Inferring thermodynamic stability relationship of polymorphs from melting data

Area of Science:

• Solid-state chemistry
• Physical chemistry
• Materials science

Background:

• Polymorphism, the existence of multiple crystal forms, significantly impacts material properties.
• Determining the thermodynamic stability relationships between polymorphs is crucial for material design and application.
• Existing methods for assessing polymorph stability can be complex or require specific experimental conditions.

Purpose of the Study:

• To develop a method for inferring thermodynamic stability relationships of polymorphs using readily available melting data.
• To establish a predictive tool for estimating Gibbs free energy difference (delta G) and transition temperatures between polymorphs.
• To validate the proposed method against established techniques and literature data.

Main Methods:

• Derivation of thermodynamic formulas to calculate Gibbs free energy difference (delta G) and its temperature slope from melting temperatures and heats of fusion.
• Extrapolation of calculated delta G values to estimate relative stability at different temperatures.
• Determination of transition (or virtual transition) temperatures by extrapolating delta G to zero to infer monotropy or enantiotropy.

Main Results:

• The method successfully estimates Gibbs free energy differences and relative stability between polymorphs using melting data.
• Extrapolation of delta G provides estimations of transition temperatures, allowing inference of monotropy or enantiotropy.
• The approach shows good agreement with methods using solubility data and aligns with the Heat of Fusion Rule.
• Application to 96 literature polymorph pairs demonstrates high accuracy, with average deviation in transition temperatures of 2%.

Conclusions:

• Melting data can be effectively utilized to predict thermodynamic stability relationships between polymorphs.
• The developed method offers a practical and accurate approach for assessing polymorph stability and phase transitions.
• This technique provides valuable insights into polymorphism, aiding in the selection and development of materials with desired properties.