CrB Structure Type Research Articles

DFT-Guided Crystal Structure Redetermination and Lattice Dynamics of the Intermetallic Actinoid Compound UIr.

UIr has been discussed as a rare example of a noncentrosymmetric, ferromagnetic superconductor crystallizing in the acentric PdBi structure type (P21, mP16). Here we present a new structure model for UIr. By means of single-crystal and powder X-ray diffraction we find UIr to crystallize in the centrosymmetric space group P21/c, in line with previous ab initio calculations. The discrepancy with the previous noncentrosymmetric model in space group P21 is explained by the occurrence of twinning. The observed twinning hints toward a high-temperature displacive phase transition of UIr to the CrB structure type (Cmcm, oS8): we discuss the lattice dynamics corresponding to this transition by crystallographic symmetry mode analysis and by density functional theory (DFT). We find that spin-orbit coupling is essential to understand this phase transition. We apply our theoretical considerations for a critical judgment of the structure models of UPt and NpIr that have been reported to crystallize isotypically with UIr. We confirm that UPt is isotypic to UIr (P21/c), whereas we predict NpIr to crystallize in the CrB structure type. Our report on the centrosymmetric crystal structure of UIr has an effect on all those theoretical models that investigated potentially novel superconducting coupling mechanisms of this compound on the basis of the noncentrosymmetric structure model.

A tenfold twin of the CrB structure type

NiZr crystallized from an amorphous matrix or solidified from an undercooled melt exhibits a tenfold twinned microstructure, which is explained by an ideal twin model utilizing special geometric properties of the CrB structure type. The model is unique in several ways: (i) it contains no adjustable parameters other than a scaling factor accounting for the smallest interatomic distance; (ii) it features an irrational shift in the translational part of the twin operation; and (iii) it has many traits commonly observed for quasicrystals, connected to the occurrence of decagonal long-range orientational order, making NiZr the first experimental example of the recently introduced concept of {\bb Z}-module twinning. It is shown how these remarkable properties of the tenfold twin's structure model are related to one another and founded in number theory as well as in the mathematical theory of aperiodic order.

Nucleation study for an undercooled melt of intermetallic NiZr.

Electrostatic levitation is applied in order to undercool liquid glass forming NiZr significantly below its melting temperature. For NiZr large undercoolings are found to be highly reproducible with this experimental method. One single NiZr sample of high purity is undercooled 200 consecutive times which leads to a distribution function of undercooling temperatures. Within a statistical approach of classical nucleation theory, the undercooling distribution is analyzed yielding parameters, e.g., a pre-exponential factor of KV ≈ 1035 m-3 s-1, which indicates homogeneous nucleation. This result is consistent with the crystallization behavior of NiZr at high undercooling and with the corresponding microstructural analysis. Since NiZr is a representative of the very common CrB structure type, with 132 isostructural phases existing, understanding its nucleation behavior adds important knowledge to the nucleation of binary alloys in general.

Magnetic properties of the orthorhombic NdPd

Abstract The equiatomic NdPd compound crystallizes in the orthorhombic CrB structure type ( oS 8, Cmcm , No. 63). The NdPd phase melts congruently at 1240 °C, as observed by differential thermal analysis; one further sharp thermal effect detected at 1040 °C is very likely due to a structural transition. We confirm the CrB prototype for the low-temperature form of NdPd. The lattice parameters of this compound are a =3.842(2) A, b =10.776(7) A, c =4.605(2) A, as obtained from Guinier powder pattern; those for the corresponding iso-structural LaPd compound, prepared as non-magnetic reference, are a =3.947(2) A, b =11.036(3) A, c =4.663(2) A. Despite the fact that NdPd has been known since long, its physical properties have not been investigated till date. Here we report the results of magnetization, heat capacity and electrical resistivity measurements performed on this compound. NdPd undergoes a single ferromagnetic transition close to 15 K, inferred from a sharp upturn in the magnetization at lower temperatures and from Arrott plots measured at selected temperatures between 1.9 and 18 K. The coercive field and remnant magnetization at 1.9 K are 320 Oe and 0.24 μ B /f.u., which become negligible at 15 K. A sharp peak in the heat capacity at ≈15 K confirms the bulk magnetic transition. Isothermal magnetization at 2 K shows a tendency to saturation, reaching a value of 1.9 μ B /f.u. at the maximum applied field of 70 kOe. The zero field resistivity shows an anomaly near 15 K, in correspondence with the magnetic and heat capacity data. A negative magnetoresistivity, typical of a ferromagnet, is observed in the magnetically ordered state in an applied magnetic field of 50 kOe. LaPd is a typical Pauli paramagnet with a Sommerfeld coefficient γ =3.9 mJ/mol K 2 .

