Electrical resistivity of the FeXMn1-XS (0 <= X <= 0.29) single crystals based on the Mott insulator alpha-MnS is experimentally studied in the temperature range of 2-300 K at ambient pressure (P = 0) and at high (up to 30 GPa) hydrostatic pressures for x = 0.12. The electron subsystem of FeXMn1-XS undergoes insulator-to-metal transitions indicated by a resistivity drop by a factor of 10(6) with an increase in the chemical pressure (X) due to the cation substitution under ambient conditions and at the hydrostatic pressure (P-C = 27 +/- 5 GPa) for x = 0.12. The results obtained show that the hydrostatic pressure - and cation-substitution induced insulator-to-metal transitions in the alpha-MnS-based Mott compounds have similar mechanisms. The dependence of the critical hydrostatic pressure P-C on the Fe content in FeXMn1-XS is established.

Neutron heavy irradiation was performed on synthetic diamonds contain nitrogen atoms in isolated substitutional form (called “type Ib diamond”) and they were annealed under a pressure of 6 GPa. A large number of nitrogen B-aggregate which consists of four substitutional nitrogen atoms symmetrically surrounding a vacancy was formed within 30 m from single nitrogen atoms. Furthermore it is observed that, in these diamonds, single nitrogen atoms coexist with the B-aggregates, which is unexplainable by the simple nitrogen aggregation model.

Studies on the Raman peaks appear at 593, 625, 651 and 1198cm(-1) in the intra-molecule hydrogen bond material 5-bromo-9-hydroxyphenalenone were performed under various pressure and temperature conditions. These peaks show two characteristic results: (1) in high pressure study, their pressure coefficients are approximately 4-6 times larger than those of other Raman peaks of this material, (2) in the latter experiment, they decrease in intensity with temperature and vanish around room temperature. These results support a model that these Raman peaks are generated by intra-molecule vibrations coupled with a proton tunneling motion.

Fe x Mn1−x S belongs to the group of strong electron correlations compounds MnO. We present here experimental results for the antiferromagnetic iron–manganese sulfide system, based on X-ray and neutron diffraction studies. The neutron diffraction investigations were carried out at ambient conditions and at hydrostatic pressures up to 4.2 GPa in the temperature range from 65 to 300 K. Our results indicate that the Néel temperature of α-MnS increases up to room temperature by applying chemical (x Fe) or weak hydrostatic pressure P. In Fe0.27Mn0.73S, the Néel temperature increases from 205(2) K (P = 0 GPa) to 280(2) K (P = 4.2 GPa) and the magnetization at 100 K decreases by a factor of 2.5 when the hydrostatic pressure increases from 0 to 4.2 GPa.

A pressure-induced phase transition in the colossal magnetoresistance (CMR) material Fe0.18Mn0.82S was studied by using infrared (IR) reflection and X-ray diffraction (XRD) at pressures up to 40 GPa at room temperature. XRD shows that the crystal structure of this sample is a NaCl-type structure at ambient pressure, that a structural change starts around 17 GPa, and that a mixed phase mixed between the NaCl-type low-pressure phase and an unknown structure high pressure phase continues up to around 25 GPa. On the other hand, the IR reflectivity increases with increasing pressure from 15 GPa and becomes remarkably high around 20 GPa. The spectra do not show any changes from 30 GPa. From these results, we conclude that the phase transition of Fe0.18Mn0.82S at room temperature starts around 15 GPa and is completed around 30 GPa and that the high-pressure phase is not a band-overlapping semimetal but a true metal.

The Cr x Mn 1 − x S single crystals have been synthesized based on manganese monosulfide as a result of cation substitution, and their magnetic properties have been studied. It has been established that the Cr x Mn 1 − x S solid solutions with a face-centered cubic NaCl structure are formed in the concentration region 0 ≤ x < 0.3. The unit cell parameter of the solid solution decreases as the degree of substitution increases due to the variation in the ionic radius of cations. These substances are antiferromagnets. An increase in the degree of cation substitution in the Cr x Mn 1 − x S solid solutions is accompanied by a decrease in the number of 3 d electrons in the d shell of manganese monosulfide and causes a decrease in the magnetic transition temperature from 149 K ( x = 0) to 96 K ( x = 0.29), which differs from previously known results.

Absorption measurements were carried out on the (NV)(-) centers in diamond at low temperature after laser illumination. It was observed that the width of the zero-phonon line becomes persistently more than two times larger. This characteristic phenomenon was studied in detail about excitation photon energy, illumination time, sample temperature and recovery nature. (c) 2007 WILEYNCH Verlag GmbH & Co. KGaA, Weinheim.

Raman studies were performed on 0-dimentional hydrogen bond system crystalline 5-bromo-9-hydroxyphenalenone and its deuterated material 5-bromo-9-deuteroxyphenalenone at 40 K. In the former, four Raman peaks which the latter lack were observed at 593, 625, 651 and 1198 cm(-1). Since their molecular structures are the same except for the hydrogen bond part, the appearance of additional peaks suggests a possibility that the origins of these peaks are intra-molecule phonons coupled with proton tunnelling motion.

The infrared reflection measurements of NaCl type manganese monochalcogenides MnO and α‐MnS were performed under high pressure at room temperature. In both case, drastic increase in reflectivity was observed from 94 and 23 GPa and it becomes pressure‐insensitive around 127 and 29 GPa, respectively. These results strongly suggest that the metallic transitions occur around the pressure regions, which are consistent with the result of recent X‐ray studies.

