A.M. Heyns

The i.r. and Raman spectra of sodium hexafluorophosphate monohydrate, NaPF6·H2O

Abstract The i.r. and Raman spectra of NaPF6·H2O and partially deuterated NaPF6·H2O have been obtained. All of the fundamental modes of the PF−6-ion have been located and the rule of mutual exclusion applies for these modes. In the Raman spectra, the number of components observed for each fundamental PF−6-mode is consistent with the number of site group components predicted. The hydrogen bonds in NaPF6·H2O are very weak and the hydrogen bond energy is of the order of 1.7 kcal/mole. The frequencies of the nine fundamental modes of the H2O, D2O and HDO molecules have been converted to harmonic values. It is shown that if the HOH valence angle is varied, and the effective harmonic force field of the water molecules calculated for each assumed value, an estimated value of this angle in NaPF6·H2O can be obtained. The librational modes of the water molecules have been located and are assigned to rocking and wagging modes.

Vibrational spectra and high-pressure polymorphism of KPF6

Abstract KPF 6 I at ambient conditions is cubic, space group Fm 3 m , with eightfold disorder of the anions. At ∼0°C it transforms to KPF 6 II, which appears to be rhombohedral and ordered. KPF 6 II further transforms to KPF 6 III at − 14·7°C. The III/II and II/I transition lines were studied as functions of pressure. The infrared and Raman spectra of these phases were studied in the range 50°C to −190°C, and polarized Raman spectra of single-crystal KPF 6 I were obtained at 25°C. The vibrational spectra of KPF 6 I are consistent with a disordered model where the PF bonds are tilted away from the crystal axes. The vibrational spectra of KPF 6 II are not, however, consistent with either of the space groups R 3 m or R 3 , usually encountered for this formula type, and indicate a lower site symmetry for the anions. The vibrational spectra of KPF 6 III indicate a complete lifting of all degeneracy as well as the failure of the rule of mutual exclusion.

Vibrational spectra, high-pressure polymorphism and force constants of KAsF6

Abstract Rhombohedral KAsF6 II (distorted CsCl-type) transforms at 101·5°C to a high-temperature phase KAsF6 I which is cubic, space group Fm3m (NaCl-type), with a0 = 8·050 A at 124°C. The anions are highly disordered in KAsF6 I, and it it probable that the fluorine atoms are offset from, and rotate about, the lines connecting the heavy atoms. The thermal expansion parameters of KAsF6 II and I were measured in the range 21–314°C. The volume change upon transition 12%. The II/I transition temperature increases steeply with pressure to 20 kbar. The i.r. and Raman spectra of these phases were studied in the range 150 to −190°C, and polarized Raman spectra of single-crystal KAsF6 II were obtained at 25°C. There is some evidence that the vibrational spectra of KAsF6 I show splitting similar to KPF6 I. This is consistent with tilting of the AsF bonds away from the crystal axes. The vibrational spectra of KAsF II are consistent with the space group R 3 . The force constants of the AsF6- ion were calculated for a number of different force fields.

A study of the infrared and Raman spectra of ammonium hexafluorophosphate NH4PF6 over a wide range of temperatures

Abstract The polarised Raman spectra of NH4PF6 as well as the infrared and Raman spectra of polycrystalline NH4PF6 and ND4PF6 were measured from 50–4000 cm−1 over a wide range of temperatures, confirming the phase transitions which occur at ~190 and ~130°K in these compounds. The spectra of Phases I and 11 are almost identical and in agreement with a face-centred cubic structure containing NH4+-ions with a low barrier to rotational freedom. Phase III is characterised by the appearance of new features in the infrared and Raman spectra and a possible structure for this phase is discussed. The rotational disorder that exists in all three phases in these compounds is reflected in their far-i.r. spectra.

Polymorphism, high-pressure phase diagram and vibrational spectra of KSbF6

Abstract KSbF6 I, stable above ∼16°C, is tetragonal, space group probably P42/mcm, with a0, c0 = 5·147,10·059 A. Low-temperature KSbF6II is cubic, space group probably I23-T3 or I213-T5 with a0 = 10·176 A. Both phases are ordered. The phase diagram of KSbF6 was determined to 7 kbar and 150°C, and reveals the appearance of a denser polymorph KSbF6 III above the III/II/I triple point at 3·2 kbar, 14°C. KSbF6 III can be expected to be rhombohedral, space group R 3 -C3i2. Infrared and Raman spectra of polycrystalline KSbF6 were studied in detail in the range 35°C to −190°C, and these data used to deduce the probable space groups of KSbF6 for phases I and II. The formerly accepted space groups of tetragonal and cubic KNbF6 and KTaF6 and cubic AgSbF6 are incorrect. KSb(OH)6 is cubic, space group probably. Pn3m-Oh4, with a0 = 8·125 A.

