X-ray and neutron powder diffraction study of the monoclinic perovskites Sr2MSbO6 (M=In, Y)

The structures of the perovskites Sr₂InSbO₆ and Sr₂YSbO₆ have been investigated by X-ray and neutron powder diffraction. Both compounds are of monoclinic distortion, space group P2₁/n, with the lattice parameters related to that of the ideal cubic-perovskite (a_p) by a_p, c≈2a_p and β≈90°. The distortions that occur in Sr₂InSbO₆ and Sr₂YSbO₆ can be viewed as due to the octahedral tilts around both the two-fold [110]_p- and the four-fold [001]_p-axis of the cubic aristotype with the oxygen's shifted away from the In/Y(III) ions towards the Sb(V) ions, creating an ordered arrangement of the alternating InO₆/YO₆ and SbO₆ octahedra.     

High-resolution neutron powder diffraction study on the structure of BaPbO3

Using high-resolution time-of-flight neutron powder diffraction, the crystal structure of BaPbO₃ has been reinvestigated at room temperature and 4.2 K. By comparing different structural models, i.e. the orthorhombic Imma and the monoclinic I2/m, it is concluded that the former one describes correctly the structure of BaPbO₃, and no Imma→I2/m phase transition exists in the temperature range investigated. The apparent monoclinic distortion is likely due to the existence of twins that introduce the micro strain resulting in anisotropic line broadening of the observed profiles.     

Magnetic ordering in La0.75Sr0.25CrO3: A neutron diffraction study

Low-temperature neutron diffraction measurements were carried out on a powder sample of the compound La_0.75Sr_0.25CrO₃ in order to elucidate its magnetic structure. Rietveld analysis of the neutron diffraction data, as a function of temperature, showed that it possesses a G-type antiferromagnetic alignment of Cr spins at all temperatures below 300 K. Down to the lowest achievable temperature, viz. 17 K, the Cr site moments were found to be the weighted average of the 75% Cr³⁺ and 25% Cr⁴⁺ spin-only ionic moments. At 17 K, the Cr site moment was 2.71(5) μ_B/Cr ion. There is no observable change in the Cr–O bond lengths as a function of temperature. The tilt angles of the CrO₆ octahedra marginally increase with decreasing temperature.     

Conductivity behaviour of a cubic/tetragonal phase stabilized nanocrystalline La2O3-ZrO2

La₂O₃ doped nanocrystalline zirconia (ZrO₂) was prepared by chemical co-precipitation method for the 3, 5, 8, 10, 15, 20 and 30 mol.% concentrations of La₂O₃. Structural studies were performed using X-ray diffraction (XRD). All the as-synthesized samples were found to be in monoclinic phase. As-synthesized samples were given heat treatment at higher temperatures for tetragonal/cubic structural phase stabilization. Sintering the samples at temperature 1173 K stabilized the tetragonal and cubic phases. A slight shift in the 100% peak of the cubic phase was observed towards the low diffraction angle indicating the substitution of the bigger La³⁺ ion into the ZrO₂ lattice. Grain sizes were found to lie between 10 and 13 nm. Electrical conductivity studies were performed on the cubic phase stabilized La₂O₃-ZrO₂ by complex impedance spectroscopy. The conductivity increases up to the dopant concentration 10 mol.% and then decreases with further increase in La₂O₃ concentration. Initial increase in conductivity is correlated to the stabilization of the cubic phase and the subsequent decrease in the conductivity with the dopant content is interpreted on the basis of the oxygen-ion movement model. Electrical conductivity has contributions from grain and grain boundary regions. But the grain boundary conductivity is slightly higher than the corresponding grain conductivity. Higher grain boundary conductivity shows higher diffusion coefficient for the atoms on the surface of the ZrO₂ grains. The possible mechanism of the oxygen ion conduction in the La₂O₃ stabilized zirconia (LSZ) is reported. The Barton, Nakajima and Namikawa (BNN) relation has been applied to the conductivity data and found that the d.c. and a.c. conductions have been correlated to each other by the same mechanism.     

