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We measured the volume of hafnium at several pressures up to 67 GPa and at temperatures between 300 to 780 K using a resistively heated diamond anvil cell with synchrotron x-ray diffraction at the Advanced Photon Source. The measured data allows us to determine the P-V-T equation of state of hafnium. The previously described [Xia et al., Phys. Rev. B 42, 6736-6738 (1990)] phase transition from hcp (α) to simple hexagonal (ω) phase at 38 GPa at room temperature was not observed even up to 51 GPa. The ω phase was only observed at elevated temperatures. Our measurements have also improved the experimental constraint on the high P-T phase boundary between the ω phase and high pressure bcc (β) phase of hafnium. Isothermal room temperature bulk modulus and its pressure derivative for the α-phase of hafnium were measured to be B0 = 112.9 ± 0.5 GPa and B0′ = 3.29 ± 0.05, respectively. P-V-T data for the α-phase of hafnium was used to obtain a fit to a thermodynamic P-V-T equation of state based on model by Brosh et al. [CALPHAD 31, 173-185 (2007)]. [ABSTRACT FROM AUTHOR]

Polycrystalline tungsten hexacarbonyl, W(CO)6, was studied using in situ Raman spectroscopy and synchrotron x-ray diffraction (XRD) at high pressures up to 60 GPa in a diamond anvil cell. High pressure causes collapse of the Oh molecular symmetry in W(CO)6 leading to decomposition of the carbonyl compound. The decomposed material has characteristic of δ(OCO), v(C=C), v(C=O), and adsorbed CO Raman features. High pressure XRD results showed the irreversible amorphization of tungsten hexacarbonyl. A solid state vibrational coupling mechanism is proposed to explain the formation of δ(OCO) units. Further, the high pressure Raman results of W, Mo, and Cr hexacarbonyls are compared and the breakdown of the Dewar-Chatt-Duncanson model in this family is qualitatively discussed. [ABSTRACT FROM AUTHOR]

The effect of pressure on the low temperature tetragonal (I4mm) to orthorhombic (Pmn21) phase transition of a potential hydrogen storage compound ammonia borane (NH3BH3) was investigated in diamond anvil cell using Raman spectroscopy. With applied pressure, the transition occurs at higher temperature, which indicates that pressure enhances the ordering of the structure. The positive Clapeyron slope of the transition was determined to be dP/dT = ∼25.7 MPa/K, indicating the transformation is of exothermic. Appearance of some of the characteristic Raman modes of orthorhombic phase requires undercooling of around ∼15 K below the transition, indicating possible existence of an intermediate phase. [ABSTRACT FROM AUTHOR]

We have studied the structural stability of NaBH4 under pressures up to 17 GPa and temperatures up to 673 K in a diamond anvil cell and formed an extended high P-T phase diagram using combined synchrotron x-ray diffraction and Raman spectroscopy. Even though few reports on phase diagram of NaBH4 are found in current literature, up to our knowledge this is the first experimental work using diamond anvil cell in a wide pressure/temperature range. Bulk modulus, its temperature dependence, and thermal expansion coefficient for the ambient cubic phase of NaBH4 are found to be 18.76(1) GPa, -0.0131 GPa K-1, and 12.5×10-5+23.2×10-8 T/K, respectively. We have also carried out Raman spectroscopic studies at room temperature up to 30 GPa to reinvestigate the phase transitions observed for NaBH4. A comparative symmetry analysis also has been carried out for different phases of NaBH4. [ABSTRACT FROM AUTHOR]

We have studied the structural stability of NaBH4 under pressures up to 17 GPa and temperatures up to 673 K in a diamond anvil cell and formed an extended high P-T phase diagram using combined synchrotron x-ray diffraction and Raman spectroscopy. Even though few reports on phase diagram of NaBH4 are found in current literature, up to our knowledge this is the first experimental work using diamond anvil cell in a wide pressure/temperature range. Bulk modulus, its temperature dependence, and thermal expansion coefficient for the ambient cubic phase of NaBH4 are found to be 18.76(1) GPa, -0.0131 GPa K-1, and 12.5×10-5+23.2×10-8 T/K, respectively. We have also carried out Raman spectroscopic studies at room temperature up to 30 GPa to reinvestigate the phase transitions observed for NaBH4. A comparative symmetry analysis also has been carried out for different phases of NaBH4. [ABSTRACT FROM AUTHOR]

