On the thermodynamic aspect of zinc oxide polymorphism: calorimetric study of metastable rock salt ZnO

Heat Capacities of Nanostructured Wurtzite and Rock Salt ZnO: Challenges of ZnO Nano-Phase Diagram

“…This result is consistent with the data in [6] and imply the difference between bulk-and nano-phase diagrams of ZnO. [6]; (b) Nano-phase diagram of ZnO representing equilibria between nanostructured phases. Isopotential curve corresponds to ∆G rs/w (p,T) = 0 (equilibrium between w-ZnO and rs-ZnO; black solid line) with the estimated error of ±0.3 kJ mol −1 for ∆H f [6] (the dashed blue lines).…”

“…At the same time, the ab initio simulations of structural transition in ZnO nanowires at high pressures (calculations using the SIESTA code) indicated that passage from bulk crystal to nanograins reduces ∆H f (0 K) from 23 kJ mol −1 down to 10 kJ mol −1 [18]. This result is consistent with the data in [6] and imply the difference between bulk-and nano-phase diagrams of ZnO. [6]; (b) Nano-phase diagram of ZnO representing equilibria between nanostructured phases.…”

“…Nano-phase diagram of zinc oxide has been calculated using the fitted experimental heat capacity values, directly-measured ∆H f [6] and thermoelastic p-V-T data on rs-ZnO [5,19] and w-ZnO [20]. The experimental data on direct and inverse transformations in ZnO at high (above 500 K) temperatures are in satisfactory agreement [7,19].…”

“…Figure 3a shows the formation enthalpy ΔHf calculated using the fitted values of heat capacities and experimental value of ΔHf (298.15 K) = 11.7±0.3 kJ mol -1 [6]. One can observe only weak temperature dependence of ΔHf.…”

“…rs-ZnO can be recovered at ambient conditions as individual (nanostructured) phase, up to the volume of ~100 mm 3 per individual high-pressure synthesis experiment [4,5]. The recovery of such a substantial sample volume allowed to conduct the measurements of a wide range of physical properties -previously unavailable -such as, for example, the standard enthalpy of the w-ZnO-to-rs-ZnO phase transformation ΔH(298.15 K) = 11.7±0.3 kJ mol -1 [6]. At the same time, the reported data should be taken with precaution, since the bulk samples with micro-sized grains are available only for w-ZnO.…”