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This work reports a new series of A\(Ln\)(HCO\(_2\))(C\(_2\)O\(_4\))\(_{1.5}\) (A = [(CH\(_3\))\(_2\)NH\(_2\)]\(^+\) and Ln\(^{3+}\) = Tb\(^{3+}\)-Er\(^{3+}\)) compounds made solvothermally. These \({Cmce}\) phases combine monovalent and divalent ligands, which enables a scarce combination of A\(^+\) and B\(^{3+}\) cations in a hybrid perovskite-like compound. The ratio of ligands leads to ordered anion vacancies, which alternate with oxalate linkers along the \(c\)-axis. The A-site cations are disordered and cannot be identified crystallographically, likely a result of the larger pores of these frameworks compared to the recently reported AEr(HCO\(_2\))\(_2\)(C\(_2\)O\(_4\)) phases. Neutron and infrared spectroscopy, supported by elemental composition, enables these cations to be identified as [(CH\(_3\))\(_2\)NH\(_2\)]\(^+\) molecules. Magnetic property measurements suggest these materials have weak antiferromagnetic interactions but remain paramagnetic to 1.8 K.

We present a study on the nuclear and magnetic structures of two iron-based garnets with magnetic cations isolated on tetrahedral sites. Ca\(_2\)YZr\(_2\)Fe\(_3\)O\(_{12}\) and Ca\(_2\)LaZr\(_2\)Fe\(_3\)O\(_{12}\) offer an interesting comparison for examining the effect of increasing cation size within the diamagnetic backbone of the garnet crystal structure, and how such changes affect the magnetic order. Despite both systems exhibiting well-pronounced magnetic transitions at low temperatures, we also find evidence for diffuse magnetic scattering due to a competition between the nearest-neighbor, next nearest-neighbor, and so on, within the tetrahedral sites. This competition results in a complex noncollinear magnetic structure on the tetrahedral sublattice creating a mixture of ferro- and antiferromagnetic interactions above the long-range ordering temperature near 20 K and suggests that the cubic site of the garnet plays a significant role in mediating the superexchange interactions between tetrahedral cations.

Herein we report the synthesis and structures of [(CH\(_3\))\(_2\)NH\(_2\)]Er(HCO\(_2\))\(_2\)(C\(_2\)O\(_4\)) and [(NH\(_2\))\(_3\)C]Er(HCO\(_2\))\(_2\)(C\(_2\)O\(_4\)), in which the inclusion of divalent oxalate ligands allows for the exclusive incorporation of A\(^+\) and B\(^{3+}\) cations in an ABX\(_3\) hybrid perovskite structure for the first time. We rationalise the observed thermal expansion of these materials, including negative thermal expansion, and find evidence for weak antiferromagnetic coupling in [(CH\(_3\))\(_2\)NH\(_2\)]Er(HCO\(_2\))\(_2\)(C\(_2\)O\(_4\)).

This work reports the ambient temperature synthesis and structural characterisation of six new first row transition metal acetylenedicarboxylate coordination polymers. The Co and two Ni compounds adopt structures in which the octahedral metals are connected into 1D chains via the acetylenedicarboxylate ligand. In contrast the Mn and two Zn compounds adopt 3D metal-organic frameworks; while the Mn compound is non-porous the two Zn structures contain dimeric or trimeric clusters connected into frameworks that are potentially porous. These two anionic metal-organic frameworks are, however, charged balanced by cations siting in their pores which greatly reduces the ability to access their porosity.

This study reports the facile synthesis, crystal structures and magnetic properties of five new Mn, Co and Cu complexes with chloride and simple amine ligands. The four hydrazinium complexes are discrete in nature while the O-methylhydroxylamine phase contains edge-sharing chains bridged by chloride ligands. Investigation of the magnetic properties of these materials reveals that two of these materials, Co(NH3NH2)2(H2O)2Cl4 and Cu(NH2OCH3)2Cl2, exhibit interesting antiferromagnetic properties arising from their low dimensional structures. Co(NH3NH2)2(H2O)2Cl4 appears to exhibit significant 2D magnetic frustration while the magnetic susceptibilities of Cu(NH2OCH3)2Cl2 are well fitted by a one-dimensional chain model. The relationships between the strength of the magnetic coupling observed in these materials and their likely exchange pathways are also discussed.

