The modulated structure of LaMo₈O₁₄

Abstract

Lanthanum octamolybdate [LaM 08014 , a = 11.129 (1), b = 10.000 (1), c = 9.218 (1) A~, basic space group C2ca, superspace group_ pc~, Mr = 1130.4, Dx = 7.32gcm -3, V= 1025.9A 3, Z=4, T= 294K, Mo Ka radiation, A =0.710730 A,, # = 134.5 cm-i, F(000) = 2020] exhibits a onedimensional modulated structure that is found to be commensurate within experimental error with a modulation wavevector q* having components 0,~,0. Both a density modulation wave, governing the occupancy probability of some atomic sites [La(1), La(2), Mo(4) and Mo (5)], and a displacive modulation wave of small but significant amplitude, acting on all atoms, are involved in the crystal. The modulated structure was refined from 3347 unique hklm reflections [I_> 30-(/)] with the REMOS program. The final R factors (based on F) of the (1845) main reflections and the (1502) first-order satellite reflections are 0.038 and 0.090, respectively. Because the q2* component of q* has the particular value and the modulation is harmonic, remarkable relations result between the atomic displacements. Within a classical approach, with use of the spacegroup description, these relations would be masked. The average structure of this crystal appears to be closely related to the structure of LaMo7.7Ot4 crystal, which is not modulated. The main result of this study is the occurrence of four distinct configurations of the Mo8 metallic clusters inside the same crystal. As suggested by the results of electron microscopy investigations, a more or less large deviation from an ideal structural order would be involved in most of the modulated crystals.

Thermoelectric properties of molybdenum oxides LnMo8O14 (Ln = La, Ce, Pr, Nd and Sm)

“…Molybdenum oxides with the formula LnMo 8 O 14 (Ln = La, Ce, Pr, Nd and Sm) [16,17,[29][30][31][32] were reported to contain Mo 8 clusters. LaMo 8 O 14 and CeMo 8 O 14 crystallize in the orthorhombic crystal system with the space group Pbcn as determined by the single crystal X-ray diffraction, although there is another different modification obtained from fused-salt electrolysis for LaMo 8 O 14 [16].…”

“…Molybdenum oxides, such as LaMo 8 O 14 [16,17], LaMo 5 O 8 [18,19], Y 4 Mo 4 O 11 [20], Pr 4 Mo 9 O 18 [21] and La 5 Mo 32 O 54 [22] containing different Mo clusters with strong metal-metal bonds between the Mo atoms, are usually semiconductors with narrow band gaps, which could be potential materials for thermoelectric applications. In this work, we will introduce a series of new ntype oxides (LnMo 8 O 14 , Ln = La, Ce, Pr, Nd and Sm) containing Mo 8 clusters showing promising thermoelectric properties.…”

LaMo8O14

“…The two related compounds LalVIo7.7Ol4 [space group Aba2,a = 9.196(1), b = 9.985(1), c = 11.171 (1)A] and LaMosOi4 [superspace group pC'2ca --Ill' a = 11.129(1), b = 10.000(1), c = 9.218(1)A, q* = b*/3] were obtained by fused-salt electrolysis in the early nineties (Leligny, Ledesert, Labbe, Raveau & McCarroll, 1990;Leligny et al, 1993). Single-crystal structure determinations carried out on these compounds revealed that their dominant structural features are layers containing biface-capped octahedral Mo8 clusters which exhibit different configurations and arrangements.…”

PrMo8O14, a Fourth Crystalline Form of the Series RMo8O14 (R = La, Ce, Pr, Nd, Sm)

“…Their structures comprise only cis-edgesharing bi-face-capped MOB clusters. The second form, which was described by Leligny et al (1993), corresponds to that shown by crystals of the stoichiometric compound LaMo8O14, synthesized by fused-salt electrolysis. The crystal structure of the latter compound is more complex due to a one-dimensional commensurate modulation of wavevector q* = b*/3 [a = 11.129(1), b = 10.000(1), c = 9.218 (1) ~,].…”