What Are the Common Crystal Forms of Tungsten Oxide?
The common crystal forms of tungsten oxide include the following:
- Monoclinic Tungsten Oxide
Characteristics of Monoclinic Tungsten Oxide
This is the most common crystal form of tungsten trioxide (WO3) at standard temperature and pressure. The monoclinic structure is a white, powdery, or fine-granular crystalline form. Its symmetry is characterized by the absence of higher-order symmetry axes, with no more than one secondary symmetry axis or plane.
Applications of Monoclinic Tungsten Oxide
Due to its stability and broad physical and chemical properties, monoclinic tungsten oxide finds applications in various fields such as catalysts, electrochromic materials, and gas sensors.
- Triclinic Tungsten Oxide
Characteristics of Triclinic Tungsten Oxide
Triclinic tungsten oxide may appear within specific temperature ranges (e.g., -50°C to 17°C). The triclinic system is characterized by the absence of higher-order symmetry axes, secondary symmetry axes, and symmetry planes. Some forms may have a center of symmetry, while others lack it entirely. Although triclinic tungsten oxide exists under certain conditions, it is less commonly encountered in practical applications compared to the monoclinic form.
- Orthorhombic Tungsten Oxide
Characteristics of Orthorhombic Tungsten Oxide
When the temperature increases to a certain range (e.g., 330°C to 740°C), tungsten oxide may transform into the orthorhombic form. This structure is characterized by the absence of higher-order symmetry axes but has at least three secondary symmetry axes and planes in total.
Applications of Orthorhombic Tungsten Oxide
Orthorhombic tungsten oxide may have unique advantages in specific applications, but its use cases are less common and more specialized.
- Tetragonal (Square) Tungsten Oxide
Characteristics of Tetragonal Tungsten Oxide
At high temperatures (e.g., above 740°C), tungsten oxide may transition to the tetragonal form. The tetragonal structure features a 4-fold symmetry axis, which serves as the vertical symmetry axis (C-axis) of the crystal. The two horizontal symmetry axes are perpendicular to the C-axis. Due to the stringent high-temperature conditions required, the tetragonal form of tungsten oxide is less commonly encountered in practical production and applications.
Summary
The common crystal forms of tungsten oxide include monoclinic, triclinic, orthorhombic, and tetragonal structures. Among these, monoclinic tungsten oxide is the most prevalent at standard temperature and pressure, offering a wide range of applications. Other crystal forms appear under specific temperature conditions and may have limited or specialized use cases.
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