Variable-temperature and variable-pressure single-crystal diffraction studies are carried out on a mixed-valence perovskite dimethylammonium (DMA) iron formate compound, with the formula [(DMA3)(H2O)][FeII3FeIII(HCOO)12], in order to investigate potential electric ordering of the DMA cation from its fourfold type of dynamic disorder at ambient conditions. Mossbauer spectroscopy is additionally carried out at ambient conditions to confirm the presence and ratio of Fe2+ and Fe3+ cations. Below 200 K, a dynamic to static disorder of the DMA cation is observed, while the crystal symmetry and iron formate framework structure remain the same. Upon application of pressure, however, a phase transition occurs that lowers the symmetry above 3.3 GPa. This work highlights the further chemical modifications that are possible within the dimethylammonium metal formates family, i.e. doping upon the A-site with neutral molecules, leading to additional opportunities to tune their physical properties.

This study examines the crystal structures of the AB(HCO2)3 (A = Li or Na and B = Mn or Co) metal-organic frameworks, which we find to adopt a chiral cubic P213 structure. This shows that the Li containing formates are isostructural with their Na analogues, extending the phase stability of this chiral architecture. The Mn containing compounds have a magnetic sublattice similar to ?-Mn, long of interest due to its highly frustrated antiferromagnetic coupling. In contrast the Co formates appear to have partially disordered alkali and transition metal cations, which prevents the formation of a clean ?-Mn-like magnetic sublattice. We have also re-examined the magnetic properties of NaMn(HCO2)3 finding it to be a simple paramagnet down to 2 K with only weak antiferromagnetic coupling.

In this study we report the synthesis, crystal structures and magnetic properties of three frameworks wherein Cu cations are bridged by formate linkers into one-dimensional motifs. One of these compounds, Cu2(HCO2)3(C3N2H4)4(NO3), contains a ladder motif but remains paramagnetic to 2 K. This is likely because of the longer superexchange pathway along its chains due to the orientation of the Jahn–Teller axis of its Cu cations. In contrast Cu(HCO2)(NO3)(NH3)2 and Cu(HCO2)(ClO4)(NH3)2 feature Cu(HCO2) chains in which the Jahn–Teller axis is oriented perpendicular to the chain direction; these exhibit antiferromagnetic order below 12 and 7 K, respectively. Their magnetic susceptibilities are well fitted by a one-dimensional chain model but further examination of their magnetic properties reveals significant inter-chain magnetic coupling and a lack of spin dynamics. This suggests that these transitions correspond to the emergence of long-range magnetic order, highlighting the importance of detailed studies of frameworks containing low dimensional motifs to gain a deeper understanding of their magnetic behaviour.

Co adipate, Co(C6H8O4), has been found to order near 10 K into a magnetic structure featuring sheets of tetrahedral Co cations coupled antiferromagnetically in two dimensions through carboxylate groups. The emergence of this order is accompanied by magnetoelastic coupling, which drives anisotropic negative thermal expansion along the a-axis below 50 K, the first time such behaviour has been observed in a metal-organic framework. The monoclinic angle, β, has also been found to decrease on cooling, passing through a metrically orthorhombic phase without a phase transition; this unusual behaviour has been rationalised in terms of the thermal expansion along the principal axes.

New quenched-in fluorite-type materials with composition (BiO 1.5)0.94-x(LaO1.5)0.06(PbO) x, x = 0.02, 0.03, 0.04 and 0.05, were synthesised by solid state reaction. The new materials undergo a number of phase transformations during heating between room temperature and 750 °C, as indicated by differential thermal analysis. Variable temperature X-ray diffraction performed on the material (BiO1.5)0.92(LaO1.5) 0.06(PbO)0.02 revealed that the quenched-in fcc fluorite-type material first undergoes a transformation to a β-Bi 2O3-type tetragonal phase around 400 °C. In the range 450-700 °C, α-Bi2O3-type monoclinic, Bi 12PbO19-type bcc and β1/β 2-type rhombohedral phases, and what appeared to be a ε-type monoclinic phase, were observed, before a single-phase fluorite-type material was regained at 750 °C. © 2010 Elsevier Ltd © 2011 Elsevier Ltd. All rights reserved.

The phase composition of two series of mixed Nb5+-Sb5+ double perovskites formed between the pairs Ba2EuNbO6-Ba2PrSbO6 and Ba2NdSbO6-Ba2NdNbO6 have been studied using synchrotron X-ray powder diffraction methods. In both series extensive phase segregation is observed demonstrating limited solubility of Sb5+ in these Nb5+ perovskites, irrespective of the precise structures of the double perovskite. Evidence for a monoclinic I2/m phase in the series formed between tetragonal I4/m Ba2EuNbO6 and rhombohedral R over(3, -) Ba2EuNbO6 is presented. It is postulated that this phase segregation is a consequence of competing bonding requirements of the Nb5+ and Sb5+ cations associated with their electronic configurations. © 2007 Elsevier Inc. All rights reserved.

The crystal structures adopted by BaPrO3 at and above ambient temperature have been examined using a combination of synchrotron X-ray and neutron diffraction. BaPrO3 has been established to undergo a series of phase transitions from P b n m orthorhombic → I b m m orthorhombic → R over(3, ̄) c rhombohedral → P m over(3, ̄) m cubic. BaPrO3 is the second A2+B4+O3 perovskite found to adopt rhombohedral symmetry in preference to the I4/mcm tetragonal structure. Analysis of the octahedral tilting through the rhombohedral to cubic phase transition indicates that this transformation is continuous and tricritical in nature. The tricritical behaviour of this transition is likely to be a result of the competition between tetragonal and rhombohedral structures to be the preferred phase, with the rhombohedral symmetry adopted by BaPrO3 being stabilised by the unusually large B-site cation. © 2008 Elsevier Ltd. All rights reserved.

The structure of the cation-ordered double perovskite Ba2HoTaO6 was examined using synchrotron X-ray powder diffraction at fine temperature intervals over the range of 90-300 K. Ba2HoTaO6 has a cubic structure in space group F m over(3, ̄) m at room temperature. A proper ferroelastic phase transition to I4/m tetragonal symmetry occurs near approximately 260 K. Analysis of the spontaneous tetragonal strain versus temperature indicated that the phase transition is second order in nature. © 2007 Elsevier Ltd. All rights reserved.

An anhydrous manganese succinate, Mn(C4H4O 4), has been synthesised hydrothermally and studied by single-crystal X-ray diffraction. It adopts a succinate pillared structure in which layers of corner-sharing MnO6 octahedra alternate with sheets that contain chains of edge-sharing octahedra. This unique 3D framework structure contains highly distorted MnO6 octahedra, which are made possible by the lack of ligand field stabilisation energy for the high-spin Mn2+ ion. Attempts to dope the structure with other divalent transition-metal ions were accordingly unsuccessful. Magnetic sus-ceptibility and heat capacity measurements indicate that Mn(C4H4O4) undergoes antiferromagnetic ordering below 12 K, with a second antiferromagnetic transition at approximately 6 K. These two antiferromagnetic phases undergo further transitions in applied fields, underlining the subtle magnetic behaviour that is possible in inorganic-organic frameworks of this structural complexity. © 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

The structure of the defect pyrochlore CsTi0.5W1.5O6 has been investigated using electron, synchrotron X-ray and neutron diffraction methods. The material is cubic a = 10.2773 Å with displacive disorder of the Cs cations along the 〈1 1 1〉 direction. The local structure, revealed by the diffuse structure in the electron diffraction patterns shows there is correlated displacement of the heavy Cs atoms along the 〈1 1 0〉 directions. The thermal expansion of the material is also described. © 2008 Elsevier Ltd. All rights reserved.

