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1. Introductory remarks

Unlike the feeling that the uninitiated could have, and in spite of the common belief prevailing in the chemistry community until some decades ago, the behaviour and properties of inorganic systems at high and very high temperature may differ significantly from the chemical behaviour we are used to dealing with at near room temperature. Since the very beginning of the research field destined to become known as “high temperature chemistry”, researchers realized that it was of special scientific interest, because high temperature behaviour of materials cannot be easily predicted by simply extrapolating the information known under ordinary temperature conditions. Indeed, a number of phenomena and factors commonly considered marginal or negligible in the “usual” room temperature chemistry (vapourization processes, entropy effects overcoming energetic driving forces, thermodynamic rather than kinetic control of processes, formation of new and unexpected molecular species and solid phases due to stabilization of odd or unusual oxidation states of elements, etc.) come on stage and their importance increases more and more with increasing temperature, playing a dominant role in many physicochemical processes. Thus, it was (and still is) commonly assumed by laypersons that gas phase systems always tend towards simplification on increasing temperature. However, different and even opposite behaviour is observed when the gas phase is in equilibrium over a condensed system since the formation of more complex species is often favoured under high temperature conditions. On the


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