Search

We have studied structural properties of La 3 Ni 2 B 2 N 3− δ samples with distinctly different values of the superconducting transition temperature by means of powder neutron diffractometry and specific heat measurements. The refinement of lattice site occupations reveals full occupations for all sites in the La 3 Ni 2 B 11 2 N 3 structure with space group I 4/ mmm except for nitrogen site N(2). For samples with T c = 13.0 K and 13.7 K we obtain for the N(2) site distinctly different occupation factors of 0.90 and 0.93, respectively. The latter confirms a direct relation between the nitrogen stoichiometry and the superconducting transition temperature. Based on the analysis of temperature dependent lattice parameters, atomic displacement factors, lattice heat capacity data and ab initio phonon density of states calculations we discuss the thermal expansion and vibrational properties of superconducting La 3 Ni 2 B 2 N 3− δ .

Acknowledgements We acknowledge the University of Aberdeen for provision of a studentship for Harriet Hopper and the Higher Education Commission of Pakistan for providing a visiting fellowship for Dr. Falak Sher.

We report results of detailed structural investigations on the system La2-xSrxCuRuO6 (0 0 respectively, neutron diffraction patterns collected at 10 K and 30 K across the characteristic transition temperature suggested by the magnetic data, do not show any extra peaks or extra intensity at lower temperature as compared to the high temperature, thus, establishing the absence of any long-range magnetic ordering in these samples. (C) 2016 Elsevier B.V. All rights reserved.

The perovskite system Pb(Fe1-xMnx)2/3W1/3O3 (0 ≤ x ≤ 1, PFMWO) has been prepared by conventional solid-state reaction under different sintering conditions. Structures and phase composition as well as thermal, magnetic and dielectric properties of the compounds have been systematically investigated experimentally and by first-principles density functional calculations. A clean perovskite phase is established at room temperature for compositions 0 ≤ x ≤ 0.4. Rietveld refinements of X-ray and neutron powder diffraction patterns demonstrate that the compounds crystallize in space group Pm-3m (0 ≤ x ≤ 0.4). The degree of ordering of the Fe and W/Mn cations was found to depend on the concentration of Mn. First-principles calculations suggest that the structural properties of PFMWO are strongly influenced by the Jahn-Teller effect. The PFMWO compounds behave as relaxor ferroelectrics at weak Mn-doping with a dielectric constant that rapidly decreases with increasing Mn content. A low temperature antiferromagnetic G-type order with propagation vector k = (1/2,1/2,1/2) is derived from neutron powder diffraction data for the samples with x ≤ 0.4. However with increasing doping concentration, the magnetic order is perturbed. First-principles calculations show that the dominant exchange coupling is antiferromagnetic and occurs between nearest neighbor Fe atoms. When the system is doped with Mn, a relatively weak ferromagnetic (FM) interaction between Fe and Mn atoms emerges. However, due to the presence of this FM interaction, the correlation length of the magnetic order is greatly shortened already at rather low doping levels.

In a recent work we have reported the formation and the crystal structure of the new family of compounds R3Pd5 (R = Sc, Y, Gd–Lu; Pu3Pd5-type, oS32, spacegroup 63, Cmcm). DFT calculations for Gd3Pd5 and heat capacity and magnetic measurements for Tb3Pd5, Dy3Pd5, Ho3Pd5 and Er3Pd5 were able to discriminate at least two distinct antiferromagnetic transitions at low temperatures; at the same time these data suggested these separate magnetic orderings to be likely associated to the two structurally different rare earth sublattices (4c and 8e, respectively). The magnetic structures of the R3Pd5 compounds, with R = Tb, Dy, Ho, Er, have been now investigated in this work. High-resolution and high-intensity neutron powder diffraction data were recorded for the four compounds of the series. The results reveal that the magnetic structures adopted are significantly diverse and their temperature dependence more complex than originally deduced from magnetic and heat capacity measurements. Tb3Pd5 is in fact the only of the four compounds where the rare earth sublattices order independently with two different magnetic propagation vectors on the two sites (k1 = [1 0 ½] for Tb2 and k2 = [kx 0 0] or k3 = [1 0 0] for Tb1). In Dy3Pd5 both sublattices follow the propagation vector k = [kx 0 0], with the second transition corresponding to a spin reorientation. In Ho3Pd5 ordering of both sublattices follow the k = [1 0 0] propagation vector; the second transition seen in the magnetic and heat capacity data corresponds to an accelerated increase in the magnetic moment of the Ho2 sublattice. The case of Er3Pd5 is special as it is the sole case where one of the rare earth sublattices does not order down to T = 1.5 K; here the second transition determined from the magnetic data corresponds to a change of the magnetic propagation vector from k1 = [kx 0 kz] to k2 = [1 0 0], both only acting on the Er2 sublattice. Irrespectively of the very similar structures not even two of the four studied R3Pd5 compounds show identical magnetic structures on either of the two sublattices. These strong variations have to be connected to changes in the RKKY interactions induced by the progressive filling of the f-orbitals of the heavy rare earth ions and the concomitant changes of the crystal electric fields.

La2−xSrxCoTiO6 (0.6 ≤ x ≤ 1.0) compound series is prepared by Sr-substitution in the A-site of the perovskite by a modified Pechini procedure under air. Charge compensation as Sr2+ content increase occurs by Co2+ oxidation to Co3+. Reduced samples are obtained by further treatment under 5%H2/Ar and characterized by Neutron Powder Diffraction. Upon redution, Co3+ to Co2+ reduction and oxygen vacancies creation are detected. Dependence of total conductivity with temperature and pO2 exhibits a typical p-type semiconducting behaviour. Results show that the higher the Sr content, the higher holes (Co3+) concentration and consequently, La2−xSrxCoTiO6 (x = 1.0) shows the highest conductivity (13.23 S/cm at 1073 K in air). The negligible reactivity with YSZ, used as the electrolyte, of symmetrical cells under oxidant conditions and the moderate thermal expansion found by XRD point to their possible use as SOFC cathodes. Thus, La1.2Sr0.8CoTiO6-based cathodes display polarization resistance of 0.9 Ω cm2 at 1073 K in oxygen, only slightly above than the current state-of-the-art.

