A new form of NaMnAsO₄

Area of Science:

• Inorganic Chemistry
• Solid-State Chemistry
• Crystallography

Background:

• The alpha-polymorph of sodium manganese(II) orthoarsenate (NaMnAsO4) adopts an olivine-type structure.
• In the alpha-polymorph, manganese(II) cations are in distorted octahedral coordination.

Purpose of the Study:

• To synthesize and characterize a new polymorph of NaMnAsO4.
• To elucidate the crystal structure and coordination environment of manganese in the new polymorph.

Main Methods:

• Hydrothermal synthesis was employed to obtain the new material.
• Crystal structure analysis was performed to determine the atomic arrangement.

Main Results:

• A novel beta-polymorph of NaMnAsO4 was successfully synthesized.
• The beta-polymorph exhibits a unique crystal structure with two distinct manganese(II) cations in [5]-coordination.
• Finite {Mn4O16} units form rows, linked by orthoarsenate groups into a framework with channels.
• Sodium cations reside within these channels with coordination numbers of six and seven.
• The beta-polymorph is isotypic with NaCoPO4 and NaCuAsO4.

Conclusions:

• The discovery of the beta-polymorph expands the known structural diversity of NaMnAsO4.
• The [5]-coordination of manganese represents a significant structural difference from the alpha-polymorph.
• The framework structure with channels offers potential for ion exchange or host-guest chemistry.