La_1.16Mo₈O₁₆: a hollandite-related compound with an incommensurate modulated structure

Abstract

The hollandite‐related structure La1.16Mo8O16, Mr = 1184.66, tetragonal, , a = 9.983 (1), c = 2.8890 (5) Å, V = 287.9 Å3, Z = 1, Dx = 6.83 g cm−3, (Mo Kα) = 0.71069 Å, μ = 126 cm−1, F(0000) = 530, room temperature, R = 0.041 for 1145 unique reflections with I3σ(I). At room temperature the compound exhibits a one‐dimensional incommensurate modulated structure with a modulation wavevector q* = 0.608 (1)c*. Both a displacive modulation wave, acting on La, Mo and O atoms, and a modulation wave governing the occupancy probability of La sites inside the tunnels are involved in the crystal. Within the superspace group , the final R values of main reflections (477) and first‐ and second‐order satellite reflections (523 and 145) are 0.030, 0.050 and 0.133 respectively. The more spectacular modulation features are the occurrence of La—La pairs in the tunnels and the formation of Mo3 triangular clusters in the double chains of edge‐sharing octahedra. Contrary to previous descriptions based upon rigid tunnels in structures of the hollandite type, the tunnels in the crystal studied are distorted in a periodic way along [001] (λ = 4.75 Å), giving rise to alternate contractions and expansions. The distortion of the double octahedral chains is considerable and probably created via the La—O(1) bonds by insertion of La atoms inside two adjacent tunnels.

Incommensurate modulations in a hollandite phase Ba x (Al, Fe)2x Ti8–2x O16 intended for the storage of radioactive wastes: a (3+1) dimension structure determination

“…The superspace group was easily identified as I4/m(00g)00, since there were no systematic extinctions, other than those related to the I cell centering. The possibility of a non centrosymmetric space group (I4(00g)00), as found in the literature [20] in another hol- Fig. 2.…”

A New Manganite with an Original Composite Tunnel Structure: Ba6Mn24O48

“…In hollandite-type compounds commensurate and incommensurate superstructure reflections and diffuse scattering are commonly observed (12)(13)(14)(15)(16)(17)22). Several models of atoms distributions within the tunnels have been proposed.…”

“…It has been indeed observed that besides the ideal tetragonal hollandite structure, there exist orthorhombic and monoclinic distortions (2,(8)(9)(10)(11). But most importantly, modulation and disorder phenomena concerning the particular arrangement of the cations in the large four-sided tunnels are commonly observed (12)(13)(14)(15)(16)(17). Moreover, a synthetic hollandite-type has been observed by dehydration of the psilomelane (Ba, H O)Mn O (18) at 550°C.…”

An electron and X-ray diffraction study of the compositely modulated barium nickel hollandite Ba x (Ni x Ti8— x )O16, 1.16 < x < 1.32, solid solution

“…For a conventional incommensurately modulated structure, this would certainly represent a rather simpler indexation scheme. Leligny et al, 1992). It is confirmed by EDPs such as the [ 1 102] f zone axis EDP shown in Fig.…”