FeB-type Structure Research Articles

New Minerals from Inclusions in Corundum Xenocrysts from Mt. Carmel, Israel: Magnéliite, Ziroite, Sassite, Mizraite-(Ce) and Yeite.

Our nanomineralogical investigation of melt inclusions in corundum xenocrysts from the Mt. Carmel area, Israel has revealed seven IMA-approved new minerals since 2021. We report here four new oxide minerals and one new alloy mineral. Magnéliite (Ti3+2Ti4+2O7; IMA 2021-111) occurs as subhedral crystals, ~4 μm in size, with alabandite, zirconolite, Ti,Al,Zr-oxide, and hibonite in corundum Grain 767-1. Magnéliite has an empirical formula (Ti3+1.66Al0.13Ti4+0.15Mg0.10Ca0.01Sc0.01)Σ2.06 (Ti4+1.93Zr0.08)Σ2.01O7 and the triclinic P1¯ Ti4O7-type structure with the cell parameters: a = 5.60(1) Å, b = 7.13(1) Å, c = 12.47(1) Å, α = 95.1(1)°, β = 95.2(1)°, γ = 108.7(1)°, V = 466(2) Å3, Z = 4. Ziroite (ZrO2; IMA 2022-013) occurs as irregular crystals, ~1-4 μm in size, with baddeleyite, hibonite, and Ti,Al,Zr-oxide in corundum Grain 479-1a. Ziroite has an empirical formula (Zr0.72Ti4+0.26Mg0.02Al0.02Hf0.01)Σ1.03O2 and the tetragonal P42/nmc zirconia(HT)-type structure with the cell parameters: a = 3.60(1) Å, c = 5.18(1) Å, V = 67.1(3) Å3, Z = 2. Sassite (Ti3+2Ti4+O5; IMA 2022-014) occurs as subhedral-euhedral crystals, ~4-16 μm in size, with Ti,Al,Zr-oxide, mullite, osbornite, baddeleyite, alabandite, and glass in corundum Grain 1125C1. Sassite has an empirical formula (Ti3+1.35Al0.49Ti4+0.08Mg0.07)Σ1.99(Ti4+0.93Zr0.06Si0.01)Σ1.00O5 and the orthorhombic Cmcm pseudobrookite-type structure with the cell parameters: a = 3.80(1) Å, b = 9.85(1) Å, c = 9.99(1) Å, V = 374(1) Å3, Z = 4. Mizraite-(Ce) (Ce(Al11Mg)O19; IMA 2022-027) occurs as euhedral crystals, <1-14 μm in size, with Ce-silicate, Ti-sulfide, Ti,Al,Zr-oxide, ziroite, and thorianite in corundum Grain 198-8. Mizraite-(Ce) has an empirical formula (Ce0.76Ca0.10La0.07Nd0.01)Σ0.94(Al10.43Mg0.84Ti3+0.60Si0.09Zr0.04)Σ12.00O19 and the hexagonal P63/mmc magnetoplumbite-type structure with the cell parameters: a = 5.61(1) Å, c = 22.29(1) Å, V = 608(2) Å3, Z = 2. Yeite (TiSi; IMA 2022-079) occurs as irregular-subhedral crystals, 1.2-3.5 μm in size, along with wenjiite (Ti5Si3) and zhiqinite (TiSi2) in Ti-Si alloy inclusions in corundum Grain 198c. Yeite has an empirical formula (Ti0.995Mn0.003V0.001Cr0.001)(Si0.996P0.004) and the orthorhombic Pnma FeB-type structure with the cell parameters: a = 6.55(1) Å, b = 3.64(1) Å, c = 4.99(1) Å, V = 119.0(4) Å3, Z = 4. The five minerals are high-temperature oxide or alloy phases, formed in melt pockets in corundum xenocrysts derived from the upper mantle beneath Mt. Carmel.

