ACID-BASE PROPERTIES AND VOLATILIITY OF FLUORIDES OF s-, p-, d-METALS

Abstract

A correlation between acid-base properties and volatility parameters (temperature and enthalpy of vaporization) of fluorides of s-, p-, and d-metals was revealed. As a characteristic of acid-base properties, the basicity parameter proposed in the work, based on the size-charge characteristics of the ions that make up the compound, is taken. The value of the basicity parameter naturally decreases from fluorides of low-valent metals (I–III) to fluorides of highvalent metals (IV–VII). The indicated cation substitution results in a significant increase in the volatility of compounds, which is explained by a change in the type of crystal structure from ionic-covalent to molecular one typical for acid-type fluorides. Metal fluorides with intermediate values of the basicity parameter (Zr(IV), Hf(IV), Al(III)) are characterized by significantly higher values of temperature and evaporation enthalpy, that is, lower volatility compared to metal fluorides with a molecular type of structure, which indicates the manifestation of ionic-covalent type of structure. The entropy of evaporation of metal fluorides with a molecular type of structure, as well as with an ionic-covalent type, is characterized by low values (80 ÷ 140 J/mol∙K), which indicates minor changes in the structure of the molecules during evaporation. On the other hand, in the case of compounds with intermediate basicity (ZrF4, AlF3), the values of the entropy of evaporation are significantly higher (over 180 J/mol∙K), which indicates significant structural changes. A method of experimental evaluation of the volatility of compounds by measuring the rate of the coating condensation on the substrate is proposed. It is shown the possibility of combining metal fluorides into complex compounds based on the principle of basicity and on the known values of the evaporation temperature and the conventional temperature. The nature of the influence of anionic substitution in Zirconium compounds with the same charge of ions (F– → Cl– → Br– → l–) and with a change in charge (F– → O2– → N3– → C4–) on the basicity and volatility of the compounds was established. If in the first case there are no significant changes in the nature of the compounds, then in the second there is a sharp increase in melting and evaporation temperatures due to an increase in basicity.