Monoclinic modification

Like sulphur, selenium exists in a number of allotropic forms. These include both crystalline, rhombic and monoclinic modifications... 

Methylsull anyT I //-tetrazole was found to crystallize in a monoclinic form, and could be sublimed into an orthorhombic form, with the structures differing in the relative polarity of the molecular layers in the two forms [47]. /) -1 o d o a ce t o p h e n one was found to crystallize in two polymorphs that both contained C—H-re points of contact, but where the contacts were shorter in one form than in the other [48]. A second monoclinic modification of the mixed salt benzimidazolium 3-carboxyphe-noxyacetate 3-carboxyphenoxyacetic acid was reported, where the acid hydrogen atom and the two monoanions comprised a carboxylate monoanion/neutral molecule in which the acid proton was disordered between the two anionic units [49]. 

Bright orange-red triclinic or monoclinic crystals density 2.676 g/cm at 25°C triclinic form converts to monoclinic modification at 241.6°C melts at 398°C decomposes at 500°C moderately soluble in cold water, 4.9 g/lOOmL at 0°C very soluble in boiling water, 102 g/lOOmL at 100°C aqueous solution acidic, a 10% solution has a pH 3.57 insoluble in alcohol. 

Figure 1 Crystal structures of (a) GeF2 and (b) the monoclinic modification of SnF2...

Both lead chromate and lead sulfochromate (the latter is a mixed-phase pigment) can be orthorhombic or monoclinic the monoclinic structure is the more stable [3.132], The greenish yellow orthorhombic modification of lead chromate is metastable at room temperature, and is readily transformed to the monoclinic modification under certain conditions (e.g., concentration, pH, temperature). The latter modification occurs naturally as crocoite. 

The crystal and molecular structures of the orthorhombic and monoclinic modifications of (Me)3Sn(Me2Dtc) have been determined (570). In both struc-... 

Thallium(I) oxide, T120, has a monoclinic modification, Q h, C2/m, a = 6.082, b = 3.52, c = 13.24 A, and p = 108.2°, with four molecules in the unit cell. Figure 5.63 shows the T1 atoms in close-packed layers in an AB sequence. Alternate O layers are filled by oxygen atoms giving the notation 2 3/2POP(m). The octahedra share edges. This is an unusual structure with oxygen atoms in octahedra. The Tl-O distances are 2.511-2.531 A. The Tl-Tl distances within a plane are 3.514 A, within... 

Above 900° C hexagonal MoTe2 transforms into a monoclinic modification. The structure of MoTe2(h) (Fig. 10) is almost identical to that... 

FlG. 4. Solid state absorption spectra of BkBr3 in its orthorhombic and monoclinic modifications at room temperature. 

Crystal structure cubic, orthorhombic and monoclinic modifications... 

Production clinkers have been found to contain cubic or orthorhombic forms of the aluminate phase, alone or in combination. The monoclinic modification has not been observed. The orthorhombic modification is also known as the prismatic, dark interstitial material, and is sometimes pseudo-tetragonal. It can arise only if sufficient alkali is available, but its formation appears to be favoured also by rapid cooling and by bulk compositions potentially able to yield a relatively high proportion of aluminate phase (M12). 

The strengthening of bonds in the monoclinic modification causes an increase of the crystallization temperature of activated gels. The activation of TiOj gel causes anatase formation and simplifies further transformation into rutile (T, decreases). 

AI2O3 and Si02 with added mineralizers (KF, LiCl) provides substantial intensification of the synthesis of barium aluminosilicate. The formation of the monoclinic modification 0fBaAl2Si2Og in reaction mixtures activated in the presence of modifying agent (5 %... 

Hexagonal celsian can be transformed into monoclinic modification by mechanical activation of the mixture with modifiers (KF, LiCl at a level of 1-5 mol. %) followed by annealing at 1100°C. 

Telluric acid is a white sohd the crystal structure of which is built up of discrete regular octahedral molecules of Te(OH)6. It can conveniently be obtained by oxidizing TeC>2 in aqueous solution by, for example, KMn04. Anhydrous telluric acid can be crystallized in cubic, tetragonal, and monoclinic modifications. Structural data exist for the cubic and monoclinic forms. They both consist of discrete Te(OH)6 molecules that are linked together by hydrogen bonds. 

In 2,3,7,8-tetramethoxythianthrene 3-III, the less stable (lower in both density and absolute value of lattice energy) monoclinic modification is obtained under kinetic conditions rapid crystallization from polar diisopropyl ether, whereas the more stable (higher density and lattice energy) orthorhombic modification is thermodynamically obtained from a non-polar hydrocarbon solvent. 

The similarity of the two crystal structures leads to very similar X-ray powder diffraction patterns (Fig. 9.3), reminiscent of the situation in terephthalic acid (Section 4.4). Careful inspection reveals that the orthorhombic modification can be distinguished by a peak (the 511 reflection in the Golovina et al. (1994) cell) at 29 = 27.24°, while the monoclinic modification can be distinguished peak (from 211) at 20 = 19.26°. 

Stam, C. H. (1972). Crystal structure of a monoclinic modification and the refinement of a triclinic modification of Vitamin A acid (retinoic acid), C20H28O2. Acta Crystallogr. B, 28, 2936 5. [162]... 

The stretch induced transformation of initially unoriented orthorhombic polyethylene (Vestolen A) into the monoclinic modification was studied by Heise, Riekel and Stach for stretching velocities down to 0.26 mm s"T The coexistence of both modifications and the formation of two domains of monoclinic material was established by the Vidicon detector. 

The a-phase is the low temperature monoclinic modification of cryolite (sodium hexafluoroalumlnate), space group P2 /n (4,... 

At room temperature, pure Zr02 is monoclinic and has a density of 5,7 gcm". When heated it undergoes a quite complex sequence of polymorphic inversions which have not yet been explained to general satisfaction. Ruh and Rockett (1970) attempted to summarize the available data in the phase diagram shown in Fig. 9. Some properties of the modifications are listed in Table 3. The monoclinic modification (baddeleyite) is stable at room temperature and under normal pressures, and is con-... 

Several stmctural modifications of the AM2(P04)3 orthophosphates are known at the present time. The realization of one or another modification depends on the kind and combination of the and A cations, on the synthesis conditions, namely temperature, nature of solvent, atmosphere and so on. For a long time, the monoclinic modification, space group C2 tc or Cc, was considered to be the most widely represented among the actinide and monovalent cation phosphates AM2(P04)3. Its formation had been observed for thorium phosphates and arsenates with A = Li, Na, K, Rb, Cs, Cu, Ag, T1 [26-33], for uranium compounds with A = Li, Na, K, Cs [26,29,30,34-38,39,40],... 

---