what is anhydrous oxide
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Within this group we shall consider the oxides of single metallic and semi-metallic elements, and complex oxides involving two or more elements of formal ionic charge not exceeding four (silica and the silicates are treated separately). These oxides are therefore distinguished from the majority of sulphates, phosphates, etc., by the fact that no separate oxy-anion can usually be distinguished in their structure. Oxides have received considerable attention from physicists concerned with the solid state: their approach, procedures, and results are described by Hadni (1967) and Mitra (1969). Since the simpler structures involve only one or two different types of bond, it becomes feasible to calculate the complete vibrational spectrum, and to check the results, at the long wavelength limit, against infrared reflection and Raman spectra obtained from single crystals. Oxide spectra are now well understood in principle, as is illustrated by calculations of the vibrations of quartz (Elcombe, 1967), rutile structures, including T1O2 and SnO2 (Katiyar et al., 1971), and perovskites (Cowley, 1964), all of which achieve favourable agreement with experimental data.
In contrast, most of these oxides present special difficulties to the chemical spectroscopist, who aims to obtain information on structure and composition from spectra derived from specimens which can usually only be examined in powdered form. Because of the intense dipole oscillations induced by the vibrations of highly ionic oxides, their powder spectra are profoundly modified by the shape and size of the particles, as discussed in Chapter