We report experimental studies of a series of BaFe₂S_3-x Se _x (0 ⩽ x ⩽ 3) single crystals and powder specimens using x-ray diffraction, neutron-diffraction, muon-spin-relaxation, and electrical transport measurements. A structural transformation from Cmcm (BaFe₂S₃) to Pnma (BaFe₂Se₃) was identified around x = 0.7 - 1. Neutron-diffraction measurements on the samples with x = 0.2, 0.4, and 0.7 reveal that the Néel temperature of the stripe antiferromagnetic order is gradually suppressed from ~120 to 85 K, while the magnitude of the ordered Fe²⁺ moments shows very little variation. Similarly, the block antiferromagnetic order in BaFe₂Se₃ remains robust for 1.5 ⩽ x ⩽ 3 with negligible variation in the ordered moment and a slight decrease of the Néel temperature from 250 K (x = 3) to 225 K (x = 1.5). The sample with x = 1 near the Cmcm and Pnma border shows coexisting, two-dimensional, short-range stripe- and block-type antiferromagnetic correlations. The system remains insulating for all x, but the thermal activation gap shows an abrupt increase when traversing the boundary from the Cmcm stripe phase to the Pnma block phase. The results demonstrate that the crystal structure, magnetic order, and electronic properties are strongly coupled in the BaFe₂S_3-x Se _x system.

Quasi-one-dimensional iron chalcogenides possess various magnetic states depending on the lattice distortion, electronic correlations, and presence of defects. We present neutron diffraction and inelastic neutron scattering experiments on the spin ladder compound BaFe2-δS1.5Se1.5 with ∼6% iron vacancies. The data reveal that long-range magnetic order is absent, while the characteristic magnetic excitations that correspond to both the stripe- and block-type antiferromagnetic correlations are observed. First-principles calculations support the existence of both stripe- and block-type antiferromagnetic short-range orders in the experimental sample. The disappearance of long-range magnetic order may be due to the competition between these two magnetic orders, which is greatly enhanced for a certain concentration of iron vacancies, which we calculate to be about 6%, consistent with the measured iron vacancy concentration. Our results highlight how iron vacancies in the iron-based spin ladder system strongly influence the magnetic ground state.

We report the synthesis and characterization of RbNi2Se2, an analog of the iron chalcogenide superconductor RbxFe2Se2, via transport, angle-resolved photoemission spectroscopy, and density functional theory calculations. A superconducting transition at Tc=1.20 K is identified. In the normal state, RbNi2Se2 shows paramagnetic and Fermi-liquid behaviors. A large Sommerfeld coefficient yields an effective electron mass of m∗≈6me. In the superconducting state, zero-field electronic specific-heat data Ces can be described by a two-gap BCS model, indicating that RbNi2Se2 is a possible multigap superconductor. Our density functional theory calculations and angle-resolved photoemission spectroscopy measurements demonstrate that RbNi2Se2 exhibits relatively weak correlations and multiband characteristics, consistent with the multigap superconductivity.