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

Abstract

The hollandite-related structure La 1.16 Mo 8 O 16 , M r = 1184.66, tetragonal, P^{I4}_{\kern3pt 1}, a = 9.983 (1), c = 2.8890 (5) Å, V = 287.9 Å 3 , Z = 1, D x = 6.83 g cm −3 , \lambda(Mo K α) = 0.71069 Å, μ = 126 cm −1 , F (0000) = 530, room temperature, R = 0.041 for 1145 unique reflections with I \geq3σ( 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 P^{I4}_{\kern3pt 1}, 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 Mo 3 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.