Gd5Si4-type Structure Research Articles

Unit cell volume reduction of Gd5(Si,Ge)4 nanoparticles controlled by bulk compressibility

The production of Gd5(Si,Ge)4 compounds in reduced dimensionality, through pulsed laser deposition (PLD), have shown their potential for practical applications. Here, we present nanoparticles ranging from 10 to 27 nm of average particle size of Gd5(SixGe1−x)4, with x = 0, 0.45 and 0.60, obtained using an Nd:Yag (1064 nm) and an Excimer KrF laser (248 nm). Synchrotron X-ray Diffraction measurements revealed a reduced unit cell volume in comparison to their bulk counterpart. The x = 0 sample presented a ∼1.99% reduction while x = 0.45 composition, a shrinkage of ∼1.81% on the unit cell volume that are a result of a structural change to a Gd5Si4-type structure [O(I)]. In contrast, x = 0.60 nanoparticles conserve the bulk crystal structure with ∼ 0.95% of volume shrinkage. As a consequence, there is a change on the magnetic transition order from a first to a second one for all nanostructures followed by a magnetocaloric response reduction. These observations unveil a direct correlation between the bulk compressibility values and the unit cell shrinkage, suggesting that the rise of a surface stress plays a major role on the particle and unit cell dimensions.

Influence ofp-Type Double-Doping on the Crystals and Electronic Structures of Two Polar Intermetallics: La4.57(1)Li0.43Ge3.80(3)In0.20and Nd4.32(1)Li0.68Ge3.87(3)In0.13

Two quaternary polar intermetallic compounds La4.57(1)Li0.43Ge3.80(3)In0.20 and Nd4.32(1)Li0.68Ge3.87(3)In0.13 were synthesized using a conventional high temperature synthetic method as we attempted to introduce the p-type double-doping of Li and In for RE and Ge in the RE5-xLixGe4-y (RE = rare-earth metals) system, and their crystal structures were characterized by single crystal X-ray diffraction experiments. The two title compounds crystallize in the orthorhombic space group Pnma (Pearson code oP16, Z = 4) with six crystallographically independent asymmetric atomic sites and adopt the Gd5Si4-type structure. Overall crystal structures of two isotypic title compounds can be described as a 1:1 assembly of the hypothetical 2-dimensional (2D) RE2(RE/Li)(In/Ge)2 layered structure adopting the Mo2FeB2-type structure and the dumbbell-shaped inter-slab (In/Ge)2 dimers bridging two such neighboring 2D layers along the crystallographic b-axis direction. The observed “direction selective” structural transformation from the Sm5Ge4-type to the Gd5Si4-type structure can be understood as a result of the simultaneous double-doping by the relatively smaller amount of Li substitution for La at the RE3 site than that in the La4LiGe4 and the partial In substitution for Ge at both of the M1 and M3 sites. The site-preference of In for two particular anionic sites were thoroughly studied using four hypothetical La4LiGe3In models having different atomic arrangements by the tight-binding linear muffin-tin orbital (TB-LMTO) method. The overall electronic structure and individual chemical bonding influenced by the given double-doping were also discussed on the basis of the density of states (DOS) and crystal orbital Hamilton population (COHP) curves analyses.

Critical magnetic behavior of magnetocaloric materials with the Gd5Si4-type structure

An extensive investigation on the magnetism of Gd5(SixGe1−x)4 compounds which stabilize in the Gd5Si4-type structure is here presented, focusing on the Gd5Si4 and Gd5Si2Ge2 compositions. Through Arrott plot analysis, the temperature dependencies of the spontaneous magnetizations are retrieved and the magnetic interaction coefficients (Jexch) are estimated. The obtained Jexch value for Gd5Si4 is 15% higher than that of Gd5Si2Ge2. Such enhancement cannot be attributed to a magneto-volume coupling only, being mainly associated with the magnetic polarization of the (Si,Ge)-(Si,Ge) interslab connections. The critical exponents were estimated (δ=3.1 and 2.9, γ=1.01 and 1.06, β=0.48 and 0.56, and n=0.67 and 0.63 for Gd5Si4 and Gd5Si2Ge2, respectively) demonstrating that both compositions belong to the same universal class. This conclusion is reinforced by the construction of a universal logarithmic scaling plot of the M(H) isothermal curves. Hence, we conclude that the magnetism of the Gd5Si4-type structure compounds is simply dominated by long range ferromagnetic interactions. This contrasts to other Gd5(SixGe1−x)4 compositions where a strong competition antiferro versus ferromagnetic interactions and short versus long range order are observed. Finally, the Gd5Si4 composition has shown a reduction of 50% on the magnetic entropy change maxima when compared with the Gd5Si2Ge2 compound studied, despite its higher magnetic interaction coefficient value.

