α- and β-Na2[BH4][NH2]: two modifications of a complex hydride in the system NaNH2–NaBH4; syntheses, crystal structures, thermal analyses, mass and vibrational spectra

Abstract

The title compound Na2[BH4][NH2]≡Na2BNH6 exists in two different modifications: the low-temperature (LT) β- and the high-temperature (HT) α-phase. The HT α-Na2BNH6 is accessible by single heating of the binaries NaNH2 and NaBH4 (molar ratio = 1:1) to the melting point of 492 K for 1 h in evacuated Pyrex ampoules. On longer annealing (T = 342 K, 10 days), the HT α-phase converts completely to the LT β-phase. The β → α transition at 371 K is very fast, but the conversion α → β is quite slow. No further phase formation was detected in the system up to a molar ratio of 5:1 (NaNH2:NaBH4). The cubic α-Na2BNH6 crystallizes in the K3SO4F-type of structure (space group: P m over(3, ̄) m, Z = 1) with a = 4.6950(1) Å. β-Na2BNH6 crystallizes in orthorhombic symmetry (space group: Pbcm, Z = 4) with a = 6.5384(2) Å, b = 6.4960(1) Å, c = 9.8512(2) Å. Both structures can directly be derived from the perovskite type structure ABO3, with the [BH4]− and [NH2]− moieties in the A and B sites, respectively. The charge balancing Na ions are occupying 2/3 of the O positions. In both cases, the presence of the two different anions in the solids was also confirmed by vibrational spectra. According to DTA and high temperature X-ray diffraction (HT-XRD) measurements, the educts NaNH2 and NaBH4 react at 430 K to form α-Na2BNH6 which remains stable until the melting point of 492 K. Between 492 and 573 K, no significant effects were detected in DTA and TG. Above 573 K, however, a sudden decomposition takes place in two steps (659 and 689 K) causing a total mass loss of 8%. The gaseous species formed during the decomposition process were followed by mass spectra (MS). The mass spectra obtained from 1:1 and 2:1 mixtures of NaNH2 and NaBH4 in the range 492 K < T < 773 K prove that the main gaseous product evolved during the thermal decomposition is H2. NH3 and H2O were also detected, but their concentrations are quite low. These findings confirm that the mixture of the complex hydrides NaNH2–NaBH4 (molar ratio ≥1:1) – comparable to the LiNH2–LiBH4 system – are potential candidates for solid hydrogen storage materials. The composition of the solid residues after the thermal treatment depends on the molar ratio of the starting binaries NaNH2 and NaBH4. While Na2BNH6 (1:1) decomposes to a mixture of NaH, Na and an unidentified amorphous solid, mixtures ≥2:1 yielded exclusively Na3BN2. The vibrational spectra of the title compounds Na2BNH6 have been measured and interpreted based on the Td and C2v symmetry of the relevant [BH4]− and [NH2]− groups. Both the (N–H) and (B–H) frequencies exhibit small but significant shifts with respect to the pure binaries.