Investigating colossal thermal expansion driven by chalcogen bonds using AB(XCN)4 (A = Co, Zn, Cd; B = Cd, Hg; X = S, Se) compounds

Abstract

The thermal expansion of eight compounds from the well-known AB(XCN)4 family of thiocyanate- and selenocyanate-bridged frameworks (A = Co, Zn, Cd; B = Cd, Hg; X = S, Se) has been studied using single-crystal X-ray crystallography and Raman spectroscopy, to investigate the influence of chalcogen bonding interactions. In the case of ZnHg(SCN)4, biaxial negative thermal expansion (−6.9(9) ppm K−1) along the a and b axes was observed, occurring elastically due to the colossal positive thermal expansion (124.3(18) ppm K−1) along the c axis driven by weak S···S interactions. Substituting the thiocyanate groups with selenocyanate groups, i.e., with ZnHg(SeCN)4, caused a decrease in the anisotropic behaviour, due to stronger Se···Se interactions suppressing the expansion (α a = −0.4(15) ppm K−1; α c  = 90.0(17) ppm K−1). Replacing Zn with Cd was found to also suppress the anisotropic behaviour—regardless of the chalcogen used—while placing Cd at the B site enhanced the anisotropic behaviour. The effect of replacing Zn with Co was less clear, likely due to their similar size. Overall, the results illustrate how weak chalcogen bonds can drive colossal thermal expansion, leading to biaxial negative or zero thermal expansion.