Structural chemistry of superconducting pnictides and pnictide oxides with layered structures

Bärnighausen Trees – A group–subgroup reference database

Group–subgroup schemes are a useful tool in crystal chemistry for systemizing crystal structures and they are an indispensable help during X-ray crystallographic studies of complex, twinned and modulated structures. Meanwhile many superstructure variants are summarized within so-called Bärnighausen trees. The present database lists relevant literature with respect to the crystallographic/group-theoretical tools and original work and gives a tabulated overview on the crystallographic fingerprints (aristotype, space group symbol, Pearson code and Wyckoff sequence) of the respective superstructures.

Structural diversity among multinary pnictide oxides: a minireview focused on semiconducting and superconducting heteroanionic materials

Incorporating different anions with varied ionic sizes and charges is a rapidly growing approach to bring out unusual physical properties among various classes of solid-state materials, pnictides and chalcogenides in particular. This minireview is focused on hetero-anionic materials based on the pnictogens, which have been demonstrated to offer an impressive diversity of crystal chemistry and electronic structures. In addition, many pnictide oxides or oxypnictides, over the course of the last decade, have been shown to exhibit a broad spectrum of superconducting, magnetic, and semiconducting properties. However, the structural diversity of the mixed-anion materials is far greater than the several known structure types, or their variants, of the well-known layered superconductive materials. Therefore, with this treatise, we aim to provide a comprehensive overview of the crystal chemistry of pnictide oxides by recounting almost 40 different structures of such ternary and multinary compounds. In addition to the structural aspects, we also highlight some of the challenges associated with the synthesis, and briefly summarize reported, to date, physical properties of this remarkable class of solids.

Coloring and distortion variants of the bcc packing and for the aristotypes BaAl4 and CeMg2Si2

The huge number of intermetallic structure types with many representatives calls for structural systemization. The combination of crystal chemistry with group theory is an efficient tool for such systemization and can be displayed in a concise and compact way via group-subgroup schemes. The present overview deals with such group-subgroup schemes (Bärnighausen trees) for coloring and distortion variants of the bcc packing as well as superstructures that derive from the aristotypes BaAl4 and CeMg2Si2.

Centrosymmetric LaRh2Ga2

Polycrystalline samples of LaRh2Ga2 with the centrosymmetric CaBe2Ge2 type structure were obtained by arc-melting. Small single crystals were grown through a special annealing sequence in an induction furnace. The structures of two different crystals were refined from diffractometer data, confirming the centrosymmetric structure. The 2c Rh sites were refined with anharmonic atomic displacement parameters.

Group-subgroup schemes for MoNi4, Nb4N5, KxFe2−ySe2, Nd10Au3As8O10 and CsInCl3: i5 superstructures of I 4/m allowing atom, charge or vacancy ordering

Isomorphic symmetry reductions are a special class of klassengleiche symmetry reductions where the subgroup has the same space group type. The present contribution highlights five different isomorphic transitions of index 5 (i5), all from I 4/m to I 4/m. These superstructure formations allow (i) atom ordering in MoNi4, (ii) vacancy ordering in Nb4N5, KxFe2−ySe2 and Nd10Au3As8O10 and (iii) charge ordering in CsInCl3 (≡Cs2InIInIIICl6). The group-subgroup schemes in the Bärnighausen formalism are discussed along with the crystal chemical consequences.

Evidence for negative thermal expansion in the superconducting precursor phase SmFeAsO

The fluorine-doped rare-earth iron oxypnictide series SmFeAsO1-x F x (0 [Formula: see text] 0.10) was investigated with high resolution powder x-ray scattering. In agreement with previous studies (Margadonna et al 2009 Phys. Rev. B. 79 014503), the parent compound SmFeAsO exhibits a tetragonal-to-orthorhombic structural distortion at [Formula: see text]  =  130 K which is rapidly suppressed by [Formula: see text] 0.10 deep within the superconducting dome. The change in unit cell symmetry is followed by a previously unreported magnetoelastic distortion at 120 K. The temperature dependence of the thermal expansion coefficient [Formula: see text] reveals a rich phase diagram for SmFeAsO: (i) a global minimum at 125 K corresponds to the opening of a spin-density wave instability as measured by pump-probe femtosecond spectroscopy (Mertelj et al 2010 Phys. Rev. B 81 224504) whilst (ii) a global maximum at 110 K corresponds to magnetic ordering of the Sm and Fe sublattices as measured by magnetic x-ray scattering (Nandi et al 2011 Phys. Rev. B 84 055419). At much lower temperatures than [Formula: see text], SmFeAsO exhibits a significant negative thermal expansion on the order of  -40 ppm · K-1 in contrast to the behaviour of other rare-earth oxypnictides such as PrFeAsO (Kimber et al 2008 Phys. Rev. B 78 140503) and the actinide oxypnictide NpFeAsO (Klimczuk et al 2012 Phys. Rev. B 85 174506) where the onset of [Formula: see text] 0 only appears in the vicinity of magnetic ordering. Correlating this feature with the temperature and doping dependence of the resistivity and the unit cell parameters, we interpret the negative thermal expansion as being indicative of the possible condensation of itinerant electrons accompanying the opening of a SDW gap, consistent with transport measurements (Tropeano et al 2009 Supercond. Sci. Technol. 22 034004).

