60 GPa. YbNiGa$_{4}$ undergoes a smooth volume contraction and shows steady increase in Yb-valence with pressure, though the Yb-valence reaches saturation around 25 GPa. In YbNiIn$_{4}$, a change in pressure dependence of the volume and a peak in Yb-valence suggest a pressure induced electronic topological transition occurs around 10-14 GPa. In the pressure region where YbNiIn$_{4}$ and YbNiGa$_{4}$ 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.