The structures of eight members of the series of double perovskites of the type Ba2LnB′O6 (Ln=La3+–Sm3+ and Y3+ and B′=Nb5+ and Ta5+) were examined both above and below room temperature using synchrotron X-ray powder diffraction. The La3+ and Pr3+ containing compounds had an intermediate rhombohedral phase whereas the other tantalates and niobates studied have a tetragonal intermediate. This difference in symmetry appears to be a consequence of the larger size of the La3+ and Pr3+ cations compared to the other lanthanides. The temperature range over which the intermediate symmetry is stable is reduced in those compounds near the point where the preferred intermediate symmetry changes from tetragonal to rhombohedral. In such compounds the transition to the cubic phase involves higher order terms in the Landau expression. This suggests that in this region the stability of the two intermediate phases is similar.

The structures of 28 compounds in the two series Ba2LnSbO6 and Ba2LnNbO6 have been examined using synchrotron X-ray and in selected cases neutron powder diffraction at, below and above ambient temperature. The antimonate series is found to undergo a sequence of phase transitions from monoclinic to rhombohedral to cubic symmetry with both decreasing ionic radii of the lanthanides and increasing temperature. Compounds in the series Ba2LnNbO6, on the other hand, feature an intermediate tetragonal structure instead of the rhombohedral structure exhibited by the antimonates. This difference in symmetry is thought to be caused by π-bonding in the niobates that is absent in the antimonates. The bonding environments of the cations in these compounds have also been examined with overbonding of the lanthanide and niobium cations being caused by the unusually large B-site cations. © 2006 Elsevier Inc. All rights reserved.

This study probes the magnetic and ferroelectric ordering of the NH4M(HCO2)3 (M = Mn(2+), Fe(2+), Co(2+) and Ni(2+)) frameworks using neutron diffraction, improving the understanding of the origins of the properties of these fascinating multiferroics. This rare study of the magnetic structure of a family of metal-organic frameworks shows that all four compounds exhibit antiferromagnetic coupling between neighbouring cations bridged by formate ligands. The orientation of the spin, however, changes in a highly unusual way across the series with the spins aligned along the c-axis for the Fe(2+) and Ni(2+) frameworks but lying in the ab plane for the other members of the series. This work also sheds new light on the nature of the ferroelectric order-disorder transition in these materials; probing changes in the ammonium cation across the transition and also shows that the Ni(2+) framework does not undergo a transition to the polar P63 phase due to the smaller size of the Ni(2+) cation. Finally trends in their anisotropic negative thermal expansion, which potentially enhances their ferroic behaviour, are quantified.

This work reports the synthesis and structures of six new AB(HCO2)3 (A = Rb+ or Cs+ and B = Mn2+, Co2+ or Ni2+) frameworks containing the largest monoatomic cations on the A-site. RbMn(HCO2)3 is found to adopt a distorted perovskite framework with a 412[middle dot]63 topology and a mixture of syn-anti and anti-anti ligands, while the remaining compounds adopt a chiral hexagonal structure with a 49[middle dot]66 topology. The structures of these frameworks clarify the effect of ionic size on the formation of the five known architectures adopted by the AB(HCO2)3 frameworks, which have attracted attention as a new class of potential multiferroics, and in particular the chiral hexagonal structure within this. This also highlights the role of molecular A-site cations in stabilising the 49[middle dot]66 topology for frameworks where such cations are too large or small to support this structure on the basis of size alone, possibly due to hydrogen bonding. The magnetic properties of the RbB(HCO2)3 and CsMn(HCO2)3 frameworks are also reported with the Rb+ compounds featuring weak ferromagnetic behaviour and the latter being purely antiferromagnetic. In conjunction with a comparison of the other isostructural AB(HCO2)3 frameworks we find that compounds adopting the 49[middle dot]66 topology have much higher magnetic ordering temperatures than those with the RbMn(HCO2)3 structure, highlighting the importance of understanding the structure-property relationships of the ternary formates.

Several double perovskite compounds of the type Ba 2 NdB′O 6− δ (B′=Sn 4+ , Nb 5+ and Sb 5+ ) were synthesised. Room temperature neutron diffraction patterns were obtained in order to determine the precise structure of these compounds. It was found that monoclinic Ba 2 NdSnO 5.5 and Ba 2 NdNbO 6 and rhombohedral Ba 2 NdSbO 6 all featured only out-of-phase tilting. The structures of each of these three compounds revealed consistent overbonding of the Nd 3+ cations most likely as a result of the significant size difference between the Nd 3+ ions and B′ cations.

