1. Supercell Crystal Structure of Na2Ga2(BO3)2O
Grace E. Brown, Dr. Robert W. Smith,
Department of Chemistry, University of Nebraska at Omaha, Omaha, NE 68182 -
Unit Cell
Abstract
Crystal Structure
Cell Parameters
Na2Ga2(BO3)2O consists of parallel sheets of [NaGa2(BO3)2]∞, separated by sodium layers. The BO3 anions are planar, in a trigonal bipyramidal arrangement about the O(2) atom.
Figure 1 is from the current published literature structure (Corbel et al.), which states that the O(2) atom occupies one of three positions around the axis of the (BO3)2O crystal, making the borate planes parallel.
In fact, the O(2) atom only occupies one bent position, creating a “ripple” in the borate planes and decreasing symmetry.
Na2Ga2(BO3)2O is an oxyborate with a reported crystal structure in the trigonal space group P 3 1c. The literature states that the three-dimensional structure is created from infinite stacking of parallel [NaGa2(BO3)2]∞ sheets separated by sodium layers. Our single-crystal X-ray diffraction data requires some modification to this structure.
The data suggests that Na2Ga2(BO3)2O actually possesses a supercell crystal structure with lower symmetry. The borate anions within the [NaGa2(BO3)2] sheets are not parallel to each other, but slightly tilted. This slight off-parallel characteristic creates very small supercell peaks in the X-ray diffraction pattern. The supercell crystal has the space group P 3 with cell parameters doubled along the a- and b-axes and tripled along the c-axis; the cell volume is thus 12 times larger than in the reported structure.
Literature:
a = 4.855(2)
c =15.441(7)
Z = 2
Supercell:
a = (1)
c = (5)
Z = 24
Systematic Absences
Literature:
h h 2ℎ l l odd
0 0 0 l l odd
Supercell:
None
Fig. 1. Na2Ga2(BO3)2O, Unit Cell (Supercell Not Included)
Symmetry
Literature:
Point Group: 3 1m
Space Group: P 3 1c (no. 163)
Supercell:
Point Group: 3
Space Group: P 3 (no. 147)
Methods and Materials
The compound was synthesized in the solid state. Stoichiometric quantities of NaNO3, Ga(NO3)3, and H3BO3 were ground and decomposed at 300 °C for 30 minutes. This product was then heated and cooled slowly, and ground multiple times, eventually reaching a high temperature of 750°C. Hexane was used during grinding to facilitate a more homogeneous phase.
X-Ray Diffraction
Conclusion and Future Directions
References
Powder x-ray diffraction and preliminary single-crystal x-ray diffraction results clearly indicate the supercell structure. The supercell is lower in symmetry, as the bent characteristic of the O(2) atom in the trigonal bipyramidal unit cause the borate planes to be non-parallel. It is in the P 3 space group. The unit cell is doubled along the a- and b-axes, and tripled along the c-axis.
For future research, the next step will be to obtain publishable single-crystal diffraction data. From this, the atomic coordinates can be derived and the structure will be defined.
Fig. 2. X-ray diffraction of Na2Ga2(BO3)2O
1. Corbel & Leblanc. “Crystal Structure of Na2M2(BO3)2O (M=Al, Ga); Comparison with Other Layered Oxyborates and SiP2O7.” J. Solid State Chem. 154, (2000).
The x-ray diffraction pattern of the Na2Ga2(BO3)2O sample GB4-1 in Figure 2 has several small peaks due to the supercell structure. These are due to the multiple angles in the structure caused by the bent symmetry of the O(2), resulting in the rippling effect of the borate plane.
Acknowledgments
Eric Villa, Creighton University, Omaha, NE
Charles Ross, University of Nebraska at Lincoln, Lincoln, NE