Temperature Neutron Powder Diffraction Data Research Articles

Magneto-orbital ordering in the divalent A -site quadruple perovskite manganites AMn7O12 ( A=Sr , Cd, and Pb)

Through analysis of variable temperature neutron powder diffraction data, we present solutions for the magnetic structures of SrMn$_7$O$_{12}$, CdMn$_7$O$_{12}$, and PbMn$_7$O$_{12}$ in all long-range ordered phases. The three compounds were found to have magnetic structures analogous to that reported for CaMn$_7$O$_{12}$. They all feature a higher temperature lock-in phase with \emph{commensurate} magneto-orbital coupling, and a delocked, multi-\textbf{k} magnetic ground state where \emph{incommensurate} magneto-orbital coupling gives rise to a constant-moment magnetic helix with modulated spin helicity. CdMn$_7$O$_{12}$ represents a special case in which the orbital modulation is commensurate with the crystal lattice and involves stacking of fully and partially polarized orbital states. Our results provide a robust confirmation of the phenomenological model for magneto-orbital coupling previously presented for CaMn$_7$O$_{12}$. Furthermore, we show that the model is universal to the $A^{2+}$ quadruple perovskite manganites synthesised to date, and that it is tunable by selection of the $A$-site ionic radius.

Magnetic Interactions in the Double Perovskites R2NiMnO6 (R = Tb, Ho, Er, Tm) Investigated by Neutron Diffraction.

R2NiMnO6 (R = Tb, Ho, Er, Tm) perovskites have been prepared by soft-chemistry techniques followed by high oxygen-pressure treatments; they have been investigated by X-ray diffraction, neutron powder diffraction (NPD), and magnetic measurements. In all cases the crystal structure is defined in the monoclinic P21/n space group, with an almost complete order between Ni(2+) and Mn(4+) cations in the octahedral perovskite sublattice. The low temperature NPD data and the macroscopic magnetic measurements indicate that all the compounds are ferrimagnetic, with a net magnetic moment different from zero and a distinct alignment of Ni and Mn spins depending on the nature of the rare-earth cation. The magnetic structures are different from the one previously reported for La2NiMnO6, with a ferromagnetic structure involving Mn(4+) and Ni(2+) moments. This spin alignment can be rationalized taking into account the Goodenough-Kanamori rules. The magnetic ordering temperature (TCM) decreases abruptly as the size of the rare earth decreases, since TCM is mainly influenced by the superexchange interaction between Ni(2+) and Mn(4+) (Ni(2+)-O-Mn(4+) angle) and this angle decreases with the rare-earth size. The rare-earth magnetic moments participate in the magnetic structures immediately below TCM.

Structural and magnetic study of Yb3+ in the perovskites Sr2YbMO6 (M=Nb, Ta, Sb)

The compounds Sr2YbNbO6, Sr2YbTaO6 and Sr2YbSbO6 have been prepared using solid state methods by heating pelleted reagents in air at temperatures up to 1400°C. Rietveld refinement against room temperature neutron powder diffraction data show that all three compounds crystallise with a cation-ordered variant of the perovskite structure in the P21/n space group. Complete cation ordering occurs between M5+ and Yb3+ over two octahedrally-coordinated sites in the structure and all compounds are stoichiometric in oxygen. The Sb–O bond lengths are similar to related perovskite compounds but differ slightly from those indicated by bond valence sums. Magnetic susceptibility data resemble Curie–Weiss paramagnetic behaviour, but can be better understood as arising from the effect of the octahedral crystal field on the 2F5/2 ground state of Yb3+ leading to a temperature dependent magnetic moment on this ion below 100K.

Ruthenium(V) Oxides from Low‐Temperature Hydrothermal Synthesis

AbstractLow‐temperature (200 °C) hydrothermal synthesis of the ruthenium oxides Ca1.5Ru2O7, SrRu2O6, and Ba2Ru3O9(OH) is reported. Ca1.5Ru2O7 is a defective pyrochlore containing RuV/VI; SrRu2O6 is a layered RuV oxide with a PbSb2O6 structure, whilst Ba2Ru3O9(OH) has a previously unreported structure type with orthorhombic symmetry solved from synchrotron X‐ray and neutron powder diffraction. SrRu2O6 exhibits unusually high‐temperature magnetic order, with antiferromagnetism persisting to at least 500 K, and refinement using room temperature neutron powder diffraction data provides the magnetic structure. All three ruthenates are metastable and readily collapse to mixtures of other oxides upon heating in air at temperatures around 300–500 °C, suggesting they would be difficult, if not impossible, to isolate under conventional high‐temperature solid‐state synthesis conditions.