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 CoMo3N, 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 CoNi2Mo3N quaternary nitride counterpart.

Rare earth ( R ) molybdate pyrochlores, R 2 Mo 2 O 7 , are of interest as frustrated magnets. Polycrystalline samples of Lu 2 Mo 2 O 7− x prepared at 1600 °C display a coexistence of cubic pyrochlore phases. Rietveld fits to powder neutron diffraction data and chemical analyses show that the miscibility gap is between a stoichiometric x =0 and an oxygen-deficient x ≈0.4 phase. Lu 2 Mo 2 O 7 behaves as a spin glass material, with a divergence of field cooled and zero field cooled DC magnetic susceptibilities at a spin freezing temperature T f =16 K, that varies with frequency in AC measurements following a Vogel–Fulcher law. Lu 2 Mo 2 O 6.6 is more highly frustrated spin glass and has T f =20 K.

We have investigated the magnetic properties of slightly doped multiferroic TbMnO3 after application of a magnetic field. The study focused on compositions TbMn1-xAxO3 (x ≤ 0.1) with A = Ga, Sc, Co and Al. The replacement of Mn by Ga, Al or Sc proved to be isovalent while the addition of Co leads to a partial charge transfer as Mn3+ + Co3+ → Mn4+ + Co2+. The samples with 10% of non-magnetic doping, TbMn0.9Sc0.1O3, TbMn0.9Al0.1O3 and TbMn0.9Ga 0.1O3, preserve the long range antiferromagnetic ordering of the Mn sublattice with, however, reduced transition temperatures compared to TbMnO3. New magnetic interactions in the Co-doped compound lead to the suppression of Mn ordering in TbMn0.9Co0.1O 3. The application of an external magnetic field produces similar metamagnetic transitions in all TbMn0.9A0.1O3 compounds that are ascribed to the Tb-sublattice. Powder neutron diffraction was used to determine the changes in the magnetic structure with applied magnetic field revealing a strong increase of F- and C-type magnetic reflections in these compounds. These results are accounted for by the anisotropic response of the Tb sublattice to a magnetic field while the Mn sublattice remains unchanged. © 2013 Published by Elsevier Masson SAS., Financial support from Spanish MEC (projects MAT2012-38213-C02-01 and MAT2011-27233-C02-02) and DGA (CAMRADS) is acknowledged.

Substitution of La3+ by Sr2+ in the perovskite La2CoTiO6 yields materials of the La2−xSrxCoTiO6 series. The dominant charge-compensating mechanism is oxidation of Co2+ if they are prepared at air. The as prepared oxides can be reduced inducing a large amount of oxygen vacancies while keeping the perovskite structure. The electrical behaviour of the La2−xSrxCoTiO6 series is dominated by p-type electronic conduction in a wide pO2 range through non-adiabatic hopping of small-polarons. The electrical conductivity increases with x, except for the x = 0.5 material which shows an unexpectedly low conductivity due to microstructural and short-range ordering effects. The highest conductivity material, La1.60Sr0.40CoTiO6, is selected to study the electrochemical properties of the series. This compound is chemically compatible with YSZ up to 1373 K, in both oxidizing and reducing atmospheres. The preliminary evaluation of the electrode performance reveals that La1.60Sr0.40CoTiO6-based electrodes exhibit polarization resistances of typically 0.8 Ω cm2 at 1073 K in oxygen, which are close to the values obtained for LSM-based cathodes. Thus, the electrochemical behaviour of this oxide as cathode is particularly encouraging since the electrode microstructure is not optimized; it is expected that an improved microstructure will perform at least similarly to the state-of-art in SOFCs materials.

Ba2LnMoO6 double perovskites have been recently shown to display a wide range of interesting magnetic and structural properties; Ba2154SmMoO6 exhibits simultaneous antiferromagnetic order and a Jahn–Teller distortion. Here we report a high temperature neutron diffraction study of Ba2154SmMoO6 from 353 to 877 K. The results evidence a tetragonal to cubic phase transition at 423 K. Above this temperature the thermal displacement parameters of the oxygen atoms are modelled anisotropically as a result of a transverse vibration of the bridging oxygen. A smooth increase in the cell parameter a is observed with temperature for Ba2154SmMoO6.

Materials of the series La2−xSrxNiTiO6−δ (0 ≤ x ≤ 0.5) have been characterized by both structural and electrochemical methods in order to assess their possible use as electrodes for SOFCs. Neutron and X-ray powder diffraction experiments have shown that they are stable under both oxidizing and reducing conditions while chemically compatible with YSZ at SOFC operating temperatures. Moreover, the thermal expansion has been determined to be isotropic with αL = 10.0(3) × 10−6 K−1. This value is similar to that found for other perovskite materials used in SOFCs. However, polarization resistances reveal modest values of electrochemical response under oxygen (1.5 Ω cm2 at 900 °C) and are quite poor in 5%H2/N2 mixtures (15 Ω cm2) at the same temperature. Nevertheless, microstructure has not been optimized fully enough to discard the material as a potential SOFC cathode.

The synthesis of the new compounds RMgSn (R = La–Nd, Sm, Gd–Tm, Lu and Y) has been recently reported. The compounds formed by La and Ce crystallise in the TiNiSi structure type ( oP 12, Pnma ), while from Nd they adopt the CeScSi-type ( tI 12, I 4/ mmm ); PrMgSn is dimorphic: its high-temperature form (HT) is TiNiSi-type while the low-temperature one (LT) is CeScSi-type. In this paper we now report the results of a neutron diffraction investigation which has been performed in order to refine the crystal as well as the magnetic structures for the RMgSn compounds with R = Ce, Pr, Nd and Tb. All these compounds see at low temperature the establishment of long range magnetic ordering with a predominantly antiferromagnetic interaction; only PrMgSn-HT orders ferromagnetically. These results agree with those from magnetic measurements recently reported. The magnetic structure of CeMgSn is of the amplitude-modulated type, the value of the magnetic propagation vector refined at 2 K is τ = [0, 0.1886(4), 0.3384(8)]. The PrMgSn-HT phase below T = 52 K adopts first a purely ferromagnetic structure, then at about T = 15 K a second magnetic coupling leads to a spin-canted magnetic structure. Both PrMgSn-LT and NdMgSn have the same antiferromagnetic commensurate magnetic structure. The TbMgSn compound below T N = 35 K orders antiferromagnetically with an equal moment cycloidal structure; however a second magnetic transition at a temperature corresponding to T N 2 = 65 K is likely also present.

