ABSTRACT:The coefficient of thermal expansion of ZrMgMo 3 O 12 has been measured and was found to be extremely close to zero over a wide temperature range including room temperature (α = (1.6 ± 0.2) × 10 −7 K −1 from 25 to 450°C by X-ray diffraction (XRD)). ZrMgMo 3 O 12 belongs to the family of AMgM 3 O 12 materials, for which coefficients of thermal expansion have previously been reported to range from low-positive to low-negative. However, the low thermal expansion property had not previously been explained because atomic position information was not available for any members of this family of materials. We determined the structure of ZrMgMo 3 O 12 by nuclear magnetic resonance (NMR) crystallography, using 91 Zr, 25 Mg, 95 Mo, and 17 O magic angle spinning (MAS) and 17 O multiple quantum MAS (MQMAS) NMR in conjunction with XRD and density functional theory calculations. The resulting structure was of sufficient detail that the observed zero thermal expansion could be explained using quantitative measures of the properties of the coordination polyhedra. We also found that ZrMgMo 3 O 12 shows significant ionic conductivity, a property that is also related to its structure.

Over the past two decades, the magnetic ground states of all rare earth titanate pyrochlores have been extensively studied, with the exception of Sm2Ti2O7. This is, in large part, due to the very high absorption cross-section of naturally-occurring samarium, which renders neutron scattering infeasible. To combat this, we have grown a large, isotopically-enriched single crystal of Sm2Ti2O7. Using inelastic neutron scattering, we determine that the crystal field ground state for Sm 3+ is a dipolar-octupolar doublet with Ising anisotropy. Neutron diffraction experiments reveal that Sm2Ti2O7 orders into the all-in, all-out magnetic structure with an ordered moment of 0.44(7) µB below TN = 0.35 K, consistent with expectations for antiferromagnetically-coupled Ising spins on the pyrochlore lattice. Zero-field muon spin relaxation measurements reveal an absence of spontaneous oscillations and persistent spin fluctuations down to 0.03 K. The combination of the dipolar-octupolar nature of the Sm 3+ moment, the all-in, all-out ordered state, and the low-temperature persistent spin dynamics make this material an intriguing candidate for moment fragmentation physics.Rare earth titanate pyrochlores of the form R 2 Ti 2 O 7 have long been a centerpiece in the study of geometricallyfrustrated magnetism [1]. In this family of materials, the magnetism is carried by the R 3+ rare earth ions, which decorate a network of corner-sharing tetrahedra.The study of this family has led to the discovery of a range of fascinating ground states such as the dipolar spin ice state, which was first observed in Ho 2 Ti 2 O 7 and Dy 2 Ti 2 O 7 [2-4]. Here local Ising anisotropy combines with dominant dipolar interactions, which are ferromagnetic at the nearest neighbour level on the pyrochlore lattice [5]. The spin ice state is characterized by individual tetrahedra obeying two-in, two-out "ice rules", wherein two spins point directly towards the tetrahedron's center and the other two spins point outwards (left inset of Fig. 1). This configuration can be achieved in six equivalent ways for a single tetrahedron, giving rise to a macroscopic degeneracy for the lattice as a whole. In other titanates, where the rare earth moments are smaller than in Ho 2 Ti 2 O 7 and Dy 2 Ti 2 O 7 , dipolar interactions become less important and exchange interactions tend to dominate. This is exactly the case when R = Sm 3+ (∼ 1 µ B ), where the magnetic moment is reduced by a factor of ten from R = Ho 3+ and Dy 3+ (∼ 10 µ B ), corresponding to dipolar interactions that are weaker by two orders of magnitude.In this letter we show that anitferromagnetically coupled Ising spins with negligible dipolar interactions give rise to an all-in, all-out (AIAO) magnetic ground state in Sm 2 Ti 2 O 7 . The AIAO structure is characterized by adjacent tetrahedra alternating between all spins pointing inwards and all spins pointing outwards (right inset of Fig. 1). Unlike the ferromagnetic spin ice state, the antiferromagnetic AIAO state does not give rise to a macroscopic degenerac...