Low temperature thermodynamical properties of ErCu2Si2

ErCu₂Si₂ crystallises in the tetragonal ThCr₂Si₂-type crystal structure. In this paper results of magnetometric, electrical transport, specific heat as well as neutron diffraction are reported. Results of electrical resistivity and specific heat measurements performed at low temperature yield existence of magnetic ordering roughly at 1.3 K. These results are in concert with neutron diffraction measurements, which reveal simple antiferromagnetic ordering between 0.47 and 1.00 K. At temperatures ranging from 1.00 up to 1.50 K an additional incommensurate magnetic structure was observed. The propagation vector k=(0;0;0.074) was proposed to describe magnetic reflections within the amplitude modulated magnetic structure. Basing on specific heat studies the crystal field levels splitting scheme and magnetic entropy were calculated.     

Neutron diffraction studies of Tb2Rh3Si5 compound

In this work neutron diffraction studies of Tb₂Rh₃Si₅ compound are reported. The compound crystallizes in the monoclinic crystal structure of Lu₂Co₃Si₅-type. At 1.5 K an antiferromagnetic ordering with a propagation vector k=(1/2;1/2;1/2) was observed. The Tb magnetic moments of 9.8(2) μ_B form a non-collinear magnetic structure. In the vicinity of Néel temperature of 8 K a change of the magnetic ordering is evidenced. The change seems to be connected with phase transition from commensurate to incommensurate sine-wave modulation of the Tb magnetic moments.     

Neutron powder diffraction study of the Nd(Ba1−xNdx)2Cu3O7+δ solid solution

The Nd(Ba_1−xNd_x)₂Cu₃O_7+δ solid solution, Nd123ss, has been investigated by neutron powder diffraction and Rietveld analysis. It is confirmed that the crystal structure of its Nd-rich limit, Nd(Ba_0.55Nd_0.45)₂Cu₃O_7.33, is satisfactorily described in the space group Bmmm (a=7.7679(3), b=3.8542(1), and c=22.9590(9) Å). The fourfold superstructure with respect to the orthorhombic cell of YBCO is due to ordering between Ba and Nd atoms in the bridging layer. Differences with previous works concern exclusively the distribution of O atoms in the ‘chain’ layer. Our results give strong indications that ordering also occurs for lower Nd contents.     

Effects of excess oxygen content on the hole-carrying CuO2-layers in Tl2(Ba1−xSrx)2Ca2Cu3Oy superconducting oxides

The effects of oxygen doping on the hole-carrying CuO₂-layers in Tl₂(Ba_1−xSr_x)₂Ca₂Cu₃O_y were studied by combined chemical and valence analysis, T_c measurements and neutron diffraction. The highest T_c is characterized by an optimal excess oxygen content, Δy, dichotomizing the under- and over-doped regions for each Sr concentration. While the average Tl valence is close to 3.0 and independent of Δy, the average Cu valence shows a linear dependence with Δy. An increase of the flatness of the CuO₂ plane, characterized by the O(2)-Cu(2)-O(2) bond angle of ∼176°, was observed at the optimal Δy.     

Some properties of the phonon spectra of transition metal disilicides VSi2, NbSi2, and TaSi2

The phonon spectra of metallic disilicides VSi₂, NbSi₂, and TaSi₂ have been studied in detail by inelastic neutron scattering at 300 K and specific heat measurements between 10 K and 250 K. The specific heat calculated from the generalised phonon density of states extracted from neutron measurements is in good agreement with the measured lattice contribution to the specific heat. The properties of the phonon spectra are discussed in relation with other data reported for these isostructural and isoelectronic disilicides.     

Magnetic properties of NdAu2Ge2

Physical properties of NdAu₂Ge₂, crystallising with the tetragonal ThCr₂Si₂-type crystal structure, were investigated by means of magnetic, calorimetric and electrical transport measurements as well as by neutron diffraction. The compound exhibits antiferromagnetic ordering below T_N=4.5 K with a collinear magnetic structure of the AFI-type. The neodymium magnetic moments are parallel to the c-axis and amount to 1.04(4) μ_B at 1.5 K. The observed magnetic behaviour is strongly influenced by crystalline electric field effect.     

