Sr₁₃□NbAs₁₁ and Eu₁₃□NbAs₁₁ – Defect Variants of the Ca₁₄AlSb₁₁ Structure with asymmetric [As₃]⁷⁻ anions

Abstract

The novel compounds Sr13NbAs11 and Eu13NbAs11 have been synthesized from SrAs, Eu5As4, Sr, Nb and As in niobium ampoules at 1173–1273 K. The tetragonal tI 200 phases are defect variants of the Ca14AlSb11 structure (space group I41/acd (no. 142); Sr13□NbAs11: a = 1649.8(2) and c = 2214.1(3); Eu13□NbAs11: a = 1632.9(8) and c = 2197.3(8) pm; Z = 8). The structures are built from the cations Sr2+, and Eu2+, respectively, and from the anions [NbAs4]7−, As3−, and the linear polyanion [As3]7−. This polyanion (isosteric to I3−) is asymmetric with d(AsAs) = 273.0 and 346.0 pm (Sr) and 274.7 and 335.6 pm (Eu), respectively. The bond lengths in the tetrahedral anions are d(NbAs) = 250.8 and 251.1 pm. The complete structural arrangement is related to that of Cu2O by forming two interpenetrating networks. The oxygen atoms are substituted by niobium centered As4 tetrahedra, and the Cu atoms are substituted by As6 octahedra (centered by Sr, Eu). The central As atoms of the polyanions connect the nets. Both As networks are enveloped by the remaining cations forming cubes, tetragonal antiprisms and capped trigonal prisms.

Structure and magnetic properties of Ca14MnP11

“…The tetragonal distortion of the Mn tetrahedron compared to the main group analogs was MnBi 11 , suggest that the electron counting be re-evaluated and postulates that charge balance is maintained if the tetrahedral unit be considered to be Mn 2+ (Bi 4 ) 11À rather than Mn 3+ (Bi 4 ) 12À . X-ray Magnetic Circular Dichroism (XMCD) experimental spectra [19] for Yb 14 MnSb 11 shows that in this compound, Mn is best considered to be Mn 2+ with an antiferromagnetically coupled spin on Sb [19]. ESR measurements on Ca 14 AlP 11 show no signal, as expected from the Zintl electron counting, and Ca 14 MnP 11 show a broad signal at g ¼ 2:00, suggesting uncompensated spin density [20].…”

“…Moreover, the moment obtained from the fit of the data is much higher than observed in any other Mn-containing compound in this series [2]. Theoretical calculations [18] for Ca 14 Table 3 Selected bond lengths [Å ] and angles [1] for Ca 14 MnP 11 Mn 2+ (Pn 4 ) 11À . In the first case there is a hole on the Mn, and all the Pn orbitals are filled.…”

“…This high-temperature magnetic ordering is unusual. Other transition metal Zintl phases synthesized with Nb as the transition metal do not show ferromagnetic ordering [13][14][15][16].…”

“…EPR and 31 P solid-state NMR on Ca 14 MnP 11 has been published [20]. That paper investigated the structural status of the P 3 trimer.…”

Exploitation of Zintl Phases in the Pursuit of Novel Magnetic and Electronic Materials

“…9 » 12 ' 15 The tetragonal Nb phases, A 13 NbSbn (Sr, Eu) are defect variants of the Cai 4 AlSbn structure with fractional cation vacancies. 9 The A 14 MnPn n compounds have been shown to have unusual magnetic and electronic properties. 12 In these transition metal Zintl compounds, the metal in the tetrahedron is formally Mn 111 , a d 4 ion.…”