Microscopic evidence for significant ionic disorder in the Yb ³⁺-chain in Yb4(As_1-x P_x)₃: ³¹P NMR studies

We report unambiguous microscopic evidence from ³¹P NMR under H_ext ≅ 7.3 T for significant ionic disorder in the Y b ³⁺ chain in Yb₄(As_1-x P_x) ₃ (x = 0.05 and 0.40), which have similar characteristic χ(T) and C_p(T, H_ext) behavior to the antiferromagnetic quantum spin chain (AFQSC) system Yb₄As₃. Our conclusion is based on the observations only below the charge-ordering transition at T₀ ≅ 292 K of clear structures in the spectrum, which can be fitted well by the superpositions of almost equally spaced five Gaussian components. Since perfect ordering of Yb³⁺ in the chain sites would lead to a single-line spectrum also below T₀, the structures should be ascribed to significant ionic disorder in the Yb³⁺ chain and resulting distribution of local configurations of Yb³⁺ in the eight nearest-neighboring Yb sites around ³¹P. Quantitative comparisons with a simple model show that about 1/3 and at least 10% of the chain sites are disordered, namely occupied by Yb²⁺, for x = 0.40 and 0.05, respectively. Combining with previous other experimental results, extreme robustness against the disorder in the Yb³⁺ chain is deduced for the characteristic χ(Ê) and C_p(T, H_ext) behavior normally attributed to the AFQSC.