AbstractThe intermetallic compounds NaAlSi and NaAlGe have been prepared and their structure has been determined by powder data. They crystallize in an anti-PbFCl-type lattice with the following cell dimensions: NaAlSi:a = 413.5 pm, c = 737.9 pm, c/a = 1.785; NaAlGe: a = 416.4 pm, c = 742.7 pm, c/a = 1.784.

A determination of the crystal structure of ammonium tetraphenylborate (ATPB) at 120 K shows that the ammonium ion in this crystal, at a site of symmetry D,,, is oriented in a way consistent with the expectation from a simple model based on Coulombic interaction. Infrared spectra of the isotopic ammonium ions obtained between 10 Kand room temperature indicate that the effect of the tetraphenylborate anion on the strength ofthe N-H bond (as measured by the stretchingfrequency) is small and independent of temperature, and that the distortion of the ammonium ion from T, symmetry is slight. From the combined evidence it is concluded that the ammonium ion in ATPB must be regarded as hydrogen-bonded, but the potential field due to the four phenyl rings surrounding the ammonium ion offers little resistance to the bending (and probably also to the librational) motion of the ion.WOLFGANG J. WESTERHAUS, OSVALD KNOP et MICHAEL FALK. Can. J. Chem. 58,1355. Une determination de la structure cristalline de tetraphenylborate d'ammonium (ATPB) a 120 K montre que l'ionammonium dans ce cristal, au site de symetrie D,,, est orient6 d'une manikre conforme a celle prkvue a partir d'un modkle simple base sur une interaction coulombienne. Les spectres infrarouge des ions ammonium isotopiques obtenus entre 10 K et la temperature ambiante indiquent que I'effet de I'anion tetraphenylborate sur la force de la liaison N-H (telle que mesuree par la frequence de valence) est faible et independant de la temperature et que la distorsion de I'ion ammonium par rapport a la symetrie T, est faible. L'ensemble de ces preuves permet la conclusion que I'ion ammonium dans I'ATPB doit &re considere comme lie par des liaisons hydrogkne, mais le potentiel dD aux quatre cycles phenyl entourant I'ion ammonium offre peu de resistance au mouvement de deformation de I'ion (et probablement aussi au mouvement de libration).[Traduit par le journal]

Investigation of the perovskites NH4MF3 (M = Mn, Co, Zn) using the NH3Dt ion as an i.r. probe has confirmed the existence of low-temperature structural transitions from a cubic (Pm3m, 2 = 1) phase to a phase of lower symmetry in all three compounds. Although in the cubic phase the site symmetry at the N site is Oh, the effective symmetry of the ammonium ion is only DZd. This is due to the tendency of the ion to establish as strong normal N-H-.-F bonds as is possible within the coordination cuboctahedron of F atoms. The actual behaviour of the ion in the cubic phase is the result of a compromise between satisfying this tendency by "static" (distortion of the ion) and dynamic (large-amplitude librational and bending motions) means. The tetragonal distortion of the ion decreases with increasing temperature. To maintain an overall cubic symmetry the directions of the S, axes of the elongated NH4 tetrahedra in the crystal must be distributed at random and, in view of the rapid reorientations of the ammonium ion known to occur in these fluorides, undergo constant change. The low-temperature phase is not derived from the cubic phase by a simple tetragonal distortion. The site symmetry at the N atom is C , , or C, and the overall hydrogen bonding in this phase is strengthened. A model is proposed which accounts for the observed entropy of transition.

In the systems K-Al, Ga, In-Si, Ge, Sn each phase of composition K8(3B, 4B)46 was prepared. The crystal structure for K8(Al, Ge)48 and K8(Ga, Ge)46 was determined by X-ray investigations. In the systems Na-Al, Ga-Si, Ge the compounds Na8(Al, Ge)46 (crystal structure determined), Na8(Ga, Si)46 and Na8(Ga, Ge)46 were prepared.

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: