Neutron powder diffraction and magnetic measurements on CsMnI3

Results of neutron powder diffraction and magnetic measurements on single crystals of CsMnI₃ are reported. Three-dimensional ordering takes place at T_c = 11.1(3) K. Above T_c very broad peaks occur in the neutron powder diffraction diagram, indicating one-dimensional correlations along the chain. Below T_c the Mn²⁺ ions are coupled antiferromagnetically along the chain. Interchain exchange leads to a 120° structure, slightly distorted due to anisotropy. One-third of the chains have their magnetic moment parallel to the c axis and the rest of the chains have magnetic moments making an angle of 50(2)° with the c axis. The magnetic moment as found from neutron diffraction extrapolated to 0 K is 3.7(1)μ_B, indicating a considerable zero-point spin reduction. The intrachain exchange $\text{J}\text{k}$ was found to be ?9.1(1)K, whereas the ratio of the inter- to intrachain interaction was determined as $\text{J′}\text{J}\text{= × 10}{}^{\mathrm{?3}}$. A spin flop occurs at H = 54 kOe on application of a magnetic field parallel to the c axis. When a field perpendicular to the c axis is applied a spin reorientation occurs at 1 kOe.     

Neutron powder diffraction on RbCrl3 and magnetic measurements on RbCrl3 and CsCrl3

β-RbCrI₃ (a = 13.772(3), b = 8.000(2), c = 7.069(2) Å β = 95.85(1)°, $\text{Z = 4,}\text{C2}\text{m}$ at 293 K) and γ-RbCrI₃ (a = 13.586(2), b = 7.923(2), c = 14.094(3) Å, β = 96.88(1)°, Z = 8, C2 at 1.2 K) are isostructural to β-RbCrCl₃ and γ-RbCrCl₃ and are both Jahn-Teller distorted BaNiO₃ structures. In both compounds elongated octahedra occur. γ-RbCrI₃ most probably has a magnetic spiral structure at 4.2 and 1.2 K. Theoretically, a spiral propagating along the b axis is expected. A model with k = 9/19b1 yielded the best result. However, no good fit was obtained possibly because of a misfit in k and canting of the magnetic moments due to anisotropy. χ vs T single-crystal measurements on β-CsCrI₃ are in accordance with its magnetic structure. The three-dimensional magnetic ordering temperature T_c is estimated as 27(1) K. From the χ vs T curves of γ-RbCrI₃, T_c could not be determined. From fits to χ vs T powder data $\text{J}\text{k}$ of CsCrI₃ and RbCrI₃ are estimated to be ?14(2) and ?11(1) K, respectively.     

Neutron powder diffraction and magnetic measurements on RbTil3, RbVI3, and CsVI3

CsVI₃ (a = 8.124(1) c = 6.774(1)Å,Z = 2, P6₃/mmc at 293 K) adopts the BaNiO₃ structure. Three-dimensional magnetic ordering takes place atT_c = 32(1)K. At 1.2 K the magnetic moment is 1.64(5) μ_B and it forms a 120° spin structure in the basal plane. RbVI₃ (a = 13.863(2) c = 6.807(1) Å,Z = 6, P6₃cmor P3¯c1 at 293 K) and RbTiI₃ (a = 14.024(3) Å,c = 6.796(2) Å,Z = 6, P6₃cm orP3¯c1 at 293 K) adopt a distorted BaNiO₃ structure, probably isostructural with KNiCl₃.T_c of RbVI₃ is 25(1) K. At 1.2 K, RbVI₃ has a spin structure similar to the one of CsVI₃ with a magnetic moment of 1.44(6) μ_B. RbTiI₃ shows no magnetic ordering at 4.2 K. It is shown that a deviation from the 120° structure is expected for compounds with a distorted BaNiO₃ structure such as RbVI₃. The cell dimensions of CsTiI₃ are reported.     