Ruthenium(V) Oxides from Low‐Temperature Hydrothermal Synthesis

Low-temperature (200 °C) hydrothermal synthesis of the ruthenium oxides Ca1.5 Ru2 O7 , SrRu2 O6 , and Ba2 Ru3 O9 (OH) is reported. Ca1.5 Ru2 O7 is a defective pyrochlore containing Ru(V/VI) ; SrRu2 O6 is a layered Ru(V) oxide with a PbSb2 O6 structure, whilst Ba2 Ru3 O9 (OH) has a previously unreported structure type with orthorhombic symmetry solved from synchrotron X-ray and neutron powder diffraction. SrRu2 O6 exhibits unusually high-temperature magnetic order, with antiferromagnetism persisting to at least 500 K, and refinement using room temperature neutron powder diffraction data provides the magnetic structure. All three ruthenates are metastable and readily collapse to mixtures of other oxides upon heating in air at temperatures around 300-500 °C, suggesting they would be difficult, if not impossible, to isolate under conventional high-temperature solid-state synthesis conditions.

Structural and Magnetic Study of Order–Disorder Behavior in the Double Perovskites Ba2Nd1–xMnxMoO6

The synthesis and structural and magnetic characterization of the site-ordered double perovskites, Ba2Nd1-xMnxMoO6, 0 < x ≤ 1, are reported in order to show the effect of doping Jahn-Teller active, S = 1/2, Mo(5+) into the structure of Ba2MnMoO6, which exhibits anomalous long-range antiferromagnetic order. Rietveld refinements against room temperature neutron powder diffraction data indicate that the tetragonal distortion present in the Ba2NdMoO6 end member persists to x ≤ 0.3. This is predominantly manifested as a tilting of the MO6 octahedra, and there is no evidence of any structural phase transitions on cooling to 1.5 K. For x > 0.3, no deviation from the ideal cubic Fm3̅m symmetry is observed. Furthermore, dc-susceptibility measurements confirm that Mn(2+) is being doped onto the Nd(3+) site, and the associated oxidation of Mo(5+) to Mo(6+). For all compositions, the Curie-Weiss paramagnetic behavior above 150 K indicates negative Weiss constants that range from -24(2) and -85(2) K. This net antiferromagnetic interaction is weakest when x ≈ 0.5, where the disorder in cation site occupancy and competition with ferromagnetic interactions is the greatest. Despite these strong antiferromagnetic interactions, there is no evidence in the dc-susceptibility of a bulk cancellation of spins for x > 0.05. Low-temperature neutron diffraction measurements indicate that there is no long-range magnetic order for 0.1 ≤ x < 0.9. Ba2Nd0.10Mn0.90MoO6 exhibits additional Bragg scattering at 2 K, indicative of long-range antiferromagnetic ordering of the Mn(2+) cations, with a propagation vector k = (1/2, 1/2, 1/2). The scattering intensities can be modeled using a noncollinear magnetic structure with the Mn(2+) moments orientated antiferromagnetically along the four different ⟨111⟩ directions.

RbxBayMn[3-(x + 2y)]/2[Fe(CN)6]·zH2O prussian blue analogues: Controlling magnetic ordering by alkaline earth metal cation substitution and magnetic field

We have controlled the nature of magnetic ordering in RbxBayMn[3-(x + 2y)]/2[Fe(CN)6]·zH2O prussian blue analogues by alkaline earth metal cation (Ba2+) substitution and application of an external magnetic field. Structural analysis, by employing Rietveld refinement of X-ray and neutron diffraction patterns, reveals a tetragonal crystal structure for all these compounds. The presence of Fe3+-C ≡ N-Mn2+ chains in the structure, and absence of both cyanide flipping (Mn2+-C ≡ N-Fe3+) and charge transfer from Mn2+ to Fe3+ (Fe2+-C ≡ N-Mn3+) have been confirmed by infrared and Mössbauer studies. The analysis of the low temperature neutron powder diffraction data reveals a long-range antiferromagnetic ordering in these compounds. The magnetic structure is layered, consisting of an antiparallel stacking of ferromagnetic sheets along the crystallographic c-direction. Within each sheet in the ab plane, the ordered moments of Mn2+ [5.04(1) μB for (x = 0.84, y = 0) and 4.99(7) μB for (x = 0.19, y = 0.3) sample at 1.5 K] and Fe3+ [0.98(3) μB for (x = 0.84, y = 0) and 0.99(3) μB for (x = 0.19, y = 0.3) sample at 1.5 K] are aligned ferromagnetically along the c-axis. Interestingly, dc magnetization study indicates that on application of an external magnetic field, an antiferromagnetic to ferrimagnetic-like phase transition occurs below ∼5 K. This phase transition becomes more and more prominent with increasing magnetic field as well as for higher Ba substitution. The role of a number of factors (such as ionic radius of Ba2+ ion and change in net ligand strength around Mn due to chemical insertion of Ba2+ ions) on the affecting nature of magnetic ordering has been discussed. The observed magnetism in these compounds is seen in light of the orbital model of superexchange interaction between magnetic ions (Mn2+ and Fe3+) mediated by cyanide ligands.