The crystal structure of α-Fe2O3 and α-Cr2O3 is usually described with the corundum-type trigonal crystal structure based on the space group R3¯c. There are, however, some observations of the magnetic ordering of both α-Fe2O3 and α-Cr2O3 that are incompatible with the trigonal symmetry. We show experimental evidence based on X-ray powder diffraction and supported by transmission electron microscopy that the symmetry of the crystal structure of both α-Fe2O3 and α-Cr2O3 is monoclinic and it is described with the space group C2/c (derived from R3¯c by removing the threefold rotation axis). The magnetic orderings of α-Fe2O3 and α-Cr2O3 are compatible with the magnetic space groups C2/c and C2/c', respectively. These findings are in agreement with the idea from Curie [(1894), J. Phys. 3, 393-415] that the dissymmetry of the magnetic ordering should be associated with a dissymmetry of the crystal structure.

Here, we report a synthetic strategy to control the B-site ordering of the transition metal-doped perovskite-type oxides with the nominal formula of BaCa(0.335)M(0.165)Nb(0.5)O(3-δ) (M = Mn, Fe, Co). Variable temperature (in situ) and ex situ powder X-ray diffraction (PXRD), selected area electron diffraction (SAED), energy dispersive X-ray spectroscopy (EDX), Fourier transform infrared spectroscopy (FTIR), scanning/transmission electron microscopy (SEM/TEM), and thermogravimetic analysis (TGA) were used to understand the B-site ordering as a function of temperature. The present study shows that BaCa(0.335)M(0.165)Nb(0.5)O(3-δ) crystallizes in the B-site disordered primitive perovskite (space group s.g. Pm3̅m) at 900 °C in air, which can be converted into the B-site 1:2 ordered perovskite (s.g. P3̅m1) at 1200 °C and the B-site 1:1 ordered perovskite phase (s.g. Fm3̅m) at 1300 °C. However, the reverse reaction is not feasible when the temperature is reduced. FTIR revealed that no carbonate species were present in all three polymorphs. The chemical stability of the investigated perovskites in CO2 and H2 highly depends on the B-site cation ordering. For example, TGA confirmed that the B-site disordered primitive perovskite phase is more readily reduced in dry and wet 10% H2/90% N2 and is less stable in pure CO2 at elevated temperature, compared to the B-site 1:1 ordered perovskite-type phase of the same nominal composition.

We report novel details regarding the reactivity and mechanism of the solid-state topotactic reduction of Sr2MnO4 using a series of solid-state metal hydrides. Comprehensive details describing the active reducing species are reported and comments on the reductive mechanism are provided, where it is shown that more than one electron is being donated by H(-). Commonly used solid-state hydrides LiH, NaH, and CaH2, were characterized in terms of reducing power. In addition the unexplored solid-state hydrides MgH2, SrH2, and BaH2 are evaluated as potential solid-state reductants and characterized in terms of their reductive reactivities. These 6 group I and II metal hydrides show the following trend in terms of reactivity: MgH2 < SrH2 < LiH ≈ CaH2 ≈ BaH2 < NaH. The order of the reductants are discussed in terms of metal electronegativity and bond strengths. NaH and the novel use of SrH2 allowed for targeted synthesis of reduced Sr2MnO(4-x) (0 ≤ x ≤ 0.37) phases. The enhanced control during synthesis demonstrated by this soft chemistry approach has allowed for a more comprehensive and systematic evaluation of Sr2MnO(4-x) phases than previously reported phases prepared by high temperature methods. Sr2MnO3.63(1) has for the first time been shown to be monoclinic by powder X-ray diffraction and the oxidative monoclinic to tetragonal transition occurs at 450 °C.