TiNiSi-type Structure Research Articles

Large magnetocaloric effect of RCoSn (R=Dy, Ho, Er and Tm) compounds

Materials with large magnetocaloric effect (MCE) at low temperatures under low field changes play an important role in the application of cryogenic magnetic refrigeration and attract extensive research interests of researchers. In this work, four ternary rare-earth-based RCoSn (R=Dy, Ho, Er and Tm) compounds were successfully synthesized and the crystal structure, magnetic properties, magnetic phase transition, and magnetocaloric effects were systematically studied. RCoSn (R=Dy, Ho, Er and Tm) compounds all crystallize in orthorhombic TiNiSi-type structure and undergo an antiferromagnetic (AFM) to paramagnetic (PM) transition with Neél temperature of 11.0, 7.9, 4.9 and 3.0 K, respectively. Large cryogenic MCE of RCoSn (R=Dy, Ho, Er and Tm) with the maximum magnetic entropy change (−ΔSMmax) of 10.6, 13.7, 17.1, 13.4 J/kg K for field change of 0–5 T was obtained. Furthermore, the value of −ΔSMmax for ErCoSn and TmCoSn is as high as 11.3 and 10.1 J/kg K for field change of 0–2 T, which is very competitive among low temperature magnetocaloric materials. The characteristic of second order magnetic phase transition indicates RCoSn (R=Dy, Ho, Er and Tm) compounds have good magnetic/thermal reversibility. These results show that RCoSn compounds, especially ErCoSn and TmCoSn, exhibit promising MCE and have great application potential in cryogenic magnetic refrigeration.

Magnetic properties of the solid solutions CePt1−x Au x Al (x = 0.1–0.9)

Abstract Members of the solid solutions CePt1–x Au x Al have been synthesized from the elements and pure samples could be obtained for the whole range of x from 0.1–0.9. Powder X-ray diffraction patterns indicated the TiNiSi-type structure (orthorhombic, space group Pnma) for the whole series, consistent with the end members CePtAl and CeAuAl. The lattice parameters and therefore also the unit cell volumes evolve in a linear fashion when going from CePtAl to CeAuAl. Magnetic susceptibility and magnetization investigations were performed for the compounds with x = 0.1, 0.2, 0.3, 0.5, 0.7 and 0.9. The results show that the ferromagnetic ground state of CePtAl gets destabilized for small degrees of substitution (x = 0.1) resulting in a lower Curie temperature. For larger values of x, changes towards an antiferromagnetic ground state are observed, in line with pure CeAuAl.

Pressure tuning reverse martensitic transformation in the Mn0.9Co0.1NiGe half-Heusler alloy

Here we investigate the structural properties of the Mn0.9Co0.1NiGe half-Heusler alloys under pressure up to 12 GPa by Synchrotron angle-dispersive x-ray diffraction (XRD). At room temperature and pressure, the compound exhibits only the hexagonal NiIn2-type structure. Lowering the temperature to 100 K at ambient pressure induces an almost complete martensitic phase transformation to the orthorhombic TiNiSi-type structure. With increasing pressure, the stable orthorhombic phase gradually undergoes a reverse martensitic transformation. The hexagonal phase reaches 85% of the sample when applying 12 GPa of pressure at T = 100 K. We further evaluated the bulk modulus of both hexagonal and orthorhombic phases and found similar values (123.1 ± 5.9 GPa for hexagonal and 102.8 ± 4.2 GPa for orthorhombic). Also, we show that the lattice contraction induced is anisotropic. Moreover, the high-pressure hexagonal phase shows a volumetric thermal contraction coefficient α v ∼ −8.9(1) × 10−5K−1 when temperature increases from 100 to 160 K, evidencing a significant negative thermal expansion (NTE) effect. Overall, our results demonstrate that the reverse martensitic transition presented on Mn0.9Co0.1NiGe induced either by pressure or temperature is related to the anisotropic contraction of the crystalline arrangement, which should also play a crucial role in driving the magnetic phase transitions in this system.