Temperature-Dependent Crystal Structures of Ti2AlN and Cr2GeC as Determined from High Temperature Neutron Diffraction

In this work, we report on the temperature-dependent crystal structures of the isostructural, layered hexagonal phases Ti2AlN and Cr2GeC determined by Rietveld analysis of high temperature neutron powder diffraction data of fully dense, polycrystalline, bulk samples in the 100° to 1100°C temperature range. For both phases, the A-group atoms, Al and Ge, vibrate with the highest amplitude and do so anisotropically within the basal plane. All bonds expand linearly with temperature, with the highest relative thermal expansion occurring in the Ti–Al and Cr–Ge bonds. The thermal expansion coefficients in the a- and c-direction are, respectively, 10.3(±0.2) × 10−6 and 9.3(±0.2) × 10−6 K−1 for Ti2AlN and 12.8(±0.3) × 10−6 and 14.6(±0.3) × 10−6 K−1 for Cr2GeC. The unit cell volume expansions observed by HTND are 10.0(±0.2) × 10−6 K−1 for Ti2AlN and 13.4(±0.3) × 10−6 K−1 for Cr2GeC.

Sr3Co2O4.33H0.84: An Extended Transition Metal Oxide-Hydride

Reaction of the n = 2 Ruddlesden-Popper oxide Sr(3)Co(2)O(5.80) with CaH(2) yields an extended oxide-hydride phase: Sr(3)Co(2)O(4.33)H(0.84). Neutron powder diffraction data reveal the material adopts a body-centered orthorhombic structure (Immm: a = 3.7551(5) Å, b = 3.7048(4) Å, c = 21.480(3) Å) in which the hydride ions are accommodated within disordered CoO(1.16)H(0.46) layers. Low temperature neutron powder diffraction data show no evidence for long-range magnetic order, suggesting the chemical disorder in the anion lattice of the material leads to magnetic frustration.

Magnetic structure and magnetic entropy change in the intermetallic compound DyCoAl

Magnetic and magnetocaloric properties of polycrystalline DyCoAl compound have been studied in fields up to 9 T. This compound orders ferromagnetically at ∼37 K (TC). A maximum magnetic entropy change of ∼−18 J/kg/K is obtained for a field change of 9 T near TC which is reasonably large. Low temperature neutron powder diffraction data indicate collinear ferromagnetic structure where Dy magnetic moments lie in the ab plane. At 10 K, the magnetic moment at Dy site is only ∼4.8μB.

Effect of the Pb2+ lone electron pair in the structure and properties of the double perovskites Pb2Sc(Ti0.5Te0.5)O6 and Pb2Sc(Sc0.33Te0.66)O6: relaxor state due to intrinsic partial disorder

We describe the preparation, the crystal structure refined from neutron powder diffraction (NPD) data, and study of the permittivity of two related double perovskites, Pb2Sc(Ti0.5Te0.5)O6 and Pb2Sc(Sc0.33Te0.66)O6. These compounds were synthesized by standard ceramic procedures; Rietveld refinements from room temperature NPD data show that the crystal structures are well defined in a cubic unit cell (space group Fm3m) with double parameter, a = 2a0 ≈ 8 Å. They contain a completely ordered array of ScO6 and (B,Te)O6 (B = Sc, Ti) octahedra sharing corners; the PbO12 polyhedra present an off-center displacement of the lead atoms along the [1 1 1] directions, due to the electrostatic repulsion between the Pb(2+) 6 s electron lone-pair and the Pb-O bonds of the cuboctahedron. Both compounds present a low temperature, highly dispersive maximum in permittivity, the position of which follows the Vogel-Fulcher relation with freezing temperatures of 156 and 99 K for Pb2Sc(Ti0.5Te0.5)O6 and Pb2Sc(Sc0.33Te0.66)O6, respectively, exhibiting a typical phenomenology of relaxors.

