Neutron Powder Diffraction Measurements Research Articles

Realizing square and diamond lattice S=1/2 Heisenberg antiferromagnet models in the α and β phases of the coordination framework, KTi(C2O4)2·xH2O

We report the crystal structures and magnetic properties of two psuedo-polymorphs of the $S=1/2$ Ti$^{3+}$ coordination framework, KTi(C$_2$O$_4$)$_2\cdot$xH$_2$O. Single-crystal X-ray and powder neutron diffraction measurements on $\alpha$-KTi(C$_2$O$_4$)$_2\cdot$xH$_2$O confirm its structure in the tetragonal $I4/mcm$ space group with a square planar arrangement of Ti$^{3+}$ ions. Magnetometry and specific heat measurements reveal weak antiferromagnetic interactions, with $J_1\approx7$ K and $J_2/J_1=0.11$ indicating a slight frustration of nearest- and next-nearest-neighbor interactions. Below $1.8$ K, $\alpha$ undergoes a transition to G-type antiferromagnetic order with magnetic moments aligned along the $c$ axis of the tetragonal structure. The estimated ordered moment of Ti$^{3+}$ in $\alpha$ is suppressed from its spin-only value to $0.62(3)~\mu_B$, thus verifying the two-dimensional nature of the magnetic interactions within the system. $\beta$-KTi(C$_2$O$_4$)$_2\cdot$2H$_2$O, on the other hand, realises a three-dimensional diamond-like magnetic network of Ti$^{3+}$ moments within a hexagonal $P6_222$ structure. An antiferromagnetic exchange coupling of $J\approx54$ K -- an order of magnitude larger than in $\alpha$ -- is extracted from magnetometry and specific heat data. $\beta$ undergoes N\'eel ordering at $T_N=28$ K, with the magnetic moments aligned within the $ab$ plane and a slightly reduced ordered moment of $0.79~\mu_B$ per Ti$^{3+}$. Through density-functional theory calculations, we address the origin of the large difference in the exchange parameters between the $\alpha$ and $\beta$ psuedo-polymorphs. Given their observed magnetic behaviors, we propose $\alpha$-KTi(C$_2$O$_4$)$_2\cdot$xH$_2$O and $\beta$-KTi(C$_2$O$_4$)$_2\cdot$2H$_2$O as close to ideal model $S=1/2$ Heisenberg square and diamond lattice antiferromagnets, respectively.

Magnetic field induced phase transition in spinel GeNi2O4

Cubic spinel GeNi2O4 exhibits intriguing magnetic properties with two successive antiferromagnetic phase transitions (TN1 12.1 and TN2 11.4 K) with the absence of any structural transition. We have performed detailed heat capacity and magnetic measurements in different crystallographic orientations. A new magnetic phase in presence of magnetic field (H > 4 T) along the [111] direction is revealed, which is not observed when the magnetic field is applied along the [100] and [110] directions. High field neutron powder diffraction measurements confirm such a change in magnetic phase, which could be ascribed to a spin reorientation in the presence of magnetic field. A strong magnetic anisotropy and competing magnetic interactions play a crucial role on the complex magnetic behavior in this cubic system.

Hidden magnetic order in the triangular-lattice magnet Li2MnTeO6

The manganese tellurate Li2MnTeO6 consists of trigonal spin lattices made up of Mn4+ (d3, S = 3/2) ions. The magnetic properties of this compound were characterized by several experimental techniques, which include magnetic susceptibility, specific heat, dielectric permittivity, electron spin resonance (ESR), nuclear magnetic resonance (NMR) and neutron powder diffraction (NPD) measurements, and by density functional calculations (DFT). The magnetic susceptibility chi(T) demonstrates very unusual behavior. It isdescribed by the Curie-Weiss law at high temperature with Curie-Weiss temperature of Theta = -74 K, exhibits no obvious anomaly indicative of a long-range magnetic ordering at low magnetic fields. At high magnetic fields, however, the character of chi(T) changes showing a maximum at about 9 K. That this maximum of chi(T) reflects the onset of an antiferromagnetic order was confirmed by specific heat measurements, which exhibit a clear lambda-type anomaly at TN around 8.5 K even at zero magnetic field, and by 7Li NMR and dielectric permittivity measurements. The magnetic structure of Li2MnTeO6, determined by neutron powder diffraction measurements at 1.6 K, is described by the 120-degree non-collinear spin structure with the propagation vector k = (1/3, 1/3, 0). Consistent with this finding, the spin exchange interactions evaluated for Li2MnTeO6 by density functional calculations are dominated by the nearest-neighbor antiferromagnetic exchange within each triangular spin lattice. This spin lattice is strongly spin frustrated with f = |Theta|/TN around 8 and exhibits a two-dimensional magnetic character in a broad temperature range above TN.