We report the synthesis of the new compounds R15Si9C with R = Sm, Gd–Er, Y and R15Ge9C with R = Ce, Pr and Nd. These compounds crystallize in the hexagonal La15Ge9Fe structure type (hP50, P63mc, Z = 2) which is an ordered superstructure of La5Ge3 (Mn5Si3-type, hP16, P63/mcm, Z = 2). The superstructure is absent in a single-phase R5Ge3C0.33 alloy with R = Sm and Gd. The addition of interstitial carbon modifies the magnetic properties significantly as regards the magnetic transition temperature and the nature of the magnetic ordering. Carbon doping induces mostly a ferromagnetic or ferrimagnetic configuration. The preponderance of ferromagnetic interactions in the superstructure compounds, with respect to the parent phases, can tentatively be attributed to a change in the sign of the RKKY interactions. The dense Kondo lattice state of Ce in ferrimagnetic Ce5Ge3 is preserved in Ce15Ge9C. Large coercive fields (∼4–5 kOe) at low temperatures are observed in most compounds, which exceed ∼50 kOe in the two Sm based compound.

We present the magnetic temperature phase diagrams of the CrB- and FeB-type orthorhombic phases of the DySi compound, determined from high-quality powder XRPD and neutron diffraction, as well as the sample microstructure as determined by HRTEM. Both phase diagrams comprise a HT (Tc1, Tc2−TN) and a LT range (5 K–Tc1, Tc2) separated by a monoclinic phase transition at Tc1=Tc2=23.5 K well below the second-order Neel transition (TN=40 K). The transition paths are for CrB-type Cmcm (HT) Tc1→C2/m11 (LT), and for FeB Pnma (HT) Tc2→P21/n11 (LT). The transitions are related to non-monotonous changes of the lattice and structural parameters displaying anomalies at Tc1, Tc2 and slight volume changes. For the CrB-type the monoclinic angle decreases smoothly from Tc1 down to 5 K and the maximum strain experienced by the crystal lattice in the (0 2 1) direction was found at Tc1. In the FeB-type, in addition to the magneto-elastic transition at Tc2=23.5 K, minor anomalies are found at 13.5 K in the temperature dependence of the monoclinic angle and the maximum strain along (0 1 1). Both temperatures mark the sequence of changes in the magnetic domain microstructure observed in FeB: below T2=23.5 K the incommensurate HT magnetic phase with q 3 ≈ ( 0 1 2 1 7 ) disproportionates into two LT phases q 3 ≈ ( 0 1 2 1 11 ) T2 and q 2 ≈ ( 0 1 2 1 6 ) coexisting in the form of domains with portions varying with T going from T2 down to 13.5 K (q2 increasing at the cost of q3). This behaviour could be related to structural inhomogeneities below the first-order magneto-elastic transition Tc2, if one assumes a broad two phase range, where the HT (Pnma) phase coexists with (P21/n) as a metastable phase at LT in the form of domains with different magnetic behaviour.

Samples of mesoporous Co 3 O 4 , created by using mesoporous silicas KIT-6 and SBA-16 as hard templates to control the growth of Co 3 O 4 have been investigated with SQUID magnetometry and neutron powder diffraction, to reveal the effects of high surface area on the magnetic and electronic properties. DC magnetic susceptibility measurements show lower Neel ordering temperatures and lower magnetic moments than in a “bulk” reference. A lower second transition temperature is also observed in the mesoporous samples, associated with the freezing of the surface (shell) magnetic moments. Measurements taken with increasing applied field at constant temperature show the materials to be antiferromagnetic as expected. Complementary parametric neutron powder diffraction studies show similar trends between the two mesoporous samples when looking at their Neel temperatures, and verify long range order within the samples.

A detailed study of Strontium substitution (up to 25% at most) with Lanthanide (La, Nd and Sm) in the Sr2CrReO6 double perovskite has been carried out by means of neutron powder diffraction (NPD), scanning electron microscopy (SEM), low magnetic field and high pulsed magnetic field magnetometry as well as transport measurements. In the present manuscript we will focus on the consequences of such substitution on structural, electronic and magnetic properties. The presented results show the lack of expected enhancement of the Curie temperature by the Lanthanide addition. This puts doubts on the possibility of electron doping in the Sr2CrReO6 system via the standard solid-state reaction growth techniques with rare-earth doping, commonly used for other double perovskite compounds., Financial support by EuroMagNET (under the EU contract RII3-CT- 2004-506239 of the 6th Framework Structuring the European Research Area, Research Infrastructures Action), as well as the Spanish Ministry of Science (through projects FIS08-03951, MAT2005-04562 and MAT2008-06567-C02 including FEDER funding) and the Polish Ministry of Science and Higher Education is also acknowledged.