Study of CeNi4Mn by neutron diffraction

We report neutron diffraction measurements on CeNi₄Mn, which has recently been identified as a soft ferromagnet with a sizeable spin-transport polarization. Our data show conclusively that the Mn atoms occupy a unique site (4c) in the unit cell, which has the symmetry of the cubic MgCu₄Sn-type structure. We infer a moment of 4.6 μ_B on Mn at 17 K, which is oriented ferromagnetically along the {101} plane. The amplitude of the Mn vibrational motion is found to be larger than that of Ce and Ni atoms at all temperatures, thereby lending support to theoretical prediction of rattling phonon modes in this compound.     

Pressure-induced structural phase transition in ReO3

We have investigated the pressure-induced structural phase transition in ReO₃ by neutron diffraction on a single crystal. We collected neutron diffraction intensities from the ambient and high pressure phases at P=7 kbar and refined the crystal structures. We have determined the stability of the high pressure phase as a function temperature down to T=2 K and have constructed the (P-T) phase diagram. The critical pressure is P_c=5.2 kbar at T=300 K and decreases almost linearly with decreasing temperature to become P_c=2.5 kbar at T=50 K. The phase transition is driven by the softening of the M₃ phonon mode. The high pressure phase is formed by the rigid rotation of almost undistorted ReO₆ octahedra and the Re-O-Re angle deviates from 180°. We do not see any evidence for the existence of the tetragonal (P4/mbm) intermediate pressure phase reported earlier.     

NMR and NQR studies on superconducting Sr2RuO4

We review our nuclear-magnetic resonance (NMR) and nuclear-quadrupole-resonance (NQR) studies in superconducting Sr₂RuO₄, which have been performed in order to investigate the gap structure and the pairing symmetry in the superconducting state and magnetic fluctuations in the normal state. The spin-lattice relaxation rate (1/T₁) of a high-quality sample with shows a sharp decrease without a coherence peak just below T_c, followed by a T³ behavior down to 0.15 K. This result indicates that the superconducting gap in pure Sr₂RuO₄ is a highly anisotropic character with a line-node gap. The Knight shift, which is related to the spin susceptibility, is unchanged in the superconducting state irrespective of the direction of the applied fields and various magnitude of the field. This result strongly suggests that the superconducting pairs are in the spin-triplet state, and the spin direction of the triplet pairs is considered to be changed by small fields of several hundred Oe.     

One-dimensional bulk ferromagnets: NdAl2 and hcp cobalt

It is shown experimentally that NdAl₂ and hcp cobalt are one-dimensional (1D) bulk ferromagnets. For hcp cobalt this is only under the condition that the sample is magnetically saturated, i.e. that all moments are aligned parallel to the hexagonal c-axis. In 1D magnets the transverse interactions need not to be zero but must be sufficiently weak such that the transverse correlation length does not diverge at the critical temperature. The transverse interactions are then not relevant and the phase transition is driven by the longitudinal interactions. On the other hand, magnetic Bragg scattering relies on finite transverse correlations. For NdAl₂ no conventional magnetic Bragg scattering is observed if all moments are aligned vertical to the scattering plane by a magnetic field. For hcp cobalt the scattering intensity is considerably reduced in this geometry instead of having its maximum. From this observation it can be concluded that the transverse correlation length is practically zero in NdAl₂ but has a finite value in hcp cobalt. The macroscopic magnetization shows normal ferromagnetic saturation.     