Ammonia borane, NH3BH3, has attracted significant interest as a promising candidate material for hydrogen storage. The effect of pressure on the bonding in NH3BH3 was investigated using Raman spectroscopy to over 20 GPa in a diamond anvil cell, and two new transitions were observed at approximately 5 and 12 GPa. Vibrational frequencies for the modes of the NH3 proton donor group exhibited negative pressure dependence, which is consistent with the behavior of conventional hydrogen bonds, while the vibrational frequencies of the BH3 proton acceptor group showed positive pressure dependence. The observed behavior of these stretching modes supports the presence of dihydrogen bonding at high pressure. In addition, the BH3 and NH3 bending modes showed an increase in spectral complexity with increasing pressure together with a discontinuity in dν/dP which suggests rotational disorder in this molecule. These results may provide guidance for understanding and developing improved hydrogen storage materials. [ABSTRACT FROM AUTHOR]

The nanocrystalline (mean grain size 20 nm) and microcrystalline magnetoresistive manganites with composition La0.7Ca0.3MnO3 were studied using a four probe electrical resistivity measurement in a strong magnetic field. The room temperature values of electrical resistivity are found to be 20 times larger for the nanocrystalline than for the microcrystalline sample, and therefore show a remarkable influence of fine grain microstructure. Such a difference reveals a more intense electron scattering inside the structurally and magnetically disordered grain boundaries in the nanocrystalline material than in the microcrystalline material. A pulsed magnetic field of 47 T reduces electrical resistivity three and eight times for the nano- and microcrystalline samples, respectively, and causes a monotonic shift of the transition temperature from TP=150 to 180 K in nanocrystalline manganite. In the microcrystalline sample TP shifts only from 250 to 270 K. The temperature dependencies of electrical resistivity above TP confirm that the polaronic model is applicable up to 47 T. The activation energy of the microcrystalline manganite decreases from about 2000 K down to 300 K between the 0 and 47 T. The high field activation energy of the nanocrystalline manganite is twice as large as that of the microcrystalline manganite. The strong negative magnetoresistance effect in both nano- and microcrystalline manganites reaches about 90% at 47 T in different temperature ranges. Most abrupt changes of magnetoresistance occurring at the lowest temperatures in magnetic fields up to 2 T are followed by a relatively slower approach to saturation at stronger fields for both microstructures. The magnetoresistance hysteresis found in the nanocrystalline manganite at low temperatures is related to the magnetization hysteresis appearing at relatively weak magnetic fields. The saturation magnetization in the grain bulk and grain boundaries is determined. The influence of magnetic viscosity effects on electrical resistivity of the nanocrystalline manganite is also analyzed. [ABSTRACT FROM AUTHOR]

We have studied the structural stability of NaBH(4) under pressures up to 17 GPa and temperatures up to 673 K in a diamond anvil cell and formed an extended high P-T phase diagram using combined synchrotron x-ray diffraction and Raman spectroscopy. Even though few reports on phase diagram of NaBH(4) are found in current literature, up to our knowledge this is the first experimental work using diamond anvil cell in a wide pressure/temperature range. Bulk modulus, its temperature dependence, and thermal expansion coefficient for the ambient cubic phase of NaBH(4) are found to be 18.76(1) GPa, -0.0131 GPa K(-1), and 12.5x10(-5)+23.2x10(-8) T/K, respectively. We have also carried out Raman spectroscopic studies at room temperature up to 30 GPa to reinvestigate the phase transitions observed for NaBH(4). A comparative symmetry analysis also has been carried out for different phases of NaBH(4).

Ammonia borane, NH(3)BH(3), has attracted significant interest as a promising candidate material for hydrogen storage. The effect of pressure on the bonding in NH(3)BH(3) was investigated using Raman spectroscopy to over 20 GPa in a diamond anvil cell, and two new transitions were observed at approximately 5 and 12 GPa. Vibrational frequencies for the modes of the NH(3) proton donor group exhibited negative pressure dependence, which is consistent with the behavior of conventional hydrogen bonds, while the vibrational frequencies of the BH(3) proton acceptor group showed positive pressure dependence. The observed behavior of these stretching modes supports the presence of dihydrogen bonding at high pressure. In addition, the BH(3) and NH(3) bending modes showed an increase in spectral complexity with increasing pressure together with a discontinuity in d nu/d P which suggests rotational disorder in this molecule. These results may provide guidance for understanding and developing improved hydrogen storage materials.