Unraveling the nature of Fe-doping mediated inter- and intra-chain interactions in Ca3Co2O6

The structural and magnetic properties of quasi-one-dimensional (1D) spin-chain compounds Ca3Co2-xFexO6 (x = 0, 0.1, 0.2 and 0.3) synthesized by a sol-gel method have been systematically studied by means of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), neutron powder diffraction (NPD), dc and ac magnetization measurements. The samples adopt a rhombohedral crystal structure with the space group R-3c in a temperature range of 5–100 K. Fe ions are found to locate at the trigonal prism Co2 crystallographic sites. Fe doping converts some of low-spin Co3+ ions at Co1 octahedral sites into Co2+ ions. For the high-temperature paramagnetic phase, the Curie paramagnetic temperature θp changes its sign from positive to negative at x = 0.2, implying that the dominant magnetic interaction is driven from ferromagnetic (FM) to antiferromagnetic (AFM) upon Fe substitution. A partial low-spin to high-spin state crossover of the Co1 ions is observed at high Fe-doping level x = 0.3. All the samples exhibit a long-range spin-density wave (SDW) AFM ordering below TN, followed by a spin-glass-like transition at Tf. These low-temperature magnetic phases were significantly suppressed upon Fe doping. The Fe substitution in Ca3Co2-xFexO6 weakened both intrachain and interchain magnetic interactions, caused by enhanced magnetic disorder due to the different magnetic characters of Ising Co and Heisenberg Fe spins.

Responsive Four-Coordinate Iron(II) Nodes in FePd(CN)4.

Metal node design is crucial for obtaining structurally diverse coordination polymers (CPs) and metal-organic frameworks with desirable properties; however, FeII ions are exclusively six-coordinated. Herein, we present a cyanide-bridged three-dimensional (3D) CP, FePd(CN)4 , bearing four-coordinate FeII ions, which is synthesized by thermal treatment of a two-dimensional (2D) six-coordinate FeII CP, Fe(H2 O)2 Pd(CN)4 ⋅4 H2 O, to remove water molecules. Atomic-resolution transmission electron microscopy and powder X-ray and neutron diffraction measurements revealed that the FePd(CN)4 structure is composed of a two-fold interpenetrated PtS topology network, where the FeII center demonstrates an intermediate geometry between tetrahedral and square-planar coordination. This four-coordinate FeII center with the distorted geometry can act as a thermo-responsive flexible node in the PtS network.

Cooperative Ru(4d)−Ho(4f) magnetic ordering and phase coexistence in the 6H perovskite multiferroic Ba3HoRu2O9

We report cooperative magnetic orderings in a 6H-perovskite multiferroic system, Ba3HoRu2O9, via comprehensive neutron powder diffraction measurements. This system undergoes long-range antiferromagnetic ordering at TN1 ~ 50 K with a propagation wave vector of K1 = (0.5 0 0), a transition temperature much higher than the previously reported one at ~10 K (TN2). Both Ru and Ho-moments order simultaneously below TN1, followed by spin-reorientations at lower temperatures, demonstrating strong Ru(4d)-Ho(4f) magnetic correlation. Below TN1 another magnetic phase with a propagation wave vector K2 = (0.25 0.25 0) emerges and coexists with the one associated with K1, which is rarely observed and suggests complex magnetism due to phase competition in the magnetic ground state. We argue that the exchange-striction arising from the up-up-down-down spin structure associated with K2-wave vector below TN2 may be responsible for the small ferroelectric polarization reported previously in this compound.

