Heat capacities of nanostructured wurtzite and rock salt ZnO: challenges of ZnO nano-phase diagram

TL;DR

This study measures the low-temperature heat capacities of nanostructured ZnO polymorphs, revealing differences between phases and providing thermodynamic insights into their phase stability and transition pressures.

Contribution

It provides the first detailed heat capacity measurements of nanostructured rs-ZnO and w-ZnO and analyzes their thermodynamic stability and phase diagram.

Findings

No significant nanostructure effect on w-ZnO Cp

Cp of rs-ZnO is lower than w-ZnO below 100 K

Equilibrium pressure between phases is about 4.6 GPa at 300 K

Abstract

Low-temperature heat capacities (Cp) of nanostructured rock salt (rs-ZnO) and wurtzite (w-ZnO) polymorphs of zinc oxide were measured in the 2-315 K temperature range. No significant influence of nanostructuring on Cp of w-ZnO has been observed. The measured Cp of rock salt ZnO is lower than that of wurtzite ZnO below 100 K and is higher above this temperature. Using available thermodynamic data, we established that the equilibrium pressure between nanocrystalline w-ZnO and rs-ZnO is close to 4.6 GPa at 300 K (half as much as the onset pressure of direct phase transformation) and slightly changes with temperature up to 1000 K.