Charge ordering and ferromagnetic transition in single-crystal Eu ₄As₃

A structural transition from the cubic anti-Th₃P₄ structure to the trigonal one has been found in the single crystal of Eu ₄As₃ as the temperature is decreased below 340 K. Anomalies in the structural, transport and thermal properties associated with this transition are very similar to those found in the charge ordering of Yb₄As₃ and Sm₄Bi₃. The absence of orbital moment for Eu²⁺ ions indicates that the orbital freedom plays no role in the charge ordering in Eu₄As₃. The ferromagnetic transition at 18 K is dominated by the exchange interaction between a divalent ion and a trivalent ion. We also discuss the role of the orbital freedom in the charge ordering of Eu₄As₃, Yb ₄As₃, and Sm₄Bi₃.