Structural and Thermodynamic Stability of the "1111" Structure Type: A Case Study of the EuFZnPn Series

Effect of antifluorite layer on the magnetic order in Eu-based 1111 compounds, EuTAsF (T = Zn, Mn, and Fe)

The 1111 compounds with an alternating sequence of fluorite and antifluorite layers serve as structural hosts for the vast family of Fe-based superconductors. Here, we use neutron powder diffraction and density-functional-theory (DFT) band-structure calculations to study magnetic order of Eu²⁺ in the [EuF]⁺ fluorite layers depending on the nature of the [TAs]^- antifluorite layer that can be non-magnetic semiconducting (T = Zn), magnetic semiconducting (T = Mn), or magnetic metallic (T = Fe). Antiferromagnetic transitions at T_N ∼ 2.4-3 K due to an ordering of the Eu²⁺ magnetic moments were confirmed in all three EuTAsF compounds. Whereas in EuTAsF (T = Zn and Mn), the commensurate k₁ = (½ ½ 0) stripe order pattern with magnetic moments within the a-b plane is observed, the order in EuFeAsF is incommensurate with k = (0 0.961(1) ½) and represents a cycloid of Eu²⁺ magnetic moments confined within the bc-plane. Additionally, the Mn²⁺ sublattice in EuMnAsF features a robust G-type antiferromagnetic order that persists at least up to room temperature, with magnetic moments along the c-direction. Although DFT calculations suggest stripe antiferromagnetic order in the Fe-sublattice of EuFeAsF as the ground state, neutron diffraction reveals no evidence of long-range magnetic order associated with Fe. We show that the frustrating interplane interaction J₃ between the adjacent [EuF]⁺ layers is comparable with in-plane J₁-J₂ interactions already in the case of semiconducting fluorite layers [TAs]^- (T = Zn and Mn) and becomes dominant in the case of the metallic [FeAs]^- ones. The latter, along with a slight orthorhombic distortion, is proposed to be the origin of the incommensurate magnetic structure observed in EuFeAsF.

Synthesis, electronic structure and physical properties of two new layered compounds, EuFAgSe and EuFAg1-δTe, featuring the active redox pair Eu2+/Ag. Dalton Trans 2020;49:7426-7435. [PMID: 32432284 DOI: 10.1039/d0dt01504k]

Systematic studies of the ZrCuSiAs (also LaOAgS or 1111) structure type resulted in the synthesis of two new fluoride chalcogenides, EuFAgSe and EuFAg1-δTe, whereas their sulfide analog, EuFAgS, could not be obtained. Both new compounds are tetragonal, P4/nmm, with cell parameters a = 4.1542(1) Å, c = 9.2182(1) Å for the selenide and a = 4.3255(1) Å, c = 9.5486(1) Å for the telluride. Rietveld refinement reveals a significant silver deficiency in the telluride (δ = 0.05), while the selenide is nearly stoichiometric. Both compounds are semiconductors as shown by diffuse reflectance spectroscopy and confirmed by density-functional calculations of the band structure. Magnetism of both compounds is predominantly driven by Eu2+, as indicated by magnetic susceptibility measurements and corroborated by 151Eu Mössbauer spectroscopy. EuFAg1-δTe and EuFAgSe are paramagnetic down to 1.8 K.

A new oxygen-free cobalt-based compound SmCoAsF with multiple magnetic transitions

A new quaternary oxygen-free cobalt-based compound SmCoAsF, which crystallizes with the tetragonal ZrCuSiAs-type structure, is synthesized via a solid state metathesis reaction.

Layered Compounds BaFMgPn (Pn = P, As, Sb, and Bi), Transition-Metal-Free Representatives of the 1111 Structure Type

Four new transition metal-free pnictide representatives of the LaOAgS structure type were predicted by DFT calculations and found in the BaFMgPn (Pn = P, As, Sb and Bi) family. The compounds adopt the tetragonal space group P4 /nmm with the unit cell parameters a/ c 4.3097(1) Å/9.5032(1) Å, 4.3855(1) Å/9.5918(1) Å, 4.5733(1) Å/9.8184(1) Å, and 4.6359(1) Å/9.8599(1) Å, respectively. According to the DFT calculations, these new compounds are semiconductors with band gaps steadily decreasing from Pn = P ( ca. 2 eV) to Pn = Bi ( ca. 1 eV). The corresponding strontium fluoride and rare-earth oxide analogs are unlikely to exist and have not been observed yet. The trends of the stability within 1111 and structurally and/or chemically related compounds based on a combined consideration of geometry and DFT calculations are discussed.