The Crystal Structure of α-La₂Mo₂O₉ and the Structural Origin of the Oxide Ion Migration Pathway

Abstract

We describe for the first time the full 3D atomic structure of room-temperature α-La2Mo2O9. The material, despite its simple chemical formula, has a remarkable 312 crystallographically unique atoms and is thus one of the most complex oxide structures reported to date. Despite this complexity, the structural results offer significant insight into the O2- migration pathway in the anion conducting high-temperature form, β-La2Mo2O9. The material contains a mixture of 4, 5, and 6 coordinated Mo sites, suggesting that variable Mo coordination number is a key factor in providing a low-energy O2- conduction pathway. We provide quantitative analysis showing that the positions in the ordered array of 216 oxygen sites of α-La2Mo2O9 are directly related to the average sites occupied in β-La2Mo2O9, providing compelling evidence that the high-temperature conducting form of the material has a time-averaged version of the low-temperature structure.

The mechanism of oxide ion conductivity in bismuth rhenium oxide, Bi28Re2O49

“…Extensive experimental and computational work has led to the proposal of several general classes of mechanism for oxide ion migration: the vacancy mechanism; the interstitial and intersticialcy mechanisms; the cooperative mechanism and the variable coordination number mechanism. [9][10][11] The mechanism occurring in fluorite-related oxide ion conductors has been thought to proceed principally via vacancies, whilst some apatites and melilite-related compounds exhibit an interstitial mechanism. [9,12] La 1-x Ba 1+x GaO 4-x/2 related oxide ion conductors exhibit a cooperative mechanism which requires exchange of oxygen between GaO 4 groups, forming Ga 2 O 7 units as intermediates in the conduction process.…”

“…[10] The crystal structure of α-La 2 Mo 2 O 9 was found to contain Mo in three different types of coordination environments (4-, 5-and 6-coordinate) and to undergo an order-disorder phase transition to the highly conducting β-La 2 Mo 2 O 9 polymorph. [11] Based on this structural work, it was proposed that the ability of cations to support variable coordination environments was important for the oxide ion migration pathways. [11] The structure of Bi 28 3 was selected for data collection.…”

Nature of the Monoclinic to Cubic Phase Transition in the Fast Oxygen Ion Conductor La2Mo2O9 (LAMOX)

“…The structure of the low temperature phase was not solved at the time of the LAMOX discovery due to its great complexity and was only solved by I.R. Evans et al in 2005 5 . This resulted in one of the most complex oxide structures reported up to now with 312 crystallographically independent atoms.…”

Interstitial Oxide Ion Order and Conductivity in La1.64Ca0.36Ga3O7.32 Melilite

“…The charge carriers are most commonly oxygen vacancies in fluorites [2,3] and perovskites. [3,4] There are fewer examples of interstitial-oxygenbased conductors such as the apatites [5,6] and La 2 Mo 2 O 9 -based materials, [7][8][9] so information on how these excess anion defects are accommodated and the factors controlling their mobility is important.The A 2 B 3 O 7 melilite structure consists of anionic layers of five-membered rings of two totally condensed (four neighboring tetrahedra linked by B-O-B bonds) and three partially condensed (three neighboring tetrahedra) BO 4 tetrahedra, separated by sheets of A cations located above the five-ring centers (Figure 1 a, and Figure S1.1 in the Supporting Information). Previously, [10] we demonstrated that A-site substitution in melilite LaSrGa 3 O 7 (A 2 = LaSr, B = Ga) affords La 1.54 Sr 0.46 Ga 3 O 7.27 , in which interstitial oxygen atoms (O int ) are located in the five-rings of the two-dimensional tetrahedral network, gives pure oxide ion conductivity of 0.02-0.1 S cm À1 over the 600-900 8C temperature range.…”

“…The charge carriers are most commonly oxygen vacancies in fluorites [2,3] and perovskites. [3,4] There are fewer examples of interstitial-oxygenbased conductors such as the apatites [5,6] and La 2 Mo 2 O 9 -based materials, [7][8][9] so information on how these excess anion defects are accommodated and the factors controlling their mobility is important.…”