CrB-type, ordered α-MnB: Single crystal structure and spin-canted magnetic behavior

Manganese monoboride has a low- (α) and a high-temperature (β) modification, as well as a defect-rich low-temperature variant (α′). The crystal structure (FeB-type structure, s.g. Pnma) and properties of high-temperature MnB are well-known. In this work, single crystals were grown via chemical vapor transport reactions, both of β-MnB and the low-temperature modification, α-MnB. This allowed for determining the crystal structure of defect-free α-MnB [CrB-type structure, s.g. Cmcm, a = 3.0098(6) Å, b = 7.6390(2) Å, and c = 2.94620(6) Å]. Furthermore, α′-MnB, the stacking fault-dominated CrB-variant, was obtained as crystalline powder and characterized by X-ray powder diffraction and transmission electron microscopy. Direction-resolved measurements of the magnetic properties of α-MnB revealed spin-canted magnetic behavior along c and ferromagnetism along a and b with a Curie temperature of 456 K; ferromagnetic β-MnB has a Curie temperature of 568 K.

New Insights into the Crystal Chemistry of FeB-Type Compounds: The Case of CeGe.

Several alkaline earth or rare earth binary monosilicides and -germanides possess complex bonding properties, such as polycation formation exceeding the scope of classical electron counting rules. In this study, we present characterization by powder and single-crystal diffraction and thermal analysis of CeGe, one of the few monogermanides crystallizing in the FeB-type structure. Comparative computational investigations for structure types experimentally observed for monogermanides and alternative structures with different structural motifs were performed to gain energetical insights into this family of compounds, underlining the preference for infinite germanium chains over other structural motifs. Formation enthalpy calculations and structural chemical analysis highlight the special position of FeB-type compounds among the monogermanides.

The crystal chemistry of ZrSi

Reported is a revised crystal structure of β–ZrSi (TlI/CrB structure type), correcting the atomic position and bond distances. The Si–Si bond length has been modified substantially from 2.723(6) Å to 2.4411(8) Å. The β-ZrSi single crystals were grown from an arc-melted button, and were characterized using single crystal X-ray diffraction. A survey of the TlI/CrB structure type shows that changes to the nomenclature would be useful, separating it into four chemically distinct subtypes: TlI, CrB, CaSi, and ThCo. β–ZrSi is an example of the CaSi subtype. α-ZrSi crystallizes in the related FeB structure type, which is also divided here into the subtypes FeB, CeSi, and YNi. The effect of electron count on the relative phase stabilities of the CaSi and CeSi subtypes is rationalized with the aid of the Zintl concept and electronic structure calculations in the Linear Muffin-Tin Orbital (LMTO) basis.

Crystal structure and magnetism of rare earth intermetallic compound Dy0.33Ho0.33Er0.33Ni

Multi-component rare earth intermetallic compound Dy0.33Ho0.33Er0.33Ni has been prepared by arc melting and characterized by powder X-ray diffraction and magnetization experiments. The sample crystallizes in orthorhombic FeB-type structure at 300 K and ferromagnetically orders around 40 K (TC). The TC is close to the average value of the ferromagnetic transition temperatures of DyNi, HoNi and ErNi compounds. Magnetization has been measured as a function of field up to 15 kOe and low field magnetocaloric effect around the magnetic transition temperature is computed.

Controllable magnetic transitions and magnetocaloric effect of Ho1-xTmxNi (0≤x≤0.8) compounds

Polycrystalline Ho1-xTmxNi (0≤x≤0.8) compounds were successfully prepared by an arc-melting method, and the crystal structure, magnetic properties and magnetocaloric effect (MCE) were investigated in detail. X-ray diffraction (XRD) results indicate that Ho1-xTmxNi (0≤x≤0.8) compounds are pure phases with FeB-type orthorhombic structure (space group Pnma). Magnetic measurements show that HoNi alloy undergoes a spin reorientation (SR) transition and a ferromagnetic (FM) to paramagnetic (PM) transition at 13.5 K and 35.5 K, respectively. Rare earth atoms Tm with small spin were used to substitute Ho atoms in HoNi compound in order to adjust the magnetic phase transitions and to further optimize the magnetocaloric effect (MCE). With increasing Tm content from 0 to 0.8, the refrigerant temperature span decreases from 41.6 K to 17.3 K. When the content of Tm is 0.3, a platform-shaped isothermal magnetic entropy change (-ΔSM) curve is obtained and the value of -ΔSM is relatively stable over 35 K. Our findings are of great importance for HoTmNi compounds in practical applications.

