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closo-Borates, such as Na2B12H12, are an emerging class of ionic conductors that show promising chemical, electrochemical and mechanical properties as electrolytes in all-solid-state batteries. Motivated by theoretical predictions, high-pressure in situ powder X-ray diffraction on Na2B12H12 was performed and two high-pressure phases are discovered. The first phase transition occurs at 0.5 GPa and it is persistent to ambient pressure, whereas the second transition takes place between 5.7 and 8.1 GPa and it is fully reversible. The mechanisms of the transitions by means of group theoretical analysis are unveiled. The primary-order parameters are identified and the stability at ambient pressure of the first polymorph is explained by density functional theory calculations. Finally, the parameters relevant to engineer and build an all-solid-state battery, namely, the bulk modulus and the coefficient of the thermal expansion are reported. The relatively low value of the bulk modulus for the first polymorph (14 GPa) indicates a soft material which allows accommodation of the volume change of the cathode during cycling.
Supporting information
CCDC references: 1870835; 1870894
sodium dodecahydro-closo-dodecaborate (epsilon)
top
Crystal data top
B12H12Na2 | c = 9.227 (3) Å |
Mr = 187.6 | V = 973.2 (5) Å3 |
Orthorhombic, Pbca | Z = 4 |
a = 9.720 (2) Å | Synchrotron radiation, λ = 0.71414 Å |
b = 10.851 (3) Å | T = 293 K |
Data collection top
One-axis goniometer diffractometer | Data collection mode: transmission |
Radiation source: Synchrotron, BM01 SNBL | Scan method: Stationary detector |
Specimen mounting: capillary | |
Refinement top
Rp = 0.005 | Profile function: TCHZ |
Rwp = 0.006 | 36 parameters |
Rexp = 0.002 | (Δ/σ)max = 0.041 |
RBragg = 0.07 | Background function: Chebychev polynomial |
3358 data points | |
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top | x | y | z | Biso*/Beq | |
Na1 | 0.3797 (12) | 0.1453 (13) | 0.8969 (12) | 1 | |
B1 | 0.33001 | −0.01575 | 0.54202 | 1 | |
B2 | 0.46118 | −0.02073 | 0.67864 | 1 | |
B3 | 0.55367 | 0.14294 | 0.44977 | 1 | |
B4 | 0.38696 | 0.08437 | 0.40051 | 1 | |
B5 | 0.42417 | 0.12054 | 0.58509 | 1 | |
B6 | 0.40186 | −0.07805 | 0.37881 | 1 | |
H1 | 0.21023 | −0.02527 | 0.57213 | 0 | |
H2 | 0.43587 | −0.03705 | 0.80476 | 0 | |
H3 | 0.59231 | 0.24416 | 0.41645 | 0 | |
H4 | 0.30622 | 0.14301 | 0.33121 | 0 | |
H5 | 0.37115 | 0.20776 | 0.64237 | 0 | |
H6 | 0.33573 | 0.86601 | 0.29174 | 0 | |
sodium dodecahydro-closo-dodecaborate (zeta)
top
Crystal data top
B12H12Na2 | c = 7.0102 (17) Å |
Mr = 187.6 | V = 382.97 (18) Å3 |
Orthorhombic, Pnnm | Z = 2 |
a = 6.301 (2) Å | Synchrotron radiation, λ = 0.71414 Å |
b = 8.671 (2) Å | T = 293 K |
Data collection top
One-axis goniometer diffractometer | Data collection mode: transmission |
Radiation source: Synchrotron, BM01 SNBL | Scan method: Stationary detector |
Specimen mounting: capillary | |
Refinement top
Rp = 0.004 | Profile function: TCHZ |
Rwp = 0.005 | 34 parameters |
Rexp = 0.002 | (Δ/σ)max = 0.196 |
RBragg = 0.03 | Background function: Chebychev polynomial |
3358 data points | |
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top | x | y | z | Biso*/Beq | |
Na1 | 0.243 (6) | −0.562 (7) | 0 | 2 (3) | |
B1 | −0.04893 | 0.90574 | 0.79903 | 1 (5) | |
B2 | 0.79039 | 0.88302 | 0 | 1 (5) | |
B3 | 0.21059 | 0.94213 | 0.87599 | 1 (5) | |
B4 | 0.04997 | 0.81026 | 0 | 1 (5) | |
H1 | −0.08347 | 0.84279 | 0.65438 | 1 (5) | |
H2 | 0.63903 | 0.80594 | 0 | 1 (5) | |
H3 | 0.36161 | 0.90475 | 0.79065 | 1 (5) | |
H4 | 0.08226 | 0.67579 | 0 | 1 (5) | |
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