Results of an experimental study of MnS, FeS, and Fe x Mn 1− x S single crystals are presented. The phase composition, the lattice parameters, and the state of paramagnetic ions in Fe x Mn 1− x S have been determined by x-ray diffraction analysis and Mössbauer spectroscopy. A sequence of transitions have been found in iron manganese sulfide with x = 0.29 at temperatures T 1 ≈ 25–50 K, T 2 ≈ 125 K, and T 3 ≈ 190 K with a change in kinetic properties and the formation of a metallic state at low temperatures T ≈ 2 K. The possibility of a Mott-Hubbard transition in Fe x Mn 1− x S sulfides with variation of the composition and the temperature is discussed.

Raman scattering spectra of a KH2PO4 (KDP) single crystal have been obtained under various temperature and pressure conditions. It was observed that (i) the peak around 1310 cm−1 is not a single peak as reported previously but consists of two components and (ii) in the ferroelectric phase the 1013, 1306 and 1315 cm−1 peaks become obscure near the ferroelectric–paraelectric phase boundary. Furthermore, they show positive pressure shifts and the pressure coefficients are obtained. An interpretation is proposed for the origins of these Raman peaks.

Infrared (IR) absorption measurements were performed at room temperature on neutron-irradiated type Ib diamonds which contain around 100 ppm substitutional nitrogen atoms in single form. It was observed that the 1332 cm(-1) absorption peak appears by irradiation but vanishes again at the dose region higher than 2 x 10(18) n/cm(2). This phenomenon is explained by using two-step charge compensation model. Furthermore three remarkable peaks are observed at 1530 and 1570 cm(-1) and at 1450cm(-1). Relations between absorption intensity and neutron dose of these peaks were studied. Intensity of these peaks increase linearly with dose; however, the 1450 cm(-1) peak intensity saturates around 2 x 10(17) n/cm(2) in contrast with the others. These results are consistent with the interstitial models proposed previously. (c) 2005 Elsevier B.V. All rights reserved.

Optical absorption and IR reflection measurements of MnO were performed under various pressures up to 140 GPa at room temperature. In IR reflection measurement, a drastic increase in reflectivity due to the metallization is observed from 94 GPa. On the other hand, in absorption, it is observed that the band gap does not close monotonously with pressure but accelerated extremely from 80 GPa, as indicated by an abrupt increase of an additional absorption tail located at the lower energy than the absorption edge. The absorption tail extends to 0 eV around 90 GPa, which is consistent with the IR reflection measurement. The pressure coefficient of the band gap shift is obtained to be -0.016 eV/GPa up to 80 GPa.

The absorption measurements of alpha-MnS were performed under high pressure up to 22GPa at room temperature. It was observed that the band gap shift monotonously with applied pressure up to 17GPa with the pressure coefficient of −0.039eV/GPa. However, in the higher pressure region, gradient of the absorption tail becomes gentle. Tip of the tail reaches to 0eV at 22GPa, which means that the metallic transition starts around the pressure. This result shows excellent agreement with the previous works.

The IR reflection measurements of α-MnS have been performed at room temperature under various pressures. It is observed that the reflectivity increases drastically at the pressure range of 23–29GPa and becomes almost constant at the higher pressure. The carrier concentrations obtained from the reflectivity spectra at the pressures higher than 29GPa are the order of 1022cm−3. Therefore it is concluded that pressure-induced semiconductor–metal transition occurs at the pressure range of 23–29GPa. This result is quite consistent with the newest results of X-ray analysis.

Raman and absorption measurements under high pressure were performed in NiO single crystals at room temperature. A drastic increase of the absorption intensity was observed in the whole visible range. Furthermore, the intensity of a defect induced Raman peak located around 500 cm–1 becomes remarkably, especially at 25 GPa. Neither of the spectral changes recover even after removing the pressure. These results indicate that a permanent lattice disorder is introduced. The remarkable spectral changes are explained to be a result of defects (especially singly charged nickel vacancies) introduced by a deformation of the crystal due to the non‐hydrostatic compression.

Pressure dependence of Raman scattering in solid oxygen was studied at 1.8 K. When the alpha phase is compressed, two libron peaks that, increasing pressure, merge smoothly with those of the epsilon phase at 8 GPa are observed around 5 GPa, besides the libron peaks due to the alpha phase. Furthermore, unknown Raman peaks are also observed at 5-9 GPa. Coexistence of these Raman peaks indicates that the pressure region from 5 to 9 GPa at low temperature does not correspond to a single phase but complicated mixed ones including alpha and epsilon components.

Pressure dependence of magnon Raman scattering was studied at 1.8 K up to 4 and 60 GPa for solid oxygen and MnO, respectively. In both cases, magnon peaks shift to the higher energy side with increasing pressure. In solid oxygen, the pressure coefficients and intensity of magnon Raman peak are observed to change simultaneously around 0.1 GPa and the peak vanishes at 2.9 GPa. In MnO, the two-magnon Raman peak vanishes around 20 GPa. These results mean that changes in spin configurations (i.e. magnetic phase transitions) occur at these pressures.

Chains of crystalline-Si nanospheres were studied by means of Raman scattering spectroscopy. We found that the one-phonon Raman scattering peak from the chains was asymmetric and broader than that from bulk Si. This phenomenon can be attributed to a phonon confinement in the silicon nanospheres. The phonon confinement became more obvious by decreasing the size of the silicon nanospheres in the chains. We also found that the Si nanospheres in the chains were under compressive stress by the covering oxide layers through the analysis of the Raman shift.