2 research outputs found
Low-Temperature Vaterite-Type LuBO<sub>3</sub>, a Vacancy-Stabilized Phase Synthesized at High Temperature
Low-temperature
vaterite-type LuBO<sub>3</sub> (Ï€-LBO) was prepared by a solid-state
reaction method at high temperature. The reasoning of the existence
of vacancy-stabilized π-LBO was investigated for the first time
using neutron diffraction patterns, Fourier transform infrared (FT-IR)
spectra, and high-resolution transmission electron microscopy. The
results clearly demonstrated that the B and O vacancies in π-LBO
came into being during the heating process. The existence of an open
B<sub>3</sub>O<sub>9</sub> ring consisting of BO<sub>3</sub> and BO<sub>4</sub> units in π-LBO due to the B and O vacancies was demonstrated
by FT-IR. The vacuum ultraviolet–ultraviolet spectroscopic
properties of π-LBO were studied in detail. In addition, the
luminescence mechanism of Ce<sup>3+</sup> in π-LBO was put forward
and discussed with that of calcite-type LuBO<sub>3</sub> (β-LBO)
Giant Negative Thermal Expansion in Bonded MnCoGe-Based Compounds with Ni<sub>2</sub>In-Type Hexagonal Structure
MnCoGe-based compounds undergo a
giant negative thermal expansion
(NTE) during the martensitic structural transition from Ni<sub>2</sub>In-type hexagonal to TiNiSi-type orthorhombic structure. High-resolution
neutron diffraction experiments revealed that the expansion of unit
cell volume can be as large as Δ<i>V</i>/<i>V</i> ∼ 3.9%. The optimized compositions with concurrent magnetic
and structural transitions have been studied for magnetocaloric effect.
However, these materials have not been considered as NTE materials
partially due to the limited temperature window of phase transition.
The as-prepared MnCoGe-based compounds are quite brittle and naturally
collapse into powders. By using a few percents (3–4%) of epoxy
to bond the powders, we introduced residual stress in the bonded samples
and thus realized the broadening of structural transition by utilizing
the specific characteristics of lattice softening enforced by the
stress. As a result, giant NTE (not only the linear NTE coefficient
α but also the operation-temperature window) has been achieved.
For example, the average α̅ as much as −51.5 ×
10<sup>–6</sup>/K with an operating temperature window as wide
as 210 K from 122 to 332 K has been observed in a bonded MnCo<sub>0.98</sub>Cr<sub>0.02</sub>Ge compound. Moreover, in the region between
250 and 305 K near room temperature, the α value (−119
× 10<sup>–6</sup>/K) remains nearly independent of temperature.
Such an excellent performance exceeds that of most other materials
reported previously, suggesting it can potentially be used as a NTE
material, particularly for compensating the materials with large positive
thermal expansions