  The valley-related multiple topological phase transitions attracted
significant attention due to their providing significant opportunities for
fundamental research and practical applications. However, unfortunately, to
date there is no real material that can realize valley-related multiple
topological phase transitions. Here, through first-principles calculations and
model analysis, we investigate the structural, magnetic, electronic, and
topological properties of VSiXN$_4$ (X = C, Si, Ge, Sn, Pb) monolayers.
VSiXN$_4$ monolayers are stable and intrinsically ferrovalley materials.
Intriguingly, we found that the built-in electric field and strain can induce
valley-related multiple topological phase transitions in the materials from
valley semiconductor to valley-half-semimetal, to valley quantum anomalous Hall
insulator, to valley-half-semimetal, and to valley semiconductor (or to
valley-metal). The nature of topological phase transition is the built-in
electric field and strain induce band inversion between the
d$_{xy}$/d$_{x2-y2}$ and d$_{z2}$ at obritals at K and K' valleys. Our findings
not only reveal the mechanism of multiple topological phase transitions, but
also provides an ideal platform for the multi-field manipulating the spin,
valley, and topological physics. It will open new perspectives for spintronic,
valleytronic, and topological nanoelectronic applications based on these
materials.

Mesoscale and Nanoscale Physics;Materials Science;Applied Physics