6h Sites Research Articles

Coexistence of distinct Ce 4f-electron states at inequivalent crystallographic sites: CePtGe2 case

In CePtGe2, which has two crystallographically inequivalent Ce sites, the coexistence of a localized magnetic 4f-electron state at one Ce site and an itinerant heavy-electron state at the second one has been suggested. We prepared a series of pseudoternary (CexY1−x)PtGe2 compounds and investigated them by X-ray diffraction, magnetization, and specific heat measurements. The structure analysis revealed that the Y ions selectively occupy the Ce sites at the 4h position, which is more suited for smaller Ce ions with the itinerant heavy-electron state. The Néel temperature, low-temperature saturation magnetization, and magnetic entropy change weakly in the interval 1.0 ≥ x ≥ 0.5 while the linear specific-heat coefficient decreases rapidly with decreasing x. Further substitution of Y for Ce leads to a rapid decrease of the Néel temperature, low-temperature saturation magnetization, a remaining low linear specific-heat coefficient, and finally vanishing antiferromagnetism. These results corroborate the scenario of the coexistence of the magnetic Ce ions with localized 4f electrons responsible for the ordered magnetic state hosted by the 4i site, whereas the Ce ions at the 4h site are characterized by itinerant heavy 4f electrons.

Synthesis, disorder and Ising anisotropy in a new spin liquid candidate PrMgAl11O19

Here we report the successful synthesis of large single crystals of triangular frustrated PrMgAl11O19 using the optical floating zone technique. Single crystal x-ray diffraction (XRD) measurements unveiled the presence of quenched disorder within the mirror plane, specifically ~7% of Pr ions deviating from the ideal 2d site towards the 6h site. Magnetic susceptibility measurements revealed an Ising anisotropy with the c-axis being the easy axis. Despite a large spin–spin interaction that develops below ~10 K and considerable site disorder, the spins do not order or freeze down to at least 50 mK. The availability of large single crystals offers a distinct opportunity to investigate the exotic magnetic state on a triangular lattice with an easy axis out of the plane.

Understanding the electrochemical properties of Mg-doped Li2MnO3: first-principles calculations.

Non-transition metal doping, especially for Mg, has been gradually employed to optimize the electrochemical performance of Li-rich cathode material Li2MnO3. However, the effects of Mg doping on the electrochemical behavior of Li2MnO3 have not been studied extensively. In this work, we investigate the effect of Mg doping at both the 2b (in the Li/Mn mixed layer) and 4h (in the Li layer) Li sites on the electrochemical properties of Li2MnO3 through first-principles calculations and ab initio molecular dynamics simulations. The local lattice structure, electronic density of states, Bader charge, delithiation voltage, lattice oxygen stability and Li diffusion kinetics are examined. Electronic structure analysis shows that Mg can activate the electrochemical activity of surrounding Mn by charge transfer, making Mn participate in charge compensation at the initial delithiation stage. Mg doping can also cause an increase in the average oxygen vacancy formation energy and hence depress the oxygen release during the delithiation process. Molecular dynamics simulations show that the diffusion kinetics of Li ions in Mg2b-Li2MnO3 is enhanced with respect to the undoped one, whereas Mg doped at the 4h site cannot improve the diffusion kinetics of Li ions. Further studies found that Mg doped at the 2b site results in a decrease in the energy barrier for the intra-layer diffusion and an increase in the energy barrier for the inter-layer diffusion of the nearby Li vacancies.

Synthesis of a new potassium-substituted lead fluorapatite and its structural characterization.