Crystal structure and magnetism of Gd5−xEuxGe4

The Gd5−xEuxGe4 phases, designed using the valence electron count (VEC)-structure relationship found in the R5T4 (R=rare-earth, alkali and alkaline-earth metal; T=main-group element) system, have been synthesized by high-temperature reactions. The Gd5−xEuxGe4 compounds with x≤0.25 and VEC≤30.75e−/formula adopt the orthorhombic Sm5Ge4-type structure (space group Pnma) with broken interslab Ge–Ge dimers (dGe–Ge>3.4Å); the phase with x=0.50 and VEC=30.5e−/formula crystallizes with the monoclinic Gd5Si2Ge2-type structure (P1121/a); and the phases with x=1.0–2.0 and VEC=30–29e−/formula have the Gd5Si4-type structure (Pnma) with the intact interslab Ge–Ge dimers (dGe–Ge<2.7Å). The divalent Eu cations are predicted to occupy the largest R site on the surface of the ∝2[R5Ge4] slabs, according to the theoretical analyses of Gd4EuGe4 and the structural studies on Y3Eu2Ge4. The antiferromagnetic ordering of Gd5Ge4 is turned into the ferromagnetic one through the Eu substitution. The ground state is ferromagnetic for all the Eu-substituted phases.

Effect of Small Amount of Zn Additions on the Magnetocaloric Properties of Gd&lt;sub&gt;5&lt;/sub&gt;Si&lt;sub&gt;2&lt;/sub&gt;Ge&lt;sub&gt;2 &lt;/sub&gt;Alloy

The structural and magnetic property of Gd5Si1.99Ge2Zn0.01and Gd5Si2Ge2alloys prepared by arc-melting the starting materials with commercial available purity (99.95wt%) was investigated by x-ray diffraction and Vibrating Sample Magnetometer. The result shows that the Gd5Si1.99Ge2Zn0.01alloy has monoclinic phase with Gd5Si2Ge2-type structure. The polymorphic orthorhombic phase with Gd5Si4-type structure coexists with the monoclinic phase in Gd5Si2Ge2alloy. The addition of small Zn element in Gd5Si2Ge2alloy results in a considerable enhancement of its magnetocaloric effect. The maximum magnetic entropy change rapidly increases from 5.03J/(kg K) to 20.70J/(kg K) for a magnetic field change from 0 to 1.5T. The magnetic order temperature is 278K in Gd5Si1.99Ge2Zn0.01alloy and 278.5K in Gd5Si2Ge2alloy respectively. The magnetocaloric effect of Gd5Si2Ge2with the small addition of Zn is significantly improved.

Structural, magnetic and magnetocaloric properties of the Gd 5Si 4− xSb x ( x=0.5–3.5) phases

Substitution of Sb for Si in the Gd 5Si 4 phase results in the formation of ternary Gd 5Si 4− x Sb x phases with three different structure types. For x≤0.38 the Gd 5Si 4-type structure ( Pnma space group) with all interslab T-T dimers intact exists ( T is Si or Si/Sb mixture), in the x=1.21–1.79 region the Sm 5Ge 4-type structure ( Pnma) with all interslab dimers broken is found. A further increase in the Sb concentration ( x=2.27–3.1) leads to the appearance of the Eu 5As 4-type structure ( Cmca) with broken but equivalent interslab T⋯T bonds. The Gd 5Si 4− x Sb x phases with 1.21≤ x≤3.1 undergo ferromagnetic transitions in the temperature range from 164 to 295 K. Magnetocaloric effect in terms of the isothermal entropy change, Δ S, reaches the maximum values of -3.7(4), -4.2(6) and -4.6(7) J/kg K for the Gd 5Si 2Sb 2, Gd 5Si 1.5Sb 2.5 and Gd 5SiSb 3 samples, respectively.

Phase relationship, microstructure and magnetocaloric effect in Gd1−x(Si0.5Ge0.5)x alloys

Microstructure and magnetocaloric effect were investigated in Gd1−x(Si0.5Ge0.5)x alloys with x = 0.38, 0.41, 0.45, 0.47 and 0.50. The phase identification and compositional analysis were carried out by a combination of x-ray powder diffraction, scanning electron microscopy and electron probe microanalysis. The Gd5(Si,Ge)4 phase was found to have an orthorhombic Gd5Si4-type structure when the coexisting phase was Gd-rich (Gd5Si3-type) and a monoclinic Gd5Si2Ge2-type structure when the coexisting phase was Gd-depleted (Gd5Ge5-type). The magnetocaloric effect was found to depend on the volume fraction and the Si/Ge ratio of the ferromagnetic 5 : 4 phase in the above series of alloys. A maximum magnetocaloric effect (ΔS)M of 14.3 J kg−1 K−1 for a magnetic field change from 0 to 5 T was obtained for the x = 0.45 alloy.