First-Order Phase Transition in BaNi2Ge2 and the Influence of the Valence Electron Count on Distortion of the ThCr2Si2 Structure Type

Structural instability has a strong influence on the understanding of superconductivity in iron-containing 122 phases. Similar to the 122 iron-based high-temperature superconductors, the intermetallic compound BaNi₂Ge₂ undergoes an orthorhombic-to-tetragonal structural phase transition. The compound was prepared by arc-melting mixtures of the elements under an argon atmosphere. Single crystals were obtained by a special heat treatment in a welded tantalum ampule. The crystal structure of the compound was investigated by powder and single-crystal X-ray diffraction. Differential thermal analysis of BaNi₂Ge₂ showed a reversible phase transition at ca. 480 °C. In situ temperature-dependent synchrotron powder X-ray diffraction studies revealed that below 480 °C the crystal structure of BaNi₂Ge₂ is orthorhombic [own structure type, space group Pnma, a = 8.3852(4) Å, b = 11.3174(8) Å, and c = 4.2902(9) Å at 30 °C] and the high-temperature phase above 510 °C belongs to the tetragonal ThCr₂Si₂-type structure [space group I4/mmm, a = 4.2664(1) Å, and c = 11.2537(3) Å at 510 °C]. The reversible first-order low-temperature ↔ high-temperature phase transition around 480 °C is associated with distortion of the [Ni₂Ge₂] layer of low-temperature modification. The anisotropy of thermal expansion of the unit cell in BaNi₂Ge₂ was analyzed. The crystal chemistry and chemical bonding are discussed in terms of linear muffin-tin orbital band structure calculations and a topological analysis using the electron localization function. In related compounds, the level of distortion of the uncollapsed tetragonal ThCr₂Si₂-type structure depends on the valence electron count (VEC).

The quaternary arsenide oxides Ce9Au5–x As8O6 and Pr9Au5–x As8O6

The quaternary gold arsenide oxides Ce9Au5−x As8O6 and Pr9Au5−x As8O6 were synthesized from the rare earth elements (RE), rare earth oxides, arsenic and gold powder at maximum annealing temperatures of 1173 K. The structures were refined from single crystal X-ray diffractometer data: Pnnm, a=1321.64(6) pm, b=4073.0(3), c=423.96(2), wR2=0.0842, 3106 F 2 values, 160 variables for Ce9Au4.91(4)As8O6 and Pnnm, a=1315.01(4), b=4052.87(8), c=420.68(1) pm, wR2=0.0865, 5313 F 2 values, 160 variables for Pr9Au4.75(1)As8O6. They represent a new structure type and show a further extension of pnictide oxide crystal chemistry. A complex polyanionic gold arsenide network [Au5As8]15− (with some disorder in the gold substructure) is charge compensated with polycationic strands of condensed edge-sharing O@RE 4/4 and O@RE 4/3 tetrahedra ([RE 4O3]2 12+) as well as RE 3+ cations in cavities.

A study of temperature dependent local atomic displacements in a Ba(Fe(1-x)Co(x))2As2 superconductor

We have studied the local structure of a Ba(Fe(1-x)Co(x))2As2 superconductor using temperature dependent extended X-ray absorption fine structure (EXAFS) measurements. Polarized EXAFS at the Fe K-edge on an optimally doped (x = 0.06) single crystal has permitted us to determine atomic displacements across the superconducting transition temperature (T(c)). The Fe-As bondlength hardly shows any change with temperature; however, the Fe-Fe sublattice reveals a sharp anomaly across T(c), indicated by a significant drop in mean square relative displacements, similar to the one known for cuprates and A15-type superconductors. We have also found a large atomic disorder around the substituted Co, revealed by polarized Co K-edge EXAFS measurements. The Co-Fe/Co bonds are more flexible than the Fe-Fe bonds with the As-height in Co-containing tetrahedra being larger than the one in FeAs4. The results suggest that the local Fe-Fe bondlength fluctuations and the atomic disorder in this sub-lattice should have some important role in the superconductivity of Ba(Fe(1-x)Co(x))2As2 pnictides.

