RESUMO
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 atT= 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 coefficientαvâ¼ -8.9(1) × 10-5K-1when 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.
RESUMO
The High-Dynamic Double-Crystal Monochromator (HD-DCM) is a mechatronic system with unique control-based architecture and deep paradigm changes as compared with traditional beamline monochromators. Aiming at unprecedented inter-crystal positioning stability in vertical-bounce double-crystal monochromators (DCMs) of the order of 10â nrad RMS (1â Hz to 2.5â kHz), and not only in fixed-energy but also in fly-scan operation, it has been developed according to a `first-time right' predictive design approach for hard X-ray beamlines at Sirius, the fourth-generation light source at the Brazilian Synchrotron Light Laboratory (LNLS/CNPEM). This work explores some of the challenges that emerge with this new technology and presents the latest commissioning results that demonstrate the unparallel performances of the HD-DCM at the undulator-based EMA (Extreme Methods of Analysis) beamline at Sirius. With the enabled fast spectroscopy fly-scan possibilities, a new energy-tuning evaluation method, based on wave-propagation simulations, becomes part of a motion-oriented analysis that is carried out to derive the multi-axis non-linear positioning problem, covering not only energy selection and fixed exit in the HD-DCM but also the emission spectrum of an adjustable-phase undulator (APU). The HD-DCM control scheme and its flexible operation modes are described in detail as well. Furthermore, a new integration topology between the HD-DCM and EMA's APU, coming already close to ultimate motion levels, is described and validated.