The magnetic structure of the Fe 2 P-type R 6 CoTe 2 phases ( R =Gd–Er, space group P 6¯2 m ) has been investigated through magnetization measurement and neutron powder diffraction. All phases demonstrate high-temperature ferromagnetic and low-temperature transitions: T C =220 K and T CN =180 K for Gd 6 CoTe 2 , T C =174 K and T CN =52 K for Tb 6 CoTe 2 , T C =125 K and T CN =26 K for Dy 6 CoTe 2 , T CN =60 K and T N =22 K for Ho 6 CoTe 2 and T CN ∼30 K and T N ∼14 K for Er 6 CoTe 2 . Between 174 and 52 K Tb 6 CoTe 2 has a collinear magnetic structure with K 0 =[0, 0, 0] and with magnetic moments along the c -axis, whereas below 52 K it adopts a non-collinear ferromagnetic one. Below 60 K the magnetic structure of Ho 6 CoTe 2 is that of a non-collinear ferromagnet. The holmium magnetic components with a K 0 =[0, 0, 0] wave vector are aligned ferromagneticaly along the c -axis, whereas the magnetic component with a K 1 =[1/2, 1/2, 0] wave vector are arranged in the ab plane. The low-temperature magnetic transition at ∼22 K coincides with the reorientation of the Ho magnetic component with the K 0 vector from the collinear to the non-collinear state. Below 30 K Er 6 CoTe 2 shows an amplitude-modulate magnetic structure with a collinear arrangement of magnetic components with K 0 =[0, 0, 0] and K 1 =[1/2, 1/2, 0]. The low-temperature magnetic transition at ∼14 K corresponds to the variation in the magnitudes of the M Er K0 and M Er K1 magnetic components. In these phases, no local moment was detected on the cobalt site. The magnetic entropy of Gd 6 CoTe 2 increases from Δ S mag =−4.5 J/kg K at 220 K up to Δ S mag =−6.5 J/kg K at 180 K for the field change Δ μ 0 H =0–5 T.

The temperature magnetic phase diagrams, of the dimorphic DySi compound, have been studied in terms of wave vectors in the range 1.5–45 K, by neutron diffraction. The polycrystalline sample consists of 26% of CrB-type ( Cmcm no. 63, all atoms at 4c site: (0, y , 1/4)) and of 74% of FeB-type ( Pnma no. 62, all atoms at 4c site: ( x , 1/4, z )). The CrB-ordering is described by the wave vector: q 1 =(0, 0, 1/2) over the entire magnetically ordered regime with a uniaxial magnetic structure along the shortest axis c. The FeB-type magnetic phase diagram reveals three distinct regions of magnetic ordering below T N and one first order transition at T 2 =23.5 K (on heating). The ordering is described by two symmetry independent magnetic vectors q 2 =(0, 1/2, 1/6) and q 3 =(0, q 3 y , q 3 z ) with a temperature variable length. At 1.5 K q 3 y ≈1/2 and q 3 z ≈1/11. The two phases coexist in the form of domains. They differ in the moment orientation of the q 3 phase that deviates by ∼22° from the b -axis in the (0, 0, 1) plane. The low temperature range ( LT ) 1.5 K— T 2 subdivides into two regions: (i) LT -1, between 1.5 K— T 1 where the relative amount of the two phases remains unchanged and in (ii) LT -2: T 1 – T 2 where the amount of the incommensurate q 3 phase increases at the cost of the commensurate q 2 amplitude modulated structure which remains unchanged but fully disappears at the first order transition at T 2 =23.5 K. The q 3 phase undergoes minor changes until 22 K and gets destabilised at T 2 where the q 3 z component jumps from the LT value q 3 z ≈1/11 to the HT value ≈1/7 and the q 3 y component increases from 0.484(1) to 0.495(1). (iii) The high temperature ( HT ) range T 2 – T N ( T N =40±1 K) is described by a single wave vector q 3 . The disproportionation of the HT magnetic phase q 3 below T 2 into two coexisting distinct phases q 2 , q 3 down to 1.5 K is an unusual phenomenon, to our knowledge observed for the first time. Various mechanisms are discussed.

A new series of non-stoichiometric sulfides Ga1−xGexV4S8−δ (0≤x≤1; δ≤0.23) has been synthesized at high temperatures by heating stoichiometric mixtures of the elements in sealed quartz tubes. The samples have been characterized by powder X-ray diffraction, SQUID magnetometry and electrical transport-property measurements. Structural analysis reveals that a solid solution is formed throughout this composition range, whilst thermogravimetric data reveal sulfur deficiency of up to 2.9% in the quaternary phases. Magnetic measurements suggest that the ferromagnetic behavior of the end-member phase GaV4S8 is retained at x≤0.7; samples in this composition range showing a marked increase in magnetization at low temperatures. By contrast Ga0.25Ge0.75V4S8−δ appears to undergo antiferromagnetic ordering at ca. 15 K. All materials with x≠1 are n-type semiconductors whose resistivity falls by almost six orders of magnitude with decreasing Ga content, whilst the end-member phase GeV4S8−δ is a p-type semiconductor. The results demonstrate that the physical properties are determined principally by the degree of electron filling of narrow-band states arising from intracluster V–V interactions.

The low-temperature magnetic ordering of the dimorphic DySi compound has been studied at 1.5 K by neutron diffraction on two polycrystalline samples. The samples comprise various amounts of the two orthorhombic modifications: CrB-type (Cmcm Nr. 63, all atoms at 4c site: (0, y, 1 4 )) and FeB-type (Pnma Nr. 62, all atoms at 4c site: (x, 1 4 , z)), both order antiferromagnetically (TN≈38 K). The CrB-type phase orders with a uniaxial structure with the wave vector q1=(0, 0, 1 2 ) requiring a doubling of the c-axis. The Dy moments point along the linear chain with the shortest distance c. At 1.5 K, the ordered moment value is 8.57(1) μB/Dy atom. Two symmetry independent wave vectors describe the 1.5 K magnetic ordering of the FeB-type phase: q2=(0, 1 2 , 1 6 ) and q3=(0, 0.484(1), 0.0892(1)), coexisting in form of domains. In both structures the magnetic moments are confined to the (0 0 1) plane at an angle of 2(2)° and 22(3)° from the shortest axis b, respectively. Both structures correspond to sine wave modulations. The amplitude of the q2 wave is mo=7.5(1) μB/Dy atom and that of q3 8.2(1) μB/Dy atom. The wave vector q2 when referring to the (a, 2b, c) cell and the wave vector q=(0, 0, 1 6 ) corresponds to a transversal modulation, which by a proper origin choice can be also described as an antiphase domain structure with two amplitudes. The moments point to the b-axis and are stacked in the sequence (+mo/2, −mo/2, −mo, −mo/2, +mo/2, +mo, …) along the c-direction, while tb acts as an antitranslation. For the q3 phase, the local moment value depends on the atom position in the wave. We also discuss the case where q3 and q2 act simultaneously in physical space.