The binary tin-rich lanthanum stannides La 2Sn 5 and La 3Sn 7—A structural and bond theoretical study

The three tin-rich lanthanum stannides LaSn 3, La 2Sn 5 and La 3Sn 7, which were known only from indexed powder diagrams, have been synthesized from melts of the elements by slow cooling of sealed sample tubes or arc melting and quenching. Their crystal structures were refined from X-ray powder diffractometer data using the Rietveld method. LaSn 3 (cubic, space group Pm 3 ¯ m , a=477.77(1) pm, Z=1, R F 2 = 0.044 ) crystallizes with the common Cu 3Au structure type, which can be described either as an ordered close packing or as a network of [Sn 6] octahedra ( closo clusters) connected via common vertices. The structures of La 2Sn 5 (Ce 2Sn 5 structure type, orthorhombic, space group Cmmm, a=463.85(3), b=3581.3(3), c=469.83(4) pm, Z=4, R F 2 = 0.075 ) and La 3Sn 7 (Gd 3Sn 7 structure type, orthorhombic, space group Cmmm, a=457.83(4), b=2628.40(2), c=467.99(4) pm, Z=2, R F 2 = 0.068 ) , which retain some similarities to ordered close packings as well, show 2D blocks of corner-sharing [Sn 6] octahedra terminated by [Sn 5] square pyramids ( nido clusters). In between these layers, isolated Sn zigzag chains similar to those observed in LaSn (CrB structure type) are interspersed. A simple v.e. count after Zintl and Wade/Jemmis for the polyanionic Sn building blocks resulting from these structure descriptions fits the observed v.e. numbers of the three compounds almost exactly. It is also possible to explain the minima of the tDOS close to E F, which have been calculated within the framework of the FP-LAPW DFT approach. However, the simple assumption of an exclusively Sn- p bonded polyanion would neglect the large Sn–La bonding contributions and the reduced amount of electron transfer from La to Sn when compared to related alkaline earth stannides.

ChemInform Abstract: Crystal Structure of the Compound DyGe3.

Abstract Earlier investigations had indicated the presence of rare earth germanium compounds of a composition RGe x with x close to 3 of unknown structure. Neutron diffraction data obtained on the corresponding dysprosium compound show that the stoichiometry is DyGe 3 . This compound crystallizes in a novel type of structure related to the CrB structure type. The structure is orthorhombic and comprises one dysprosium site and three germanium sites. It can be thought to be built up from DyGe by inserting slabs of trigonal prisms consisting of germanium atoms into the CrB type unit cell of DyGe.

(Ca/Sr)Au xCd 1− x: Stacking variants of the CrB–FeB series

The structural chemistry of binary 1:1 alkaline earth metallides A II M ( M = p -block or late transition element) is dominated by planar M zig-zag chains, which are stacked in different orientations (CrB ( c) to FeB ( h) type) and with variable stacking distances (types I and II). As a case study of the electronic influences, the substitution of Au against Cd in the respective Ca and Sr aurides was examined by means of experimental, crystallographic and computational methods. Starting from CaAu, up to 11% of Au can be substituted by Cd without a change in the CrB structure type (orthorhombic, space group Cmcm, a = 398.2 ( 1 ) , b = 1122.6 ( 6 ) , c = 460.9 ( 2 ) pm , Z = 4 , R 1 = 0.0303 ). Starting from SrAu (stacking sequence ( hc ) 2 ( h 2 c ) 2 ), depending on the proportion of the Cd substitution a successive change to structures with increased hexagonality is observed: In SrAu 0.93 Cd 0.07 (monoclinic, space group P 2 1 / m , a = 621.3 ( 4 ) , b = 472.4 ( 2 ) , c = 1216.1 ( 9 ) pm , β = 96.97 ( 5 ) ∘ , Z = 6 , R 1 = 0.0467 ) the stacking sequence is h 2 c , i.e. the hexagonality is 66.67%. A slightly more increased Cd content in SrAu 0.78 Cd 0.22 (orthorhombic, space group Pnma , a = 3243.3 ( 8 ) , b = 474.17 ( 8 ) , c = 626.20 ( 9 ) pm , Z = 16 , R 1 = 0.0682 ) drives the hexagonality to 75%, with a ( h 3 c ) 2 stacking sequence known from several rare earth nickel compounds. Further Cd substitution is not possible. However, in the Cd-rich section of the two series, where the CsCl / β -brass structure type occurs for both alkaline earth elements, a small Au substitution, as determined from powder data by Rietveld refinements, is possible. The substitution limit and the stability ranges of the CsCl and the CrB type can be rationalized from the calculated band structures. Geometrical and electronic criteria are used to compare and discuss the stability ranges in a structural map.