La3Cu4P4O2 and La5Cu4P4O4Cl2: synthesis, structure and 31P solid state NMR spectroscopy

The phosphide oxides La3Cu4P4O2 and La5Cu4 P4O4Cl2 were synthesized from lanthanum, copper(I) oxide, red phosphorus, and lanthanum(III) chloride through a ceramic technique. Single crystals can be grown in a NaCl/KCl flux. Both structures were refined from single crystal X-ray diffractometer data: I4/mmm, a = 403.89(4), c = 2681.7(3) pm, wR2 = 0.0660, 269 F2 values, 19 variables for La3Cu4P4O2 and a = 407.52(5), c = 4056.8(7) pm, wR2 = 0.0905, 426 F2 values, 27 variables for La5Cu4P4O4Cl2. Refinement of the occupancy parameters revealed full occupancy for the oxygen sites in both compounds. The structures are composed of cationic (La2O2)2+ layers and covalently bonded (Cu4P4)5– polyanionic layers with metallic characteristics, and an additional La3+ between two adjacent (Cu4P4)5– layers. The structure of La5Cu4P4O4Cl2 comprises two additional LaOCl slabs per unit cell. Temperature-dependent magnetic susceptibility studies revealed Pauli paramagnetism. The phosphide substructure of La3Cu4P4O2 was studied by 31P solid state NMR spectroscopy. By using a suitable dipolar re-coupling approach the two distinct resonances belonging to the P2 4– and the P3– units could be identified.

New quaternary arsenide oxides with square planar coordination of gold(I) - structure, (197)Au Mössbauer spectroscopic, XANES and XPS characterization of Nd10Au3As8O10 and Sm10Au3As8O10

The quaternary gold(I) arsenide oxides Nd10Au3As8O10 and Sm10Au3As8O10 were synthesized in sealed quartz ampoules from the rare earth (RE) elements, their appropriate sesquioxides, arsenic, arsenic(III) oxide and finely dispersed gold at maximum annealing temperatures of 1223 K. Both structures were refined from X-ray single crystal diffractometer data at room temperature and at 90 K. Nd10Au3As8O10 and Sm10Au3As8O10 crystallize with a new structure type that derives from the BaAl4 structure through distortions and formation of ordered vacancies. The structures consist of stacked polycationic [RE10O10](10+) layers with oxygen in tetrahedral rare earth coordination and polyanionic [Au(I)3(As2)4](10-) layers with gold in square planar or rectangular planar coordination of four arsenic dumbbells (255 pm As1-As2). In contrast to the well known ionic rare earth oxide layers, the gold arsenide layers rather show covalent bonding and account for the metallic nature of these two new arsenide oxides. This is confirmed by electronic structure calculations and resistivity measurements. The oxidation state of gold was investigated by (197)Au Mössbauer, X-ray absorption near edge structure (XANES) and photoelectron (XPS) spectroscopy. Due to missing comparative gold arsenide compounds, the monovalent gold phosphide oxides RE2AuP2O were measured for comparison. The XANES measurements additionally comprise monovalent gold arsenides REAuAs2. The XPS study contains BaAuAs as reference compound instead. Combination of all data clearly indicates Au(I), which was not observed in square planar coordination up to now. Temperature dependent magnetic susceptibility data show Curie-Weiss paramagnetism for Nd10Au3As8O10 and no magnetic ordering down to 2.5 K. Sm10Au3As8O10 shows the typical Van Vleck type paramagnetism for samarium compounds along with a transition to an antiferromagnetically ordered state at TN = 8.6 K.

Intra-unit-cell nematic charge order in the titanium-oxypnictide family of superconductors

Understanding the role played by broken-symmetry states such as charge, spin and orbital orders in the mechanism of emergent properties, such as high-temperature superconductivity, is a major current topic in materials research. That the order may be within one unit cell, such as nematic, was only recently considered theoretically, but its observation in the iron-pnictide and doped cuprate superconductors places it at the forefront of current research. Here, we show that the recently discovered BaTi2Sb2O superconductor and its parent compound BaTi2As2O form a symmetry-breaking nematic ground state that can be naturally explained as an intra-unit-cell nematic charge order with d-wave symmetry, pointing to the ubiquity of the phenomenon. These findings, together with the key structural features in these materials being intermediate between the cuprate and iron-pnictide high-temperature superconducting materials, render the titanium oxypnictides an important new material system to understand the nature of nematic order and its relationship to superconductivity.