Prismatic crystals of partially potassium substituted lead fluorapatite Pb5.09Ca3.78K1.13(PO4)6F0.87 were grown through a solid-state reaction. The structural study conducted by single-crystal X-ray diffraction revealed that the compound crystallizes in the hexagonal P63/m space group, with unit cell parameters a = b = 9.7190(5) Å, c = 7.1700(6) Å and V = 587.37(7) Å3(Z = 1), as well as final values amounting to R and wR of 0.0309 and 0.0546, respectively. The structural refinement demonstrated that Pb occupies both the (6h) and (4f) structural sites of hexagonal fluorapatite, K occupies the (6h) site, and Ca is placed on the (4f) site. Powder X-ray diffraction study indicated the absence of additional phases or impurities. Chemical analysis using atomic absorption spectrometry and energy-dispersive X-ray spectroscopy confirmed the expected chemical formula. The electrical conductivity measured over a wide temperature range was found to be governed by the ion mobility mechanism in the tunnel along the c axis (probably attributed to the fluorine ion located there). We, therefore, could infer from the analysis of the complex impedance spectra that the electrical conductivity of our apatite depends essentially on the temperature and frequency, which produces a relaxation phenomenon and semiconductor-like behavior. Moreover, the strong absorption in the UV-Visible region was substantiated through studies of the optical properties of the developed sample. Fluorescence spectra exhibited emissions in the orange regions when excited at 375 nm. The findings of the phenomena resulting from the emission and conduction of the apatite in question suggest its potential for application in various technological fields such as photovoltaic cells, optoelectronics, photonics, LED applications, catalysis and batteries.

Improved hydrogen ab-/desorption performance of Ti–Cr based alloys via dual-effect of oxide reduction and element substitution by minor Al additive

Hysteresis of hydrogen storage alloys essentially hinders the compressibility and operation efficiency of metal hydride hydrogen compressors. In this work, minor Al additive was added into Ti–Cr based hydrogen compression alloys through induction levitation melting, and the apparent hysteresis was successfully ameliorated. The XRD and SEM results indicate homogeneous element distribution of a main C14 Laves phase of Ti0.96Zr0.06Cr1.0Mn0.6Fe0.4Alx (x = 0, 0.01, 0.02, 0.04, 0.08) alloys, and vanishing of a minor oxide phase in case the Al additive is introduced. As Al stoichiometry increases in the alloys, the hydrogenation plateau decreases obviously but the dehydrogenation plateau hardly changes. Meanwhile, the hydrogen capacity ascends initially but descends slightly. Additionally, no severe performance degradation is induced. The mechanism for overall performance improvement can be concluded as dual functions of Al additive: one can be oxide reduction of other elements during melting determined by XPS and ICP, and the other can be partial Al substitution for the B-side elements, leading to relieved deformation energy surrounding the vacant Al-containing interstitials. As to hydrogenation kinetics, the alloys exhibit accelerated hydrogenation rate with increased Al stoichiometry due to decreased activation energy and hydrogenation plateau. Furthermore, the first-principles calculation results indicate random Al substitution for the B-side atoms, and anisotropic expansion of the unit cell can be attributed to Al substitution at 6h sites. This work reveals a simple and effective way to enhance hydrogen storage performance of Ti–Cr based alloys.

Redetermination of the crystal structure of yttrium chromium tetra-boride, YCrB4, from single-crystal X-ray diffraction data.

The structural parameters of yttrium chromium tetra-boride YCrB4 were refined based on single-crystal X-ray diffraction data. YCrB4 is ortho-rhom-bic, having a space group of type Pbam (No. 55) and with lattice parameters of a = 5.9425 (2), b = 11.4831 (4), c = 3.4643 (1) Å. The Y and Cr atoms are located at Wyckoff 4h sites (x, y, 0) and B atoms at the Wyckoff 4g sites (x, y, 1/2). The first structural investigation of YCrB4 was performed using a single crystalline sample [Kuz'ma, (1970 ▸). Kristallografiya. 15, 372-374]. The present study successfully refined all the positional and atomic displacement parameters of the Y, Cr, and B atoms.

