Synthesis, crystal structure and magnetic properties of Tm8S11

Peculiarities of crystal structures of rare-earth metal sulfides with the period 4 Å

The crystal structures of rare-earth metal sesquisulfides and some related compounds (Tm5S7, Tm8S11, Tm15S22) of orthorhombic or monoclinic symmetry with the mirror plane perpendicular to the translation of 3.7 Å – 4.1 Å have been studied using an analysis of close-packed atomic nets. The cation and the anion matrices have been shown to possess a similar construction for all compounds. A system of parallel mirror planes (∼2 Å apart) causes all atoms to be located only in these planes; in fact, this means that a crystal structure becomes pseudo two-dimensional. It was found that in each structure there were sets of cation close-packed crystallographic planes perpendicular to the mirror planes; when intersecting they form a node net close to the ideal 36 net. Because of the proximity of the neighbouring mirror planes the cation node in one mirror plane corresponds with a vacancy in the adjacent mirror plane. Hence the cation matrices can be represented by a single-layer hexagonal structure whose basal planes split into the two mirror planes with half the cations moving onto each of the mirror planes (the AA′ type according to our classification). The anion matrices have a similar construction but they are less regular and on a smaller scale. Each structure type has an inherent law governing the distribution of atoms and vacancies in the net and the proper conditions for the compatibility between the cation and anion matrices that determine the unit cell dimensions. So, the system of the nearest mirror planes is the main factor which regulates the atomic arrangement, that is, keeps to the minimum the number of arbitrarily changed coordinates or, more properly, degrees of a freedom of the structure.

Ternary erbium chromium sulfides: structural relationships and magnetic properties

Single crystals of four erbium-chromium sulfides have been grown by chemical vapor transport using iodine as the transporting agent. Single-crystal X-ray diffraction reveals that in Er(3)CrS(6) octahedral sites are occupied exclusively by Cr(3+) cations, leading to one-dimensional CrS(4)(5-) chains of edge-sharing octahedra, while in Er(2)CrS(4), Er(3+), and Cr(2+) cations occupy the available octahedral sites in an ordered manner. By contrast, in Er(6)Cr(2)S(11) and Er(4)CrS(7), Er(3+) and Cr(2+) ions are disordered over the octahedral sites. In Er(2)CrS(4), Er(6)Cr(2)S(11), and Er(4)CrS(7), the network of octahedra generates an anionic framework constructed from M(2)S(5) slabs of varying thickness, linked by one-dimensional octahedral chains. This suggests that these three phases belong to a series in which the anionic framework may be described by the general formula [M(2n+1)S(4n+3)](x-), with charge balancing provided by Er(3+) cations located in sites of high-coordination number within one-dimensional channels defined by the framework. Er(4)CrS(7), Er(6)Cr(2)S(11), and Er(2)CrS(4) may thus be considered as the n = 1, 2, and infinity members of this series. While Er(4)CrS(7) is paramagnetic, successive magnetic transitions associated with ordering of the chromium and erbium sub-lattices are observed on cooling Er(3)CrS(6) (T(C)(Cr) = 30 K; T(C)(Er) = 11 K) and Er(2)CrS(4) (T(N)(Cr) = 42 K, T(N)(Er) = 10 K) whereas Er(6)Cr(2)S(11) exhibits ordering of the chromium sub-lattice only (T(N) = 11.4 K).