Neutron powder diffraction study of the crystal and magnetic structures of BiNiO3 at low temperature

The crystal and magnetic structures of the charge ordered perovskite BiNiO₃ have been studied at temperatures from 5 to 300 K using neutron diffraction. Rietveld analysis of the data shows that the structure remains triclinic (space group ) throughout the whole temperature range. Bond-valence sum calculations based on the Bi-O and Ni-O bond distances confirm that the charge distribution is Bi³⁺_0.5Bi⁵⁺_0.5Ni²⁺O₃ down to 5 K. The magnetic cell is identical to that of the triclinic superstructure and a G-type antiferromagnetic model gives a good fit to the magnetic intensities, with an ordered Ni²⁺ moment of 1.76(3) μ_B at 5 K. However, BiNiO₃ is ferrimagnetic due to the inexact cancellation of opposing, inequivalent moments in the low symmetry cell.     

Neutron powder diffraction study of the magnetic and crystal structures of SrFe2(PO4)2

The crystal and magnetic structures of SrFe²⁺₂(PO₄)₂ have been determined by neutron powder diffraction data at low temperatures (space group P2₁/c (no. 14); Z=4; a=9.35417(13) Å, b=6.83808(10) Å, c=10.51899(15) Å, and β=109.5147(7)° at 15 K). Two magnetic phase transitions were found at T₁=7.4 K (first-order phase transition) and T₂=11.4 K (second-order phase transition). The transition at T₂ was hardly detectable by dc and ac magnetization measurements, and a small anomaly was observed by specific heat measurements. At T₁, strong anomalies were found by dc and ac magnetization and specific heat. The structure of SrFe₂(PO₄)₂ consists of linear four-spin cluster units, Fe2-Fe1-Fe1-Fe2. Below T₁, the propagation vector of the magnetic structure is k=[0,0,0]. The magnetic moments of the inner Fe1-Fe1 atoms of the four-spin cluster unit are ferromagnetically coupled. The magnetic moment of the outer Fe2 atom is also ferromagnetically coupled with that of the Fe1 atom but with spin canting. The four-spin cluster units form ferromagnetic layers parallel to the [−101] plane, while these layers are stacked antiferromagnetically in the [−101] direction. Spin canting of the outer Fe2 atoms provides a weak ferromagnetic moment of about 1 μ_B along the b-axis. The refined magnetic moments at 3.5 K are 4.09 μ_B for Fe1 and 4.07 μ_B for Fe2. Between T₁ and T₂, a few weak magnetic reflections were observed probably due to incommensurate magnetic order.     

Neutron powder diffraction on α-Tl4CrI6 and β-Tl4CrI6

α-Tl₄CrI₆ (a = 9.132(1), c = 9.667(1) Å, $\text{Z = 2,}\text{P4}\text{mnc}$ at 293 K) adopts a distorted Tl₄HgBr₆ structure. In α-Tl₄CrI₆ there occurs a random distribution of Jahn-Teller distorted octahedra which are elongated perpendicular to the c axis. Between 77 and 4.2 K a phase transition occurs. In β-Tl₄CrI₆ (a = 12.941(3), b = 12.596(3), c = 9.602(2) Å, Z = 4, Cccm at 4.2 K) the directions of elongation of the octahedra are ordered. The structure is very much related to that of α-Tl₄CrI₆. A three-dimensional magnetic ordering takes place at 2.7(2) K. The magnetic space group at 1.2 K is C_I22′2′. The magnetic moments (3.48(6) μ_B) are parallel to (0 0 1) and have an angle of 41(9)° with the a axis. Four magnetic sublattices are present, forming two independent magnetic lattices which have no interaction due to the antiparallel ordering.     

Neutron diffraction, specific heat and magnetic susceptibility of Ni3(PO4)2

The Ni₃(PO₄)₂ phosphate was synthesized by the ceramic method in air atmosphere. The crystal structure consists of a three-dimensional skeleton constructed from Ni₃O₁₄ edge-sharing octahedra, which are interconnected by (PO₄)³⁻ oxoanions with tetrahedral geometry. The magnetic behavior was studied on powdered sample by using susceptibility, specific heat and neutron diffraction data. The nickel(II) orthophosphate exhibits a three-dimensional magnetic ordering at approximately 17.1 K. However, its complex crystal structure hampers any parametrization of the J-exchange parameter. The specific heat measurements of Ni₃(PO₄)₂ exhibit a three-dimensional magnetic ordering (λ-type) peak at 17.1 K. Measurements above T_N suggest the presence of a small short-range order in this phase. The total magnetic entropy was found to be 28.1 KJ/mol at 50 K. The magnetic structure of the nickel(II) phosphate exhibits ferromagnetic interactions inside the Ni₃O₁₄ trimers which are antiferromagnetically coupled between them, giving rise to a purely antiferromagnetic structure.     

