Commensurate structure of Ca(2)CoSi(2)O(7), a new twinned orthorhombic structure

Area of Science:

• Crystallography
• Materials Science
• Solid State Chemistry

Background:

• The melilite group minerals exhibit complex crystal structures.
• Understanding variations in these structures is crucial for materials science applications.
• Previous studies on Ca(2)CoSi(2)O(7) have suggested certain structural arrangements.

Purpose of the Study:

• To elucidate the crystal structure of the commensurate phase of dicalcium cobalt disilicate (Ca(2)CoSi(2)O(7)).
• To refine the structure using a specialized least-squares program for twinned crystals.
• To compare the derived structure with previously reported findings.

Main Methods:

• Derivation of the crystal structure from a (3+2)-dimensional modulated structure.
• X-ray diffraction analysis of twinned Ca(2)CoSi(2)O(7) crystals at 170 K.
• Refinement using a newly developed least-squares program for twinned crystal data.

Main Results:

• The crystal structure is orthorhombic with space group P2(1)2(1)2 and dimensions a = 23.510 (4), b = 23.510 (4), c = 5.025 (1) Å.
• The structure features a regular arrangement of sixfold coordinated calcium polyhedra and CoO(4) tetrahedra along the [001] direction.
• The observed distribution of these structural units differs from prior reports.

Conclusions:

• The crystal structure of Ca(2)CoSi(2)O(7) in its commensurate phase has been successfully determined.
• The use of a specialized refinement program was essential due to crystal twinning.
• The findings provide new insights into the structural variations within the melilite group.