Some properties of the a1 compounds are in table 711

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Unformatted text preview: ride TliN is known, and the azides TI'N3 and T1'[Tl1''(N3)~]; the phosphides T13P, TIP3 and TlP5 have been reported but are not well characterized. With AS, Sb and Bi thallium forms alloys and intermetallic compounds Tl3X, T17Bi2 and TlBi2. The 111-V semiconductors can all be made by direct reaction of the elements at high temperature and under high pressure when necessary. Some properties of the A1 compounds are in Table 7.11 from which it is clear that there are trends to lower mp and energy band-gap E , with increasing atomic number. Analogous compounds of Ga and In are grey or semi-metallic i n appearance and show similar trends (Table 7.12). These data should be compared with those for Si, Ge, Sn and Pb on p. 373 and for the isoelectronic 11-VI semiconductors of Zn, Cd and Hg with s, Se and Te (p. 1210). In addition, GaN is obtained by reacting Ga and NH3 at 1050" and InN by reducing and nitriding In203 with NH3 at 630". The Table 7.11 Some properties of A1 111-V compounds Property Colour MPK E ,kJ AlN Pale yellow >2200 decomp 41 1 AlP Yellow 2000 236 AlAs Orange 1740 208 AlSb - 1060 145 (a)Energygap between top of (filled) valence band and bottom of (empty) conduction band (p. 332). To convert from W mol-' to eV atom-' divide by 96.485. Table 7.12 Comparison of some 111-V semiconductors Property GaP GaAs GaSb InP InAs InSb MPPC E ,M mol-'(a) 1465 218 1238 138 712 69 1070 130 942 34 525 17 (a)See note to Table 7.11. 256 Aluminium, Gallium, Indium and Thallium nitrides show increasing susceptibility to chemical attack, AIN being inert to both acids and alkalis, GaN being decomposed by alkali, but not acid, and InN being decomposed by both acids and alkalis. Most of the other 111-V compounds decompose slowly in moist air, e.g. A1P gives Al(OH)3 and PH3. As a consequence, semiconductor devices must be completely encapsulated to prevent reaction with the atmosphere. The great value of 111-V semiconductors is that they extend the range of properties of Si and Ge and by judicious mixing in ternary phases they permit a continuous interpolation of energy band gaps, current-carrier mobilities and other characteristic properties. Some of their uses are summarized in the Panel on p. 258. Other compounds containing AI-N or Ga-N bonds, including heterocyclic compounds and cluster organometallic compounds, are considered in section 7.3.6. Some unusual stereochemistries While it remains true that tetrahedral and octahedral coordination modes are the predominant stereochemistries adopted by the group 13 metals, nevertheless increasing diversity is being achieved by carefully selecting appropriate electronic and geometric features to enhance the stabilization of unusual stereochemistries. Some representative examples follow. Trigonal planar A1 is found in the [A1Sb3I6“anions” in [Cs6K3Sb(AISb3)], which is formed by heating a stoichiometric mixture of 6Cs, 3KSb and AlSb in a sealed Nb ampoule at 677”C.(65) The Ga analogue was prepared similarly. The plan...
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