Antiferroquadrupolar Ordering in the Pseudo-Binary Tetragonal Compounds Ho_{1 - X}Dy_xB₂C₂

Antiferroquadrupolar (AFQ) and antiferromagnetic (AFM) transitions in Ho_{1 - x}Dy_xB₂C₂ (0 ≤ x ≤ 1.0) have been investigated on the single-crystalline samples by magnetization and specific heat measurements. As the Dy concentration increases, the both transition temperatures increases from T_Q = 4.5 K and T_N = 5.9 K for x = 0 to T_Q = 24.7 K and T_N = 15.3 K for x = 1. For x = 0.2 and 0.3, there appears a single transition at which the AFQ and AFM transitions occur simultaneously. For 0.4 ≤ x, the two transition temperatures are reversed. The temperature dependence of specific heat is interpreted well by assuming that the entropy of Rln 3 is released through the successive transitions in HoB₂C₂, that is, the 4f ground state of Ho³⁺ ion is a quasi-triplet. It is supposed that the Ho³⁺ ion has a rather small quadrupolar moment compared with the Dy³⁺ quadrupole.