Role of Fe addition in Ni–Mn–Ga–Co–Cu–Fe ferromagnetic shape memory alloys for high-temperature magnetic actuation

The Ni–Mn–Ga ferromagnetic shape memory alloys (FSMAs) are a group of active materials that undergo martensitic transformations (MTs) induced by temperature, stress and/or magnetic fields, resulting in large recoverable mechanical deformations. Their fast response and high energy density make them ideal candidates for implementation in sensors and actuators. Recently, the development of high-temperature FSMAs (HTFSMAs), working at temperatures over 373 K, has become an important task to meet the current requirements of modern technologies. The magnetic and magnetoelastic properties of FSMAs are very sensitive to the interactions between the magnetic moments of atoms that, in turn, depend on the atomic positions within the lattice. Here we investigate a series of the polycrystalline Ni45Mn25-xGa20Co5Cu5Fex (x = 0, 1, 2 and 5 at%) HTFSMAs. Their MT and Curie temperatures, the crystal structures of the martensitic and austenitic phases, the temperature evolutions of the lattice parameters of both phases and the atomic site occupancies have been studied by means of powder neutron diffraction measurements, complemented by standard characterization techniques. Based on the atomic site occupancies and additional measurements of the saturation magnetization, the influence of the structure and atomic ordering on the magnetism in these materials is analysed. The most promising candidate for high temperature magnetic actuation is the alloy with Fe 5 at%, following a combination of the high MT and Curie temperatures, around 370 K and 440 K, respectively, with a low tetragonality ratio, c/a ≈ 1.16, in the proximity of MT.

Extremely low thermal conductivity from bismuth selenohalides with 1D soft crystal structure

Materials with intrinsically low thermal conductivity are of fundamental interests. Here we report a new sort of simple one-dimensional (1D) crystal structured bismuth selenohalides (BiSeX, X = Br, I) with extremely low thermal conductivity of ~0.27 W m−1 K−1 at 573 K. The mechanism of the extremely low thermal conductivity in 1D BiSeX is elucidated systematically using the first-principles calculations, neutron powder-diffraction measurements and temperature tunable aberration-corrected scanning transmission electron microscopy (STEM). Results reveal that the 1D structure of BiSeX possesses unique soft bonding character, low phonon velocity, strong anharmonicity of both acoustic and optical phonon modes, and large off-center displacement of Bi and halogen atoms. Cooperatively, all these features contribute to the minimal phonon transport. These findings provide a novel selection rule to search low thermal conductivity materials with potential applications in thermoelectrics and thermal barrier coatings.

Unveiling ferrimagnetic ground state, anomalous behavior of the exchange-bias field around spin reorientation, and magnetoelectric coupling in YbCr1−xFexO3 ( 0.1≤x≤0.6 )

We present a comprehensive experimental study of the magnetic structure, magnetic and dielectric properties of the rare-earth orthochromite-orthoferrite solid-solution series $\mathrm{YbC}{\mathrm{r}}_{1\ensuremath{-}x}\mathrm{F}{\mathrm{e}}_{x}{\mathrm{O}}_{3}$ ($0.1\ensuremath{\le}x\ensuremath{\le}0.6$). Room-temperature synchrotron x-ray diffraction analysis reveals the absence of any superlattice reflections, which excludes the formation of a $B$-site-ordered double-perovskite-like phase and establishes the complete solid solubility of Fe at the Cr site within the framework of orthorhombic Pbnm structure. We demonstrate that canted antiferromagnetic ground state of $\mathrm{YbCr}{\mathrm{O}}_{3}$ is converted to a ferrimagnetic with Fe doping, in addition to an increase in the magnetic ordering temperature. An unusual, second magnetic transition (first-order in nature) appears for $x\ensuremath{\ge}0.3$ samples below the ferrimagnetic transition temperature (e.g., at 70 K for $x=0.4$), which is identified as the spin reorientation of transition metal ions from the neutron powder diffraction measurements, and primarily, driven by the $f\text{\ensuremath{-}}d$ exchange interaction. A clear evidence of the anomalous behavior of coercivity and exchange bias field is found around the spin reorientation temperature, which is characterized by a significant change in the magnetocrystalline anisotropy due to spin reorientation of transition metal ions. Temperature-dependent dielectric data exhibit the magnetoelectric coupling as well as a ferroelectric relaxor-like state at the onset of ferrimagnetic ordering. Here, we reveal the anomalous behavior of the exchange bias field and significant magnetoelectric coupling around the spin reorientation and ferrimagnetic transitions, respectively, in $\mathrm{YbC}{\mathrm{r}}_{1\ensuremath{-}x}\mathrm{F}{\mathrm{e}}_{x}{\mathrm{O}}_{3}$.

