The first eight years of operation of the Cold Neutron Chopper Spectrometer (CNCS) at the Spallation Neutron Source in Oak Ridge is being reviewed. The instrument has been part of the facility user program since 2009, and more than 250 individual user experiments have been performed to date. CNCS is an extremely powerful and versatile instrument and offers leading edge performance in terms of beam intensity, energy resolution, and flexibility to trade one for another. Experiments are being routinely performed with the sample at extreme conditions: T ≲ 0.05 K, p ≳ 2 GPa, and B = 8 T can be achieved individually or in combination. In particular, CNCS is in a position to advance the state of the art with inelastic neutron scattering under pressure, and some of the recent accomplishments in this area will be presented in more detail.

The cold neutron chopper spectrometer at the Spallation Neutron Source—A review of the first 8 years of operation

The first eight years of operation of the Cold Neutron Chopper Spectrometer (CNCS) at the Spallation Neutron Source in Oak Ridge is being reviewed. The instrument has been part of the facility user program since 2009, and more than 250 individual user experiments have been performed to date. CNCS is an extremely powerful and versatile instrument and offers leading edge performance in terms of beam intensity, energy resolution, and flexibility to trade one for another. Experiments are being routinely performed with the sample at extreme conditions: T~0.05K, p>=2GPa and B=8T can be achieved individually or in combination. In particular, CNCS is in a position to advance the state of the art with inelastic neutron scattering under pressure, and some of the recent accomplishments in this area will be presented in more detail.

The Cold Neutron Chopper Spectrometer at the Spallation Neutron Source - A Review of the first 8 Years of Operation

Inelastic neutron scattering (INS) spectra are presented for chromium(II) Tutton salts, as a function of the temperature and pressure. Transitions are observed between the levels of the 5Ag (Ci) ground term and the data modeled with a conventional S = 2 spin Hamiltonian. At 10 K and ambient pressure, the zero-field-splitting parameters of the ammonium salt, (ND4)2Cr(D2O)6(SO4)2, are determined as D = −2.431(4) cm-1 and E = 0.091(4) cm-1, evolving to D = −2.517(4) cm-1 and E = 0.127(5) cm-1 upon application of 7.5(1.0) kbar of quasi-hydrostatic pressure. By contrast, the change in the INS spectrum of the rubidium salt in this pressure range is comparitively minor. The results are interpreted using a 5E⊗e vibronic-coupling Hamiltonian, in which low-symmetry strain, perturbing the adiabatic potential-energy surface, is pressure-dependent. It is argued that, for the ammonium salt, the change with pressure of the anisotropic strain impinging upon the [Cr(D2O)6]2+ cation is sufficient to cause a switch of the l...

Pressure-induced switch of the direction of the unique Jahn-Teller axis of the chromium(II) hexaqua cation in the deuterated ammonium chromium Tutton salt.

The possibilities of the use of Eu3+ in extracting information of the pressure effects on the nature of its crystal site in the NH4.Eu(SO4)2 catalytic host are closely inspected through the study of emission spectra for applied pressures up to 87 kbar. The phenomenological crystal field analysis of these spectra reveals clear discontinuities, at approximately 30 kbar, the sharper ones, and then at approximately 70 kbar, in crystal field strength trends, which taken together with structure-based simulations of crystal field interactions indicate well-defined pressure-induced anisotropic distortions in Eu3+ local environments.

Effects of high pressure on the luminescence spectra of Eu(SO4)2.NH4 microcrystals: anisotropically induced structural distortions.

The europium germanate NaEu3(GeO4)2(OH)2 was prepared under mild hydrothermal conditions. In situ synchrotron X-ray diffraction analysis and photoluminescence spectroscopic measurement in a diamond anvil cell (DAC) under high pressure were performed to study its structural stability as well as the luminescent properties of the Eu3+ ions. The synchrotron X-ray diffraction analysis revealed that the structure of NaEu3(GeO4)2(OH)2 was stable under pressure that ranged from 1 atm to 20 GPa with only a little distortion, and the unit-cell compression occurred more readily along the a axis than along the c axis. When the pressure was reduced to ambient pressure, the distortion and the unit cell recovered to the original state. Photoluminescence spectroscopic studies under high pressure showed that the redshift of the 5D0 7FJ (J = 0, 1, 2) transitions and the pressure imposed on the sample had a good linear relationship. Also, the intensity of the red emission lines was independent of the pressure in the range from 9 to about 20 GPa.

NaEu3(GeO4)2(OH)2: A High-Pressure-Stable Photoluminescent Lanthanide Germanate

The crystal fields (CFs) of the binary rare-earth compounds PrAl3 and NdAl3 have been examined at ambient pressure by means of inelastic neutron scattering. The CF of the latter compound has also been measured under hydrostatic pressure (p = 0.84 GPa). The observed substantial changes of the CF under pressure are discussed within the framework of first-principles density functional theory calculations.

https://iopscience.iop.org/article/10.1088/0953-8984/15/19/325/pdf

Crystal-field excitations in PrAl3 and NdAl3 at ambient and elevated pressure

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Crystal-field excitations in PrAl3 and NdAl3 at ambient and elevated pressure