Pressure-induced structural modifications of rare-earth hafnate pyrochlore

Abstract

Complex oxides with the pyrochlore (ABO) and defect-fluorite ((A,B)O) structure-types undergo structural transformations under high-pressure. Rare-earth hafnates (AHfO) form the pyrochlore structure for A = La-Tb and the defect-fluorite structure for A = Dy-Lu. High-pressure transformations in AHfO pyrochlore (A = Sm, Eu, Gd) and defect-fluorite (A = Dy, Y, Yb) were investigated up to ~50 GPa and characterized by in situ Raman spectroscopy and synchrotron x-ray diffraction (XRD). Raman spectra at ambient pressure revealed that all compositions, including the defect-fluorites, have some pyrochlore-type short-range order. In situ high-pressure synchrotron XRD showed that all of the rare earth hafnates investigated undergo a pressure-induced phase transition to a cotunnite-like (orthorhombic) structure that begins between 18 and 25 GPa. The phase transition to the cotunnite-like structure is not complete at 50 GPa, and upon release of pressure, the hafnates transform to defect-fluorite with an amorphous component. For all compositions, in situ Raman spectroscopy showed that disordering occurs gradually with increasing pressure. Pyrochlore-structured hafnates retain their short-range order to a higher pressure (30 GPa vs. <10 GPa) than defect-fluorite-structured hafnates. Rare earth hafnates quenched from 50 GPa show Raman spectra consistent with weberite-type structures, as also reported for irradiated rare-earth stannates. The second-order Birch-Murnaghan equation of state fit gives a bulk modulus of ~250 GPa for hafnates with the pyrochlore structure, and ~400 GPa for hafnates with the defect-fluorite structure. DyHfO is intermediate in its response, with some pyrochlore-type ordering, based on Raman spectroscopy and the equation of state, with a bulk modulus of ~300 GPa. As predicted based on the similar ionic radius of Zr and Hf, rare-earth hafnates show similar behavior to that reported for rare earth zirconates at high pressure.

Bright aspects of defects and dark traits of dopants in the photoluminescence of Er2X2O7:Eu3+ (X = Ti and Zr) pyrochlore: an insight using EXAFS, positron annihilation and DFT

“…This is also observed in Y 2 Sn 2Àx Zr x O 7 with Zr substitution as well as the thermally induced structural evolution in Gd 2 Hf 2 O 7 15,39. Similar short range order and the partial retention of bands attributed to pyrochlore are also observed in the supposedly fluorite-type rare earth hafnates of Dy and Yb under ambient conditions 40.…”

Effect of Molten Salt Synthesis Processing Duration on the Photo- and Radioluminescence of UV-, Visible-, and X-ray-Excitable La2Hf2O7:Eu3+ Nanoparticles

“…On the other hand, the DFP has mainly one active Raman mode, that is Γ DFP = F 2g , because the seven O 2– ions are randomly oriented over the eight anionic sites in this particular phase, leading to a high level of structural disordering. 37,38 Phase transition from the OPP ( Fd 3̅ m (A 2 B 2 O 6 O′, space group, Z = 8) to DFP (AO 2 , Fm 3̅ m , Z = 4)) proceeds by the disappearance of A 1g and E g Raman modes and decrease in the number of F 2g mode from 4 to 1. The ionic radius ratio ( r A / r B ) plays an important role in determining the type of structure that the A 2 B 2 O 7 composition is going to attain.…”

A Comparison of Order-Disorder in Several Families of Cubic Oxides

“…The process of disordering evolves through the formation of cation antisite pairs and Frenkel defects as discussed previously ( Maram et al, 2015 ). The various conditions that induce disorder in pyrochlore are ionic radii of A and B cations, ball milling, temperature, pressure, and radiation damage ( Subramanian et al, 1983 ; Heremans et al, 1995 ; Wuensch et al, 2000 ; Eberman et al, 2002 ; Lian et al, 2003b ; Liu et al, 2004 ; Rushton et al, 2004 ; Fuentes et al, 2005 ; Zhang et al, 2005 ; Zhang and Saxena, 2005 ; Moreno et al, 2006a ; Moreno et al, 2006b ; Zhang et al, 2006 ; Mandal et al, 2007 ; Mandal and Tyagi, 2007 ; Zhang et al, 2007 ; Zhang et al, 2009 ; Zhang F. X. et al, 2010 , Zhang et al, 2010 J ; Lang et al, 2010 ; Sanjuán et al, 2011 ; Sayed et al, 2011 ; Shlyakhtina and Shcherbakova, 2011 ; Zhang et al, 2015 ; Rittman et al, 2017b ; Turner et al, 2017 ; Fuentes et al, 2018 ; Wright et al, 2021 ).…”