Pre-existing orthorhombic embryos-induced hexagonal–orthorhombic martensitic transformation in MnNiSi1–x (CoNiGe) x alloy

The thermal–elastic martensitic transformation from high-temperature Ni2In-type hexagonal structure to low-temperature TiNiSi-type orthorhombic structure has been widely studied in MnMX (M = Ni or Co, and X = Ge or Si) alloys. However, the answer to how the orthorhombic martensite nucleates and grows within the hexagonal parent is still unclear. In this work, the hexagonal–orthorhombic martensitic transformation in a Co and Ge co-substituted MnNiSi is investigated. One can find some orthorhombic laths embedded in the hexagonal parent at a temperature above the martensitic transformation start temperature (M s). With the the sample cooing to M s, the laths turn broader, indicating that the martensitic transformation starts from these pre-existing orthorhombic laths. Microstructure observation suggests that these pre-existing orthorhombic laths do not originate from the hexagonal–orthorhombic martensitic transformation because of the difference between atomic occupations of doping elements in the hexagonal parent and those in the pre-existing orthorhombic laths. The phenomenological crystallographic theory and experimental investigations prove that the pre-existing orthorhombic lath and generated orthorhombic martensite have the same crystallography relationship to the hexagonal parent. Therefore, the orthorhombic martensite can take these pre-existing laths as embryos and grow up. This work implies that the martensitic transformation in MnNiSi1−x (CoNiGe) x alloy is initiated by orthorhombic embryos.

Equiatomic intermetallic barium compounds BaTX (T=Pd, Pt, Au; X=Mg, Zn, Cd) with TiNiSi and ZrBeSi type structure

AbstractThe equiatomic ternary BaTX (T=Pd, Pt, Au; X=Mg, Zn, Cd) phases were obtained by reaction of the elements in sealed tantalum ampoules. The compounds were characterized through their X‐ray powder patterns. BaAuMg and BaTCd (T=Pd, Pt, Au) crystallize with the TiNiSi type structure, while the compounds BaTZn (T=Pd, Pt) adopt the ZrBeSi type. The structures of BaAuMg and BaPdZn were refined from single crystal X‐ray diffractometer data: Pnma, a=801.34(5), b=467.71(8), c=924.07(10) pm, wR2=0.0955, 736 F2 values, 20 variables for BaAuMg and P63/mmc, a=449.68(2), c=901.62(4) pm, wR2=0.1172, 145 F2 values, 8 variables for BaPdZn. The structures derive from the aristotype AlB2. The gold and magnesium atoms form puckered Au3Mg3 hexagons with 284–328 pm Au−Mg while the Pd3Zn3 hexagons in BaPdZn are planar (260 pm Pd−Zn). Comparison of all known BaTX phases shows a range of the valence electron count (VEC) from 13 (BaMnGe) to 18 (BaAuP). The BaTX phases crystallize with seven different structure types and there is no obvious correlation with VEC.

TiNiSi-type YbIrMg with trivalent ytterbium

Abstract YbIrMg was obtained from a reaction of the elements in an equiatomic ratio in a sealed tantalum ampoule in a muffle furnace. The TiNiSi-type structure of YbIrMg was refined from single crystal X-ray diffractometer data: Pnma, a = 695.72(3), b = 405.84(6), c = 840.57(7) pm, wR = 0.0431, 511 F 2 values and 21 variables. A small degree of Mg/Ir mixing within the three-dimensional [IrMg] network leads to the refined composition YbIr1.024(4)Mg0.976(4). The course of the cell volume in the REIrMg series (Iandelli plot) points to trivalent ytterbium in YbIrMg.

Achievement of reversible table-like magnetocaloric effect in MnCo1-Gd Ge alloys around room temperature

