物理课件28-2.pptVIP

28.3 Bonds in crystals In ionic solids force ~ positive negative ions f ~ 1/r2 In covalent solids and metallic solids forces ~ between ions and electrons. Molecular solids polar f ~ 1/r3 H2O (ice, snowflakes) non-polar f ~ 1/r7 inert gas, halogens, and fullerball C60... looks like domes designed by architect Buckminster Fuller. Fullerene ball or buckyball 富勒球 巴基球 Alkali-metal-doped crystals:MxC60 (M = alkali metal, K, Rb) is superconducting TC ~28 K Ionic crystals fcc (face-centered cubic), e.g., NaCl bcc (body-centered cubic), e.g., Cscl The potential is basically Coulombic a = 1.747 6 for fcc crystals; a = 1.762 7 for bcc crystals Pauli principle tends to keep those filled subshells from overlapping. This means that there exists a repulsive “potential” bcc 12 CsI bcc 10.5 CsCl fcc 9.5 KBr fcc 9 KCl fcc 9.5 NaI fcc 8 NaCl fcc 7 LiCl fcc 6 LiF structure n Crystal NaCl ? Na+ + Cl– 7.84 eV Na 5.14 eV Cl 3.61 eV Atomic cohesive energy –V0 is the ionic cohesive energy ~ 7.84eV l = 32.6mm (near infrared region) f ~ 1/r2 more stable than molecular solids (1/r3, 1/r7) (cohesive energy~ eV) vaporization temperature ~ 40× 300K, actual values are more like 1 000 to 2 000K transparent to visible light (ions: filled shells) absorb infrared radiation (l = 32.6mm ) soluble in polar liquids poor electrical conductivity than metallic solids 28.4 Free electron model for metals §27.4 Contribution of electrons to heat capacity of solids §19.3 susceptibility with classical model Thermal emission of electrons f in section 22.2 The electrons can leave the well if