The role of the Pb2+ 6s lone pair in the structure of the double perovskite Pb2ScSbO6

The new double perovskite Pb2ScSbO6 was synthesized by standard ceramic procedures; the Rietveld refinement of room temperature neutron powder diffraction data shows that the crystal structure is well defined in the space group Fm3[combining macron]m. It contains a completely ordered array of alternating ScO6 and SbO6 octahedra sharing corners; the PbO12 polyhedra present an off-center displacement of the lead atoms along the [111] direction, due to the electrostatic repulsion between the Pb2+ 6s lone pair and the Pb-O bonds of the cuboctahedron. Dielectric permittivity measurements show a peak near 343 K, with a Curie-Weiss response above this temperature, which suggests an antiferroelectric behavior. Finally we present a DFT study of the electronic structure of Pb2ScSbO6, showing a great difference between the electronic density within SbO6 and ScO6 octahedra.

Oxide ion distribution and conductivity in Bi 7Nb 2 − 2 xY 2 xO 15.5 − 2 x

A detailed investigation using X-ray and neutron powder diffraction as well as ac impedance spectroscopy in the substituted bismuth oxide system Bi 7Nb 2 − 2 x Y 2 x O 15.5 − 2 x is presented. Combined refinement of variable temperature X-ray and neutron powder diffraction data reveal both compositional and temperature dependencies of the oxide ion distribution in this system. Oxide ions are found to be distributed over three crystallographically distinct sites, with one site exclusively associated with the dopant cations Nb 5+ and Y 3+ leading to a distorted octahedral geometry for these ions. Changes in the oxide ion distribution are correlated with a non-linear thermal expansion as well as non-Arrhenius behaviour of total conductivity at high temperatures.

Order–disorder transition in the complex lithium spinel Li 2CoTi 3O 8

Li 2CoTi 3O 8 has an ordered Li 2BB′ 3O 8 spinel structure, space group P4 332, at room temperature with 3:1 ordering of Ti and Li on the octahedral sites, and Li, Co disordered over the tetrahedral site. Rietveld refinement of variable temperature neutron powder diffraction data has shown an order–disorder phase transition in Li 2CoTi 3O 8 which commences at ∼500 °C with Li and Co mixing on the tetrahedral and 4-fold octahedral sites and is complete at a first order structural discontinuity at ∼915 °C. The fraction of Ti on the 12-fold octahedral site exhibits a small decrease with increasing temperature, which may suggest that the disordering involves all three cations. Above 930 °C, the structure, space group Fd3¯ m, has Li, Co and Ti sharing a single-octahedral site and Li, Co sharing a tetrahedral site, although Co still exhibits a preference for tetrahedral coordination. A labelling scheme for ordered and partially ordered 3:1 spinels is devised which focuses on the occupancy of the Li, B cations.

Structural study and proton conductivity in Yb-doped BaZrO 3

Traditional solid state sintering has been used to prepare the perovskite BaZr 0.9Yb 0.1O 3 − δ . Analysis of X-ray powder diffraction data shows that an increase of the unit cell parameter, a, was observed compared to undoped BaZrO 3. Rietveld analysis of room temperature neutron powder diffraction data confirmed cubic symmetry (space group Pm-3m). Dynamic thermogravimetric analysis (TGA) indicates that the hydration process occurs below 200 °C. The strong O–H stretch band, 2500–3500 cm − 1 , in the infrared absorbance spectrum clearly manifests the presence of protons in the hydrated material. Proton conductivity was investigated on hydrated and as-prepared samples under dry and wet atmospheres, respectively.

Phase Transitions of the Magnetoelectric A2NiMoO6 (A = Ba, Sr) and Ca2NiWO6 by Neutron Diffraction

The magnetic ferroelectric double perovskites A 2 NiMoO 6 (A = Ba, Sr), and Ca 2 NiWO 6 have been characterised using neutron powder diffraction (NPD) data. The Ba 2 NiMoO 6 compound has previously been reported in cubic or hexagonal symmetry which led to investigation of polymorphism. High temperature NPD data of the Sr 2 NiMoO 6 compound indicates a continuous structural phase transition from tetragonal to cubic symmetry at approximately 550 K. The reduction in symmetry is linked to spontaneous polarization allowing the structural perturbation to be described as an improper ferroelectric phase transition. The Ca 2 NiWO 6 compound shows similar behaviour as Sr 2 NiMoO 6 above 700 K, possibly P21/n → (I2/m) → 14/m → Fm-3m.

