TY - JOUR

T1 - The Rigidity of the (BH4)-Anion Dispersed in Halides AX, A = Na, K; X = Cl, Br, I, and in MBH4 with M = Na, K, Rb, Cs

AU - Assi, Zeina

AU - Schneider, Alexander Gareth

AU - Ulpe, Anna Christina

AU - Bredow, Thomas

AU - Rüscher, Claus Henning

N1 - Funding Information: Funding: ZA was funded by DAAD Ref 441: A/10/97515. CHR used LUH-internal support and RU764/6-1, AGS and ACU were funded within the DFG project of BR1768/8-1.

PY - 2022/4/6

Y1 - 2022/4/6

N2 - The B–H bond length of the borohydride anion (BH4−) in alkali metal borohydrides MBH4 with M = Na, K, Rb, Cs, and diluted in different alkali halide matrices, was investigated experimentally by infrared spectroscopy (FTIR) and theoretically using first principles calculations. The peak positions in IR absorption spectra of NaBH4 pressed at 754 MPa in halides NaX and KX with X = Cl, Br, I show significant variations indicating ion exchange effects between the halide and NaBH4. For NaBH4 in NaBr, NaI, KBr and KI pellets, the peak positions indicate that BH4− could be highly diluted in the AX matrix, which renders an isolation of BH4− in AX (i-BH4−). For NaBH4 in NaCl and KCl pellets, a solution of BH4− in AX occurred only after a further thermal treatment up to 450◦C. The observed peak positions are discussed with respect to the lattice parameter (a0), anion to cation ratio (R = rA/rX), standard enthalpy of formation (∆fH) and ionic character (Ic ) of the halides. A linear relation is obtained between ν3 (i-BH4−) and the short-range lattice energies of AX. Density functional theory (DFT) calculations at generalized gradient approximation (GGA) level were used to calculate the IR vibrational frequencies ν4, ν3 and ν2 + ν4 for series of compositions Na(BH4)0.25X0.75 with X = Cl, Br, I, and MBH4. The theoretical and experimental results show the same trends, indicating the rigidity of the B–H bond length and the failure of Badger’s rule.

AB - The B–H bond length of the borohydride anion (BH4−) in alkali metal borohydrides MBH4 with M = Na, K, Rb, Cs, and diluted in different alkali halide matrices, was investigated experimentally by infrared spectroscopy (FTIR) and theoretically using first principles calculations. The peak positions in IR absorption spectra of NaBH4 pressed at 754 MPa in halides NaX and KX with X = Cl, Br, I show significant variations indicating ion exchange effects between the halide and NaBH4. For NaBH4 in NaBr, NaI, KBr and KI pellets, the peak positions indicate that BH4− could be highly diluted in the AX matrix, which renders an isolation of BH4− in AX (i-BH4−). For NaBH4 in NaCl and KCl pellets, a solution of BH4− in AX occurred only after a further thermal treatment up to 450◦C. The observed peak positions are discussed with respect to the lattice parameter (a0), anion to cation ratio (R = rA/rX), standard enthalpy of formation (∆fH) and ionic character (Ic ) of the halides. A linear relation is obtained between ν3 (i-BH4−) and the short-range lattice energies of AX. Density functional theory (DFT) calculations at generalized gradient approximation (GGA) level were used to calculate the IR vibrational frequencies ν4, ν3 and ν2 + ν4 for series of compositions Na(BH4)0.25X0.75 with X = Cl, Br, I, and MBH4. The theoretical and experimental results show the same trends, indicating the rigidity of the B–H bond length and the failure of Badger’s rule.

KW - alkali metal borohydride

KW - BH in halide

KW - first principle calculations

KW - infrared spectroscopy

UR - http://www.scopus.com/inward/record.url?scp=85128638387&partnerID=8YFLogxK

U2 - 10.3390/cryst12040510

DO - 10.3390/cryst12040510

M3 - Article

AN - SCOPUS:85128638387

VL - 12

JO - CRYSTALS

JF - CRYSTALS

SN - 2073-4352

IS - 4

M1 - 510

ER -