Institutional Research Information Service
UCL Logo
Please report any queries concerning the funding data grouped in the sections named "Externally Awarded" or "Internally Disbursed" (shown on the profile page) to your Research Finance Administrator. Your can find your Research Finance Administrator at https://www.ucl.ac.uk/finance/research/rs-contacts.php by entering your department
Please report any queries concerning the student data shown on the profile page to:

Email: portico-services@ucl.ac.uk

Help Desk: http://www.ucl.ac.uk/ras/portico/helpdesk
Publication Detail
Equilibrium and metastable phase transitions in silicon nitride at high pressure: A first-principles and experimental study
We have combined first-principles calculations and high-pressure experiments to study pressure-induced phase transitions in silicon nitride (Si 3 N 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 3 N 4 , while phonon softening in the α-Si 3 N 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 3 N 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.
Publication data is maintained in RPS. Visit https://rps.ucl.ac.uk
 More search options
UCL Researchers
Dept of Chemistry
University College London - Gower Street - London - WC1E 6BT Tel:+44 (0)20 7679 2000

© UCL 1999–2011

Search by