• The table-like magnetocaloric effect in MnCo 1- x Gd x Ge alloys are achieved. • Two successive magnetic transitions appear in the MnCo 1- x Gd x Ge ( x = 0.04, 0.06). • The refrigerant capacity reach 282 J/kg in MnCo 0.94 Gd 0.06 Ge with Δ H = 50 kOe. • The relative cooling power reach 335 J/kg in MnCo 0.94 Gd 0.06 Ge with Δ H = 50 kOe. • The maximum temperature span are 128 K in MnCo 0.94 Gd 0.06 Ge with Δ H = 50 kOe. The structural, magnetic and magnetocaloric properties in the Gd doped MnCo 1- x Gd x Ge alloys were investigated. With the increase of Gd content, the coexistence of Ni 2 In-type hexagonal structure and TiNiSi-type orthorhombic structure appear from the pure Ni 2 In-type hexagonal structure of MnCoGe. All samples undergo the second-order magnetic transition (SOMT). MnCo 1- x Gd x Ge ( x = 0.04, 0.06) have two successive magnetic transitions and a large reversible table-like MCE has been achieved around room temperature (RT). Moreover, the relative cooling power ( RCP ) and effective refrigeration temperature span (δ T FWHM ) with the magnetic field change of 50 kOe reach 335 J/kg and 128 K, respectively.

Magnetic and structural properties of Fe-substituted MnCoGe with Ni2In-type structure

Site occupation of Fe atoms, magnetic and structural properties in MnCo1-xFexGe (x = 0.2, 0.5, 0.7 and 1.0) were investigated. Two phase co-existence of the hexagonal Ni2In-type structure (P-phase) and the orthorhombic TiNiSi-type structure was observed at room temperature for x = 0.2. For x = 0.5, 0.7 and 1.0, MnCo1-xFexGe was confirmed to be the P-phase. With increasing x from 0.2 to 1.0 for the P-phase, the lattice parameter ap slightly increased by only 0.14%, but cp decreased by 2.1%. As a result, when x was increased from 0.2 to 1.0, the unit cell volume of the P-phase decreased monotonously by 1.8%. The saturation magnetic moment and Curie temperature of MnCo0.5Fe0.5Ge were 2.16 μB/f.u. and 205 K, respectively, which decreased monotonically with increasing x. 57Fe Mössbauer spectroscopy indicated that 90–94% of the substituted Fe atoms occupied the Co-site but 10–6% of the Fe also occupied the Mn-site in the P-phase. For x = 0.5, 0.7 and 1.0, the hyperfine field at the Co-site was 8.29–9.44 T, which was larger than that at the Mn-site. The sign of quadruple splitting at the Co-sites changed from positive to negative with the increase of x, suggesting that the electric field gradients at Co-sites changed.

Flux Growth, Crystal Structures, and Electronic Properties of the Ternary Intermetallic Compounds Ca3Pd4Bi8 and Ca3Pt4Bi8

Reaction of the elements yielded Ca3Pt4Bi8 and CaPtBi, which are, to the best of our knowledge, the first reported ternary Ca–Pt–Bi compounds. The compounds crystallize isostructural to the Pd analogs Ca3Pd4Bi8 (own structure type) and CaPdBi (TiNiSi structure type), respectively. Employing a multistep temperature treatment allows for the growth of mm-sized single crystals of Ca3Pd4Bi8 and Ca3Pt4Bi8 from a Bi self-flux. Their crystal structures can be visualized as consisting of a three-dimensional extended polyanion [M4Bi8]6– (M = Pd, Pt), composed of interlinked M–Bi chains propagating along the c direction, and Ca2+ cations residing in one-dimensional channels between the chains. First-principles calculations reveal quasi-one-dimensional electronic behavior with reduced effective electron masses along [001]. Bader analysis points to a strong anionic character of the M species (M = Pd, Pt) in Ca3M4Bi8. Thus, it is more appropriate to address the compounds Ca3Pd4Bi8 and Ca3Pt4Bi8 as a palladide and platinide, respectively. Magnetization measurements indicate diamagnetic behavior with no indications for superconductivity down to 2 K. Electrical resistivity data are consistent with metallic behavior and suggest predominant electron–phonon scattering.

