Structural investigation of the negative thermal expansion in yttrium and rare earth molybdates

Abstract

The Sc(2)(WO(4))(3)-type phase (Pbcn) of Y(2)(MoO(4))(3), Er(2)(MoO(4))(3) and Lu(2)(MoO(4))(3) has been prepared by the conventional solid-state synthesis with preheated oxides and the negative thermal expansion (NTE) has been investigated along with an exhaustive structural study, after water loss. Their crystal structures have been refined using the neutron and x-ray powder diffraction data of dehydrated samples from 150 to 400 K. The multi-pattern Rietveld method, using atomic displacements with respect to a known structure as parameters to refine, has been applied to facilitate the interpretation of the NTE behavior. Polyhedral distortions, transverse vibrations of A· · ·O-Mo (A = Y and rare earths) binding oxygen atoms, non-bonded distances A· · ·Mo and atomic displacements from the high temperature structure, have been evaluated as a function of the temperature and the ionic radii.

Polymorphism in Ho2(MoO4)3

“…On the other hand, we have found that the less dense sample γ (dehydrated)-Ho 2 (MoO 4 ) 3 exhibits negative thermal expansion from 400 to 780 K, after water loss, as it is observed with other similar molybdates (Guzmán-Afonso et al, 2011) and from 1125 K to melting. We have calculated the linear thermal expansion coefficient α L (negative value) until 780K for γ-Ho 2 (MoO 4 ) 3 , and we have confirmed that α L is less negative when the ionic radius decreases (Guzmán-Afonso et al, 2011). The calculated value was -6.25x10 -6 K -1 ; thus, it shows the largest negative linear expansion coefficient for this family of compounds.…”

“…The β'-β phase transition is similar to other studied compounds: when the ionic radius decreases, the positive linear expansion coefficient is positive, although the c parameter decreases when the temperature increases . Negative thermal expansion has been found for the γ-phase, and the negative linear expansion coefficient decreases when the ionic radius decreases, as observed for similar compounds (Guzmán-Afonso et al, 2011). However, an anomalous behavior has been detected during the coexistence of phases.…”

“…The corresponding paraelectric-paraelastic phase (β) belongs to the P-42 1 m space group and arises after a phase transition at about 450 K. The heavier molybdates with the Sc 2 (WO 4 ) 3 structure type have a Pbcn space group; this structure is known as γ-phase and exhibits negative thermal expansion (NTE) (Guzmán-Afonso et al, 2011). A possible ferroelastic phase (γ'phase), with positive thermal expansion, is expected at very low temperatures but it has only been found in some isostructural compounds (Evans and Mary, 2000).…”

Effect of pressure onLa2(WO4)3with a modulated scheelite-type structure

“…We used projector-augmented wave pseudopotentials [18,19] and the PBEsol prescription for the exchange and correlation energy within the generalized gradient approximation [20]. The 5s 2 , 5p 6 , 6d 1 , and 6s 2 electrons of La, the 5p 6 , 5d 4 , and 6s 2 of W, and the 2s 2 and 2p 4 of O were allowed to relax in the calculations, with the remaining electrons being kept frozen at the atomic cores. To ensure a total-energy convergence of 1 meV per formula unit, we used plane-wave basis sets with an energy cutoff of 520 eV and dense Monkhorst-Pack grids [21] appropriate for each structure considered-for example, a 4 × 4 × 2 grid (resulting in 20 independent k points) was used for the ambient-conditions α phase with two formula units in the primitive unit cell, and a 3 × 2 × 2 grid (eight k points) was used for the Bi 2 (MoO 4 ) 3 -type structure with four formula units in the unit cell.…”

“…14, and Z = 4 are found in related compounds like Bi 2 (MoO 4 ) 3 , Sb 2 (SO 4 ) 3 , and As 2 (SO 4 ) 3 [1]. Trimolybdates and tritungstates with heavier RE cations (RE = Ho-Lu) adopt the γ form of Sc 2 (WO 4 ) 3 and display negative thermal expansion (NTE) [4][5][6]. A metastable ferroelectric phase with the β form of Gd 2 (MoO 4 ) 3 appears for intermediate * Corresponding author: javierl@marengo.dfis.ull.es molybdates with RE = Nd-Dy [4], but this has never been observed in tritungstates.…”