The Polymorphism of Alkali Metal Formates, Part 3 A Raman Spectroscopic, X-Ray and Thermoanalytical Study of the Polymorphism of Lithium Formate

Abstract

The crystal structure of anhydrous lithium formate, published recently by Kansikas and Hermansson, is in fact that of a quenchable high-temperature modification (LiHCOO–I). The low-temperature modification LiHCOO–II can be obtained by thermal dehydration of the monohydrate, LiHCOO·H2O, or by drying it over P4O10. LiHCOO–II persists up to 228 ¼C, at which temperature it transforms to LiHCOO–I, which in turn is stable up to its melting point at 272 °C. LiHCOO–I is also the modification which crystallizes when a melt of lithium formate is cooled down, in spite of the fact that solidification occurs only at 165 ¼C, i.e. at a temperature well within the stability region of LiHCOO –II. The Raman spectra of the different forms of lithium formate are discussed in some detail. Single crystals of LiHCOO–II, obtained by slow evaporation of a saturated solution of anhydrous LiHCOO in absolute methanol, are hexagonal, space group P63, with a = 14.915(2) Ä, c = 5.543(1) Å and Z = 18. All lithium atoms are tetrahedrally coordinated by oxygen. The L iO4 tetrahedra are connected by corner-sharing and by bridging formate groups to form a complicated three dimensional network comprising tunnels running along [0001]. N oindications for an at least partial occupation of these tunnels have been found. © 1992, Walter de Gruyter. All rights reserved.