Correlation between Crystal Structure and Thermoelectric Properties of Sr1−xTi0.9Nb0.1O3−δ Ceramics

Polycrystalline Sr1−xTi0.9Nb0.1O3−δ (x = 0, 0.1, 0.2) ceramics have been prepared by the solid state method and their structural and thermoelectric properties have been studied by neutron powder diffraction (NPD), thermal, and transport measurements. The structural analysis of Sr1-xTi0.9Nb0.1O3−δ (x = 0.1, 0.2) confirms the presence of a significant amount of oxygen vacancies, associated with the Sr-deficiency of the materials. The analysis of the anisotropic displacement parameters (ADPs) indicates a strong softening of the overall phonon modes for these samples, which is confirmed by the extremely low thermal conductivity value (κ ≈ 1.6 W m-1 K−1 at 823 K) found for Sr1−xTi0.9Nb0.1O3−δ (x = 0.1, 0.2). This approach of introducing A-site cation vacancies for decreasing the thermal conductivity seems more effective than the classical substitution of strontium by rare-earth elements in SrTiO3 and opens a new optimization scheme for the thermoelectric properties of oxides.

Jahn-Teller and geometric frustration effects on the structural and magnetic ground states of substituted spinels (Ni,A)Cr2O4 (A = Mn/Cu)

Comparative study of the effects of substitution (25%) of Jahn-Teller (JT) active Cu2+ and non JT active Mn2+ cations, at the Ni2+ site, on the geometric frustration (GF) and cooperative JT distortion (CJTD) in multiferroic NiCr2O4 has been carried out with the help of low-temperature magnetization, x-ray diffraction and time of flight (TOF) neutron powder diffraction (NPD) measurements. On Mn2+ substitution the CJTD, which causes elongation of the NiO4 tetrahedra in I41/amd phase of NiCr2O4, gets suppressed at room-temperature (RT) and stabilizes the Fd-3m phase in (Ni0.75Mn0.25)Cr2O4. On the contrary the Cu2+ substitution inverts the effect of CJTD and the tetrahedra get flattened at RT, which in turn oppositely distorts the I41/amd lattice in Ni0.75Cu0.25Cr2O4. The frustration index decreases on substitution. Intriguingly, the tetragonal to orthorhombic phase transition and the antiferromagnetic ordering, which are clearly evident in the pristine NiCr2O4 and the Cu2+ substituted sample respectively at 30 K and 70 K, have not been observed for the Mn2+ substituted sample. This has been attributed to the change in Cr3+- Cr3+ direct magnetic-interaction on structural transition. However, the long-range ferrimagnetic order remains intact for all the three compositions i.e. NiCr2O4, Ni0.75Mn0.25Cr2O4 and Ni0.75Cu0.25Cr2O4 occurring at 75 K, 70 K and 95 K, respectively. The Mn2+ substituted compound shows short-range magnetic correlation at low-temperatures, as evident from the diffuse and broad hump in the intensity-vs-d spacing plot of TOF NPD data. This short-range magnetic order has been attributed to the known spiral magnetic order.

Magnetic and structural properties of Ni-substituted magnetoelectric Co4Nb2O9

The magnetic and structural properties of polycrystalline Co$_{4-x}$ Ni$_x$ Nb$_2$ O$_9$ (x=1,2) have been investigated by neutron powder diffraction, magnetization and heat capacity measurements, and density functional theory (DFT) calculations. For x=1, the compound crystallizes in the trigonal P$\bar{3}$c1 space group. Below T$_N$ = 31 K it develops a weakly non-collinear antiferromagnetig structure with magnetic moments in the ab-plane. The compound with x=2 has crystal structure of the orthorhombic Pbcn space group and shows a hard ferrimagnetic behavior below T$_C$ =47 K. For this compound a weakly non-collinear ferrimagnetic structure with two possible configurations in ab plane was derived from ND study. By calculating magnetic anisotropy energy via DFT, the ground-state magnetic configuration was determined for this compound. The heat capacity study in magnetic fields up to 140 kOe provide further information on the magnetic structure of the compounds.

