ABO3 perovskites show cubic structure at high temperatures where the
A-site cation is having 12 coordination with oxygen ions and
B-site cation is having 6 coordination with oxygen ions
But whereas in low temperature stable phase of ABO3 structures (particularly in rhombohedral sodium bismuth titanate)
What is the effective A-site cation coordination (6 or 9 or any other) in NBT?
Please give me brief answer with references.
Hi Santosh Kumar Behara ,
In short, the coordination number (CN) decreases from 12 (cubic phase) to approximately 8.4393 (rhombohedral phase). A more complete answer can be found below. I have attached a figure to help with my explanation. I have also included Crystallographic Information Files (CIFs) for both the cubic and rhombohedral phases.
On the right, you can see a temperature-composition phase diagram; 0 mol% on the x-axis corresponds to BNT. Upon cooling from high temperature (>350 C), BNT undergoes two structural phase transitions from cubic to pseudocubic (at 350 C) and pseudocubic to rhombohedral (at approximately 200 C).
On the left, you can see the structures for the cubic and rhombohedral structures, which I have obtained from the Inorganic Crystal Structure Database (ICSD) and visualized using VESTA (http://jp-minerals.org/vesta/en/). Purple, gold, blue, and red atoms correspond to Bi, Na, Ti, and O, respectively. Purple, gold, and blue polyhedra correspond to those of Bi, Na, and Ti, respectively. These polyhedra help to visualize the coordination environment of Bi and Na.
For the cubic phase, each Bi and Na are equidistant from 12 O's. Therefore, the CN of Bi and Na in the cubic phase is 12. For the rhombohedral phase, each Bi and Na are surrounded by 12 O's, however, they are closer to some and further from others. As a result, the effective CN of Bi and Na in the rhombohedral phase is 8.4393. This non-integer, effective CN is calculated using the bond valence model (https://www.iucr.org/resources/data/datasets/bond-valence-parameters) in VESTA (obtained by choosing the "polyhedral" structural model style and double clicking on the polyhedra).
Dear Robert Wexler
Thank you for the detailed explanation. Yes I am agreeing with these results. But I have some doubts still.
- Generally, in distorted perovskites like CaTiO3 and BiFeO3, the A-site (Ca/Bi) coordination with oxygen will change as the BO6 rotates during the phase transformation from high-temperature cubic to low temperature orthorhombic/rhombohedral structures. In distorted orthorhombic perovskites such as CaTiO3, the coordination of A-site cation is reported to be 8 or 9. As the rhombohedral structure is even more distorted than the one that is orthorhombic, is it not acceptable to take a still lower “nearest neighbor coordination number” of 6 rather than 8 or 9?
https://materialsproject.org/materials/mp-39681/
- From the above mentioned link, in first principle calculations of NBT (rhombohedral structure), the coordination number of A-site cation is mentioned as 6.
Please explain me these two comments. Thank you very much for your valuable time.
Hi Santosh Kumar Behara ,
The structure you have sent me is a double perovskite (AA'B2O6) and not ABO3, which you have written in your original question.
Upon inspection of this structure, I agree with your assessment that the CNs of Bi and Na are approximately 6. Using the bond valence model I mentioned above, I get 5.8720 for Bi and 6.9786 for Na.
Hai Robert Wexler
May I know which software you have used for the Bond-valence analysis in the context of NBT ?
Thanks in advance
- Badges
- Science topic
- Similar topics
- Filing