Experimental data on thermionic current density and electron work function of YB₁₂ (100) at T = 1218 – 1978 K in high vacuum (p < 10^-4 Pa) are first introduced. Temperature dependences of the thermionic current density and effective electron work function are presented without extrapolation to the zero-field currents because of the anomalous Schottky effect. The temperature dependences of theYB₁₂ electron work function can be described by linear functions at certain temperature intervals. Preferential boron evaporation and additional ion bombardment by the residual gases ions and evaporated boron ions results in appearance of new phases depleted of boron on the YB₁₂ surface. As a result, an YB₄ – YB₆ double layer on the YB₁₂ single crystal surface has formed. The appearance of the new boride phases, depleted of boron, on the emitting surface causes an increase of thermionic current density compared with individual YB₁₂.