On the Origin of the Negative Thermal Expansion Behavior of YCu

Among the intermetallics and alloys, YCu is an unusual material because it displays negative thermal expansion without spin ordering. The mechanism behind this behavior that is caused by the structural phase transition of YCu has yet to be fully understood. To gain insight into this mechanism, we experimentally examined the crystal structure of the low-temperature phase of YCu and discuss the origin of the phase transition with the aid of thermodynamics calculations. The result shows that the high-temperature (cubic CsCl-type) to low-temperature (orthorhombic FeB-type) structural phase transition is driven by the rearrangement of three covalent bonds, namely, Y-Cu, Y-Y, and Cu-Cu, which compete for the bonding energy and phonon entropy. At low temperatures, the mixing of Y and Cu does not take place easily because of the weak attractive force between these atoms expected from the small negative mixing enthalpy. This causes all three interactions to take part in the bonding, and Y and Cu are segregated to form an FeB-type structure, which is stabilized by internal energy. At higher temperatures, Cu ions are bound loosely with Y ions due to the large Y-Cu distance (3.01 Å), which results in large vibration entropy and stabilizes a CsCl-type crystal structure. In addition, the CsCl-type structure is reinforced by the Y-Y interaction between next-nearest neighbors, resulting in a smaller unit cell volume. The crystal structure has the simple cubic framework of Y containing Cu ions bound loosely at the cavity sites. The calculated frequency of the Y-like phonon modes is much higher than that of the Cu-like modes, indicating the presence of Y-Y covalent interactions in the CsCl-type phase.

Magnetocaloric effect in textured rare earth intermetallic compound ErNi

Melt-spun ErNi crystallizes in orthorhombic FeB-type structure (Space group Pnma, no. 62) similar to the arc-melted ErNi compound. Room temperature X-ray diffraction (XRD) experiments reveal the presence of texture and preferred crystal orientation in the melt-spun ErNi. The XRD data obtained from the free surface of the melt-spun ErNi show large intensity enhancement for (1 0 2) Bragg reflection. The scanning electron microscopy image of the free surface depicts a granular microstructure with grains of ∼1 μm size. The arc-melted and the melt-spun ErNi compounds order ferromagnetically at 11 K and 10 K (TC) respectively. Field dependent magnetization (M-H) at 2 K shows saturation behaviour and the saturation magnetization value is 7.2 μB/f.u. for the arc-melted ErNi and 7.4 μB/f.u. for the melt-spun ErNi. The isothermal magnetic entropy change (ΔSm) close to TC has been calculated from the M-H data. The maximum isothermal magnetic entropy change, -ΔSmmax, is ∼27 Jkg-1K-1 and ∼24 Jkg-1K-1 for the arc-melted and melt-spun ErNi for 50 kOe field change, near TC. The corresponding relative cooling power values are ∼440 J/kg and ∼432 J/kg respectively. Although a part of ΔSm is lost to crystalline electric field (CEF) effects, the magnetocaloric effect is substantially large at 10 K, thus rendering melt-spun ErNi to be useful in low temperature magnetic refrigeration applications such as helium gas liquefaction.

