CsCl-type structure

There is a lively controversy concerning the interpretation of these and other properties, and cogent arguments have been advanced both for the presence of hydride ions H" and for the presence of protons H+ in the d-block and f-block hydride phases.These difficulties emphasize again the problems attending any classification based on presumed bond type, and a phenomenological approach which describes the observed properties is a sounder initial basis for discussion. Thus the predominantly ionic nature of a phase cannot safely be inferred either from crystal structure or from calculated lattice energies since many metallic alloys adopt the NaCl-type or CsCl-type structures (e.g. LaBi, )S-brass) and enthalpy calculations are notoriously insensitive to bond type. 

A martensitic transformation from a cubic CsCl-type structure by 110 (lT0> type shears occurs for NlxAli. alloys in the composition range 0.615 < x < 0.64. Precursive... 

According to investigations performed by Pakhomov and Kaidalova [204], the crystal structure of NF NbC consists of infinite chains made up of distorted octahedral ions (NbOF4 ) linked by oxygen atoms. Ammonium cations, NH/, occupy the spaces between the chains, as shown in Fig. 30. The packing of the structural units in the NH4NbOF4 crystal can be described as a CsCl type structure in which CF ions are replaced by Nb02F4 complexes and Cs" ions are replaced by ammonium ions. 

Figure 11.7 shows schematically the resulting calculated variation of H with p for the NaCl-type and the CsCl-type phases of CaO. The NaCl-type structure, which is stable at low pressures, is the rock salt structure in which the Ca and O atoms are 6-coordinate. In the CsCl structure, stable at high pressures, both cation and anion are 8-coordinate. In the static limit where the entropy is set to zero, the thermodynamically most stable phase at any pressure is that with the lowest value of H at the thermodynamic transition pressure, ptrs, the enthalpies of the two phases are equal. For CaO the particular set of potentials used in Figure 11.7 indicates a transition pressure of 75 GPa between the NaCl-type and CsCl-type structures, which compares with experimental values in the range 60-70 GPa. 

Intuitively, one would expect a volume contraction on forming a strongly bonded compound from the elements. Indeed, Richards 190, 191) regarded heats of formation as heats of compression. The fractional volume contraction, AV = (molecular volume - 2 atomic vol-ume)/2(atomic volume), has been related to formation heats for NaCl or CsCl type structures 151). Even nonpolar compounds in the condensed state cohere in close-packed arrays. The packing density of difluorine, derived from the ratio of the van der Waals envelope to the molecular volume, is especially low, and a larger contraction would be expected for fluorides than for other halides. This approach has yet to be systematically examined. 

Unlike CsAu (Cs+Au , CsCl type structure) and others [20] which are aurides with well defined Au ions, CsHg and similar amalgams are not simple mercu-rides but have more complicated structure and bonding properties, as was shown in Section 2.4.2.2. 

In several Fe, Co, Ni alloy systems, phases having structures pertaining to the inter-related Laves type and a and // types are formed (often homogeneous in certain ranges of compositions). For compositions around 1 1, a number of solid solution phases with the CsCl-type structure and (with semi-metals) with the NiAs-type are found. 

The W body-centred cubic structure can be compared with the simple cubic CsCl-type structure (which can be obtained from the W type by an ordered substitution of the atoms) and with the MnCu2Al-type structure ( ordered superstructure of the CsCl type) see Fig. 3.31 and notice the typical eight (cubic) coordination. 

Figure 7.20. Section sequence parallel to the base plane of the cP2-CsCl type structure.

The characteristic sections of a few adjacent cells are shown in Fig. 7.22 for these structures. A perspective view of the AlCu2Mn structure and a comparison with the CsCl-type structure. 

The alloy /J NiAl is a solid with a CsCl-type structure in which one atom is located at the corners, and the second atom at the center of the unit cell. The valence-electron to atom ratio is often quoted as 1.5, using a counting scheme in which the transition metal has zero valence and the A1 is considered as trivalent. 

Figure 9.25. The CsCl-type structure of a-IrV. A view of the structure looking down the c axis is shown. The larger shaded circles are Ir atoms.

Transformation Twinning of Z 2(CsCl)-Type Structure Based on an Inhomogeneous Shear Model Frederick E. Wang... 

The atomic mechanism, based on the previously proposed inhomogeneous shear, leading to the formation of twinning and antiphase boundaries in TiNi with the CsCl-type structure is described. The twinning mechanism described herein explains the electrical resistivity anomaly due to incomplete thermal cyclings observed previously in TiNi. This explanation is in keeping, in a qualitative manner, with the "memory effects observed in relation to the electrical resistivity anomaly. 

As early as 1943, Sommer (101) reported the existence of a stoichiometric compound CsAu, exhibiting nonmetallic properties. Later reports (53, 102, 103,123) confirmed its existence and described the crystal structure, as well as the electrical and optical properties of this compound. The lattice constant of its CsCl-type structure is reported (103) to be 4.263 0.001 A. Band structure calculations are consistent with observed experimental results that the material is a semiconductor with a band gap of 2.6 eV (102). The phase diagram of the Cs-Au system shows the existence of a discrete CsAu phase (32) of melting point 590°C and a very narrow range of homogeneity (42). 

Alpha silver iodide (a-Agl), a fast ion conductor, is one of the different polymorphic structures of Agl showing a cubic structure [51], where I occupies anionic positions, that is, the Cl- sites in the CsCl-type structure (see Figure 2.19). On the other hand, the low temperature phase, that is, p-Agl, exhibits a hexagonal wurtzite-type structure. 

An ab initio molecular-orbital calculation for the NH3-HC1 dimer was performed and the existence in it of a hydrogen bond was predicted [98-700]. This result was confirmed experimentally when it was found that three N-H bonds are shorter than the fourth one [707], NH4C1 crystallizes in the CsCl-type structure, where NH4 has tetrahedral symmetry, because of the stabilization effect of the Madelung energy... 

BaO CsCl-type structure all cubic holes filled by Ba2+ cations ... 

It appears that the difference in coordinating capability of the anions in the above structures is associated with differences in polarizability of the F ligands in the MF. In general, the CsCl type structure occurs only when at least one of the ions is highly polarizable. In those hexafluorometalates which can be unambiguously described as salts A+MF the small, less polarizable (harder) cations such as Li " and Na generally... 

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