Electronic structure and magnetocaloric properties of Ce2Fe17−xCox compounds upon Co substitution

Electronic structure, magnetic properties and magnetocaloric effect of Ce2Fe17−xCox alloys upon Co substitution have been systematically investigated. The Ce2Fe17−xCox (x = 0.33–1) crystallizes in a rhombohedral Th2Zn17-type 2:17 R main phase and minor CeFe2/α-Fe secondary phases. The Ce2Fe17−xCox shows a second-order phase transition characteristic and Co substitution contributes to the enhancement in the Curie temperature (TC) and the magnetocaloric properties. With x increasing from 0.33 to 1, the TC raises from 261.9 to 335.6 K, the maximum magnetic entropy change (ΔSMmax) at a field change of 0–2 T enhances by 14.9% from 2.28 to 2.62 J∙kg−1K−1, and the relative cooling power (RCP) increases from 164.8 to 171.3 J∙kg−1, respectively. According to first-principles calculations, Co atoms energetically occupy the 18h site and enhances the inter-site exchange coupling parameters (Jij) due to the strong hybridization and coupling between Co 3d and Fe 3d electrons. Furthermore, the spin-down total density of states (DOS) reveals a pseudogap developed across the Fermi level (EF) located at around − 0.09 – +0.20 eV with increasing Co content from 0.33 to 1. This pseudogap contributes to the stabilization of ferrimagnetic state and the improvement of the magnetic entropy change. Understanding the underlying electronic and magnetic behavior mechanisms is helpful for developing high-performance magnetocaloric materials.

Determining Factors in Triggering Hysteretic Oxygen Capacities in Lithium-Excess Sodium Layered Oxides.

Oxygen redox (OR) reactions in sodium layered oxide cathodes have been studied intensively to harness their full potential in achieving high energy density for sodium-ion batteries (SIBs). However, OR triggers a large hysteretic voltage during discharge after the first charge process for OR-based oxides, and its intrinsic origin is unclear. Therefore, in this study, an in-depth reinvestigation on the fundamentals of the reaction mechanism in Na[Li1/3Mn2/3]O2 with a Mn/Li ratio (R) of 2 was performed to determine the factors that polarize the OR activity and to provide design rules leading to nonhysteretic oxygen capacity using first-principles calculations. Based on thermodynamic energies, the O2-/O22- and O2-/On- conditions reveal the monophasic (0.0 ≤ x ≤ 4/6) and biphasic (4/6 ≤ x ≤ 1.0) reactions in Na1-x[Li2/6Mn4/6]O2, but each stability at x = 5/6 is observed differently. The O-O bond population elucidates that the formation of an interlayer O-O dimer is a critical factor in triggering hysteretic oxygen capacity, whereas that in a mixed layer provides nonhysteretic oxygen capacity after the first charge. In addition, the migration of Li into the 4h site in the Na metallic layer contributes less to the occurrence of voltage hysteresis because of the suppression of the interlayer O-O dimer. These results are clearly elucidated using the combined-phase mixing enthalpies and chemical potentials during the biphasic reaction. To compare the Mn oxide with R = 2, Na1-x[Li1/6Mn5/6]O2 tuned with R = 5 was investigated using the same procedure, and all the impeding factors in restraining the nonhysteretic OR were not observed. Herein, we suggest two strategies based on three types of OR models: (i) exploiting the migration of Li ions for the suppression of the interlayer O-O dimer and (ii) modulating the Mn/Li ratio for controlling the OR participation, which provides an exciting direction for nonhysteretic oxygen capacities for SIBs and lithium-ion batteries.