In this paper we report a preliminary neutron diffraction investigation of the proton conducting La 0.99 Ca 0.01 NbO 4 material. Measurements have been acquired from room temperature up to 750 °C under dry and wet atmosphere. The main result of this investigation is the observation of the existence of a broad temperature interval where the low- T monoclinic phase coexists with the high- T tetragonal phase. Even at the highest investigated temperature small fractions of the monoclinic phase have been detected under the experimental conditions employed here.

The crystallographic and magnetic structures of polycrystalline Ba2(Fe,W)2O6 and Ba2(Fe,W0.5Mo0.5)2O6 double-perovskites have been investigated by X-ray and neutron powder diffraction (NPD), magnetization and Mossbauer spectroscopy. The samples were synthesized by conventional solid-state reaction at temperatures about 1273 K. The compounds crystallize in the cubic structure with space group F m 3 ¯ m . The magnetic structures were determined by neutron powder diffraction between 5 K and 310 K. Evidence for an antiferromagnetic behavior has been observed for Ba2(Fe,W)2O6 with TN = 24.7 K and a two-phase separation for Ba2(Fe,W0.5Mo0.5)2O6 into an antiferromagnetic structure of the W-type with TN = 24.7 K and the ferromagnetic Mo-type with TC = 270 K.

In this paper we have studied, by means of high-resolution neutron powder diffraction and magnetic susceptibility, the structural and magnetic features of selected samples of the HoBaCo2O5+δ layered cobaltite in the low oxygen content range 0.02≤δ≤0.22. The results show a strong antiferromagnetic contribution at room temperature coupled to an intermediate spin state of the Co3+ ions.

For the first time, the 7M modulated structure, frequently observed in ferromagnetic shape memory Ni–Mn–Ga martensitic phases, is solved by powder diffraction analysis. Two polycrystalline samples with composition Ni2Mn1.2Ga0.8 and Ni2.15Mn0.85Ga, respectively, showing a 7M martensitic state stable at room temperature, were studied. The determination of the modulated crystal structure of Ni2Mn1.2Ga0.8 martensite was achieved by refining the X-ray powder diffraction pattern by the Rietveld method. The basic structure belongs to monoclinic symmetry. The crystal structure, solved within the superspace approach, is found to show an incommensurate 7M modulation with q = 0.308c∗. The Rietveld refinement for Ni2.15Mn0.85Ga martensite on the basis of neutron powder data surprisingly provides a very similar incommensurate 7M structure with the same periodicity and analogous modulation function. The incommensurate structure presents typical displacive modulation with several analogies with the Zhdanov (5, 2 ¯ )2 stacking sequence.

Magnetic structures and magnetic phase transitions in natural-layered TbMn2(GexSi1−x)2 compounds have been studied by magnetisation, low-field AC-susceptibility, electrical resistivity and neutron-diffraction experiments. Non-collinear magnetic structures were observed in the concentration range 0 < x < 0.88. In compounds with x

We present a comparative investigation of the Bi manganites with commensurate composition x = 1 4 : Bi0.75Sr0.25MnO3 and Bi0.75Ca0.25MnO3. μSR and diffraction data confirm different structural, magnetic and electronic transitions in the two compounds. The robust charge-order phase detected with Sr do not occur in Bi0.75Ca0.25MnO3, which present inhomogeneous magnetism and short-range charge order. In both compounds the application of magnetic fields produces a continuous progressive canting of the moments, visible even for relatively small fields ( μ 0 H ⪡ 1 T ) .

In this paper we report studies on the correlation between the presence and distribution of cation vacancies in doped manganites (La,M)MnO3+delta (where M = Na, Ca) and their magnetic properties. Results indicate that cation vacancies are distributed differently for the different crystal structures and dopant ion type. In particular it is shown that knowledge of the total vacancies concentration alone is not enough to fully characterize the physical properties of manganites and that their distribution between the A and B sites of the perovskite structure plays a crucial role which should be taken into account in future studies., 27 pages, 5 figure. To appear in J. Solid State Chem

Texture correction of isostatically pressed molybdite powders has been carried out using the D1A high-resolution fixed wavelength 25-detector powder diffractometer at the Institut Laue-Langevin. All of the samples were spun during the data collections to improve particle statistics. In the present study, the GSAS Rietveld program was used for analysis of each pattern, in which the generalized spherical harmonic (GSH) is generated using selection rules depending on the crystal symmetry of the phase under investigation. The default sample texture symmetry was chosen to be cylindrical and the maximum of eight harmonic orders was selected. The results indicate that the correct crystal structure can be obtained when applying corrections to intensities using the GSH description.

By means of high-resolution neutron powder diffraction at 10 K , we have characterized the structural details of LaxSr2−xFeMoO6 (0⩽x⩽0.5) and CaxSr2−xFeMoO6 (0⩽x⩽0.6) series of compounds. The Curie temperature (TC) raises in the La series and slightly decreases in the Ca one. The enhancement of TC in the La series is attributed to the effect of the conduction band-filling when doping with La.

The magnetic structure of Fe 2 Mo 3 O 12 was determined by neutron powder diffraction and magnetization studies. The magnetic unit cell is identical with the monoclinic chemical one ( Z =8), and the magnetic space group is P2 1 /a. The magnetic moments are ferromagnetically aligned along the b -axis for each sublattice corresponding to each Fe-site. There are four different Fe-sites, and the magnetic moments are parallel within two of them, but antiparallel to those of the other two sublattices, so that a ferrimagnetic structure results. The dominant antiferromagnetic supersuperexchange couplings are those along Fe–O–O–Fe paths with the two bridging oxygens belonging to one [MoO 4 ]-tetrahedron and forming large Fe–O–O and O–O–Fe angles.