CaAgxZn1– x: Zwischen CrB- und FeB-Strukturtyp – Eine Studie zu elektronischen Einflüssen auf die Stapelung von Zick-Zack-Ketten in polaren Erdalkalimetall-Monometalliden / CaAgxZn1−x: Between CrB and FeB Structure Type – A Study on Electronic Factors Influencing the Stacking of Zig-zag Chains in Polar Alkaline Earth Monometallides

Depending on both electronic (valence electron numbers) and geometric (atom size ratios) characteristics of the contributing elements, the 1 : 1 compounds AIIM of the heavier alkaline earth elements A and electron-rich transition metals M form the well known CrB or FeB structure types. Both structure types exhibit M zig-zag chains, which are stacked in different orientations. In systematic studies of the pseudo-binary section CaAgxZn1−x four new ternary phases with different stacking variants between the CrB (cubic stacking, c) and the FeB (hexagonal stacking, h2) structure type have been prepared and characterized on the basis of single crystal X-ray data. Starting from CaAg (CrB type, orthorhombic, space group Cmcm, a = 405.22(7), b = 1144.7(2), c = 464.43(11) pm, Z = 4, R1 = 0.0197), up to 24% of Ag (CaAg0.76Zn0.24: a = 408.6(2), b = 1144.3(5), c = 460.7(2) pm, R1 = 0.0208) can be substituted by zinc without a change in the structure type. Close to the 1 : 1 ratio of Ag and Zn, the HT-TbNi structure type with the stacking sequence (hc2)2, i. e. 33% hexagonality (CaAg0.52Zn0.48: orthorhombic, space group Pnma, a = 2345.47(6), b = 454.370(10), c = 609.950(10) pm, Z = 12, R1 = 0.0298) is formed, followed by the SrAg type with 50% hexagonality (CaAg0.48Zn0.52: orthorhombic, space group Pnma, a = 1571.0(2), b = 451.50(7), c = 609.80(9) pm, Z = 8, R1 = 0.0733). The amount of hexagonal stacking is further increased with increasing Zn content in CaAg0.33Zn0.67 (Gd0.7Y0.3 structure type, h2c stacking, 67% hexagonality, monoclinic, space group P21/m, a = 610.39(9), b = 448.53(5), c = 1195.7(2) pm, β = 96.829(14)◦, Z = 3, R1 = 0.0221). Finally, a pure hexagonal stacking sequence, i. e. the FeB structure type (orthorhombic, space group Pnma, Z = 4) is observed from CaAg0.14Zn0.86 (a = 804.57(2), b = 443.050(10), c = 611.350(10) pm, R1 = 0.0131) to CaAg0.06Zn0.94 (a = 806.1(3), b = 441.0(2), c = 610.4(3) pm, R1 = 0.0255). Intriguingly, the series ends with the binary compound CaZn, which again crystallizes with the CrB structure type exhibiting cubic stacking of the zig-zag chains only (0% hexagonality). In an accompanying computational study, the chemical bonding in the series Ca(Ge/Ga/Zn/Ag) of isotypic binary metallides with variable valence electron numbers has been analyzed using FP-LAPW band structure methods. The electronic structures of the two border stacking variants are compared using the crystal data of CaZn (CrB type) and CaAg0.06Zn0.94 (FeB type). Geometrical and electronic criteria are used to compare and discuss the stability ranges of the different stacking variants inbetween the CrB and the FeB structure type found in polar intermetallic 1 : 1 phases.

Structure and bonding of ternary gallides CaGa1−x(Si/Sn/Al/In)x with the CrB type and related structures

