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One standard description of sodium-rich NaCl is removal of lattice chloride into a NaCl reservoir, leaving a positively charged chloride vacancy and an electron:This Kröger–Vink notation equation conserves atoms, chloride sites and effective charge. The electron can be trapped in the vacancy, forming an electrically neutral F center:Thus sodium excess introduces electronic defects and can give coloration. It does not imply that every added electron is a mobile conduction electron: electronic transport depends on trapping and thermal release. Chloride vacancies can also assist anion transport. An alternative microscopic incorporation route is a sodium interstitial plus an electron, but the vacancy–electron model describes the usual alkali-metal-colored halide.

The oxygen-pressure exponents can be explained by a change in the dominant charge state of metal vacancies, with holes carrying the p-type current. For a divalent metal oxide, a doubly negative metal vacancy is $V_{\mathrm{M}}^{\prime \prime }$ in Kröger–Vink notation. A conventional oxygen-incorporation reaction isTreating regular oxygen-site activity as constant, the law of mass action gives $K_2=[V_{\mathrm{M}}^{\prime \prime }]p_h^2/p_{{\mathrm{O}}_2}^{1/2}$. If these are the compensating defects, charge neutrality gives $p_h=2[V_{\mathrm{M}}^{\prime \prime }]$, hence $p_h^3\propto p_{{\mathrm{O}}_2}^{1/2}$ and $\sigma \propto p_{{\mathrm{O}}_2}^{1/6}$, assuming nearly constant hole mobility.

For singly negative vacancies, useNow $p_h=[V_{\mathrm{M}}^{\prime }]$ and $p_h^2\propto p_{{\mathrm{O}}_2}^{1/2}$, so $\sigma \propto p_{{\mathrm{O}}_2}^{1/4}$. The charge-state equilibrium $V_{\mathrm{M}}^{\prime }\rightleftharpoons V_{\mathrm{M}}^{\prime \prime }+h^{\bullet }$ gives $[V_{\mathrm{M}}^{\prime \prime }]/[V_{\mathrm{M}}^{\prime }]=K_{\mathrm{ion}}/p_h$: the singly charged fraction can therefore dominate as oxygen pressure and hole concentration increase. This is a consistent dilute-defect model, not a unique experimental identification of every defect. Changes in mobility, defect association or other compensating defects require independent tests.