Synthesis and Characterization of Superconducting Ca₁-xNaxFFeAs

A representative of the fluoride-containing iron pnictide high-temperature superconductors, namely CaFFeAs, was doped with sodium up to the composition Ca_0.86Na_0.14FFeAs for the first time. Single crystals with an edge length in the range of 0.1 - 2.0 mm were obtained via solid-state and flux syntheses, respectively. The composition of the crystals was verified by means of single crystal X-ray diffractometry and energy dispersive X-ray spectroscopy (EDX). Measurements of the electrical resistivity, as well as the magnetization on a crystal of Ca_0.89Na_0.11FFeAs both show a transition to the superconducting state on cooling to 34.5 K. Investigations of the upper critical fields reveal an anisotropy ratio of about five. The lattice parameters and molar volumes increase with rising sodium content. This effect is clearly observable for the c-axis and the volume, whereas the increase of the a-axis is rather minor.

Study of the electronic and magnetic properties as a function of isoelectronic substitution in SmFe(1-x)RuxAsO0.85F0.15

We have studied the electronic and magnetic properties of SmFe(1-x)RuxAsO0.85F0.15 (x = 0, 0.05, 0.25, 0.33, 0.5) by high-resolution x-ray absorption and x-ray emission spectroscopy. The local Fe magnetic moment (μ) tends to decrease for a small Ru substitution, but it shows a clear increase with further substitution. It appears that impurity scattering prevails in reducing the μ with small Ru substitution due to an extended Ru d-band. A nanoscale phase separation, that decouples the FeAs layers from the spacer layers, drives the increase of μ at higher Ru substitution. The results provide important information on nanoscale phase separation due to isoelectronic substitution in the active layers of iron-based 1111-superconductors and its effect on the local magnetic properties.

Ce4Ag3Ge4O(0.5)--chains of oxygen-centered [OCe2Ce(2/2)] tetrahedra embedded in a [CeAg3Ge4] intermetallic matrix

The oxidation of an intermetallic phase under high-pressure/high-temperature conditions led to the synthesis of Ce4Ag3Ge4O(0.5) exhibiting [OCe2Ce(2/2] tetrahedral chains, in which the oxygen atoms statistically occupy the tetrahedral centres. Starting from a 1:1:1 CeAgGe precursor (NdPtSb type), a multianvil high-pressure/high-temperature experiment at 11.5 GPa and 1250-1300 °C revealed Ce4Ag3Ge4O(0.5), crystallizing in the space group Pnma with the following lattice parameters: a = 2087.3(4), b = 439.9(1), and c = 1113.8(2) pm. Magnetic measurements showed Curie-Weiss behavior above 100 K with an experimental magnetic moment of 2.42 µB per Ce atom, close to the value for the free Ce(3+) ion, clearly indicating trivalent cerium in Ce4Ag3Ge4O(0.5). Full potential GGA+U band structure calculations resulted in metallic properties and a magnetic ground state with one unpaired 4f-electron per cerium in agreement with the experiments.

Crystallographic and magnetic phase transitions in the layered ruthenium oxyarsenides TbRuAsO and DyRuAsO

The crystallographic and physical properties of TbRuAsO and DyRuAsO at and below room temperature are reported, including full structure refinements from powder X-ray diffraction data and measured electrical and thermal transport properties, magnetic susceptibility, and heat capacity. Both compounds are isostructural to LaFeAsO (ZrCuSiAs-type, P4/nmm) at room temperature. However, DyRuAsO undergoes a symmetry-lowering crystallographic phase transition near 25 K, and adopts an orthorhombic structure (Pmmn) below this temperature. This structural distortion is unlike those observed in the analogous Fe compounds. Magnetic phase transitions are observed in both compounds which suggest antiferromagnetic ordering of lanthanide moments occurs near 7.0 K in TbRuAsO and 10.5 K in DyRuAsO. The nature of the structural distortion as well as thermal conductivity and heat capacity behaviors indicate strong coupling between the magnetism and the lattice. The behaviors of both materials show magnetic ordering of small moments on Ru may occur at low temperatures.