Effect of rapid quenching on the magnetism and magnetocaloric effect of equiatomic rare earth intermetallic compounds RNi (R = Gd, Tb and Ho)

Magnetocaloric effect (MCE) in RNi (where R=Gd, Tb and Ho) compounds has been studied in their arc-melted and melt-spun forms. The compound GdNi has the orthorhombic CrB-type structure (Space group Cmcm, No. 63) and the compound HoNi has the orthorhombic FeB-type structure (Space group Pnma, No. 62) at room temperature regardless of their synthesis condition. However, arc-melted TbNi orders in a monoclinic structure (Space group P21/m, No. 11) and when it is rapidly quenched to a melt-spun form, it crystallizes in an orthorhombic structure (Space group Pnma, No. 62). The arc-melted GdNi, TbNi and HoNi compounds order ferromagnetically at ∼69K, ∼67K and ∼36K (TC) respectively. While the melt-spun GdNi shows about 6K increase in TC, the ordering temperature of TbNi remains nearly the same in both arc-melted and melt-spun forms. In contrast, a reduction in TC by about 8K is observed in melt-spun HoNi, when compared to its arc-melted counterpart. Isothermal magnetic entropy change, ∆Sm, calculated from the field dependent magnetization data indicates an enhanced relative cooling power (RCP) for melt-spun GdNi for field changes of 20kOe and 50kOe. A lowered RCP value is observed in melt-spun TbNi and HoNi. These changes could have resulted from the competing shape anisotropy and the granular microstructure induced by the melt-spinning process. Tailoring the MCE of rare earth intermetallic compounds by suitably controlled synthesis techniques is certainly one of the directions to go forward in the search of giant magnetocaloric materials.

Structural and Magnetic Properties of MnxB100−x Alloys

Mn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">100-x</sub> alloys (x = 30-60) were fabricated by arc melting technique. Structural analysis showed that alloy with Mn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">55</sub> B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">45</sub> atomic composition has a single-phase orthorhombic FeB-type MnB structure. As-synthesized alloy has a saturation magnetization of 130 emu/g at 300 K, which is the largest value previously reported. Saturation magnetization increases rapidly from 3.5 to 130 emu/g as x increases from 30 to 55. Further increase of the x causes the reduction of the saturation magnetization to 50 emu/g. Magnetic properties of the MnB alloy have been also evaluated by first-principles density functional theory calculations. Electronic structure calculations were performed using full potential linearized augmented plane wave method and a spin polarized generalized gradient approximation. A 1.914 μ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</sub> magnetization per MnB unit was calculated, which corresponds to 158 emu/g at 0 K. Curie temperature (T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> ) of the Mn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">55</sub> B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">45</sub> was measured as 566 K which is in good agreement with the values reported for MnB alloy.

La2NiSb – A Ternary Ordered Version of the Bi3Ni Type with Highly Polar Bonding

Abstract The lanthanum-rich antimonide La2NiSb was synthesized by annealing a cold-pressed pellet of the elements in a sealed silica glas tube at 1120 K. La2NiSb was characterized by powder and single-crystal X-ray diffraction: ordered Bi3Ni type, Pnma, Z =4, a=825.6(3), b=452.2(2), c=1195.5(4) pm, wR=0.0695, 856 F2 values, 26 variables. The nickel atoms form infinite zigzag chains (259 pm Ni-Ni) with trigonal-prismatic lanthanum coordination for each nickel atom. The antimony atoms cap the rectangular faces of the lanthanum prisms (336 pm La-Sb) and thereby coordinate also the nickel atoms (271 pm Ni-Sb). These rods run parallel to the b axis and form a herringbone pattern, similar to the FeB-type structure of GdNi. Although metallic conductivity is expected for La2NiSb from DFT-based band structure calculations, the real-space bonding analysis shows prominent localization of electrons on antimonide anions and positively charged lanthanum cations. The chain substructure is strongly bonded by polar covalent Ni-Sb and multicenter Ni-Ni interactions. The nickel atoms, which are involved in multicenter bonding with adjacent nickel and lanthanum atoms, provide a conductivity pathway along the prismatic strands. 121Sb Mössbauer spectroscopic data at 78 K show a single signal at an isomer shift of -7.62(3)mms-1, supporting the antimonide character. La2NiSb shows weak paramagnetism with a susceptibility of 2.5 x 10-3 emu mol-1 at room temperature.

