  Using first principles calculations, we study the ground-state structure of
bulk proton-exchanged lithium niobate, which is also called hydrogen niobate
and is widely used in waveguides. Thermodynamics helps to establish the most
favorable nonpolar surface as well as the water-deficient and water-rich phases
under different ambient conditions, which we refer to as "dehydrated" and
"rehydrated" phases, respectively. We compute the low-frequency dielectric
response and the optical refractive indices of hydrogen niobate in different
phases. The dielectric constant is greatly enhanced compared to lithium
niobate. At shorter wavelengths, the refractive indices vary between each phase
and have a sharp contrast to lithium niobate. Our study characterizes the
structures and thermal instabilities of this compound and reveals its excellent
dielectric and optical properties, which can be important in the future
application in waveguides.
