The many phases of CaC2

Abstract

Polymorphic CaC2 was prepared by reacting mixtures of CaH2 and graphite with molar ratios between 1:1.8 and 1:2.2 at temperatures between 700 and 1400 °C under dynamic vacuum. These conditions provided a well controlled, homogeneous, chemical environment and afforded products with high purity. The products, which were characterized by powder X-ray diffraction, solid state NMR and Raman spectroscopy, represented mixtures of the three known polymorphs, tetragonal CaC2-I and monoclinic CaC2-II and -III. Their proportion is dependent on the nominal C/CaH2 ratio of the reaction mixture and temperature. Reactions with excess carbon produced a mixture virtually free from CaC2-I, whereas high temperatures (above 1100 °C) and C-deficiency favored the formation of CaC2-I. From first principles calculations it is shown that CaC2-I is dynamically unstable within the harmonic approximation. This indicates that existing CaC2-I is structurally/dynamically disordered and may possibly even occur as slightly carbon-deficient phase CaC2-δ. It is proposed that monoclinic II is the ground state of CaC2 and polymorph III is stable at temperatures above 200 °C. Tetragonal I represents a metastable, heterogeneous, phase of CaC2. It is argued that a complete understanding of the occurrence of three room temperature modifications of CaC2 will require a detailed characterization of compositional and structural heterogeneities within the high temperature form CaC2-IV, which is stable above 450 °C. The effect of high pressure on the stability of the monoclinic forms of CaC2 was studied in a diamond anvil cell using Raman spectroscopy. CaC2-II and -III transform into tetragonal CaC2-I at about 4 and 1GPa, respectively.