Polymorphie und Pseudosymmetrie von Li2CoCl4

Abstract

Abstract The crystal structures of Li2CoCl4 at room temperature and at 400° C were redetermined by X-ray measurements of a twinned trilling crystal and by neutron powder diffraction. The low-temperature polymorph Li2CoCl4 oC14 crystallizes in an ordered NaCl defect structure of SnMn2S4 type (space group Cmmm, Z = 2, a = 724.1(1), b = 1027.8(1) and c = 358.69(3) pm, RI = 5.2%). Because of a pronounced pseudosymmetry this structure could only be solved by combination of several independent methods taking into account group theoretical aspects of the phase transitions involved. The high-temperature polymorph Li2CoCl4 cF56 obtained above 310°C crystallizes in the Li2MnBr4 HTM I type (space group Fd[unk]m, Z = 8, a = 1043.53(7) pm (400°C), RI = 5.1 %). Whether a fully disordered deficient NaCl structure (aristotype) is reached before melting has not been fully established. The mechanisms of the phase transitions are discussed in terms of symmetry reduction and ordering of the metal ions.