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By using first-principles calculation and quasi-harmonic approximation method, we have systematically investigated the thermal expansion properties of tetragonal phase of PbTiO 3 , focusing on the correlation between the A1(TO) mode and negative thermal expansion (NTE). It is found that volume of the unit cell shows positive thermal expansion in a axis direction, and NTE in c axis direction from 0 K to 900 K. It is also found that the A1(TO) mode is softening when the volume shrinks by the phonon calculation. Calculating of Gruneisen parameters show that the A1(TO) mode makes a main contribution to the NTE, and this vibrational mode is the Pb, Ti and O atoms’ relative motion in c axis direction, which lead to a shrink of the unit cell in this direction. Our calculations clarify the correlation between the phonon mode and NTE of PbTiO 3 in c axis direction at the atomic level, and provide an important theoretical basis for further understanding of the mechanism of the NTE.

By using first-principles calculations, we have systematically investigated the hydration in A2M3O12 (A = Y, Er, Yb, Lu, In, Sc; M = Mo, W) family materials, focusing on the role of the ionic radius and electronegativity of A3+ cations on the hydration of the materials. Our calculations showed that O end of the inserted H2O prefers to bind to A3+ or M6+ cations depending on the ionic radius and electronegativity of A3+ cations. As the radius of A3+ cations decreasing from R(Y3+) = 0.90 to R(Sc3+) = 0.75 A, the binding site of H2O is shifted from the position near the A3+ cations to that between the A3+ and the M6+ cations, finally to the position between the two M6+ cations. The binding energy is generally decreased as R decreasing (exception for A = Sc). Meanwhile, in all the cases two H of H2O tend to point to the nearby bridge O atoms sharing by AO6 octahedra and MO4 tetrahedra, and form hydrogen bonds, which play a key role in the binding between H2O and the materials. It was also found that the tungstates generally exhibit weak hygroscopicity comparing to the corresponding molybdates. Our results revealed the relationship between the hygroscopicity and the ionic radius of the A3+ cations, and provided a theoretical guidance in synthesis of A2M3O12 family materials with weak hygroscopicity.

Negative thermal expansion (NTE) in isostructural cubic ReO3 and ScF3 were studied using first-principles calculations within the quasiharmonic approximation. The reproduced coefficients of NTE (CNTE) is in reasonable agreement with experiment and the NTE behavior was attributed to the vibrational modes which have the minimum negative values of Gruneisen parameters at M(0.5, 0.5, 0) and R(0.5, 0.5, 0.5) in Brillouin zone. The calculated CNTE of ReO3 is an order of magnitude smaller than that of ScF3 due to the stiffer covalent bonds in ReO3 as well as the vibrational modes which will generate a sharp energy-increasing antibonding state. Consequently, the NTE was suppressed in ReO3. Meanwhile, larger lattice constant and smaller bulk modulus than those of ReO3 also imply that ScF3 is expected to have larger coefficient of negative thermal expansion.