X-ray and Neutron Diffraction Studies of SrTe₂FeO₆Cl, an Oxide Chloride with Rare Anion Ordering

Abstract

The oxychloride SrTe 2 FeO 6 Cl is obtained by hightemperature solid-state synthesis under inert conditions in closed reaction vessels. The compound crystallizes in a novel monoclinic crystal structure that is described in the space group P12 1 /n1 (No. 14). The unit cell parameters, a = 10.2604(1) Å, b = 5.34556(5) Å, c = 26.6851(3) Å, and β = 93.6853(4)°, and atomic parameters were determined from synchrotron diffraction data, starting from a model that was obtained from single-crystal X-ray diffraction data. The anion lattice exhibits a rare ordering of oxide and chloride ions: onedimensional zig-zag ladders of chlorine (squarelike motif) are surrounded by an oxygen matrix. Two different iron sites coordinated solely to oxygen are present in the structure, one octahedral and one square pyramidal, both distorted. Similarly, two different strontium coordinations are present; the first homoleptic coordinated to eight oxygen atoms and the second heteroleptic coordinated to four oxygen and four chlorine atoms in a fac-like manner. The lone pair of Te(IV) is directed toward the larger chlorine atoms. Magnetic susceptibility measurements confirm that Fe is +3 (d 5 ) in the highspin electronic configuration, exhibiting an almost ideal spin-only moment, μ eff = 5.65 μ B Fe −1 . The slightly negative Weiss constant (θ CW = −39 K) suggests dominating antiparallel spin-to-spin coupling in the paramagnetic temperature range, agreeing with an observed long-range antiferromagnetic spin ordering below Neél temperature, T N ∼ 13 K, and a broad second order-like anomaly in the specific heat measurement data. Low-temperature neutron diffraction data reveal that the antiferromagnetic ordered phase is Ctype, with a k-vector (1/2, 1/2, 0) and ordered moment of 4.14(7) μ B . The spin structure can be described as antiferromagnetic ordered layers stacked along the a-axis, forming layers of squares that alternate along the c-axis.

Sr2Te4V2O13Cl2, a Layered Structure Forming a Canted Antiferromagnetic Ground State

“…The combination of a p-block element containing a stereochemically active lone pair and mixed anions was chosen deliberately to synthesize novel low-dimensional crystal structures. This approach has led to the discovery of multiple low-dimensional structures, ,, which can be explained by the fact that the lone pair is directed toward the lower charged anion, very often a monovalent halide ion X – , as opposed to the divalent oxygen O 2– , leading to large areas without bonding, giving rise to the low-dimensional structures. , This combination of p-block elements containing a stereochemically active lone pair and magnetic ions can also lead to magneto-electric coupling, a possibility for the polar space group Pca 2 1 . Similarly, a sharp increase in magnetic susceptibility at low temperatures (Figure ), the observation of a spin-flop-like process in the field-dependent magnetization (Figure ), and a significant release of entropy (heat) (Figure ) indicate that Sr 2 Te 4 V 2 O 13 Cl 2 is an ordered canted antiferromagnet below the expected ordering temperature of ∼3 K, which can be described by the Dzyaloshinskii–Moriya interaction. , …”