Drastic Influence of Synthesis Conditions on Structural, Magnetic, and Magnetocaloric Properties of Mn(Fe,Ni)(Si,Al) Compounds

Mn compounds presenting magneto-structural phase transitions are currently intensively studied for their giant magnetocaloric effect; nevertheless, several parameters remain to be further optimized. Here, we explore the Mn(Fe,Ni)(Si,Al) series, which presents two advantages. The Mn content is fixed to unity ensuring a large saturation magnetization, and it is based on non-critical Si and Al elements instead of the more commonly employed Ge. Structural and magnetic properties of MnFe0.6Ni0.4Si1-xAlx compounds are investigated using powder X-ray diffraction, SEM, EDX, DSC, and magnetic measurements. We demonstrate that a magneto-structural coupling leading to transformation from ferromagnetic with orthorhombic TiNiSi-type structure to a paramagnetic hexagonal Ni2In-type phase can be realized for 0.06 < x ≤ 0.08. Unfortunately, the first-order transition is relatively broad and incomplete, likely as the result of insufficient sample homogeneity. A comparison between samples synthesized in different conditions (as-cast, quenched from 900 °C, or quenched from 1100 °C) reveals that Mn(Fe,Ni)(Si,Al) samples decompose into a Mn5Si3-type phase at intermediate temperatures, preventing the synthesis of high-quality samples by conventional methods such as arc-melting followed by solid-state reaction. By identifying promising MnFe0.6Ni0.4Si1-xAlx compositions, this study paves the way toward the realization of a giant magnetocaloric effect in these compounds using alternative synthesis techniques.

Phase equilibria in the Er–Zr–Ni system and crystal structure of R1+xZr1-xNi (R=Er, Tm, Lu) compounds

The isothermal section of the Er–Zr–Ni phase diagram in the Er–Zr–ErNi region at 870 K has been studied by means of X-ray phase and structural analyzes and energy-dispersive X-ray spectroscopy. The mutual solubility of the components Er, Zr and Ni at 800 ° С was established: Zr dissolves up to 3 at. % Er and practically does not dissolve Ni; Ni practically does not dissolve Er and Zr; Er dissolves up to 12 at. % Zr and practically does not dissolve Ni. The following solid solutions of substitution were detected: Er 3- x Zr x Ni (0≤ x ≤0.24; structure type (ST) Fe 3 C, space group (SG) Pnma, a = 6.804–6.801(1) Ǻ, b = 9.430–9.431(3) Ǻ, c = 6.245–6.241(2) Ǻ); Er 3- x Zr x Ni 2 (0≤ x ≤0.04; ST Er 3 Ni 2 , SG R , a = 8.472–8.451(2) Ǻ, c = 15.680–15.665(4) Ǻ). Two ternary compound with the narrow homogeneity range Er x Zr 1- x Ni 2 (0.12≤ х ≤ 0.24) and Er 1+ x Zr 1− x Ni (0.33 ≤ x ≤ 0.48) occurs in the system. The Er x Zr 1- x Ni 2 (0.12 ≤ х ≤ 0.24) compound crystallizes in the cubic MgCu 2 type of structure, а = 6.959(6)–6.987(5) Å). The crystal structure of Er 1+ x Zr 1− x Ni (0.33≤ x ≤ 0.48) compound was investigated by means of EDX and powder X-ray diffraction. It crystallizes in the TiNiSi ST (SG Pnma , No. 62, oP 12, а = 6.8175(7)–6.8253(8) Å, b = 4.6494(5)–4.6575(6) Å, c = 8.1021(9)–8.113(1) Å). In Er 1+x Zr 1−x Ni, the Er/Zr statistical mixture leading to nonequiatomic compositions occupies the position corresponding to the nickel site of the TiNiSi structure type. The isostructural rare earth compounds R 1+x Zr 1−x Ni (0.33≤ x ≤ 0.48; R=Tm, Lu) have been synthesized and their homogeneity ranges were refined. Keywords: ternary system, phase equilibria, intermetallic compounds, crystal structure.

Doping site induced alteration of incommensurate antiferromagnetic ordering in Fe-doped MnNiGe alloys