Anisotropic magnetic properties and giant magnetocaloric effect of single-crystal PrSi

A single crystal of PrSi was grown by the Czochralski method in a tetra-arc furnace. Powder x-ray diffraction of the as-grown crystal revealed that PrSi crystallizes in an FeB-type structure with space group Pnma (No. 62). The anisotropic magnetic properties were investigated by means of magnetic susceptibility, isothermal magnetization, electrical transport, and heat capacity measurements. Magnetic susceptibility data clearly indicate the ferromagnetic transition in PrSi with a ${T}_{C}$ of 52 K. The relative easy axis of magnetization was found to be the [010] direction. Heat capacity data confirm the bulk nature of the transition at 52 K and exhibit a huge anomaly at the transition. A sharp rise in the low-temperature heat capacity has been observed (below 5 K) which is attributed to the ${}^{141}$Pr nuclear Schottky heat capacity arising from the hyperfine field of the Pr moment. The estimated Pr magnetic moment 2.88 ${\ensuremath{\mu}}_{\mathrm{B}}$/Pr from the hyperfine splitting is in agreement with the saturation magnetization value obtained from the magnetization data measured at 2 K. From the crystal electric field analysis of the magnetic susceptibility, magnetization, and heat capacity data it is found that the degenerate $J=4$ Hund's rule derived state of the Pr${}^{3+}$ ion splits into nine singlets with an overall splitting of 284 K, the first excited singlet state separated by just 9 K from the ground state. The magnetic ordering in PrGe appears to be due to the exchange-generated admixture of low-lying crystal field levels. The magnetocaloric effect (MCE) has been investigated from magnetization data along all three principal crystallographic directions. The large magnetic entropy change, $\ensuremath{-}\ensuremath{\Delta}{S}_{M}=$22.2 J/kg K, and the relative cooling power, RCP = 460 J/kg, characteristic of the giant magnetocaloric effect are achieved near the transition temperature (${T}_{C}$ = 52 K) for $H=$ 70 kOe along $[010]$. Furthermore, the PrSi single crystal exhibits a giant MCE anisotropy.

Structural stability of intermetallic compound YCu studied by ab initio total energy calculation

Abstract High pressure effect on the structural polymorphy of YCu was studied based on the first principle density functional theory. The calculated values of lattice parameters are in good agreement with the available experimental results. The geometry optimization results indicate the anisotropy of the compressibility of FeB-type YCu under high pressure. It is remarkable that the compressibilities along the b - and c -axes experience an abnormality at about 3.3 GPa. Energy calculation results demonstrate that YCu is stable in the FeB-type structure at low temperature and ambient pressure, which is in good agreement with the experimental results. And there exists a structural phase transition from FeB-type structure to CsCl-type structure at the transition pressure of approximate 6.7 GPa. This is the quantitative theoretical prediction of structural phase transition, and it still awaits the experimental confirmation.

Anisotropic magnetic properties and superzone gap formation in CeGe single crystal

Single crystals of CeGe and its non-magnetic analog LaGe have been grown by the Czochralski method. The CeGe compound crystallizes in the orthorhombic FeB-type crystal structure with the space group Pnma (#62). The anisotropic magnetic properties have been investigated for well oriented single crystals by measuring the magnetic susceptibility, electrical resistivity and heat capacity. It has been found that CeGe orders antiferromagnetically at 10.5 K. Both transport and magnetic studies have revealed large anisotropy, reflecting the orthorhombic crystal structure. The magnetization data at 1.8 K reveal metamagnetic transitions along the [010] direction at 4.8 and 6.4 T and along the [100] direction at a critical field of 10.7 T, while the magnetization along the [001] direction increases linearly without any anomaly up to a field of 16 T. From the magnetic susceptibility and the magnetization measurements it has been found that the [010] direction is the easy axis of magnetization. The electrical resistivity along the three crystallographic directions exhibits an upturn at TN, indicating superzone gap formation below TN in this compound. We have performed crystal electric field analysis on the magnetic susceptibility and the heat capacity data and found that the ground state is a doublet, and the energies of splitting from the ground state to the first and second excited doublet states were estimated to be 39 and 111 K, respectively.

