Modulated structure of La2Co1.7 from neutron and X-ray diffraction data

Area of Science:

• Solid-state chemistry
• Crystallography
• Materials science

Background:

• Understanding the atomic arrangement in intermetallic compounds is crucial for predicting their properties.
• Lanthanum cobalt (La-Co) compounds are of interest due to their potential magnetic and electronic applications.
• Previous studies suggest complex structural behaviors in related systems.

Purpose of the Study:

• To elucidate the crystal structure of La(2)Co(1.7) using advanced diffraction techniques.
• To investigate the nature of atomic modulations and their impact on the crystal structure.
• To determine the precise chemical composition and refine structural models.

Main Methods:

• Single-crystal neutron diffraction at low and room temperatures.
• Single-crystal X-ray diffraction at room temperature.
• Analysis using Laue white-beam and CCD detectors.
• Indexing and structure solution in superspace groups.

Main Results:

• The average structure of La(2)Co(1.7) is hexagonal (P6(3)/mmc).
• Satellite reflections indicate a 3+1 dimensional incommensurately modulated structure.
• Cobalt atoms display positional disorder due to steric constraints, adopting a non-commensurate periodicity with lanthanum.
• Refined models include a sawtooth function and a composite system, yielding a composition of La(2)Co(1.8(1)).

Conclusions:

• The La(2)Co(1.7) crystal exhibits an incommensurately modulated structure.
• Cobalt atoms are not precisely located in ideal octahedral sites, leading to a different periodicity than lanthanum.
• The structure can be described as a composite system with modulated lanthanum and non-periodically displaced cobalt atoms.