Magnetovolume effect and magnetic transition in Gd5Si4

Magnetocaloric compound Gd5Si4 has a higher Curie temperature TC=336K than that of Gd pure metal (TC=294K). High temperature synchrotron x-ray powder diffraction experiments have been performed for Gd5Si4 from 300to400K. Rietveld fitting of the results indicate that the compound retains the orthorhombic Gd5Si4-type structure with a space group of Pnma in the above temperature range, where the unit cell is composed of two well defined Gd–Si slabs connected to each other by Si–Si bonds along the b axis. The unit cell volume expands almost linearly with increasing temperature below and above TC. However, the lattice thermal expansion is anisotropic and discontinuous around TC. The lattice constants change from a=7.4834(1)Å, b=14.7476(3)Å, and c=7.7493(2)toa=7.4846(2)Å, b=14.7452(4)Å, and c=7.7560(1) as the temperature is increased from 325to345K. b contracts by about 0.02% while c expands by about 0.08%. Electronic structure calculations show that the anisotropic lattice expansion near TC changes the valence electron distribution between Gd and Si and thus to the Ruderman-Kittel-Kasuya-Yosida interaction, which affects the magnetic ordering and the magnetic transition in Gd5Si4.

Crystal structure and phase relationships in the pseudobinary system Nd5Si4–Nd5Ge4

Phase relationships at ambient temperature in the pseudobinary system Nd5Si4–Nd5Ge4 were studied by X-ray powder diffraction (XRD). Four structurally distinct phase regions exist in this system. The Nd5Si4-based solid solution in which Ge statistically substitutes for Si crystallizes in the tetragonal Zr5Si4-type structure with space group P41212. The Nd5Ge4-based solid solution in which Si statistically substitutes for Ge crystallizes in the orthorhombic Gd5Ge4-type structure with space group Pnma. There are two ternary intermediate phases, which crystallize in the orthorhombic Gd5Si4-type structure with space group Pnma and the monoclinic Gd5Si2Ge2-type structure with space group P1121/a. The Rietveld powder diffraction profile fitting technique was used for refinement of the crystal structure. The lattice parameters, atomic positions and interatomic distances for Nd5Si4, Nd5Si2.8Ge1.2, Nd5Si2Ge2 and Nd5Ge4 were derived.

Formation of completely miscible solid-solution Gd 5(Si 1− xGe x) 4 induced by ferromagnetic exchange interaction

The crystal structure and magnetic properties of Gd 5(Si 1− x Ge x ) 4 compounds (0≤ x≤1.0) are investigated by means of temperature dependent X-ray powder diffraction and magnetic measurements. A crystallographic and magnetic phase diagram of the pseudobinary system Gd 5Si 4–Gd 5Ge 4 is constructed. An intriguing observation is that Gd 5(Si 1− x Ge x ) 4 alloys form a completely miscible solid-solution at low temperature (< T C), crystallizing in the Gd 5Si 4-type structure as indicated by the well-defined linear dependence of lattice parameters on germanium concentration x, and decomposing into three partially miscible solid-solutions above the Curie temperature, i.e. Gd 5Si 4 based ( x<0.5), Gd 5Ge 4 based ( x≥0.8) and Gd 5Si 2Ge 2 based (0.5≤ x<0.8) solid-solutions. This unusual feature can be understood by taking into account competing effects of ferromagnetic exchange interaction and lattice strain in the solid-solution.

On the Sr-Pb system

The Sr-Pb system was studied by thermal analysis and X-ray analysis. Seven intermediate phases were found to exist. Two of these phases melt congruently: Sr 2Pb (1155 °C); SrPb 3 (675 °C). The remaining five phases form peritectically: Sr 5Pb 3 (1054 °C); Sr 5Pb 4 (943 °C); SrPb (785 °C); Sr 2Pb 3 (717 °C); Sr 3Pb 5 (645 °C). Two eutectics are formed, at 87.5 at.% Sr (725 °C) and at 30.5 at.% Sr (627 °C). The crystal structures of the compounds Sr 2Pb (anti-PbCl 2 type), Sr 5Pb 3 (Cr 5B 3 type), SrPb (CrB type) and SrPb 3 (Ti 3Cu type) are confirmed; Sr 5Pb 4 crystallizes with the Gd 5Si 4-type structure.