Abstract The polar binary alkaline earth ( A II ) tetrelides A II M IV are known to crystallize with the CrB type, where the two-bonded M atoms of planar zig-zag chains (6 v.e./M) are surrounded by stretched trigonal prisms of A cations. In contrast, in the only known isotypic trielide, CaGa (with only 5 v.e./M) these prisms are substantially compressed. Hence, the phase widths of mixed gallides/tetrelides A II Ga 1 − x M x IV ( M IV = Si , Sn) forming the CrB structure type and mixed trielides CaGa 1 − x ( Al / In ) x of related types were studied synthetically, structurally and by theoretical calculations. All compounds were synthesized from stoichiometric mixtures of the elements and characterized by single crystal X-ray diffraction. DFT band structure calculations were carried out for the binary border compounds. In mixed tetrelides CaGa 1 − x Si x , the phase width of the CrB type reaches from CaSi up to x = 0.42 ; in CaSn the maximum of Ga substitution is 0.41, whereas nearly no tetrel substitution is observed in CaGa. The geometrical changes (inter- and intra-chain M–M distances) during these substitutions (involving reduction of v.e./M) as well as the observed substitution limits and the changes of the bonding character in the planar chains are explained by the results of the band structure calculations. Partial exchange of Ga in CaGa against Al or In results in a series of mixed trielides with large monoclinic commensurately modulated superstructures of the CrB type: these phases, CaGa 0.67 Al 0.33 (space group C 2 / c , a = 2717.7 ( 12 ) , b = 1159.9 ( 3 ) , c = 607.4 ( 2 ) pm, β = 92.42 ( 3 ) ° , Z = 36 , R 1 = 0.0374 ), CaGa 0.84 In 0.16 (space group P 2 1 / c , a = 1222.9 ( 8 ) , b = 1167.9 ( 5 ) , c = 612.0 ( 3 ) pm, β = 95.24 ( 3 ) ° , Z = 16 , R 1 = 0.0558 ) and CaGa 0.55 In 0.45 (space group P 2 1 / c , a = 936.1 ( 6 ) , b = 1188.1 ( 9 ) , c = 618.5 ( 3 ) pm, β = 101.73 ( 4 ) ° , Z = 12 , R 1 = 0.0793 ) show only very small phase widths. In their structures the b axis is unchanged with respect to the CrB type and the enlargement of the a – c basis can be described by a general transformation matrix. In all three structures, the M chains are of wavelike form, varying the inter-chain distances such that Al and In atoms tend to approach each other, whereas Ga tends to form short intra-chain distances while avoiding close inter-chain contacts.

Crystal structures of the compounds RAl0.5−xSi0.5+x (R = La, Ce, Pr, Nd, Sm, Gd), R3Al4Si6 (R = La, Pr), and RAlSi2 (R = Pr, Nd)

Nine ternary compounds were found in R–Al–Si systems (R=rare-earth element) and their crystal structures were determined by X-ray powder diffraction. Compounds RAl0.5−xSi0.5+x with CrB structure type (oS8, Cmcm) were found for R=La, Ce, Pr, Nd, Sm; compounds R3Al4Si6 with Ce3Al4Si6 structure type (hP13, P-3m1) for R=La, Pr; and compounds RAlSi2 with CeAlSi2 structure type (hP8, P-3m1) for R=Pr, Nd. The structure of the compound PrAl0.5Si0.5 was refined from X-ray powder diffraction data (RB=0.038): Pr in 4(c), y=0.3566(1), and a statistical mixture Al0.5Si0.5 in 4(c), y=0.0731(5). Within the rows of isostructural compounds, the volumes of the unit cells regularly decrease on increasing the atomic number of the rare-earth element.

Structure and Properties of the New PhosphidesM2M′P (M=Zr, Hf;M′=Co, Ni) and Their Relations to ZrNi and HfNi

The isostructural title compounds can be obtained by arc-melting of cold-pressed mixtures ofMP,M, andM′ in the ratio 1:1:1 (M=Zr, Hf;M′=Co, Ni). Their crystal structure (space groupP21/m, No. 11,Z=2) can be regarded as a new superstructure of the CrB structure type. Thus, the structure ofM2NiP can be derived formally from ZrNi or HfNi (both CrB type) by replacing every second Ni atom by P in such a way that P–P contacts are minimized. Condensed channels formed by zirconium and hafnium, respectively, build up an extended early-transition-metal framework. The late-transition-metal atoms and P are situated in singly capped trigonal prisms of theMsublattice, forming –Ni–P– zigzag chains. The expectation of metallic properties is confirmed by Extended Hückel calculations and experimentally observed Pauli paramagnetism. The differences between Co and Ni on the one hand and between Zr and Hf on the other hand are discussed, as well as the changes from MNi toM2NiP.

Hf2NiP: The Planned Modification of an Intermetallic Phase by (Formal) Substitution of Nickel by Phosphorus

Planned synthesis in solid-state chemistry: Formal substitution of every other Ni atom in HfNi by P led, as predicted, to the synthesis of Hf2NiP, which crystallizes in a monoclinic superstructure of the CrB structure type (structure depicted on the right; • = Ni, ˆ = P, ◯ = Hf). The resulting structural and physical differences between the phases HfNi and Hf2NiP are discussed.