Grounded in fact: Inorganic-organic frameworks with 3D Li-O-Li connectivity can form solid solutions through mechanochemical synthesis. High-resolution synchrotron powder X-ray diffraction and cross-polarization solid-state NMR spectroscopy demonstrate complete ligand mixing in the resulting binary and ternary systems (see picture for trends in unit cell volume (V) of the ternary system {Li2(suc)x(mal)y(met)z} n). Copyright © 2013 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim.

The structures of seven new transition metal frameworks featuring Mn, Co, or Zn and either the meso or chiral D and L isomers of the 2,3-dimethylsuccinate ligand are reported. Frameworks that exhibit two-dimensional covalently bonded layers with weak interlayer interactions can be made with all three cations by incorporation of the chiral isomers of the 2,3-dimethylsuccinate ligand. The formation of such structures, suitable for the creation of nanosheets via exfoliation, is, however, not as ubiquitous as is the case with the 2,2-dimethylsuccinate frameworks since frameworks that incorporate the meso-2,3-dimethylsuccinate ligand form three-dimensional structures. This clear distinction between the formation of structures with covalent connectivity in two and three dimensions, depending on the choice of 2,3-dimethylsuccinate isomer, is due to the different conformations adopted by the backbone of the ligand. The chiral isomer prefers to adopt an arrangement with its methyl and carboxylate groups gauche to the neighboring functional groups of the same type, while the meso-ligand prefers to adopt trans geometry. A gauche-arrangement of the methyl groups places them on the same side of the ligand, making this geometry ideal for the formation of layered structures; a trans-relationship leads to the methyl groups being further apart, reducing their steric hindrance and making it easier to accommodate them within a three-dimensional structure. The ease of exfoliation of the layered frameworks is examined and compared to those of known transition metal 2,2-dimethylsuccinate frameworks by means of UV-vis spectroscopy. It is suggested that layered frameworks with more corrugated surfaces exfoliate more rapidly. The size, structure, and morphology of the exfoliated nanosheets are also characterized. The magnetic properties of the paramagnetic frameworks reveal that only the three dimensionally covalently bonded phases containing meso-2,3-DMS in trans-arrangements order magnetically. These frameworks are antiferromagnets at low temperatures, although the Co compound undergoes an unusual antiferromagnetic to ferromagnetic transition with increasing applied magnetic field.

The structures and magnetic properties of two transition metal frameworks that feature a mixture of two linear dicarboxylate ligands are reported. Compounds 1, Mn 2(C 4H 4O 4)(C 6H 8O 4)(H 2O) 4· 2H 2O, and 2, Co 6(C 4H 4O 4) 4(C 6H 8O 4)(OH) 2(H 2O) 4·5H 2O, contain a mixture of succinate and adipate ligands but adopt significantly different structures. Compound 1 features layers of MnO 6 dimers, intra-connected by carboxylate groups, with neighbouring dimers connected to each other via the adipate ligands in one direction and succinate ligands in the other. Extensive hydrogen bonding in the third dimension provides the main force holding layers together. Framework 2 has inorganic layers of CoO 6 octahedra arranged into rings of 14 members each, with adipate ligands providing inter-layer connectivity. The structures of these two compounds are compared to Mn and Co dicarboxylate frameworks containing only one type of organic ligand, including Co(C 6H 8O 4), compound 3, whose structure is reported in this work for the first time; they are found to be significantly different from those that form under similar conditions. Both compounds order magnetically near 2 K. Compound 1 is an antiferromagnet, in which the intra-dimer coupling dominates the magnetic behaviour, while framework 2 is most likely a canted antiferromagnet. Both compounds undergo magnetic phase transitions with increasing applied magnetic fields, at 14 kOe and 0.35 kOe in 1 and 2, respectively. The transition in the Mn compound is a simple spin flop but in the Co compound the suppression of the long range ordered state is also accompanied by the elimination of the ferromagnetic component of its magnetic interactions. © 2012 The Royal Society of Chemistry.

