- Cadmium tungstate is an inorganic compound with the formula CdWO₄, composed of cadmium(II) cations (Cd²⁺) and tungstate anions (WO₄²⁻).
- It is a dense, crystalline material with significant industrial importance, especially as a scintillator for X-ray and gamma-ray detection. Its unique combination of high density, optical transparency, and luminescence under radiation makes it a material of choice in medical imaging and security scanning systems.
- Crystal Structure and Bonding: CdWO₄ crystallizes in a monoclinic wolframite-type structure (space group P2/c). In this arrangement, Cd²⁺ is coordinated by eight oxygen atoms, forming a distorted polyhedron, while tungsten exists as WO₆ octahedra. These octahedra share edges to form chains along the crystal lattice, with cadmium ions positioned between them to maintain charge balance. The crystal field effects around tungsten significantly influence the compound’s optical and scintillation properties.
- Physical Properties
- Appearance – Transparent to translucent crystals, colorless to pale yellow; often grown as large, optically clear single crystals for detector use.
- Density – ~7.9–8.0 g/cm³, relatively high due to tungsten’s atomic mass.
- Melting point – Around 1325–1350 °C; stable at high temperatures in air.
- Hardness – Mohs hardness ~4–4.5, moderate brittleness.
- Optical properties – High refractive index (~2.2) and strong absorption in the UV; photoluminescent under ionizing radiation with a primary emission peak around 480–520 nm (blue-green region).
- Scintillation decay time – ~12–15 µs, slower than some scintillators but with good energy resolution and low afterglow.
- Preparation
- Solid-state synthesis – Mixing cadmium oxide (CdO) or cadmium carbonate (CdCO₃) with tungsten trioxide (WO₃), followed by high-temperature firing (~800–1000 °C).
- Czochralski or Bridgman growth – Pulling large, high-quality single crystals from the melt, crucial for optical and scintillation applications.
- Hydrothermal growth – Less common, used for producing fine powders or nanocrystals.
- Applications
- Scintillators – CdWO₄ is widely used as a scintillation crystal for detecting X-rays and gamma rays, particularly in medical computed tomography (CT) scanners and security baggage inspection systems. Its high density and effective atomic number (Z_eff ≈ 64) make it highly efficient for stopping high-energy photons.
- Medical imaging – In CT systems, CdWO₄ crystals convert X-ray photons into visible light, which is then detected by photodiodes.
- Nuclear radiation monitoring – Suitable for detecting various types of ionizing radiation in research and industry.
- Optical applications – Investigated for use in nonlinear optics and photonic devices due to its refractive and luminescent properties.
- Chemical and Thermal Stability: CdWO₄ is highly stable under normal environmental conditions and resists attack by water and most acids, though it dissolves in strong mineral acids, forming soluble cadmium and tungsten compounds. It is stable in air at elevated temperatures but will eventually decompose into CdO and WO₃ under extreme heating.
- Toxicology and Safety Considerations
- Cadmium hazard – Highly toxic and carcinogenic; inhalation of dust or vapor can cause severe respiratory and systemic effects.
- Tungsten hazard – Generally less toxic but can cause lung irritation and accumulation in tissues.
- Safety measures – Growth, grinding, or machining of CdWO₄ crystals must be performed with dust control, local exhaust ventilation, and appropriate PPE.
- Environmental Impact: While tungsten itself poses low environmental risk, the cadmium content in CdWO₄ makes it a regulated hazardous material. Disposal and recycling must follow strict hazardous waste protocols to prevent soil and water contamination.
