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

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 °C, 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 °C, 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) Å, c = 5.543(1) Å and Z = 18. All lithium atoms are tetrahedrally coordinated by oxygen. The LiO₄ tetrahedra are connected by corner-sharing and by bridging formate groups to form a complicated three dimensional network comprising tunnels running along [0 0 0 1 ]. No indications for an at least partial occupation o f these tunnels have been found.