Refinement of the structures of GeS, GeSe, SnS and SnSe

The orthorhombic structures (Pbnm-Di~; No. 62) of GeS, GeSe, SnS and SnSe have been refined (X-ray diffractometer data: 319 hkl, R = 0.029; 370 hkl, R = 0.061 ; 386 hkl, R = 0.062; 506 hkl, R = 0.076). The bond distances are: Ge S = 2.438(1), 2.448(2) A; Ge-Se = 2.574(2),2.564(3) A; Sn-S = 2.665(2),2.627(4) A; SnSe = 2.793(2), 2.744(3) A. The structures of these compounds reveal systematic variations of bond lengths, bond angles and of nonbonding distances. These properties as well as their temperature dependence compared to those of related compounds allow to treat these structures as different configurations of a hypothetical reaction path of a phase transition GeS type ---+ TII type ---+ NaCl type.