# Charge Ordering and Incommensurate Modulations in the Metamagnetic Layered Manganese Oxysulfide Sr2MnO2Cu3.5S3

## TL;DR

This paper studies the structural and magnetic properties of a manganese oxysulfide compound, revealing charge ordering and magnetic transitions at low temperatures.

## Contribution

The paper reports on the discovery of checkerboard charge ordering and incommensurate modulations in Sr2MnO2Cu3.5S3, along with its magnetic phase transitions.

## Key findings

- Below 190 K, Sr2MnO2Cu3.5S3 exhibits checkerboard charge-ordering of Mn2+ and Mn3+ ions.
- The compound undergoes a metamagnetic transition at 1.1 T, changing from antiferromagnetic to ferromagnetic interlayer ordering.
- Local order of Mn sites is observed in the higher-temperature charge-disordered regime.

## Abstract

Sr2MnO2Cu3.5S3 contains
mixed-valent Mn ions Mn2+/3+ in axially elongated MnO4S2 octahedra connected via apical sulfide anions
to copper-deficient antifluorite-type Cu4‑δS3 layers where δ ∼ 0.5. Copper deficiency
is charge-compensated by oxidation of Mn 3d states resulting in mixed-valency.
The compound is tetragonal in P4/mmm at ambient temperatures (a = 4.016345(1) Å, c = 11.40708(5) Å). Below 190 K, superlattice reflections
in diffraction data and an increase in resistivity, signal checkerboard
charge-ordering of Mn2+ and Mn3+. The superstructure
approximates to a √2a × √2a × 2c expansion of the room temperature
cell in space group P42/nmc. However, satellite reflections signal a (3 + 2)­D incommensurate
modulation of Cu site occupancies in the Cu-deficient sulfide layers
coupled with displacements of the sulfur positions; overall the superstructure
below 190 K requires description in superspace group P42/nmc(a,0,0)­0000­(0,a,0)­00s0. Analysis of total scattering measurements along
with pair distribution functions supports the charge-ordered low temperature
model and reveals local order of distinct Mn sites within the higher-temperature
charge-disordered regime. Below T
N = 27
K, long-range magnetic ordering is A-type antiferromagnetic with distinct
moments for Mn2+ and Mn3+ ions directed perpendicular
to the MnO2 planes and ordered ferromagnetically. Long-range
antiferromagnetic order results from interlayer antiferromagnetic
coupling. A metamagnetic transition at 1.1 T corresponds to a change
to long-range interlayer ferromagnetic ordering via a spin-reorientation
of magnetic moments and is associated with a slight decrease in the
charge separation between the Mn sublattices, consistent with observations
on mixed-valent perovskite and Ruddlesden–Popper-type oxide
manganites.