Sohd state phase transitions

X-ray powder diflraction (XRPD) provides considerably less structural information than single crystal X-ray diflraction however, it is generally considered to be the gold standard through which salt and co-crystal forms may be identified and polymorphism and solvate formation confirmed in polycrystalline soHds. Modem powder diffractometers, when equipped with temperature and/or humidity control accessories, greatly expand the range of applications of powder diffraction to include studies of solid state phase transitions, desolvation (dehydration), rehydration, melting and recrystallization. ... 

The last phase transition is to the soHd state, where molecules have both positional and orientational order. If further pressure is appHed on the monolayer, it collapses, owiag to mechanical iastabiHty and a sharp decrease ia the pressure is observed. This coUapse-pressure depends on the temperature, the pH of the subphase, and the speed with which the barrier is moved. 

Pure metallic cobalt has a soHd-state transition from cph (lower temperatures) to fee (higher temperatures) at approximately 417°C. However, when certain elements such as Ni, Mn, or Ti are added, the fee phase is stabilized. On the other hand, adding Cr, Mo, Si, or W stabilizes the cph phase. Upon fcc-phase stabilization, the energy of crystallographic stacking faults, ie, single-unit cph inclusions that impede mechanical sHp within the fee matrix, is high. 

In this review, we shall introduce some basic concepts about sohd-state NMR of half-integer quadrupolar nuclei and discuss the most useful and promising methods presently available to study them. These include older methods, such as Double Rotation (DOR) and Dynamic Angle Spinning (DAS), and novel techniques including MQMAS, Quadrupolar Phase Adjusted Spinning Sidebands QPASS, SateUite Transition (ST) MAS and Inverse-STMAS NMR, and Fast Amplitude Modulation (FAM). We also discuss several techniques based on dipolar interactions between quadrupolar and spin-1/2 nuclei, such as Cross-Polarization (CP) MQMAS, MQ Heteronuclear Correlation Spectroscopy... 

Salje EKH, Devarajan V, Bismayer U, Guimaraes DMC (1983) Phase transitions in Pb3(Pi xAsx04)2 influence of the central peak and flip mode on the Raman scattering of hard modes. J Physics C Sohd State Phys 16 5233-5243... 

The first report of the existence of fullerenes in 1985 [1], and the subsquent discovery in 1990 of a method to produce them in macroscopic amounts [2], paved the way to a new era of carbon science that involves curved surfaces on the nanoscopic scale. As is well known, the aggregation of fuUerene molecules at moderate temperatures and pressures leads to molecular sohds termed ful-lerites. The (buckminsterfuUerene) and Cyg fullerenes and the corresponding fullerites are the easiest to produce, and for this reason they have been the subject of most experimental works. Certain aspects of the solid-state science of fullerenes (e.g., crystal structures, phase transitions, formation of exo- and... 

Interpolation or intercalation seeIntercalation Chemistiy) is said to occur when additional species are placed into a host structure to change either composition or properties. At one extreme, intercalation can refer to the insertion of guest molecules into cage structures such as that of the zeolites see Zeolite, or between the layers of laminated compounds such as the clays see Silicon Inorganic Chemistry. At the other extreme, the insertion of small atoms such as C or N into metal phases to form interstitial alloys see Alloys , Carbides Transition Metal Solid-state Chemistry, Nitrides Transition Metal Sohd-state Chemistry, is included in the category. A large variety of structures can be found in such materials, and... 

But we omit the generalizations of equilibrium stabilities for phase transitions [1, 103-106] (for them typically criteria stability like (3.256), (3.257) are not valid), for more general materials (say sohds), and the more complicated problem of stabUity of nonequilibtium states (e.g. the vast field of dissipative structures [24, 37, 80, 107-109]) because most of these issues do not concern our (one-phase) model or are now in the stage of intensive and not completely resolved research see also Rem. 31 in Chap. 4. 

Figure 11.27 Freezing , because for most substances the sohd phase is denser than the hquid phase and increasing the pressme will eventually drive a phase transition from the hquid to the sohd state (provided the temperature is below the critical temperatme)...

If produced by burning a CaO (or CaC0)+Si02 blend at a high temperature, the C2S modification that is stable at this temperature and is formed initially in a sohd-state reaction tends to convert gradually to -C2S upon cooling. The phase transitions occur in the sequence a >a However, the transition is usually incomplete, and a... 

Kiefer B, Stixrude L (2001) Phase transition in MgSiOs perovskite. Nature submitted Kieffer SW (1979) Thermodyanamics and lattice vibrations of minerals, I mineral heat capacities and their relationship to simple lattice vibrational modes. Rev Geophys Space Phys 17 1-19 Kittel C (1996) Introduction to sohd state physics. Wiley, New York... 

Palasyuk T, Tkacz M (2007) Pressure-induced structural phase transition in rare-earth tiihydiides. Sohd State Commun 141 354-358... 

O. Kanert, H. Schulz, J. Albers, Nuclear magnetic resonance smdy of the cubic-to-tetragonal phase-transition in BaTi03, Sohd State Commun. 91 (1994) 465—469. 

M. Vijayakumar, J.W. Traer, J.F. Britten, G.R. Goward, Investigations of the phase transition and proton dynamics in rubidium methane phosphonate studied by sohd-state NMR, J. Phys. Chem. C 112 (2008) 5221-5231. 

The theory of step-by-step descent in symmetry due to the Jahn-Teller effect may be appUed to real sohd-state systems where the symmetry of the unit cell is determined by the symmetry of the Jahn-Teller active centre. Phase transitions in such cases where the Jahn-Teller mechanism is determining must correspond to the descent in symmetry via one or more chains originatmg at the same parent symmetry group G. ... 

---