Neutron diffraction performed on Nd 6 Fe 13− x Al 1+ x ( x =1–3) and La 6 Fe 11 Al 3 powder samples has shown a selective distribution of the Al atoms among the five non-rare-earth positions filling up successively the 4a Si and the 16l 2 Fe4 sites of the prototype Nd 6 Fe 13 Si structure with increasing x . Traces of Al could be detected at the 16k Fe site when x =3. The magnetic ordering is associated in all cases with the wave vector q =(0 0 1) ( I p magnetic lattice with anticentering translation). All Nd compounds display almost easy axis behaviour below T N and spin reorientation transitions at lower temperatures from easy axis to easy cone. A full set of neutron data in the entire magnetically ordered regime has been analysed to derive the exact magnetic structural parameters and possible spin reorientation transitions. The largest ordered moment values were observed for Nd 6 Fe 12 Al 2 at 1.5 K (3.2(2) μ B /Nd atom, close to the free ion value, and 2.1 μ B /Fe atom) while no detectable moment value could be found for the Fe4 site for higher Al concentrations. The iron moment values are in satisfactory agreement with Mossbauer and magnetization results.

A comparative investigation of the charge modulation tendencies in R–M–MnO3 and Bi–Sr–MnO3 manganites reveals remarkable differences in these series of materials. By means of magnetic, magnetotransport, synchrotron X-ray and neutron diffraction, as well as pulsed high magnetic fields data, we compare the systematic electronic and magnetic properties of several rareearth-based manganites and bismuth-based Sr manganites. The differences between these types of systems are rationalized in terms of the role that the different electronic structure of bismuth and rare earth plays in these materials.

We report that the charge-ordered phase of Bi0.75Sr0.25MnO3 presents a transition temperature T CO ≃600 K , well above the ordering temperature in half-doped Bi0.50Sr0.50MnO3 (T CO ≃525 K ). Magnetization, calorimetry, and neutron diffraction measurements evidence that, in underdoped Bi0.75Sr0.25MnO3, an off-stoichiometry corresponding to a Mn3+/Mn4+ ratio of 3 increases TCO by ∼75 K with respect to half-doping. This unusual feature confirms the active role of Bi in this family, the only known manganites presenting charge order well above room temperature.

Specific heat, magnetic susceptibility, magnetization and neutron diffraction measurements were carried out on Ce 3 Cu 4 Sn 4 and Ce 3 Cu 4 Ge 4 compounds (structure type Gd 3 Cu 4 Ge 4 , Immm, 2 Ce sites). The linear coefficient of specific heat is γ =330 mJ/mol Ce K 2 for the stannide and γ =225 mJ/mol Ce K 2 for the germanide, indicative for heavy fermion behaviour. The Ruderman–Kittel–Kasuya–Yosida exchange interactions in the presence of the magnetocrystalline anisotropy induce magnetic ordering, which is complex and proceeds in several steps. For both compounds below the Neel temperature T N =10.3 K incommensurate magnetic arrangements are favoured. They lock into a commensurate structure with the wave vector q 3 =(0 0 0) at T 2 ( T 2 =7.3 K for stannide and T ′ 2 =7.8 K for germanide). Due to a strong anisotropy the Ce magnetic moments are aligned along the x -axis in the T 2 – T 3 interval, but at T 3 = T ′ 3 =2.6 K a reorientation of the moments into the xy -plane occurs. The evolution of the magnetic order and the moment values differ for the two inequivalent crystallographic Ce sites. The ordered Ce magnetic moment is 2.05(2) μ B at the 2d site and 1.10(2) μ B at the 4e site for germanide. For stannide the corresponding values are 0.75(1) μ B and 1.94(2) μ B . Two different magnetic arrangements are possible for Ce 3 Cu 4 Sn 4 at low temperatures.

The series of Sr 2 Fe 1− x Cr x MoO 6− y has been synthesized and studied by means of a wide set of experimental techniques. These samples belong to the family of A 2 BB′O 6 double perovskites with a ferrimagnetic ground state. The replacement of Fe by Cr leads to samples with a high oxygen deficiency and to increasing cation disorder between B and B′ sites. The magnetic moments of these samples decrease as the iron content does. Electrical properties change as well strongly along the series: while Sr 2 FeMoO 6 is metallic, the rest of the samples show a semiconducting behavior with an activation energy that increases as the content of Cr does. Only Fe-rich samples exhibit large negative magnetoresistance with the low-field response characteristic of a half-metallic system. The Cr-rich samples display instead a small linear negative magnetoresistance.

High-resolution synchrotron X-ray and neutron powder diffraction of Bi 1− x Ca x MnO 3 ( x ≥0.75) has shown that these systems present a phase segregation coinciding with the electronic localization of the e g electrons. The very precise results obtained provide evidence of a mechanism for phase segregation based on subtle compositional heterogeneity effects.

The new ternary selenide Cr4TiSe8 crystallises in the V5S8 type structure. The determination of the metal atom distribution over the three crystallographically independent sites using neutron scattering experiments reveals that the Ti atoms exclusively occupy the sites within the full metal atom layers with a pronounced preference for one of the two possible sites. The magnetic properties in the high temperature region exhibit Curie–Weiss behaviour with a paramagnetic Curie temperature θ of −300(5) K indicating strong antiferromagnetic exchange interactions. Between about 100 and 300 K the susceptibility data pass a very broad maximum and a qualitatively explanation is presented that base on antiferromagnetic exchange interactions between Cr atoms forming linear trimers, dimers, or Cr atoms being isolated. Below about 50 K the zero field cooled and field cooled susceptibility curves diverge suggesting spin glass behaviour. In the low temperature neutron diffraction patterns no additional peaks are observed which gives further evidence for the occurrence of a spin glass. Below 120 K the background of the diffraction patterns becomes modulated due to the short range magnetic interactions with sizes of the domains ranging from about 15 to 38 A. Low temperature single crystal X-ray investigations show a remarkable change of the contraction of the lattice parameters below about 220 K that may be due to the onset of the strong magnetic exchange interactions.

