Heat capacity and phonon mean free path of wurtzite GaN

We report on lattice specific heat of bulk hexagonal GaN measured by the heat flow method in the temperature range 20-300 K and by the adiabatic method in the range 5-70 K. We fit the experimental data using two temperatures model. The best fit with the accuracy of 3 % was obtained for the temperature independent Debye's temperature $\theta_{\rm D}=365$ {\rm K} and Einstein's temperature $\theta_{\rm E}=880$ {\rm K}. We relate these temperatures to the function of density of states. Using our results for heat conduction coefficient, we established in temperature range 10-100 K the explicit dependence of the phonon mean free path on temperature $\it{l}_{\rm ph}\propto T^{-2}$. Above 100 K, there is the evidence of contribution of the Umklapp processes which limit phonon free path at high temepratures. For phonons with energy $k_{\rm B}\times 300$ {\rm K} the mean free path is of the order 100 {\rm nm}Comment: 5 pages, 4 figure

Equilibrium and metastable phase transitions in silicon nitride at high pressure: A first-principles and experimental study

peer reviewedWe have combined first-principles calculations and high-pressure experiments to study pressure-induced phase transitions in silicon nitride (Si 3N 4). Within the quasi-harmonic approximation, we predict that the α phase is always metastable relative to the β phase over a wide pressure-temperature range. Our lattice vibration calculations indicate that there are two significant and competing phonon-softening mechanisms in the β-Si 3N 4, while phonon softening in the α-Si 3N 4 is rather moderate. When the previously observed equilibrium high-pressure and high-temperature β → γ transition is bypassed at room temperature (RT) due to kinetic reasons, the β phase is predicted to undergo a first-order structural transformation to a denser P6̄ phase above 39 GPa. The estimated enthalpy barrier height is less than 70 meV/atom, which suggests that the transition is kinetically possible around RT. This predicted new high-pressure metastable phase should be classified as a "postphenacite" phase. Our high-pressure x-ray diffraction experiment confirms this predicted RT phase transition around 34 GPa. No similar RT phase transition is predicted for α-Si 3N 4. Furthermore, we discuss the differences in the pressure dependencies of phonon modes among the α, β, and γ phases and the consequences on their thermal properties. We attribute the phonon modes with negative Grüneisen ratios in the α and β phases as the cause of the predicted negative thermal expansion coefficients (TECs) at low temperatures in these two phases, and predict no negative TECs in the γ phase. © 2011 American Physical Society