Expanding the Averievite Family, (MX)Cu5O2(T5+O4)2 (T5+ = P, V; M = K, Rb, Cs, Cu; X = Cl, Br): Synthesis and Single-Crystal X-ray Diffraction Study

Abstract

Averievite-type compounds with the general formula (MX)[Cu5O2(TO4)], where M = alkali metal, X = halogen and T = P, V, have been synthesized by crystallization from gases and structurally characterized for six different compositions: 1 (M = Cs; X = Cl; T = P), 2 (M = Cs; X = Cl; T = V), 3 (M = Rb; X = Cl; T = P), 4 (M = K; X = Br; T = P), 5 (M = K; X = Cl; T = P) and 6 (M = Cu; X = Cl; T = V). The crystal structures of the compounds are based upon the same structural unit, the layer consisting of a kagome lattice of Cu2+ ions and are composed from corner-sharing (OCu4) anion-centered tetrahedra. Each tetrahedron shares common corners with three neighboring tetrahedra, forming hexagonal rings, linked into the two-dimensional [O2Cu5]6+ sheets parallel to (001). The layers are interlinked by (T5+O4) tetrahedra (T5+ = V, P) attached to the bases of the oxocentered tetrahedra in a “face-to-face” manner. The resulting electroneutral 3D framework {[O2Cu5](T5+O4)2}0 possesses channels occupied by monovalent metal cations M+ and halide ions X−. The halide ions are located at the centers of the hexagonal rings of the kagome nets, whereas the metal cations are in the interlayer space. There are at least four different structure types of the averievite-type compounds: the P-3m1 archetype, the 2 × 2 × 1 superstructure with the P-3 space group, the monoclinically distorted 1 × 1 × 2 superstructure with the C2/c symmetry and the low-temperature P21/c superstructure with a doubled unit cell relative to the high-temperature archetype. The formation of a particular structure type is controlled by the interplay of the chemical composition and temperature. Changing the chemical composition may lead to modification of the structure type, which opens up the possibility to tune the geometrical parameters of the kagome net of Cu2+ ions.

Structural and chemical complexity of minerals: an update

“…The list of twenty most structurally complex minerals known so far given in Table 3 reveals the following most important complexity-generating mechanisms in minerals: the presence of large (sometimes nanometre-scale) clusters such as polyoxometalates or related finite-cluster structures (Krivovichev, 2020b) isolated from each other; such multinuclear atomic units possess a large number of atoms with different topological functions; the examples are ewingite, morrisonite, vanarsite, bouazzerite and postite; as a rule, natural polyoxometalate minerals are also highly hydrated, with the exception of arsmirandite and lehmannite, which are anhydrous polyoxocuprates formed in volcanic fumaroles (Britvin et al , 2020); the presence of large clusters linked together to form three-dimensional frameworks; the examples are ilmajokite and paddlewheelite (both minerals are also highly hydrated, which contributes greatly to their structural complexities); the formation of complex three-dimensional modular frameworks formed by cages of different sizes and topologies; this complexity type corresponds to paulingite-group minerals, fantappièite and sacrofanite (members of the sodalite–cancrinite ABC series (Bonaccorsi and Nazzareni, 2015; Chukanov et al , 2021), tschörtnerite, mendeleevite-(Ce) and rowleyite; the formation of complex layers with different combinations of modules (chains or rings); this type is characteristic for layered uranyl minerals (such as vandendriesscheite and gauthierite) and layered silicates (parsettensite); a high hydration state in salts with complex heteropolyhedral units (alfredstelznerite and voltaite-group minerals); the formation of ordered superstructures of relatively simple structure types; the examples are meerschautite (which is the only sulfide species in the list and was described by Biagioni et al (2016) as an expanded derivative of owyheeite) and manitobaite (that has a fivefold superstructure relative to alluaudite (Tait et al , 2011)); for further discussion on the relations between superstructures and complexity see Krivovichev et al (2019a, 2021), Kornyakov et al (2021) and Kornyakov and Krivovichev (2021). …”

“…the formation of ordered superstructures of relatively simple structure types; the examples are meerschautite (which is the only sulfide species in the list and was described by Biagioni et al (2016) as an expanded derivative of owyheeite) and manitobaite (that has a fivefold superstructure relative to alluaudite (Tait et al , 2011)); for further discussion on the relations between superstructures and complexity see Krivovichev et al (2019a, 2021), Kornyakov et al (2021) and Kornyakov and Krivovichev (2021).…”