The i.r. and Raman spectra of tetraethylammonium hexafluoroantimonate (C2H5)4NSbF6

Abstract The vibrational spectra of (C 2 H 5 ) 4 NSbF 6 (I) at ambient temperature are compatible with cubic symmetry but there are signs that the symmetry of the crystal is lower than Fm 3 m -O 5 h , the space group predicted from X-ray powder diffraction methods. Torsional modes of the methyl groups have been identified but could not be assigned unambiguously. A rotational barrier of approximately 2kcal/mol has been calculated for the threefold reorientational motions of these groups. These motions change very little when the phase transition temperature (∼26O K) is traversed. In (C 2 H 5 ) 4 NSbF 6 (II) most of the degenerate -CH 2 , -CH 3 , CC and SbF modes are split into two components. The interionic lattice modes reflect the disorder which exists in the crystal at ∼300 K but at lower temperatures in (C 2 H 5 ) 4 NSbF 6 (II) well-defined features have been observed.

The infrared and raman spectra of tetramethylammonium hexa-fluorophosphate, (CH3)4NPF6, and tetramethylammonium hexa-fluoroantimonate, (CH3)4NSbF6

Abstract The powder diffraction pattern of (CH 3 ) 4 NSbF 6 has been obtained and can be indexed on the basis of a primitive tetragonal unit cell. It is therefore very likely that (CH 3 ) 4 NSbF 6 is isostructural with (CH 3 ) 4 NPF 6 which belongs to the space group P 4/ nmm - D 7 4 h . The vibrational spectra of these two compounds were recorded over a wide temperature range and are discussed in relation to the above-mentioned crystal structure as well as the spectra of other (CH 3 ) 4 N + -compounds. The vibrational bands of the cations in these salts show that they are only slightly distorted and weakly hydrogen-bonded to the respective anions. The spectra of the anions can be interpreted in terms of ordered groups showing the predicted splittings of bands. An approximately linear correlation is found between the translational lattice modes observed in the i.r. spectra and the reduced masses of a number of isostructural (CH 3 ) 4 N + compounds while the A 2 u .- E u . splitting of these modes decreases as the strength of hydrogen bonding in the crystal decreases.

High-pressure/high-temperature phase relations and vibrational spectra of CsSbF6

CsSbF6(II) under ambient conditions is trigonal, space group D3d5-R3m. At 187.8°C it undergoes a phase transition with an enthalpy change of 5.267 ± 0.316 kJ mole−1, to phase CsSbF6(I). CsSbF6 decomposes with loss of fluorine at atmospheric pressure at high temperatures, but under pressure the decomposition is prevented and a melting point of 310°C at atmospheric pressure can be inferred. The III phase boundary and melting curve were studied as functions of pressure. The infrared and Raman spectra of CsSbF6(II) were studied in the temperature range of −256 to 20°C, at ambient pressure. The crystal chemistry of the CsSbF6 and its relationship with other related compounds is discussed.

X-ray and Raman studies of single crystals of CsSbF6

Crystals of CsSbF6 belong to the rhombohedral space groupR¯3-C3i2 witha=7.904(1)andc=8.261(1) Å,V=446.95 Å3,Z=3,Dc=4.11 gcm−3. The antimony atom is surrounded by six fluorine atoms in a nearly perfect octahedral configuration with Sb-F 1.875(9) Å, while 12 fluorine atoms surround the cesium atom with closest contact 3.116 Å. Polarized Raman spectra of single crystals of CsSbF6 have been obtained, and it is shown that these results can be interpreted in terms of a unimolecular rhombohedral structure. The small distortion from an octahedral arrangement for the SbF6− group is clearly reflected in the spectra. The Raman results are in better agreement with the space groupR¯3m-D3d5 than withR¯3-C3i2, but this conclusion must be regarded with caution since the two features in the vibrational spectra of CsSbF6 which can be used to distinguish between the two structures are weak and ill-defined.

The vibrational spectra and crystallographic properties of CsPF6

Abstract Indications are that CsPF 6 (I) at ambient conditions is cubic with a possible space group of Fm 3 m -O 5 h . A slight distortion of the unit cell cannot, however, be ruled out. Assuming Fm 3 m symmetry the Raman spectra of CsPF 6 (I) are consistent with a disordered model in which the PF − 6 ions are tilted away from the crystallographic axes. The phase transition which occurs below 90 K in CsPF 6 is reflected in the vibrational spectra and further significant changes occur below 60 K particularly in the Raman bands. It is not yet clear whether these changes represent the establishment of long-range order or whether a further phase of CsPF 6 exists below 60 K. A possible structure for CsPF 6 at very low temperatures is discussed.