Daniel Antonio

The Hardest Superconducting Metal Nitride

Transition–metal (TM) nitrides are a class of compounds with a wide range of properties and applications. Hard superconducting nitrides are of particular interest for electronic applications under working conditions such as coating and high stress (e.g., electromechanical systems). However, most of the known TM nitrides crystallize in the rock–salt structure, a structure that is unfavorable to resist shear strain, and they exhibit relatively low indentation hardness, typically in the range of 10–20 GPa. Here, we report high–pressure synthesis of hexagonal δ–MoN and cubic γ–MoN through an ion–exchange reaction at 3.5 GPa. The final products are in the bulk form with crystallite sizes of 50 – 80 μm. Based on indentation testing on single crystals, hexagonal δ–MoN exhibits excellent hardness of ~30 GPa, which is 30% higher than cubic γ–MoN (~23 GPa) and is so far the hardest among the known metal nitrides. The hardness enhancement in hexagonal phase is attributed to extended covalently bonded Mo–N network than that in cubic phase. The measured superconducting transition temperatures for δ–MoN and cubic γ–MoN are 13.8 and 5.5 K, respectively, in good agreement with previous measurements.

Technetium Tetrachloride Revisited: A Precursor to Lower-Valent Binary Technetium Chlorides

Technetium tetrachloride has been prepared from the reaction of technetium metal with excess chlorine in sealed Pyrex ampules at elevated temperatures. The product was characterized by single-crystal and powder X-ray diffraction, transmission electron microscopy, scanning electron microscopy, and alternating-current magnetic susceptibility. Solid TcCl(4) behaves as a simple paramagnet from room temperature down to 50 K with μ(eff) = 3.76 μ(B). Below 25 K, TcCl(4) exhibits an antiferromagnetic transition with a Néel temperature (T(N)) of ∼24 K. The thermal behavior of TcCl(4) was investigated under vacuum at 450 °C; the compound decomposes stepwise to α-TcCl(3) and TcCl(2).

Pressure induced structural transition and enhancement of superconductivity in Co doped CeFeAsO

The superconducting CeFe1−xCoxAsO (Co=0.1) oxyarsenide with a transition temperature (Tc) 11.4 K has been investigated by in situ high pressure synchrotron x-ray diffraction, magnetization, and resistivity measurements. The experiments performed at 10 K up to 6 GPa and at room temperature (RT) up to 55 GPa indicate large anisotropic lattice compression. A pressure induced structural change to a collapsed tetragonal structure is observed above 10 GPa at RT. We report here the enhancement of Tc from 11.4 to 12.3 K with a small increase in pressure up to 0.4 GPa and is first observed in an electron doped Ce-1111 system.

Effect of Pressure on Valence and Structural Properties of YbFe2Ge2 Heavy Fermion Compound--A Combined Inelastic X-ray Spectroscopy, X-ray Diffraction, and Theoretical Investigation.

The crystal structure and the Yb valence of the YbFe2Ge2 heavy fermion compound was measured at room temperature and under high pressures using high-pressure powder X-ray diffraction and X-ray absorption spectroscopy via both partial fluorescence yield and resonant inelastic X-ray emission techniques. The measurements are complemented by first-principles density functional theoretical calculations using the self-interaction corrected local spin density approximation investigating in particular the magnetic structure and the Yb valence. While the ThCr2Si2-type tetragonal (I4/mmm) structure is stable up to 53 GPa, the X-ray emission results show an increase of the Yb valence from v = 2.72(2) at ambient pressure to v = 2.93(3) at ∼9 GPa, where at low temperature a pressure-induced quantum critical state was reported.

Antiferromagnetism in Pr 3 In

We present neutron diffraction, magnetic susceptibility and specific heat data for a single-crystal sample of the cubic Cu3Au structure compound Pr3In. Antiferromagnetic order occurs below TN=12 K with propagation vector 0,0,0.5± where 1/12. The neutron diffraction results can be approximated with a model where the moments in each of the three Pr sublattices form ferromagnetic sheets perpendicular to and alternating in sign along the propagation direction, with a 12-unit-cell square-wave modulation. The very small specific heat anomaly that we observe at TN=12 K may be a consequence of the fact that the ordered moment is induced in the 1 singlet when TTN. The magnetic susceptibility indicates that in addition to the antiferromagnetic transition at 12 K, there may be a transition near 70 K below which there is a very small remnant magnetization 0.005B.

High pressure effects on U L3 x-ray absorption in partial fluorescence yield mode and single crystal x-ray diffraction in the heavy fermion compound UCd11

We report a study of high pressure x-ray absorption (XAS) performed in the partial fluorescence yield mode (PFY) at the U L3 edge (0–28.2 GPa) and single crystal x-ray diffraction (SXD) (0–20 GPa) on the UCd11 heavy fermion compound at room temperature. Under compression, the PFY-XAS results show that the white line is shifted by +4.1(3) eV at the highest applied pressure of 28.2 GPa indicating delocalization of the 5f electrons. The increase in full width at half maxima and decrease in relative amplitude of the white line with respect to the edge jump point towards 6d band broadening under high pressure. A bulk modulus of K0 = 62(1) GPa and its pressure derivative, K0 = 4.9(2) was determined from high pressure SXD results. Both the PFY-XAS and diffraction results do not show any sign of a structural phase transition in the applied pressure range.

High Pressure Materials Physics Research at UNLV

High-pressure studies on strongly correlated-electron systems allow the study of the relationship between structural, elastic, electronic, and magnetic properties of d-and f-band systems. The High Pressure Science and Engineering Center (HiPSEC) at UNLV performs interdisciplinary research on a wide variety of materials at high pressures. One such system, YbB2 displays antiferromagnet order at ambient pressure. We present heat capacity measurements at high magnetic fields to 9 T and structural measurement at pressures up to 5 GPa on YbB2.

Antiferromagnetism in Pr3In

We present neutron diffraction, magnetic susceptibility and specific heat data for a single-crystal sample of the cubic Cu3Au structure compound Pr3In. Antiferromagnetic order occurs below TN=12 K with propagation vector 0,0,0.5± where 1/12. The neutron diffraction results can be approximated with a model where the moments in each of the three Pr sublattices form ferromagnetic sheets perpendicular to and alternating in sign along the propagation direction, with a 12-unit-cell square-wave modulation. The very small specific heat anomaly that we observe at TN=12 K may be a consequence of the fact that the ordered moment is induced in the 1 singlet when TTN. The magnetic susceptibility indicates that in addition to the antiferromagnetic transition at 12 K, there may be a transition near 70 K below which there is a very small remnant magnetization 0.005B.

Antiferromagnetism in Pr{sub 3}In

We present neutron diffraction, magnetic susceptibility and specific heat data for a single-crystal sample of the cubic Cu3Au structure compound Pr3In. Antiferromagnetic order occurs below TN=12 K with propagation vector 0,0,0.5± where 1/12. The neutron diffraction results can be approximated with a model where the moments in each of the three Pr sublattices form ferromagnetic sheets perpendicular to and alternating in sign along the propagation direction, with a 12-unit-cell square-wave modulation. The very small specific heat anomaly that we observe at TN=12 K may be a consequence of the fact that the ordered moment is induced in the 1 singlet when TTN. The magnetic susceptibility indicates that in addition to the antiferromagnetic transition at 12 K, there may be a transition near 70 K below which there is a very small remnant magnetization 0.005B.