Mission Statement

Exploring nuclear and magnetic structures as a function of temperature, pressure, magnetic field, and electric field.

Instrument Description

The Four-Circle Diffractometer goniometer has a full χ circle with a 4–800 K closed-cycle helium refrigerator. The detector is a scintillator-based 2D Anger Camera. The upper 2Θ limit is 160°. A multilayer-[110]-wafer silicon monochromator with the reflection from planes of the <011> zone ensures sharp diffraction peaks in specified ranges of detector angles by control of the horizontal radius of curvature. Any plane from the <011> zone can be set in Bragg position, but only the (331), (220) with (440), and (111) with (333) reflections are of practical interest. For the fixed monochromator angle of 47.5°, these reflections provide principal incident wavelengths of 1.005 Å, 1.546 Å, and 2.541 Å, respectively. A PC-based LabView system provides user-friendly diffractometer control and data acquisition. The beam size is 6.3 mm in diameter, and the minimum measured crystal size is 0.02 mm3. The maximum crystal dimension is usually limited to 5 mm. The flux on the sample can be up to 2.2 × 107 n/cm2/s. The horizontal bending of the monochromator can be changed to optimize the Q-resolution or flux depending upon the needs of the measurement. The longer wavelength of 2.541 Å has ~5% λ/3 contamination and is mainly used for polarized neutron diffraction (S-bender will filter out the high-order contamination). The 1.546 Å-wavelength has the highest flux but with ~1.4% λ/2 contamination (PG filter is available to reduce the contamination below 10-4), is mainly used for determining magnetic structures. The 1.005 Å wavelength is monochromatic and is good for precisely determining both nuclear and magnetic structures although the flux is 8 times lower than the highest flux at 1.546 Å.

Applications

The HB-3A Four-Circle Diffractometer has the mission to explore nuclear and magnetic structures as a function of temperature, pressure, magnetic field, and electric field. The instrument is particularly suitable for studying magnetic structures, phase transitions and possible accompanied structural changes, as well as measuring order parameters and exploring the phase diagram. It is also suitable for a wide range of small-unit-cell crystallography studies from structure refinement and solution to charge and nuclear density mapping. It can be used to study superlattice structures and atomic anharmonicity. Users have researched problems in physics, materials science, chemistry, and mineralogy. Recent specific areas of study include magnetic structure and nuclear superstucture in iron pnictide superconductors, magnetic and nuclear structures of possible quantum magnets, Weyl semimetals, topological insulators, multiferroic oxide phase transitions and diagrams, magnetic/orbital frustration in spinels, new permanent magnets, temperature dependence of atomic displacement parameters in battery and thermoelectric materials, structural phase transitions of photo-voltaic hybrid perovskites, hydrogen bonding in rock-forming minerals, crystallography of novel scintillators, and diffuse scattering.

Specifications

Beam Spectrum:Thermal
Monochromators:Double focusing silicon
Monochromator angle:47.5°
Incident Wavelength:1.005 Å (Si-133), 1.546 Å (Si-022), 2.541 Å (Si-111)
Goniometer:Huber, full χ circle, with 4 - 800 K CCR
Scattering angle:-27° < 2θ < 160°
Detector:2D detector (scintillation Anger camera with 1 mm spatial resolution)
Crystal size requirement:> 0.1 mm3, maximum crystal dimension 5mm
Flux at sample:2.2 × 107 n/cm2/s
Polarized neutron diffraction:
S-bender super mirror polarizer
Polarization ratio 95%