Two New Arsenides, Eu₇Cu₄₄As₂₃ and Sr₇Cu₄₄As₂₃, With a New Filled Variety of the BaHg₁₁ Structure

Abstract

Two new ternary arsenides, namely, Eu7Cu44As23 and Sr7Cu44As23, were synthesized from elements at 800 °C. Their crystal structure represents a new filled version of the BaHg11 motif with cubic voids alternately occupied by Eu(Sr) and As atoms, resulting in a 2 × 2 × 2 superstructure of the aristotype: space group Fm3̅m, a = 16.6707(2) Å and 16.7467(2) Å, respectively. The Eu derivative exhibits ferromagnetic ordering below 17.5 K. In agreement with band structure calculations both compounds are metals, exhibiting relatively low thermopower, but high electrical and low thermal conductivity.

Experimental and Theoretical Studies on the Crystal Structure of Ternary Copper Arsenides A2Cu3As3 (A = Sr, Eu)

“…Although the magnetic behavior of Eu 2 Cu 3 As 3 suggests ferromagnetic (FM) coupling among the Eu 2+ ions, it should be addressed here that an antiferromagnetic (AFM) order was also observed below 24 K. Weak ferromagnetic interactions are frequently seen for europium pnictide Zintl phases such as Eu 7 Cu 44 As 23 , Eu 5 Sn 2 As 6 , EuGa 2 As 2 , and Eu 3 Ga 2 P 4 , which feature small positive Weiss constants without any clear magnetic ordering temperature (i.e., 21 K for EuGa 2 As 2 and 24.2 K for Eu 3 Ga 2 P 4 ) . However, some europium compounds such as Eu 3 Ge 2 As 4 and EuMn 2 P 2 exhibit both FM and AFM coupling in their structures .…”

“…The A–Cu–Pn ternary system (A = alkaline‐earth or rare‐earth metal, Pn = P, As) constitutes a huge family, and the relevant structural units are very flexible . In addition to the compounds based on simple CuPn 4 tetrahedra such as CaCu 2 As 2 and BaCu 2 As 2 , other copper‐rich or clathrate‐like analogues with very complex anions such as Ba 8 Cu 18– x As 10 and Eu 7 Cu 44 As 23 are also commonly seen, and the electrons in these compounds cannot be counted by the octet rule. The probable reason is that the Cu 3d orbitals are not usually in strict closed‐shell configurations.…”

Effect of Transition Metal Substitution on the Structure and Properties of a Clathrate-Like Compound Eu7Cu44As23

“…The only difference is that we had to increase the annealing time by 48 h in order to reach equilibrium. The largest homogeneity range was observed for T = Ni, with an almost linear (Figure 2) decrease of the unit cell parameter from a = 16.6707(2) Å [11] for the undoped phase to a = 16.3719(1) Å for the sample with x = 20 (the sample with the nominal composition Eu 7 Cu 24 Ni 20 As 23 contains up to 5% EuNi 5 As 3 ). The homogeneity range for Co and Fe were found to be narrower, with the substitution limit of x = 8.…”

“…The procedure was essentially the same as for the previously reported A 7 Cu 44 As 23 (A = Eu, Sr) [11]. Prior to use, the Fe, Co, and Ni powders were annealed in hydrogen to remove surface oxide.…”

“…It is worth noting that the pseudo-layered compounds typically feature antiferromagnetic (AFM) ordering [7], whereas ferromagnetic (FM) ordering is rare—Eu 2 Cu 6 P 5 and EuCu 4 P 3 being the only examples [9]. On the contrary, compounds with 3D structures frequently display FM ordering, as in the clathrate compound Eu x Ba 8− x Cu 16 P 30 , exhibiting a superstructure of the type-I clathrate [10], and in the recently discovered clathrate-like compound Eu 7 Cu 44 As 23 [11]. Whereas the former is a typical clathrate compound whose structure and properties can be rationalized using the Zintl–Klemm approach, the latter phase is an unbalanced metallic compound.…”