High-pressure polymorphism of titanium dioxide.

Area of Science:

• Materials Science
• Solid-State Physics
• Crystallography

Background:

• Titanium dioxide (TiO2) exists in various polymorphs, with the rutile structure being thermodynamically stable under ambient conditions.
• Understanding phase transitions in TiO2 under extreme conditions is crucial for materials science and geophysics.
• Previous studies have explored TiO2 phase diagrams, but direct transitions under simultaneous high pressure and temperature require further investigation.

Purpose of the Study:

• To investigate the direct phase transition of titanium dioxide from the rutile to the alpha lead dioxide form.
• To characterize the compressibility and molar volume of the alpha lead dioxide form of TiO2.
• To identify potential sources of error in calculating equilibrium pressures between TiO2 polymorphs.

Main Methods:

• In situ X-ray diffraction under simultaneous high pressure and high temperature.
• Room temperature compressibility studies.
• Analysis of molar volume convergence and its effect on thermodynamic calculations.

Main Results:

• Direct transition from rutile to the alpha lead dioxide form of TiO2 was observed under high P-T conditions.
• Compressibility data for the alpha lead dioxide form showed anomalous behavior.
• Molar volume of the alpha lead dioxide form closely approached, but did not equal, that of the rutile form.
• An unexpectedly large error was identified in equilibrium pressure calculations at 298 K.

Conclusions:

• The direct rutile to alpha lead dioxide transition in TiO2 is experimentally confirmed.
• Anomalous compressibility of the alpha lead dioxide phase introduces significant uncertainty in thermodynamic phase boundary determinations.
• Precise determination of equilibrium pressures requires careful consideration of molar volume convergence and potential experimental artifacts.