Neutron powder diffraction study of phase transitions in Sr2SnO4

The phase transitions in Sr₂SnO₄ at high temperature have been studied using high resolution time-of-flight powder neutron diffraction. The room temperature structure of Sr₂SnO₄ is orthorhombic (Pccn), which can be derived from the tetragonal K₂NiF₄ structure by tilting the SnO₆ octahedra along the tetragonal [100]_T- and [010]_T-axes with non-equal tilts. At the temperature of about 423 K, it transforms to another orthorhombic structure (Bmab) characterized by the SnO₆ octahedral tilt around the [110]_T-axis. At still higher temperatures (∼573 K) the structure was found to be tetragonal K₂NiF₄-type (I4/mmm).     

Neutron powder diffraction study of nuclear and magnetic structures of oxidized and reduced YBa2Fe3O8+w

YBa₂Fe₃O_8+w has been investigated by neutron powder diffraction as function of temperature and oxygen nonstoichiometry close to the limits of the homogeneity range, −0.24<w<0.12. The nonstoichiometry in this triple perovskite is achieved either by accommodation of extra oxygen atoms (w>0) in the structural layers of Y, or by creating oxygen vacancies (w<0) between the BaO layers in the ab plane of the octahedrally coordinated Fe. Moderate amounts of these defects do not alter the long-range crystallographic symmetry, which is best described in space group P4/mmm at all temperatures. However, in the most reduced samples, oxygen vacancies order and the nuclear structure becomes orthorhombic (Pmmm), thus showing a mirror-like similarity to the behavior of YBa₂Cu₃O_6+w′ upon oxidation. The effects of nonstoichiometry on these related crystal structures are discussed in terms of bond-valence sums. The cooperative magnetic structure for all compositions is based on a larger cell related to the nuclear cell by the transformation matrix ), having orthorhombic symmetry when the nuclear structure is tetragonal and monoclinic symmetry when the nuclear structure is orthorhombic. The iron moments are coupled antiferromagnetically in all three directions, the Néel temperature is almost constant as a function of w, and so is also the low-temperature saturation moment μ_AF≈4.0μ_B.     

Neutron diffraction and electrochemical studies on LiIrSn4

Large quantities of single phase, polycrystalline LiIrSn₄ have been synthesised from the elements by melting in sealed tantalum tubes and subsequent annealing. LiIrSn₄ crystallises with an ordered version of the PdGa₅ structure: I4/mcm, a=655.62(8), . The lithium atoms were clearly localised from a neutron powder diffraction study: R_P=0.147 and R_F=0.058. Time-dependent electrochemical polarisation techniques, i.e. coulometric titration, chronopotentiometry, chronoamperometry and cyclic voltammetry were used to study the kinetics of lithium ion diffusion in this stannide. The range of homogeneity (Li_1+ΔδIrSn₄, −0.091?δ?+0.012) without any structural change in the host structure and the chemical diffusion coefficient (∼10⁻⁷-10⁻⁹ cm²/s) point out that LiIrSn₄ is a first example of a large class of intermetallic compounds with lithium and electron mobility. Optimised materials from these ternary lithium alloys may be potential electrode material for rechargeable lithium batteries.     