Neutron scattering and AC susceptibility measurements have been made on isotopically enriched 154 SmMn 2 ( Ge 0.9 Si 0.1 ) 2 . A temperature versus pressure magnetic phase diagram is proposed. The incommensurate cone structures detected in SmMn2Ge2 at low temperatures and pressures do not appear in the phase diagram of the Si-substituted system. There is no evidence of a change in the magnetic structure associated with a small peak in susceptibility observed at around 145 K which has previously been interpreted in terms of changes to the cone structure. However, as with SmMn2Ge2, the non-collinear magnetic structures of these magnetic phases are such that a larger antiferromagnetic component appears within (0 0 1) ab -planes in the ferromagnetic states than in the antiferromagnetic state. With increasing Si substitution, the antiferromagnetic phase becomes the magnetic equilibrium state over a wider temperature range. The observed changes in the Mn–Mn spacing and magnetic coupling are largely consistent with previous related observations, although under a pressure of ∼8 kbar, low-temperature magnetic structures are found below the critical lattice spacing of a =4.031 A which are similar to those observed at ambient pressure.

We present the results of neutron powder diffraction experiments with the D2B instrument at Institut Laue Langevin on Al 2 (WO 4 ) 3 and Y 2 (WO 4 ) 3 over the temperature range 20 to 800°C. The coefficients of thermal expansion are determined to be α a =−1.31×10 −6 °C −1 , α b =5.94×10 −6 °C −1 , and α c =−9.94×10 −6 °C −1 for Al 2 (WO 4 ) 3 and α a =−10.35×10 −6 °C −1 , α b =−3.06×10 −6 °C −1 , and α c =−7.62×10 −6 °C −1 for Y 2 (WO 4 ) 3 . As in the case of Lu 2 (WO 4 ) 3 , the expansivity is unusual in the case of Y 2 (WO 4 ) 3 , as the negative thermal expansion is present along all three axes.

The magnetic structures of TlCr5Se8 and Tl0.2Cr5Se8 were determined at 2 K using neutron scattering experiments. In the antiferromagnetic state the stoichiometric sample shows monoclinic symmetry with the magnetic cell exhibiting a doubling of the c-axis. All Cr atoms have magnetic moments in the b direction with magnitudes in accordance with Cr(III). As a consequence of direct and indirect superexchange, antiferromagnetic as well as ferromagnetic links between the different Cr atoms are observed. In Tl0.2Cr5Se8 in the antiferromagnetic state the symmetry is still monoclinic also showing a doubled c-axis. The Cr atoms have magnetic moments in the a–c-plane. Drastic changes of the Cr–Cr interatomic distances lead to a spin structure being significantly different from that observed for the stoichiometric sample. At about 70 K additional magnetic reflections which are due to the occurrence of an intermediate magnetic phase are observed in the powder pattern. The transition between the antiferromagnetic and the intermediate phase is accompanied by abrupt changes of the lattice parameters.

Variable-temperature powder neutron diffraction data demonstrate that FeV{sub 2}S{sub 4} undergoes a transition to a long-range magnetically ordered state at 135(7) K, in agreement with magnetic susceptibility data. High-resolution neutron diffraction data collected at 1.9 K reveal that magnetic ordering results in a doubling of the crystallographic unit-cell dimensions (I2/m a = 5.8303(2), b = 3.2761(1), c = 11.2398(4) {angstrom}, {beta} = 92.046(2){degree}) in the a and c directions and that the magnetic structure is described by a propagation vector of ({1/2}, 0, {1/2}). Cations in an ordered defect layer, 76% of which are Fe(II), possess an average ordered moment of 1.86(5) {mu}{sub B}, which is directed at an angle of 75{degree} to the layer. Cation-cation interactions reduce the average moment of cations in the MS{sub 2} unit to 0.17(4) {mu}{sub B}. The complex magnetic structure involves essentially collinear antiferromagnetic ordering between nearest-neighbor cations.

Powder neutron diffraction data for NixCr3−xS4(x= 1 4 , 1 2 , 3 4 ) collected over the temperature range 1.8≤T≤286 K demonstrate that all phases exhibit long-range magnetic order at low temperatures. The magnetic ordering temperature is composition dependent and increases with decreasing nickel content (205, 210, and 218 K forx= 3 4 , 1 2 , and 1 4 , respectively). The magnetic structure determined at 1.8 K involves a doubling of the unit cell in both theaand thecdirections. Moments associated with cations in a fully occupied metal layer, directed parallel to the cation layers, show little compositional dependence, whereas those associated with cations in an ordered vacancy layer are directed toward the anion layers and have a maximum value at the composition corresponding tox= 1 2 . At temperatures between 1.8 K and the ordering temperature, materials with compositions corresponding tox= 1 4 ; and 3 4 show a gradual 90° reorientation of the direction of the vacancy layer moments such that at temperatures just below the ordering temperature, moments lie parallel to the crystallographicb-axis.

The magnetic phase diagram of the compound ErNiSi2 of CeNiSi2 structure type (Cmcm, space group) has been studied by neutron diffraction and magnetic measurements. The paramagnetic neutron data confirmed the sample homogeneity. The analysis of the data collected in the magnetically ordered regime (TN=3.4 K) showed the existence of two distinct types of magnetic ordering associated with the wave vectors q1=(1/2, 0, 0) and q2=(qx, qy, qz) which are not related by symmetry. The q1 phase corresponds to a uniaxial antiferromagnetic arrangement of the Er moments along b associated with a 2a cell enlargement. The refined moment value at 1.5 K is μEr=7.6(1) μB. This ordering is stable only below T1=1.8 K. Above T1, a first-order transition accompanied by hysteresis effects, leads to the appearance of a second set of magnetic reflections. These reflections are associated with a wave vector in a general reciprocal lattice position q2=(qx, qy, qz) that has a temperature dependent length and orientation. At T1 the two phases coexist and are present in equal amounts. The wave vector components of the q2 phase at T1 are qx=0.1262(3), qy=0.022(2) and qz=0.2273(3) r.l.u. The structure corresponds to a sine wave modulated structure with an amplitude of 5.31 μB polarized along b. This structure dominates the high-temperature region, T>T1. Below T1 it is still present as a metastable phase but its relative amount decreases with decreasing temperature. The same holds for the q1 phase which exists as a metastable phase above T1. The coexistence region on heating spans the interval T1±0.45 K.

