A detailed high pressure X-ray diffraction and Raman spectroscopy study is carried out on monolayer WS2 and nanocrystalline WS2. The monolayer sample is obtained by liquid exfoliation. Photoluminescence and Raman measurements show it to consist of a monolayer. Careful analysis of ambient and high pressure data indicates the emergence of a triclinic phase at about 5.8 GPa in patches embedded in the parent hexagonal phase. This raises a question mark over the structural purity of the exfoliated monolayer materials beyond certain stress conditions. Raman mode values and their full width at half maximum of the monolayer sample show anomalous changes at about 27 GPa, the pressure where the sample completely gets converted to the triclinic structure indicating the importance of strain in structural as well as electronic properties of two dimensional materials.

We have carried out detailed experimental investigations on polycrystalline CuO using dielectric constant, dc resistance, Raman spectroscopy and X-ray diffraction measurements at high pressures. Observation of anomalous changes both in dielectric constant and dielectric loss in the pressure range 3.7–4.4 GPa and reversal of piezoelectric current with reversal of poling field direction indicate to a change in ferroelectric order in CuO at high pressures. A sudden jump in Raman integrated intensity of Ag mode at 3.4 GPa and observation of Curie-Weiss type behaviour in dielectric constant below 3.7 GPa lends credibility to above ferroelectric transition. A slope change in the linear behaviour of the Ag mode and a minimum in the FWHM of the same indicate indirectly to a change in magnetic ordering. Since all the previous studies show a strong spin-lattice interaction in CuO, observed change in ferroic behaviour at high pressures can be related to a reentrant multiferroic ordering in the range 3.4 to 4.4 GPa, much earlier than predicted by theoretical studies. We argue that enhancement of spin frustration due to anisotropic compression that leads to change in internal lattice strain brings the multiferroic ordering to room temperature at high pressures.

Fe 4 Nb 2 O 9 was recently reported to be a new magnetoelectric material with two distinct dielectric anomalies located at T N ≈ 90 K for an antiferromagnetic transition and T str ≈ 77 K of unknown origin, respectively. By analyzing low-temperature neutron-powder-diffraction data, here we determined its magnetic structure below T N and uncovered the origin of the second dielectric anomaly as a structural phase transition across T str . In the antiferromagnetically ordered state below T N , both Fe1 and Fe2 magnetic moments lying within the weakly and strongly buckled honeycomb layers are arranged in a fashion that the three nearest neighbors are directed oppositely. Upon cooling below T str , the symmetry of crystal structure is lowered from trigonal P-3c1 to monoclinic C2/c, in which a weak sliding of the metal octahedral planes introduces a monoclinic distortion of ∼1.7 • . The magnetic structure is preserved in the low-temperature monoclinic phase, and the Fe magnetic moment increases from 2.1(1)μ B at 95 K to 3.83(4)μ B at 10 K assuming an equal moment configuration at Fe1 and Fe2 sites. The magnetic point group and linear magnetoelectric tensor at each temperature region are determined. From a symmetry-related tensor analysis, the microscopic origins of the magnetoelectric effects between T N and T str are proved to be spin-current and d-p hybridization mechanisms.

Plastic bending of organic crystals is a well-known, yet mechanistically poorly understood phenomenon. On three structurally related epimers, derivatives of galactose, glucose, and mannose, it is demonstrated here that small changes in the molecular structure can have a profound effect on the mechanical properties. While the galactose derivative affords crystals which can be easily bent, the crystals of the derivatives of glucose and mannose are brittle and do not bend. Structural, microscopic, and mechanical evidence is provided showing that hydrogen bonding of water molecules is the key element for sliding over the slip planes in the crystal and accounts for the plastic bending.

We report a comprehensive high-pressure study on the monoclinic TlFeSe2 single crystal, which is an antiferromagnetic insulator with quasi-one-dimensional crystal structure at ambient pressure. It is found that TlFeSe2 undergoes a pressure-induced structural transformation from the monoclinic phase to an orthorhombic structure above P c ≈ 13 GPa, accompanied with a large volume collapse of ΔV/V 0 = 8.3%. In the low-pressure monoclinic phase, the insulating state is easily metallized at pressures above 2 GPa; while possible superconductivity with T c onset ∼ 2 K is found to emerge above 30 GPa in the high-pressure phase. Such a great tunability of TlFeSe2 under pressure indicates that the ternary AFeSe2 system (A = Tl, K, Cs, Rb) should be taken as an important platform for explorations of interesting phenomena such as insulator-metal transition, dimensionality crossover, and superconductivity.

High pressure behaviour of nanocrystalline YCrO is investigated up to 10 GPa using electrical, magnetic, synchrotron x-ray diffraction and Raman spectroscopy measurements. High pressure dielectric constant measurements show a sharp peak at 4.5 GPa, though the sample is found to be in ferroelectric phase up to the highest pressure of our study from piezoelectric current measurements. X-ray diffraction measurements show absence of any structural phase transition, however anomalies are observed in the unit cell structural parameters at about 4.3 GPa and the Y-atom position shows a maximum shift at the same pressure. In the absence of any structural transition, anomalous behaviour of relevant Raman modes with minimum in the Raman band width at about same pressure indicate towards a spin-phonon interaction. AC magnetic measurements in the toroid anvil cell show an anomalous enhancement of magnetic moment above 4 GPa indicating a collective magnetic response of nanoparticles.

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High pressure Raman spectroscopy, X-ray diffraction, and dielectric measurements have been carried out in Ba1−xSrxTiO3 (x = 0.05 and 0.1). Detailed structural analysis revealed a single phase transition from tetragonal P4mm to cubic Pm3m symmetry. Increase in Sr ion concentration resulted in decrease in the phase transition pressure. The dielectric measurements showed considerable lowering of transition pressure which has been attributed to bulk behaviour of the material.