Anomalous magnetic ground state in PrSi evidenced by the magnetocaloric effect

Previous investigations into the physical properties of PrSi have shown this compound to order ferromagnetically at 54 K. However, the magnetic ground state has not yet been determined unambiguously. PrSi crystallizes in the orthorhombic FeB-type structure. Typically, the crystalline electric field (CEF) would uplift the degeneracy of the J=4 Pr3+ free-ion ground state multiplet, yielding nine (normally non-magnetic) singlets. It is known that magnetic order in such systems may be established via the admixture of two singlets into a doublet state, analogous to TmNi, which crystallizes in the same structure. From symmetry considerations, collinear ferromagnetically ordered moments should lie parallel to the crystallographic b-axis. However, neutron diffraction experiments have shown that the easy magnetic axis lies in the ac-plane. This is the first suggestion that the ground state in PrSi is not determined by the CEF alone. Here, we investigate the ground state properties of PrSi by analyzing its specific heat. A Schottky contribution associated with the thermal population of CEF-split energy levels is absent from the 4f-electron contribution to the specific heat and the magnetic configurational entropy points to a full nine-fold degenerate J=4 multiplet magnetic ground state. The strongest evidence for such a ground state is found when the magnetocaloric effect (MCE) in this system is considered. Furthermore, the MCE indicates the presence of a significant higher order exchange term in the magnetic Hamiltonian.

Lanthan-Triel/Tetrel-ide La(Al,Ga)x(Si,Ge)1-x. Experimentelle und theoretische Studien zur Stabilität intermetallischer 1 : 1-Phasen / Lanthanum Triel/Tetrel-ides La(Al,Ga)x(Si,Ge)1−x. Experimental and Theoretical Studies on the Stability of Intermetallic 1:1 Phases

Systematic studies of the phase formation at the binary sections LaSi - LaGa and LaGe - LaAl have been carried out by means of synthetic, crystallographic and bond theoretical methods. The hightemperature forms of the two binary monotetrelides LaSi and LaGe crystallize with the FeB structure type, whereas LaGa forms the related CrB type and LaAl the significantly different CeAl type. Starting from LaSi/LaGe, the FeB type (orthorhombic, space group Pnma, Z = 4, a = 839.2(1)/842.7(1), b = 399.9(1)/412.3(1), c = 606.2(2)/612.2(1) pm, R1 = 0.0356/0.0298) remains stable only down to a valence electron number per M atom (M = Si, Ge, Al, Ga) of 6.9 (LaGa0.10Si0.90: a = 840.14(7), b = 404.12(12), c = 608.5(2) pm, R1 = 0.0513; LaAl0.15Ge0.85: a = 845.40(7), b = 414.08(13), c = 614.08(14) pm, R1 = 0.0213). In the system LaGaxSi1−x, the stability range of the CrB type (orthorhombic, space group Cmcm, Z = 4) starts at a gallium proportion of 25% (LaGa0.25Si0.75: a = 450.03(8), b = 1140.5(2), c = 406.05(6) pm, R1 = 0.0163) and extends to the border compound LaGa (v. e./M = 6). The CrB type also occurs in the system La-Al-Ge, but is in this case only formed around the 1 : 1 composition in between LaAl0.42Ge0.58 (a = 455.90(12), b = 1161.1(3), c = 418.05(9) pm, R1 = 0.0474) and LaAl0.61Ge0.39 (a = 454.89(10), b = 1168.8(2), c = 420.41(11) pm, R1 = 0.0447). These stability ranges, the variations of several key geometric parameters such as the M-M distances or the heights of the trigonal prisms, and the main aspects of the chemical bonding in these lanthanum monometallides are analyzed using FP-LAPWband structure methods. The structure of the new compound La2Al2Ge (V2B3 structure type, orthorhombic, space group Cmcm, a = 416.69(4), b = 2719.7(4), c = 450.46(5) pm, Z = 2, R1 = 0.0458) combines the structural elements of the CrB/FeB structure family (two-bonded M atoms) with the trigonally planar bonded M atoms of the ThSi2 type in a fully ordered Al and Ge atom distribution and thus without phase width

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.