The structures of fourteen new lanthanide frameworks (La, Ce, Eu, Tb, Y and Lu) containing the 2,2- or 2,3-dimethylsuccinate ligands are reported. While the majority of the known 2,2-dimethylsuccinate frameworks feature two dimensionally bonded layers, capped by hydrophobic methyl groups, several of these new frameworks adopt quite different architectures. These include one dimensional inorganically connected chains (La and Ce) with only non-covalent interactions in the other two dimensions, and three dimensional covalently bonded frameworks (Eu and Lu) with spaces in their structure to accommodate the bulky methyl groups. The new 2,3-dimethylsuccinate frameworks (La and Y) adopt three-dimensional covalently bonded frameworks. The factors affecting the formation of structures with different dimensionalities are examined and compared to previously reported transition metal frameworks. In addition, the sequence of phases formed with changing lanthanide size, concentrations and temperatures are rationalised. The luminescent properties of several 1- and 2-D frameworks doped with Eu and Tb are reported, with the Y host exhibiting the most intense emission.

The effect of cation valency on the complex structures of divalent and trivalent transition metal gallates has been examined using a combination of neutron and synchrotron X-ray powder diffraction, single-crystal X-ray diffraction and XANES spectroscopy. In the divalent frameworks, M(C(7)H(4)O(5))·2H(2)O (M = Mn, Co and Ni), it was found that charge balance was achieved via the presence of protons on the meta-hydroxyl groups. It was also established that these compounds undergo a discontinuous phase transition at lower temperatures, which is driven by the position of the extra-framework water molecules in these materials. By contrast, in the trivalent Fe gallate, Fe(C(7)H(3)O(5))·2H(2)O, it was found that the stronger bonding between the meta-hydroxy oxygen and the cations leads to a weakening of the bond between this oxygen and its proton. This is turn is thought to lead to stronger hydrogen bonding with the extra-framework water. The lattice water is disordered in the Fe(III) case, which prevents the phase transition found in the M(II) gallates. Refinement against the neutron diffraction patterns also revealed that the relatively mild microwave synthesis of gallate frameworks in D(2)O led to an extensive deuteration of the ortho-hydrogen sites on the aromatic ring, which may suggest a more versatile method of deuterating aromatic organics. The antiferromagnetic structure of Co gallate has also been determined.

Two new series of basic Mn dicarboxylate frameworks, Mn2(CO 2(CH2)nCO2)(OH)2 (where n = 0, 2, 4 and 5) and Mn4(CO2(CH2) nCO2)3(OH)2 (where n = 3 and 5), have been synthesised and the evolution of their structures with changing length of the dicarboxylate ligand was examined. Compounds in the Mn 2(CO2(CH2)nCO2)(OH) 2 series contain 2 dimensionally inorganically connected layers, which are bridged in the third dimension by the dicarboxylate ligand. While the compounds in this series with n = 0 or 2 contain only octahedrally coordinated Mn, the frameworks with longer ligands, n = 4 or 5, have a 1:1 ratio of octahedrally and trigonal bipyramidally coordinated cations. Structures in the Mn4(CO2(CH2)nCO2) 3(OH)2 series contain inorganically connected double chains of MnOx polyhedra, which comprise an equal number of octahedra and trigonal bipyramids. Trigonal bipyramidal coordination environments are very rarely found in dicarboxylate frameworks and the roles of the longer dicarboxylate ligands and the d5 electronic configuration of Mn 2+ in their formation are discussed. The magnetic properties of Mn2(CO2(CH2)2CO2)(OH) 2 have also been examined. It undergoes two magnetic transitions. The higher temperature transition to a two dimensionally ordered antiferromagnetic phase occurs around 44 K, in low applied fields, and is followed by a transition to a three dimensionally ordered canted antiferromagnetic state near 36 K. The Néel temperature of this phase is unusually high for a transition metal dicarboxylate and the factors thought to support this are examined. © The Royal Society of Chemistry 2011.