The magnetic structure of Fe-doped MnNiGe alloys of nominal compositions ${\mathrm{MnNi}}_{0.75}{\mathrm{Fe}}_{0.25}\mathrm{Ge}$ and ${\mathrm{Mn}}_{0.85}{\mathrm{Fe}}_{0.15}\mathrm{NiGe}$ has been explored through detailed neutron powder diffraction (NPD) study in ambient and high-pressure (6 kbar) conditions. Both these alloys crystallize with ${\mathrm{Ni}}_{2}\mathrm{In}$-type hexagonal structure (with $P{6}_{3}/mmc$ space group) at room temperature and undergo a martensitic transition to low-temperature TiNiSi-type orthorhombic structure (with space group $Pnma$). Both the alloys show incommensurate antiferromagnetic ordering at the low-temperature martensitic phase. However, the modulation of the magnetic structures depends strongly on the doping site of Fe. The incommensurate propagation vector $\mathbf{k}$ for ${\mathrm{MnNi}}_{0.75}{\mathrm{Fe}}_{0.25}\mathrm{Ge}$ and ${\mathrm{Mn}}_{0.85}{\mathrm{Fe}}_{0.15}\mathrm{NiGe}$ alloys are found to be (0.1790(1),0,0) and (0.1543(3),0,0), respectively at 1.5 K, and these remain almost unchanged with increasing sample temperature under ambient conditions. The application of the external pressure results in a significant effect on both martensitic transition temperature and low-temperature magnetic structure. The propagation vectors $\mathbf{k}$ for both the alloys show a monotonic decrease with increasing sample temperature in the presence of external pressure. Interestingly, no sign of the magnetically ordered hexagonal austenite phase was observed in any alloys down to the lowest measurement temperature.

The solid solution TbNiIn1−x Ga x

Abstract Phase formation in the solid solution TbNiIn1−x Ga x at 873 K was investigated in the full concentration range by means of powder X-ray diffraction and EDX analysis. The samples were synthesized by arc-melting of the pure metals with subsequent annealing at 873 K for one month. The influence of the substitution of indium by gallium on the type of structure and solubility was studied. The solubility ranges have been determined and changes of the unit cell parameters were calculated on the basis of powder X-ray diffraction data: TbNiIn1–0.4Ga0–0.6 (ZrNiAl-type structure, space group P 6 ‾ 2 m $P\bar{6}2m$ , a = 0.74461(8)–0.72711(17) and c = 0.37976(5)–0.37469(8) nm); TbNiIn0.2–0Ga0.8–1.0 (TiNiSi-type structure, space group Pnma, а = 0.68950(11)–0.68830(12), b = 0.43053(9)–0.42974(6), с = 0.74186(10)–0.73486(13) nm). The crystal structures of TbNiGa (TiNiSi type, Pnma, a = 0.69140(5), b = 0.43047(7), c = 0.73553(8) nm, wR2=0.0414, 525 F 2 values, 21 variables), TbNiIn0.83(1)Ga0.17(1) (ZrNiAl type, P 6 ‾ 2 m $P\bar{6}2m$ , a = 0.74043(6), c = 0.37789(3) nm, wR2 = 0.0293, 322 F 2 values, 16 variables) and TbNiIn0.12(2)Ga0.88(2) (TiNiSi type, Pnma, a = 0.69124(6), b = 0.43134(9), c = 0.74232(11) nm, wR2 = 0.0495, 516 F 2 values, 21 variables) have been determined. The characteristics of the solid solutions and the variations of the unit cell parameters are briefly discussed.

Low thermal conductivity and semimetallic behavior in some TiNiSi structure-type compounds