Electronic structures and related properties of Ce2Fe17-based magnetocaloric material upon Nd/Si substitution

The electronic structures and magnetocaloric properties of Ce2Fe17-based materials doped with Nd/Si have been investigated by means of experiments and first-principles calculations based on density functional theory (DFT). Ce1.6Nd0.4Fe17−xSix alloys crystallize in a rhombohedral Th2Zn17-type 2:17R main structure with trace amounts of the α-Fe secondary phase, and Si substitution for Fe contributes to the 2:17R phase. Compared with the parent Ce2Fe17 compound, Nd/Si substitution increases the Curie temperature (TC) to 273 K for Ce1.6Nd0.4Fe17 and to 297 K for Ce1.6Nd0.4Fe16.67Si0.33. DFT calculations reveal that Nd/Si addition increases the c/a values and Fe-Fe interatomic distances at the 6c site, thus leading to an enhanced TC. In addition, Si atoms in Ce2Fe17-based alloys energetically prefer to occupy the 18h site. The local density of states and charge density difference map for a single Si and its first-nearest-neighbor Fe atoms confirm a strong hybridization between the electronic orbitals of Si and Fe atoms, leading to slight lattice contraction and a decrease in the formation energy of the 2:17R phase. The Ce1.6Nd0.4Fe17−xSix alloys exhibit a maximum isothermal magnetic entropy change (ΔSM) of 3.7–4.1 J/kg K, working temperature range (∆TFWHM) of 90.5–95.5 K, and a relative cooling power (RCP) of 350–371 J/kg at μ0∆H = 5 T, suggesting that Nd/Si substitution contributes to the optimization of Ce2Fe17-based alloys as potential room-temperature magnetic refrigerants.

Crystal structure of the τ11 Al4Fe1.7Si phase from neutron diffraction and ab initio calculations

The intermetallic τ11 Al4Fe1.7Si phase is of interest for high-temperature structural application due to its combination of low density and high strength. We determine the crystal structure of the τ11 phase through a combination of powder neutron diffraction and density functional theory calculations. Using Pawley and Rietveld refinements of the neutron diffraction data provides an initial crystal structure model. Since Al and Si have nearly identical neutron scattering lengths, we use density-functional calculations to determine their preferred site occupations. The τ11 phase exhibits a hexagonal crystal structure with space group P63/mmc and lattice parameters of a = 7.478 Å and c = 7.472 Å. The structure comprises five Wyckoff positions; Al occupies the 6h and 12k sites, Fe the 2a and 6h sites, and Si the 2a sites. We observe site disorder and partial occupancies on all sites with a large fraction of 80% Fe vacancies on the 2d sites, indicating an entropic stabilization of the τ11 phase at high temperature.

Appearance of pentavalent Fe ion as a result of a charge disproportionation in Fe-substituted Li2MnO3

20% Fe substituted Li2MnO3 (Li1+x(Fe0.2Mn0.8)1-xO2, 0 < x < 1/3) was prepared using coprecipitation–calcination method. X-ray Rietveld analyses showed that most Fe and Mn ions were on both 4g and 2b sites and small amount of them occupy on 2c and 4h sites in monoclinic Li2MnO3-type structure. Neutron Rietveld analysis revealed that Mn ion was mainly on the 4g site, whereas Fe ion was on both 4g and 2b sites. Temperature dependence of magnetization detected a cusp at 30 K for the sample, originating from its antiferromagnetic nature. The 57Fe Mössbauer spectrum at 300 K consisted of two symmetric doublets with different isomer shifts. From the isomer shifts, Fe ion existed as 3+δ and 4+δ states (0 < δ < 1). Below 200 K, the Fe4+δ component split to two doublets having two isomer shift values assigned as 4+ and 5+ states. At 3 K, the spectrum consisted of magnetically ordered 3+, 4+, and 5+ irons and paramagnetic 4+ state iron. The magnetically ordered pentavalent and tetravalent Fe ions can be stabilized at a highly distorted octahedral 4g site, whereas magnetically ordered trivalent and paramagnetic tetravalent Fe ions exist mainly on the less distorted octahedral 2b site.

Uncovering the Structural Evolution in Na-Excess Layered Cathodes for Rational Use of an Anionic Redox Reaction.

