Pressure-induced phase transitions in Na₂B₁₂H₁₂, structural investigation on a candidate for solid-state electrolyte

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.

Computing details

sodium dodecahydro-closo-dodecaborate (epsilon)

Crystal data

B12H12Na2	c = 9.227 (3) Å
Mr = 187.6	V = 973.2 (5) Å^3
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

One-axis goniometer diffractometer	Data collection mode: transmission
Radiation source: Synchrotron, BM01 SNBL	Scan method: Stationary detector
Specimen mounting: capillary

Refinement

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 (Ų)

	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)

Crystal data

B12H12Na2	c = 7.0102 (17) Å
Mr = 187.6	V = 382.97 (18) Å^3
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

One-axis goniometer diffractometer	Data collection mode: transmission
Radiation source: Synchrotron, BM01 SNBL	Scan method: Stationary detector
Specimen mounting: capillary

Refinement

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 (Ų)

	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)