Motivated by recent advances in half-Heusler based thermoelectric materials, we investigated the phase stability and thermoelectric properties of compounds ZrNiSi, ZrNiGe, HfNiSi, NbCoSi, and ZrNiSb, some of which were recently reported in literature as promising half-Heuslers for thermoelectric applications using the first-principles density functional theory based calculations. Here, we show that all the named compounds actually crystallize with the orthorhombic TiNiSi structure type, which remains stable above room temperature up to at least 1100 K. In ZrNiSb, $5%$ excess Zr is required to obtain the pure orthorhombic phase. Our first-principles electronic band structure calculations reveal that they are semimetals. In ZrNiSi, ZrNiGe, and HfNiSi, the Fermi surface consists of small electron and hole pockets with electrons as the majority charge carriers. In NbCoSi and ZrNiSb, the majority carriers are holes. A pseudogaplike feature is observed in the electronic density of states with Fermi energy (${E}_{F}$) located either slightly below (ZrNiSi, ZrNiGe, and HfNiSi) or above the pseudogap (NbCoSi). In ZrNiSb no pseudogap is observed; however, the density of states at ${E}_{F}$ is still small. The electrical conductivity ($\ensuremath{\sigma}$) near room temperature is of the order of ${10}^{3}\phantom{\rule{4pt}{0ex}}\mathrm{S}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$, which is intermediate between that of the degenerate semiconductors and metallic alloys. Near room temperature the thermopower is negative for $\mathrm{ZrNi}X$ ($X$ = Si, Ge) and HfNiSi, and positive for NbCoSi and ZrNiSb as predicted theoretically. The average value of Seebeck coefficient is small, of the order of $10\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{V}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$. Despite reasonably high electrical conductivity, the thermal conductivity ($\ensuremath{\kappa}$) of these compounds is found to be generally low ($<15\phantom{\rule{4pt}{0ex}}\mathrm{W}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ near $300\phantom{\rule{4pt}{0ex}}\mathrm{K}$). In ${\mathrm{Zr}}_{1.05}\mathrm{Ni}\mathrm{Sb}$, which has the highest electrical conductivity ($\ensuremath{\approx}4000\phantom{\rule{4pt}{0ex}}\mathrm{S}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$), $\ensuremath{\kappa}$ is as low as $\ensuremath{\approx}4\phantom{\rule{4pt}{0ex}}\mathrm{W}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ at $300\phantom{\rule{4pt}{0ex}}\mathrm{K}$, of which almost 70% is estimated to be due to the electronic contribution resulting in a lattice contribution which is $<1\phantom{\rule{4pt}{0ex}}\mathrm{W}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$. This uncommon combination of high electrical conductivity and low thermal conductivity is interesting and invites further attention.

New intermetallics R1+xZr1−xNi (R = Er–Tm, x ~ 0.5) with the TiNiSi type of structure

A new series of isostructural rare earth compounds R1+xZr1−xNi (R = Er, Tm, Lu; x ~ 0.5) was synthesized from the elements by arc melting and subsequent annealing at 870 K for 1400 h. The crystal structures of the intermetallic compounds were investigated by means of single-crystal X-ray diffraction. They all crystallize in the TiNiSi structure type (space group Pnma, No. 62, oP12). In R1+xZr1−xNi, the R/Zr statistical mixture leading to nonequiatomic compositions occupies the position corresponding to the nickel site of the TiNiSi structure type. The calculated shortest interatomic distances are close to the sums of the single-bond covalent radii of respective elements. Electronic structure calculations performed with the tight-binding LMTO method revealed the non-zero density of states at the Fermi level and suggest metallic character. R1+xZr1−xNi (R = Er, Tm, Lu; x ~ 0.5) undergoes no long-range magnetic ordering down to 2 K.

Magnetism and Thermomechanical Properties in Si Substituted MnCoGe Compounds

MnCoGe-based compounds have been increasingly studied due to their possible large magnetocaloric effect correlated to the magnetostructural coupling. In this research, a comprehensive study of structure, magnetic phase transition, magnetocaloric effect and thermomechanical properties for MnCoGe1−xSix is reported. Room temperature X-ray diffraction indicates that the MnCoGe1−xSix (x = 0, 0.05, 0.1, 0.15 and 0.2) alloys have a major phase consisting of an orthorhombic TiNiSi-type structure with increasing lattice parameter b and decreasing others (a and c) with increasing Si concentration. Along with M-T and DSC measurements, it is indicated that the Tc value increased with higher Si concentration and decreased for structural transition temperature Tstr. The temperature dependence of the magnetization curves overlaps completely, indicating that there is no thermal hysteresis, and it is shown that the transition is the second-order type. It is also shown that the decreased magnetization on the replacement of Si for Ge decreases the value of −ΔSM from −ΔSM~8.36 J kg−1 K−1 at x = 0 to −ΔSM~5.49 J kg−1 K−1 at x = 0.2 with 5 T applied field. The performed Landau theory has confirmed the second-order transition in this study, which is consistent with the Banerjee criterion. The magnetic measurement and thermomechanical properties revealed the structural transition that takes place with Si substitution of Ge.

A Noncentrosymmetric Polymorph of LuRuGe.