The antiferromagnetic structures of the Mn succinate framework, $\text{Mn}({\text{C}}_{4}{\text{H}}_{4}{\text{O}}_{4})$, have been determined using neutron diffraction. The structure comprises alternating layers containing chains of edge-sharing $\text{Mn}(\text{II}){\text{O}}_{6}$ octahedra and sheets of corner-sharing $\text{Mn}(\text{II}){\text{O}}_{6}$ octahedra, respectively, with a layer separation of $\ensuremath{\sim}7.5\text{ }\text{\AA{}}$. At 10 K the edge-sharing ${\text{MnO}}_{6}$ octahedral chains order antiferromagnetically into a collinear sinusoidal spin structure with a propagation vector ${k}_{2}=(0,\ensuremath{-}0.5225,0)$, in which individual edge-sharing ${\text{MnO}}_{6}$ chains are ferromagnetically ordered. The sheets of corner-sharing ${\text{MnO}}_{6}$ octahedra order magnetically at 6 K, adopting the antiferromagnetic structure expected for a square arrangement of cations with a propagation vector ${k}_{8}=(\ensuremath{-}1,0,1)$. The ordering of these sheets at a lower temperature than the chains is consistent with their longer nearest-neighbor superexchange pathway. The magnetic structure of the edge-sharing layers is unaffected by the 6 K phase transition, indicating that the orderings of the two different layers are essentially independent of each other.

The synthesis and structures of the perovskites Ln1/3TaO3 are described. As the size of the Ln cation is reduced, the compounds display a sequence of structure: P4/mmm/La → Cmmm/Ce−Gd → Pmma/Tb, Dy → Pmc21/Ho, Er. Although apparently tetragonal in P4/mmm, electron diffraction patterns of Tm1/3TaO3 reveal this has a complex incommensurate structure. Likewise Gd1/3TaO3 appears metrically tetragonal, but electron diffraction and synchrotron X-ray powder diffraction demonstrate this is actually orthorhombic. The suppression of the spontaneous orthorhombic strain in Gd1/3TaO3 is thought to be due to the proximity to the first-order Cmmm−Pmma transition. Variable temperature studies show both Tb1/3TaO3 and Dy1/3TaO3 undergo a first-order Cmmm−Pmma transition upon heating.

Compounds in the double perovskites series Ba{sub 2}LnSn{sub x}Sb{sub 1-x}O{sub 6-{delta}} (Ln=Pr and Tb) have been synthesised and structurally characterised using synchrotron X-ray and neutron powder diffraction. It was found that the two end-members of the Ba{sub 2}PrSn{sub x}Sb{sub 1-x}O{sub 6-{delta}} series both adopt rhombohedral symmetry but the antimonate is a fully ordered double perovskite while the stannate has no B-site cation ordering. X-ray absorption near-edge structure (XANES) and near-infrared spectroscopy indicate that the Pr cations gradually change oxidation state from Pr{sup 3+} to Pr{sup 4+} with increasing x and that this is likely to be the cause of the loss of B-site ordering. Similarly, both Ba{sub 2}TbSbO{sub 6} and Ba{sub 2}TbSnO{sub 6-{delta}} are cubic with B-site ordering present in the former but absent in the latter due to the oxidation state change of the Tb from Tb{sup 3+} to Tb{sup 4+}. Multiple linear regression analysis of the Pr and Tb L{sub III}-edge XANES indicates that the rate of Ln{sup 3+} transforming to Ln{sup 4+} is such that there are no oxygen vacancies in Ba{sub 2}PrSn{sub x}Sb{sub 1-x}O{sub 6-{delta}} but in Ba{sub 2}TbSn{sub x}Sb{sub 1-x}O{sub 6-{delta}} there is a small amount of oxygen vacancies, with a maximum of {delta}{approx}0.05 present.more » - Graphical abstract: Powder diffraction and spectroscopic techniques have been used to investigate the series Ba{sub 2}LnSn{sub x}Sb{sub 1-x}O{sub 6-{delta}} (Ln=Pr or Tb). It was found that increased Sn{sup 4+} doping leads to oxidation of the Ln{sup 3+} cations to Ln{sup 4+}. The X-ray absorption near-edge structure of the Ln L{sub III}-edge indicates that this oxidation state change occurs gradually such that there are few oxygen vacancies present.« less