CaAg – SrAg – BaAg: Zwischen CrB‐ und FeB‐Strukturtyp

AbstractCaAg – SrAg – BaAg: Between CrB and FeB Structure TypeDepending 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 against each other. In systematic studies of the pseudo‐binary sections CaxSr1−xAg and SrxBa1−xAg three 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 determined on the basis of single crystal x‐ray data. Starting from CaAg (CrB type, stacking c), up to 25 percent of Sr (i.e. in Ca0.75(2)Sr0.25(2)Ag: orthorhombic, space group Cmcm, a = 413.4(1), b = 1157.8(3), c = 467.4(2) pm, Z = 4, R1 = 0.0453) can be substituted without a change in the structure type. At a 1:1 ratio of Ca and Sr, a new structure type with stacking sequence (hc5)2 is formed (Ca0.48(3)Sr0.52(3)Ag: space group Pnma, a = 4800.1(1), b = 471.8(12), c = 624.11(9) pm, Z = 24, R1 = 0.0881). Further reduced cubic stacking ((hc2)2) is observed in the Sr rich region with Ca proportions between 39 and 25(2) % (space group Pnma, x = 0.39(1)/0.25(2): a = 2412.2(3)/2434.67(4), b = 472.75(5)/473.97(8), c = 626.17(7)/629.2(3) pm, Z = 12, R1 = 0.0430/0.0489) where the HT‐TbNi structure type is formed. The stability range of the SrAg structure type ((hc)2) starts at a Ca content of 5 % (Ca0.05Sr0.95Ag: space group Pnma, a = 1643.1(2), b = 476.79(7), c = 636.27(9) pm, Z = 8, R1 = 0.0318). From a new second modification of SrAg (space group Pnma, a = 4131.5(4), b = 478.79(6), c = 638.81(6) pm, Z = 20, R1 = 0.0394) up to a Ba proportion of 8 percent, the α‐CaCu structure type with sequence (hch2c)2 is formed. The stability range of the FeB structure with purely hexagonal stacking extends from Sr0.84(1)Ba0.16(1)Ag (a = 830.78(12), b = 479.35(8), c = 644.93(11) pm, Z = 4, R1 = 0.0413) to the border phase BaAg. In an accompanying computational study, the chemical bonding in the binary compounds CaAg, SrAg and BaAg is analyzed using FP‐LAPW band structure methods. Together with geometrical criteria, the results are used to compare and discuss the stability of the different stacking variants of the polar intermetallic 1:1 phases inbetween the CrB and the FeB structure type.

The Densest Packing of AB Binary Hard-Sphere Homogeneous Compounds across all Size Ratios

This paper considers the homogeneous packing of binary hard spheres in an equimolar stoichiometry, and postulates the densest packing at each sphere size ratio. Monte Carlo simulated annealing optimizations are seeded with all known atomic inorganic crystal structures, and the search is performed within the degrees of freedom associated with each homogeneous AB structure type. Structures isopointal to the FeB structure type are found to have the highest packing fraction at all sphere size ratios. The optimized structures match or improve on the best previously demonstrated packings of this type, and show that compound structures can pack more densely than segregated close-packed structures at all radius ratios less than 0.62.

PAC study of martensitic transformations in RCu compounds

The martensitic transformation from the cubic CsCl-type to the orthorhombic FeB-type structure of RCu (R=Gd, Tb and Y) compounds has been investigated by perturbed angular correlation (PAC) measurements of the nuclear electric quadrupole interaction (QI) at 111Cd probe nuclei on the Cu site as a function of temperature. The QI data indicate that the martensitic transition extends over a finite temperature range and affects only part of the samples. The transition ranges are 190 K< T<250 K, 80 K< T<150 K and 60 K< T<120 K for GdCu, YCu and TbCu, respectively, with saturation values between 40 and 60 percent. The temperature dependence of the QI parameters in the FeB-type structure is independent of the R constituent. The order of magnitude of the magnetic hyperfine interaction in the FeB-type structure has been estimated from the low temperature PAC spectra.