Alloying and Doping Control in the Layered Metal Phosphide Thermoelectric CaCuP

Area of Science:

• Materials Science
• Solid State Physics
• Thermoelectric Materials

Background:

• Calcium copper phosphide (CaCuP) is a promising low-cost, low-density thermoelectric material.
• Previous studies achieved a figure of merit (zT) of 0.5 at 792 K.
• Performance was limited by high lattice thermal conductivity and intrinsic p-type doping.

Purpose of the Study:

• To investigate methods for enhancing the thermoelectric performance of CaCuP.
• To explore thermal and electronic tunability through isovalent alloying and stoichiometry control.

Main Methods:

• Isovalent alloying of CaCuP with arsenic (As) to form the CaCuP1-xAsx solid solution series.
• Phase boundary mapping by deviating from the 1:1:1 stoichiometry to control doping levels.
• Calculation of the Lorenz number using the single parabolic band approximation.

Main Results:

• Isovalent alloying with As reduced lattice thermal conductivity but increased p-type doping, hindering zT improvement.
• Phase boundary mapping revealed that increasing p-type doping was more feasible than decreasing it.
• Calculations indicated multiband behavior in highly doped samples, explaining unrealistic Lorenz number predictions.
• The best thermoelectric performance, with zT approaching 0.6 at 873 K, was achieved with CaCuP and slightly Cu-enriched CaCu1.02P.

Conclusions:

• Isovalent alloying with As is not effective for improving CaCuP's thermoelectric figure of merit.
• Stoichiometric control offers a viable route for tuning doping levels in CaCuP.
• Optimized CaCuP compositions demonstrate potential for efficient thermoelectric energy conversion.