The temperature dependence of the anisotropy field of LaFe 12 O 19 was measured from 4.2 K up to the Curie temperature. Below 110 K LaFe 12 O 19 exhibits a structural change from the hexagonal to an orthorhombic structure with spacegroup Cmcm. The origin is related to a change of valence at the 2a site from Fe 3 + to Fe 2 + in order to achieve charge compensation, since La 3 + replaces Sr 2 + in this material. This becomes visible in the isomer shift as detected with Fe 57 Mossbauer spectroscopy at the Fe 2a site. In addition an anomalous behaviour of the hyperfine field was observed, indicating the presence of an unquenched orbital momentum. This explains the strong increase of the anisotropy field at low temperatures and the high values of the magnetostriction constant, observed in the experiment.

Neutron diffraction performed on Pr 6 Fe 13 Ag and Pr 6 Fe 13 Au powder samples reveals a collinear antiferromagnetic ordering of the four Fe and the two Pr sublattices associated with the wave vector q =(001) ( I P magnetic lattice with anticentering translation). The moments of all sublattices located at ( xy 0) layers, sandwiched between successive X layers perpendicular to z at z =−0.25, 0.25, are confined to the same direction within the ( xy 0) plane. They change their direction collectively when going to the next Ag(Au) layers at z =0.25, 0.75. The Pr moments at 1.5 K are 2.5(2)μ B and 2.6(1)μ B /Pr atom, while the average Fe ordered value is 2.2μ B /Fe atom, in agreement with Mossbauer results. Detailed results are presented for Pr 6 Fe 13 Ag.

The relation between crystal structure, microstructure and magnetic structure in TbFe6Ge6 is derived on the basis of neutron powder diffraction for samples annealed at 925, 800 (part I) and 650°C. All samples display a density modulation of only the guest subsystem (Tb/Tb* and Ge5/Ge5*) described by the wave vector q =( 1 2 , 1 2 , 0) of the orthohexagonal YCo6Ge6 structure. The occupancy of the Tb site is 1 for the sample with Tan=925°C and decreases to 0.830(6) for the sample with Tan=650°C. At high temperatures (HT) T1

The magnetic structure of the compound DyNiSi 2 (CeNiSi 2 type of structure) has been studied by neutron diffraction and magnetic measurements. The paramagnetic neutron data confirmed the structure reported earlier for this compound. The analysis of the data collected in the magnetically ordered regime ( T N =24 K) showed the occurence of a two step magnetic ordering described by the presence of three wave vectors. In a very narrow region immediately below T N the ordering is described by a transversal sine wave modulated structure associated with the wave vector q 2 =1/5 b *. Below 22 K a phase transition takes place manifested by the appearance of higher harmonics pertaining to the wave vectors q 1 =(010)=5 q 2 and q 3 =3 q 2 . The I ( q 1 ) intensities increase at the cost of the I ( q 2 ) and I ( q 3 ) ones. The intensities I ( q 1 ) and I ( q 2 ) coexist down to 1.5 K in varying proportions while the only very weak reflection of q 3 disappears down to 1.5 K. The wave vector q 1 =(010) pertains to a collinear moment arrangement of the Dy moments along c with the magnetic space group C P (22′2 1 ′/m′cm)i′ S 57 391 . The refined moment value at 1.5 K is μ Dy =7.5(1) μ B .

Powder neutron diffraction data for NiCr2S4have been collected over the temperature range 1.8≤T≤286 K. The Cr3S4structure (space groupI2/m) is adopted at all temperatures. Refinement of 286 K data (a=5.8954(14) A,b=3.4066(8) A,c=11.0589(22) A,β=91.362(4)°) demonstrates that the distribution of nickel and chromium cations between sites in a fully occupied metal layer and sites in an ordered vacancy layer is close to that expected for the normal structure type. Data collected at 1.8 K (a=5.8621(5) A,b=3.4051(2) A,c=11.0297(9) A,β=91.607(6)°) reveal the presence of long-range magnetic order, involving a doubling of the unit cell in both theaandcdirections. Cations in the fully occupied metal layer possess an ordered magnetic moment of 1.57(6)μBwhich is directed parallel to the cation layers. Cations in the vacancy layer have a moment of 1.36(8)μBwhich is directed toward the anion layers. The magnetic ordering temperature was determined as 180(5) K. These results are correlated with magnetic susceptibility and electrical conductivity data.

The magnetic ordering of the novel orthorhombic compound Er3Ge4 (Cmcm) with two Er sites has been studied by neutron diffraction and magnetic measurements.Er3Ge4 orders antiferromagnetically below TN = 7.3 K. The magnetic moments of the Er1 atoms (at 8(f)) make an angle of 35.5(3)° with the y-axis while the moments of the Er2 atoms (at 4(a)) are perpendicular to it within the (O, y, z) plane. This triangular antifferomagnetic momentsarrangement of the two sublattices in the plane (O, y, z) relates to the stacking of the rare earth triangular prism slabs along the a axis. Adjacent prisms along b or c are shifted by x = 1 2 and display different chiralities. The 1.5 K ordered magnetic moment values of Er1 and Er2 are 7.32(5) μB/Er and 6.37(6) μB/Er, respectively. The two Er sublattices order simultaneously but display a completely different thermal behaviour indicating the existence of competing interactions. The magnetic space group Cp ( 2 2′2′ m ′ c m ) (Sh57391) has a primitive lattice associated with the wave vector q = (0 1 0).