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Metal closo-borates have recently received significant attention due to their potential applications as solid-state ionic conductors. Here, the synthesis, crystal structures, and properties of (NH4)2B10H10·xNH3 (x = 1/2, 1 (α and β)) and (NH4)2B12H12·xNH3 (x = 1 and 2) are reported. In situ synchrotron radiation powder X-ray diffraction allows for the investigation of structural changes as a function of temperature. The structures contain the complex cation N2H7+, which is rarely observed in solid materials, but can be important for proton conductivity. The structures are optimized by density functional theory (DFT) calculations to validate the structural models and provide detailed information about the hydrogen positions. Furthermore, the hydrogen dynamics of the complex cation N2H7+ are studied by molecular dynamics simulations, which reveals several events of a proton transfer within the N2H7+ units. The thermal properties are investigated by thermogravimetry and differential scanning calorimetry coupled with mass spectrometry, revealing that NH3 is released stepwise, which results in the formation of (NH4)2BnHn (n = 10 and 12) during heating. The proton conductivity of (NH4)2B12H12·xNH3 (x = 1 and 2) determined by electrochemical impedance spectroscopy is low but orders of magnitude higher than that of pristine (NH4)2B12H12. The thermal stability of the complex cation N2H7+ is high, up to 170 °C, which may provide new possible applications of these proton-rich materials.

Metal

Ammine metal borohydrides show potential for solid-state hydrogen storage and can be tailored toward hydrogen release at low temperatures. Here, we report the synthesis and structural characterization of seven new ammine metal borohydrides

Synthesis, crystal structures, and thermal and magnetic properties of the complete series of halide-free rare-earth (RE) metal borohydrides are presented. A new synthesis method provides high yield and high purity products. Fifteen new metal borohydride structures are reported. The trends in crystal structures, thermal behavior, and magnetic properties for the entire series of RE(BH 4 ) x are compared and discussed. The RE(BH 4 ) x possess a very rich crystal chemistry, dependent on the oxidation state and the ionic size of the rare-earth ion. Due to the lanthanide contraction, there is a significant decrease in the volume of the RE 3+ -ion with increasing atomic number, which correlates linearly with the unit cell volume of the α- and β-RE(BH 4 ) 3 polymorphs and the solvated complexes α-RE(BH 4 ) 3 ·S(CH 3 ) 2 . The thermal analysis reveals a one-step decomposition pathway in the temperature range from 247 to 277 °C for all RE(BH 4 ) 3 except Lu(BH 4 ) 3 , which follows a three-step decomposition pathway. In contrast, the RE(BH 4 ) 2 decompose at higher temperatures in the range 306 to 390 °C due to lower charge density on the rare-earth ion. The RE(BH 4 ) 3 show increasing stability with increasing Pauling electronegativity, which contradicts other main group and transition metal borohydrides. The majority of the compounds follow Curie-Weiss paramagnetic behavior down to 3 K with weak antiferromagnetic interactions and magnetic moments in accord with those of isolated 4f ions. Some of the RE(BH 4 ) x display varying degrees of temperature-dependent magnetic moments due to low-lying excited stated induced by crystal field effects. Additionally, a weak antiferromagnetic ordering is observed in Gd(BH 4 ) 3 , indicating superexchange through a borohydride group.