In situ X-ray powder diffraction, synthesis, and magnetic properties of InVO3

We report the first synthesis and high-temperature in situ X-ray diffraction study of InVO₃. Polycrystalline InVO₃ has been prepared via reduction of InVO₄ using a carbon monoxide/carbon dioxide buffer gas. InVO₃ crystallizes in the bixbyite structure in space group Ia−3 (206) with a=9.80636(31) Å with In³⁺/V³⁺ disorder on the (8b) and (24d) cation sites. In situ powder X-ray diffraction experiments and thermal gravimetric analysis in a CO/CO₂ buffer gas revealed the existence of the metastable phase InVO₃. Bulk samples with 98.5(2)% purity were prepared using low-temperature reduction methods. The preparative methods limited the crystallinity of this new phase to approximately 225(50) Å. Magnetic susceptibility and neutron diffraction experiments suggest a spin-glass ground state for InVO₃.     

Neutron powder diffraction study of tetragonal Li7La3Hf2O12 with the garnet-related type structure

We have successfully synthesized a polycrystalline sample of tetragonal garnet-related Li-ion conductor Li₇La₃Hf₂O₁₂ by solid state reaction. The crystal structure is analyzed by the Rietveld method using neutron powder diffraction data. The structure analysis identifies that tetragonal Li₇La₃Hf₂O₁₂ has the garnet-related type structure with a space group of I4₁/acd (no. 142). The lattice constants are a=13.106(2) Å and c=12.630(2) Å with a cell ratio of c/a=0.9637. The crystal structure of tetragonal Li₇La₃Hf₂O₁₂ has the garnet-type framework structure composed of dodecahedral La(1)O₈, La(2)O₈ and octahedral HfO₆. Li atoms occupy three types of crystallographic site in the interstices of this framework structure, where Li(1) atom is located at the tetrahedral 8a site, and Li(2) and Li(3) atoms are located at the distorted octahedral 16f and 32g sites, respectively. These Li sites are filled with the Li atom. The present tetragonal Li₇La₃Hf₂O₁₂ sample exhibits bulk Li-ion conductivity of σ_b=9.85×10⁻⁷ S cm⁻¹ and grain-boundary Li-ion conductivity of σ_gb=4.45×10⁻⁷ S cm⁻¹ at 300 K. The activation energy is estimated to be E_a=0.53 eV in the temperature range of 300-580 K.     

Neutron powder diffraction experiments on the layered triangular-lattice antiferromagnets RbFe(MoO4)2 and CsFe(SO4)2

Powder specimens of the layered triangular-lattice antiferromagnets RbFe(MoO₄)₂ and CsFe(SO₄)₂ were prepared and neutron powder diffraction experiments were carried out in order to determine the magnetic structure. The magnetic structure of both compounds is the so-called 120^° structure in the triangular plane and is incommensurate between the planes. The ordered moments are confined in the basal ab-plane. It is also found that RbFe(MoO₄)₂ exhibits structural phase transition at around 190 K from to .     

Neutron and X-ray diffraction study on polymorphism in lithium orthotantalate,Li3TaO4

The structures of the low-and high-temperature modifications of lithium orthotantalate, Li₃TaO₄, have been determined by neutron and X-ray diffraction methods. The low-temperature, or β, phase has symmetry $\text{C2}\text{c}$ and lattice parameters a₁ = 8.500(3), b₁ = 8.500(3), c₁ = 9.344(3)Å, and β = 117.05(2)°. The high-temperature, or α, phase has symmetry P2 and lattice parameters a_h = 6.018(1), b_h = 5.995(1), c_h = 12.865(2)Å, and β_h = 103.53(2)°. Both structures are ordered. The β-phase has a rock salt-type structure with a 3 : 1 ordering of the Li⁺ and Ta⁵⁺ ions. Its structure can be generated from the low-temperature modification by means of a complex pattern of shifts of the Ta⁵⁺ ions.     

