10.5.4 Factors Affecting the Position of RO Vibration Band
In conclusion, it should be noted that the vibrational spectra of zeolites may indirectly serve their phase identification, although sometimes it is a complicated
process. The position of the bands associated with the characteristic pseudolattice
vibrations is influenced by:
• Multiplicity of rings (its increase causes the presence of bands with ever lower
wavelengths);
• Al:Si ratio (its increase also reduces the wave number of bands);
• Type of vibration associated with the band;
• Type of extra-framework cations;
• The degree of deformation and the multitude of ring conformations;
• Structure ordering.
These factors cause that the ranges in which the characteristic bands for particular types of units occur may change and overlap, and determining the exact
limits of such ranges in the spectra for some groups is difficult.
10.6 SBU Versus Periodic Structure
Recent advances in computational techniques and increased availability of computers with ever-increasing computing power allow constructing increasingly
complex and computationally demanding theoretical models that better describe the
actual structures and thus provide much more precise information on the properties
of zeolites. Despite the undoubted advantage of a very short calculation time, the
approach in which small fragments of the structure are used to interpret the whole
vibrational spectrum of a solid state materials makes it impossible to determine the
effect of long-range ordering (due to translational symmetry of crystal lattice) on the
spectrum [59], especially in far-infrared region, where bands associated with crystal
lattice and cationic vibrations should be expected.
Despite the aforementioned dynamic increase in the available computing power,
computational methods used routinely in ab initio calculations for periodic systems
are so far rarely used in the description of zeolitic structures, inter alia due to the
high complexity of these systems: a large number of symmetrically non-equivalent
atoms in the unit cell, common partial occupation of some of Wyckoff’s positions
in asymmetric unit cell by extra-framework cations, and water present in real
systems, e.g., in the form of hydration shells of non-tetrahedral cations. The few
reported results are based on significant approximations; e.g., Creighton et al. [60],
using the Wilson method modified for crystals calculated the theoretical vibrational
spectrum of pure silica form of sodalite (Si 12 O 24 ), while Iyer and Singer [61, 62] in
their works devoted to crystalline lattices of zeolite A and sodalite stated that there
is no direct correlation between the individual structure elements and the properties
314
M. Król et al.
In conclusion, it should be noted that the vibrational spectra of zeolites may indirectly serve their phase identification, although sometimes it is a complicated
process. The position of the bands associated with the characteristic pseudolattice
vibrations is influenced by:
• Multiplicity of rings (its increase causes the presence of bands with ever lower
wavelengths);
• Al:Si ratio (its increase also reduces the wave number of bands);
• Type of vibration associated with the band;
• Type of extra-framework cations;
• The degree of deformation and the multitude of ring conformations;
• Structure ordering.
These factors cause that the ranges in which the characteristic bands for particular types of units occur may change and overlap, and determining the exact
limits of such ranges in the spectra for some groups is difficult.
10.6 SBU Versus Periodic Structure
Recent advances in computational techniques and increased availability of computers with ever-increasing computing power allow constructing increasingly
complex and computationally demanding theoretical models that better describe the
actual structures and thus provide much more precise information on the properties
of zeolites. Despite the undoubted advantage of a very short calculation time, the
approach in which small fragments of the structure are used to interpret the whole
vibrational spectrum of a solid state materials makes it impossible to determine the
effect of long-range ordering (due to translational symmetry of crystal lattice) on the
spectrum [59], especially in far-infrared region, where bands associated with crystal
lattice and cationic vibrations should be expected.
Despite the aforementioned dynamic increase in the available computing power,
computational methods used routinely in ab initio calculations for periodic systems
are so far rarely used in the description of zeolitic structures, inter alia due to the
high complexity of these systems: a large number of symmetrically non-equivalent
atoms in the unit cell, common partial occupation of some of Wyckoff’s positions
in asymmetric unit cell by extra-framework cations, and water present in real
systems, e.g., in the form of hydration shells of non-tetrahedral cations. The few
reported results are based on significant approximations; e.g., Creighton et al. [60],
using the Wilson method modified for crystals calculated the theoretical vibrational
spectrum of pure silica form of sodalite (Si 12 O 24 ), while Iyer and Singer [61, 62] in
their works devoted to crystalline lattices of zeolite A and sodalite stated that there
is no direct correlation between the individual structure elements and the properties
314
M. Król et al.