Understanding Rechargeable Battery Function Using In Operando Neutron Powder Diffraction.

The performance of rechargeable batteries is influenced by the structural and phase changes of components during cycling. Neutron powder diffraction (NPD) provides unique and useful information concerning the structure-function relation of battery components and can be used to study the changes to component phase and structure during battery cycling, known as in operando measurement studies. The development and use of NPD for in operando measurements of batteries is summarized along with detailed experimental approaches that impact the insights gained by these. A summary of the information gained concerning battery function using in operando NPD measurements is provided, including the structural and phase evolution of electrode materials and charge-carrying ion diffusion pathways through these, which are critical to the development of battery technology.

Fast Ionic Conductivity in the Most Lithium-Rich Phosphidosilicate Li14SiP6

Solid electrolytes with superionic conductivity are required as a main component for all-solid-state batteries. Here we present a novel solid electrolyte with three-dimensional conducting pathways based on "lithium-rich" phosphidosilicates with ionic conductivity of σ > 10-3 S cm-1 at room temperature and activation energy of 30-32 kJ mol-1 expanding the recently introduced family of lithium phosphidotetrelates. Aiming toward higher lithium ion conductivities, systematic investigations of lithium phosphidosilicates gave access to the so far lithium-richest compound within this class of materials. The crystalline material (space group Fm3m), which shows reversible thermal phase transitions, can be readily obtained by ball mill synthesis from the elements followed by moderate thermal treatment of the mixture. Lithium diffusion pathways via both tetrahedral and octahedral voids are analyzed by temperature-dependent powder neutron diffraction measurements in combination with maximum entropy method and DFT calculations. Moreover, the lithium ion mobility structurally indicated by a disordered Li/Si occupancy in the tetrahedral voids plus partially filled octahedral voids is studied by temperature-dependent impedance and 7Li NMR spectroscopy.

Structural, magnetic and electronic properties of Ti-doped BaFeO3-δ exhibiting colossal dielectric permittivity

We report a systematical investigation of the structural, magnetic, electronic, and dielectric properties of BaFe1-xTixO3-δ (x = 0.05, 0.10, 0.15 and 0.20) samples synthesized via the solid-state reaction method. Rietveld refinement using a combination of room-temperature X-ray (XRD) and neutron powder diffraction (NPD) data confirms single phases with a 6H-type structure for all the samples. X-ray absorption spectroscopy (XAS) study reveals a coexistence of Fe3+ and Fe4+, whose ratio remains almost unchanged with increasing Ti concentration. Magnetization measurements reveal a low-temperature inhomogeneous magnetic state with coexisting ferromagnetic (FM), AFM clusters and a paramagnetic phase. Moreover, magnetization of x = 0.10 shows a pinched hysteresis loop behavior, suggesting the coexistence of hard and soft FM phases in samples with high Ti concentrations. Low-temperature NPD measurements also confirm the absence of any long-range magnetic order in the samples. A colossal dielectric permittivity and two relaxation processes are observed for most of the studied samples. There is a change in electric conduction mechanism from the small polaron hopping to the Mott's variable-range hopping with increasing Ti concentration.

High-temperature neutron powder diffraction studies on atomic structure of (1−x)PbTiO3-xBi(Zn0.5Ti0.5)O3

The atomic structure of (1−x)PbTiO3-xBi(Zn0.5Ti0.5)O3 (x=0.1, 0.2, 0.25) is investigated across the tetragonal to cubic phase transition by neutron powder diffraction measurements up to temperature T=750°C. Both the long-range average structure and the local atomic distributions are studied from Rietveld refinements and atomic pair distribution function (PDF) analysis combined with reverse Monte-Carlo (RMC) modeling, respectively. In the tetragonal phase, the average structure is featured with a large shift of oxygen, which is responsible for an off-centering of cations. With increasing concentration x, the shift of the planar oxygen along the c-axis increases further. The increment of the oxygen displacement is larger than that of the Ti/Zn displacement. This makes the off-centering of cations larger at higher x, which enhances the stability of the tetragonal phase with higher transition temperature. On thermal displacements, the heaviest ion Pb/Bi in the structure exhibits the largest value and is also increased by the concentration x in both the tetragonal and cubic phases. The local structure from PDF and RMC modeling shows that the abnormal thermal displacements are due to a large distribution of Pb off-centering and a bond-length mismatch between Ti–O and Zn–O bonds. Based on these complementary local and average structures, a splitting between Ti and Zn displacements is discussed.

