Pressure-induced order–disorder transition in Gd1.5Ce0.5Ti2O7 pyrochlore

An experimental study on ordered pyrochlore structured Gd1.5Ce0.5Ti2O7 (Fd3¯m) was carried out up to 45 GPa by synchrotron radiation X-ray diffraction and Raman spectroscopy. Experimental results show that Gd1.5Ce0.5Ti2O7 transfers to a disordered cotunnite-like phase (Pnma Z = 4) at approximately 42 GPa. Compared with the end member Gd2Ti2O7, the substitution of Ce3+ for Gd3+ increases the transition pressure and the high-pressure stability of the pyrochlore phase. This pressure-induced structure transition is mainly controlled by cationic order–disorder modification, and the cationic radius ratio rA/rB may also be effective for predicting the pyrochlore oxides' high-pressure stability. Two isostructural transitions are observed at 6.5 GPa and 13 GPa, and the unit-cell volume of Gd1.5Ce0.5Ti2O7 as a function of pressure demonstrates its compression behaviour is rather complex.


Recommendation?
Accept with minor revision (please list in comments)

Comments to the Author(s)
In this work, the authors report an experimental study on ordered pyrochlore structured Gd1.5Ce0.5Ti2O7 by synchrotron radiation X-ray diffraction and Raman spectroscopy. An orderdisorder transition is observed, and the transition is controlled by cationic order-disorder modification. This manuscript could be considered for acceptance after a minor revision. 1. To better understand the phase transition behavior, especially the isostructural transitions at lower pressures, the authors could perform some theoretical calculations if possible. 2. The grammatical mistakes and other errors in the manuscript should be carefully revised.

Are the interpretations and conclusions justified by the results? Yes
Is the language acceptable? Yes

Do you have any ethical concerns with this paper? No
Have you any concerns about statistical analyses in this paper? No

Recommendation?
Accept as is

Comments to the Author(s)
In the current manuscript Niu and co-workers performed high-pressure experiments for (Gd0.667Ce0.333)2Ti2O7, a pyrochlore. They observed a phase transition at about 45GPa. They also reported two iso-structural transitions at 6.5 GPa and 13 GPa. The topic is interesting for people working in the related fields. The experimental details were described well. The results might be of interest for some people. The measured data were analyzed in a clear way. The manuscript was written concisely. The text is in the scope of the Journal. Therefore I'd like to propose acceptance of this manuscript for publication in Royal Society Open Science.

30-Jul-2019
Dear Dr Niu: Title: Pressure-Induced Order-Disorder Transition in Gd 1.5 Ce 0.5 Ti 2 O 7 Pyrochlore Manuscript ID: RSOS-190842 Thank you for submitting the above manuscript to Royal Society Open Science. On behalf of the Editors and the Royal Society of Chemistry, I am pleased to inform you that your manuscript will be accepted for publication in Royal Society Open Science subject to minor revision in accordance with the referee suggestions. Please find the reviewers' comments at the end of this email.
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Comments to the Author: (There are no comments.) ********************************************** Reviewer comments to Author: Reviewer: 1 Comments to the Author(s) In this work, the authors report an experimental study on ordered pyrochlore structured Gd1.5Ce0.5Ti2O7 by synchrotron radiation X-ray diffraction and Raman spectroscopy. An orderdisorder transition is observed, and the transition is controlled by cationic order-disorder modification. This manuscript could be considered for acceptance after a minor revision. 1. To better understand the phase transition behavior, especially the isostructural transitions at lower pressures, the authors could perform some theoretical calculations if possible. 2. The grammatical mistakes and other errors in the manuscript should be carefully revised.
Reviewer: 2 Comments to the Author(s) In the current manuscript Niu and co-workers performed high-pressure experiments for (Gd0.667Ce0.333)2Ti2O7, a pyrochlore. They observed a phase transition at about 45GPa. They also reported two iso-structural transitions at 6.5 GPa and 13 GPa. The topic is interesting for people working in the related fields. The experimental details were described well. The results might be of interest for some people. The measured data were analyzed in a clear way. The manuscript was written concisely. The text is in the scope of the Journal. Therefore I'd like to propose acceptance of this manuscript for publication in Royal Society Open Science.
Author's Response to Decision Letter for (RSOS-190842 Theoretical calculations to reveal the high pressure behavior mechanism of the Gd 1.5 Ce 0.5 Ti 2 O 7 pyrochlore is a challenging topic. However, we are sorry to say that we don't have enough resources to perform it. The reasons are following: First is the confusing magnetic structure. In Gd 2 Ti 2 O 7 pyrochlore, because of the topology of the crystal structure, the spins of the Gd 3+ are geometrically frustrated. The magnetic structure of Gd 2 Ti 2 O 7 is still a currently active research topic. What's more, Ce 3+ replacing Gd 3+ will doubtfully effect the interaction of Gd 3+ . So to determine the ground state magnetic structure of the Gd 1.5 Ce 0.5 Ti 2 O 7 requires a lot of testing, and there is rarely experimental evidence can be consulted. Second, to perform the calculations on the Gd 1.5 Ce 0.5 Ti 2 O 7 pyrochlore's high pressure behavior, we lack enough computing resources. The crystal structure of the high pressure stable cotunnite-like phase is a highly disordered structure. Both cations and anions are randomly distributed on 4c positions in the cotunnite-like structures, and 1/8 of the anions are missing. In previous studies (Xiao et al, 2009, Xiao et al, 2010, the structure of the cotunnite-like phase is established by the special quasirandom structure approach by using the Monte-Carl simulated. Unfortunately, we don't have enough source now. Besides, researchers believe that the similar isostructural transition in Sm 2 Zr 2 O 7 may be caused by the anion disorder (Zhang et al, 2007), and describing this anion disordered structure also requires a lot of computing resources. So it is hard for us to carry out theoretical calculations for better understand the high pressure behavior of the Gd 1.5 Ce 0.5 Ti 2 O 7 , but we have also found that it is an interesting and challenging topic worth carrying out. We will perform the theoretical research in the future.