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

The hollandite-related structure La_1.16Mo₈O₁₆, M_r = 1184.66, tetragonal, P^I4 ₁, a = 9.983 (1), c = 2.8890 (5) Å, V = 287.9 Å³, Z = 1, D_x = 6.83 g cm^-3, (Mo K[alpha]) = 0.71069 Å, [mu] = 126 cm^-1, F(0000) = 530, room temperature, R = 0.041 for 1145 unique reflections with I3[sigma](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 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 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] ([lambda] = 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.