The reactivity of lattice nitrogen within the Ni2Mo3N and NiCoMo3N phases

In this study, the reactivity of bulk lattice nitrogen within the filled β-Mn structured Ni2Mo3N phase has been investigated by application of powder neutron diffraction and heterolytic nitrogen isotopic exchange measurements. In contrast to Co3Mo3N, despite the similarity in the N immediate local environment comprising NMo6 octahedra, its reactivity is found to be limited and this lower reactivity was maintained upon the introduction of a significant proportion of cobalt to yield its filled β-Mn structured CoNiMo3N quaternary nitride counterpart.

Crystal growth and magnetic structure of MnBi2Te4

Millimeter-sized MnBi$_2$Te$_4$ single crystals are grown out of Bi-Te flux and characterized by measuring magnetic and transport properties, scanning tunneling microscope (STM) and spectroscopy (STS). The magnetic structure of MnBi$_2$Te$_4$ below T$_N$ is determined by powder and single crystal neutron diffraction measurements. Below T$_N$=24\,K, Mn$^{2+}$ moments order ferromagnetically in the \textit{ab} plane but antiferromagnetically along the crystallographic \textit{c} axis. The ordered moment is 4.04(13) $\mu_{B}$/Mn at 10\,K and aligned along the crystallographic \textit{c}-axis. The electrical resistivity drops upon cooling across T$_N$ or when going across the metamagnetic transition in increasing fields below T$_N$. A critical scattering effect was observed in the vicinity of T$_N$ in the temperature dependence of thermal conductivity. However, A linear temperature dependence was observed for thermopower in the temperature range 2K-300K without any anomaly around T$_N$. These indicate that the magnetic order in Mn-Te layer has negligible effect on the electronic band structure, which makes possible the realization of proposed topological properties in MnBi$_2$Te$_4$ after fine tuning of the electronic band structure.

Dynamic Magnetic-Transformation-Induced Exchange Bias in[α−Fe2O3]0.1[FeTiO3]0.9

Up to now, for the conventional exchange bias (EB) systems there has been one pinning phase and one pinned phase, and the pinning and pinned phases are inherent to the material and do not mutually transform into each other. Interestingly, we show here that EB is observed in a special system (Fe2O3)0.1(FeTiO3)0.9 (HI9) different from the conventional EB system. Neutron powder diffraction and magnetic measurement confirm that for HI9: i) two types of short range antiferromagnetic ordering coexist, ii) there are two pinning phases and one pinned phase, iii) the pinned phase is not intrinsic to the structure but can be dynamically produced from the pinning phase with the help of an external magnetic field. Consequently, two anomalous EB behaviors are observed: i) both the coercivity (HC) and the exchange bias field (HE) simultaneously decrease to zero at 30 K, ii) for a high cooling field (Hcool) HE decreases logarithmically with increasing Hcool. Using Arrott plots it is confirmed that the first order magnetic phase transformation (FOMPT) from the AFM Fe2+ to ferromagnetic (FM) Fe2+ and the second order magnetic phase transformation (SOMPT) for the process whereby the FM Fe2+ aligns with the external field direction coexist in HI-9. The Morin transition and FOMPT cause the anomalous EB behaviors. This work may provide fresh ideas for research into EB behavior.

The magnetic structure of L10 ordered MnPt at room temperature determined using polarized neutron diffraction

Neutron scattering studies have been carried out on single crystal films of [001] orientated L10 ordered MnPt grown epitaxially on MgO (001) using DC magnetron sputtering. Polarized neutron diffraction studies at room temperature show that the moments in ordered MnPt are aligned perpendicular to the [001] axis with a tilt of 45° to the [100] axis and not parallel to [100] as inferred from previous powder neutron diffraction measurements.