A new paradigm based on an anionic O2-/On- redox reaction has been highlighted in high-energy density cathode materials for sodium-ion batteries, achieving a high voltage (∼4.2 V vs Na/Na+) with a large anionic capacity during the first charge process. The structural variations during (de)intercalation are closely correlated with stable cyclability. To determine the rational range of the anion-based redox reaction, the structural origins of Na1-xRu0.5O1.5 (0 ≤ x ≤ 1.0) were deduced from its vacancy (□)/Na atomic configurations, which trigger different interactions between the cations and anions. In the cation-based Ru4+/Ru5+ redox reaction, the □ solubility into fully sodiated Na2RuO3 predominantly depends on the crystallographic 4h site when 0.0 ≤ x ≤ 0.25, and the electrostatic repulsion of the linear O2--□-O2- configuration is accompanied by the increased volumetric strain. Further Na extraction (0.25 ≤ x ≤ 0.5) induces a compensation effect, leading to Na2/3[Na□Ru2/3]O2 with the □ formation of 2b and 2c sites, which drastically reduce the volumetric strain. In the O2-/On- anionic redox region (0.5 ≤ x ≤ 0.75), Na removal at the 4h site generates a repulsive force in O2--□-O2- that increases the interlayer distance. Finally, in the 0.75 ≤ x ≤ 1.0 region, the anionic O charges are unprotected by repulsive forces, and their consumption causes severe volumetric strain in Na1-xRu0.5O1.5. Coupling our mechanistic understanding of the structural origin with the □- and Na-site preferences and the electrostatic interaction between lattice O and vacancies in Na1-xRu0.5O1.5, we determined the rational range of the anionic redox reaction in layered cathode materials for rechargeable battery research.

Dysprosium magnesium silicate apatite featuring field and temperature stable slow magnetization relaxation.

Dy–Mg silicate Dy8Mg2(SiO4)6O2 has been prepared by high-temperature solid state reaction. It has an apatite type structure (P63/m) with the Dy atoms fully occupying the 6h site and being in random distribution with the Mg atoms at the 4f site. The compound reveals dual magnetization relaxation with widely varying contributions from fast (FR) and slow (SR) relaxation paths controlled by field and temperature. The SR path is stabilized by a strong magnetic field, exhibits a weak dependence of relaxation time τ on field and temperature, and sustains large τ of a few seconds up to a temperature of 40 K and under a field of 50 kOe. The analysis of the electronic structure and comparison with the known Dy-doped phosphate apatites suggests that the Orbach and Raman processes are suppressed.

Formation and magnetic properties of hexagonal DO19 phase in Fe2MnGa: An experimental and theoretical investigation

The formation and magnetic properties of Fe2MnGa hexagonal DO19 phase were investigated in detail. Hexagonal Fe2MnGa can be stabilized by furnace cooling, while the FCC phase can be obtained by quenching from high temperature. Strong ferromagnetic character is identified in Fe2MnGa DO19 phase. Its saturation magnetic moment Ms at 5 K is as large as 4.40 μB/f.u. and Curie temperature TC is 563 K. First-principles calculations indicate that Fe and Mn atoms tend to occupy the 6h sites randomly in the DO19 lattice. The DO19 type Fe2MnGa shows a normal ferromagnetic metal character in its electronic structure. Similar to the experimental results, the calculated total magnetic moment is also quite large and mainly comes from the contributions of Fe and Mn spin moments. Both of them are larger than 1.8 μB and are parallel coupled. These properties are quite different from the antiferromagnetic character in FCC Fe2MnGa.