Proton conductivity and low temperature structure of In-doped BaZrO 3

The proton conductivity and structural features of In 3+ substituted BaZrO 3 samples, i.e., BaZr 1− x In x O 3− δ , were investigated. Rietveld analysis of low temperature (10 K) neutron powder diffraction data collected on as-prepared and deuterated samples confirmed cubic symmetry (space group Pm-3m) for all compositions. The level of oxygen vacancies refined in the as-prepared samples were in good agreement with the values expected to conserve charge neutrality, whilst an increase in oxygen occupancy, reflecting the incorporation of OD − species, was obtained for the deuterated materials. An expansion of the unit cell parameter, a, was observed as a function of In 3+ doping as well as after the deuteration reaction. The conductivity of pre-hydrated and dry samples was measured using impedance methods. For 25% In-doped BaZrO 3, the low T (300 °C) conductivity of the heating cycle of the dried sample was greater than that of the cooling cycle of the pre-hydrated sample indicating a greater number of protons in the nominally dry sample. In contrast, the conductivity values were similar at higher temperatures e.g. T > 500 °C where proton conduction is not dominant.

Synthesis and structural characterization of perovskite type proton conducting BaZr 1− xIn xO 3− δ (0.0 ≤ x ≤ 0.75)

Solid state sintering has been used to prepare the cubic perovskite structured compounds BaZr 1− x In x O 3− δ (0.0 ≤ x ≤ 0.75). Analysis of X-ray powder diffraction (XRPD) data reveals that the unit cell parameter, a, increases linearly with an increased Indium concentration. XRPD data was also used to demonstrate the completion of sample hydration, which was reached when the materials showed a set of single-phase Bragg-peaks. Dynamic thermogravimetric analysis (TGA) data showed that approx. 89% of the total number of available oxygen vacancies can be filled in BaZr 1− x In x O 3− δ for x = 0.50, and that the maximum water uptake occurs below 300 °C. Rietveld analysis of the room temperature neutron powder diffraction (NPD) data confirmed the average cubic symmetry (space group Pm-3m), and an expansion of the unit cell parameter after the hydration reaction. The strong O–H stretch band, 2500–3500 cm − 1 , in the infrared absorbance spectrum clearly manifests the presence of protons in the hydrated material. Proton conductivity of hydrated BaZr 1− x In x O 3− δ , x = 0.75 was investigated during heating and cooling cycles under dry argon atmosphere. The total conductivity during the heating cycle was nearly two orders of magnitude greater than that of cooling cycle at 300 °C, whilst these values were similar at higher temperatures i.e. T > 600 °C.

High temperature phase transition of the magnetoelectric double perovskite Sr 2NiMoO 6 by neutron diffraction

The magnetic ferroelectric double perovskite Sr 2NiMoO 6 has been characterised using neutron powder diffraction (NPD) data and magnetic susceptibility measurements. High temperature NPD data indicate that the material undergoes a continuous structural phase transition from tetragonal ( I4 /m) to cubic ( Fm 3 ¯ m) at approximately 550 K. The transition is linked to the level of rotation of the NiO 6 and MoO 6 octahedra, which form perfectly segregated sub-lattices in the material. The reduction in symmetry at 550 K is linked to the onset of a spontaneous dipole in the material allowing the structural perturbation to be described as an improper ferroelectric phase transition. The appearance of low angle reflections in the 20 K dataset indicates the presence of an antiferromagnetic ordered ground state. The magnetic moment was refined to 1.88(8) μ B confirming that the magnetism originates from the Ni 2+ (S = 1) ions.

The crystal structure of CaLaMnFeO6 double perovskite

A single-phase, polycrystalline sample of the double perovskite CaLaMnFeO6 was prepared by solid-state synthesis and characterized with neutron powder diffraction. The compound belongs to the class of AA′BB′O6 perovskites with a random distribution of Mn and Fe atoms over the B-cation sub-lattice. Rietveld refinement of the room temperature neutron powder diffraction data revealed that the crystal structure of CaLaMnFeO6 is disordered with orthorhombic symmetry (space group Pbnm) having lattice parameters a=5.4468(5) Å, b=5.4386(5) Å, c=7.6862(5) Å. The crystal structure is distorted due to the small size of the La+3 and Ca+2 cations, which force the (Mn/Fe)O6 octahedra to tilt in order to optimize the 12-coordinated (La/Ca)–O bond distances.