Physical properties of new S = 3/2 triangular-lattice compounds LiCrSe2, LiCrTe2, and NaCrTe2 have been investigated by X-ray diffraction and magnetic measurements. These compounds crystallize in the ordered NiAs-type structure, where alkali metal ions and Cr atoms stack alternately. Despite their isomorphic structures, magnetic properties of these three compounds are different; NaCrTe2 has an A-type spin structure with ferromagnetic layers, LiCrTe2 is likely to exhibit a helical spin structure, and LiCrSe2 shows a first-order-like phase transition from the paramagnetic trigonal phase to the antiferromagnetic monoclinic phase. In these compounds and the other chromium chalcogenides with a triangular lattice, we found a general relationship between the Curie-Weiss temperature and magnetic structures. This relation indicates that the competition between the antiferromagnetic direct d-d exchange interaction and the ferromagnetic superexchange interaction plays an important role in determining the ground state of chromium chalcogenides.
Anomalous successive
structural transitions in layered 1T-CrSe2 with an unusual Cr4+ valency
were investigated by synchrotron X-ray diffraction. 1T-CrSe2 exhibits dramatic structural changes in in-plane
Cr–Cr and interlayer Se–Se distances, which originate
from two interactions: (i) in-plane Cr–Cr interactions derived
from Peierls-like trimerization instabilities on the orbitally assisted
one-dimensional chains and (ii) interlayer Se–Se interactions
through p–p hybridization. As a result, 1T-CrSe2 has the unexpected ground state of an antiferromagnetic
metal with multiple Cr linear trimers with three-center–two-electron
σ bonds. Interestingly, partial substitution of Se for S atoms
in 1T-CrSe2 changes the ground state from
an antiferromagnetic metal to an insulator without long-range magnetic
ordering, which is due to the weakening of interlayer interactions
between anions. The unique low-temperature structures and electronic
states of this system are determined by the competition and cooperation
of in-plane Cr–Cr and interlayer Se–Se interactions.
Very thin aluminum-doped zinc oxide (AZO) films with a well-defined (0001) orientation and a surface roughness of 0.357 nm were deposited on amorphous glass substrates at a temperature of 200 °C by radio frequency magnetron sputtering, which are promising, particularly in terms of orientation evolution, surface roughness, and carrier transport, as buffer layers for the subsequent deposition of highly (0001)-oriented AZO polycrystalline films of 490 nm thickness by direct current (DC) magnetron sputtering. Sintered AZO targets with an Al2O3 content of 2.0 wt. % were used. DC magnetron sputtered AZO films on bare glass substrates showed a mixed (0001) and the others crystallographic orientation, and exhibited a high contribution of grain boundary scattering to carrier transport, resulting in reduced Hall mobility. Optimizing the thickness of the AZO buffer layers to 10 nm led to highly (0001)-oriented bulk AZO films with a marked reduction in the above contribution, resulting in AZO films with improved Hall mobility together with enhanced carrier concentration. The surface morphology and point defect density were also improved by applying the buffer layers, as shown by atomic force microscopy and Raman spectroscopy, respectively.
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