Neutron diffraction experiments on CsCrI3 at 300, 77, and 1.2 K

CsCrI₃ has been investigated by neutron powder diffraction at room temperature and 77 and 1.2 K. It undergoes a phase transition at 150 K due to the cooperative Jahn-Teller effect. The high-temperature form, α-CsCrI₃ (hexagonal, space group $\text{P6}{}_3\text{mmc}\text{, a = 8.127(1)}$Å, c = 6.944(1)Å, Z = 2), adopts the BaNiO₃ structure with a local Jahn-Teller distortion. The low-temperature form, β-CsCrI₃ (orthorhombic, space group Pbcn, a = 8.102(1)Å, b = 13.792(1)Å, c = 6.900(1)Å, Z = 4), has a structure not yet been reported for a Jahn-Teller distorted BaNiO₃ structure. It is shown that the low-temperature form can be derived from the BaNiO₃ structure by means of a canting of triangles, formed by the three common I^? ions of two adjacent CrI₆^4? octahedra. The magnetic structure of β-CsCrI₃ at 1.2 K is found to consist of an antiparallel sequence of ferromagnetic (0 0 1) planes with a magnetic moment in the ∥1 0 0∥ direction of 3.16 μ_B.     

High-temperature powder synchrotron diffraction studies of synthetic cryolite Na3AlF6

A high-resolution synchrotron diffraction study of the structures of a synthetic sample of cryolite Na₃AlF₆ from room temperature to 800°C is reported. At room temperature Na₃AlF₆ is monoclinic and the structure is described in space group P2₁/n. Heating the sample to 560°C results in only minor changes to the structure. A first-order transition from this monoclinic structure to a high-temperature cubic structure is observed near 567°C. The cubic structure is characterized by disorder of the fluoride atoms.     

High-resolution neutron powder diffraction study on the phase transitions in BaPbO3

Phase transitions that occurred in perovskite BaPbO₃ have been investigated using high-resolution time-of-flight neutron powder diffraction. The structure at room temperature is orthorhombic (space group Imma), which is derived from the cubic aristotype by tilting the PbO₆ octahedra around the two-fold axis (tilt system a⁰b⁻b⁻). The orthorhombic structure shows anisotropic line broadening attributed to the presence of micro twins. At above about 573 K, BaPbO₃ undergoes a discontinuous phase transition to a tetragonal structure (space group I4/mcm) with the tilting of the PbO₆ octahedra being about the four-fold axis of the cubic aristotype (tilt system a⁰a⁰c⁻). With further increasing the temperature, BaPbO₃ experiences a continuous phase transition to a simple cubic structure (space group Pm3¯m) at above about 673 K. The later phase transition is characterised by a critical exponent of β=0.36, depicted by the three-dimensional Heisenberg universality class. The earlier reported Imma→I2/m phase transition above room temperature has not been observed.     

Neutron diffraction study of phase transitions in perovskite-type strontium molybdate SrMoO3

The structure of a polycrystalline sample of SrMoO₃ has been investigated using powder neutron diffraction from 5 to 300 K, to reveal two structural phase transitions, the first from the cubic structure with a=3.97629(3) Å to a tetragonal structure in I4/mcm near 266 K and the second to an orthorhombic Imma phase below 125 K. The average Mo-O distance is essentially independent of temperature. The temperature dependence of the octahedral tilting appears typical of a tricritical phase transition.     

Neutron diffraction study on the spinel Cu0.72Co2.28O4

A neutron powder diffraction refinement of the positional and thermal structure parameters of Cu_0.72Co_2.28O₄ spinel has been carried out. It has been found that the spinel is inverse, with inversion parameter approximately equal to one half of the copper content value.     

Synchrotron and neutron powder diffraction study of phase transition in weberite-type Nd3NbO7 and La3NbO7

La₃NbO₇ and Nd₃NbO₇ are insulating compounds that have an orthorhombic weberite-type crystal structure and undergo a phase transition at about 360 and 450 K, respectively. The nature of the phase transitions was investigated via heat capacity measurements, synchrotron X-ray and neutron diffraction experiments. It is here shown that above the phase transition temperature, the compounds possess a weberite-type structure described by space group Cmcm (No. 63). Below the phase transition, the high temperature phase transforms into a weberite-type structure with space group Pmcn (No. 62). The phase transformation primarily involves the off-center shifting of Nb⁵⁺ ions inside the NbO₆ octahedra, combined with shifts of one third of the Ln³⁺ (Ln³⁺=La³⁺ and Nd³⁺) ions at the center of the LnO₈ polyhedra towards off-center positions. The phase transition was also proven to have great impacts on the dielectric properties.