Adsorption and structural properties of hydroxy- and new lacunar apatites

Abstract Design of novel phosphorus apatites, K2Pb6Zn2(PO4)6 and K2Pb4Ca4(PO4)6, zinc hydroxyapatite Ca8Zn2(PO4)6(OH)2, as well as hydroxyapatite Ca10(PO4)6(OH)2, and its corresponding calcined powder were conducted by means of two different synthesis protocols, i.e. solid state and wet chemical methods. The prepared materials adopt P63/m (No 176) as a space group. Structural refinements for K2Pb6Zn2(PO4)6 and K2Pb4Ca4(PO4)6 lacunar apatites, and that for the calcined hydroxyapatite, were performed using Rietveld method. For K2Pb6Zn2(PO4)6, the refinement results showed that the majority of Pb2+ cations occupy (6h) sites. This study examines the impact of the synthesis conditions on the physico-chemical properties and on the surface reactivity of the prepared materials. The analysis of the interactions of the samples with respect to a protein model (Bovin Serum Albumin), evaluated under the same experimental conditions, revealed a fast-kinetic process for the precipitated samples, compared to the crystals obtained by solid chemical reaction. Thus, the equilibrium conditions were reached in less than 20 min for the former while 6 h of contact were requested for the latter. However, all the obtained isotherms show a Langmuirian shape. The difference in the adsorption parameters of the materials, i.e. constant affinity and amount adsorbed at saturation, is discussed in terms of surface characteristics, chemical composition, crystals size and specific surface area. It is concluded that the interaction with the Bovin Serum Albumin (BSA) molecules is highly sensitive to the microstructure of the crystals.

Elaboration, Vibrational Study and Thermal Behavior of Lacunar Apatites NaPb3−xCdxCa(PO4)3 (0 ≤ x ≤ 1)

In this work we are interested in the synthesis of a new solid solution type NaPb3–xCdxCa(PO4)3; (0 ≤ x ≤ 1). The lacunar apatite series are synthesized by a solid-state reaction and studied by X-ray diffraction, infrared and Raman scattering spectroscopy, thermal gravimetric analysis, and differential scanning calorimetry. The crystal structure of these compounds was refined using the Rietveld method. These compounds crystallize in the hexagonal system (space group P63/m) with a number of units per crystal lattice Z = 2. The structure is formed by [PO4]3− tetrahedra and Pb2+/Ca2+/Cd2+ ions, which constitute the basic skeleton of the structure. The Pb2+/Ca2+/Cd2+ ions occupy the 6h sites, whereas the 4f sites are occupied in half by the Pb2+/Ca2+/Cd2+ ions and the other half by the Na+ ions. The observed frequencies in the Raman and infrared spectra are explained and discussed based on the factor group analysis and by comparison with similar apatites. The vibrational spectroscopy results are in good agreement with the X-ray diffraction measurements. The internal modes of (PO4)3− tetrahedra are assigned and corroborate well with the factor group analysis for the symmetry P63/m. We also investigated the thermal stability of these apatite materials by differential scanning calorimetry.

Synthesis and investigation of electrochemical performance of mixed valent Li4FeMoO6 as positive electrode material in rechargeable lithium ion batteries

Lithium rich layered cathode materials containing additional lithium in metal layer are of current interest as they exhibit capacities exceeding 250 mAh g−1. Here, we report Li4FeMoO6, a new lithium rich cathode material containing mixed valent cations with a stoichiometry of Li4Fe2+0.45Fe3+0.55Mo5+0.55Mo6+0.45O6. XRD data shows that Li4FeMoO6 crystallizes in monoclinic structure (C2/m space group) similar to Li2MnO3 and hence in the layered structure notion the formula can be written as Li[Li0.33Fe0.33Mo0.33]O2. XRD analysis reveals that Li and Fe are disordered over 4h and 2d sites, whereas 2a sites are only filled with Mo atoms. HRTEM and electron diffraction studies indicate presence of structural disorder and planar lattice defects such as stacking faults (SFs) or twinning. During electrochemical cycling in the potential range of 1.5 ̶ 4.6 V, Li4FeMoO6 exhibits initial charge capacity of 313 mAh g−1. It shows 90% reversible capacity for the first cycle and after 100 cycles the capacity retention is about 75.5%. Above 4.6 V, Mo dissolution is observed which brings down the capacity retention to 44% in 100 cycles. Interestingly, Li4FeMoO6 show high rate performance which can be attributed to good conductivity arising from the presence of mixed valent Mo5+/Mo6+ and Fe2+/Fe3+.