We report a new polymorph of LuRuGe, obtained in indium flux. This phase exhibits the noncentrosymmetric ZrNiAl-type structure with the space group P6̅2m as determined by single-crystal X-ray diffraction. This polymorph can convert into another centrosymmetric polymorph (TiNiSi-type structure, space group Pnma) at high temperatures. We performed electrical transport, magnetization, and specific heat measurements on this new phase. It shows metallic behavior with a Hall sign change from negative at 2 K to positive at 125 K. LuRuGe exhibits Pauli paramagnetism as the ground state with no local magnetic moments from either the Ru or Lu site. The Debye temperature Θ = 348 K and electronic coefficient γe = 3.6 mJ K-2 mol-1 are extracted from the low-temperature specific heat data in LuRuGe. We also carried out first-principles density functional theory calculations to map out the electronic band structure and density of states. There are several electronic bands crossing the Fermi level, supporting a multiband scenario consistent with the Hall sign change. The density of states around the Fermi level is mainly from Ru 4d and Ge 4p electrons, indicating a strong hybridization between those atomic orbitals.

Specific heat of Cu substituted GdNiSi

The specific heat of magnetic regenerator materials affects the efficiency of cryocoolers below 10 K. GdNiSi with the orthorhombic TiNiSi-type structure shows antiferromagnetic order at 11 K. To tune the magnetic transition temperature below 10 K, Cu substituted GdNiSi alloys were prepared. Powder X-ray diffraction and specific heat measurements were performed to study the effect of Cu substitution on the crystal structure and magnetic transition temperature. Increasing Cu content, the secondary phase which may be the hexagonal BeZrSi-type GdNi1−x Cu x Si is observed. The peak of the specific heat for x ≤ 0.2 was shifted higher temperature (13 K) due to the magnetic transition, though the unit cell volume of the TiNiSi-type structure slightly increased with increasing x. This result possibly originates from the increase of the RKKY interaction for the sinusoidal curve due to an increase of the intersite distance of Gd-ions.

Structural transition and antiferromagnetic ordering in the solid solution CePd1−x Au x Al (x = 0.1–0.9)

Abstract The intermetallic solid solution CePd1−x Au x Al (x = 0.1–0.9) has been synthesized from the elements by arc-melting and subsequent annealing in induction followed by tube furnaces. The samples were characterized using the Guinier powder diffraction technique and the structures of the nominal compositions CeAuAl and CePd0.2Au0.8Al were refined from single crystal X-ray diffractometer data. For small values of x = 0.1–0.3, the compounds crystallize in the hexagonal ZrNiAl-type structure (space group P 6 ‾ $‾{6}$ 2m), while for x = 0.5–0.9 the orthorhombic TiNiSi-type structure (space group Pnma) was observed. In both structure types, the transition metal and aluminum atoms form a complex polyanionic network with the cerium atoms filling the respective cavities. The transition metal atoms are in both cases surrounded in the shape of a tri-capped trigonal prism, the connectivity of these units, however, is different. Temperature-dependent magnetic susceptibility measurements of all compounds indicated a stable trivalent oxidation state for the cerium atoms along with antiferromagnetic ordering around T N ∼ 3 K.

Optimization of Criteria for an Efficient Screening of New Thermoelectric Compounds: The TiNiSi Structure-Type as a Case-Study.

High-throughput calculations can be applied to a large number of compounds, in order to discover new useful materials. In the present work, ternary intermetallic compounds are investigated, to find new potentially interesting materials for thermoelectric applications. The screening of stable nonmetallic compounds required for such applications is performed by calculating their electronic structure, using DFT methods. In the first section, the study of the density of states at the Fermi level, of pure elements, binary and ternary compounds, leads to empirically chose the selection criterion to distinguish metals from nonmetals. In the second section, the TiNiSi structure-type is used as a case-study application, through the investigation of 570 possible compositions. The screening leads to the selection of 12 possible semiconductors. The Seebeck coefficient and the lattice thermal conductivity of the selected compounds are calculated in order to identify the most promising ones. Among them, TiNiSi, TaNiP, or HfCoP are shown to be worth a detailed experimental investigation.