Crossover from a heavy fermion to intermediate valence state in noncentrosymmetric Yb₂ Ni₁₂ (P,As)₇

We report measurements of the physical properties and electronic structure of the hexagonal compounds Yb₂ Ni₁₂ Pn₇ (Pn = P, As) by measuring the electrical resistivity, magnetization, specific heat and partial fluorescence yield X-ray absorption spectroscopy (PFY-XAS). These demonstrate a crossover upon reducing the unit cell volume, from an intermediate valence state in Yb₂ Ni₁₂ As₇ to a heavy-fermion paramagnetic state in Yb₂ Ni₁₂ P₇, where the Yb is nearly trivalent. Application of pressure to Yb₂Ni₁₂P₇ suppresses T_FL, the temperature below which Fermi liquid behavior is recovered, suggesting the presence of a quantum critical point (QCP) under pressure. However, while there is little change in the Yb valence of Yb₂Ni₁₂P₇ up to 30 GPa, there is a strong increase for Yb₂Ni₁₂As₇ under pressure, before a near constant value is reached. These results indicate that any magnetic QCP in this system is well separated from strong valence fluctuations. The pressure dependence of the valence and lattice parameters of Yb₂ Ni₁₂ As₇ are compared and at 1 GPa, there is an anomaly in the unit cell volume as well as a change in the slope of the Yb valence, indicating a correlation between structural and electronic changes.