Syntheses, crystal structures, and comparisons of rare-earth oxyapatites Ca2 RE 8(SiO4)6O2 (RE = La, Nd, Sm, Eu, or Yb) and NaLa9(SiO4)6O2.

Six different rare-earth oxyapatites, including Ca2 RE 8(SiO4)6O2 (RE = La, Nd, Sm, Eu, or Yb) and NaLa9(SiO4)6O2, were synthesized using solution-based processes followed by cold pressing and sinter-ing. The crystal structures of the synthesized oxyapatites were determined from powder X-ray diffraction (P-XRD) and their chemistries verified with electron probe microanalysis (EPMA). All the oxyapatites were isostructural within the hexa-gonal space group P63/m and showed similar unit-cell parameters. The isolated [SiO4]4- tetra-hedra in each crystal are linked by the cations at the 4f and 6h sites occupied by RE 3+ and Ca2+ in Ca2 RE 8(SiO4)6O2 or La3+ and Na+ in NaLa9(SiO4)6O2. The lattice parameters, cell volumes, and densities of the synthesized oxyapatites fit well to the trendlines calculated from literature values.

Elaboration, vibrational study and thermal behavior of lacunar apatites NaPb3–xCdxCa(PO4)3 (0 ≤ x ≤ 1)

In this work we are interested in the synthesis of a new solid solution type NaPb3–xCdxCa(PO4)3; (0 ≤ x ≤ 1). The lacunar apatite series are synthesized by a solid-state reaction and studied by X-ray diffraction, infrared and Raman scattering spectroscopy, thermal gravimetric analysis, and differential scanning calorimetry. The crystal structure of these compounds was refined using the Rietveld method. These compounds crystallize in the hexagonal system (space group P63/m) with a number of units per crystal lattice Z = 2. The structure is formed by [PO4]3– tetra­hedra and Pb2+/Ca2+/Cd2+ ions, which constitute the basic skeleton of the structure. The Pb2+/Ca2+/Cd2+ ions occupy the 6h sites, whereas the 4f sites are occupied in half by the Pb2+/Ca2+/Cd2+ ions and the other half by the Na+ ions. The observed frequencies in the Raman and infrared spectra are explained and discussed based on the factor group analysis and by comparison with similar apatites. The vibrational spectroscopy results are in good agreement with the X-ray diffraction measurements. The internal modes of (PO4)3– tetrahedra are assigned and corroborate well with the factor group analysis for the symmetry P63/m. We also investigated the thermal stability of these apatite materials by differential scanning calorimetry.

Good comprehensive performance of Laves phase Hf1-xTaxFe2 as negative thermal expansion materials

Negative thermal expansion (NTE) materials can compensate for the normal positive thermal expansion (PTE) of most materials, and thus have great potential applications. Itinerant magnetic Laves phase compounds Hf1-xTaxFe2 with x ∼0.16–0.22 exhibit an abrupt volume shrink as large as ΔV/V ∼1% at the ferromagnetic (FM) to antiferromagnetic phase transition. Here we report that by reducing the Ta concentration the sharp volume change was gradually modified to a continuous one and moved to room temperature. NTE was optimized in x = 0.13, showing a linear NTE coefficient as large as −16.3 ppm/K over a broad window of 105 K (222 K - 327 K). As revealed by Electron Spin Resonance, the broadened NTE window is closely coupled with the asynchronous FM orderings of Fe moments at 6h and 2a Fe sites. In addition to good mechanical properties (i.e., Young's modulus, compressive strength and Vickers hardness), their thermal and electrical conductivities are superior to other metallic NTE materials, suggesting their wide applications as PTE compensators.