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

Abstract

We have studied the effect of pressure on the structural and vibrational properties of lanthanum tritungstate La 2 (WO 4 ) 3 . This compound crystallizes under ambient conditions in the modulated scheelite-type structure known as the α phase. We have performed x-ray diffraction and Raman scattering measurements up to a pressure of 20 GPa, as well as ab initio calculations within the framework of the density functional theory. Up to 5 GPa, the three methods provide a similar picture of the evolution under pressure of α-La 2 (WO 4 ) 3 . At 5 GPa, we begin to observe some structural changes, and above 6 GPa we find that the x-ray patterns cannot be indexed as a single phase. However, we find that a mixture of two phases with C2/c symmetry accounts for all diffraction peaks. Our ab initio study confirms the existence of several C2/c structures, which are very close in energy in this compression range. According to our measurements, a state with medium-range order appears at pressures above 9 and 11 GPa, from x-ray diffraction and Raman experiments, respectively. Based upon our theoretical calculations we propose several high-pressure candidates with high cationic coordinations at these pressures. The compound evolves into a partially amorphous phase at pressures above 20 GPa.

Role of rare earth sites and vacancies in the anomalous compression of modulated scheelite tungstates RE2 ( WO4)3

“…To our knowledge the following molybdates with α phase are the only ones studied up to now: Nd 2 (MoO 4 ) 3 , Tb 2 (MoO 4 ) 3 [19,20], and Eu 2 (MoO 4 ) 3 [21,22]. The only tungstate studied under high pressure is La 2 (WO 4 ) 3 , which undergoes two distinguishable phase transitions at high pressure compatible with the results of first-principles calculations [23].…”

Experimental and theoretical study ofα–Eu2(MoO4)3under compression

“…Moreover, the β-angle increases following the anticlockwise rotation of the MoO 4 tetrahedra. This increase is also observed in α-La 2 (WO 4 ) 3 [15] up to the first pressureinduced phase transition, while in α-Tb 2 (MoO 4 ) 3 the opposite behavior is observed [11], with tetrahedra undergoing a clockwise rotation in the pressure range from 7.5 to 12.7 GPa. The evolution of all the oxygen-bridge angles (see figure 3(b)) helps to explain the rotation and deformation of both tetrahedra and therefore the unusual compression of the cell parameters.…”

“…This may be the result of the reversibility of the rotations and deformations of the MoO 4 groups described above. Note that this is not the case of La 2 (WO 4 ) 3 and probably other light RE tritungstates with the α-phase, where vacancies and RE atoms are disordered and the structure is not recovered at ambient conditions [15].…”

“…More recently, PIA in both αand β′-phases has been explained as a spatial self-organization of the oxygen clouds around the Mo and RE subnetworks despite their different behaviors before amorphization [13,14]. On the other hand, the compression of α-phase in La 2 (WO 4 ) 3 and Tb 2 (MoO 4 ) 3 has been recently analyzed [11,15]. The first compound undergoes two phase transitions prior to PIA [15], while the second one reaches the amorphization without phase transitions [11].…”

“…On the other hand, the compression of α-phase in La 2 (WO 4 ) 3 and Tb 2 (MoO 4 ) 3 has been recently analyzed [11,15]. The first compound undergoes two phase transitions prior to PIA [15], while the second one reaches the amorphization without phase transitions [11].…”