Infrared spectra of the ammonium ion in crystals. Part XIV. Hydrogen bonding and orientation of the ammonium ion in fluorides, with observations on the transition temperatures in cubic cryolite, elpasolite, and perovskite halides

Abstract

The ir spectra of the isotopic probe ion NH3Dt have been used to obtain information about the symmetry, orientation, and hydrogen-bonding involvement of the ammonium ion, between 10 K and room temperature, in NH4F, NH4HF,, (NH4)2[Cr(H20),]F,, NH4PF6, (NH4),SiF7, the elpasolites (NH4)?BFeF6 (B = Na, K) and CslNH4MF6 (M = Fe, Al), and the cryolites (NH4),MF6 (M = Al, Cr, Fe). Several of these fluorides exhibit low-temperature transitions, some of which are evident in the probe-ion spectra. It is shown that relating the isotopically isolated ND stretching and bending frequencies to the N F distances and to the coordination numbers of the ammonium ion reveals important trends in the dependence of the behaviour of the ion on its immediate environment in the crystal. A detailed discussion is presented of the effect of ionic size and the geometric tolerance factor r on the transition temperatures of cubic cryolite, perovskite, and elpasolite halides, as well as on the anisotropy of the principal thermal amplitudes of the halogen atom in such compounds. The relation between T,, and the frequency of the ND stretching absorption of NH,D' in the ammonium representatives of these classes of halides is also explored. In Part VI (2) an effort was made to explore, systematically, the dependence of the strength and type of hydrogen bonding of the ammonium ion in a variety of halides on factors such as the nature of the acceptor atom X, the coordination number of the ammonium ion and the N-H .-.X angle. Whilst the gross features of this dependence were successfully identified, the compounds were too few to obtain a detailed picture of the overall situation. In particular, the number of fluorides included was not sufficiently large if one considers that the fluorides determine the observed trend in a greater measure than the other halides, which exhibit a relatively narrow range of behaviour and form a fairly homogeneous group.In the present work we have supplemented these results by obtaining the v, and v,,, spectra of the NH3Dt probe ion in additional fluorides, including some recently described (3)(4)(5) ' For Part XIII, see ref. 1. 'Present address:

High‐Temperature VIS‐IR Spectroscopy of NH4‐Phyllosilicates

“…However, NH 4 + can be part of the phyllosilicate mineral lattice (as an interlayer cation) or can be adsorbed on the mineral surface. Spectral data of ammonium‐bearing minerals and rocks have been previously reported (e.g., Berg et al., 2016; Bishop, Banin, et al., 2002; Ehlmann et al., 2018; Farmer & Russell, 1967; Ferrari et al., 2019; Knop, Oxton, & Falk, 1979; Knop, Westerhaus, & Falk, 1980; Knop, Westerhaus, Falk, & Massa, 1985; Krohn & Altaner, 1987; Pironon et al., 2003) and show features related to the presence of nitrogen complexes at 1.56, 2.05, 2.12, 3.06, 3.25, 3.55, 4.2, 5.7, and 7.0 μm (Bruno & Svoronos, 1989; Robinson, 1974). Some of these spectra have been used in spectral models to retrieve the minerals present on the Ceres surface (e.g., De Sanctis, Ammannito, et al., 2015; Marchi et al., 2019; Raponi et al., 2019).…”

Alkylammonium lead halides. Part 2. CH3NH3PbX3 (X = Cl, Br, I) perovskites: cuboctahedral halide cages with isotropic cation reorientation

“…ref. 21). It is associated with the volume of the X12 cage available to the cation (usually expressed in terms of the geometric tolerance factor t = (r, + rX)/(rM + r x ) d 2 ) and also with T,,, (Fig.…”

“…This situation thus resembles the cation coordination in CH3NH3PbX3 but is more restrictive as regards reorientation: it is best understood by comparing CH3NH3PbX3(I) and the antiCaF2 (CH3NH3),TeX6 with the corresponding ammonium salts, NH4MF3 and (NH4)2MX6, which are essentially isostructural with the respective CH3NH3+ salts and have been extensively studied. From ir studies of the isotopically isolated NH3D+ ion (21,41,42) it is known that in the anti-CaF2 (NH4)2MX6 the N-H bonds avoid pointing at the comers of the '?he C19 structure type is topologically related to the C1 type, whereas the C6 type is not (38).…”

Transformation of germanium to fluogermanates

“…The peak at 577cm -1 and its shoulders were observed in hexagonal α-GeO 2 quartz crystals [16] but also Ge-F mode frequencies could contribute to this band of multiple peaks [17,20,21]. By analogy to previous FTIR studies on (NH 4 ) 2 SiF 6 cryptocrystals and ptype Germanium, all the major features of the spectrum at 3240 cm -1 , 483 cm -1 , 725 cm -1 and 1425 cm -1 have been attributed to stretching, bending and rocking modes of N-H vibrations in NH 4 + and GeF 6 ions [17][18][19][20][21]. There is no indication for the presence of Ge bonded to hydrogen inside the material as evidenced from the absence of Ge-H stretching modes in FTIR spectra.…”