Feng, JingShian, SamuelXiao, BingClarke, David2014-09-302014Feng, Jing, Samuel Shian, Bing Xiao, and David R. Clarke. 2014. “First-Principles Calculations of the High-Temperature Phase Transformation in Yttrium Tantalate.” Physics Review B 90, no. 9: 094102.1098-0121http://nrs.harvard.edu/urn-3:HUL.InstRepos:12965656The high-temperature phase transition between the tetragonal (scheelite) and monoclinic (fergusonite) forms of yttrium tantalite (YTaO4 ) has been studied using a combination of first-principles calculations and a Landau free-energy expansion. Calculations of the Gibbs free energies show that the monoclinic phase is stable at room temperature and transforms to the tetragonal phase at 1430 °C, close to the experimental value of 1426±7 °C. Analysis of the phonon modes as a function of temperature indicate that the transformation is driven by softening of transverse acoustic modes with symmetry Eu in the Brillouin zone center rather than the Raman-active Bg mode. Landau free-energy expansions demonstrate that the transition is second order and, based on the fitting to experimental and calculated lattice parameters, it is found that the transition is a proper rather than a pseudoproper type. Together these findings are consistent with the transition being ferroelastic.en-USJournal Article2014-09-08Jing Feng, Samuel Shian, Bing Xiao and David R. Clarke2014-09-3010.1103/PhysRevB.90.094102