Hexagonal phase tungsten trioxide (WO3) exhibits unique structural characteristics. Below is a detailed analysis of its crystal structure:
Overview of the Hexagonal Phase WO3 Crystal Structure
Basic Configuration
The crystal structure of hexagonal phase WO3 is based on a distorted cubic ReO3 structure. This structure results from the absence of A-site cations in the ABO3-type perovskite framework. Specifically, six oxygen atoms form an octahedron with a tungsten (W) atom positioned at the center of the WO6 octahedron.
Layered Structure
The crystal structure of hexagonal phase WO3 can also be described as a layered arrangement. Each layer comprises WO6 octahedra connected at their vertices to form hexagonal rings. These adjacent hexagonal rings align to form triangular motifs, creating one-dimensional triangular channels. The growth direction of this layered structure aligns with the crystallographic c-axis.
Relationship Between Crystal Structure and Temperature
Stability Range of the Hexagonal Phase
Hexagonal phase WO3 is stable within a specific temperature range, typically between 200°C and 400°C.
- Below 200°C, the product predominantly consists of orthorhombic WO3.
- At around 300°C, the characteristic peaks of the hexagonal phase begin to appear.
- Above 450°C, these peaks completely vanish, transitioning the material into monoclinic WO3.
Microstructural Features of the Hexagonal Phase WO3
Channel Structures
The rigid tungsten-oxygen framework of hexagonal phase WO3 includes WO6 octahedra connected to form hexagonal and triangular rings. These octahedra stack perpendicularly to the c-axis, forming two types of one-dimensional channels:
- Channels formed by hexagonal rings.
- Channels formed by triangular rings.
Both types of channels are parallel to the c-axis, making hexagonal phase WO3 an excellent host for ion insertion.
Color and Impurities
Pure hexagonal phase WO3 typically appears white. However, the presence of impurities, such as monoclinic WO3, can give it a yellowish hue.
Application Areas of Hexagonal Phase WO3
Due to its unique crystal structure and physical-chemical properties, hexagonal phase WO3 has potential applications in various fields:
- Tungsten and Tungsten Carbide Production: It can be used as a precursor for producing tungsten powder and tungsten carbide powder, essential for manufacturing hard alloy products (e.g., cutting tools and molds).
- Photoelectrocatalysis: Its superior photoelectrocatalytic performance enables applications in photocatalytic degradation and gas sensing.
Summary
The crystal structure of hexagonal phase WO3 features a distorted cubic ReO3 framework and layered configuration. Its stability is highly temperature-dependent, and it contains unique one-dimensional channels aligned with the c-axis. These structural traits make hexagonal phase WO3 highly promising for applications in materials science and engineering.
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