Crystal structure of the compound DyGe 3

Earlier investigations had indicated the presence of rare earth germanium compounds of a composition RGe x with x close to 3 of unknown structure. Neutron diffraction data obtained on the corresponding dysprosium compound show that the stoichiometry is DyGe 3. This compound crystallizes in a novel type of structure related to the CrB structure type. The structure is orthorhombic and comprises one dysprosium site and three germanium sites. It can be thought to be built up from DyGe by inserting slabs of trigonal prisms consisting of germanium atoms into the CrB type unit cell of DyGe.

Structure and magnetic properties of the α-FeB phase obtained by mechanical working

The crystal structure and the magnetic properties of the low-temperature α-FeB phase obtained by working in a mill are investigated by X-ray diffraction analysis, Mossbauer study, and magnetic measurements. The crystal structure of α-FeB is determined as a CrB(Bf) structure type with a = 0.2951 nm, b = 0.7564 nm, and c = 0.2953 nm. Parameters of the hyperfine interactions in α-FeB are also determined. The Mossbauer data analysis suggests that the observed change of Heff in α-FeB in comparison with the value for the β-modification is related with the change of orientation of the magnetic moments of Fe atoms making an angle of ≈40° with respect to the crystallographic b-axis. It is shown that β- and α-FeB have close values of magnetization and Curie temperature Tc = 580 K. [Russian Text Ignored.]

The stable and metastable phase relations in the HfBe alloy system

Ultra-rapid liquid-quenching (> 10 5 °C s −1), in splat-foil and continuous ribbon form, has been found to produce fully glassy structures in the HfBe system. The glass formation range is 30–60 at.% Be which brackets the eutectic composition of ~ 42 at.% Be. Extended solubility of Be in the b.c.c. terminal solid solution was also observed. A metastable ordered phase was found at 50 at.%; m- HfBe is an orthorhombic B f , CrB-structure type, that is crystallographically related to the stable HfBe 2 phase. Calculations of the critical cooling rate for glass formation were carried out and computed values of T ̇ c are in reasonable agreement with experimental estimations. The composition dependences of certain physical properties of the glasses were also determined.

Metallic glass formation and properties in Zr and Ti alloyed with Be—I the binary Zr-Be and Ti-Be systems

Ultra-rapid liquid-quenching (> 10 5°C s −1) has been found to produce fully glass structures in the binary Zr-Be and Ti-Be alloy systems over composition ranges that bracket the respective eutectics Zr-34.5 at.%Be and Ti-37.5 at.%Be. Both systems also exhibit extended solubility of Be in the Zr and Ti b.c.c. terminal solid solutions. In addition, these studies revealed the existence of metastable ordered phases at 50 at.%Be in the two systems: m- ZrBe (orthorhombic B ƒ, CrB structure type) and m- TiBe (cubic B2, CsCl structure type). The experiments clearly indicate that critical cooling rates for glass formation in Ti-Be alloys are significantly higher than in Zr-Be alloys; consequently, only the Zr-Be glasses are readily fabricable in continuous ribbon form. Various criteria for establishing glass forming tendency (GFT) were examined and detailed calculations of critical cooling rates (Ṫ) using the Davies-Uhlmann kinetic formulations were carried out and compared with the experimental results. The composition dependences of certain physical properties of the glasses were also examined and are discussed.

CrB-type equiatomic compounds of europium, ytterbium and alkaline-earth metals with Si, Ge, Sn, Pb

Some MX compounds of Eu and Yb with Si, Ge, Sn, and Pb have been prepared together with the corresponding compounds of the alkaline-earth elements. The crystal structures of YbSi, EuSi, BaSi, EuGe, SrGe, BaGe, EuSn, SrSn, BaSn, and SrPb have been determined. These compounds crystallize with the CrB structure type and their lattice constants, atomic parameters and interatomic distances are reported.

Alkaline earth silicides, germanides and stannides with CrB structure type

Alkaline earth silicides, germanides and stannides with CrB structure type

Gadolinium and dysprosium intermetallic phases. I. The crystal structures of DyGa and GdPt and their related compounds

The intermetallic phnses DyGa, DyGe, GdGa, and GdGe are isostructural with the CrB structure type. The phases GdPt, DyPt, GdNi, and DyNi have structures which are only slight distortions of the FeB structure type. The structures were determined by a combinatibn of single crystal and powder methods. (auth)