Antiferroquadrupolar ordering in the pseudo-binary tetragonal compounds Ho1-xDyxB2C2

Antiferroquadrupolar (AFQ) and antiferromagnetic (AFM) transitions in Ho1-x DyxB2C2 (0 less than or equal to s less than or equal to 1.0) have been inv estigated on the single-crystalline samples by magnetization and specific h eat measurements. As the Dy concentration increases, the both transition te mperatures increases from T-Q = 3.5 K and T-N = 5.9K for x = 0 to T-Q = 24. 7K and T-N = 15.3K for x = 1. For x = 0.2 and 0.3, there appears a single t ransition at which the AFQ and AFM transitions occur simultaneously. For 0. 4 less than or equal to x, the two transition temperatures are reversed. Th e temperature dependence of specific heat is interpreted well by assuming t hat the entropy of R ln 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.