NaCl Type Phase

Room temperature values of the lattice constant a (in A), the measured density Dexp, and the calculated density Dcaic derived from lattice constant (both in g/cm )  

For MSe, M = La to Er except Sm and Eu, the unit cell volume is an almost linear function of the melting point (see figure in paper), Kutolin, Smirnova [24]. 

Along the homogeneity range, from MSe to MSeo.75, the lattice constant (in A) changes regularly with composition between the limiting values given  

Guittard et al. [9]. The lattice constants of TmSe in the homogeneity range increase from -5.63 A for Tmo.87Se to -5.71 A for Tm osSe, see p. 320. 

Bond lengths M -Se for 6-coordinated M were calculated by Poix [25], values in A  

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As a final example, we may mention the NaCl-type phases formed in the V-0 systems. The V01 x phase is homogeneous in the composition range 42-57 at.% O. Lattice parameter determination in combination with density measurements evidenced that, in the structure, vacancies occur in both V and O sublattices through the entire range of composition. At the stoichiometric composition VO there are = 15% of sites vacant in each sublattice. 

This structure type with the axial ratio c/a close to 1 is an example of the Hagg interstitial phases formed when the ratio between non-metal and metal radii is less than about 0.59. The structure can be described as a 3D array of trigonal prisms of W atoms (contiguous on all the faces). Alternate trigonal prisms are centred by C atoms. Compounds belonging to this structure type are for instance IrB, OsB, RuB, MoC, WC (compare, however, with the NaCl-type phase), NbN, WN, MoP, etc. 

It is obvious that the stability of the NaCl-type phase in these solid-solution systems increases with the content of NbN or ZrN, whose number of valence electrons is one or two less than that of MoN. That is to say, the stability of the NaCl-type phase in these systems decreases with increasing the number of valence electrons, Ny. In the higher Ny regions, the NaCl-type phase in the MoN-NbN system transforms to the WC-type phase in NH3 gas and to y-Mo2N-type phase in N2 gas. These phase transformations take place in the samples with x < 0.5, ie., Ny > 10, of the Moj Zr N and in the samples with X < 0.38, ie., Ny > 10.38, of the Moj jjNb,(N system. The annealed Nb-rich films showed a small reduction in lattice parameter, although the NaCl-type structure was remained until 1173K. 

N) have high melting point and hardness, even though they exhibit metallic conductivity. However, the Bilz s model cannot give a good explanation for the instability of the NaCl-type phase in the compounds with Ny > 10 and the contribution of d-electrons of metals to these bands is not yet clear, because his band model is too simple to discuss the physical and chemical properties. 

In order to clarify the relation between Ny and r fr, the structural type are plotted as functions of these two parameters for the present systems together with the other related compounds of A/(C, N) in Fig. 3. In this figure the data for the present systems obtained after the annealing under NH3 at 1173K was used. It is seen that there are two regions of structural type according to sample s Ny and r. r, one is the NaCl-type and another is the WC-type phase. In the case of Ny less than 10, only the NaCl-type phase with stoichiometric composition is formed independently of At A ve=10, the... 

WC-type phase and the other hexagonal phases are formed in addition to the NaCl-type phase. In the case of Ny larger than 10, the WC-type phase, the... 

NaCl-type phase with nonstoichiometric composition and y-MojN-type phase, are formed. The WC-type region and the other region can be separated simply by r lr, eg., in the region of larger than 0.53, only the WC-type phase is formed, in which there exists not only simple nitrides and carbides, such as WN, MoN, OsC and RuC, but also solid-solution compounds, such as MoN-NbN, MoN-TiN, TiN-CoN and TiN-NiN systems. It is of interest to note that Tio 7C00 3N and Tip yNif, 3N have the WC-structure while the end members, TiN, CoN and NiN do not take the WC structure. 

Similar phenomena were observed in AIN the onset of the B3 -> B1 type transformation changes from 20-23 GPa in the bulk material, to 14.5 GPa in nano-crystals and 24.9 GPa in nano-wire samples [179]. Bulk and nanocrystaUine GaN also have been studied by high-pressure X-ray diffraction, revealing the phase transitions from the wurtzite to the NaCl-type phase at 40 GPa for the bulk and 60 GPa for the nanocrystals [180],... 

The Eu and Yb pnictides in comparison with the alkaline-earth pnictides. Phases marked with contain partly or exclusively (the NaCl-type phases)... 

As is known in the case of binary systems, when their composition is close to equiatomic, d-ina-via metals form, as a rule, stable phases with Bl-type structures. But, as was observed by Vereshchagin and Kabalkina (1979) using a high-pressure treatment, it is possible to get B1 B2 (NaCl-CsCl)-type structural transitions with some oxides. The question of whether this could happen with d-met carbides was discussed by Ivanovsky et al (1988). These authors carried out LMTO band structure calculations for hypothetical TiC, VC and CrC compounds with a B2 structure. The lattice parameters were determined from the condition that the unit cell volumes of the CsCl- and NaCl-type phases were equal. In order to consider the influence of uniform isotropic compression the B2 VC calculations were carried out for crystal lattice volumes of 5 and 10% less than the equilibrium one. 

In and T1 monohalides are isoelectronic with the Sn and Pb monochalcogenides. The character of their structures, however, is different, although geometrically, the two structures are closely related. Common to both groups is, furthermore, the occurrence of NaCl-type phases and NaCl-type derivatives (see Table 51). CsCl-type modifications, however, are known only for the In and T1 halides, similar to the alkali halides and rare-earth monochalcogenides, but no such modifications are reported for the isoelectronic Sn and Pb chalcogenides. 

Lattice constants, calculated and measured densities at 23 °C for several samples of the NaCl-type phase Agi jSn,+,Se2 y. The measured values represent the average of six determinations for each composition. 

Near solid solution limit two NaCl-type phases after pressure release (the phase with the larger constant resulted from conversion of wurtzite phase underpressure, see Fig. 103). 

Under pressure (p 45 kbar) transformation to NaQ structure and conversion to zincblenpressure released, except near the solid solution limit, where the NaCl phase remained. Therefore two NaCl-type phases resulted from the high pressure conversion. See also Fig. 103. 

Fig. 116. CeS,, Si CeSi, Ge,. Lattice constant of the NaCl-type phase vs. composition pOGhel].

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