Crystal structure and structural transition caused by charge-transfer phase transition for iron mixed-valence complex (n-C3H7)4N[FeFe(dto)3] (dto=C2O2S2)

(n-C₃H₇)₄N[Fe^IIFe^III(dto)₃] shows a new type of first order phase transition called charge-transfer phase transition around 120 K, where the charge transfer between Fe^II and Fe^III occurs reversibly. Recently, we have succeeded in obtaining single crystals of the title complex and determined the crystal structure at room temperature. Crystal data: space group P6₃, Z=2. Moreover, we have investigated the structural transition caused by the charge-transfer phase transition by means of powder X-ray diffraction measurement. When the temperature is decreased, the a-axis, which corresponds to the hexagonal ring size in two-dimensional honeycomb network structure of [Fe^IIFe^III(dto)₃]_∞, contracts by 0.1 Å at the charge-transfer transition temperature (T_CT), while the c-axis, perpendicular to the honeycomb network layer, elongates by 0.1 Å at T_CT. Consequently, when the temperature is decreased, the unit cell volume decreases without noticeable anomaly around T_CT, which is responsible for the quite small vibrational contribution to the entropy change, compared with usual spin crossover transition. Thus, the charge-transfer phase transition around 120 K for (n-C₃H₇)₄N[Fe^IIFe^III(dto)₃] is regarded as spin entropy driven phase transition.     

Determination of antisite defects in double perovskite Sr2FeMoO6 by diffraction method: Simulation studies

The crystal structure of double perovskite Sr₂FeMoO₆ synthesized via solid-state reaction at 1280 °C under a reduction atmosphere is refined by Rietveld technique based on X-ray powder diffraction (XRD) data in 2θ range of 15-140°. An antisite content (AS), i.e. Fe on the Mo sites (=Mo on the Fe sites), of 12.1(1)% is derived. In reference to the refinement results, a series of X-ray and neutron powder diffraction (NPD) data with different antisite contents, ranging from 0 (completely ordered) to 50% (completely disordered), are generated with a Poisonian noise added and subjected to Rietveld refinements with the same initial values for the refinable parameters. The AS is reproduced satisfactorily from the refinement of XRD data and the combined refinement of XRD and NPD data with a relative deviation smaller than 4%, whereas the relative deviation of AS derived from the refinement of NPD data can be as large as 50%. However, the atomic occupancies and isotropic temperature factors of all the atoms can be reasonably reproduced from the refinement of NPD data and the combined refinement of XRD and NPD data, whereas these data can be reproduced only for cations (Sr, Fe, Mo) from the refinement of XRD data. The present simulation studies shed light on understanding the controversial statements derived from XRD and NPD work regarding the antisite defects in Sr₂FeMoO₆, which in turn is indispensable for understanding the mechanism of large room temperature low-field magnetoresistance of the compound.     

Phase coexistence in annealed CaMn7O12

The structural phase transition in annealed CaMn₇O₁₂ has been investigated by using high resolution synchrotron radiation powder diffraction. There is a phase coexistence phenomenon: two different crystallographic phases coexist in the material between 410 and 458 K. The first one is trigonal and it has a charge ordering (CO) of the Mn³⁺ and Mn⁴⁺ ions, while the second one is cubic and charge delocalized (CD). The volume fraction of the CD phase increases with temperature from 22% at 418 K up to 100% at 468 K. Both phases have domains of at least 150 nm at each temperature. The annealing of CaMn₇O₁₂ relaxed a part of the strains in the lattice, but did not influence the phase coexistence phenomenon.     

High Curie temperature in B-site ordered Sr2CrWO6 epitaxial thin films

Epitaxial and c-axis oriented double perovskite Sr₂CrWO₆ thin films were prepared on SrTiO₃ (100) and LaAlO₃ (100) substrates by pulsed-laser deposition. Structural, magnetic and transport properties were found to be sensitive to the gas conditions employed during the deposition. A small amount of oxygen along with Ar during the deposition was found to be essential for B-site ordering; such films displayed lattice parameters close to the bulk value and display ferromagnetic metallic behavior. The Curie temperature observed above 500 K in these films is higher than bulk Sr₂CrWO₆ samples. Films grown without oxygen were observed to have long c-parameter and no B-site ordering; they were non-magnetic and semiconducting.