Amorphization in rare earth tungstates with modulated scheelite-type structure under pressure

Abstract

A large number of known AB compounds adopt the B1 (NaCl) structure. Under pressure, most of them transform to the B2 (CsCl) structure directly or via intermediate phase(s). The computational investigations suggest that the dense B2 phase is favored at high pressure for silver halides with the B1-structure (AgX, X=Cl, Br and I). The experimental studies have shown that AgCl transforms from B1 to B2 at 17 GPa, 200 ℃ [1]. In-situ X-ray work [1] showed two intermediate phases existing between the B1-B2 phase transition in AgCl, with the structure of KOH and TlI, respectively. As for AgBr and AgI, both are known to undergo B1-KOH-type structural transitions [2] in the pressure range between 13-16 GPa. In this study, we extended the experimental conditions to higher pressure to investigate the phase transitions in AgBr and AgI and to find out if they share the same topological transition as AgCl under high pressure. The silver halides were studied in the diamond anvil cell (DAC) by angular dispersive X-ray powder diffraction technique. The highpressure phase transformations of AgCl, AgBr and AgI were investigated up to 28, 41 and 56 GPa, respectively. In AgCl, a phase transition sequence B1 → KOH → TlI → B2 was observed at high pressure, room temperature. The pathway of transformations in AgBr was found to be B1→ (unknown phase) → KOH → TlI at high pressure. The "unknown phase" observed in AgBr was found for the first time and has been detected in three consecutive runs carried out in this study. At 35.5 GPa, we applied the laser heating (estimated temperature ~ 500 ℃) and found that TlI-type AgBr maintained the same structure type (TlI). For AgI, the KOH-type high-pressure phase was observed up to 27 GPa. Upon further compression to the pressure of 56 GPa, and after annealing by laser heating (estimated temperature ~ 500 ℃), AgI did not transform to the B2 phase. Based on the current experimental results, we conclude that the topological transition of B1-KOH-TlI-B2 type AgCl can be achieved by applying pressure along. The occurrence of the TlI phase in AgBr at high pressures, shows that the structural transition sequence of B1-B2 in AgBr is similar to the one found in AgCl. Yet, an additional new low-pressure phase is found to exist in AgBr between the B1 and KOH phase. Computational studies confirm the same transition sequence for the B1-B2 transition of AgI [3]. Within the experimental data resolution, KOH-type AgI is identified to be stable up to 27 GPa, at least. Compared with AgCl and AgBr, the stability range of the KOHtype AgI is surprisingly wide, an effect which has also been noted in the computational study [3]. However, our x-ray diffraction data of AgI above 40 GPa do not confirm the TlI structure type, which was predicted to be the stable polymorph in the computational studies [3]. In general, the transition pressures of this multiple series of solidsolid phase transformation are increasing with decreasing ionicity of this series of AgX compounds. The ionicities for AgCl, AgBr and AgI are 0.869, 0.847 and...

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

“…This view is (15) supported by the fact that similar patterns persist up to the maximum experimental pressure reached of 17.8 GPa. This second phase transition would take place at ∼9-10 GPa in agreement with our Raman and theoretical results discussed in following sections, as well as with our own measurements performed in other RE tritungstates [27]. Even though the measured high-pressure diffractograms were inadequate for Rietveld refinement, the new phase seems disordered and without long-range three dimensional order, but not completely amorphous.…”

“…These changes may be related to some kind of atomic positional disorder, so that other phases with similar structures might be also present in this mixture. We have observed a similar behavior in other RE tungstates, which also adopt the α phase [27].…”

“…5 shows that the volume of the α structure can be contracted along more than one possible path. Moreover, we have also recently observed a quite different pressure dependence of the cell parameters of α-Eu 2 (MoO 4 ) 3 [34]. All this may explain the hysteresis observed in the decompression from 7.4 GPa (Fig.…”