The structures of compounds in the perovskite series Ba{sub 2}LnSn{sub x}Nb{sub 1-x}O{sub 6-{delta}} (Ln=Pr and Tb and x=0, 0.1, 0.2, ..., 1.0) have been examined using synchrotron X-ray and neutron diffraction. It was found that niobate members of both series feature full B-site cation ordering but that this order is lost with increasing x. X-ray absorption near-edge structure (XANES) and near-infrared spectroscopies indicate that the oxidation state of the lanthanide cations gradually changes from Ln{sup 3+} to Ln{sup 4+} with increased Sn{sup 4+} doping. This is believed to be the cause of the loss of B-site ordering. Least squares analysis of the XANES spectra suggests that the rate of the transformation of Ln{sup 3+} cations to the tetravalent state is such that the Pr series contains no oxygen vacancies while the Tb series may contain a very small amount of vacancies, with {delta}{approx}0.02. - Graphical abstract: The series Ba{sub 2}LnSn{sub x}Nb{sub 1-x}O{sub 6-{delta}} (Ln=Pr and Tb) has been investigated using powder diffraction and spectroscopic techniques. Similarly to the analogous Sb{sup 5+} containing compounds it was found that Sn{sup 4+} doping leads to the gradual oxidation of the Ln{sup 3+} cations to the Ln{sup 4+} state. Quantitative analysis of the Lnmore » L{sub III}-edge absorption spectra indicates that few or no oxygen vacancies are present.« less

The P 6 3 ( a =2 a p , b =2 b p , c = c p ) crystal structure reported for BaAl 2 O 4 at room temperature has been carefully re-investigated by a combined transmission electron microscopy and neutron powder diffraction study. It is shown that the poor fit of this P 6 3 ( a =2 a p , b =2 b p , c = c p ) structure model for BaAl 2 O 4 to neutron powder diffraction data is primarily due to the failure to take into account coherent scattering between different domains related by enantiomorphic twinning of the P 6 3 22 parent sub-structure. Fast Fourier transformation of [0 0 1] lattice images from small localized real space regions (∼10 nm in diameter) are used to show that the P 6 3 ( a =2 a p , b =2 b p , c = c p ) crystal structure reported for BaAl 2 O 4 is not correct on the local scale. The correct local symmetry of the very small nano-domains is most likely orthorhombic or monoclinic.

The perovskite Sr2MnSbO6 has been synthesized using conventional ceramic techniques and structurally characterized using high-resolution powder X-ray and neutron diffraction. The structure is tetragonal in space group I 4 / m. The octahedra were found to feature Jahn-Teller (JT) distortion due to the presence of Mn3+, and this is identified as strongly contributing to the octahedral tilting. Evidence for B-site cation ordering is presented however there is extensive anti-site disorder. The disordering of the Mn3+ and Sb5+ cations is believed to be a result of the similar size of these two cations and the polarizability of the Sb5+ cation. The structure was found to undergo a transition to cubic symmetry at 521 °C with removal of the octahedral tilting leading to the quenching of the JT distortion. This phase transition was found to be continuous and tricritical in nature. © 2006 Elsevier Inc. All rights reserved.

High-resolution neutron and synchrotron X-ray powder diffraction studies are reported for the six oxides AB2O4 (A=Ca2+, Sr2+ and Ba2+ and B=Al3+ and Ga3+). These oxides all adopt a stuffed tridymite type structure, the precise nature of which depends on both the A- and B-type cations. Bond valence calculations reveal a range of values for the various A-type cations, in all cases at least one site is significantly underbonded. Conversely the tetrahedral B-type sites invariably exhibit unexceptional bond valencies. Attempts to dope the oxides with various lanthanides to the 1% level invariably resulted in some segregation into alternate phases located at the grain boundaries. The identity of the impurity phases is presented and the importance of bond valencies in understanding this segregation is highlighted.