Monopnictides

The transition-metal monopnictides MPn with the MnP-type structure discussed above contain strong M-M and weak Pn-Pn bonds. Compounds richer in Pn can also be examined by XPS, such as the binary skutterudites MPn , (M = Co, Rh, Ir Pn = P, As, Sb), which contain strong Pn-Pn bonds but no M-M bonds [79,80], The cubic crystal structure consists of a network of comer-sharing M-centred octa-hedra, which are tilted to form nearly square Pnn rings creating large dodecahedral voids [81]. These voids can be filled with rare-earth atoms to form ternary variants REM Pnn (RE = rare earth M = Fe, Ru, Os Pn = P, As, Sb) (Fig. 26) [81,82], the antimonides being of interest as thermoelectric materials [83]. 

Monopnictides, plutonium, 7.9 691 Monopolar electrodes, 9 621 Mono/poly soluble dyes, 7 373t Monopotassium phosphate (MKP),... 

Protactinium dipnictides (X = As, Sb) have been synthesized by reaction of As or Sb vapour with metal hydride at 400-700 Pa3As4 was obtained by thermal dissociation of PaAs2 at 840 °C, Pa3Sb4 from the corresponding dipnictide at 1200 °C. Monopnictides of protactinium were not obtained by thermal dissociation of higher compounds. The diantimonides of the transuranium elements Np, Pu, Am dissociate between 700-800 °C into the monocompounds. Monopnictides of the higher transuranium elements have been obtained at the pg scale with and 0 by thermal dissociation. 

We take as an example the case of plutonium monopnictides PuP, PuAs, PuSb, the susceptibility of which is well fitted by Eq, (17). Results are given in Table 8 while bare data are in agreement with the full J multiplet value ( 1 pP in the intermediate coupling), the corrected value is substantially smaller and points towards a Fg ground state (see Table 8) in agreement with more detailed discussions ... 

Table 8. Bare ( ) and corrected values for the constant susceptibility and the effective moment of plutonium monopnictides...

O. Vogt and K. Mattenberger, Magnetic measurements on rare earth and actinide monopnictides and... 

The berkelium monopnictides have been prepared on the multimicrogram scale by direct combination of the elements (138). In all cases, the lattice constants of the NaCl-type cubic structures were smaller than those of the corresponding curium monopnictides but comparable to those of the corresponding terbium compounds. This supports the semimetallic classification for these compounds. One additional report of BkN has appeared (139). The lattice parameter derived from the sample exhibiting a single phase was 0.5010 0.0004 nm, whereas that extracted from the mixed-phase sample of BkN resulting from incomplete conversion of a hydride was 0.4948 0.0003 nm. Clearly, additional samples of BkN should be prepared to establish more firmly its lattice constant. 

In this section we present experimental results on the temperature dependence of elastic constants for intermetallic rare-earth compounds in which magnetoelastic effects due to the presence of crystal fields are dominant. There are systematic studies of these effects for given structures across the rare-earth series. Examples are the rare-earth monopnictides, especially the rare-earth antimonides (RSb), the rare-earth dialuminides (RAlj) and rare-earth compounds with the CsCl structure. From such experiments one obtains the single-ion magnetoelastic coupling constants gj. across the series and in a few cases the quadrupolar coupling constant gf [eq. (38)] too. The case of a cooperative Jahn-TeUer effect will be treated separately in sect. 2.4.3. The examples presented here can be explained mostly with the single-ion strain susceptibility Xr [ <1- (35) instead of eq. 

Rare-earth antimonides RSb. Among the rare-earth monopnictides the RSb system has been studied in greatest detail. For a discussion of physical properties of the pnictides see Hulliger (1979). 

Deformation potential coupling constants are of the order of fip, (Ziman 1960). To observe deformation potential effects in the temperature dependence of elastic constants several conditions have to be met as discussed above dpA(,(0) must be large and - Eq has to be of the order of k T. This excludes normal metals and only d-band metals with rather narrow bands can exhibit this behavior. Typical examples have been given above. In intermetallic rare-earth compounds simple density of states arguments show why elastic constant effects can be observed only for CsCl-type and Th3P4-type materials. In table 4 electronic specific heat values are listed for various rare earth compounds. This is an updated list of a previous work, see Liithi et al. (1982). This table indicates that monopnictides and monochalcogenides have smaller values of y than CsCl- and Th3P4-structure materials, i.e., the 5d band of the former structure is more hybridized than in the latter. 

The experimental lattice parameters as a function of lanthanide atomic number show the famous lanthanide contraction, the decrease of the lattice parameter across the lanthanide series, with the exception of the two anomalies for Eu and Yb, as seen in Figure 1 (top panel). What is plotted there b actually the atomic sphere radius S (in atomic units) as a function of the lanthanide element A similar behaviour is abo observed, for example, for lanthanide monochalcogenides and monopnictides, whose lattice parameters are abo shovm in Figure 1 (middle and bottom panels). 

At ambient conditions, the lanthanide monopnictides RX (X = N, P, As, Sb, Bi) and monochalcogenides RX (X = 0, S, Se, Te, Po) crystallize in the NaCl structure. Given the combination of an electropositive R-ion with an electronegative pnic-tide or chalcogenide, one might assume that an ionic picture can be applied here. But based on the observed properties, Rhyne and McGuire (1972) proposed a classification distinguishing between the so-called valence balanced compoimds... 

At ambient conditions, the Yb monopnictides and monochalcogenides crystallize in the B1 structure. As outlined, the Yb pnictides are all foimd to be well described by the nominally trivalent scenario, where the effective valence varies from 2.88 in YbN to 2.69, 2.63, and 2.53 in YbP, YbAs, and YbSb, respectively. Experimentally, the position of the/ band is found 0.2 eV above the Fermi level in YbN, YbP, and YbAs (Degiorgi et al., 1990,1993). Other experiments have revealed heavy-electron behaviour in Yb pnictides (Ott et al., 1985 Sakon et al., 1992 Takeda et al., 1993), but this can be a reflection of sample non-stoichiometry (Degiorgi et al., 1990,1993). The discrepancy between the present electronic structure and the pictme provided by Degiorgi et al. (1990,1993) can be due to the LSD approximation, since the position of the narrow band in the theory is solely determined by the LSD potential (no correlation correction). LDA - - U calculations on YbN (Larson et al., 2007) include a positive correlation shift of the unoccupied f-states that leads to an ideal trivalent Yb ion in accordance with Degiorgi et al. (1990,1993). 

To summarize, most of the lanthanide monopnictide systems are characterized... 

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