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A heterometallic single-source molecular precursor Li2Mn2(tbaoac)6 (1 , tbaoac = tert -butyl acetoacetato) has been specifically designed to achieve the lowest decomposition temperature and a clean conversion to mixed-metal oxides. The crystal structure of this tetranuclear molecule was determined by single crystal X-ray diffraction, and the retention of heterometallic structure in solution and in the gas phase was confirmed by nuclear magnetic resonance spectroscopy and mass spectrometry, respectively. Thermal decomposition of this precursor at the temperatures as low as 310 oC resulted in a new metastable oxide phase formulated as lithium-rich, oxygen-deficient spinel Li1.5Mn1.5O3.5. This formulation was supported by a comprehensive suite of techniques including thermogravimetric/differential thermal analysis, elemental analysis, inductively coupled mass spectrometry, iodometric titration, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy studies, and Rietveld refinement from powder X-ray diffraction data. Upon heating to about 400 oC, this new low-temperature phase disproportionates stoichiometrically, gradually converting to layered Li2MnO3 and spinel Li1+x Mn2-x O4 (x < 0.5). Further heating to 750 oC results in formation of thermodynamically stable Li2MnO3 and LiMn2O4 phases. 

KBiNb 2 O 7 was prepared from RbBiNb 2 O 7 by a sequence of cation exchange reactions which first convert RbBiNb 2 O 7 to LiBiNb 2 O 7 , before KBiNb 2 O 7 is formed by a further K-for-Li cation exchange. A combination of neutron, synchrotron X-ray and electron diffraction data reveal that KBiNb 2 O 7 adopts a polar, layered, perovskite structure (space group A 11 m ) in which the BiNb 2 O 7 layers are stacked in a (0, ½, z ) arrangement, with the K + cations located in half of the available 10-coordinate interlayer cation sites. The inversion symmetry of the phase is broken by a large displacement of the Bi 3+ cations parallel to the y -axis. HAADF-STEM images reveal that KBiNb 2 O 7 exhibits frequent stacking faults which convert the (0, ½, z ) layer stacking to (½, 0, z ) stacking and vice versa , essentially switching the x - and y -axes of the material. By fitting the complex diffraction peak shape of the SXRD data collected from KBiNb 2 O 7 it is estimated that each layer has approximately a 9% chance of being defective – a high level which is attributed to the lack of cooperative NbO 6 tilting in the material, which limits the lattice strain associated with each fault. 

Solid state compounds which exhibit non-centrosymmetric crystal structures are of great interest due to the physical properties they can exhibit. The ‘hybrid improper’ mechanism – in which two non-polar distortion modes couple to, and stabilize, a further polar distortion mode, yielding an acentric crystal structure – offers opportunities to prepare a range of novel non-centrosymmetric solids, but examples of compounds exhibiting acentric crystal structures stabilized by this mechanism are still relatively rare. Here we describe a series of bismuth-containing layered perovskite oxide phases, RbBiNb 2 O 7 , LiBiNb 2 O 7 and NaBiNb 2 O 7 , which have structural frameworks compatible with hybrid-improper ferroelectricity, but also contain Bi 3+ cations which are often observed to stabilize acentric crystal structures due to their 6s 2 electronic configurations. Neutron powder diffraction analysis reveals that RbBiNb 2 O 7 and LiBiNb 2 O 7 adopt polar crystal structures (space groups I 2 cm and B 2 cm respectively), compatible with stabilization by a trilinear coupling of non-polar and polar modes. The Bi 3+ cations present are observed to enhance the magnitude of the polar distortions of these phases, but are not the primary driver for the acentric structure, as evidenced by the observation that replacing the Bi 3+ cations with Nd 3+ cations does not change the structural symmetry of the compounds. In contrast the non-centrosymmetric, but non-polar structure of NaBiNb 2 O 7 (space group P 2 1 2 1 2 1 ) differs significantly from the centrosymmetric structure of NaNdNb 2 O 7 , which is attributed to a second-order Jahn-Teller distortion associated with the presence of the Bi 3+ cations.